Introduction

To mitigate the negative impacts caused by climate change, society must actively develop and promote more sustainable technologies, with particular emphasis on industrial sectors that generate significant environmental impact. In Brazil, for instance, the agricultural, mining, and construction industries generate hundreds of millions of tons of waste annually (Lopes et al., 2023; Oliveira et al., 2020). The activities of the construction industry play a significant role in carbon dioxide (CO₂) emissions, contributing worldly approximately 25% of the total CO₂ released into the atmosphere (Farias; Marinho, 2020). This emission is related to building materials, construction activities, and operational energy demand, with the latter representing about 42% of global energy consumption (Durante; Callejas; Amaral, 2020). Additionally, the construction industry consumes around 30% of the raw materials available worldwide, and 17% of global freshwater (Dixit; Culp; Fernéndez-Solís, 2013; Giannetti et al., 2018; Souza; Ghisi, 2020).

Regarding construction materials, Portland cement alone is responsible for approximately 7% of global CO₂ emissions (Lopes et al., 2023; Carvalho et al., 2023a). Consequently, several authors seek alternatives to reduce its consumption, such as using fillers (Carvalho et al., 2023b), and/or adopting supplementary cementitious materials (Defaveri et al., 2022; Bayraktar et al., 2019). For the complete replacement of Portland cement, recent studies propose the use of geopolymer binders (alkali-activated or acid-activated) (Carvalho et al., 2023a; Goryunova et al., 2023). Geopolymer concrete can emit 60 to 80% less CO₂ than Portland cement concrete (Duxson et al., 2007). Like Portland cement, geopolymers can be used for the production of pastes (Su; Zhong; Peng, 2021), mortars (Zhang et al., 2020), and concretes (Ahmad et al., 2022).

The present study focusses on the properties of mortars, coating (or plastering) mortars more specifically, used for finishings of masonry-based envelope systems in many countries. In recent years, researchers have sought to reduce the environmental impacts of coating mortars, often replacing their components with waste materials. Some examples include steelmaking slag, quartzite tailings, iron ore tailings, fly ash, copper slag, glass waste, and crumb rubber (Mendes et al., 2020a; De Rossi et al., 2018; Sivasakthi; Jeyalakshmi; Rajamane, 2021; Guitiérrez; Villaquirá-Caicedo; Guzmán-Aponte, 2020; Wongsa et al., 2018). Other researchers have prioritized their energy efficiency aspect, using thermally insulating waste materials to improve the envelope’s thermal performance (Martins et al., 2024; Becker; Effiting; Schackow, 2022; Koksal; Mutluay; Gencel, 2020).

The envelope’s construction materials act on the climatic independence of the internal rooms from the external environment (Mendes et al., 2022; Passos; Caraseck; Amaral, 2016). Therefore, a building envelope with enhanced thermal performance reduces energy consumption for indoor conditioning and improves thermal comfort for users (Lamberts; Dutra; Pereira, 2014). Thermal resistance and thermal capacity are the most relevant envelope properties regarding this thermal performance (Lamberts; Dutra; Pereira, 2014).

Thermal resistance (Tr) refers to the rate of heat transfer through a certain element. It is a function of the element’s thermal conductivity (λ) and thickness (t) (Equation 1) (ABNT, 2022). An envelope with high thermal resistance slows down the heat transfer rate and makes the indoors less susceptible to external climate. In turn, thermal capacity (Tc) is a measure of an element’s ability to change its temperature in response to applied energy, determined by its specific gravity (ρ), thickness (t), and specific heat (c) (Equation 2) (ABNT, 2022). A building envelope with high thermal capacity results in reduced indoor temperature fluctuations.

The envelope’s thermal performance is directly related to these two properties (among others), and the bioclimatic aspects of the analyzed region (Lamberts; Dutra; Pereira, 2014). In hot climates, for example, depending on factors such as building design, solar orientation, and ventilation, buildings envelopes with high thermal resistance and/or high thermal capacity can promote overheating in the indoor spaces (Rakontonjanahary; Scholzen; Waldmann, 2020).

Energy simulations are usually adopted to equalize all these influencing parameters and operationalize various types of analyses, strategies, and scenario simulations (Stoffel et al., 2023). They are computational modeling tools that predict certain properties (e.g., energy demand for climate control, internal temperature, and relative humidity) based on a realist building model coupled with the climatic file of the studied region. There are a few techniques to evaluate how these values relate to the building’s overall thermal performance, with the two most adopted ones being the Thermal Load method (TL) and the Degree-Hour method (DH) (Mendes et al., 2024).

