**Contents**



## **About the Editors**

**Carlos Thomas** is an Associate Professor at the Materials Science and Engineering Laboratory in the Civil Engineering School of the University of Cantabria, Spain. He received his International Ph.D. Cum Laude and his Extraordinary Doctorate Award in 2013. His research activity has focused on the evaluation of construction, demolition and industry wastes for the manufacture of recycled mortars and concrete. In the last five years, he has participated as a Principal Investigator in 5 R+D+i projects and as a researcher in another 10 R+D+i projects, with both public and private funding, related to recycled materials. He is the author of 20 Q1 JCR papers, one of them certified as one of the 25 most downloaded in ScienceDirect, and more than 80 congress communications and conferences. He has undertaken research stays in France, Germany, Brazil and Portugal and he is one of the promoters of the "Spanish Recycled Concrete Network" that has led to the publication of this book. carlos.thomas@unican.es

**Jorge de Brito** is a Full Professor of Civil Engineering at Instituto Superior Tecnico (IST), ´ University of Lisbon, Portugal, where he received his Ph.D. in 1993. He is the author of over 500 research articles in peer-reviewed journals, and 8 books, including "Recycled Aggregate in Concrete: Use of Industrial, Construction and Demolition Waste" (Springer) and "Sustainable Construction Materials: Recycled aggregates" (Woodhead). He was co-editor of "Handbook of Recycled Concrete and Demolition Waste" (Woodhead) and "New Trends on eco-efficient and recycled concrete" (Woodhead). He is Editor-in-Chief of the *Journal of Building Engineering* (Elsevier) and is part of the Editorial Board of 45 peer-reviewed journals. He belongs to several CIB, CEN, RILEM and IABSE committees. He was the Head of the CERIS Research Centre (around 400 researchers) in 2017–2018 and is the Head of the Department of Civil Engineering, Architecture and Georesources, and Director of the Eco-Construction and Rehabilitation Doctoral Program from IST, University of Lisbon. jb@civil.ist.utl.pt.

**Valeria Corinaldesi** is a Full Professor of "Materials Science and Technology" at Universita` Politecnica delle Marche (Italy). She completed her M.Sc. Degree in Civil Engineering at the University of Ancona (Italy) in 1998; her Ph.D. in Materials' Engineering at the University of Bologna (Italy) in 2002. She is the co-author of more than 300 publications, mainly on peer-reviewed international journals and conference proceedings and the co-inventor of 3 national patents and 1 european patent. Her overall scientific production received more than 3000 citations with H-index = 32, Orcid code: 0000-0001-9372-5032. She contributed to more than 50 international conferences as a speaker and/or member of either scientific or organizing committees. She is a referee for many international scientific journals indexed on WoS and Scopus databases; in particular, she is a member of the Editorial Board of the 'Journal of Building Engineering'. She is a Member of the World Road Association-PIARC (Italian National Committee), Ministry of Infrastructures and Transports, of the American Concrete Institute—Italy Chapter, of RILEM TC "Structural behavior and innovation of recycled aggregate concrete". v.corinaldesi@staff.univpm.it.

### *Editorial* **Special Issue High-Performance Eco-Efficient Concrete**

**Carlos Thomas 1,\*, Jorge de Brito <sup>2</sup> and Valeria Corinaldesi <sup>3</sup>**


#### **1. Introduction**

The benefits of recycling in the construction sector have been widely demonstrated and are unquestionable. The use of recycled aggregates, steel slags, and low-impact cements implies an important reduction of the environmental footprint, and eco-efficient concretes made with them must be a priority. However, these materials show, in some cases, losses of mechanical and durability behavior compared with natural materials. This is why we must invest our efforts in finding high-performance eco-efficient concretes that can compete—or even surpass—traditional concrete. To achieve this, research and dissemination of its results is essential. The objective of this Special Issue is to group the most recent and relevant research in relation to high-performance eco-efficient concrete into a single document. Subsequently, the possibility of publishing a book with the contributions of all authors will be assessed.

So far, 16 papers have been published in the Special Issue out of a total of 21 submitted. The next sections provide a brief summary of each of the papers published.

#### **2. High-Frequency Fatigue Testing of Recycled Aggregate Concrete**

Sainz-Aja et al. [1] show that concrete fatigue behavior has not been extensively studied, in part because of the difficulty and cost. Some concrete elements subjected to this type of load include the railway superstructure of sleepers or slab track, bridges for both road and rail track, and the foundations of wind turbine towers or offshore structures. In order to address fatigue problems, a methodology was proposed that reduces the lengthy testing time and high cost by increasing the test frequency up to the resonance frequency of the set formed by the specimen and the test machine. After comparing this test method with conventional frequency tests, it was found that tests performed at a high frequency (90 Hz) were more conservative than those performed at a moderate frequency (10 Hz); this effect was magnified in those concretes with recycled aggregates coming from crushed concrete (RC-S). In addition, it was found that the resonance frequency of the specimen– test machine set was a parameter capable of identifying whether the specimen was close to failure.

#### **3. Mechanical and Durability Properties of Concrete with Coarse Recycled Aggregate Produced with Electric Arc Furnace Slag Concrete**

Tamayo et al. [2] show the search for more sustainable construction materials, capable of complying with quality standards and current innovation policies, aimed at saving natural resources and reducing global pollution, is one of the greatest present societal challenges. In this study, an innovative recycled aggregate concrete (RAC) is designed and produced based on the use of a coarse recycled aggregate (CRA) crushing concrete with electric arc furnace slags as aggregate. These slags are a by-product of the steelmaking

**Citation:** Thomas, C.; de Brito, J.; Corinaldesi, V. Special Issue High-Performance Eco-Efficient Concrete. *Appl. Sci.* **2021**, *11*, 1163. https://doi.org/10.3390/app11031163

Received: 1 January 2021 Accepted: 25 January 2021 Published: 28 January 2021


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**Copyright:** © 2021 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 (https:// creativecommons.org/licenses/by/ 4.0/).

industry and their use, which avoids the use of natural aggregates, is a new trend in concrete and pavement technology. This paper has investigated the effects of incorporating this type of CRA in concrete at several replacement levels (0%, 20%, 50%, and 100% by volume), by means of the physical, mechanical, and durability characterization of the mixes. The analysis of the results has allowed the benefits and disadvantages of these new CRAs to be established, by comparing them with those of a natural aggregate concrete (NAC) mix (with 0% CRA incorporation) and with the data available in the literature for concrete made with more common CRA, based on construction and demolition waste (CDW). Compared to NAC, similar compressive strength and tensile strength values for all replacement ratios have been obtained. The modulus of elasticity, the resistance to chloride penetration, and the resistance to carbonation are less affected by these CRA than when CRA from CDW waste is used. Slight increases in bulk density over 7% were observed for total replacement. Overall, functionally good mechanical and durability properties have been obtained.

#### **4. High Performance Self-Compacting Concrete with Electric Arc Furnace Slag Aggregate and Cupola Slag Powder**

Sosa et al. [3] present the development of self-compacting concretes with electric arc furnace slags is a novelty in the field of materials and the production of high-performance concretes with these characteristics is a further achievement. To obtain these high-strength, low-permeability concretes, steel slag aggregates and cupola slag powder are used. To prove the effectiveness of these concretes, they are compared with control concretes that use diabase aggregates, fly ash, and limestone supplementary cementitious materials (SCMs, also called fillers), and intermediate mix proportions. The high density SCMs give the fresh concrete self-compacting thixotropy using high-density aggregates with no segregation. Moreover, the temporal evolution of the mechanical properties of mortars and concretes shows pozzolanic reactions for the cupola slag. The fulfillment of the demands in terms of stability, flowability, and mechanical properties required for this type of concrete, and the savings of natural resources derived from the valorization of waste, make these sustainable concretes a viable option for countless applications in civil engineering.

#### **5. Economic and Technical Viability of Using Shotcrete with Coarse Recycled Concrete Aggregates in Deep Tunnels**

This work [4] analyzes the technical and economic viability of using coarse recycled aggregates from crushed concrete in shotcrete, as a primary lining support in tunnels. Four incorporation ratios of coarse natural aggregate (CNA) with coarse recycled concrete aggregates from concrete (CRCA) were studied: 0%, 20%, 50%, and 100%. The mechanical properties of the dry-mix shotcrete were obtained in an independent experimental campaign. Initially, the technical viability of CRCA shotcrete was validated for deep rock tunnels, based on the convergence-confinement method. Two cases were studied to determine the equivalent thickness for each combination of replacement ratio using CRCA shotcrete: (i) similar stiffness and (ii) similar yield stress. Subsequently, an economic assessment was performed. The stiffness criterion increased the thickness below 10% in both the 20% and 50% replacement ratios, which shows their technical viability with very marginal cost increase (<5%). On the other hand, the maximum pressure criteria required higher increments, close to 30% in the 50% replacement ratio. A full replacement was proven impracticable in both analyses.

#### **6. Experimental Characterization of Prefabricated Bridge Deck Panels Prepared with Prestressed and Sustainable Ultra-High Performance Concrete**

Enhanced quality and reduced on-site construction time are the basic features of prefabricated bridge elements and systems [5]. Prefabricated lightweight bridge decks have already started finding their place in accelerated bridge construction (ABC). Therefore, the development of deck panels using high strength and high performance concrete has become an active area of research. Further optimization in such deck systems is possible using prestressing or replacement of raw materials with sustainable and recyclable materials. This research involves experimental evaluation of six full-depth precast prestressed high strength fiber-reinforced concrete (HSFRC) and six partial-depth sustainable ultrahigh performance concrete (sUHPC) composite bridge deck panels. The composite panels comprise UHPC prepared with ground granulated blast furnace slag (GGBS) with the replacement of 30% cement content overlaid by recycled aggregate concrete made with replacement of 30% of coarse aggregates with recycled aggregates. The experimental variables for six HSFRC panels were depth, level of prestressing, and shear reinforcement. The six sUHPC panels were prepared with different shear and flexural reinforcements and sUHPC-normal/recycled aggregate concrete interface. Experimental results exhibit the promise of both systems to serve as an alternative to conventional bridge deck systems.

