Incorporation of spent bleaching earth and glass sediment as an alternative raw material for the manufacture of eco-products based on fired clay

2.1. Materials

In this study, the following were selected as raw materials: commercial clay, two spent bleaching earth (SBE and SBC) from the bleaching process in the refining of oils and fats of animal and vegetable origin from a food product manufacturing company, and glass sediment from a safety glass processing and transformation company for architecture, located in the Department of Valle del Cauca in Colombia. These materials can be seen in Figure 1. The adaptation of the clay was carried out in a jaw crusher and subsequently, a grinding process was carried out, using a ball mill for a period of two (2) hours. The glass sediment (GS) was also subjected to a grinding process in a ball mill, and in the case of the spent bleaching earth maceration process was carried out in a porcelain mortar, until obtaining a particle size passing through sieve No. 100 (<0.149 mm).

Figure 1 a) Clay (C), b) Spent bleaching earth (SBE), c) Spent bleaching earth with lime (SBC), and d) glass sediment (GS). 

The chemical composition was determined by X-ray fluorescence (XRF). The results can be seen in Table 1. In general, the solid waste contemplated in this study reported significant amounts of SiO₂. In the case of the spent bleaching earth (SBE), the MgO content was greater than 16%, the CaO in the spent bleaching earth with lime (SBC) and in the glass sediment (GS) meanwhile exceeded 9% by weight, while the Al₂O₃ was around 3 % in the spent bleaching earth. As reported by Eliche-Quesada et al. (26Eliche-QuesadaD, Martínez-MartínezS, Pérez-VillarejoL, Iglesias-GodinoFJ, Martínez-GarcíaC, Corpas-IglesiasFA. 2012. Valorization of biodiesel production residues in making porous clay brick. Fuel Process. Technol.103:166-173. 10.1016/j.fuproc.2011.11.013), the presence of oxides such as K₂O, Na₂O, Fe₂O₃, CaO, and MgO favor vitrification and are considered fluxes, so they can influence the densification behavior of the ceramic materials during firing and therefore help reduce the process temperature, thereby improving the properties of the ceramics. It is noteworthy that the spent bleaching earth had a high loss on ignition (L.O.I.), greater than 25%, associated mainly with the presence of organic compounds, in this case from the oil. However, waste that contains fuel also contributes to saving energy in firing, while waste containing oxides improves the ceramic properties (27MonteiroSN, VieiraCMF. 2014. On the production of fired clay bricks from waste materials: A critical update. Constr. Build. Mater.68:599-610. 10.1016/j.conbuildmat.2014.07.006).

The density of the powders was determined in an Anton Paar Ultrapyc 3000 helium pycnometer according to the ASTM C329 standard (ASTM International 2016). Table 1 shows that the glass sediment has a higher density (2.53 g/cm³) compared to the spent bleaching earth. A modular particle size analyzer with Mastersizer 2000 laser diffraction technology that includes a Hydro 2000MU accessory was used to determine the particle size of the clay and the glass sediment, obtaining an average particle diameter of 8.558 µm and 36.994 µm, respectively. The granulometry results revealed that for the glass sediment, 90% of the sample remained below a particle size of 63.716 µm, whereas for the clay, it was 24.040 µm. Differential thermal analysis was carried out in a TA Instruments V2.5H universal equipment (SDTQ 600) at a heating rate of 10 °C/min. in airflow of 100ml/min up to a maximum temperature of 1100 °C for the clay and 1000°C for the spent bleaching earth and the Infrared Spectroscopy analysis was carried out on the PerkinElmer Spectrum 100 FTIR equipment with the KBr tablet preparation procedure.

