**Advancements in Development and Characterization**

Editors

**Pawel Sikora Sang-Yeop Chung**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Editors* Pawel Sikora Technische Universitat Berlin ¨ Germany West Pomeranian University of Technology Szczecin Poland

Sang-Yeop Chung Department of Civil and Environmental Engineering, Sejong University Republic of Korea

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Crystals* (ISSN 2073-4352) (available at: https://www.mdpi.com/journal/crystals/special issues/ Cement Composites).

For citation purposes, cite each article independently as indicated on the article page online and as indicated below:

LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. *Journal Name* **Year**, *Article Number*, Page Range.

**ISBN 978-3-03943-657-6 (Hbk) ISBN 978-3-03943-658-3 (PDF)**

© 2020 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications.

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### **Contents**


#### **Rahwan Hwang, Il-Wha Lee, Sukhoon Pyo and Dong Joo Kim**

Influence of the Aggregate Surface Conditions on the Strength of Quick-Converting Track Concrete Reprinted from: *Crystals* **2020**, *10*, 543, doi:10.3390/cryst10060543 .................. **149**

#### **Zuzana Marcalikova, Radim Cajka, Vlastimil Bilek, David Bujdos and Oldrich Sucharda**

Determination of Mechanical Characteristics for Fiber-Reinforced Concrete with Straight and Hooked Fibers

Reprinted from: *Crystals* **2020**, *10*, 545, doi:10.3390/cryst10060545 .................. **163**

### **About the Editors**

**Pawel Sikora** is an Assistant Professor at the Faculty of Civil and Environmental Engineering of the West Pomeranian University of Technology, Szczecin. He received his Ph.D. in civil engineering from Warsaw University of Technology. He has vast teaching and research experience and is the co-author of numerous papers, mainly, in the field of modification of cementitious composites with nanomaterials and lightweight concretes. Currently, he is conducting his postdoctoral research stay at the Technical University of Berlin within the Marie-Skłodowska Curie Actions – Individual Fellowship (Horizon 2020) related to 3D printing of concrete.

**Sang-Yeop Chung** is working as an assistant professor in the Department of Civil and Environmental Engineering at Sejong University, Seoul, Korea. He attained his Ph.D. degree at Yonsei University in Korea and worked as a postdoctoral researcher at the same University. Prof. Chung also worked at Technische Universitat Berlin (TU Berlin) in Germany until February ¨ 2019. He mainly studies the micro-scale material characterization and property evaluation of engineering materials (e.g., concrete, lightweight aggregate, metals). Throughout his academic career, Prof. Chung has received awards from the German Academic Exchange Service (DAAD) and the National Research Foundation of Korea (NRF) and has published many research papers in renowned international journals.

### *Editorial* **Cement-Based Composites: Advancements in Development and Characterization**

**Pawel Sikora 1,2,\* and Sang-Yeop Chung 3,\***


Received: 16 September 2020; Accepted: 17 September 2020; Published: 17 September 2020 -

**Abstract:** This Special Issue on "Cement-Based Composites: Advancements in Development and Characterization" presents the latest research and advances in the field of cement-based composites. This special issue covers a variety of experimental studies related to fibre-reinforced, photocatalytic, lightweight, and sustainable cement-based composites. Moreover, simulation studies are present in this special issue to provide the fundamental knowledge on designing and optimizing the properties of cementitious composites. The presented publications in this special issue show the most recent technology in the cement-based composite field.

**Keywords:** cement-based composite; experimental studies; numerical simulation; sustainability

#### **1. Introduction**

Concrete, a composite material composed of cement, water, aggregates, and often admixtures, is the most produced human-made material in the world. This material is an indispensable element of modern societies and is used in most of today's constructed engineering structures. Concrete structures need to satisfy specific characteristics in terms of mechanical performance and long-term durability so that they can be used without serious consideration of maintenance for many decades. Therefore, proper methods to produce advanced high-performance composites are actively required. Due to their composite nature, the choice of proper individual components and their interaction and compatibility plays a vital role in shaping the final properties of cement-based composites. In addition, there is a strong need to develop sustainable cementitious composites as well as their alternatives to tackle the challenge of increasing total anthropogenic carbon dioxide emissions. Therefore, numerical approaches to modeling and evaluating a material's characteristics and properties can also be used to accelerate the material's development.

This Special Issue on "Cement-Based Composites: Advancements in Development and Characterization" presents the latest research and advances in the field of cement-based composites.

#### **2. Contents of this Special Issue**

Marcalikova et al. [1] performed a study on the effects of two types of fibers (hooked and straight) on the quantitative and qualitative parameters of concrete. The fibers were hooked and straight. The influence of the fibers type and content on the mechanical properties in fiber-reinforced concrete was analyzed by functional dependence. Comprehensive evaluations of the mechanical properties of compressive strength, splitting tensile strength, bending tensile strength, and fracture energy

were performed. Moreover, the resulting load-displacement diagrams and summary recommendations for the structural use and design of fiber-reinforced concrete structures were presented.

Smarzewski [2] conducted his research on local bond strength of short length specimens in high-performance concrete (HPC) and basalt fibre reinforced high-performance concrete (BFRHPC). As the main variables in this study, the basalt fibre volume content, concrete cover, bar diameter, and rib geometry were chosen. Moreover, additional factors were the directions of the casting and loading. For different ranges of BFRHPC strength, relationships for bond strength concerning the splitting tensile strength were obtained. The study showed that the bond strength increased with the splitting tensile strength and compressive strength of BFRHPC specimens with the 12 mm and 16 mm bar respectively. Moreover, the bond strength of BFRHPC was lower for the bar with the greater distances between the lugs on the bar.

In the study by Jiang et al. [3], a new mixture composition of steel fiber-reinforced MgO concrete (SFRMC) was proposed and evaluated to combine positive effects of both fiber-reinforced concrete and expansion concrete. The influence of steel fiber and MgO on the strength and chloride diffusion resistance of concrete was evaluated by splitting tensile test and chloride diffusion test. The results showed that the combined action of steel fiber and MgO reduced the porosity of concrete and the chloride diffusion coefficient, which could not be achieved by steel fiber and MgO separately. Moreover, mercury intrusion porosimeter (MIP) and scanning electron microscopy (SEM) confirmed that newly developed concretes exhibit lower porosities than reference concrete.

Cheng et al. [4] performed a study on the unconfined compression strength of polypropylene fiber-reinforced composite cemented clay. Three main factors, including polypropylene fiber content, composite cement content, and curing time on the unconfined compressive strength of fiber-reinforced cemented clay, were studied. The authors concluded that the incorporation of fibers can increase the compressive strength and residual strength of cement-reinforced clay as well as the corresponding axial strain when the stress peak is reached compared with cement-reinforced clay. The compressive strength of fiber-composite cement-reinforced marine clay increases with the increase of curing time and composite cement content. Moreover, differences in the failure mode in specimens were found: cement-reinforced clay is a brittle failure, while the failure mode of fiber-reinforced cemented clay is a plastic failure.

Lehner et al. [5] evaluated the chloride ion diffusion coefficient of self-compacting concrete with steel-fibre reinforcement. Data was compared with Ordinary Portland Cement (OPC). Three different procedures of diffusion coefficient calculation were presented namely: rapid chloride penetration test accelerated penetration tests with chloride, and surface measurement of electrical resistivity using Wenner probe. The resulting diffusion coefficients obtained by all methods are compared and evaluated regarding the basic mechanical properties of concrete mixtures. Meaningful relations between the testing methods and steel fibers content were established.

Zhong et al. [6] performed a study on visible light catalysis of graphite carbon nitride-silica composite material and its potential as a surface treatment of cement. Graphite carbon nitride-silica composite materials were synthesized by thermal polymerization using nanosilica and urea as raw materials. The effect of nanosilica content and specific surface area were investigated towards optimizing the material composition. The surface of cement-based materials was treated with graphite carbon nitride-silica composite materials by the one-sided immersion and brushing methods for the study of photocatalytic performance. By comparing the degradation effect of Rhodamine B, it was found that the painting method is more suitable for the surface treatment of cement. In addition, through the reaction of calcium hydroxide and graphite carbon nitride-silica composite materials, it was found that the combination of graphite carbon nitride-silica composite materials and cement is through C–S–H gel. Another approach towards the production of photocatalytic cementitious composites was proposed by Wang et al. [7]. TiO2@SiO2 core-shell nanocomposites with different coating thicknesses were synthesized by varying the experiment parameters. Authors reported that the introduction of SiO2 coatings accelerated the rhodamine B removal to a certain extent, owing to its

high surface area; however, more SiO2 coatings decreased its photocatalytic efficiencies. The cement matrix treated with TiO2@SiO2 core-shell nanocomposites showed good photocatalytic efficiency and durability after harsh weathering processing. A reaction mechanism was revealed by the reaction of TiO2@SiO2 nanocomposites with Ca(OH)2.

Abd Elrahman et al. [8] performed a study towards the development of insulating light-weight cementitious composites in order to reduce the energy loss and consumption in buildings. Three different approaches towards incorporating air voids in cement pastes were proposed by introducing: aluminum powder, air-entraining agent, and hollow microspheres. A comprehensive evaluation of oven-dry density, compressive strength, porosity, water absorption, and thermal conductivity was performed. Moreover, X-ray micro-computed tomography (micro-CT) was adopted to investigate the microstructure of the air-entrained cement pastes. Meaningful relations between each type of air-entraining method were established. The experimental results obtained showed that specimens with an admixture of hollow microspheres can improve the compressive strength of cement composites compared to other air-entraining admixtures at the same density level. It was also confirmed that the incorporation of aluminum powder creates large voids, which have a negative effect on specimens' strength and absorption.

Hwang et al. [9] studied the effects of the aggregate surface conditions on the compressive strength of quick-converting track concrete (QTC). The compressive strength of QTC and interfacial fracture toughness (IFT) were investigated by changing the amount of fine abrasion dust particles (FADPs) on the aggregate surface from 0.00 to 0.15 wt% and the aggregate water saturation from 0 to 100%. Significant relations between the effects of aggregate water saturation on the compressive strength of the QTC and IFT were reported in correspondence to the amount of FADPs. As an outcome of this comprehensive research, fundamental knowledge of the importance of the aggregate surface conditions for the strength development of QTC was presented.

Is,toan et al. [10] performed a study on the production of lightweight mortars containing hemp shiv, volcanic rocks, and white cement. A complex study covering the evaluation of the chemical, acoustic, thermal, and mechanical properties as well as fire (heat) resistance were performed. Interesting relations between the constituents and effects on the various material on the selected properties were comprehensively evaluated. As an outcome of this study, sustainable cementitious mortar with increased indoor comfort performance was produced.

Ren et al. [11] performed a study towards understanding the underlying mechanism of inhibition of the alkali-carbonate reaction (ACR) using fly ash. Authors reported that when the alkali equivalent (equivalent Na2Oeq) of the cement is 1.0%, the addition of 30% fly ash can significantly inhibit the expansion in low-reactivity aggregates, while for moderately reactive aggregates, the expansion rate can also be reduced by adding 30% of fly ash. The study showed that fly ash refines the pore structure of the cement paste, thus the alkali migration rate in the curing solution to the interior of the concrete microbars is reduced. As the content of fly ash increases, the concentrations of K<sup>+</sup> and Na+, and the pH value in the pore solution gradually decrease. This makes the ACR in the rocks slower, such that the cracks are reduced, and the expansion due to the ACR is inhibited.

In addition to the experimental studies, Kim et al. [12] proposed a numerical method for inversely estimating the spatial distribution characteristic of a material's elastic modulus using the measured value of the observation data and the distance between the measurement points. In this study, the structural factors with randomness are typically modeled as having a certain probability distribution (probability density function) and a probability characteristic (mean and standard deviation). To overcome the limitations of previous studies with uncertainties, Kim et al. propose a method to numerically define the spatial randomness of the material's elastic modulus and confirm factors such as response variability and response variance so that the material properties can be predicted using the proposed method.

The above-mentioned articles are of cement-based composites with different approaches, and these studies can contribute to the development of advanced cement-based materials significantly.

**Author Contributions:** Writing—original draft, P.S. and S.-Y.C.; Writing—review & editing, P.S. and S.-Y.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** P.S. wishes to acknowledge that this special issue was launched thanks to his Postdoctoral research stay which has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 841592.