#### **7. Alkali Activated Paste and Concrete Based on of Biomass Bottom Ash with Phosphogypsum**

There is a growing interest in the development of new cementitious binders for building construction activities. In this study, biomass bottom ash (BBA) was used as an aluminosilicate precursor and phosphogypsum (PG) was used as a calcium source [6]. The mixtures of BBA and PG were activated with the sodium hydroxide solution or the mixture of sodium hydroxide solution and sodium silicate hydrate solution. Alkali activated binders were investigated using X-ray powder diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) test methods. The compressive strength of hardened paste and fine-grained concrete was also evaluated. After 28 days, the highest compressive strength reached 30.0 MPa—when the BBA was substituted with 15% PG and activated with NaOH solution—which is 14 MPa more than control sample. In addition, BBA fine-grained concrete samples based on BBA with 15% PG substitute activated with NaOH/Na2SiO3 solution showed higher compressive strength compered to when NaOH activator was used 15.4 MPa and 12.9 MPa respectfully. The NaOH/Na2SiO3 activator solution resulted reduced

#### **8. Mechanical Properties and Freeze–Thaw Durability of Basalt Fiber Reactive Powder Concrete**

Basalt fiber has a great advantage on the mechanical properties and durability of reactive powder concrete (RPC) because of its superior mechanical properties and chemical corrosion resistance. In this paper, basalt fiber was adopted to modified RPC, and plain reactive powder concrete (PRPC), basalt fiber reactive powder concrete (BFRPC) and steel fiber reactive powder concrete (SFRPC) were prepared [7]. The mechanical properties and freeze–thaw durability of BFRPC with different basalt fiber contents were tested and compared with PRPC and SFRPC to investigate the effects of basalt fiber contents and fiber type on the mechanical properties and freeze–thaw durability of RPC. Besides, the mass loss rate and compressive strength loss rate of RPC under two freeze–thaw conditions (fresh-water freeze–thaw and chloride-salt freeze–thaw) were tested to evaluate the effects of freeze–thaw conditions on the freeze–thaw durability of RPC. The experiment results showed that the mechanical properties and freeze–thaw resistance of RPC increased as the basalt fiber content increase. Compared with the fresh-water freeze–thaw cycle, the damage of the chloride-salt freeze–thaw cycle on RPC was great. Based on the freeze–thaw experiment results, it was found that SFRPC was sensitive to the corrosion of chloride salts and compared with the steel fiber, the improvement of basalt fiber on the freeze–thaw resistance of RPC was great.

#### **9. A Low-Autogenous-Shrinkage Alkali-Activated Slag and Fly Ash Concrete**

Alkali-activated slag and fly ash (AASF) materials are emerging as promising alternatives to conventional Portland cement. Despite the superior mechanical properties of AASF materials, they are known to show large autogenous shrinkage, which hinders the wide application of these eco-friendly materials in infrastructure. To mitigate the autogenous shrinkage of AASF, two innovative autogenous-shrinkage-mitigating admixtures, superabsorbent polymers (SAPs) and metakaolin (MK), are applied in this study [8]. The results

show that the incorporation of SAPs and MK significantly mitigates autogenous shrinkage and cracking potential of AASF paste and concrete. Moreover, the AASF concrete with SAPs and MK shows enhanced workability and tensile strength-to-compressive strength ratios. These results indicate that SAPs and MK are promising admixtures to make AASF concrete a high-performance alternative to Portland cement concrete in structural engineering.

#### **10. Properties of Foamed Lightweight High-Performance Phosphogypsum-Based Ternary System Binder**

The potential of phosphogypsum (PG) as secondary raw material in construction industry is high if compared to other raw materials from the point of view of availability, total energy consumption, and CO2 emissions created during material processing. This work [9] investigates a green hydraulic ternary system binder based on waste phosphogypsum (PG) for the development of sustainable high-performance construction materials. Moreover, a simple, reproducible, and low-cost manufacture is followed by reaching PG utilization up to 50 wt.% of the binder. Commercial gypsum plaster was used for comparison. Highperformance binder was obtained, and on a basis of it, foamed lightweight material was developed. Low water-binder ratio mixture compositions were prepared. Binder paste, mortar, and foamed binder were used for sample preparation. Chemical, mineralogical composition, and performance of the binder were evaluated. Results indicate that the used waste may be successfully employed to produce high-performance binder pastes and even mortars with a compression strength up to 90 MPa. With the use of foaming agent, lightweight (370–700 kg/m3) foam concrete was produced with a thermal conductivity from 0.086 to 0.153 W/mK. Water tightness (softening coefficient) of such foamed material was 0.5–0.64. The proposed approach represents a viable solution to reduce the environmental footprint associated with waste disposal.

#### **11. Effect of Fly Ash as Cement Replacement on Chloride Diffusion, Chloride Binding Capacity, and Micro-Properties of Concrete in a Water Soaking Environment**

This paper [10] experimentally studies the effects of fly ash on the diffusion, bonding, and micro-properties of chloride penetration in concrete in a water soaking environment based on the natural diffusion law. Different fly ash replacement ratio of cement in normal concrete was investigated. The effect of fly ash on chloride transportation, diffusion, coefficient, free chloride content, and binding chloride content were quantified, and the concrete porosity and microstructure were reported through mercury intrusion perimetry and scanning electron microscopy, respectively. It was concluded from the test results that fly ash particles and hydration products (filling and pozzolanic effects) led to the densification of microstructures in concrete. The addition of fly ash greatly reduced the deposition of chloride ions. The chloride ion diffusion coefficient considerably decreased with increasing fly ash replacement, and fly ash benefits the binding of chloride in concrete. Additionally, a new equation is proposed to predict chloride-binding capacity based on the test results.

#### **12. Durability Assessment of Recycled Aggregate HVFA Concrete**

The possibility of producing high-volume fly ash (HVFA) recycled aggregate concrete represents an important step towards the development of sustainable building materials. In fact, there is a growing need to reduce the use of non-renewable natural resources and, at the same time, to valorize industrial by-products, such as fly ash, which would otherwise be sent to the landfill. The present experimental work [11] investigates the physical and mechanical properties of concrete by replacing natural aggregates and cement with recycled aggregates and fly ash, respectively. First, the mechanical properties of four different mixtures have been analyzed and compared. Then, the effectiveness of recycled aggregate and fly ash on reducing carbonation and chloride penetration depth has been also evaluated. Finally, the corrosion behavior of the different concrete mixtures, reinforced with either bare or galvanized steel plates, has been evaluated. The results obtained show that highvolume fly ash (HVFA) recycled aggregate concrete can be produced without significative

reduction in mechanical properties. Furthermore, the addition of high-volume fly ash and the total replacement of natural aggregates with recycled ones did not modify the corrosion behavior of embedded bare and galvanized steel reinforcement.

#### **13. Mechanical Properties and Flexural Behavior of Sustainable Bamboo Fiber-Reinforced Mortar**

In this study [12], a sustainable mortar mixture is developed using renewable byproducts for the enhancement of mechanical properties and fracture behavior. A highvolume of fly ash—a by-product of coal combustion—is used to replace Portland cement, while waste by-products from the production of engineered bamboo composite materials are used to obtain bamboo fibers and to improve the fracture toughness of the mixture. The bamboo process waste was ground and size-fractioned by sieving. Several mixes containing different amounts of fibers were prepared for mechanical and fracture toughness assessment, evaluated via bending tests. The addition of bamboo fibers showed insignificant losses of strength, resulting in mixtures with compressive strengths of 55 MPa and above. The bamboo fibers were able to control crack propagation and show improved crack-bridging effects with higher fiber volumes, resulting in a strain-softening behavior and mixture with higher toughness. The results of this study show that the developed bamboo fiber-reinforced mortar mixture is a promising sustainable and affordable construction material with enhanced mechanical properties and fracture toughness with the potential to be used in different structural applications, especially in developing countries.

#### **14. Industrial Low-Clinker Precast Elements Using Recycled Aggregates**

Increasing amounts of sustainable concretes are being used as society becomes more aware of the environment. This paper [13] attempts to evaluate the properties of precast concrete elements formed with recycled coarse aggregate and low clinker content cement using recycled additions. To this end, six different mix proportions were characterized: a reference concrete; two concretes with 25%wt. and 50%wt. substitution of coarse aggregate made using mixed construction and demolition wastes; and others with recycled cement with low clinker content. The compressive strength, the elastic modulus, and the durability indicator decrease with the proportions of recycled aggregate replacing aggregate, and it is accentuated with the incorporation of recycled cement. However, all of the precast elements tested show good performance with slight reduction in the mechanical properties. To confirm the appropriate behavior of New Jersey precast barriers, a test that simulated the impact that simulated the impact of a vehicle was carried out.

#### **15. Photocatalytic Recycled Mortars: Circular Economy as a Solution for Decontamination**

The circular economy is an economic model of production and consumption that involves reusing, repairing, refurbishing, and recycling materials after their service life. The use of waste as secondary raw materials is one of the actions to establish this model. Construction and demolition waste (CDW) constitute one of the most important waste streams in Europe due to its high production rate per capita. Aggregates from these recycling operations are usually used in products with low mechanical requirements in the construction sector. In addition, the incorporation of photocatalytic materials in construction has emerged as a promising technology to develop products with special properties, such as air decontamination. This research [14] aims to study the decontaminating behavior of mortars manufactured with the maximum amount of mixed recycled sand without affecting their mechanical properties or durability. For this, two families of mortars were produced, one consisting of traditional Portland cement and the other of photocatalytic cement, each with four replacement rates of natural sand by mixed recycled sand from CDW. Mechanical and durability properties, as well as decontaminating capacity, were evaluated for these mortars. The results show adequate mechanical behavior, despite the incorporation of mixed recycled sand, and improved decontaminating capacity by means of NOx reduction capacity.

#### **16. Durability of High Volume Glass Powder Self-Compacting Concrete**

The transport characteristics of waste glass powder incorporated self-compacting concrete (SCC) for a number of different durability indicators are reported in this paper [15]. SCC mixes were cast at a water to binder ratio of 0.4 using glass powders with a mean particle size of 10, 20, and 40 μm, and at cement replacement levels of 20, 30, and 40%. The oxygen permeability, electrical resistivity, porosity, and chloride diffusivity were measured at different ages from 3 to 545 days of curing. The amount and particle size of the incorporated waste glass powder was found to influence the durability properties of SCC. The glass incorporated SCC mixes showed similar or better durability characteristics compared to general purpose (GP) and fly ash mixes at similar cement replacement level. A significant improvement in the transport properties of the glass SCC mixes was observed beyond 90 days.

#### **17. High-Durability Concrete Using Eco-Friendly Slag-Pozzolanic Cements and Recycled Aggregate**

Clinker production is very energy-intensive and responsible for releasing climaterelevant carbon dioxide (CO2) into the atmosphere, and the exploitation of aggregate for concrete results in a reduction in natural resources. This contrasts with infrastructure development, surging urbanization, and the demand for construction materials with increasing requirements in terms of durability and strength. A possible answer to this is eco-efficient, high-performance concrete. This article [16] illustrates basic material investigations to both, using eco-friendly cement and recycled aggregate from tunneling to produce structural concrete and inner shell concrete, showing high impermeability and durability. By replacing energy- and CO2-intensive cement types by slag-pozzolanic cement (CEM V) and using recycled aggregate, a significant contribution to environmental sustainability can be provided while still meeting the material requirements to achieve a service lifetime for the tunnel structure of up to 200 years. Results of this research show that alternative cements (CEM V), as well as processed tunnel spoil, indicate good applicability in terms of their properties. Despite the substitution of conventional clinker and conventional aggregate, the concrete shows good workability and promising durability in conjunction with adequate concrete strengths.

