*Article* **Experimental Study with Plaster Mortars Made with Recycled Aggregate and Thermal Insulation Residues for Application in Building**

**Daniel Ferrández 1,\*, Manuel Álvarez 1, Pablo Saiz <sup>2</sup> and Alicia Zaragoza <sup>1</sup>**


**Abstract:** The high demand for natural resources and increased industrial activity is driving the construction sector to search for new, more environmentally friendly materials. This research aims to analyse plaster mortars with the incorporation of construction and demolition waste (CDW) to move towards a more sustainable building sector. Three types of aggregates (natural, recycled concrete and recycled from ceramic walls) and two types of insulation waste (expanded polystyrene with graphite and mineral wool) have been added to the plaster matrix to evaluate its mechanical and physical properties and its suitability in the elaboration of prefabricated materials. The results show how plaster mortars made with recycled aggregates have higher mechanical resistance than conventional plaster without incorporating sand. The incorporation of crushed mineral wool residues improves the flexural strength of plaster mortars and their application in the execution of prefabricated panels. Likewise, the expanded polystyrene residues reduce the final density of mortars, improving their behaviour against water absorption and reducing the final thermal conductivity of plaster material.

**Keywords:** plaster mortar; recycled aggregates; thermal insulation; building

#### **1. Introduction**

The construction sector is included in the six factors (cropland, grazing land, fishing grounds, forest products, carbon and built-up land) that make up the ecological footprint of humanity [1]. For this reason, it is necessary to involve efforts to develop new construction materials based on circular economy criteria [2]. To this end, the European Union has defined a firm line of action that is included in the "European Green Deal" presented by the Commission Communication of 11 December 2019 [3]. This document includes as a main element the efficient use of resources in construction, as well as ensuring that less waste is produced. For this reason, more and more researchers are channelling their studies towards the scope of sustainable construction and the use of raw materials from construction and demolition waste (CDW) management [4,5].

These CDW are the main source of waste generation in modern society [6]. Thus, the application of the 3Rs principle (reduce-reuse-recycle) avoids the generation of waste and converts it back into resources, thus closing the circle in industrial ecosystems [7]. With regard to the composition of these CDW, according to reports published by the European Commission [8], most of the waste comes from the demolition of concrete structures (12–40%) and factory works made with ceramic materials (8–54%), and the main recycling process of these residues is the generation of aggregates for construction [9]. Additionally, the volume of waste from insulating materials is growing due to European initiatives to improve the energy efficiency of buildings and their rehabilitation [10].

**Citation:** Ferrández, D.; Álvarez, M.; Saiz, P.; Zaragoza, A. Experimental Study with Plaster Mortars Made with Recycled Aggregate and Thermal Insulation Residues for Application in Building. *Sustainability* **2022**, *14*, 2386. https://doi.org/ 10.3390/su14042386

Academic Editor: Jorge de Brito

Received: 31 January 2022 Accepted: 16 February 2022 Published: 19 February 2022

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**Copyright:** © 2022 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/).

Concrete and ceramic material waste are generally inert and low polluters; however, they occupy a large volume in landfills and have a strong impact on waste management in cities [11]. The crushing, treatment, and preparation of the waste to produce recycled aggregates is currently widespread [12]. In general, aggregates from CDW have a wide field of application in the production of masonry mortars, totally or partially replacing the natural aggregate [13]. Regarding its properties for use in building, it should be highlighted that recycled aggregates have a lower density and a higher coefficient of friability than natural aggregates. This has repercussions in a lower resistance to compression of mortars that incorporate them [14]. The recycled ceramic aggregate has a higher water absorption coefficient compared to recycled concrete aggregate and natural aggregate [15]. This means that the recycled aggregates also have a high water content in fines and other impurities derived from the manufacture [16]. Recycled ceramic aggregate from brick waste and factory works has been proven to have good properties for improving the mechanical resistance of plaster mortars for use in rehabilitation and restoration works of architectural heritage [17]. On the other hand, recycled concrete aggregates have also been used by several authors to increase the density of gypsum compounds, expanding their field of application towards the production of prefabricated building materials [18].

Furthermore, residues from thermal insulation have also been frequently used to improve the physical and mechanical properties of mortars [19]. In the case of expanded polystyrene (EPS), this has been used in various investigations as a substitute for aggregates for the manufacture of mortars with the intention of reducing the final density of the materials and lowering their thermal conductivity [20,21]. It is a material that can be added to the manufacturing process of plaster plates and panels, improving its thermal performance to produce prefabricated elements, although reducing its mechanical resistance to bending [22]. On the contrary, resistance to bending is implemented if insulating mineral wool fibre residues are incorporated [23]. Piña et al. demonstrated the possibility of incorporating this type of crushed waste as a partial replacement for aggregates, presenting good mechanical behaviour and good stability against fire [24,25].