**Acknowledgments:** We thank all the authors who contributed to this special issue for preparing interesting papers as well as reviewers who shared their valuable time for reviewing these publications. P.S. is Supported by the Polish Foundation for Science.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*

### **An Investigation of the Mechanical and Physical Characteristics of Cement Paste Incorporating Di**ff**erent Air Entraining Agents using X-ray Micro-Computed Tomography**

#### **Mohamed Abd Elrahman 1,2, Mohamed E. El Madawy 2, Sang-Yeop Chung 3,\*, Stanisław Majer 4, Osama Youssf 2,5 and Pawel Sikora 1,4**


Received: 13 December 2019; Accepted: 4 January 2020; Published: 6 January 2020

**Abstract:** Improving the thermal insulation properties of cement-based materials is the key to reducing energy loss and consumption in buildings. Lightweight cement-based composites can be used efficiently for this purpose, as a structural material with load bearing ability or as a non-structural one for thermal insulation. In this research, lightweight cement pastes containing fly ash and cement were prepared and tested. In these mixes, three different techniques for producing air voids inside the cement paste were used through the incorporation of aluminum powder (AL), air entraining agent (AA), and hollow microspheres (AS). Several experiments were carried out in order to examine the structural and physical characteristics of the cement composites, including dry density, compressive strength, porosity and absorption. A Hot Disk device was used to evaluate the thermal conductivity of different cement composites. In addition, X-ray micro-computed tomography (micro-CT) was adopted to investigate the microstructure of the air-entrained cement pastes and the spatial distribution of the voids inside pastes without destroying the specimens. The experimental results obtained showed that AS specimens with admixture of hollow microspheres can improve the compressive strength of cement composites compared to other air entraining admixtures at the same density level. It was also confirmed that the incorporation of aluminum powder creates large voids, which have a negative effect on specimens' strength and absorption.

**Keywords:** lightweight cement paste; air entraining agents; hollow microspheres; aluminum powder; pores; micro-CT; thermal insulation; compressive strength

#### **1. Introduction**

Nowadays, rapid population growth and development have led to a significant increase in energy demand. However, fossil fuels-which are the main energy resource-are limited and subject to being depleted in the next few decades. In addition, their use has had a harmful impact on the environment. They are responsible for climate change and environmental pollution; fossil fuels are the primary resource for CO2 emission globally [1]. To cope with these challenges, it is imperative to make optimum use of the available energy resources and to reduce energy consumption as much as possible. In reference to thermal comport, buildings consume about 40% of global energy consumption and are consequently responsible for about one-third of global carbon emissions [2]. High efficiency energy utilization in buildings has therefore become a principle policy for many countries. Recently, there has been increasing interest in improving the thermal insulation of buildings in order to save energy and to reduce the environmental problem. In case of existing structures, this goal can be mostly achieved by insulating the already existing structures (e.g. masonry walls), which occurs mostly during the restoration period [3]. However, for a purpose of erecting the new structures several insulating materials were developed for decreasing the energy losses from buildings and to meet structural stability and strength requirements. Lightweight cement-based materials are one of the most ideal materials for such applications, because of its high thermal insulation efficiency as well as its reasonable mechanical characteristics [4–9].

Today, lightweight concrete is available in a wide range of densities and strengths which makes it suitable for various applications. The basic advantages of lightweight concrete are its low thermal conductivity, which makes it ideal for energy conservation, as well as its low density, which reduces the dead load of structures. Cellular concrete is a type of lightweight concrete composed of cement, water, entrained air and sometimes fine aggregate [10–12]. Generally, two methods have been developed for entraining air in concrete; physically, with the use of pre-formed foam (foamed concrete) and chemically, through the use of an air-entraining agent (aerated concrete). Cellular concrete can be designed to have a density of 200 to 1800 kg/m3, depending on whether its applications are structural or non-structural [13]; though the density of cellular or lightweight concrete is relatively low, these materials have advantages to be used for both purposes. For example, structural lightweight concrete is largely used for building located in seismic area to reduce the mass subjected to seismic loads [14,15], and non-structural lightweight concrete is generally used for insulation and noise reduction [16,17].

Aerated concrete can be autoclaved (AAC) or non-autoclaved (NAAC) depending on the curing method used. In general, NAAC is known to be beneficial than AAC in the following aspects; NAAC is less expensive in production and shows better acoustic protection, fire safety, and better insulation than AAC [18–20]. On the basis of these advantages, between these approaches, the goal of the present research is to examine the properties of non-autoclaved aerated concrete, produced using different air-entraining agents (AEAs). NAAC is manufactured by creating macro porosity in a cement matrix, with the help of an expansive agent which reacts with the water and lime in the suspension [21]. This reaction liberates gas which expands the fresh concrete and generate pores, which reduce the weight and thermal conductivity of aerated concrete after hardening. The formed bubbles are nearly spherical [22] and have a diameter of about 10 μm to 1 mm [23] depending on the surfactant used. In addition to reducing the weight of concrete, air entraining agents are widely used to enhance concrete resistance to damage caused by freezing and thawing cycles. The type of air entraining agent has an important influence on the properties of aerated concrete [24,25]. Aluminum powder is normally used to obtain gas bubbles in concrete. In addition, various companies have developed different types of air entraining agents and foaming agents for normal and light-weight concretes.

To investigate the effect of different materials on aerated concrete, several researchers used various materials. Schackow et al. [26] used vermiculite and EPS to identify their effects on mechanical and thermal properties, and pre-saturated bentonite was adopted to produce ultra-lightweight concrete with numerous pores [27]. Super absorbent polymers (SAPs) was utilized as physical air entrainment in cement mortars [28]. Naratha et al. [29] have studied the effects of supplementary materials of the strength and thermal conductivity of non-autoclaved aerated concrete. Aluminum powder was added as a pore-forming agent in the amount of 0.2 wt.% of the binder content. A moderate-strength concrete, in the range of 13–23 MPa and with a density of about 1800 kg/m3, was developed. They reported that aerated concrete incorporating fly ash and silica fume has considerable advantages and low energy consumption, compared to autoclaved aerated concrete. In relation to this, Ramamurthy and

Narayanan [30] have showed that fly ash plays a pivotal role in aerated concrete without deteriorating compressive strength. Aguilar et al. [31] have studied the microstructure of aerated cement pastes with fly ash, metakaolin and sepiolite additions; concluding that the addition of fly ash increases closed porosity and produces a higher amount of calcium silicate hydrates (C-S-H), as detected using differential thermal analysis (DTA) and scanning electron microscopy (SEM).

Several kinds of research have been carried out to investigate the influence of various parameters on the properties of aerated concrete. For instance, Narayanan and Ramamurthy [25] have reviewed the structure and properties of aerated concrete which is more homogeneous than normal concrete because of the absence of coarse aggregates. They classified the aerated concrete according to the pore formation method, the binder type and the curing method. They reported that curing and pore-formation methods are the main factors controlling the properties and microstructure of aerated concrete. On the other hand, Yang et al. [32] and Chen et al. [33] have claimed that the water to solid ratio is the most important factor in controlling the properties of aerated concrete. As the water to solid ratio increases, the density and (to a lesser extent) compressive strength decrease [34,35]. It has also been asserted that there is a linear relationship between aluminum powder content and concrete density, with density decreasing as aluminum content increases [25].

Lightweight concrete can be used for structural and non-structural purposes depending on its characteristics. EN 206 defines structural lightweight concrete as a material with dry density ranges between 800 and 2000 kg/m3 with a compressive strength higher than 13 MPa (LC12/13). Cement industry is one of the main parts for CO2 emissions worldwide. To produce sustainable cement-based materials and to reduce greenhouse gases, alternative cementitious materials need to be used to replace cement without harmful impacts on concrete properties. For this purpose, fly ash can be used as alternative material of cement with high replacement level. The research presented here aims to produce lightweight cement paste and evaluate its mechanical, physical and thermal properties, so as to provide the most effective material with good material properties. Hence, proposed material can be further applied as a base for lightweight concrete production. For this purpose, we produced a set of lightweight pastes with different additives, such as aluminum powder, an air entraining agent, and hollow microspheres. Their physical properties including density, absorption, compressive strength, and thermal conductivity were then evaluated and compared using sensitive measurement tools. To figure out the pore characteristics, which strongly affect material properties, X-ray micro-computed tomography (micro-CT) was adopted and utilized to visualize the pore distribution inside the specimens. Micro-CT is a nondestructive testing method that uses X-ray and can describe the inner structure of an object without damaging the specimen. For lightweight cementitious composites, micro-CT can be utilized to investigate the characteristics of microstructural features [36–40]. With the results obtained, the effectiveness of each additives in achieving better mechanical and thermal lightweight composites properties was compared.

#### **2. Preparation of Specimens**

#### *2.1. Materials*

Ordinary Portland cement CEM I 42.5 N provided by HeidelbergCement GmbH (Germany) and fly ash (EFA-Füller HP, Baumineral, Germany) were used in this investigation, conforming to EN 197-1 and EN 450-1, respectively. The physical properties and chemical composition of cement and fly ash were measured experimentally in the laboratory, as shown in Table 1. The cement: fly ash ratio was kept at 1:3 by mass, for all mixes. The water/binder ratio was constant for all the mixes (w/b = 0.50). The w/b ratio was selected to achieve a workable cement composite with slump flow ranges between 420–480 mm according to EN 13350-5.

Three methods were used to generate air voids inside the cement paste. The first was the chemical expansion method, using aluminum powder with 0.5, 1, 2, and 3 wt.% binder content. The second method involved adding a tenside-based air entraining agent, with the same ratios as in the case of the aluminum powder: 0.5, 1, 2, and 3 wt.%. The third method involved adding prefabricated hollow plastic air bubbles (microspheres), with 0.5, 1, 2, and 3 wt.% mass of binder. Hollow microspheres are materials with spherical voids inside a plastic shell, having a diameter of 25–60 mm (median: 35 mm) and a density of 0.2 g/cm3 [40]. Each method of creating artificial voids produces a different volume of air inside the paste. The dosage of the materials used for producing the air was therefore kept constant to investigate their effects on paste porosity.


**Table 1.** Chemical composition and physical properties of the raw materials [wt.%].

#### *2.2. Mix Proportions and Specimen Preparation*

In this investigation, a total of thirteen cement pastes were prepared and produced (Table 2). Specimens with different additives were denoted as AA (air entraining agent), AL (aluminum powder), and AS (hollow microspheres), with the numbers after the specimen names presenting the amount of air entraining agent. To produce the specimens, a standard mortar mixer with 5 liter capacity and two mixing speed (140 rpm and 285 rpm according to EN 196-3) was used for mixing the components. After weighing the proportions of each material, fly ash and cement were firstly dry mixed for 1 min at the low speed, to ensure better distribution and homogeneity of the mixture. The water and air entraining agents were then added within thirty seconds and further mixed for 1 min at low speed. Consequently, there was a minute break to remove the cement paste adhering to the sides and the bottom of the bowl and to place it in the middle of the bowl. Finally, the mixer was run at high speed for 2 min. The mixing procedures for all the mixes were carried out in the same manner. Steel molds of 40 <sup>×</sup> 40 <sup>×</sup> 160 mm<sup>3</sup> were used for manufacturing cement pastes. After casting, the specimens were covered with a plastic sheet and kept in chamber with controlled temperature and relative humidity of 21 ± 1 ◦C and 99%, respectively. After 24 h of casting, the samples were demolded. They were then cured under water at a temperature of 20 ± 1 ◦C. At the age of testing, the prisms were cut to small cubes 40 <sup>×</sup> 40 <sup>×</sup> 40 mm3. The samples were surface dried prior to undertaking compressive strength tests. The first mix was designed without any air entraining agent, having been designated as the reference mix.

**Table 2.** Mix compositions of the used cement paste specimens.


#### **3. Measurements of Characteristic Properties**

#### *3.1. Material Properties*

#### 3.1.1. Material Density

Dry density (ρ*dry*) was determined according to EN 12390-7. After curing under water for 28 days, the submerged mass was measured (under water) to determine the actual volume of the samples. The saturated samples were then dried in an oven at 105 ◦C, to a constant mass. From the dry mass and volume obtained, the following formula was implemented to calculate the dry density (Equation (1)):

$$
\rho\_{dry} = m\_{dry}/V \tag{1}
$$

where *mdry* is the mass of the sample (kg) and *V* is the volume of the sample (m3).