**Author Contributions:** Conceptualization, C.T.; methodology, C.T. and J.d.B.; validation, C.T., J.d.B. and V.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Thanks are due to all of the authors and peer reviewers for their valuable contributions to this Special Issue. The MDPI management and staff are also to be congratulated for their untiring editorial support for the success of this project.

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

#### **References**


## *Article* **High-Durability Concrete Using Eco-Friendly Slag-Pozzolanic Cements and Recycled Aggregate**

#### **Klaus Voit 1, Oliver Zeman 2,\*, Ivan Janotka 3, Renata Adamcova <sup>4</sup> and Konrad Bergmeister <sup>2</sup>**


Received: 14 October 2020; Accepted: 18 November 2020; Published: 23 November 2020

**Abstract:** Clinker production is very energy-intensive and responsible for releasing climate-relevant carbon dioxide (CO2) into the atmosphere, and the exploitation of aggregate for concrete results in a reduction in natural resources. This contrasts with infrastructure development, surging urbanization, and the demand for construction materials with increasing requirements in terms of durability and strength. A possible answer to this is eco-efficient, high-performance concrete. This article illustrates basic material investigations to both, using eco-friendly cement and recycled aggregate from tunneling to produce structural concrete and inner shell concrete, showing high impermeability and durability. By replacing energy- and CO2-intensive cement types by slag-pozzolanic cement (CEM V) and using recycled aggregate, a significant contribution to environmental sustainability can be provided while still meeting the material requirements to achieve a service lifetime for the tunnel structure of up to 200 years. Results of this research show that alternative cements (CEM V), as well as processed tunnel spoil, indicate good applicability in terms of their properties. Despite the substitution of conventional clinker and conventional aggregate, the concrete shows good workability and promising durability in conjunction with adequate concrete strengths.

**Keywords:** green cements; slag-pozzolanic cement; CEM V; tunnel spoil recycling; high durability

#### **1. Introduction**

#### *1.1. Eco-E*ffi*cient Cement Production*

The demand for concrete—the world´s most used construction material—continues to remain at a high level notwithstanding recent global economic fluctuations. In doing so, the concrete industry acts as one of the main contributors to CO2 emissions accounting for approx. 5 to 7% of global anthropogenic CO2 emissions (see [1–4]), whereby Portland cement production accounts for approx. 90% of the quoted share [5]. The rapid increase in the recent global cement production is driven by China, which produced 2.35 gigatons of cement in 2015. This corresponds to 55% of the global amount of cement produced, contributing approximately 13 to 15% of China's total CO2 emissions. In the future, due to population growth, cement production worldwide is projected to increase between 12% and 23% by the year 2050 [6].

Such production quantitieswouldlead to a global emission, depending on the author, of approximately 1.3 to 1.8 billion tons of CO2 per year [2,7]. Further, CO2 emission during cement production is

derived from two sources, having a similar share: (1) Process-related CO2 accounting for the energy demand by the use of fuels and electric energy mainly for drying, grinding, and mostly from clinker burning; (2) the emission of embodied CO2 (ECO2) during CaCO3 decomposition during heating, when the chemically bound CO2 from the carbonate rock is degassing [2,5]. Summed up, the amount of generated CO2 per kilogram of produced clinker varies between 0.65 and 1.0 kg CO2 per kg clinker [8–11] depending on the fuel type and the basis of electricity production. The process-related global CO2 emission of concrete has been estimated at approximately 83 kg CO2 per ton by [12], while ECO2 accounts for an additional amount of approximately 95 to 135 kg/ton, varying depending on the specific concrete design [5,13,14], adding up to the generation of roughly 200 kg CO2 per ton of concrete.

In view of the importance of these issues, measures have been taken to reduce the environmental implications of cement and concrete production by using various strategies. The options range from customized concrete mixing design and plant technology possibilities toward energy saving [15] to carbon capture strategies [16,17]. As a very central point with regard to embodied CO2, the usage of alternative raw materials with the absence of carbonates in their mineral content is effective. This means the reduction in the portion of Portland cement replacing the clinker by alternative binder compositions (e.g., fly ash, furnace slag, or natural pozzolans) producing blended or—when using more than one blending material—so-called composite cements [18–20]. A current strategy in this context is the substitution of clinker by waste-based cementing materials, i.e., [19,21]. In the present article, the approach of slag-pozzolanic cements (CEM V) according to [22] is pursued.

#### *1.2. Aggregate Recycling*

Additionally, next to or supplementary to clinker optimization, aggregate recycling has a positive effect on the environmental impact of concrete production. Aggregate production is not particularly significant concerning CO2 emission and energy consumption compared to cement production, but due to the large quantities and the high percentage by weight (approx. 80 wt%) of conventional concrete consisting of aggregate, the impact on natural resources is considerably high. Referring to this, aggregate material in particular in close range to the demand with no other intended use can be considered a potential substitute for natural aggregate, provided all crucial technical and legal requirements are met. This applies to construction and demolition waste (CDW), as well as natural excavated rock material. The use of construction and demolition waste as aggregate is the content of current research, e.g., [21,23,24].

Likewise, excavated rock material from earthworks and tunneling is intended to be recycled as aggregate for concrete [25,26]. Due to the fact that, particularly during tunneling, large volumes of concrete are used, there are efforts to reduce the ecological impact of such construction projects at the same time. As for concrete, this concerns cement, as discussed in Section 1.1, and the aggregate used. Regarding the reuse of tunnel spoil, there are numerous related studies examining the questions of reuse possibilities and the suitability of excavated rock for aggregate production. Aspects of tunnel muck recycling and tunnel spoil application opportunities have been demonstrated [27,28]. Therefore, the type of tunnel driving method has a major influence on excavated rock characteristics; this question is evaluated by [29–31]. For high-quality concrete production, careful planning, efficient rock classification [32], and rock material management [33], as well as technical considerations focusing on material analysis and data management, are fundamental [34], not to forget the juridical considerations, as tunnel spoil can initially be considered as waste from a legal point of view, which is done by [35,36].

Tunnel structures have high construction costs and, once in operation, are counted among critical infrastructure. Therefore, for these structures, a service lifetime of some hundred years is assumed. Therefore, durability is a primary focus of the admixture designs. Besides static requirements, a reliable performance capability must be guaranteed for this time period. To meet this requirement, high-performance concrete in terms of durability and density is generally applied in tunnel structures [37]. During construction of the Swiss pioneer projects of the Base Tunnels at the Lötschberg and Gotthard massif,

approximately 40% and 35%, respectively, of the tunnel spoil was recycled mainly for the production of aggregate for concrete [38–40]. A concrete quality with high resistance against various environmental influences allowed the design for a projected service lifetime of 100 years [38].

This research addresses aggregate recycling from tunneling from the example of the high-priority infrastructure project of the Brenner Base Tunnel. The Brenner Base Tunnel is an approx. 55 km-long, flat railway tunnel project acting as a connecting link between Austria and Italy and the main element of the new Brenner railway from Munich to Verona. The tunnel is currently under construction, with a service lifetime designed for a lifespan of 200 years. This increased service lifetime is achieved by a high durability of concrete, accomplished by the high quality of raw materials and good processing of concrete, whereby a high density and impermeability of concrete have the greatest impact [37]. In addition, preliminary tests combining recycled tunnel spoil with slag-pozzolanic cements were conducted.

#### *1.3. Aims, Materials, and Methods*

The overall aims of the presented research can be summarized and listed as follows:


To reach the desired objectives, binder and concrete manufacture, as well as intensive material testing, was performed including the following: (1) Various testing methods were applied to characterize at first the cement admixtures and cement properties of the produced CEM V cement types:


Subsequently, (2) standard concrete compositions using the generated cements were tested to identify the concrete characteristics:


In a third step, (3) standard aggregate for concrete was replaced by recycled aggregate from nonstandard crushed metamorphic rock arising during tunnel excavation. These concrete mixtures—now consisting of a newly developed CEM V cement admixture and processed and recycled aggregate from schist rocks—were also examined with regard to workability (fresh concrete density, flow spread, air content), strength (compressive and bending tensile strength, as well as the fracture energy), and particularly with regard to durability, as the designed concrete composition may be used in tunnel constructions, where the recycled aggregate is generated, exhibiting long service lifetimes of up to 200 years.

At all investigation levels, an additional comparison to standard cement was made.

#### **2. Slag-Pozzolanic CEM V Characteristics**

#### *2.1. General*

To produce cements with reduced CO2 impact, the strategy of Portland cement substitution by additional cementitious constituents, in this case, fly ash (FA), granulated blast-furnace slag (S), and natural pozzolana (P), in accordance with the cement standard EN 197-1 [22], was pursued in this research to produce slag-pozzolanic cement CEM V.

In contrast to the previous edition of EN 197-1, which was valid until Spring 2018, the 27 products in the family of common cements were extended to 39 products until now, inter alia, adding the main cement type CEM VI "composite cement" (having a limestone content from 6 to 20 wt%), and the former CEM V "composite cement" had been renamed to CEM V "slag-pozzolanic cement." The latter consists of 40 to 64 wt% clinker, 18–30 wt% blast-furnace slag, and 18–30 wt% Pozzolana and siliceous fly ash in the case of CEM V/A, and of 20 to 38 wt% clinker, 31–49 wt% blast-furnace slag, and 31–49 wt% Pozzolana and siliceous fly ash for CEM V/B, whereby minor additional constituents (gypsum and limestone for instance) are possible up to 5 wt% for both types.

The increase concerning cement products expresses the research performance and interest of the cement industry in additional cement types with the substitution of clinker by different constituents. The use of these additional cementitious constituents modifies the characteristics of the produced concrete. In the case of CEM V cements, early strength is naturally reduced by the pozzolanic reaction of SiO2 and Al2O3 from fly ash or natural pozzolana and the latent hydraulic reaction of blast furnace slag, and therefore, is not suitable for fast-track construction. This can be overcome by the additional application of silica fume (SF) [5,41]. Then again, improvement in the long-term performance of concrete is indicated by using blended cements due to the delayed growth of calcium silicate hydrate (CSH) and aluminate phases, growing into the pores, providing an increased density of the concrete structure, e.g., [42–44].