The TL method directly assesses the energy efficiency of the building by measuring the energy consumption required for indoor conditioning within certain comfort temperature ranges throughout the year (Mendes et al., 2024). TL results are usually shown in energy units (J), convertible to kWh, which can, thus, be related to the building’s energy bill. This method requires simulating a heating, ventilation, and air conditioning (HVAC) system. Thus, all thermodynamic evaluations from this method are linked to the use of an active strategy for indoor conditioning, the HVAC system (Lamberts; Dutra; Pereira, 2014), unlike the DH method. The most common procedure for the DH method assesses the amount of “thermal discomfort” throughout the year by accounting for the temperature hourly difference (in °C) when the internal room temperature surpasses the upper or lower limits set for thermal comfort (Mendes et al., 2024).

In the present scenario, although many authors have developed coating mortars based on waste materials with promising physical and thermal properties, only a few articles evaluate how these properties influence the buildings’ overall thermal performance (Franco et al., 2019; Mendes et al., 2020a). No comprehensive study was found comparing all the available residue-based mortars found in the literature. To investigate the full potential of theses mortars, we sought to understand the impact that these mortars’ properties and their thickness can have on the thermal performance of the building. Thus, this study innovates by evaluating all the Portland-cement-based and geopolymer-based coating mortars produced with waste available in the current literature, from a case study in different climates in Brazil.

Materials and methods

To reach our goal, we conducted energy simulations to investigate the thermal performance of a low-income building, with its walls coated with the studied waste-based mortars. The thickness of these mortars was ranged from 10mm to 30mm (in steps of 5mm). The simulations were carried out using EnergyPlus (v22.1.0); considering one entire reference year for 3 Brazilian cities: Curitiba (cold climate), Belém (hot climate), and São Paulo (mild climate). Figure 1 presents a flowchart of the research methodology.

Waste-based coating mortars

Nine types of mortars manufactured with waste were considered, in addition to a conventional Portland-cement mortar with high insulating capacity, for comparison (referred to in this study as “CM - Insulating mortar” (Becker; Effiting; Schackow, 2022)), summing up 10 coating mortars. These mortars were selected from a review of recent articles through an exploratory search (Prodanov; Freitas, 2013). The Scopus database was adopted for this purpose since it is the largest database of peer-reviewed articles, and most publications in the construction industry are stored in this database (Hong; Chan, 2014; Tariq; Hu; Zayed, 2021).

Table 1 presents the mortars assessed and their thermophysical properties: thermal conductivity and density (specific gravity). The specific heat of the mortars was set at 1000 J/(kg·K) (ABNT, 2005) avoiding biased results, since some articles did not provide it. Table 1 also presents the range of thermal resistance and thermal capacity properties as a function of thickness, which varied between 0.01 m and 0.03 m in the analysis, according to Equation 1 and Equation 2. CM refers to “Portland-cement mortars” and GM to “geopolymer mortars”.

Simulated building

The simulated building comprises a 40m²-single-family dwelling (Figure 2). The wall system consists of ceramic blocks laid with conventional laying mortar, coated on both sides with the studied mortars, and painted with common paint, as seen in Figure 3. The construction systems adopted in the modeling for the roof and floor are detailed in Table 2. The doors were modeled in wood with a thickness of 30 mm, and the window glass was standard clear glass with a thickness of 3 mm, following the standard thermophysical properties of EnergyPlus (2022).

The building’s thermal performance was analyzed in three Brazilian cities with different climates: Curitiba (located in the state of Paraná), São Paulo (São Paulo), and Belém (Pará) (Figure 4), considering the climatic files available in the Brazilian standard NBR 15575 (ABNT, 2024). According to the Brazilian standard NBR 15220 (ABNT, 2022) and data from the Brazilian Institute of Geography and Statistics (IBGE, 2002), the city of Curitiba belongs to bioclimatic zone 1 (BZ1), presenting a “very cold temperate climate”, São Paulo (BZ3) is a city with a “mild temperate climate”, and Belém (BZ8) presents a “hot equatorial climate”.

Thermal performance evaluation

The building’s thermal performance evaluation was conducted through two techniques: the Thermal Load method (TL) and the Degree-Hour method (DH), as described by Mendes et al. (2024). Tables 3 and 4 present the settings adopted in the simulations for each method (TL and DH, respectively).

When evaluating the building’s thermal performance via the TL method, the HVAC system was considered turned on only during occupied hours. The model that ensures the lowest energy demand to condition the living room and bedrooms (evaluated in kWh) throughout the year achieves the best thermal performance.