#### 3.1.2. Compressive Strength

For the compressive strength determination, samples with dimensions of 40 <sup>×</sup> <sup>40</sup> <sup>×</sup> 40 mm<sup>3</sup> were used. The surface of the samples with aluminum powder was not smooth or homogeneous; therefore, the samples were polished before testing to remove lumps and indentations and to make their sides flat. A modern digital crushing machine (Zwick Roell, Berlin, Germany) was used for performing the compressive strength tests. Six cubes of each mix were tested, with the mean value taken under consideration in this investigation.

#### 3.1.3. Thermal Properties

The thermal conductivity of the cement paste specimens was evaluated experimentally using a Hot Disk device (Hot Disk AB, Göteborg, Sweden) which meets the requirements of ISO 22007-2 [41]. In this method, two samples from the same mix were used, with the sensor laid between the samples. Before measuring, the samples were polished to make them flat to ensure that no gaps between the sensor and the surfaces of the samples. Subsequently, the samples were dried in an oven at a temperature of 105 ± 5 ◦C, until reaching a constant mass so as to remove moisture, which has a critical influence on the thermal properties of cement-based materials. The measurements were repeated three times for each sample at different places, with the mean value considered. More details about thermal property measurement using the Hot Disk can be found in [40].

#### 3.1.4. Effective Water Porosity

The effective water porosity of the aerated cement pastes, being the accessible porosity by water, was measured using the simple method of water displacement. In this method, the sample must be saturated under water for at least 24 h. After measuring the wet mass, the submerged mass under water was determined. The saturation samples were then dried in an oven at 105 ± 5 ◦C, until constant mass. From the information of the mass in both dry (*mdry*) and saturated states (*msat*), and with the mass under water (*munder*), the effective water porosity (*P*) can be calculated with the following equation (Equation (2)) [42]:

$$P(\%) = \frac{m\_{\text{sat}} - m\_{dry}}{m\_{\text{sat}} - m\_{\text{under}}} \times 100\% \tag{2}$$

#### 3.1.5. Water Absorption

The water uptake capabilities of a partially immersed specimen can be measured according to ISO standard (15148). This method involves placing a dried specimen in a container of water, such that the immersion depth of the specimen is 5 mm. The primary force that pulls water up into the specimen is capillary suction. Increases in the specimen mass are determined at set time periods. Several measurements were taken over a period of up to 24 h, and a straight line was fitted to the

plot of the increase in mass, versus the square root of time. From this relationship, it was possible to determine the absorption coefficient of the cement-based materials.

#### *3.2. Microstructure Evaluation Using X-Ray Micro-Computed Tomography (Micro-CT)*

X-ray micro-computed tomography (micro-CT), a nondestructive investigation method, was used to estimate the spatial distribution of pores within the cement-based materials. This technique allows a series of cross-sectional images and microstructures to be obtained. Detailed pore characteristics, such as pore size distribution and pore density contour, were also examined using micro-CT images and imaging tools [43]. More details about this method can be found in [39], with only a brief explanation of the CT imaging procedure presented in this paper. Figure 1 shows the imaging process used in this study. First, a region of interest (ROI) is selected from an original image for effective examination of the sample. The original and ROI images were 8-bit images, represented by a value ranging from 0 (black) to 255 (white) in grayscale. Each image was composed of 1000 × 1000 pixels with a 27.0 μm pixel size. A binary image for classifying solids and pores was then generated, using the Otsu method [44] and an appropriate threshold value. The 3D sample for describing the pore structure was generated by consequent stacking of a series of binary images. Using the 3D images of the specimens, material characteristics, such as porosity and the spatial distribution of pores, can be effectively examined.

**Figure 1.** X-ray CT imaging process of the specimen with air-entraining admixture: (**a**) the original CT image, (**b**) region of interest (ROI), (**c**) binary image in 2D, (**d**) 3D binary image (Note: in the binary images, the white represents the solid phase, and the black regions are pores inside the specimen).

#### **4. Results and Discussions**

#### *4.1. Material Properties of the Specimens*

In this section, several properties of specimens with different additives and dosages are presented and discussed. The methods in Section 3.1 were adopted for each property.

#### 4.1.1. Density

Figure 2 shows the experimental results of the dry density of cement composites. It can be clearly seen that in all cases, the density decreased as the amount of AA increased. Both AA and AS specimens showed similar effects on the density of the cement pastes; falling from 1.34 t/m<sup>3</sup> (reference - A0) to 1.06 t/m3 and 1.04 t/m3. for mixes with 3 wt.% of AA and AS, respectively. However, the aluminum powder was much more effective than either AA or AS in reducing density. Using 0.5% AL reduced it from 1.34 to 0.85 t/m3. However, with a higher addition of AL, up to 3 wt.%, the density decreased only slightly more, to 0.75 t/m3, as can be seen for the results of the AL3 specimen.

**Figure 2.** Dry density values of the specimens with different additives.

#### 4.1.2. Compressive Strength

Compressive strength of concrete depends mainly on its major components; aggregate, cement paste, and transition zone in between. This study focuses on the properties of cement matrix which affect mechanical properties of cement-based materials significantly. Compressive strength of cement pastes was measured as an indicator of mechanical properties after curing for 28 days. Figure 3 shows the experimental results of the compression test on different pastes. In this figure, it is clearly presented that the incorporation of air voids into cement composites significantly decreases the compressive strength depending on the dosage and type of air entraining method. The AL specimens with aluminum powder show the lowest strength, which dropped from 30 MPa for reference mix without aluminum powder to 3.2 and 1.7 MPa for pastes with 0.5 and 3 wt.% aluminum powder, respectively. The same trend of the strength reduction with inclusion of air voids can be found in the case of the AA mixes, although the amount of the change is less. With the use of 3 wt.% of AA, the strength decreased to 18 MPa which represents about 60% of the reference mix. The influence of hollow microspheres (AS) on strength reduction is similar to that of the AA mixes. With addition of 3 wt.% of hollow microspheres, the compressive strength decreased to 15 MPa.

**Figure 3.** Compressive strength of the specimens with different additives: (**a**) compressive strength vs. powder content; (**b**) compressive strength vs. density.

Moreover, it is obvious from the results that 0.5 wt.% of aluminum powder reduced the strength significantly. This indicates that aluminum powder is more effective in producing air voids even with small dosage which reduces the density as well as the compressive strength. The influences of the other methods, AA and AS, on density and strength are not significant than that of aluminum powder even with higher dosages, as can be seen in Figure 3a. The relationship between the strength and dry density of cement pastes is presented in Figure 3b. The experimental results confirm that dry density has clear relationship with the compressive strength of cement pastes. The strength deterioration in the samples is directly attributable to increases in porosity volume and reductions in the solid structure volume. The strength retrogression was more pronounced in the case of aluminum powder, which was not only related to an increase in porosity volume, but also to the large size of the pores created, as detected by micro-CT measurements, which are presented below.

#### 4.1.3. Effective Water Porosity

Water was used to evaluate cement paste porosity experimentally, using the water displacement method. Figure 4 illustrates the experimental results of total porosity at an age of 28 days. It can be seen that the reference mix (A0) had the lowest porosity of about 38 vol.%. For the other mixes, porosity increased with air content. With 3 wt.% AA, it increased to 50.1 vol.%; however, with 3 wt.% AS, it reached 51.7 vol.%. The increase in porosity was much higher in the mixes with aluminum powder. It increased from 38 vol.% without aluminum powder to 65 % with 3 wt.%. It is clear in the figure that the addition of 0.5 wt.% of aluminum powder increased the porosity to 60 vol.% which was more effective than 3 wt.% of AA or AS, where the porosity was about 50.1 vol.% and 51.7 vol.%, respectively. In addition, the experimental results indicate that higher dosages of aluminum powder do not significantly increase porosity in comparison to lower dosages; an increase in aluminum powder dosage from 0.5 wt.% to 3 wt.%, increased porosity from 60 to 65 vol.%. It can be seen from Figure 4 that increasing AA and AS dosages increased porosity gradually. However, in the case of AL, porosity increased primarily at the beginning, with the rate of increase obviously decreasing later with the increment in the AL content.

**Figure 4.** Water porosities of the specimens with different additives.

#### 4.1.4. Water Absorption

Figure 5 presents the water absorption coefficient values of the different cement pastes. Mix A0, without entrained air, had the lowest water absorption, of about 0.61844 kg/(m2·h0.5). However, mix AA3 with 3 wt.% AA had the highest water absorption coefficient, of about 2.10211 kg/(m2·h0.5). In contrast to the porosity results, mixes with the aluminum powder had a lower absorption than mixes with the AA. For mixes with the hollow microspheres (AS), the absorption was higher than in the control mix, though a marginal increase in absorption was observed with an increase in AS dosage. Compared to AA mixes, those with the AS had a lower absorption coefficient. The capillary suction force controls absorption rate and quantity, in the method used to measure water absorption (water uptake). As a result, pores in the capillary range are more effective in increasing the water absorption coefficient. In the case of aluminum powder, most of the pores were large in size, and

as a result, the water absorption of these mixes was smaller than in the other mixes, where most of the pores were in the size range of the capillary pores. It is clear from the results that, the water absorption of composites made with hollow microspheres is increasing up to 0.5 wt.%. With increment of AS content no significant increase in the absorption can be detected. This can be attributed to the nature of microspheres which are closed pores with weak shell and cannot create a continuous passage for fluids to transfer. Most of these pores do not participate in increasing the water absorption as can be reflected from the experimental results and this can be considered as an advantage of this material compared to other air entraining materials.

**Figure 5.** Water absorption of the specimens with different additives.

#### 4.1.5. Thermal Conductivity

Figure 6 presents the experimental thermal conductivity results of the different cement paste samples. The thermal conductivity of the reference mix (A0) was about 0.347 W/(m·K). The thermal conductivity decreased as the AA content increased, reaching a value of 0.29 W/(m·K) at 3 wt.%. The experimental results for the mixes with hollow microspheres (AS) were similar to those with AA; conductivity fell from 0.32 W/(m·K) with 0.5 wt.% AS to 0.23 W/(m·K) with 3 wt.%. Aluminum powder seems to have a remarkable influence on thermal conductivity; with the addition of 0.5 wt.%, thermal conductivity dropped to 0.18 W/(m·K), while an addition of 3 wt.% aluminum powder reduced thermal conductivity to 0.12 W/(m·K).

**Figure 6.** Thermal conductivity of the specimens with different additives: (**a**) thermal conductivity vs. powder content; (**b**) thermal conductivity vs. density.

Thermal conductivity is a material property related directly to dry density, as well as to the moisture content of the material. In this experiment, all of the samples were dried to a constant mass; as a result, the main factor influencing thermal conductivity was dry density [10]. Mixes with aluminum powder had the lowest thermal conductivity, due to reduced density as compared to other mixes, as can be seen in Figures 2 and 6. Heat transfer in any materials depends on its components: solid structure and voids. With decreasing material density, pore volume increases, and the heat transfer decreases; this influence can be clearly detected from the results of thermal conductivity of cement pastes with different densities.

#### *4.2. Pore Characteristics Using X-Ray CT*

The inner structures of the cement pastes specimens with different admixtures were examined using X-ray CT images. Figure 7a shows the binarized 3D micro-CT images of A0, AA3, AL3, and AS3 specimens, which were obtained without damaging the specimens. In the binary figures, the white region represents the solid part of the specimen, while the black region inside the specimen presents the pores. From this figure, the pore structure inside each specimen can be easily identified. In Figure 7a, AA and AS specimens with air entraining agent and hollow microspheres solid, respectively, have a relatively high porosity as compared to the reference (A0) specimen, with their spatial distribution seemingly relatively uniform. However, the AL specimen with aluminum powder had distinctively larger pores and porosity than the other specimens; it denotes that aluminum powder had a more significant effect on securing the pores inside the specimens, than did the other admixtures.

**Figure 7.** Target specimens (A0, AA3, AL3, AS3) and their 3D binarized CT images (solid structures): (**a**) 3D binarized CT images (solid structures) of A0, AA3, AL3, and AS3 specimens (Note: in the binary images, the white denotes solid phase, and the black parts represent the pores of each specimen); (**b**) description of pore densities of the specimens (Note: the color in each figure denotes pore density values between 0 (blue) to 1 (red)).