However, in many European countries, the application of CEM V cement is not allowed (e.g., Austria) or is restricted (e.g., Germany) by national concrete standards (ÖNORM B 4710-1 [45] in the case of Austria or DIN 1045-2 [46] in the case of Germany), mainly due to the lack of experience. Therefore, in the course of this research, material characterization of the following four manufactured CEM V cement types was carried out to examine the performance of CEM V cements and concrete produced with CEM V cement:


In doing so, a significant reduction in the cement clinker content to a level of 45.1 wt% (1), 26.9 wt% (2), 52.9 wt% (3), and 30.9 wt% (4) for each individual cement type was achieved [47]. The proportional composition of the cementitious constituents in the case of (1) and (2) was 40 wt% slag, 50 wt% fly ash, and 10 wt% limestone. In the case of (3) and (4), the additives were added in equal shares in both cases.

#### *2.2. Basic Cement Qualities*

#### 2.2.1. Cementitious Constituents

Subsequent to the manufacturing of the different CEM V cement types, testing of the main cement properties was performed (see also [47]). In a first step, the main chemical properties of the cementitious constituents of (3) and (4) were compared by X-ray fluorescence to illustrate the fundamentally different chemical composition of fly ash (FA), blast furnace slag (S), and the pozzolana zeolite (P) (see Table 1). In the case of FA and the natural pozzolana zeolite (P), SiO2 and Al2O3 contents are—compared to S—high, suggesting high pozzolanic activity (see also Table 2).

**Table 1.** Chemical properties of cementitious constituents of (3) CEM V/A (S-V) 32.5 R and (4) CEM V/B (S-P) 32.5.


**Table 2.** Pozzolanic activity of cementitious constituents (by Frattini-test).


Blast furnace slag (S) shows similarly high values of SiO2 with 41.2 wt% and a high CaO and MgO content of 37.2 and 10.1 wt%, respectively. Loss on ignition is relatively high in the case of Pozzolana, because of the chemically bound water from clinoptilolite [(Na, K, Ca)2–3Al3(Al, Si)2Si13O36·12H2O].

XRD of the cementitious constituents' analysis again provides further information on the degree of crystallinity by interpretation of the peak visualization: Broader, less distinct peaks suggest low crystallinity, while explicit and high peaks indicating the opposite. XRD diagrams of the raw materials used show the following (Figure 1): FA consists of well-crystallized SiO2 and portions of anorthite (Calcium Feldspar), while the Ca- and Mg-aluminosilicates are diffuse and show a low crystalline grade without any distinct higher-intensity peak (Figure 1). Blast furnace slag (S) can be described as poorly crystallized, showing two major peaks indicating ferrite and belite phases, and also consisting of other, different burned phases (see Figure 1). By contrast, the natural pozzolana zeolite (P) consists of the well-crystallized tectosilicate Clinoptilite (numerous high-intensity single peaks with the main intensity at angles 2Θ of 11.3 and 22.5).

Pozzolanic activity of the cementitious constituents was tested using Frattini´s testing method according to [48] by the solution of 20 g of cement in 100 mL of water at 40 ◦C for varying periods (1, 7, and 28 d), measuring the calcium and hydroxide ion values, calculating the oxide-bounded Ca, and comparing the bounded CaO to the calcium oxide solubility isotherm curve (% of bounded CaO) (Table 2).

**Figure 1.** XRD diagrams displaying angle 2Θ-intensities of cementitious constituents fly ash (V) and furnace slag (S), whereby: Alite—3CaO × SiO2; Belite—2CaO × SiO2; Celite—2CaO × Al2O3; Ferrite—4CaO × Al2O3 × Fe2O3.

Table 2 clearly shows that CaO is very rapidly and most strongly bound by the pozzolana zeolite, bounding approximately 97% of CaO after a period of 28 days. This is achieved by the high SiO2 and low CaO content, cf. Table 1, resulting in a high pozzolan activity.

#### 2.2.2. Cement Properties and Composition

Following the manufacture of the four different above-mentioned CEM V cement types (1), (2), (3), and (4) (see Section 2.1), cement testing was conducted to determine the key characteristics [49]. Fundamental characteristics are cement true density and specific surface expressed as a Blaine-value, illustrated in Table 3 for the manufactured CEM V cement types, as well as for the CEM I basic product.


**Table 3.** Cement true density and Blaine value.

An important parameter with regard to chemical composition and environmental impact is the CaO-content of the various cement products. The lower the content of CaO, the lower the amount of ECO2 (see Section 1.1) emitted. On the other hand, the CaO-containing components in hydrated cement are most sensitive to any aggressive attack; therefore, reducing the CaO content in the blended cement is a key condition for the expected improvement in the durability of the cement composites, see Section 2.4. Figure 2 illustrates the CaO content derived by X-ray fluorescence.

**Figure 2.** CaO content of the various CEM V cement types.

Figure 1 clearly shows the reduced amount of CaO in CEM V cements (replaced by SiO2 and Al2O3 from cementitious constituents, see Table 1) compared to Portland clinker CEM I. Particularly, the CEM V/B with a much higher proportion of additives shows a comparable low CaO content.

Cement analysis via X-ray diffraction (Figure 3a,b) shows an amorphous content in each case, expressed by a slightly broader and diffuse appearance of the peaks, whereby the glassy, noncrystalline phases originate from blast furnace slag and fly ash, cf. Figure 1. Referring to this, cement type (1) and (2) present the highest amorphous content. Furthermore, CEM V/B (S-P) shows the lowest level of amorphous components due to the well-crystallized Clinoptilite (see also [47]).

(**a**)

**Figure 3.** XRD diagrams displaying angle 2Θ-intensities of (**a**) CEM V/A types and reference cement CEM I and (**b**) CEM V/B types and reference cement CEM Iafter 365 days curing at 20 ◦C under water, whereby: CH—portlandite Ca(OH)2; Cc—calcite, Q—quartz SiO2, An—anorthite (CaAl2Si2O8).

(**b**)

#### *2.3. Cement Mortars Properties*

To evaluate fresh and hardened cement paste properties, fresh mortar and the derived test specimens according to [50] were produced, cf. [47,49]. Therefore, test sand was mixed with cement at a ratio 3 to 1 and the corresponding amount of water to reach a water/cement ratio of 0.5 [49]. The fresh mortar properties slump, density, and air content, as well as setting time and soundness according to [51], are illustrated in Table 4, providing information about performance and workability of the tested cement types.


**Table 4.** Fresh mortar properties of different CEM V cements.

Cement types (3) and (4) show comparatively low slump values, indicating the need of superplasticizers to reach a reasonable workability of the fresh mortar. By using a super-plasticizer with a dosage of approximately 0.5 wt% of the cement weight, the slump values were increased to the same level compared to (1) and (2). The fresh mortar density of CEM V/B types is lower than that of the CEM V/A types due to the lower density of the cementitious constituents compared to Portland cement clinker. The density of cement type (4) is noticeably low, due to the porous tectosilicate structure of zeolite that is probably also responsible for the comparably high air content. Setting time of the CEM V mortars is naturally delayed due to the posterior pozzolanic and latent hydraulic reactions of the additives. Soundness of all cement types falls significantly below the limit value of 10 mm according to [51], showing satisfactory volume consistency.

The compressive and bending tensile strength of hardened mortar specimens were determined for a basic mechanical characterization according to [50] with due regard to strength development with time; see Figure 4 for the concrete compressive strength and Figure 5 for the bending tensile strength.

**Figure 4.** Compressive strength development of standard mortar specimens according to [51] (each value as a mean value of 3 individual tests), after 2, 7, 28, 56, and 90 days.

**Figure 5.** Bending flexural strength development of standard mortar prisms according to [51] (each value as mean value of 3 individual tests), after 2, 7, 28, 56, and 90 days.

The strength development of the considered cement types delivered the expected behavior: Standard cement CEM I 32.5 R reaches high early strength, though the CEM V/A cement types reach an equal strength level between 7 and 56 days of curing time. CEM V/B types lag behind but still show an increase in strength even after 56 days in contrast to CEM I and CEM V/A types.

Considering bending flexural strength (Figure 5), all cement types reach a similar strength level after 28 days of water curing. Subsequently, cement type (2) CEM V/B (S-V) is the only cement type still exhibiting a significant increase in flexural bending strength.

By using a super-plasticizer with a dosage of 0.5 wt% of the cement weight to improve the fresh mortar slump value of individual mixtures (cf. Table 4), the 28 day compressive strength for cement type (3) and (4) could be increased by approximately 10%, reaching a compressive strength of 52.1 and 40.3 N/mm2, respectively.

#### *2.4. Durability Aspects*

Pore structure and the resulting permeability are key indicators to evaluate the durability of hardened cement paste and concrete. The appearance of the pore structure of cement paste and concrete is mainly influenced by the cement type. The delayed reaction of pozzolanic or hydraulic additives leads to the growth of additional CSH-phases—by consuming and reducing the Ca(OH)2<sup>−</sup> content and a lower heat of hydration. As a result, the permeability and porosity of the paste or concrete is considerably reduced, as well as thermally induced cracking, which ensures an increased resistance against sulfate and chloride ions attack. Furthermore, due to the low availability of alkali in FA and S, the alkali reaction of cement with reactive components from the aggregate is inhibited, e.g., [52,53].

Pore structure was analyzed in detail for the tested cement types from hardened cement paste [47]. Therefore, the focus was laid on total pore content and the percentage of macro-pores with a diameter >50 nm, because the latter has a negative impact on concrete structure, particularly with regard to durability. Using mercury porosimetry according to [54,55], pore characterization was performed as shown in Figures 6 and 7.

**Figure 6.** Development of total porosity (**a**) and percentage of macro-porosity (**b**) for a curing time of 28 and 365 days.

(a)

**Figure 7.** *Cont.*

(b)

**Figure 7.** Comparison of pore-size distribution after 28 (**a**) and 365 days (**b**) curing time.

As illustrated in Figure 6, total porosity and percentage of macro-pores are clearly reduced during hydration of the Portland cement and—in the case of the CEM V cement types—the added cementitious constituents in the period from 28 to 365 days of curing. Therefore, CEM I 32.5 R shows clearly the lowest total porosity (Figure 7a) with CEM V/B cement types approximately having 1.5 times as much total pore volume. However, CEM I cement also has the highest amount of macro pores (Figure 6b) at 28 and 365 days of curing time, nevertheless—due to its comparably low total porosity and highest short-term reactivity (because of the highest clinker content and lack of latent hydraulic and pozzolanic additives)—showing the highest compressive strength at 28 days age compared to the CEM V cement types.