In contrast, the DH method was evaluated throughout all days of the year, 24 hours a day, as it does not involve energy expenditure to condition the environment. Therefore, this method also measures, indirectly, the potential of pathological manifestations associated with temperature variations, such as condensation, mold, expansion, and degradation, which will not be addressed in the present study. Additionally, by measuring the thermal variations 24/7, the DH method considers the thermal comfort of plants and pets that remain in the residence outside standardized occupancy hours. Similarly to the TL method, in the DH method, the model with the least discomfort in the most critical room throughout the year (the lowest degree-hours) demonstrates the best thermal performance.

Both methods are influenced by the occupants’ comfort temperature range, which varies according to standards and researchers’ preferences (Mendes et al., 2024). The DH method is designed to evaluate the thermal performance of the building according to its passive thermal comfort (or discomfort), i.e., without involving energy consumption for indoor climate control. Therefore, the temperature range defined for user comfort was set at 18 °C – 26 °C, reflecting a broader comfort interval based on the building›s passive performance. These values are supported by NBR 15575 (ABNT, 2024) In contrast, the TL method focuses on the energy consumption required to maintain the building’s thermal comfort through active heating and cooling systems. In this case, the setpoints were defined more narrowly (20 °C – 25 °C) to evaluate the energy efficiency of the envelope, considering normative values (ASHRAE, 2020) and stricter comfort expectations. The adoption of different temperature ranges followed the most adopted practices worldwide (Mendes et al., 2024) and thus allows for a more comprehensive understanding of the building’s comfort and energy efficiency needs.

Considering the three cities, the 10 types of coating mortar, the five thickness variations, and the two methods of thermal performance evaluation, 300 models were simulated. The creation of simulation input files and the compilation of their results were automated using Visual Basic for Applications (VBA) in Excel.

Results

Figures 5 and 6 present the results obtained in the thermal performance analysis for the residence in Curitiba (BZ1), São Paulo (BZ3), and Belém (BZ8), according to the TL method and the DH method, respectively. The discussion of these results is provided in the following sections.

Impact of variation in mortar thickness on building thermal performance

Figures 7 and 8 present the data in boxplot charts for the three analyzed cities, where each boxplot represents the aggregation of the five thicknesses of each analyzed mortar. In this section, the variation on thermal performance according to the mortars’ thickness is discussed.

Cold regions

In both methods, for the cold climate (BZ1: Curitiba), increasing the thickness of coating mortar improved the thermal performance of the building. This is observed by the reduction the model’s energy consumption for conditioning and reduced degree-hours of discomfort. This behavior is related to the increased thermal resistance and thermal capacity that thicker coating mortars provide to the building envelope (Lamberts; Dutra; Pereira, 2014), as shown from Equation 1 and Equation 2. Thus, in cold climates, the reduction of the heat flow rate (increased thermal resistance) between the internal rooms and the exterior environment was beneficial. Increasing the thermal capacity of the envelope also contributed to improving the thermal performance of the building, preventing the rooms from deviating from the comfort/setpoint temperatures. Additionally, in cold places, envelope materials with increased thermal capacity serve as “heat batteries”, heating up during the day, with solar radiation and external high temperatures, and maintaining this heat during cold night hours.

In the cold climate of Curitiba (BZ1), for both methods, the mortar CM – Quartzite + Steelmaking Slag stood out with the highest variation, varying 385 kWh (45%) and 1723 (90%) degree-hours when the mortar thickness was increased from 10 mm to 30 mm. Meanwhile, GM – Low-Calcium Fly Ash + Copper Slag was responsible for the smallest alteration in the building’s thermal performance (234 kWh and 1115 degree-hours). Contrasting these results with Table 1, one can notice that in cold climates, the greatest impact of varying mortar thickness occurs for those that manage to combine low thermal conductivity and high density, as exemplified by CM – Quartzite + Steelmaking Slag mortar (thermal conductivity 0.29 W/(m·K) and density 1721 kg/m³).

Mild regions

The results observed in the mild climate (BZ3: São Paulo) are similar to those observed in the cold climate (BZ1). Mendes et al. (2022), also found similar correlations when analyzing the sensitivity of thermal properties and thickness of conventional coating mortars in the thermal performance of buildings in cold and mild climates in Brazil.