The use of gypsum or plaster mortars to produce prefabricated elements is well known since the incorporation of aggregates improves the mechanical rigidity of the material and increases its resistance [26]. In this sense, Santa Cruz-Astorqui et al. analysed the behaviour of some original 40 × 20 × 10 cm blocks composed of a sandwich panel with two plasterboards and a core of plaster and EPS, showing that this type of prefabricated material has good deformation capacity under external stresses [27]. To improve this deformation capacity, studies have been carried out by various authors in which they have opted for the incorporation of fibres into plaster matrix, together with the incorporation of insulation residues to reduce the final thermal conductivity of the panels produced [28]. Finally, it is worth highlighting the study by Dolezelova et al., where the importance of the shape of aggregates for the manufacture of plaster mortars has been demonstrated, where sands with a rougher particle surface are the ones with a higher quality for the manufacturing of mortars, facilitating the adherence of the conglomerate to its surface and improving resistance to compression [29].

In short, these studies seek to reduce the environmental impact exerted by the construction sector during the process of execution, rehabilitation, and demolition of buildings, seeking ways to prevent and recover CDW in line with the objective established by Directive 2009/98/ EC [30]. The main objective of this paper is to study the technical feasibility of plaster mortars made with recycled aggregate and thermal insulation residues, as no study has been found that shows the possible effect of combining these two types of CDW in plaster mortars for the elaboration of precast concrete products. To do this, an experimental campaign is developed in which, on the one hand, a mechanical and physical characterization is carried out of the material to produce mortars, and, on the other hand, the possible application of these materials for their use in panels and prefabricated building blocks is studied.

#### **2. Methodology**

This section presents the materials used to manufacture mortars, as well as dosages and a description of the experimental program carried out.

#### *2.1. Materials Used*

To carry out the experimental campaign of this research, the following raw materials have been used in the preparation of mortars: plaster, water, natural sand, recycled aggregates, and thermal insulation residues (Figure 1).

**Figure 1.** (**a**) Plaster E-35; (**b**) mineral wool insulation; (**c**) insulation of expanded polystyrene with graphite; (**d**) recycled concrete aggregate; (**e**) mixed ceramic recycled aggregate.

#### 2.1.1. Binder

E-35 construction plaster was used as a conglomerating material for mixing mortars [31]. It is a material commonly used in construction for wall coverings, the production of prefabricated panels, and the execution of plates for false ceilings [32]. Equations (1) and (2) show the basic scheme of reactions to obtain this raw material [33]:

$$\text{CaSO}\_4\cdot2\text{H}\_2\text{O} \rightarrow \text{CaSO}\_4\cdot\frac{1}{2}\text{H}\_2\text{O} \,(\text{a},\text{\beta}) \,+\,\frac{3}{2}\text{H}\_2\text{O} \tag{1}$$

$$\text{CaSO}\_4 \cdot \frac{1}{2}\text{H}\_2\text{O} \rightarrow \text{CaSO}\_4 + \frac{1}{2}\text{H}\_2\text{O} \tag{2}$$

More specifically, Table 1 shows the characteristics of E-35 plaster used in accordance with the UNE-EN 13279-1:2009 standard [34], which has been supplied by Placo Saint Gobain, S.A. (Madrid Spain). Additionally, X-ray fluorescence analysis revealed that the composition of this raw material is as follows: CaSO4 (99.7%), Al (0.022%), Fe (0.035%), Si (0.068%), Sr (0.157%), and P (0.01%).

**Table 1.** Physical properties of plaster E-35.


#### 2.1.2. Thermal Insulation Waste

To improve the thermal behaviour of plaster mortars produced in this research, residues of two types of thermal insulation were used: expanded polystyrene with the addition of graphite and insulating mineral wool. The physical characteristics of these materials provided by URSA Ibérica Aislantes, S.A. (Madrid, Spain), are shown in Table 2.


**Table 2.** Physical properties of the different types of thermal insulation used.

Before being used in the mortar mix, both types of insulators had to be prepared, as can be seen in Table 2. In the case of expanded polystyrene with graphite, this was separated manually until individual spheres with a mean diameter of four millimetres were obtained, as has been done previously by other researchers [35]. Likewise, mineral wool fibre also had to be crushed and separated manually to a length of 12 mm, following the recommendations of other researchers [36].