For a more detailed investigation of the pore structures using the 3D images, porosity, spatial pore density, and pore size distribution were also examined qualitatively and quantitatively. The porosity of each specimen was calculated based on the number of pore voxels in the 3D images; the obtained total porosities of the specimens were 14.5 vol.% (A0), 22.3 vol.% (AA3), 35.1 vol.% (AL3), and 19.9 vol.% (AS3). The porosity computed from the micro-CT measurement was much smaller than that of the water porosity because of the resolution limit of the used images. Since pores even in the order of a few micrometers or even less can be detected using the experimental approach, micro-CT porosity only concerns an analysis of pores larger than the image resolution (27.0 μm); the difference thus results from the minimum measurable pore size of each method. Despite their limitations, micro-CT images can be used to visualize pore structures both quantitatively and qualitatively. From the porosities

obtained, the effects of each admixture on material porosity can be examined. The AL3 specimen shows the largest porosity which causes low thermal conductivity and mechanical strength. In the same manner, the AS3 specimen with the lowest porosity among the cases shows the largest compressive strength in Figure 3; these results confirmed that the porosity data obtained from micro-CT presents consist result with the measured properties of the specimens.

The spatial distribution of the pores in the specimens have been clearly described in Figure 7b, where the pore density of each specimen has been represented in visual form. To calculate pore density, the local porosity of the specimen was calculated for the whole specimen, after which computed values were assigned for every voxel of the specimen. For example, in the figure for the AL3 specimen, the red region indicates a region with a high local porosity, while the blue one represents a region with low local porosity. From these contours, the spatial distribution of pores in each specimen can be identified. In general, the pore distribution in the AL3 specimen can be considered as relatively uniform, showing an anisotropic trend, because the larger pore density values are distributed in a specific region. A similar phenomenon has been observed by Shabbar et al. [45] where aerated concretes containing higher aluminum powder contents (less than 0.5 wt.%) exhibited more anisotropic voids. From the results, both general trend of pore clustering as well as the effect of air entraining agents on the pore characteristics of the materials.

Figure 8 shows the pore size distribution in each specimen. From this figure, the relative pore sizes of the specimens can be compared, with all the specimens with air-entraining admixtures containing higher pore volume as well as pore sizes, particularly in the case of the AL3 specimen. It can therefore be confirmed that the pore distribution in the AL3 specimen was anisotropic, meaning that the use of aluminum powder in cement-based materials can induce a relatively large and anisotropic pore distribution. In addition, the data obtained shows consistent results in regard to other material properties. For example, the general trends regarding compressive strength and thermal conductivity, as shown in Figures 3 and 6, are similar to the pore size distribution and porosity results in Figures 7 and 8; both properties decreased as the amount of air-entraining agent or porosity increased. This result demonstrates that the use of micro-CT can be successfully utilized to describe the correlation between the material properties and the pore characteristics of cement-based materials.

**Figure 8.** Pore size distribution of A0, AA3, AL3, and AS3 specimens.

Physical and chemical characteristics of fine materials affect the performance of air entraining agent (AA) as well as aluminum powder (AL) significantly, while their influence on hollow microsphere (AS) is very limited. The performance of AA and AL depends on the pH value of the pore solution and on the presence of other supplementary materials. The presence of fly ash influences the performance of AA negatively because the carbon particles in the fly ash absorb the agent and reduce their reactivity; consequently, higher dosages are needed. In addition, the carbon particles destabilize entrained air bubbles and change its volume in fresh status of cement-based materials [22]. In the case of aluminum powder, the alkali contents and cement fineness affect its performance significantly [46]. Other parameters, such as temperature, pH of the pore solution, and the consistency of the paste, influence the reactivity of aluminum powder [47]. In contrast, hollow microspheres are more stable and do not rely on the chemical composition of the used materials strongly. Only over-mixing for long period can destroy the voids and change their distribution. To avoid these problems in this investigation, cement and fly ash with a constant proportion of 1:3 was used for all mixes so that no influences can be occurred due to chemical composition. In addition, the mixing sequence and period were the kept the same for all mixes.

#### **5. Conclusions**

In this investigation, three different materials: aluminum powder (AL), air entraining agent (AA) and hollow microspheres (AS) were adopted to create pores inside cement pastes. The same amounts of each material were used to produce lightweight pastes with pores, with the effects of their dosage on material properties and characteristics investigated. The cement pastes' physical and mechanical properties were measured experimentally. In addition, paste microstructures were characterized using micro-CT images. The remarks below serve as a conclusion:


As the results of this study have shown, each air-entraining agent has both advantages and disadvantages depending on thermal, mechanical, and absorption characteristics. The appropriate additive can therefore be selected according to the characteristics of the material at hand. In addition to the results of this study, the systematic investigation tools used here can be utilized in the further development of advanced lightweight cement-based materials as well as special building materials with particular objectives. In addition, the investigation of specimens with the same porosity but different air entraining agents can be conducted as a further study.

**Author Contributions:** Conceptualization, S.-Y.C. and M.A.E.; methodology, M.A.E. and M.E.E.M.; software, S.-Y.C.; validation, M.A.E., S-Y.C. and M.E.E.M.; formal analysis, M.A.E., S.-Y.C., and P.S.; investigation, M.A.E., M.E.E.M., S.M. and O.Y.; resources, P.S. and S.M.; data curation, S.-Y.C., M.A.E. and P.S.; writing—original draft preparation, M.A.E., S.-Y.C. and M.E.E.M.; writing—review and editing, S.-Y.C., M.A.E., P.S., O.Y. and S.M.; visualization, P.S. and S.-Y.C.; supervision, M.A.E. and M.E.E.M.; project administration, P.S. and M.A.E.; funding acquisition, P.S. and M.E.E.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 841592.

**Acknowledgments:** This research is supported by the German Egyptian Mobility Program for Scientific Exchange and Excellence Development (GE-SEED). P.S. is supported by the Foundation for Polish Science (FNP). The authors would like to express their appreciation to Paul H. Kamm (Helmholtz Centre Berlin) for his assistance in micro-CT imaging.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Rhodamine B Removal of TiO2@SiO2 Core-Shell Nanocomposites Coated to Buildings**

#### **Dan Wang 1, Zhi Geng 1, Pengkun Hou 1, Ping Yang 2, Xin Cheng 1,\* and Shifeng Huang 1,\***


Received: 13 January 2020; Accepted: 28 January 2020; Published: 31 January 2020

**Abstract:** Surface application of photocatalyst in cement-based materials could endow it with photocatalytic properties, however, the weak adhesion between photocatalyst coatings and the substrates may result in poor durability in outdoor environments. In this study, TiO2@SiO2 core-shell nanocomposites with different coating thicknesses were synthesized by varying the experiment parameters. The results indicate that SiO2 coatings accelerated the rhodamine B removal to a certain extent, owing to its high surface area; however, more SiO2 coatings decreased its photocatalytic efficiencies. The cement matrix treated with TiO2@SiO2 core-shell nanocomposites showed good photocatalytic efficiency and durability after harsh weathering processing. A reaction mechanism was revealed by the reaction of TiO2@SiO2 nanocomposites with Ca(OH)2.

**Keywords:** photocatalysis; core-shell structure; TiO2/SiO2 composite nanoparticles; cement-based materials

#### **1. Introduction**

The application of TiO2 nanoparticles in cementitious materials has gained considerable interest and research, which could endow it with photocatalytic properties, such as self-cleaning [1], air-purifying [2], and antibacterial properties [3]. P25, a commercial TiO2 nanoparticle, was widely employed in building materials, owing to its small size, excellent photocatalytic efficiency, being toxic-free, accessible, and affordable merits [4,5]. Poon [6] reported that P25 was used to modify the concrete surface of self-compacting mortars, and it was found to be effective in rhodamine B (RhB) degradation under UV and strong halogen light irradiation. Sánchez [7] mixed P25 into cement mortar using different dosages, and the mortars show outstanding degradation of NOx gases.

Traditionally, photocatalytic materials are introduced into cement-based materials via a direct mixing method. The obtained TiO2-containing cement-based materials show accelerated cement hydration [8], lower porosity, improved mechanical properties [9], self-cleaning [10], air-purifying [11], and antibacterial properties [3]. However, the incorporation method faces a serious restriction in having limited photocatalytic efficiency and some conditions. Boule [12] studied TiO2-containing cement-based materials prepared using different techniques, and the results show that the TiO2-cement mixture has significantly less efficiency than TiO2 slurries for the degradation of 3-nitrobenzenesulfonic acid and 4-nitrotoluenesulfonic acid. This is attributed to the reduction in the active surface. Meanwhile, the incorporation method is not applied in existing buildings and natural stone structures.

Enriching the surface of cement-based materials with a photocatalyst seems to be a viable option. However, the weak adhesion of the photocatalytic material to its substrate remains a big concern [13]. When the photocatalyst is exposed to harsh weather, the issue of adhesion gains more importance than it deserves. A binder of cement-based materials with a photocatalyst could be considered in order to improve durability. SiO2 as an inorganic material is a valuable resource to prevent the release of the TiO2 photocatalyst from the surface of cement-based materials to the environment, owing to its pozzolanic activity. Meanwhile, the adding of SiO2 as a heterogeneous catalysis, such as TiO2@SiO2 and SiO2@TiO2, may improve the photocatalytic efficiency in comparison to pure TiO2 [14–16]. Stephan [17] obtained silica-titania core-shell composites, and the results show that some core-shell particles have higher photocatalytic efficiency than pure nano-titania photocatalysts. Mendoza [18] studied a cement matrix treated with TiO2 and TiO2–SiO2, which showed high RhB photodegradation conversions (above 80%). Sikora [19] reported that silica/titania (mSiO2/TiO2) core-shell nanocomposites improved the compressive strength, reduced the water absorption of cement mortars, and exhibited relatively good bactericidal properties.

In this study, TiO2@SiO2 core-shell composites with different coating thicknesses were designed and synthetized. SiO2 coating accelerated RhB removal to a certain extent, and improved photocatalytic durability by test efficiencies before and after curing and an accelerated weathering process. A reaction mechanism was revealed by the reaction of TiO2@SiO2 nanocomposites with Ca(OH)2.

#### **2. Experimental Details**

#### *2.1. Materials*

White Portland cement (WC, P.W 42.5R) was provided by the Aalborg cement company (Jinan, China), and the major chemical compositions of WC are shown in Table 1. Tetraethyl orthosilicate (TEOS), aqueous ammonia (25%), RhB, calcium hydroxide (Ca(OH)2), and absolute ethanol of chemical grade were purchased from China National Pharmaceutical Group Corporation (Beijing, China).

**Table 1.** Major chemical compositions of withe Portland cement.


#### *2.2. Synthesis of TiO2@SiO2 Nanocomposites*

Commercial TiO2 nanoparticles (P25) were used as the core structure, and the SiO2 shell was obtained by the typial Stöber method. P25 (0.1 g) was dispersed into the solution of water (100 mL) and ethanol (80 mL) with an ultrasonic dispersion instrument for 30 min. Aqueous ammonia (1 mL) and TEOS (1 mL) were dropped above the mixture, respectively, under stirring for 8 h at 25 ◦C. TiO2@SiO2 nanocomposites were collected from the solution by a centrifugation method. All the experimental processes were the same, except for the experimental temperature, and nanocomposites obtained at 0 ◦C were prepared. Experimental parameters, such as temperature and the amount of P25, were adjusted to prepare different nanocomposites, and the detailed experimental data were listed in Table 2.

**Table 2.** Experimental parameters of TiO2@SiO2 nanocomposites.


#### *2.3. Cement Paste Preparation, Curing and Surface Treatment*

WC pastes (4 cm × 4 cm × 16 cm) were prepared, and the weight ratio of water to cement was 0.35. The paste specimens were put into a curing chamber at 95% relative humidity and 20 ± 2 ◦C for 28 days. After that, theses specimens were cut into slices with a size of 4 cm × 4 cm × 2 cm and then dried at 50 ◦C for 24 h. Pure P25 (0.025 g) and TiO2@SiO2 nanocomposites were dispersed into water (2 mL), respectively, and the dosages of nanocomposites were calculated to keep the same concentration of TiO2, compared to pure P25 suspension. The obtained P25 and TiO2@SiO2 suspensions were sprayed on one surface (4 cm × 4 cm) of silices.