In the climate of São Paulo (BZ3), according to the TL method, CM – Steelmaking Slag has a more pronounced impact on the building’s energy consumption (224 kWh; 21%) as its thickness varies. Varying the thickness of CM – Quartzite + Steelmaking Slag (which had the most influence in BZ1) results in an energy reduction of 214 kWh (20%). The CM – Quartzite + Steelmaking Slag mortar ensured the greatest variation in the building’s thermal performance in BZ3 when evaluated by the DH method (1405 degree-hours; 58%). Similar to BZ1, the mortar GM – Low-Calcium Fly Ash + Copper Slag showed the least impact on the building’s thermal performance in BZ3 when varying its thickness, both in the TL method (114 kWh; 10%) and the DH method (996 degree-hours; 31%). Due to the resemblance of São Paulo’s climate to the climate in Curitiba (Figure 4), the greatest impacts of varying the mortars’ thickness occurred with those that combine low thermal conductivity and high density.

Warm regions

In the hot location (BZ8: Belém), according to the TL method, no energy was demanded for heating for any of the analyzed mortars. Likewise, the DH method did not point any degree-hours of discomfort from cold. As expected for the local hot climate, this location showed the highest absolute values of energy demand for cooling (TL) and degree-hours from heat (DH) compared to any other city.

The TL method presented a distinct behavior from the previous results. In this method, thicker layers of most analyzed mortars increased the building’s energy consumption, while thinner layers improved thermal performance. In BZ8, except for the CM – Polystyrene Foam Powder mortar (brown dot, highlighted in the image), there was a strong direct correlation between the thickness of the mortars and energy consumption. Larger thicknesses promoted overheating, given that the heat was “stored” in the mortars with high thermal capacity, and “trapped” inside due to their low heat transfer rate.

This phenomenon becomes clearer when one notices the defined occupancy pattern (Table 3), leading the HVAC system to operate predominantly in the late afternoon, extending through the night until the early morning. With this schedule, the internal environment gradually overheats during periods of high temperatures (morning and early afternoon, mainly). When the external climate start to cool down, the insulating mortars prevent natural cooling, requiring more energy for mechanical cooling. So, for the present case study, in hot regions, envelope materials that prevent heat exchange increased the cooling demand.

For the CM – Polystyrene Foam Powder mortar, an opposite behavior was recorded, i.e., the increase in thickness ensured a reduction in energy consumption to acclimate the building - a better thermal performance. The CM – Polystyrene Foam Powder mortar was developed by Koksal, Mutuay and Gencel (2020), with exceptionally lower thermal conductivity (0.09 W/(m·K)) and density (393 kg/m³) than the other analyzed cement-based or geopolymer-based mortars (Table 1). This mortar has only 7.8% of the thermal conductivity and 20.1% of the density of the conventional cement-based mortars presented by Brazilian standard NBR 15200 (ABNT, 2005) (1.15 W/(m·K) and 1950 kg/m³).

As result, the properties of CM – Polystyrene Foam Powder mortar succeed achieving a high thermal resistance and a low thermal capacity, making it suitable for scenarios where high temperatures are prevalent. With this mortar, increasing the thermal resistance of the envelope (i.e., larger thicknesses) reduced the influence of the outdoor conditions in the indoor environments. Furthermore, the relatively low thermal capacity of this mortar ensured a lower risk of overheating probably because 1) heat would not be “stored” in the walls and 2) the response of the internal rooms to mechanical cooling was fast.

The results obtained by the DH method in BZ8: Belém are inverse to those obtained by the TL method. Most mortars showed that increasing thickness led to a reduction in discomfort degree-hours. Some reasons for these contradictory behaviors lie in the procedures used to evaluate thermal performance by the TL and DH methods. In the TL method, the model was configured with windows and doors closed throughout the year, the HVAC system operating according to occupancy patterns (Table 3), and the temperature setpoint adjusted to 25 °C (cooling) and 20 °C (heating). This situation aimed to simulate the occupancy of a family with an HVAC system installed at home, who turns it on mainly when there are people at home. On the other hand, in the DH method, the model was built to open windows and doors when necessary to cool the indoors with natural ventilation, in addition to accounting for occupant discomfort throughout the whole day, with limit temperatures of 26 °C for heat discomfort and 18 °C for cold discomfort. This situation aimed to simulate the occupancy of a family that does not use an HVAC system in their home, as well as to ensure good thermal performance in the residence throughout the day (regardless if the occupants were home).

For all regions, the increased thermal resistance and thermal capacity resulting from larger thicknesses led to an increased overall thermal performance in most cases. This behavior probably occurred because the increased thermal resistance allowed for lower heat exchange between indoors and outdoors, while the increased thermal capacity avoided rapid heating during daylight hours. A more in-depth analysis of the influence of these two properties will be made in the next section.