Figure 7a,b again compare the pore size distribution at 28 and 365 days of curing time, illustrating the change in pore structure. The pore structure of concretes made with CEM V-type cements shows typical symptoms of pore structure refinement in comparison with the reference CEM I concrete. During ongoing hydration, macro-pores with a diameter >50–100 nm retreat in favor of pores with smaller diameters, in the case of the CEM V-cements, more significantly compared to the CEM I cement, leaving CEM I with the highest percentage of macro-pores and demonstrating the capability of CEM V cements' pore volume densification by pozzolanic reactions of the additives. This fact indicates the increase in the volume fraction of less permeable micropores, which is reflected in the increase in the total porosity but, on the contrary, in the decrease in permeability coefficient of concrete.

In addition, carbonation is an important topic concerning durability in terms of the passivation of the reinforcement steel. Slag-pozzolanic cements are known for their lowered resistance to carbonation compared to standard Portland cement by the consumption of the strong alkaline Ca(OH)2 during pozzolanic reactions as a counter-effect to that mentioned above [5,56]. The various cement types show pH values between 12.35 and 12.47 from their aqueous solution, indicating highly alkaline conditions. Additional pH-testing of concrete samples using phenolphthalein indicator solution after 56 days (7 days curing in water, followed by air storage in the laboratory at 20 ◦C and approx. 60% relative humidity) was performed in addition. Therefore, phenolphthalein pH-indicator solution was sprayed on cut-in-half concrete cubes. Areas with a pH value higher than 9.2 turn pink, whereas the concrete surface with a lower pH value stays colorless. Testing showed a carbonation depth <0.5 mm for all CEM V cement types, indicating sufficient passivation, inter alia, because of its dense and highly impermeable concrete structure, keeping in mind that in this test stand, the curing conditions were not aggressive and the curing time was very short. The permeability subject of concrete made of CEM V-cements is further discussed in Sections 3 and 4.

#### **3. Concrete Properties Using Slag-Pozzolanic Cement**

#### *3.1. Concrete Formula and Fresh Concrete Properties*

To evaluate and compare the performance of the different manufactured slag-pozzolanic cement types, concrete specimens were produced using a standard mixture (Table 5) with a water-binder- ratio of 0.5, only varying the type of cement. In the case of cement types (3) CEM V/A (S-V) 32.5 R and (4) CEM V/B (S-P) 32.5 N, the addition of superplasticizer was necessary to reach a good workability of the fresh concrete with a flow spread of approximately 50 mm (consistency class F3 or F4 according to [57]).

**Table 5.** Concrete standard mixture for comparison of produced cement types.


For all mixtures, fresh concrete properties were determined in the laboratory and are summarized in Table 6.



In the case of (1) CEM V/A (S-V) 32.5 R and (3) CEM V/A (S-V) 32.5 R, the consistency is plastic, not reaching a soft concrete consistency, indicating an additional demand for superplasticizers. Fresh concrete density is reasonable with the exception of (4) CEM V/B (S-P) 32.5 N, showing a low density because of the high air content.

#### *3.2. Mechanical Properties of Concrete Using Slag-Pozzolanic Cement Types*

With regard to hardened concrete, compressive strength, bending tensile strength, and modulus of elasticity were evaluated for a basic characterization of the different cement types.

Compressive strength was tested on 150 mm concrete cubes after 2, 28, 90, and 365 days of water storage at 20 ± 1 ◦C to present the varying strength development of the different cement or rather concrete types (Figure 8). As expected, the slag-pozzolanic cement types start at a low early strength level. CEM V/A cement types reach the strength level of the reference cement CEM I after 28 days and show a strength increase even between 90 and 365 days of concrete age. CEM V/B cements lag significantly behind, reaching a significantly lower strength level; strength development runs nearly flat after the age of 90 days.

**Figure 8.** Concrete compressive strength using different cement types at 2, 28, and 90 days of water curing (each value as mean value of 3 individual tests).

The modulus of elasticity was determined as the secant under compression (the test specimen is loaded under axial compression, stresses and strains are recorded, and the slope of the secant to the stress–strain curve is determined) according to [58]. Test specimens (concrete prisms with dimensions of 150 × 150 × 600 mm) were water-stored at 20 ± 1 ◦C (the age-dependent pathway is displayed in Figure 9). The modulus of elasticity for the reference cement CEM I starts at approx. 30 GPa at the highest level and shows a stronger increase with time than the concrete types made with slag-pozzolanic cement. All CEM V cement types reach a similar level of approx. 36 GPa at a curing time of 365 days under water (see [49]).

**Figure 9.** Modulus of elasticity (static) using different cement types at 2, 28, and 90 days of water curing (each value as mean value of 3 individual tests).

#### *3.3. Durability Aspects: Frost and Permeability of CEM V Concretes*

The negative effect of supplementary cementitious additives concerning frost resistance is well known, e.g., [59,60]. In this case, frost resistance could not be laboratory-confirmed during frost-testing, either. Testing was performed under intensified conditions without adding artificially entrained air additionally to the air content shown in Table 6. Frost resistance was evaluated indirectly via reduction in the bending flexural strength after 50 frost-thawing cycles in demineralized water. One frost-thawing cycle takes 24 h, whereby the temperature is lowered from the initial temperature of 20 ◦C to −20 ◦C within 12 h, then kept at −20 ◦C for 4 h and subsequently raised again to the initial temperature within 8 h. Temperature measurement is performed in the center of the concrete test specimen. Frost testing was performed after a period of 90 days of water curing at 150 x 150 x 600 mm samples. The results are illustrated in Figure 10: Bending flexural strength decreases strongly after frost impact, declining to a level between approx. 45 and 17% in the case of (1) CEM V/A (S-V) 32.5 R to less than 10%, indicating structural damage of the test specimen.

**Figure 10.** Bending flexural strength without and with frost impact evaluated after 90 days of water curing (each value as mean value of 3 individual tests).

Additional optimization of pore structure (porosity is reduced by the additives) and mixing design (high contents of additives increase the demand of water) will be necessary to improve frost resistance of the tested admixtures to prevent structural degradation of the concrete. An opportunity was used to build a concrete test track (washing station for trucks; Figure 11a,b) in a concrete factory in Austria in an alpine region (+815 m above the Adriatic Sea) using cement type (1) CEM V/A (S-V) 32.5 R. The concrete track itself is a horizontal structure experiencing a lot of water impact and mechanical exposure to truck tires. In winter, the input of chlorides by the truck tires is also most likely.

**Figure 11.** Test track construction using (1) CEM V/A (S-V) 32.5 R using a cement content of 400 kg/m<sup>3</sup> and a water-binder ratio of 0.45 during (**a**) concreting work and (**b**) after finishing the construction.

Considering the results from Figure 10, the mixture was adapted as the following: Cement content was increased to 400 kg/m3, water-binder ratio was 0.45, and plastifying admixture was added to reach a flow consistency of F3 to F4 according to [57], additionally using an air-entraining admixture to reach 5% of air content. The practical realization went well and the concrete structure showed no sign of damage after winter 2019/2020, imposing at least 25 frost-thawing-cycles, implying the frost resistance of the applied concrete mixture.

Permeability of the various concrete mixtures was tested via water pressure impact. Pressure testing was conducted for concrete cubes after 90 days of water curing at 20 ± 1 ◦C and at atmospheric pressure according to [61]. Pressure testing was done at a water pressure of 500 kPa for the duration of 72 h. Afterward, the maximum depth of penetration was measured in millimeters. Water penetration depths for the different concrete mixtures with different CEM V-cements are illustrated in Figure 12.

**Figure 12.** Water penetration depth (in mm) after water pressure test (each value as mean value of 3 individual tests).

As evident from Figure 8, water penetration lies in the range of 16 to 23 mm, whereby the water penetration was reduced by using the slag-pozzolanic cements. Especially, cement type (4) CEM V/B (S-P) 32.5 N shows a strongly reduced water penetration.

#### **4. Concrete Applying Recycled Aggregate and Slag-Pozzolanic Cement**

#### *4.1. General*

A new ecological approach was applied by merging the slag-pozzolanic cements with recycled rock aggregate from tunnel driving to maximize the environmental sustainability. For this reason, basic material characteristics were determined for a filling concrete mixture using three different rock types from excavation and a structural concrete mixture using aggregate from calcareous schist to evaluate the performance of the combination of CEM V cements and recycled rock material. Detailed information on the rock characteristics are given in [28,62,63].

#### *4.2. Concrete Mixtures and Mechanical Characterization*

#### 4.2.1. Tentative Testing

The concrete mix for filling concrete without requirements of concrete exposure classes according to [57], using CEM V cement types as binder and recycled aggregate from tunneling as aggregate, applies a low cement level of 220 kg per m3 of concrete, and the water-binder ratio was set at 0.55 to reach a flow spread of approx. 45 mm (consistency class F3 according to [57]). Maximum grain size was 31.5 mm, whereby for the grain fraction of 0–4 mm, standard quartz sand was used (to

keep the water demand of the mixture at a manageable level concerning workability of the fresh concrete, because the high mica content of the finer fractions of recycled aggregates would increase water demand of the fresh concrete mix, see [28]), while fractions 4/8, 8/16, and 16/31.5 mm were produced from recycled rock material.

Three different types of aggregate were used from the tunnel driving of the Brenner Base Tunnel to determine the 28 day compressive strength (after 7 days of water storage and subsequent air storage at 20 ± 1◦C until 28 days of concrete age; Figure 13):


**Figure 13.** Compressive strength of various concrete mixtures varying cement and aggregate type (each value as mean value of 3 individual tests) after 28 days.

Figure 9 confirms the trend illustrated in Figure 6 (CEM V/B cement types show the lowest strength development after 28 days of curing time), additionally demonstrating the influence of the aggregate type: Quartz phyllite aggregate shows the lowest, calcareous schists aggregate shows mediocre, and gneiss aggregate shows the highest compressive strength due to rock characteristics, cf. [28].

#### 4.2.2. Inner Lining Concrete

In order to use concrete with slag-pozzolanic binder and recycled aggregate, a concrete strength level of at least 50 N/mm2 and a low permeability were defined as the crucial requirements. As calcareous schists are quantitatively dominant at the Brenner Base tunnel, cf. [28], concrete testing was performed using standard quartz aggregate for the grain fraction 0/4 mm and calcareous schists for 4/8, 8/16, and 16/31.5 mm. Cement content using (2) CEM V/B (S-V) 32.5 N was increased to a level of 330 kg per m3 of fresh concrete, and the water-binder-ratio was set to 0.52. Superplasticizer was used to reach a flow spread of class F4 according to [57]. Test specimens were stored for 7 days under water with subsequent air storage until 28 days of concrete age. Fresh and hardened concrete properties are illustrated in Table 7, whereby fracture energy GF was determined by the cut-through-tensile splitting test.



The results from Table 7 show encouraging results concerning basic mechanical properties of the concrete composition, achieving good concrete density and workability of the fresh concrete, as well as a good strength level reaching concrete strength class C40/50 according to [57]. Figure 14 illustrates a concrete cube specimen after compressive strength testing.