2.2. Production of material from fired clay

The manufacture of fired clay masonry involves four basic steps, summarized as: a) Selection and preparation of the mixture, b) Molding, c) Drying, and d) Firing (7Muñoz VelascoP, Morales OrtízMP, Mendívil GiróM, and Muñoz VelascoL. 2014. Fired clay bricks manufactured by adding wastes as sustainable construction material - A review. Constr. Build. Mater.63:97–107. 10.1016/j.conbuildmat.2014.03.045). In the first stage, the quantities of residue added to the mixes were established (0, 10, 30, and 50% by weight). Table 2 shows the compositions by weight of each of the mixes. The amount of water added was 30% by weight, an amount corresponding to the plastic limit of the clay determined by the ASTM D4318 standard (ASTM International, 2018). Once the compositions were established, experimental units were prepared in the form of 2x2x2 cm cubes in an artisanal manner, where the molding was carried out manually. It is important to highlight that in the mixes with the incorporation of spent bleaching earth (SBE), the workability was superior. The samples were subjected to a drying process for 24 hours. Finally, the heat treatment of the samples was carried out in an electric oven at 1000 °C, for 3 h.

The heat treatment of the samples involved four stages: 1) Preheating to 300 °C and elimination of water physically bound to the clay, 2) Heating up to 700 °C, in which the water chemically bonded to the clay is eliminated, 3) Maturation of the product at 1000 °C, and 4) Tempering of the piece, cooling in an oven. At this stage, it is important to consider the firing interval, that is, the temperature range between the beginning of vitrification (formation of the glassy phase) and the beginning of deformation since the characteristics of the paste depend on it and must be as wide as possible. The optimal firing temperature must be within this range, not too close to the beginning of vitrification so that the material is not too porous, and not too close to the beginning of deformation so that the piece is not deformed (28BarranzuelaJ.2014. Proceso productivo de los ladrillos de arcilla producidos en la region piura. Tesis pregrado en Ingeniería Civil, Universidad de Piura, Perú. Retrieved fromhttps://pirhua.udep.edu.pe/backend/api/core/bitstreams/5eea42bf-0da2-43bd-b308-b8eb99ea51a3/content.). The appearance of the samples after the firing process is evident in Figure 2, where a color change is observed with the incorporation of residues compared to the 100% clay sample (C). The samples with the presence of glass sediment (GS) presented a reddish appearance.

Figure 2 Appearance of the samples after the firing process. 

The methodology used to determine the physical and mechanical properties of fired clay pieces was taken from the Colombian regulations NTC 4017, Methods for sampling and testing of masonry units and other clay products, where the selection and preparation of the sample procedures are set out (ICONTEC, 2018), a modified adoption of ASTM C67:2017, Standard Test Methods for Sampling and Testing Brick and Structural Clay Tile. The compressive strength of the brick was evaluated in a Universal Testing Machine: INSTRON 3369 with a 50 KN compression capacity. For this test, three specimens of each mixture were taken with measurements of 20x20x20 mm. All specimens were subjected to a progressively increasing normal stress, applying a centered load with a speed of 1 mm/min. The physical characteristics (apparent density (ρa), water absorption (A24h), and volume of permeable pores (Vpp)) of the samples were determined according to the provisions of the NTC 4017:2018 standard (29ICONTEC, NTC 4017. 2018. Métodos para muestreo y ensayos de unidades de mampostería y otros productos de arcilla.) and were complemented by the technique with the ASTM C642-13 standard (30ASTM International, ASTMC642–13. 2013. Standard test method for density, absorption and voids in hardened concrete.). Finally, the thermal conductivity measurement was carried out by the transient plane source (TPS) method by the ISO 22007-2 Standard in a Hot Disk Thermal Constants Analyzer model TPS 500 S from Thermtest - Thermal conductivity instruments.

3.1. Characterization of the raw material

The results of the thermal analysis carried out on the clay and spent bleaching earth are reported in Figure 3, where a total mass loss for the clay of 10% was observed. In the case of the spent bleaching earth, the mass losses were 35% for spent bleaching earth with lime (SBC) and 37.97% for spent bleaching earth without lime (SBE).