For the DH method and the hot region (BZ8: Belém), the CM - Polystyrene Foam Powder mortar once again presented the opposite behavior. The mortar CM – Biomass Fly-Ash + Metakaolin (pink dot, highlighted in the image) showed an almost neutral behavior, slightly leaning towards the increasing trend observed for the CM - Polystyrene Foam Powder mortar (brown dot, also highlighted). The CM – Polystyrene Foam Powder mortar is the most thermally insulating (low thermal conductivity of 0.09 W/(m·K)) and the one that stores the least heat (low density of 393 kg/m³). Increasing this mortar’s thickness would prevent the heat flow from the external environment, but it would also promote a higher “heat storage” (which would be “trapped” inside due to the higher thermal resistance). The second phenomena (“trapped heat”) outweighed the first (better insulation). A similar reasoning of balancing effects can be attributed to the CM – Biomass Fly-Ash + Metakaolin mortar.

Rakotonjanahary et al. (2020) studied the overheating of a residential unit during a very hot period of the year, in Luxembourg. The authors found that the risk of overheating was only reduced with adequate shading and natural ventilation. Similarly, Baba, Wang and Zmeureanu (2022) identified a similar behavior in their thermal performance analysis of old buildings and energy-efficient buildings in Canada. The authors found that without adequate ventilation, the risk of overheating in energy-efficient buildings could be higher than in older ones, where increasing the thermal resistance of vertical envelopes (walls and windows) and reducing the air infiltration rate had a high contribution to increasing internal temperature.

In the hot climate of Belém (BZ8), by both methods, the CM – Quartzite mortar ensured the greatest variation in thermal performance with the thickness alterations. According to the TL method, the building’s cooling energy consumption varied by 260 kWh (3%). Additionally, the DH method indicated a variation of 787 degree-hours (2%) in user discomfort. The mortar that promoted the least variations on building’s thermal performance was CM – Biomass Fly Ash + Metakaolin, with only 44 kWh (0.6%) of variation on energy demand and 115 degree-hours (0.3%) reduction in discomfort. Thus, for the simulated building, in the hot climate, the greatest impact of varying mortar thickness occurred with those that combine low thermal resistance with high thermal capacity, i.e., high thermal conductivity and high density, as exemplified by CM – Quartzite mortar (0.94 W/(m·K) and 1754 kg/m³, respectively). Relative variations are small, being the smallest among all analyzed climates, but the absolute value of these differences is significant, as it is the region that consumed the most energy and had the most degree-hours of discomfort.

Best and worst scenarios of mortars and thicknesses in building thermal performance

Table 5 shows the mortars that provided the best and worst thermal performances in Curitiba (BZ1), São Paulo (BZ3), and Belém (BZ8), according to the two analyzed methods.

The difference between the best and worst mortar corresponded to 611 kWh in energy consumption, which translates to R$392.75 (US$79.36 = €72.98), and a total of 2757 degree-hours of discomfort. This indicates that, in the analyzed scenario, the correct selection of mortar type and its thickness can improve the building’s thermal performance by 47% (TL method) and 59% (DH method). The CM - Insulating mortar, with a 30 mm thickness (proven to offer the best thermal performance for this mortar type), consumed 207 kWh more (approximately equivalent to R$133.06, US$26.89, or €24.73) and experienced 845 more degree-hours compared to CM – Polystyrene Foam Powder with the same thickness (which outperformed all others).

Cold regions

In both methods, the mortar that provided the best building’s thermal performance in the cold climate of Curitiba (BZ1) was CM – Polystyrene Foam Powder with a thickness of 30 mm. For the modelled dwelling, the building with this mortar consumed 694 kWh of energy for indoor conditioning and presented only 1940 degree-hours of user discomfort throughout the year. This mortar stands out with the highest values of thermal resistance (0.333 (m²·K)/W) and the lowest values of thermal capacity (11.79 kJ/(m²·K)), indicating that its ability to reduce heat transfer was predominant for improving the building’s thermal performance.

The second-best performer (not in the table) was the CM – Quartzite + Steelmaking Slag of 30 mm (0.103 (m²·K)/W of thermal resistance, 749 kWh and 2120 degree-hours) and the third-best performer was CM - Steelmaking Slag of 30 mm (0.042 (m²·K)/W of thermal resistance, 793 kWh and 2339 degree-hours). These mortars do not present high values of thermal resistance, but they compensate it with high thermal capacity values (51.63 kJ/(m²·K) and 73.26 kJ/(m²·K), respectively). Hence, in colder climates, within a certain range of mortar thermal resistance, the mortars’ thermal capacity becomes more relevant. This phenomenon was also noticed by Mendes et al. (2022).