**Figure 14.** Concrete cube test specimen after compressive strength testing according to Table 7.

Water penetration testing was performed according to [64] with an initial water pressure of 175 kPa for the first three days, subsequently increasing the water pressure to 700 kPa up to the 14th day. A water penetration depth of 19 mm could be determined, demonstrating a very low permeability comparable to the findings illustrated in Figure 8.

#### **5. Summary and Conclusions**

Cement production has a major contribution to global CO2 emission. Composite cements with a partial substitution of CO2-intensive clinker by additives, like slag, fly ash, or pozzolana, are a promising approach to make cement manufacture more environmentally friendly, reducing the CO2-impact by up to two-thirds. In this research, slag-pozzolanic cements (CEM V) were produced and evaluated regarding their composition and characteristics. To evaluate their performance as binder, various concrete admixtures were produced and tested, whereby the mechanical properties regarding strength and durability were of central importance. In order to further increase the life cycle assessment of concrete using CEM V cements with slag-pozzolanic additives, conventional aggregate was partly exchanged by recycled rock aggregate originating from tunnel driving of the Brenner Base Tunnel. The results obtained in this research could encourage both the national standardization and the cement/construction industry for future applications because of its environmental, economic, and durability characteristics. In the course of this work, the following conclusions could be drawn:



In conclusion, it can be said that alternative cement types with reduced clinker content have their legitimacy for many application areas regarding their special characteristics discussed in this paper. Recycling of tunnel excavation material has already been proven at different construction projects and works well considering the essential framework conditions. In both cases, there is often a lack of experience impeding the practical implementation of these two alternatives.

**Author Contributions:** Conceptualization, K.V. and O.Z.; methodology, K.V., I.J., R.A. and K.B.; validation, K.V. and O.Z.; formal analysis, K.V. and I.J.; investigation, K.V. and I.J.; writing—original draft preparation, K.V. and O.Z.; writing—review & editing, I.J. and K.B.; visualization, K.V. and O.Z.; supervision, K.B.; project administration, K.V., I.J. and K.B.; funding acquisition, I.J. and K.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the EUROPEAN UNION European Regional Development Fund. Open-access funding provided by BOKU Vienna Open Access Publishing Fund. The authors would like to acknowledge the financial and technical support.

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

#### **References**


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## *Article* **Durability of High Volume Glass Powder Self-Compacting Concrete**

#### **Samia Tariq 1, Allan N. Scott 1,\*, James R. Mackechnie 1,2 and Vineet Shah <sup>1</sup>**


Received: 9 October 2020; Accepted: 5 November 2020; Published: 13 November 2020

**Abstract:** The transport characteristics of waste glass powder incorporated self-compacting concrete (SCC) for a number of different durability indicators are reported in this paper. SCC mixes were cast at a water to binder ratio of 0.4 using glass powders with a mean particle size of 10, 20 and 40 μm and at cement replacement levels of 20, 30 and 40%. The oxygen permeability, electrical resistivity, porosity and chloride diffusivity were measured at different ages from 3 to 545 days of curing. The amount and particle size of the incorporated waste glass powder was found to influence the durability properties of SCC. The glass incorporated SCC mixes showed similar or better durability characteristics compared to general purpose (GP) and fly ash mixes at similar cement replacement level. A significant improvement in the transport properties of the glass SCC mixes was observed beyond 90 days.

**Keywords:** glass powder; self-compacting concrete; durability

#### **1. Introduction**

The durability of concrete can be defined as its ability to resist weathering action, chemical attack, abrasion and any other mechanism of deterioration while preserving its original form. The degree of deterioration is strongly related to the resistance of the cover layer to transport mechanisms, such as permeation, absorption, and diffusion of gas and liquid [1]. The use of supplementary cementitious materials (SCMs) in concrete helps in increasing resistance towards most of the durability related issues. The benefits of using silica fume, fly ash, slag and calcined clay has been widely demonstrated [2–6]. The use of waste glass powder as a potential SCM has also received significant interest in the last two decades [4,7–9]. The early age compressive strength for concrete containing glass powder is generally lower than the equivalent PC concrete. Similar strength values for glass powered concrete however have been reported at 90 days for replacement levels of up to 30% [10–12]. Further grinding of the glass powder can reduce the average particle size and help to offset the lower early age compressive strength [10,13]. Increasing the fineness and proportion of glass powder however has been shown to negatively affect the workability of the mix [14,15]. The irregular angular shape of the glass particles is responsible for the increased water demand compared to PC mixes. Numerous studies have reported the implications of using glass powder on the fresh and mechanical properties of concrete; however, limited information is available on the durability performance of such concrete.

The rapid development in the infrastructure sector around the world is expected to increase the demand for cement from current the levels of ~4 billion tonnes to ~6 billion tonnes by 2050 [16]. The large carbon footprint associated with the cement industry makes it imperative to develop sustainable solutions that promote the judicious use of materials and increased service life of concrete structures with minimal maintenance. The global supply of waste glass represents approximately ~3%

(130 million tonnes) of the total cement consumption [17]. While this a relatively small proportion it does provide a possible option for diverting a waste stream from land fill and may be of particular benefit in areas that have limited supplies of more traditional SCMs such as fly ash, slag or silica fume. Establishing the durability characteristics of glass powder therefore is imperative to ensure its usage in the concrete industry together with the potential economic and environmental benefits.

Concrete containing glass powder is often investigated for susceptibility of alkali silica reaction (ASR) due to the presence of high alkali contents associated with glass. Research has shown that the ASR could be mitigated by using finely ground glass powder in concrete typically smaller than 75 μm [8,11,15,18]. Moreover, using glass powder as a replacement for cement could help in reducing the deleterious expansion associated with ASR.

Shayan and Xu (2006) [15] measured the permeation characteristics of glass powder concrete mixes using rapid chloride penetration test (RCPT). A lower chloride penetrability was reported for glass mixes as compared to the control mix. Nassar and Soroushian (2012) [19] also reported lower total charge passing through glass concrete. This property was attributed to the refined pore structure resulting from the densification of microstructure due to the pozzolanic reaction of glass. Lee et al. (2018) [20] however, reported higher RCPT value and higher chloride diffusion coefficient measured using NT 492 for concrete containing 20% glass powder at 28 days as compared to PC. The chloride diffusivity of the glass powder mixes was found to decrease as the hydration progressed. Sales et al. (2017) [21] showed a reduction in permeability and increase in electrical resistivity of mortars with an increase in the cement replacement level for fine glass powder particles. Schwarz et al. (2008) [8] compared the moisture transport properties of concrete containing fly ash and glass powder at 10% cement replacement level. The glass powder modified concrete performed better than fly ash and control mix at both early and later ages.

Although a number of researchers have studied the durability of glass powder concrete for a wide spectrum of issues including ASR, abrasion and sulfate attack, limited information is available on the effect of different fineness and replacement levels of glass powder on the transport properties in concrete. In this study, the major durability indicators: porosity, permeability, resistivity, shrinkage and chloride migration for glass powder incorporated concrete are investigated from 3 to 545 days of curing. The durability indicators provide a practical way to identify the likely performance of glass powder containing SCC [22,23].

#### **2. Methodology and Experiments**

#### *2.1. Materials*

Crushed waste glass bottles were supplied by the Glass Packing Forum. The coarse crushed glass (G) was thoroughly cleaned and subsequently ground in a ball mill to a mean particle size of 10 μm (10G), 20 μm (20G) and 40 μm (40G). General purpose (GP) Portland cement conforming to ASTM Type II and two different fly ashes one belonging each to Class C (FAC) and Class F (FAF) were used in the study. The chemical composition of raw materials measured using X-ray fluorescence (XRF) is given in Table 1. Rounded alluvial Greywacke sandstone was used as the fine and coarse aggregate (sand fineness modulus (FM) = 2.7, coarse aggregate maximum aggregate size of 13 mm). The particle size grading of the aggregates is summarized in Table 2.


**Table 1.** Chemical composition of raw materials (%/100 g).

**Table 2.** Particle size grading of fine and coarse aggregates.


#### *2.2. Sample Preparation*

A total of 8 different self-compacting concrete (SCC) mixes were investigated in this study at a water to binder ratio of 0.4. A polycarboxylic ether-based superplasticizer was used to achieve the target flow of 700 +/− 50 mm for the SCC mixes. The raw materials were dry mixed in a 90 L orbital pan mixer for two minutes. Approximately 80% of the required water was added over a period of one minute while mixing. The remaining 20% of water, which was premixed with super plasticizer, was then added to the mixer. After the addition of all the water and superplasticizer, the material was mixed for an additional 3 to 5 min. The high binder content helped to prevent segregation of the control mix, while the use of glass powder and fly ash further aided in the stability of the mix such that viscosity modifiers or stabilizing admixtures were not required for any of the mixes. The superplasticizer dosage was varied to ensure a consistent spread for each mix. Cylindrical specimens of 100 mm diameter and 200 mm height were cast for durability testing and prisms of 75 × 75 × 280 mm were cast to measure the drying shrinkage of concrete. After casting, the samples were placed in a temperature control room maintained at 20 ◦C and 65% relative humidity for 24 h. Thereafter, the specimens were demolded and cured in lime saturated water until the age of testing. Table 3 shows the mix design details. The GP, FAF30% and FAC30% were the control mixes for comparison with the glass powder (G) SCC mixes.

**Table 3.** Details of mix proportion and concrete mix design.


#### *2.3. Experiments*

#### 2.3.1. Drying Shrinkage Test

Drying shrinkage of concrete was determined in accordance with ASTM C157 (2010) [24], which consists of measuring the drying shrinkage of prismatic specimens, having dimensions of 75 mm × 75 mm × 280 mm, subject to a controlled drying environment. The specimens were removed from the curing tank at the age of 7-days after casting. Immediately after removing the specimens and wiping their surfaces dry, they were placed in the comparator. Afterward the initial measurement all the specimens were placed in the drying room at a temperature of 23 ◦C and relative humidity of 50%. Drying shrinkage measurements of three replicate specimens were taken at 7, 14, 28, 56, 90 and 180 days of air-drying and an average value of drying shrinkage in micro-strain was reported for each drying age. The shrinkage data provides an indication of long-term cracking risk, which could directly influence the transport characteristics through concrete.