The DTA curves of the clay show endothermic peaks at 56 °C and 243 °C that can be related to its dehydration. The peak at 508 °C can be associated with a dehydroxylation process (31ZuluagaD, HenaoAP, GarcíaDF, RodríguezJE, HoyosAM, LopezME, GómezC. 2016. Caracterización térmica, química y mineralógica de un tipo de arcilla roja propia de la región andina colombiana, empleada para la producción de ladrillos para construcción. Rev. Colomb. Mater.9:53-56. 10.17533/udea.rcm.326494, 32MuñozJ, MuñozR, MancillP, RodríguezJ. 2007. Estudio del procesamiento cerámico de las arcillas de la vereda ‘La Codicia’ (Guapi, Colombia) para potencializar su uso en la elaboración de piezas cerámicas. Rev. Fac. Ing.42:68-78. 10.17533/udea.redin.18837). Around 900 °C, several exothermic effects are observed attributable to the crystallization of high-temperature phases (25Eliche-QuesadaD. Corpas-IglesiasFA, Pérez-VillarejoL, Iglesias-GodinoFJ. 2012. Recycling of sawdust, spent earth from oil filtration, compost and marble residues for brick manufacturing. Constr. Build. Mater.34:275-284. 10.1016/j.conbuildmat.2012.02.079). The DTA curve of the spent bleaching earths is typical of a solid fuel. In the DTA curve of the waste, two weight losses are observed between 335 and 457 ºC associated with the combustion processes of the organic matter, which coincides with that reported by Eliche-Quesada and Corpas-Iglesias (1Eliche-QuesadaD, Corpas-IglesiasFA. 2014. Utilisation of spent filtration earth or spent bleaching earth from the oil refinery industry in clay products. Ceram. Int.40(10, B):16677–16687. 10.1016/j.ceramint.2014.08.030). The endothermic reaction between 656 and 850 °C may be attributed to the thermal decomposition of the carbonate present in the sample (25Eliche-QuesadaD. Corpas-IglesiasFA, Pérez-VillarejoL, Iglesias-GodinoFJ. 2012. Recycling of sawdust, spent earth from oil filtration, compost and marble residues for brick manufacturing. Constr. Build. Mater.34:275-284. 10.1016/j.conbuildmat.2012.02.079). The importance of thermal analyses of a clay material lies in the fact that they reveal the behavior of the sample as the temperature changes. Through the thermal profile, it is therefore known how to implement a firing curve or propose a heating program appropriate to the sample characteristics. By controlling the firing curve, it can be ensured that the sintering processes are carried out in a controlled manner and the greatest densification of the pieces is achieved, achieving products that meet the quality standards demanded by the market (33CáceresVI, Sánchez-MolinaJ, Chaparro-GarcíaAL. 2017. Evaluación de arcillas caoliniticas-illiticas provenientes de la formación guayabo del área metropolitana de Cúcuta, Norte de Santander, Colombia. Rev. ION30(1):117-127. 10.18273/revion.v30n1-2017009).

Figure 3 DTA-TG and DTG curves: a) Clay, b) SBC, and c) SBE. 