Conversely, the mortar with the worst performance was GM – Low-Calcium Fly Ash + Copper Slag with a thickness of 10 mm. The building with this mortar consumed 1305 kWh of energy for environment conditioning and generated 4697 degree-hours of discomfort. As can be seen in Table 1, this mortar has both low thermal resistance (0.017 (m²·K)/W) and thermal capacity (6.11 kJ/(m²·K)) values. Similar mortars with low thermal resistance and low thermal capacity followed this trend.

Mild regions

In the mild climate of São Paulo (BZ3), CM – Steelmaking Slag with a thickness of 30mm stood out with a good result: 825 kWh in energy consumption and 939 degree-hours of discomfort. This time, this mortar was the best among all tests performed in this region. This trend was also observed in similar mortars characterized by high thermal capacity. For instance, GM – Metakaolin + Glass Waste with 30 mm of thickness was the second-best result performer (830 kWh and 958 degree-hours). Despite having one of the lowest thermal resistances (0.034 (m²·K)/W), it had the highest thermal capacity in the sample set (74.52 kJ/(m²·K)), behaving close to CM – Steelmaking Slag with 30 mm of thickness. Moreover, CM – Quartzite + Steelmaking Slag with a thickness of 30 mm ensured the third-best result (830 kWh and 999 degree-hours). Its thermal resistance is relatively high (0.103 (m²·K)/W), as well as its thermal capacity (51.63 kJ/(m²·K)).

We can infer that, for mild climates, considering the analyzed mortars, those with properties that ensure less fluctuation in internal temperature (relatively high thermal capacity) promoted more positive influence on the building’s thermal performance. These results also suggest that the mortars’ thermal resistance plays a lesser role in the thermal performance of buildings in mild climates compared to cold ones (Mendes et al., 2022).

In turn, like in BZ1, GM – Low-Calcium Fly Ash + Copper Slag with a thickness of 10 mm ensured the lowest thermal performance result for the analyzed building in BZ3 (1123 kWh and 3213 degree-hours). As previously reported, this mortar has relatively low values of thermal resistance (0.017 (m²·K)/W) and capacity (6.11 kJ/(m²·K)).

The difference between the best and worst scenarios of mortar’s type and thickness in São Paulo (BZ3) corresponded to 298 kWh in energy consumption (R$193.10 = US$39.02 = €35.88) and 2273 degree-hours of discomfort for users. It represents a variation on the building’s thermal performance by 26% (TL method) and 71% (DH method). Comparing CM – Steelmaking Slag mortar with a thickness of 30mm (the best overall) with the CM – Insulating mortar with 30 mm (thickness with the best result for this mortar type), the CM – Steelmaking Slag mortar presented a reduction of 87 kWh in the energy demand (R$56.37 = US$11.39 = €10.48) and 484 degree-hours in discomfort hours.

Warm regions

For TL method, the best mortar in Belém (BZ8) was the CM – Polystyrene Foam Powder mortar with a thickness of 30 mm, ensuring a consumption of 6825 kWh of energy for building conditioning. The explanation for this positive result has already been explored previously, when the high thermal insulation capacity of CM – Polystyrene Foam Powder was discussed.

In addition to that, the CM – Biomass Fly Ash + Metakaolin mortar and the GM – Low-Calcium Fly Ash + Copper Slag mortar, both with a thickness of 10 mm, achieved good results in the building’s thermal performance (7031 kWh and 7074 kWh, respectively). CM – Biomass Fly Ash + Metakaolin mortar has 0.050 (m²·K)/W of thermal resistance, which is relatively low and GM – Low-Calcium Fly Ash + Copper Slag mortar has 0.150 (m²·K)/W of thermal resistance, which is relatively high. This shows again that the thermal transmittance was not the main property influencing in the building’s thermal performance, in hot climates (Mendes et al., 2022). However, these mortars have the lowest thermal capacity among the other ones (6.00 kJ/m²·K and 6.11 kJ/m²·K, respectively), promoting less “heat storage” and reducing overheating.

In contrast, the mortar GM - Metakaolin + Glass Waste (thermal resistance of 0.034 (m²·K)/W and thermal capacity of 74.52 kJ/(m²·K)), with a thickness of 30 mm obtained the worst results (7649 kWh). Thus, mortars with high thermal capacity hinder the indoor temperature reduction in the TL method.