#### *2.4. Characterization*

Transmission electron microscope (TEM, FEI G2F20, Hillsboro, OR, USA) was employed to observe the morphologies of TiO2@SiO2 nanocomposites. Scanning electron microscope (SEM, ZEISS EVO LS15, Germany) was applied to obtain morphological images of the treated cement. N2 adsorption-desorption isotherms were performed at −196 ◦C using a multi-function adsorption instrument (BET, Beijing Builder Company MFA-140, Beijing, China). The specific surface area was calculated by the Brunaur–Emmett–Teller method. Solid-state 29Si MAS NMR spectra (Bruker AV600, Germany) were acquired on a Bruker AV600 spectrometer.

#### *2.5. Photocatalytic Degradation of RhB*

The test of RhB degradation was carried out to evaluate the self-cleaning performance of TiO2@SiO2 and coated WC pastes. In photocatalytic tests of TiO2@SiO2 powders, P25 (0.01 g) and nanocomposites were dispersed in an RhB solution (20 mL, 10 mg/L), and the TiO2 dosages of nanocomposites kept the same concentration. The suspension was further stirred for 30 min in the dark and was then irradiated by a UV lamp (20 W). Part of the solution (2 mL) was taken every 15 min until 105 min, and was centrifuged to remove nanocomposites. The concentration of the obtained solution was tested using a UV/Vis spectrometer at a wavelength of 463 nm to investigate the degradation efficiency. To measure the RhB degradation of coated WC pastes, the surface coated with nanocomposites was sprayed with 2 mL of RhB solution (80 mg/L). RhB contaminated specimens were irradiated by the UV lamp, and color variations (Δ*E*) before and after irradiation were recorded using a portable sphere spectrophotometer (RM200, X-Rite). The efficiencies of RhB removal was calculated by the comparison of the color variations before (Δ*E*(0)) and after several hours (Δ*E*(*t*)) of UV light irradiation. The detailed calculation method was mentioned in References [20,21].

A lab-simulated weathering system was used to mimic rain water in an outdoor environment. Circular tap water with a flow rate of 130 mm/h was produced by pumping it from a water tank in order to simulate the rain condition, and the process lasted 5 days.

#### **3. Results and Discussion**

TiO2@SiO2 nanocomposites with different coating thicknesses were prepared to investigate its effect on photocatalytic efficiency and the bonding force with the cement-based matrix. In order to obtain TiO2@SiO2 nanocomposites with different coating thicknesses, experimental parameters, such as the temperature and dosage ratio of TEOS and P25, were adjusted to prepare different nanocomposites. Samples 1, 3, 4, and 6 were selected to investigate the effect of experimental parameters on the coating thicknesses of TiO2@SiO2 nanocomposites. The TEM images of these nanocomposites are shown in Figure 1. Compared to the TEM images of four nanocomposites, the SiO2 shell of Samples 1 and 4 is obviously present. The results indicate that the dosage ratio of TEOS and P25 decides the molar ratio of SiO2 and TiO2, and the higher value promotes the formation of the SiO2 shell. The enlarged insets of Figure 1a,c and show that the two nanocomposites both have core-shell structures, and the main shell thicknesses of Samples 1 and 4 are about 3.92 and 6.13 nm, respectively. During the preparation process

of the coating, the rates of hydrolysis and condensation processes decreased at lower temperatures, and SiO2 preferentially deposited on the surface of TiO2 particles, benefiting the process of more coating.

**Figure 1.** TEM images of Sample 1 (**a**), Sample 3 (**b**), Sample 4 (**c**), and Sample 6 (**d**).

The photocatalytic activities of TiO2@SiO2 nanocomposites with different shell thicknesses and P25 were evaluated by monitoring the degradation of RhB under UV-light irradiation, and the results are shown in Figure 2. After dark reaction for 30 min, the degradation rates of TiO2@SiO2 nanocomposites were higher than that of pure P25. Meanwhile, the values increase with the increase in the shell thickness, mainly owing to the absorption of the SiO2 coating. Samples 2, 3, and 6 have higher degradation efficiencies than pure P25 when the irradiation time was 105 min, indicating that the SiO2 coating accelerates RhB removal to a certain extent. This may be owing to the high surface area of SiO2, which adsorbed more dyes to the benefit of more dye degradation [14]. Samples 1 and 4 have higher SiO2 coating thicknesses, however, their photocatalytic efficiencies are the lowest. This is probably because much of the coating may hinder the transport of photons and decrease light absorption [18]. During the preparation process of TiO2@SiO2 nanocomposites, temperature may be an influencing factor for the thickness and density of the coating, which further affects the photocatlytic activity of nanocomposites. The nanocomposites prepared at 25 ◦C have higher degradation rates than nanocomposites synthetized at 0 ◦C, which may be due to the difference in the coating thickness and morphology.

**Figure 2.** Photocatalytic degradation of RhB under UV light by P25 and TiO2@SiO2 nanocomposites.

Samples 1–3 and P25 were sprayed on the surface of WC pastes, and the specimens coated with Samples 1–6 and P25 were named WC 1–6 and WC P25, respectively. The surface microstructures are shown in Figure 3. The figures indicate that the surfaces of hardened cement pastes treated with Samples 1–3 are denser than those of untreated cement pastes and the paste treated with P25. The results may be due to the gelatinization of SiO2 or the pozzolanic reaction of SiO2 and cement matrix. The adding of SiO2 shell improves the surface quality of cement-based materials, and it may protect against the release of TiO2 in a harsh weathering environment. In order to investigate the photocatalytic properties of WC specimens, the RhB degradation rates of these specimens were first measured after the TiO2@SiO2 nanocomposites were sprayed on the surface of the WC pastes. After that, these specimens were put into a curing chamber for two weeks. TiO2@SiO2 nanocomposites may react with the WC matrix due to their pozzolanic activity, and the degradation rates were measured again to investigate the effect of reaction productions on the photocatalytic efficiency. If the pozzolanic reaction occurs, the production could decrease the release of TiO2 after exposure to a harsh weathering process. Therefore, after the weather process, degradation rates of specimens were tested. The results are shown in Figure 4, and Table 3 shows the detailed data from the figure. RhB degradation rates of WC 1 and WC 4 are the highest, and the tendency is different from the degradation rates of TiO2@SiO2 nanocomposites. This probably because adsorption action is more effective on the RhB degradation after nanocomposites are applied on the surface of WC pastes. After curing, their degradation rates significantly increase. C-S-H gel with relatively larger BET surface areas may be formed due to the pozzolanic reaction, which may adsorb more RhB dye. The results indicate that the reaction production has no negative effect on photocatalytic efficiency. Instead, it increases BET surface areas to promote RhB degradation. The production may decrease the release of TiO2 and improve the durability of photocatalytic activity. After the weather process, the RhB removals of all specimens were decreased, and the reduction of WC P25 was obvious. WC 1 and WC 4 show the minimum photocatalytic efficiencies, which was due to more SiO2 coating of Samples 1 and 4. Meanwhile, WC 4 has the minimum value due to its larger thickness of the SiO2 coating mentioned above. The results show that adding SiO2 could improve the adhesion between the coating and substrates, and the durability of the photocatalytic property, in particular for a harsh weathering process. The RhB degradations of WC 2 and WC 3 are similar, mainly owing to their similar BET surface areas, as mentioned above.

**Figure 3.** SEM images of harden cement pastes (**a**) and samples treated with Samples 1–3 (**b**–**d**) and P25 (**e**).

**Figure 4.** RhB degradation rates of WC 1-6 and WC P25 before (**a**,**d**) and after curing (**b**,**e**) and accelerated weather process (**c**,**f**).



To more specifically study the pozzolanic reaction of TiO2@SiO2 nanocomposites with the cement matrix, experiments of nanocomposites with Ca(OH)2 (a hydration product of cement) were carried out to investigate the formation of the reaction production. Sample 2 was selected and was mixed with Ca(OH)2 saturated solution by stirring for seven days. TEM image of the reaction product of Sample 2 and Ca(OH)2 is shown in Figure 5. A foil-like structure was formed after the reaction, which may be a C-S-H gel. The particles are deposited on the surface of a foil-like structure. The SiO2 shell reacted with Ca(OH)2, and the product core was first deposited on the particles. The product core further grew to a foil-like structure. This may explain why the adding of SiO2 decreased the release of TiO2 and improved the durability of the photocatalytic activity.

**Figure 5.** TEM image of reaction product of Sample 2 and Ca(OH)2.

The objective of 29Si NMR spectrum is to assess the composition of the foil-like structure and Qn structure of the Si tetrahedron. Figure 6 shows the spectra of Sample 2 and its reaction production with Ca(OH)2. The spectrum of Sample 2 shows two signals centered at 102.5 and 113.1 ppm, which were associated with the Q3 and Q4 units in the SiO2 structure, according to previous reports [22,23]. After the reaction with Sample 2 and Ca(OH)2, the two main peaks shifted to 81.7 and 88.0 ppm, which were assigned to Q1 and Q2 silicate connections. The results indicate a decrease in the polymerization degree of silicon and the formation of a C-S-H gel. After the reaction, the SiO2 coating transformed to a C-S-H gel, which could decrease the release of TiO2 after exposure to a harsh weathering process.

**Figure 6.** NMR spectra of Sample 2 (**a**) and its reaction production (**b**) with Ca(OH)2.

#### **4. Conclusions**

TiO2@SiO2 nanocomposites were prepared using different experiment parameters, and their RhB degradations were measured. The results show that SiO2 coating accelerates the RhB removal to a certain extent, owing to the high surface area of SiO2, which adsorbed more dyes to the benefit of more dye degradation. More SiO2 coating may hinder the transport of photons and decrease light absorption, which further decreases photocatalytic efficiencies. After these nanocomposites were applied on the surface of the WC pastes, their RhB degradation rates are higher than WC P25, which is probably because adsorption action is more effective on the RhB degradation. After curing, their degradation rates significantly increase. C-S-H with relatively large BET surface areas may be formed due to the pozzolanic reaction, which may adsorb more RhB dye. After the weather process, the RhB removals of all specimens decreased, and the reduction of WC P25 was obvious. Meanwhile, WC 4 has the minimum value. Adding SiO2 could improve the adhesion between coating and substrates, and the durability of the photocatalytic property, in particular for a harsh weathering process. The experimental data of nanocomposites with Ca(OH)2 further prove the formation of C-S-H gels and reveal the reaction of the mechanism of nanocomposites and the cement matrix.

**Author Contributions:** X.C. and D.W. conceived and designed the study; D.W. and Z.G. performed the experiments; D.W. wrote the paper; P.Y., S.H., and X.C. reviewed and edited the manuscript. Resourse: P.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Program for Taishan Scholars Program, Case-by-Case Project for Top Outstanding Talents of Jinan, Distinguished Taishan Scholars in Climbing Plan, National Natural Science Foundation of China (Grant No. 51632003 and 51902129), the National Key Research and Development Program of China, (Grant No. 2016YFB0303505), and the 111 Project of International Corporation on Advanced Cement-based Materials (No. D17001).

**Acknowledgments:** This work was supported by the Program for Taishan Scholars Program, Case-by-Case Project for Top Outstanding Talents of Jinan, Distinguished Taishan Scholars in Climbing Plan, National Natural Science Foundation of China (Grant No. 51632003 and 51902129), the National Key Research and Development Program of China, (Grant No. 2016YFB0303505), and the 111 Project of International Corporation on Advanced Cement-based Materials (No. D17001).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*

### **Experimental Investigations on the Performances of Composite Building Materials Based on Industrial Crops and Volcanic Rocks**

#### **Raluca Is,toan \*, Daniela Roxana Tămas,-Gavrea and Daniela Lucia Manea**

Department of Civil Engineering and Management, Faculty of Construction, Technical University of Cluj-Napoca, 28 Memorandumului Street, 400114 Cluj-Napoca, Romania; roxana.tibrea@ccm.utcluj.ro (D.R.T.-G.); daniela.manea@ccm.utcluj.ro (D.L.M.) **\*** Correspondence: ralucafernea@ccm.utcluj.ro

Received: 16 January 2020; Accepted: 8 February 2020; Published: 11 February 2020

**Abstract:** Interdisciplinary and sustainability represent the main characteristics of this paper due to the fact that this research is offering a connection between two main areas—agronomy and construction, by using hemp shiv for the design of new building materials, which can increase the sustainability level of the building industry. For this reason, the main scope of this study is based on the investigation of a new category of composite building materials—lightweight mortars based on hemp shiv, volcanic rocks and white cement—which contribute to a positive environmental impact and help to increase indoor comfort. A complex report was carried out on two segments. The first one is focused upon the characteristics of the raw materials from the composition of the new materials, while the second segment presents a detailed analysis of these composites including morphological and chemical investigation, pyrolytic and fire behavior, compression and flexural strengths, and acoustic and thermal characteristics. The proposed recipes have as a variable volcanic rocks, while the hemp and the binder maintain their volumes and properties. The results were analyzed according to the influence of volcanic rocks on the new composites.