The DTA curves of the clay show endothermic peaks at 56 °C and 243 °C that can be related to its dehydration. The peak at 508 °C can be associated with a dehydroxylation process (31ZuluagaD, HenaoAP, GarcíaDF, RodríguezJE, HoyosAM, LopezME, GómezC. 2016. Caracterización térmica, química y mineralógica de un tipo de arcilla roja propia de la región andina colombiana, empleada para la producción de ladrillos para construcción. Rev. Colomb. Mater.9:53-56. 10.17533/udea.rcm.326494, 32MuñozJ, MuñozR, MancillP, RodríguezJ. 2007. Estudio del procesamiento cerámico de las arcillas de la vereda ‘La Codicia’ (Guapi, Colombia) para potencializar su uso en la elaboración de piezas cerámicas. Rev. Fac. Ing.42:68-78. 10.17533/udea.redin.18837). Around 900 °C, several exothermic effects are observed attributable to the crystallization of high-temperature phases (25Eliche-QuesadaD. Corpas-IglesiasFA, Pérez-VillarejoL, Iglesias-GodinoFJ. 2012. Recycling of sawdust, spent earth from oil filtration, compost and marble residues for brick manufacturing. Constr. Build. Mater.34:275-284. 10.1016/j.conbuildmat.2012.02.079). The DTA curve of the spent bleaching earths is typical of a solid fuel. In the DTA curve of the waste, two weight losses are observed between 335 and 457 ºC associated with the combustion processes of the organic matter, which coincides with that reported by Eliche-Quesada and Corpas-Iglesias (1Eliche-QuesadaD, Corpas-IglesiasFA. 2014. Utilisation of spent filtration earth or spent bleaching earth from the oil refinery industry in clay products. Ceram. Int.40(10, B):16677–16687. 10.1016/j.ceramint.2014.08.030). The endothermic reaction between 656 and 850 °C may be attributed to the thermal decomposition of the carbonate present in the sample (25Eliche-QuesadaD. Corpas-IglesiasFA, Pérez-VillarejoL, Iglesias-GodinoFJ. 2012. Recycling of sawdust, spent earth from oil filtration, compost and marble residues for brick manufacturing. Constr. Build. Mater.34:275-284. 10.1016/j.conbuildmat.2012.02.079). The importance of thermal analyses of a clay material lies in the fact that they reveal the behavior of the sample as the temperature changes. Through the thermal profile, it is therefore known how to implement a firing curve or propose a heating program appropriate to the sample characteristics. By controlling the firing curve, it can be ensured that the sintering processes are carried out in a controlled manner and the greatest densification of the pieces is achieved, achieving products that meet the quality standards demanded by the market (33CáceresVI, Sánchez-MolinaJ, Chaparro-GarcíaAL. 2017. Evaluación de arcillas caoliniticas-illiticas provenientes de la formación guayabo del área metropolitana de Cúcuta, Norte de Santander, Colombia. Rev. ION30(1):117-127. 10.18273/revion.v30n1-2017009).

The Fourier transform infrared (FTIR) spectroscopy results are reported in Figure 4. As can be seen, the spectra of the spent bleaching earth with and without lime are similar. The difference lies mainly in small shifts and the intensity of the bands in each of the spectra, in addition to the presence of a band at 872 cm^-1 for the spent bleaching earth sample with lime.

Figure 4 Fourier transform infrared (FTIR) spectroscopy curves. 

The bands between 3698 and 3621 cm^-1 for the clay and spent bleaching earth are associated with the stretching of the O-H bond, which according to Chaisena and Rangsriwatananon (34ChaisenaA, RangsriwatananonK.. 2004. Effect of thermal and acid treatment on some physico-chemical properties of lamping diatomite. J. Sci. Technol11:289–299. Retrieved from https://www.thaiscience.info/journals/Article/SJST/10890793.pdf) occurs due to the presence of kaolinite, bands 3433 and 3473 cm^-1, corresponding to the stretching of the O-H bond of water (H₂O) present in the samples, and around 1635 cm^-1 to the bending of H₂O. In the clay spectrum, the band at 1115 cm^-1 corresponds to the Si-O-Si stretching. The peaks that appear at 911 and 872 cm^-1 for the spectrum of clay and spent bleaching earth with lime, respectively, correspond to the stretching of Si-OH and/or bending of Al-Al-OH (kaolinite) inter tetrahedral. The bands between 780 and 798 cm^-1 belong to the stretching of the Si-O-Al, (Al, Mg)-O-H bond and the stretching of the Si-O-(Mg, Al) bond. Finally, the bands between 471-694 cm^-1 were associated with the bending of the O-Si-O bond, while the bands between 536-546 cm^-1 of the clay and spent bleaching earth spectra correspond to the bending of the Si-O-Al bond (33CáceresVI, Sánchez-MolinaJ, Chaparro-GarcíaAL. 2017. Evaluación de arcillas caoliniticas-illiticas provenientes de la formación guayabo del área metropolitana de Cúcuta, Norte de Santander, Colombia. Rev. ION30(1):117-127. 10.18273/revion.v30n1-2017009-36TorresJ, Mejía de GutiérrezR, CastellóR, VizcaynoC. 2011. Análisis comparativo de caolines de diferentes fuentes para la producción de metacaolín. Rev. LatinoAm. Metal. Mater.31(1):35-43. Retrieved fromhttps://ve.scielo.org/scielo.php?script=sci_arttext&pid=S0255-69522011000100006&lng=es&nrm=iso&tlng=es).