In BZ8, the DH method presents a completely different scenario. In a setting without mechanical air conditioning, and 24-h assessment, having high envelope thermal capacity proves more advantageous, preventing abrupt changes in indoor temperature. In this case, GM - Metakaolin + Glass Waste mortar with 30 mm, which has the highest thermal capacity (74.52 kJ/(m²·K)), provided the best thermal performance (33381 degree-hours). In turn, CM – Polystyrene Foam Powder mortar with a thickness of 30 mm, which has the lowest thermal capacity (11.79 kJ/(m²·K)), promoted the worst thermal performance (36605 degree-hours). The same trend was observed for other studied mortars.

These conflicting results in BZ8 shed light on the fact that the evaluation method plays a crucial role in choosing strategies to improve the thermal performance of the building. Therefore, in a building equipped with a cooling system, the results from the TL method are more suitable. There was a difference of 824 kWh (11%) for building conditioning (R$792.59 = US$160.16 = €147.29), comparing the best and worst waste-based mortar and their thickness. The Insulation Mortar with a thickness of 10 mm (which was the best mor this mortar type) consumed 319 kWh (R$306.84 = US$62.00 = €57.02) more than CM – Polystyrene Foam Powder with a thickness of 30 mm.

Conversely, if a cooling system is not planned for the building, the results obtained by the DH method are more appropriate, showing a reduction of 3224 degree-hours of discomfort for occupants throughout the year when comparing between the best and worst scenarios. Comparing the best option to the CM - Insulating mortar with 30 mm, it can reduce user discomfort by 681 degree-hours (9%).

Ideally, both metrics could and should be combined. In other words, it is desirable that the building first passively achieves maximum thermal comfort (DH method) and, whenever still required, be actively conditioned (TL method). Further investigations on this topic are recommended to understand how the mortars’ properties will perform considering both active and passive measures.

Discussion of the utilization of waste on mortar thermal properties and subsequent building thermal performance

The present results showed that varying the coating mortar’s thickness and type can significantly impact the building’s overall thermal performance. These variations are closely linked to the thermophysical properties of each mortar, as also highlighted by Mendes et al. (2022).

The thermal resistance of an element increases with thickness and decreases with thermal conductivity (Equation 1); while the thermal capacity increases with density, thickness, and specific heat (Equation 2). Therefore, thickness directly influences two crucial thermophysical properties of the building envelope, which play key roles in its thermal performance.

The density of mortar, depending on the aggregate used in its manufacturing, also correlates both with thermal conductivity and thermal capacity. For example, polystyrene, with its lightweight closed-cell air-filled structure (i.e., low density), exhibits low thermal conductivity and low density (Becker, Effiting e Schackow, 2022; Koksal, Mutluay e Gencel, 2020). On the other hand, materials like steelmaking slag offer low thermal conductivity alongside high density due to their mineralogical composition, making them highly amorphous and heavyweight (Franco et al., 2019; Mendes et al., 2020b; Martins et al., 2024).

The thermal capacity of mortars plays a crucial role in the thermal performance of buildings, largely determined by their specific heat. Despite its significance, there is a notable lack of research providing specific heat values for non-conventional mortars, representing a significant gap in the literature. However, specific heat can be obtained cost-effectively through adiabatic calorimetry (Mendes et al., 2022).

Therefore, energy simulations become essential for understanding how each mortar behaves in different climates. As these mortars are derived from waste materials, their replicability may vary, especially in Brazil, a country with continental area where waste materials lack quality control (Carvalho et al., 2023b). Simulation and computational methods aid in product development and the integration of waste materials into production cycles, promoting the circular economy.

The highest-performing mortars were crafted from waste materials, outperforming even those specifically designed for insulation (CM - Insulating mortar). Notably, these mortars are often more cost-effective and provide substantial social and environmental advantages (Lopes et al., 2023; Rentier; Cammeraat, 2022; Dobiszewska, 2017). By refraining from using river sand for conventional mortar production and repurposing waste materials that are usually discarded, these mortars offer a dual win. This underscores the potential for waste materials to improve thermal performance and enhance sustainability in Brazilian construction projects, provided they undergo careful selection and processing.

Geopolymer-based mortars, made entirely from waste materials, show comparable thermal performance to cement-based mortars. They offer a more sustainable solution for construction, reducing carbon emissions associated with transportation and production. Notably, Portland cement manufacturing is a significant carbon emitter, a concern lessened with geopolymer-based mortars. Geopolymer-based composites also outperform in physical and mechanical properties, boasting higher compressive strength, lower permeability, and shrinkage (Carvalho et al., 2023a).