### 2.1.3. Aggregates

Three different types of aggregates have been used in this research: natural aggregate (NA), recycled aggregate from concrete waste (RAcon) and recycled mixed ceramic aggregate from the demolition of masonry walls (RAmix). A physical characterization of these sands for the manufacture of mortars is shown in Table 3.

**Table 3.** Physical characterization of aggregates.


A comparison between densities of recycled and natural aggregates can be seen in the analysis in Table 3. Density in recycled aggregates is lower than in natural aggregates. This allows the elaboration of lighter prefabricated elements for use in construction, although it also results in lower mechanical performance [42]. It is also worth noting the high content of fines in these recycled sands, although for the specific case of this investigation, aggregates were sieved and particles retained in the sieves with diameters of 1 mm (60%) and 0.5 mm (40%) were used in order to obtain a homogeneous mixture. Finally, the greater water absorption of these recycled sands is also highlighted, this has its repercussion in a lower workability of mortars during the kneading process, the absorption of the RAmix being greater compared to the RAcon in accordance with other previous studies [43].

Regarding the chemical composition, this was obtained by X-ray fluorescence using Bruker S2 Puma equipment and is shown in Table 4.

**Table 4.** X-Ray fluorescence assay.


Table 4 shows the higher SiO2 content of RAcon with respect to RAmix, which in turn has a higher CaO and SO3 content because of gypsum impurities derived from the masonry wall cladding [44]. A higher Al2O3 content is also observed in the RAmix because of the ceramic origin of the bricks used in the execution of masonry works [45].

Finally, Table 5 shows the analysis performed by X-ray diffraction with the help of a Siemens D5000 diffractometer with a graphite monochromator Cu-Kα (λ = 1.540598 Å). The results obtained are shown in Table 5 where it is observed that the predominant crystalline phases for the two types of recycled aggregates used are quartz and calcite [46].

**Table 5.** Analysis by X-ray diffraction, where each (\*) shows the relative abundance of each type of mineral crystalline phase.


#### 2.1.4. Water

For the mixing of different dosages, drinking water from Canal de Isabell II (Madrid, Spain) has been used, which has been used successfully in other previous works [47]. The main characteristics of this type of water are its softness (25 mg CaCO3/l) and neutral pH between 7 and 8 [48]. In addition, the following elements can be found in its chemical composition, as listed in Table 6.

**Table 6.** Composition of drinking water in the community of Madrid.


#### *2.2. Experimental Program*

2.2.1. Dosages Used

Throughout this investigation, the following notation presented in Equation (3) has been used to refer to the different types of mortars:

$$\text{E0.8 -- Aggrage -- Isolation}\tag{3}$$

where E0.8 refers to the water/plaster ratio used to prepare the different dosages, Aggregate refers to the type of sand used, which can be of three types: NA (natural sand), RAcon (recycled concrete aggregate) and RAmix (mixed recycled aggregate), and, finally, Isolation refers to the type of thermal insulation waste incorporated: graphite-incorporated expanded polystyrene (EPS) or insulating mineral wool (MW).

For the elaboration of the different types of mortars used in this investigation, the dosages collected in Table 7 have been used. In all cases, the mixtures were carried out following the same techniques and methods that are collected in the UNE-EN standard 12379-2:2014 [49].

**Table 7.** Proportions of each material used in dosages.


It should be noted that in all the mixtures listed in Table 7, residues were manually mixed with plaster powder and then gradually poured into the water to start the mixing process. In addition, samples were kept at a temperature of 22 ± 2 ◦C and a relative humidity of 60 ± 5%. After seven days of storage under laboratory conditions, samples were dried in an oven at a constant temperature of 40 ± 2 ◦C for 24 h, as recommended in the UNE-EN 12379-2:2014 standard [49].

#### 2.2.2. Instruments and Experimental Plan

In this research, an experimental campaign has been carried out that can be divided into three phases: (1) mechanical characterization, (2) physical characterization of plaster mortars produced, and, later, (3) study of the suitability of these materials for the manufacture of panels and prefabricated blocks. A diagram of the tests carried out is shown in Table 8.

**Table 8.** Planning of the tests carried out in the laboratory.


On the other hand, to determine the effect of the incorporation of sands of a different nature and the different types of thermal insulation residues incorporated in plaster mortars, a study on the analysis of variance (ANOVA) has been carried out. Table 9 shows the factors and levels that have been taken into consideration in the design of experiments.

All the tests performed for statistical analysis have been carried out for a significance level of 5%. For the diagnosis of the model, it has been verified that the residuals of each response variable meet the conditions of normality, homoscedasticity, and independence [54]. A multiple range test has also been included to observe the existence or not of homogeneous groups between the different types of mortars included.

**Table 9.** Factors and levels used for the analysis of variance (ANOVA).