3.2. Water absorption, porosity, and apparent density tests

The results of the water absorption, porosity, and apparent density tests are reported in Figure 5. According to the provisions of the NTC 4205-2: 2009 standard, the maximum requirements for immersion in water for 24 hours that fired clay masonry units must meet should be 20%. If the samples exceed this value, the standard suggests determining whether or not the firing temperature was sufficient to form stable ceramic phases through differential thermal analysis tests of the raw material (37ICONTEC, NTC 4205-2.2009. Unidades de mampostería de arcilla cocida. Ladrillos y bloques cerámicos. Parte 2: Mampostería no estructural. 1–10.). As seen in Figure 5, only the 100% clay samples and those with the addition of glass sediment (GS) had an absorption within the parameters established by the standard, unlike the samples with the incorporation of spent bleaching earth. This is associated with the high content of organic matter present in the spent bleaching earth (>34% by weight), where it was expected that during the firing process, due to the combustion of the oil present in the samples, there would be an increase in the porosity of the ceramic pastes as the percentage of addition of the residue in the mixture increased (27MonteiroSN, VieiraCMF. 2014. On the production of fired clay bricks from waste materials: A critical update. Constr. Build. Mater.68:599-610. 10.1016/j.conbuildmat.2014.07.006). The lower water absorption of bricks with glass sediment is the result of a greater amount of local liquid phase at high temperatures, which contributes to an increase in the density of the fired brick and a decrease in the volume of open and closed pores (16Chagas LicurgoJS, Fontes VieiraCM, CandidoVS, MonteiroSN. 2015. Improvement of clay ceramic properties by glass polishing sludge incorporation. Mater. Sci. Forum.820:432437. 10.4028/www.scientific.net/MSF.820.432, 18SilvaRV, de BritoJ, LyeCQ, DhirRK. 2017. The role of glass waste in the production of ceramic-based products and other applications: A review. J. Clean. Prod.167:346-364. 10.1016/j.jclepro.2017.08.185, 38SinghA, ChandelMK. 2022. Valorization of fine fraction from legacy waste as fired bricks: A step towards circular economy. J. Clean. Prod.331:129928. 10.1016/j.jclepro.2021.129918).

Figure 5 Apparent density (black), porosity (red) and water absorption (blue) results. 

The degree of porosity of the ceramic bricks depends on the nature, properties, and processing of the initial clay. The highly porous materials can produce lower mechanical performance, and higher water absorption, although the thermal insulation is improved (18SilvaRV, de BritoJ, LyeCQ, DhirRK. 2017. The role of glass waste in the production of ceramic-based products and other applications: A review. J. Clean. Prod.167:346-364. 10.1016/j.jclepro.2017.08.185, 25Eliche-QuesadaD. Corpas-IglesiasFA, Pérez-VillarejoL, Iglesias-GodinoFJ. 2012. Recycling of sawdust, spent earth from oil filtration, compost and marble residues for brick manufacturing. Constr. Build. Mater.34:275-284. 10.1016/j.conbuildmat.2012.02.079, 39WangS, GaineyL, MackinnonIDR, AllenC, GuY, XiY. 2023. Thermal behaviors of clay minerals as key components and additives for fired brick properties: A review. J. Build. Eng.66:105802. 10.1016/j.jobe.2022.105802). In this case, the organic nature of the spent bleaching earth, in addition to contributing to the ceramic firing process by saving energy in manufacturing and fuel consumption, can give rise to a more porous microstructure, which can contribute to improving the thermal insulation capacity of the final product.