**Keywords:** sustainability; hemp; sound absorption; thermal conductivity; fire characteristics; mechanical properties; microscopy; thermal analysis; X-ray diffraction

#### **1. Introduction**

Sustainability and building materials are two different parts that together can contribute to a new perspective on the building industry. Constructions today play an important and critical role in developing a sustainable society [1]. For this reason, sustainable development represents a new challenge for each sector to become environmentally-friendly, by protecting the planet's limited resources and preventing additional pollution. During the last years of activity, the construction sector has supported the tendency to build in a more responsible way, so far as environmental and human life protection are concerned. New keywords define buildings such as passive, green, eco-friendly, sustainable and so forth. Different natural and traditional resources such as straw bale [2], hemp [3], flax [4], cob [5], sheep wool [6] and so forth are reconsidered as building materials. Moreover, new composite materials based on waste, such as glass [7,8], rubber [9] and so forth, have been studied and investigated. The use of the hemp shiv as a raw building material is not new; it was rediscovered after 1990 [10]. Its main characteristics concern improving sound and thermal performance, and having a positive impact on the environment, air-permeability, preventing condense formation and waterproofing [11]. New approaches with hemp shiv—which is regarded as a waste product obtained after the extraction of the fibers for the textile industry—have been taken into

consideration in the construction sector. There is a significant number of studies on hemp shiv with lime, which is a composite material named hempcrete [12–14], but there are only few studies that concern hemp shiv with cement [15–19].

Due to the fact that hemp shiv presents poor fire resistance, a new approach can be made with volcanic rocks, which are natural products with higher fire resistance [20]. These materials are usually found in the composition of mortars used in the rehabilitation process of the walls, because of their increased mechanical resistances [21]. They also present sound and thermal properties, as well as a low density, which will help the workability of the mortar. Durable and with higher strength properties for structural application [20], volcanic rocks are low cost materials [22] that also present fire retarding properties for steel structures [23].

Therefore, the purpose of this paper is to analyze the impact of the volcanic rocks when these are incorporated in hemp–cement composite materials used as interior mortars. These building materials are developed to be suitable not only for eco-friendly construction but also in regular buildings made from concrete, steel, and so forth, where they can increase indoor comfort.

#### **2. Materials and Methods**

#### *2.1. Raw Materials*

The composition of the new materials is designed to increase indoor comfort by developing new types of lightweight mortars, obtained as prefabricated plates. In order to understand the characteristics of the new composites, hemp shiv, natural and treated; volcanic rocks, perlite and vermiculite; and binders, white cement and hydrated lime, were analyzed.

#### 2.1.1. Hemp Shiv

The hemp shiv used for the new composites was received from HempFlax Romania (Pianu De Jos, Alba, Romania). The chemical structure of the hemp shiv is represented by cellulose 44%, hemicellulose 18%–27%, lignin 22%–28%, extractives (fatty acids, waxes, sterols, triglycerides, steryl esters, glycosides, fatty alcohols, terpenes, phenolics, simple sugars, alkaloids, pectins, gums and essential oils) 1%–6% and ash 1%–2% according to Reference [24]. The percentages may differ depending on the hemp variety used. Hemp shiv presents a natural structure, shown in Figure 1a, similar to wood fibers, with dimensions from 1 to 8 mm, as follows: 4–8 mm (48.75%), 2–4 mm (22.5%), 1–2 mm (25%) and 0.25–1 mm (4.75%). The sorting procedure was made using the Controls sort system with 6 sieves of 0.25; 0.5; 1; 2; 4 and 8 mm. The relative humidity of the hemp shiv was approximately 7% and the density was 100 kg/m3. The density was found to be similar to that found in the investigation of Reference [13]. The microscopic image of the hemp shiv presented in Figure 1b shows the cellular porous microstructure with two different sizes of longitudinal channels. According to References [25,26], the small ones range from 20–50 μm and are called xylem rays, whereas the bigger ones are on average around 50–100 μm and are named vessels. The surface walls of the vessel show perforations that permit a connection with the xylem ray, making it possible for the moisture to pass from one cell to the other. Regularly, the vessels are individually arranged and the pore frequency is about 20.8 vessels/mm2 [26]. The thermal analyses of the natural hemp shiv, presented in Figure 1c, showed a total degradation of 81.20%. The hemp shiv lost 3.6% on the first temperature interval from 20–200 ◦C and 77.6% on the second one, from 200 and 1000 ◦C. Compared with References [27,28], the second temperature interval is defined by the thermal combustion of hemicellulose and cellulose. The X-ray diffraction (XRD) of the hemp shiv was presented in a previous study [19], where the higher peak was recorded at 2 θ = 22.66◦, similar to the studies of References [25,29]. The peak represents the crystallographic plane of the cellulose. Because of the similarity with the other two studies mentioned before, the hemp shiv presents crystalline and amorphous phases. The cellulose represents the crystalline part of the hemp shiv. The hemicellulose and lignin make the amorphous part, due to the hydroxyl group [25,29].

**Figure 1.** Industrial crop (**a**) natural hemp shiv; (**b**) natural hemp microscopy; (**c**) natural hemp thermal analysis; (**d**) treated hemp shiv; (**e**) treated hemp microscopy; (**f**) treated hemp thermal analysis.

The hemp shiv used in the recipes was treated with a lime solution, intended to improve the antiseptic and the fire characteristics of biomass. The treatment procedure consisted of soaking the hemp shiv under agitation for 5 min in a lime milk solution, until all the hemp shiv was covered by the solution. Successively, the hemp was dried under normal laboratory conditions at 24 ◦C and a humidity of 40%. Applying the lime solution to the hemp did not modify the density of the fibers, as highlighted in Reference [30]. The density kept the same value of 100 kg/m3. Instead, the relative humidity changed, by decreasing to 4%. Figure 1d shows the visual aspect of the hemp shiv covered with lime solution, meanwhile Figure 1e captures the microscopic surface of hemp shiv at a scale of 500 μm. The image presents similarities with the studies in References [31,32]. The thermal analyses of the treated hemp shown in Figure 1f showed an improvement of the pyrolytic behavior of this plant, the degradation percentage being 64.15%. By treating the hemp shiv with lime solution, the mass loss was smaller by 17% at higher temperatures, in comparison with the natural ones. The diagram of the treated hemp shiv showed three temperature intervals. The first one, from 20 to 200 ◦C, presented 4.24% degradation of the shiv which is attributed to the water released [33]. The deterioration from the second and the third temperature interval, from 200 to 580 ◦C (34.74%) and 580 to 100 ◦C (25.37%) is characteristic of the lime degradation process [12]. The X-ray diffraction diagram of the treated hemp shiv indicated the same shape of graph as with the natural hemp shiv from Reference [19]. The difference consisted in the identification of new crystalline compounds—Ca(OH)2 calcium hydroxide and CaCO3 calcium carbonate. The central peak remains in the same position but varies in intensity and secondary peaks appear where the main compound is made of the carbonyl group. A remark regarding these changes is made in Reference [30], which assumes that by applying the lime treatment a part of the amorphous components of hemp, such as hemicelluloses, pectins and waxes, are removed.

#### 2.1.2. Volcanic Rocks

The volcanic rocks were procured by Procema Perlit (Jilava, Ilfov, Romania). The rocks were selected based on composition, to reinforce the fire behavior of the composite materials, but also to grow the porosity of the elements. They are natural products, with significant thermal properties due to the porous structure. The volcanic rocks, perlite and vermiculite were chosen for the new compositions. The two igneous rocks were obtained after the cooling of volcanic eruptions. The main constituents of the perlite rock are SiO2, Al2O3, K2O, Na2O; when hydrated it can also contain TiO2, CaO, MgO, and Fe2O3 in small quantities [34]. When it comes to vermiculite, the main components are SiO2, Al2O3, Fe2O3, CaO, MgO, K2O but also some small quantities of Li, Cr, Ti, and Ni [35].

Perlite has a granular aspect, as shown in Figure 2a, with white color and it is usually used in the constructions sector as a lightweight aggregate for mortars, screeds and thermal insulation concrete. According to the manufacturer's datasheet, perlite has a density around 100–140 kg/m3 (SR EN 1097-3/2002), for a grain size between 0–2.5 mm (SR EN 933/2002) and a thermal conductivity of about 0.055 W/mK (SR EN 12667/2002). The natural humidity is about 1% (SR EN 1097-6/2002), the crushing resistance is 0.10 N/mm2 (SR EN 13055/2003) and the fire reaction is class A1. The microscopic image presented in Figure 2b illustrates the exterior porous surface of a perlite grain. According to Reference [36], the unexpanded perlite can increase volume by 10 to 20 times more, when heated over 700 ◦C, becoming an expanded perlite. The water molecules from the initial stage evaporate during the heating process, cracks appear as a consequence of the steam leak, therefore the resulting cavities will define the perlite structure [34,36,37]. The X-ray diffraction graph of perlite, from Reference [19], resembles the curve shape of powder perlite studied in Reference [38], indicating an amorphous structure already identified in previous studies [39,40]. The crystalline phase of perlite is provided by the compound of SiO2 and the amorphous structure is given by the aluminosilicate compounds [39].

**Figure 2.** Volcanic rocks (**a**) perlite natural structure; (**b**) perlite microscopy; (**c**) vermiculite natural structure; (**d**) vermiculite microscopy.

The expanded vermiculite is known as a granular product, of brown color, shown in Figure 2c, which has a mica aspect, and is rich in iron ions, magnesium and silicates. The physical characteristics of vermiculite presented in the manufacturer's datasheet, indicate a grain size from 0–0.5 mm (max. 5%) and 0.5–3 mm (95%), a density of around 110–130 kg/m3, whereas the absorption is around 60%–70% of the volume. It is usually used in horticultural substrates. Therefore, by including this product in the recipe, it was intended to analyze its behavior in composite materials. The microscopic image of the vermiculite grain captured in Figure 2d showed a lamellar structure formed during the speedy heating process [20,37,41–43]. The X-ray diffraction of vermiculite [19] shows a high peak with an intensity of 800 and a diffraction angle of 2 θ = 27.4◦. The crystallographic determination of expanded vermiculite showed that this volcanic rock presents an incomplete crystallinity, having a considerable number of amorphous compounds [20,42]; therefore, it resembles the perlite. The thermal analysis of the volcanic rocks cannot be shown due to the fact that some errors appeared in the investigation process. Some studies from the scientific literature showed that the degradation process of the perlite is defined by three temperature intervals with a maximum percentage of degradation around 2% [44,45]. Reported to the vermiculite, the degradation process is around 9% [20]. These characteristics of the volcanic rocks, that is, resistance to higher temperatures, will help decrease the degradation process of the composite hemp based materials.

#### 2.1.3. Binders

The hydrated lime used for the treatment of the hemp shiv was CL80-S according to EN 459-1, produced by Carmeuse (Brasov, Romania). The chemical composition of the lime powder, according to the manufacturer's datasheet, is CaO + MgO (min.80%), CO2 (max.15%), MgO (max.5%) and SO3 (max. 2%). Other laboratory tests concerned density (about 520 kg/m3), porosity (74%) and compactness (26%). Lime powder is presented in Figure 3a, and the microscopy for this power at the scale of 500 μm can be observed in Figure 3b. The thermal analysis is presented in Figure 3c. The maximum percentage of degradation recorded by lime powder was 35.11%. The decomposition analysis was defined by two intervals of temperature. The first one, between 20–540 ◦C, where the sample recorded a decrease of mass by 11% while on the second temperature interval of 540–1000 ◦C was 24%. For the X-ray diffraction [19] the graph form showed only the presence of crystalline compounds—calcium hydroxide and calcium carbonate—which are characteristic for lime [38,46].