Still, considering the proximity of construction sites to waste material is crucial. Lopes et al. (2023) conducted a comprehensive mapping of the 49 most adopted waste materials in cement-based composites across Brazil. Comparing their findings with the waste materials investigated in this study, most were found in Curitiba (located in the state of Paraná), São Paulo (São Paulo), and Belém (Pará). Notably, iron ore tailings were exclusively present in Pará, although they did not exhibit superior thermal performance results overall. Conversely, kaolin waste and copper slag were found solely in Pará, emerging as the only study locality where they demonstrated promising results according to the DH and TL methods, respectively. Polystyrene foam powder, while not included in Lopes et al. (2023) mapping, is widely available in Brazil due to its common usage.

Conclusions

The present study investigated the impact of thickness variations in 10 cement-based and geopolymer mortars, made from recycled materials, on the thermal performance of a 40m²-single-family dwelling in three different bioclimatic zones: Curitiba (BZ1; cold), São Paulo (BZ3; mild), and Belém (BZ8; hot). The Thermal Load (TL) and Degree Hours (DH) methods were applied in the results of energy simulations on EnergyPlus software.

The main results include:

1. in cold climates, mortars with low thermal conductivity and high density, resulting in high thermal resistance and capacity, respectively, showed improved thermal performance. Varying the thickness of the mortars alone can modify the thermal performance of the building in up to 45% (TL method) and 90% (DH method);

2. in mild climates, the mortar’s impact on the building’s thermal performance is most significant when it possesses high density primarily, and preferably high thermal conductivity as well. This is because in such conditions, for the evaluated building, thermal capacity proved to be more influential than thermal transmittance to the overall performance;

3. hence, in cold and mild climates, increasing the thickness of the coating mortar reduces the energy consumption and the degree-hours of discomfort for users, due mainly to the increased thermal capacity;

4. in warm climates, the thermal performance of buildings decreased, in most times, as the thickness of mortars was increased, using the TL method. In contrast, using the DH method, an opposite effect was observed: better thermal performances were recorded with thicker mortars. Varying the mortars’ thicknesses influenced the building’s thermal performance up to 3% (TL method) and 2% (DH method);

5. in warm climates, according to the TL method, reducing the mortar’s thermal capacity improves the building’s thermal performance more than enhancing its thermal resistance. A reduced thermal capacity allows for a quicker cooling of the indoor environment, as the envelope ‘stocks’ less heat, reducing the overheating phenomenon. If the mortar has low thermal capacity, it must possess exceptional insulation capability, as demonstrated by CM – Polystyrene Foam Powder, which exhibits a very low thermal conductivity of 0.09 W/(m·K);

6. in warm climates, the results observed with the DH method were contrary to those seen with the TL method. This result highlights how the difference in the evaluation method can lead to entirely different design assumptions, and the importance of using simulation parameters as close to the reality as possible;

7. the differences between the best and worst mortars reached 824 kWh (R$792.59 = US$160.16 = €147.29) in the most extreme condition just by selecting the mortars and adjusting their thickness. This result highlights the impact of coating mortars on the energy efficiency and thermal performance of buildings;

8. in most cases, waste-based mortars, especially those with Steelmaking Slag and Polystyrene Foam Powder, showed a better performance than the CM – Insulation mortar, developed with conventional (new) materials exclusively for thermal performance purposes. This result show that residues have great potential as replacement for natural materials, even improving the final properties of the composite in some cases;

9. geopolymer-based mortars can perform as well as, or even better than, cement-based mortars if their properties are well-suited to the climate, with significantly lower CO₂ emissions; and

10. one significant research gap identified was the absence of specific heat capacity values for non-conventional construction materials. This property significantly impacts the thermal capacity of the composite material produced and warrants further investigation.

In conclusion, the type (thermal properties) and thickness of these waste-based mortars noticeably affect the overall thermal performance of buildings. As this study is based on a low-income dwelling, the authors recommend the simulation of different buildings and regions to validate the generalizability of the results. Such efforts contribute to society by advocating for the use of materials that enhance thermal comfort in buildings, reduce electricity costs, and promote the circular economy by reintegrating waste materials into the production cycle.

Acknowledgments

We acknowledge UFOP (Universidade Federal de Ouro Preto) and UFJF (Universidade Federal de Juiz de Fora) for their support. We would like to express our gratitude to the institutions that offered financial support for this work: CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior), PhD scholarship awarded to Aldo Ribeiro de Carvalho, Júlia Assumpção Castro and Vítor Freitas Mendes, finance code 001; CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, grant 305818/2023-6 - PQ Researcher - for Julia Mendes Finally, we acknowledge the research groups CIDENG-CNPq and ATIVE-CNPq for the support and collaboration.

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