#### 2.3.2. Oxygen Permeability Test

The ease of movement of fluids through a porous structure under an externally applied pressure can be determined from the permeability test. The oxygen permeability test in this study was carried out as described in [25]. This test method measures the pressure decay of oxygen passing through a core of concrete placed in a falling head permeameter. Two 25 mm thick cores were cut from the centre section of two SCC cylinders to obtain a total or 4 sample cores for testing. These slices were kept in an oven at 50 ◦C for drying until their weight became constant, as suggested in ASTM C642 (2008) [26]. The oven-dried specimens were subjected initially to oxygen at a pressure of 100 kPa and the pressure decay with time was monitored. The test was automated by using pressure transducers attached to the data logger and was continued for either 8 h or until pressure dropped to 50 kPa, whichever approached first. The coefficient of permeability was calculated by conducting a linear regression analysis on the best-fit line achieved by plotting values of ln(Po/Pt) against t, where Po is initial pressure reading at the start of the test, and Pt are subsequent pressure readings at times 't' measured from the time of reading of initial pressure. Four specimens were used to measure coefficients of oxygen permeability at the curing ages of 3, 7, 28, 90, 180, 365 and 545 days and an average value for each curing age was reported.

#### 2.3.3. Porosity

In this study, same specimens used for the oxygen permeability test were also used for porosity measurement. After the end of the permeability test, the specimens were vacuum saturated in tap water and the volume of permeable voids was found according to the procedure described in ASTM C642 (2008) [26].

#### 2.3.4. Electrical Resistivity Test

Resistivity test provides a rapid indication of the likely resistance of concrete to the penetration of chloride ions and the likely subsequent rate of corrosion. This test method consists of measuring the electrical current passed through 25 mm thick slices extracted from concrete cylinders. The same specimens used for oxygen permeability and porosity tests were used to perform the resistivity measurements. The disc specimens were placed between two parallel stainless steel plates. Sponges saturated with 5M NaCl was used to make electrical contact between concrete disks and steel plates. An alternating current was applied across the specimen and the voltage was measured.

#### 2.3.5. Bulk Diffusion Test

The apparent chloride diffusion coefficient was determined by bulk diffusion conforming to ASTM C1556. Two samples with a thickness of 75 mm were sliced from all cast cylinders at ages of 28, 90, 180, 365 and 545 days. The sides and bottom of the test specimens were sealed with an epoxy coating leaving one concrete end face exposed. The sealed specimens were vacuum-saturated in a calcium hydroxide solution, rinsed with tap water, and placed in a sodium chloride solution (165 g/L of NaCl) for at least 35-days. When the exposure time was over, the test specimens were removed from the sodium chloride solution and ten thin layers from 0 mm to 20 mm (2 mm each) were ground off parallel to the exposed face of the specimens. Then, the acid-soluble chloride content of 4 gm sample obtained from each layer was determined. The apparent chloride diffusion coefficients was calculated using Fick's law as described in ASTM C1556.

#### **3. Results**

#### *3.1. Compressive Strength*

The compressive strength results for different mixes from 28 to 545 days, along with the T500 times, are summarized in Table 4. The early age compressive strength for all the fly ash and glass powder mixes were lower than the GP control. The 20G30% and 20G40% replacement levels together with the 40G30% replacement had the three lowest 28 day strengths with values between 43 and 47 MPa. As hydration continued the gap between the GP control and FA and glass powder mixes narrowed. The glass powder mixes shows the slowest strength development taking more than one year time to attain similar strength values. In particular, the glass powder mixes with the largest particle size were the slowest to gain strength. Figure 1 shows the compressive strength of mixes normalized with respect to GP mix at the respective age. The continued strength development is attributed to the formation of C-S-H arising from the pozzolanic activity of glass powder. The observations show the importance of long-term investigation for such binder systems that have slower pozzolanic activity. Similar, compressive strength characteristics have been reported for glass powder concrete mixes in literature [11,27,28].


**Table 4.** Compressive strength (MPa) at different ages and flow time (Sec).

The fresh properties of concrete can have a significant impact up on the long-term durability of a structure. The presence of voids, as a result of compaction difficulties for instance, can be just as important as either the type of binder or w/c ratio. The primary goal of this investigation was to examine the effects of different glass powder replacement levels on the durability properties of the concrete. As such, the target flow for all the mixes was set at 700 mm and the amount of superplasticizer was allowed to vary to maintain the flow and minimize differences in the workability. It can be seen from the T500 times reported in Table 4, that there was relatively little variation with the times ranging from 3.1 s (FAF30%) to 5.5 s (GP) with most of the glass powder mixes between 4 and 5 s. The differences between either the yield shear stress as indicated by slump flow (700 mm) or plastic viscosity as indicated by T500 for the different mixes therefore were relatively small.

**Figure 1.** Relative strength of mixes normalized with respect to general purpose (GP) mix at the respective age.

#### *3.2. Drying Shrinkage*

Drying shrinkage of concrete is characterized by the time-dependent volume decrease due to moisture migration and transfer when exposed to a relatively lower humidity environment. Figure 2 shows the drying shrinkage strain of the control mixes and glass powder mixes at 30% replacement level for different particle size. GP concrete mix showed higher shrinkage measurements compared to the fly ash and glass powder SCC mixes. The finer pore structure and consumption of larger quantity of water in the pozzolanic reaction could reduce the evaporation of water due to drying. Mehdipour et al. (2018) [29] reported a larger transformation of free water to bound water in cement binders containing SCMs and hence a subsequent reduction in drying shrinkage due to the lower availability of water for evaporation. The drying shrinkage of glass mixes decreased as the glass particle size became finer, which can be related to the denser microstructure of concrete due to the presence of finer glass, which suppressed drying shrinkage. The 40G30% mix showed higher drying shrinkage compared to fly ash or the finer particle size glass powder mixes. The higher dry shrinkage could be due to overall higher porosity (Section 3.3) of the concrete because of the slower pozzolanic reaction of the coarser glass particles leading to a greater evaporation of water.

Figure 3 shows the drying shrinkage strain of the mixes at different glass powder replacement level. The shrinkage strain was found to increase with an increase in the replacement level. However, the drying shrinkage of 20G40% was similar to GP at 180 days. The higher permeable void volume and lower consumption of water in the hydration process at the higher replacement level could be the reason for higher shrinkage. Shayan and Xu [15] also reported an increase in drying shrinkage strain with increase in glass replacement level. Overall the results indicate the glass powder SCC mixes produced acceptable drying shrinkage values, below 0.075% after 56-days drying and met the requirements of the Australian Standard AS 3600 [30].

**Figure 2.** Drying shrinkage of self-compacting concrete (SCC) mixes at 30% cement replacement (these figures show drying time not age and the initial point needs to start at zero days).

**Figure 3.** Drying shrinkage of SCC mixes containing glass powder with 20 μm particle size at different replacement levels.

#### *3.3. Oxygen Permeability*

Figure 4 shows the oxygen permeability coefficient for the SCC mixes at different curing ages. The GP concrete showed the highest permeability coefficient at all the ages. The permeability coefficient

of fly ash concrete mixes were similar to GP concrete at 3 and 7 days, however, thereafter the permeability coefficient continued to reduce for the fly ash mixes as hydration progressed whereas the change in GP concrete was minimal. The lower permeability for fly ash mixes is associated with refinement in pore structure due to the pozzolanic reaction, which in turn increases the tortuosity of the network and thus hinders the free movement of the fluid [31,32]. The coefficient of permeability for glass powder mixes at the same replacement level was seen to be affected by their particle size. A lower resistance to gaseous permeation was observed for the coarser glass powder mixes. Similar to fly ash mixes, the permeability coefficient continued to reduce with time. Moreover, due to the long-term nature of the pozzolanic reaction, the advantages associated with glass replacement were more obvious after a long period of curing. The 10G30% and 20G30% both showed lower permeability coefficients compared to FAF30%. The finer particle size of glass powder helps in substantial pore refinement and hence creating an impermeable and denser microstructure [33]. In addition, the impervious nature of glass particle could also help in reducing the permeability. The 40G30% had a higher permeability as compared to GP at early ages (3, 7 and 28 days), however, the permeability coefficient reduced with progress in hydration. This shows that for coarser particle size, glass powder takes longer for the pozzolanic reaction to progress, which is also evident from the compressive strength data. Sales et al. (2017) [21] also reported similar observations of a reduction in oxygen permeability for mortar samples comprising of glass powder. Chaid et al. (2014) <sup>33</sup> attributed the reason for lower permeability in glass powder concrete to the formation of a denser microstructure. In line with the above results, Figure 5 shows the effect of glass replacement level on the permeability coefficient. The lower the replacement level the greater was the permeation resistance. The 20G40% mix exhibited a higher permeability than FAF30% until 365 days, however it showed similar value at 545 days, implying glass powder mixes even at higher replacement level could achieve similar or better transport characteristics in the long run.

**Figure 4.** Coefficient of permeability SCC mixes at 30% cement replacement.

The results of the oxygen permeability against compressive strength for the different mixes is provided in Figure 6. For same compressive strength concrete, lower permeability values were observed for glass mixes irrespective of their fineness and replacement level as compared to either the GP or FAC30% mixes. It is well known that the compressive strength is not a good predictor of durability performance particularly when comparing different binder types [34,35]. The compressive strength however can provide an indication of likely relatively performance when comparing concrete with the same general mix design. A correlation of 0.82 was obtained for power based relationship between compressive strength and permeability for glass mixes (K <sup>=</sup> 2.18 <sup>×</sup> <sup>10</sup>−<sup>9</sup> *<sup>f</sup>* <sup>−</sup>1.33 *<sup>c</sup>* ).

**Figure 5.** Coefficient of permeability of SCC mixes containing glass powder with 20 μm particle size at different replacement levels.

**Figure 6.** Relationship between compressive strength and coefficient of permeability of all the SCC mixes.

#### *3.4. Porosity*

The void content of concrete measured using water absorption test is one of the most common indicators used for assessing the durability of concrete. Figures 7 and 8 show the measured porosity for concrete mixes at different ages. The porosity reduced with continued hydration for in all the concrete mixes. The 10G30% SCC mix demonstrated a lower porosity as compared to other glass mixes at similar replacement level as well as the control mixes. Along with the pozzolanic reaction that assisted in the refinement of pore structure, the finer particle size of glass powder could help to improve particle packing in the system, which resulted in a denser and less porous concrete [19]. The findings signify the importance of particle size distribution as the presence of finer particles in the spaces between larger particles also acts to 'refine' the porosity. The porosity of 40G30% concrete mix was similar to GP despite having lower compressive strength. This implies that the pozzolanic reaction lead to precipitation of hydrates in the capillary pores which restricted the free movement of water. The volume of permeable voids increased with an increase in the glass content. The similar porosity values of 20G40% and 40G30% across ages show that the properties of concrete mixes with at higher replacement level of cement with glass powder could be maintained by varying the fineness of the glass powder. Shayan and Xu (2006) [15] reported similar results of an increase in volume of permeable voids with an increase in the glass powder content. Conversely, Du and Tan (2017) [36] showed a minimal change in porosity for up to 30% cement replacement level with glass powder, however beyond this replacement level an increase in porosity was reported as observed in 20G40% mix. A nonlinear inverse relationship was observed between coefficient of permeability and porosity as shown in Figure 9.