White cement was chosen with the main purpose of increasing the mechanical and fire characteristics of the new composite materials, but at the same time to offer a proper aesthetic side. The Portland white cement CEM I 52.5 produced by Devnya Cement was used, according to the BSS EN 197-1:2011. The cement powder is characterized by a white color, with a more intense brightness than the hydrated lime, seen when comparing Figure 3d with Figure 3a. The microscopic representation for the white cement powder was captured at the scale of 500 μm, shown in Figure 3e. The decomposition process of the cement powder was determined with the thermal analysis investigation. The maximum percentage of lost mass by the samples was 4.35%. The degradation process was defined by three temperature intervals: 0–240 ◦C (1.08%), 240–500 ◦C (0.70%) and 500–1000 ◦C (2.56%). The X- ray diffraction [19] indicated high contents of CaO and SiO2, which define the main crystalline compounds of the cement—tricalcium silicate and dicalcium silicate—while the Al is found in small quantities, which determine the presence of the mineralogical compound of tricalcium aluminate, confirmed also by References [47–50]. Also, the X-ray diffraction of the white cement captures the presence of TiO2, which is responsible for the white pigment of the Portland cement [47].

**Figure 3.** Binders (**a**) lime powder; (**b**) lime microscopy; (**c**) lime thermal analysis; (**d**) white cement powder; (**e**) white cement microscopy; (**f**) white cement thermal analysis.

#### *2.2. Preparation of New Composite Materials*

The mixing proportions of the new mortars were set up based on a previous study [18]. The composition of the analyzed samples started from the standard cement mortar formula. The sand grains were replaced with hemp shiv, obtaining three new compositions with a different hemp shiv volumes. The conclusion of the previous paper showed that the mix with a higher quantity of hemp has better acoustical and thermal characteristics. Therefore, to improve the obtained characteristics of this composition, a new set of recipes was analyzed. The new element introduced in the mortar mix was the volcanic rock, aiming to increase the fire properties of the materials, but also to obtain a lighter structure. Two types of volcanic rocks were proposed; perlite and vermiculite. Based on these new elements, three different compositions were analyzed, according to Table 1.


The dosages of the three mixtures are expressed in volumes. The ratio between binder and hemp was 1:3, the cement to volcanic rocks ratio was 1:2 and the binder to water ratio was 1:1. The mixing sequence followed the traditional part of mortar recipe, by adding the volcanic rocks at the end of

#### 36

the process. Mixing was performed with a mechanical mixer. The composition was placed into the molds in a single layer and manually compacted. For each determination, different molds were used as required by the test procedure. The samples were kept in the molds for 24 h in laboratory conditions with a temperature of 24 ◦C and a humidity of 40%. The three compositions were tested after 28 days.

#### *2.3. Visual Analysis*

The visual aspect of the compositions is shown in Figure 4. The mixtures present as homogeneous, with a uniform distribution of hemp fibers. Being designed for building interior mortars, white cement was chosen as a binder in order to comply with the aesthetic requirements. The compositions have different shades, depending on the volcanic rock used; therefore, the perlite compositions have a lighter shade than those with vermiculite, which, due to the mica appearance of the rock, are defined by a brown color.

**Figure 4.** Visual composition (**a**) M1 (C + C3 + P2); (**b**) M2 (C + C3 + V2); (**c**) M3 (C + C3 + PV2).

#### *2.4. Chemical Characterization*

The modal composition of the samples was determined by using a non-destructive method called X-ray diffraction. The technique was helpful in the identification and quantification of crystalline compounds. The samples were analyzed in the form of powder. The method consisted of recording the reflected radiation after the propagation of X-rays on the surface of the sample with a fixed wavelength and intensity. Depending on the arrangement of the atoms in the structure, the materials were classified into crystalline and noncrystalline compositions. The crystallinity of the materials is given if the atoms are disposed of in a repeated way over large atomic distances while the amorphous compounds are defined by a random display of the atoms [51]. The crystallinity of the samples was measured using a Shimadzu 6000 XRD diffractometer. The diffraction angle used was between 2 θ = 10–70◦ and the applied wavelength was equal to λ = 1.54182 Å.

#### *2.5. Morphological Analysis*

The microstructural morphology of the samples was evaluated with a Scanning Electron Microscope, model VEGA, and with the software Tescan. The samples were broken into small pieces and, before the scanning process, they were sputter-coated with a layer of gold, with a thickness of 10nm, using the turbo molecular pumped coating system Q150T ES.

#### *2.6. Thermogravimetry Analysis*

The thermal analysis of the composite materials was proposed due to the fact that the ratio between hemp and binder was 3:1. The large volume of hemp from the compositions required more attention on the pyrolitic behavior of the existing biomass. Thermogravimetry analysis (TGA) is a method which presents the modifications in materials, both at physical or chemical levels, due to the change of temperature [52]. The analysis was carried out using a TGA/SDTA 851e\_METTLER TOLLEDO device. The samples were tested in a nitrogen atmosphere (N2), at a gas flow rate of 60 mL/min and a heat rate of 20 ◦C/min in the temperature range of 25–1000 ◦C. The analysis was performed using 10 mg of each composition.

#### *2.7. Bending Core Cohesion*

The fire behavior of the samples was determined using the method of bending core cohesion according to SR EN 520/A1:2010. The method consists of applying a bending moment to the sample while the lateral surface is affected by the flames of two burners. By heating, the bending moment causes a flexion on the sample [53]. Samples were of 300 <sup>×</sup> 45 <sup>×</sup> 12.5 mm3 size. According to the test requirements the sample needs to maintain its structure during 15min at a constant temperature of 950 ◦C.

#### *2.8. Mechanical Properties*

The mechanical efficacy of the volcanic rocks in the composition of the composite materials was subject to flexural and compression forces. The tests showed the capacity of the materials to react to the action of exterior forces and were operated according to SR EN 196-1:2016 at 3, 7, 14 and 28 days [54]. The flexural strength was analyzed on samples with 40 <sup>×</sup> 40 <sup>×</sup> 160 <sup>3</sup> size, using an automatic flexural tensile tester L15 Controls. The samples tested for compressive strength were the remaining prisms after the flexural strength. The test was carried out with a hydraulic press of 250KN Tecnotest.

#### *2.9. Thermal Properties*

The thermal behavior of the composite based on hemp–volcanic rocks–cement was determined according to SR EN 12667:2002 [55]. The test was performed using a heat flow meter of type FOX 200 (TA Instruments). The thermal conductivity determination consisted of applying a variable heat flux to a sample measuring 150 <sup>×</sup> <sup>150</sup> <sup>×</sup> <sup>30</sup> 3, fitted between two plates. The report obtained after each sample test indicated the values of the thermal conductivity coefficient and thermal resistance.

#### *2.10. Acoustic Properties*

The sound absorption coefficient defines the capacity of the materials to quantify the dissipation energy of a sound. The acoustic measurements were made according to SR EN ISO 10534-2 [56], using the transfer function method with two microphones. The test was carried out with the help of the Kundt Impedance tube, on a range frequency from 50 Hz to 6400 Hz. Two types of samples, of 28 mm and 100 mm diameter, were analyzed. For the low frequency range 0 to 1600 Hz, the test was operated on the high diameter samples and for the high frequency range from 500 to 6400 Hz, on the small diameter samples. The thickness of the samples was 30 mm. According to Reference [41], the value of NRC- noise reduction coefficient can be calculated based on sound absorption coefficient. The arithmetic mean of the sound absorption coefficient on the standardized frequency ranges of 250 Hz, 500 Hz, 1000 Hz and 2000 Hz can be expressed with the following formula [57]:

$$NRC = \frac{\alpha 250 + \alpha 500 + \alpha\_{1000} + \alpha 2000}{4} \ [-] \tag{1}$$

#### **3. Results and Discussions**

#### *3.1. Chemical Characterization*

The modal composition is presented in Figure 5. The X-ray diffraction testing involved the identification of crystalline compounds, characteristic to each composition. The analysis of materials based on hemp shiv, volcanic rocks and white cement shows that crystalline compounds from the initial non-hydrated phase were found inside the composition, but also new formations that appeared during the hydration process.

**Figure 5.** X-Ray diffraction (XRD) of the composite materials (**a**) M1 (C + C3 + P2); (**b**) M2 (C + C3 + V2); (**c**) M3 (C + C3 + PV2); (**d**) comparative analysis.

The main compounds found in the three materials were alite, belite, celite, portlandite, calcite, wollastronite, titanite, perovskite, powellite and yelimite. The first three compounds (alite, belite and celite) define the cement powder. Therefore, their identification in the composite samples can be related to the reduced amount of water during the hydration process. The ratio of water-cement used was 1:1, while the ratio of raw materials (hemp + rocks) to binder was 5:1. The explanation could be emphasized by the fact that the volume of water in the composition was not used only for the hydration of the cement paste, but was also retained by the structure of the volcanic rocks and the hemp, which has a high level of absorption. Reference [37] confirmed that vermiculite and perlite, due to their exfoliated structure, require more water, though perlite is more absorbent that vermiculite. This fact can be related to Diagram 5 (b), of the composite M2, where the compounds of alite and belite were absent, which can demonstrate that the hydration process was more complete. The cement hydration leads to new crystalline formations. From the reaction of alite and belite with water, portlandite Ca(OH)2 and gels C-S-H calcium silicate hydrate are formed, and by the hydration of celite with gypsum, ettringite 3CaO·Al2O3·3CaSO4·32H2O is formed [58]. This information can be confirmed with the diagram of vermiculite composite where portlandite appears. The calcium carbonate CaCO3 present on the diagrams appears as a result of the aging process of the cement paste, the X-ray analysis being performed one year after the casting of the compositions. The rest of the compounds found in the cement paste structure are wollastronite, dioside, titanite, perovskite, powellite, yelimite. Their

occurrence can be explained by the hydration reactions through cement compounds, hemp and volcanic rock compounds. A common compound for the three mixtures is wollastronite CaSiO3 which is responsible for increasing the mechanical performances [59]. The titanite CaSiTiO5 and perovskite CaTiO3 are also present due to TiO2, contained in the white cement power and in the volcanic rocks. Diopside is a compound found in M3, shown in Figure 5c, which has a monoclinic structure pyroxene mineral with a high melting point of 1391 ◦C [60]. The analysis of graphs M1, shown in Figure 5a, and M2, shown in Figure 5b, shows that the crystalline formations are different, but their compounds are found in mixture M3. From Figure 5d, by analyzing the peaks of the compositions, it can be shown that the structure of the M1 recipe is the one with the best contoured peaks.

#### *3.2. Petrographic Features*

The scanning electron microscopy (SEM) images of the three mixtures are presented in Figure 6. In order to have an overview of each microscopic structure of the composites, the SEM representations were taken to various scales, of 2 mm, 200 μm, 50 μm, and 5 μm. The morphology of the composites presented at the 2 mm scale, shown in Figure 6a–c, showed a general texture in which the hemp fibers are incorporated in a binder matrix, while at the scale of 200 μm, the cracks on the surface of the composite are more visible and the images of volcanic rocks become more prevalent. The need to obtain a small piece of composite to perform this microscopic analysis determined the application of a breaking force on the structure of the composite, which could explain some of the cracks that appeared on the surface of the material. The hemp shiv has a tendency to fix itself to the cement matrix, due to the streaks given by the woody surface. Figure 6f presents the M2 composition at 200 μm, with the channels still visible, after the hemp detachment, supports the previous observation. At the same resolution, the compositions of M1, shown in Figure 6d, and M3, shown in Figure 6e, showed the porous texture of these materials, it being easy to associate the initial SEM images of the perlite, shown in Figure 2b, and vermiculite, shown in Figure 2d, with the images from the matrix of the composite materials. Increasing the scan resolution at 50 μm, (see Figure 6g–i) allows a more accurate observation of the cement matrix covering the surface of the raw materials. The most interesting images were taken from scale 5 μm, (Figure 6j–l) showing the new structures obtained after the cement hydration. The cement hydration process is defined when the mix between the cement and water decrease the plasticity and increase the rigidity in the hardening stage [59]. The resulting compounds after the hydration of the composite materials are C-S-H gels, CH (portlandite or calcium hydroxide), ettringite, monosulfatem unhydrated, cement particles, air voids, according to References [61,62]. The first three compounds are responsible for the strength properties. The C-S-H gel has the role of filling in the pore space of the composite structures and is responsible for the strength and durability of the cement-based materials, by creating a reticular network between the cement molecules. The CH structure is represented by large crystals with hexagonal prism forms [61,63,64], while the ettringite structure is similar to needle-shaped crystals [65]. The study showed [66] that the ratio of water/cement can also impact significantly the expansion of the new hydrated crystals. When the ratio is high—in our situation, it is 1:1—there is a considerable percentage of water and space in the structure of the composites, which allows the widespread development of the new crystals. At the scale of 5 μm, a large surface covered with acicular formations can be observed; different studies refer to them as ettringite crystals. The expansive surface of ettringite can be explained through Reference [65], which states that the formation of the crystals is due to the consumption of yelimite, anhydrite, gypsum, and Ca(OH)2. Some of these crystalline compounds were found in the X-ray diagrams of the new composites based on cement, hemp and volcanic rocks.