As the porosity of the concrete increased the permeability of the concrete also increased. As with the use of compressive strength as an indicator for durability, comparisons of overall porosity can sometimes be misleading. A single total porosity number does not provide the pore size distribution or tortuosity of the pore structure. As seen in Figure 9 a porosity of 9% would results in a coefficient of permeability of approximately 2 <sup>×</sup> <sup>10</sup>−<sup>11</sup> for the GP binder system, while the glass mixes were closer to <sup>1</sup> <sup>×</sup> <sup>10</sup><sup>−</sup>11.

**Figure 7.** Porosity of SCC mixes at 30% cement replacement at different ages.

**Figure 8.** Porosity of SCC mixes containing glass powder with 20 μm particle size at different replacement levels.

**Figure 9.** Relationship between coefficient of permeability and porosity.

#### *3.5. Resistivity*

The determination of concrete electrical resistivity has become an important technique in the evaluation of risk of corrosion associated with concrete structures [37,38]. The resistivity of concrete is affected by numerous factors such as pore solution composition, degree of hydration, pore structure, moisture content and temperature [39]. Figure 10 compares the resistivity of control mixes and glass powder mixes at 30% replacement. In agreement with the permeability and porosity results, the electrical resistivity of all the SCC mixes increased with curing age. The increase in resistivity is due to the continuous evolution of pore structure parameters due to hydration. The permeable void space reduced due to the precipitation of the hydration products in the capillary space, which hinder the mobility of ions, thereby resulting in an increase in resistivity of concrete. Carsana et al. (2014) [27] reported six times lower resistivity for glass powder mortar as compared to GP.

**Figure 10.** Electrical resistivity of SCC mixes at 30% cement replacement at different ages.

The replacement of cement with fly ash and glass powder resulted in an increase in electrical resistivity. The electrical resistivity of SCC containing glass powder increased as the glass particle size became finer. A comparison of FAF30% and 20G30%, with both having a similar particle size, showed higher electrical resistivity in glass powder containing mixes. Figure 11 shows a reduction in electrical resistivity of with increase in glass replacement level. However, even at 40% replacement 20G40% showed better electrical resistance as compared to PC.

The higher resistance towards flow of ions in SCM incorporated concrete is generally attributed to a finer pore structure due to their pozzolanic reaction. A smaller threshold pore diameter has been reporter for glass powder mixes containing up to 45% cement replacement compared to GP cement using MIP measurement techniques [36]. The smaller the threshold pore diameter, the more tortuous is the microstructure, which directly hinders the movement of ions. It should be noted, however, that the resistivity of the concrete is also affected by the composition of the pore solution and the pore solution composition of the concrete varies with w/c ratio, the degree of hydration, and use of SCMs [38–40]. Nevertheless, changes in the pore structure of the concrete are generally considered to have a greater effect on the measured electrical resistivity than changes in the pore solution composition and concentration [40]. The permeability measurements, which are not dependent on the pore solution, showed a similar behavior, implying the higher resistance offered in glass powder SCC is primarily due to pore structure modification. Evaluation of the pore solution at the various replacement levels would need to be conducted to accurately separate the contribution of the refinement in the pore structure from any modification to the pore solution composition.

The electrical resistivity and porosity values of SCC mixes at different ages are compared in Figure 12. An inverse relationship was observed between the two indicators. As expected the higher the porosity the lower was the electrical resistivity offer by the concrete. It is interesting to note that for the electrical resistivity vs. porosity all the mixes seemed to show reasonably good agreement unlike the permeability vs. strength comparison or the permeability vs. porosity comparison where the GP samples clearly showed a difference in behavior.

**Figure 11.** Electrical resistivity of SCC mixes containing glass powder with 20 μm particle size at different replacement levels.

**Figure 12.** Relation between electrical resistivity and porosity.

#### *3.6. Chloride Di*ff*usion Coe*ffi*cient*

The apparent chloride diffusion measured at the curing ages of 28, 90, 180, 365 and 545 days for control mixes and 30% glass powder SCC mixes are shown in Figure 13, while the results for the different levels of glass replacement are provide in Figure 14. The GP concrete showed the highest chloride diffusion coefficient among all mixes. The fly ash and glass powder mixes showed lower diffusion coefficient than GP at 28 days and continued to decrease with curing age. Du and Tan (2017) [36] reported a reduction of 90% in the chloride diffusion coefficient for 30% glass powder incorporated concrete as compared to GP control. The denser microstructure accompanied with refined pores and lower connectivity contribute to the reduce diffusivity. Kamali and Ghahremaninezhad (2015) [28] also reported a reduction in chloride penetrability with increase in glass powder content in concrete. The time-dependent changes in diffusion coefficient due to continued hydration is often represented by diffusion decay index (m) or ageing factor. Taking 28 days diffusion coefficient as reference, the diffusion decay coefficient was calculated at the ages 90, 180, 365 and 545 using the following equation:

$$\frac{D\_t}{D\_{ref}} = \left(\frac{t\_{ref}}{t}\right)^m$$

where *Dt* (m2/s) is the diffusion coefficient at time t (days), *Dre f* (m2/s) is the diffusion coefficient at *tre f* 28 days.

**Figure 13.** Chloride diffusion coefficient of SCC mixes at 30% cement replacement at different ages.

**Figure 14.** Chloride diffusion coefficient of SCC mixes containing glass powder with 20μm particle size at different replacement levels.

The average diffusion decay index across all ages for different mixes are given in Table 5. The diffusion decay index values of glass powder mixes are larger than those for the fly ash mixes. The decay index of 40G30% and 20G40% are similar to FAF30% indicating glass powder could bring similar changes in the pore structure that restricts the movement of chloride ions even with coarser particle size and higher replacement level. The higher ageing factor values for glass SCC mixes could improve the service life of structure subjected to chloride-induced corrosion.


**Table 5.** Diffusion decay index for different SCC mixes.

A good correlation was observed between chloride diffusion coefficient and electrical resistivity (Figure 15). From the results, it can be inferred that electrical resistivity provides a reasonable estimate of the diffusion coefficient value, which is easier to perform and much faster. As with the strength vs. permeability comparison, the chloride diffusion coefficient vs. resistivity relationship is most accurate when evaluating similar mix deign compositions. Where prequalification of mixes for use in marine construction applications is conducted based on bulk chloride diffusion testing, the resistivity testing may provide a good indication of individual batch performance for SCC containing glass powder.

**Figure 15.** Relation between electrical resistivity and chloride diffusion coefficient.

The results from chloride diffusion coefficient and the time dependent decay coefficient "*m*" were in good agreement with the previous durability index tests of porosity, permeability and resistivity, such that there was a decrease in measured performance with an increase in the glass particle size and replacement level. The replacement of 20% glass powder resulted in the lowest chloride diffusion coefficient even below that of the finer glass powder at 30% replacement levels. Some percentage of glass powder is clearly necessary to refine the pore structure of the concrete, but additions beyond 20% appear to be less effective. The beneficial effect of having a more finely ground glass powder was observed at the 30% replacement level, but no data was available in this study beyond this level for the most finely ground powder. It is likely that there is improved performance below a 30% replacement, with a more finely ground glass powder, however further work is needed to identify the optimal binder replacement level and powder size.

#### Comparison of Service-Life in Chloride Exposure Condition

In order to understand the effect of the higher diffusion decay index of glass powder concrete mixes, the service-life in terms of corrosion initiation time was predicted using Life 365® software [41]. A concrete cover depth of 50 mm, a threshold concentration of 0.05% (by mass of concrete) and a marine splash zone environment were used to estimate the time until de-passivation of the steel. The diffusion coefficient (D28) and diffusion decay coefficient (m) obtained from the reported experiments were also used in the model. Figure 16 shows the predicted time for the initiation of corrosion for the different concrete mixes. A distinct advantage in terms of increased time until corrosion initiation was observed for glass powder containing mixes. The predicted service-life for glass powder concrete mixes is greater than GP concrete irrespective of the particle size of the glass powder or the replacement levels. At a similar replacement level of 30%, an increase of approximately 2 to 3.5 times the service-life is observed for glass powder concrete mixes (10G30% and 20G30%) compared to FAF30%. A further comparison of the effect of different binder replacement levels and types on the time to initiation is provided in Table 6 for chloride thresholds levels of 0.1% and reinforcement covers of 75 mm.

**Figure 16.** Comparison of corrosion initiation time for different concrete mixes.

**Table 6.** Comparison of time to corrosion initiation for different binder replacement levels, covers and threshold concentrations.


The service life of real structures is affected by numerous variables. As previously noted the workability of the concrete in the fresh state can influence the degree of compaction and thereby the permeability of the concrete. The binder type, w/c ratio and level of curing are additional parameters that can all dramatically affect the quality of the cover concrete and the overall service life. In addition to the material and construction components, the site specific exposure condition is another crucial component. While a chloride threshold concentration of 0.05% and 0.1% by mass of concrete were used in this example, the actual chloride threshold concentration (Ct) can vary significantly even in the same structure as the use of a single chloride threshold value may not be appropriate for estimate actual performance. The example was intended for illustrative purposed to show the potential impact of glass powder on the durability of reinforced concrete structures assuming all other variables remained constant.

The wide range of tests carried out to corroborate the usage of glass powder as cement replacement shows promising results. The durability indicators (porosity, resistivity and oxygen permeability) of glass SCC mixes all indicate excellent durability according to the classification developed by Alexander et al. [22,23]. The results show that by optimizing the glass powder properties and replacement level durable concrete with mechanical and durable performance better than GP and fly ash mixes can be obtained. The usage of waste glass powder could bring positive economic and environmental benefits for concrete construction applications.

#### **4. Conclusions**

This study presented an overview of the influence of glass powder particle size and replacement level on the durability indicators for self-compacting concrete. Oxygen permeability, porosity, electrical resistivity, chloride diffusivity and drying shrinkage of glass modified SCC were compared with GP and fly ash mixes. The main findings from the study can be summarized as follows:


**Author Contributions:** S.T. performed the experimental testing, analysed the results and prepared the initial draft of the paper under the guidance of her supervisors A.N.S. and J.R.M.; A.N.S. and J.R.M. both provided advice and guidance on research direction and assisted with the interpretation of the data in addition to writing of the manuscript. V.S. helped with analysis of the data and in the preparation and revision of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was partially funded by the Building Research Association of New Zealand (BRANZ).

**Acknowledgments:** The authors would like to acknowledge the contributions of BRANZ, the Glass Packaging Forum and Golden Bay Cement for their support.

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

#### **References**


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