**Figure 6.** Scanning electron microscopy (SEM) of the composite materials at different scales at 2 mm (**a**–**c**); at 200 μm (**d**–**f**); at 50 μm (**g**–**i**) and at 5 μm (**j**–**l**).

#### *3.3. Thermal Analysis*

The thermal behavior of the composites, based on hemp shiv, volcanic rocks and white cement, is presented in Figure 7a–c. A first observation of the diagrams refers to the curves representation, which shows a degradation rate around 1% between the three composites. The lowest degradation percentage is registered by the composition M2 with 31%, followed by M3 with 32.63%, and the highest percentage being obtained by M1 with 33.24%, Figure 7d. In the degradation process, four temperature ranges were identified. Table 2 shows the amount of mass lost by each composite material, by temperature range.

**Figure 7.** Thermal analysis of the composite materials (**a**) M1 (C + C3 + P2); (**b**) M2 (C + C3 + V2); (**c**) M3 (C + C3 + PV2); (**d**) comparative analysis.

**Table 2.** Mass loss of the composite.


The first temperature range (0–220 ◦C) is characterized by a loss of mass between 4.63% and 4.98% per composition, especially around the temperature of 103 ◦C. The highest percentage is attributed to M3. From 220 ◦C to 400 ◦C, the mass loss was similar to the previous, with an exception—M1 degraded

quicker, as shown in Figure 7a. On the third interval the maximum percentage loss was 2.94% for M1, at around 450 ◦C. For the temperature range from 500 to 1000 ◦C there was approximately a 20% mass loss at the temperature of 767 ◦C. According to the scientific literature, the thermal analysis of the cement paste is characterized by the discomposure of ettrinigite at a temperature of 120–130 ◦C, the C-S-H gel below 150 ◦C, the non-hydrated gypsum at about 140 to 170 ◦C and CH (portlandite) between 420 and 550 ◦C [67]. Reference [28] presented the thermal behavior of the hemp shiv treated with CaOH2 (lime solution) and identified two temperature ranges for the degradation, the first one around 337 ◦C—when the depolymerisation of hemicellulose or pectin took place—and the second one, between 377–399 ◦C, when the cellulose decomposition occurred.

The perlite thermal decomposition presented in Reference [68] is defined by two stages, up to 120 ◦C the volcanic rock loses the water, and before 600 ◦C the dehydroxylation process occurs. Vermiculite, according to Reference [69], loses the water molecules under 127 ◦C. After that, it releases the hydroxyl from the interlayers. An observation of the four temperature ranges for the above mentioned composite materials in correlation with the scientific literature is that from 0–220 ◦C the composite mortars lose the water molecules, which affects the decomposition of the ettringite, CSH and non-hydrated gypsum. From 220 to 400 ◦C, the hemp shiv is decomposed by losing cellulose and hemicellulose. From 400 to 500 ◦C the portlandite crystals disappear and from 500 to 1000 ◦C the volcanic rocks release the hydroxyl groups.

#### *3.4. Bending Core Cohesion*

The composite materials were tested for bending core cohesion and the results complied with the test requirements, as shown in Figure 8b. The samples were tested at their thickness real scale, considered as prefabricated plates. The materials were embedded in a device in which the side faces were positioned between two burners, as shown in Figure 8a. After 15 min, at a temperature of 950 ◦C, the three compositions remained intact. The opening produced by the flame was 7.4 cm for M1, 7 cm for M2 and 6.5 cm for M3, according to Figure 8c. Positioned vertically (see Figure 8d) the samples show the degree of fire penetration inside the material. Therefore, it can be noticed that the sample with hemp-perlite-vermiculite is more resistant due to the fact that the fire penetration is less expanded comparative with the other samples. The most damaged sample was the perlite composition.

**Figure 8.** Bending core cohesion of the materials (**a**) testing device; (**b**) graphic results; (**c**) horizontal faces of the samples; (**d**) vertical faces of the samples

#### *3.5. Mechanical Properties*

The results of the flexural and compressive strengths are presented in Figure 9a,b. The tests were performed at 3, 7, 14 and 28 days, with the flexural strength device, shown in Figure 9c and the hydraulic press, shown in Figure 9d, for compressive strength.

**Figure 9.** Mechanical properties (**a**) flexural strength; (**b**) compressive strength; (**c**) flexural strength device; (**d**) hydraulic press.

For the flexural strength, the three composites show values between 1.6–3 N/mm2, while for the compressive strength the values are between 3.69 and 4.53 N/mm2. As a general observation, the results obtained were lower than the standard cement mortar values, because of the hemp shiv and volcanic rocks incorporated in the matrix of the new elements. reference [37] states that the air from the pores of the volcanic rocks is responsible for the decrease in the mechanical properties of the new materials. Another aspect is that the hemp and volcanic rocks absorb the water, but do not react to it, so the only element that creates bonds and connects all raw materials is the cement [37]. There is a continuous increase of the compressive and flexural strengths during the tests. A common observation related to the mechanical resistances refers to the impact of the volcanic rocks upon the structure of the composites. M1 sample evolves the least, the values recorded being close to one another. There is a slight gradual increase at 3, 7 and 14 days, followed by a decrease at 28 days, although the difference is not particularly significant. The decreased values obtained for compressive strength at 28 days may be explained by the environmental conditions. Due to this fact the hemp-based materials were designed as prefabricated plates, and the investigated samples were kept in laboratory conditions. The porous volcanic rocks from the samples, absorbed more water, which affected the proper hydration of the cement until 28 days In contrast, M2 has a relatively small increase in values between 3 and 7 days, after which the resistance, both at bending and at compression, doubles at 14 days. At 28 days, the value of the bending resistance increases compared to 14 days value, while the compression strength values decrease. Using both perlite and vermiculite in M3 exhibits an improvement of the mechanical strengths, the flexural and compression values registering a constant increase with similar

or higher values compared to the other two compositions. An explanation on the performances of perlite and vermiculite composites could be shown in the X-ray analysis, where the crystal compound of diopside was identified. According to Reference [45], diopside presented a lower degradation rate and increased the mechanical performances. The hemp shiv size can be also taken into consideration for the poor values of the mechanical strength, as References [70,71] showed that, the finer the hemp shiv particles, the stronger the boundaries around the fibers created by the binder will be, and the higher the mechanical characteristics of the composite. The values obtained for the compressive strength are similar to those in Reference [72].

#### *3.6. Thermal Properties*

The thermal properties of the composite materials are defined by bulk density, thermal conductivity and thermal resistance. The thermal conductivity and the thermal resistance were investigated using the Fox 200, Figure 10a, while for the bulk density of the samples, their mass was divided by their volume. The bulk density of the materials is between 600 and 750 kg/m3, Figure 10b, the thermal conductivity varies between 0.124 to 0.162 W/mK, Figure 10c and thermal resistance values are between 0.189 and 0.244 m2K/W, Figure 10d.

**Figure 10.** Thermal properties (**a**) device; (**b**) density; (**c**) thermal conductivity; (**d**) thermal resistance

Due to the fact that the compositions are expressed in volumes, the differences that occur are based on the type of volcanic rock used. In the pre-testing phase, the density of the two types of rocks was determined. The results show that perlite was lighter than vermiculite, based on the values obtained (130 kg/m3 for perlite, 140 kg/m3 for vermiculite). Therefore, the best thermal performance is recorded by M1 with 0.124 W/mK and a density of 612 kg/m<sup>3</sup> compared to M2. The same observation was made also in Reference [73]. The values of the thermal conductivity are directly proportional to the density of the materials, while the thermal resistance is inversely proportional to the thermal conductivity. The M3 composition shows a lower density than that of M2, which can be related to the different particle size distribution of both fillers. Further investigation on the particle size distribution of these rocks should be carried out. The addition to volcanic rocks in hemp-cement mortar compositions results in an increase of thermal performance, to around 50% [18]. In the analysis of their mechanical strengths compared to the thermal performance of the materials one can see that the addition of perlite increases the thermal performance of a material and at the same time decreases the values of mechanical strength. A consideration regarding the improvement of the thermal properties could be connected to the mixing process, according to Reference [73]. If mixing time is increased, the volcanic rocks are broken, which will increase the density and decrease the value of the thermal conductivity. Regarding the influence of the hemp in the compositions, it can be considered that using a smaller dimension of the wood fibers will grow the thermal properties due to the fact that the materials will be defined by a finer porous structure [72].

#### *3.7. Acoustic Properties*

The acoustic properties of the new composite materials are defined by the sound absorption coefficient according to the frequency range between 0-6400 Hz; they are tested using the Kundt tube, as shown in Figure 11a.

**Figure 11.** Acoustic properties (**a**) Kundt tube; (**b**) sound absorption coefficient 0–6400 Hz; (**c**) sound absorption coefficient on frequency standard bands; (**d**) noise reduction coefficient graph

The sound absorption coefficient is defined by the absorbed and incident sound energy [74]. The diagrams of the sound absorption coefficients on a frequency range from 0 to 6400 Hz are presented in Figure 11b. The capacity of a material to absorb the sound is defined by the type of binder,

its percentage, the size of the raw materials and the thickness of the new material [75]. The data obtained after the measurements, shown in Figure 11b, showed that the maximum value of sound absorption from the three composites is around 0.35 on a frequency of around 1500 Hz, for M3. Analyzing the sound absorption coefficient on the standard frequency bands, show in Figure 11c, one can see that M1 and M2 gradually increase sound absorption up to 1000 Hz and then they reduce it. Composite M3 presents increasing values until 2000 Hz followed by almost 0.1, on the last frequency standard band Figure 11d shows the noise reduction coefficient which confirms that the most absorbent sample is defined by M2 because of its higher porous structure confirmed through the SEM images, shown in Figure 2b,d. Less absorbent are M3 and M1. This is confirmed by Reference [76], that smaller particles increase the reflection and larger particles increase the attenuation and reduce the reflection. Comparing these results with other studies [77] it was remarked that the sound absorption coefficient depends on the particle size of the hemp and the type of binder. The cement is regarded as a reflexive binder; thus hemp and volcanic rocks will have the main role in the absorption of the sound. At the same time, increasing the volume of binder, which has low mechanical properties, will negatively influence the porosity and will decrease sound absorption. The size of the hemp shiv can influence the sound absorption coefficient of the material: if the size of the hemp shiv is smaller, the sound absorption will increase [77]. Considering all the information found in the scientific literature regarding the size of the particles, the binder, the thickness of the composite, and the air gap, further research shall be carried out in order to analyze all the parameters that influence the sound absorption coefficient of the new industrial crops composites.

#### **4. Conclusions**

The new hemp shiv, volcanic rock and cement based mortars studied in this paper were designed to respond to sustainable conditions, but also to increase indoor comfort performance. The main conclusions resulting from the study are:


**Author Contributions:** Conceptualization, R.I.; methodology and investigation, R.I., D.R.T.-G., D.L.M.; writing—original draft preparation, R.I.; writing—review and editing R.I., D.R.T.-G.; funding acquisition, D.R.T.-G. All authors have read and agreed to the published version of the manuscript.

**Funding:** The results presented in this paper were obtained in the framework of the GNaC 2018 ARUT grant "Innovative solutions for the acoustic comfort in open space offices", research Contract no. 3223/06.02.2019, with the financial support of the Technical University of Cluj-Napoca.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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