3.3. Compressive strength

Compressive strength is a crucial quality for bricks to be of use in the construction sector, as these materials primarily experience compressive loads; in contrast, flexural strength and tensile strength are typically assessed in porcelain samples (38SinghA, ChandelMK. 2022. Valorization of fine fraction from legacy waste as fired bricks: A step towards circular economy. J. Clean. Prod.331:129928. 10.1016/j.jclepro.2021.129918, 39WangS, GaineyL, MackinnonIDR, AllenC, GuY, XiY. 2023. Thermal behaviors of clay minerals as key components and additives for fired brick properties: A review. J. Build. Eng.66:105802. 10.1016/j.jobe.2022.105802). As established in NTC 4205-2 (ICONTEC, 2009), the minimum compressive strength that non-structural masonry units must meet is 10 MPa per unit, and in the case of structural masonry according to NTC 4205-1: 2009, the minimum suggested strength is 15MPa. Considering what is reported in Figure 6, the fired clay samples and the clay mixtures with the incorporation of 10% of spent bleaching earths (CSBE1 and CSBC1), exceed the values established for structural masonry units, while the CSBE3 sample that has an incorporation of 30% of lime-free spent bleaching earth exceeds the value established only for non-structural masonry units. In general, the incorporation of spent bleaching earth reduces the compressive strength, compared to the standard fired clay sample, which may be associated with the increase in porosity, coinciding with what was reported by Singh and Chandel (38SinghA, ChandelMK. 2022. Valorization of fine fraction from legacy waste as fired bricks: A step towards circular economy. J. Clean. Prod.331:129928. 10.1016/j.jclepro.2021.129918) who associate the decrease of the compressive strength to the firing of the organic materials present in the mixtures that give rise to porous structures.

Figure 6 Compressive strength results. 

Noteworthy is the best performance achieved by the samples with the incorporation of glass sediment in percentages of 20% (CGS2), showing compressive strengths greater than 29 MPa, exceeding 93% of the minimum value established in NTC 4205 for structural masonry. This coincides with what was reported by Mohammad et al. (4M.Mohammad et al.2022. Physical and mechanical properties of fired industrial waste-clay bricks from clam shells and soda lime silica glass. Mater. Today Proc.75(1):151-155. 10.1016/j.matpr.2022.10.275) and is attributed to the fact that the incorporation of glass reduces the distance between particles, leading to greater cohesion of the developed composite. The mix with the incorporation of 10% of the SBC and 20% of GS (CSBC1GS2) increases the compressive strength and even exceeds the values obtained in the reference sample (C) by up to 19%. This increase could possibly be associated with the formation of crystalline phases between the calcium of the SBC and the glass.

3.4. Thermal properties

Figure 7 reports the results of thermal conductivity, thermal diffusivity, and specific heat of the eco-products. The average thermal conductivity of the samples with 100% clay content (C) was 0.55 W/mK. With the increase in the percentage of addition of SBE, a trend is evident in the decrease of up to 35% in the thermal conductivity of these samples compared to the control sample (C). This trend is also observed in the properties of thermal diffusivity. The incorporation of glass sediment (CGS2), for its part, increases the thermal conductivity values (0.97 W/mK) by 77%. This coincides with what was reported by Xin et al. (19XinY, RobertD, MohajeraniA, TranP, PramanikBK. 2023. Transformation of waste-contaminated glass dust in sustainable fired clay bricks. Case Stud. Constr. Mater.18:e01717. 10.1016/j.cscm.2022.e01717) who found that the incorporation of glass waste in percentages of 5, 10, and 15% in the bricks increased by 46%, 50%, and 56% respectively, with the increase in the addition percentage, compared to the average thermal conductivity of the brick (0.63 W/mK). When the glass sediment is incorporated with spent bleaching earth with lime (CSBC1GS2), the value obtained for conductivity (0.75 W/mK) is 36% higher compared to the fired clay sample, but there is a decrease of up to 29% compared to the sample with the incorporation of only glass sediment (CGS2). The increase is also observed in the specific heat for the samples with the incorporation of glass sediment, although the incorporation of SBE in the CSBC1GS2 sample decreases the thermal diffusivity concerning sample C.

Figure 7 Thermal properties of the products. 

The increase in conductivity is mainly associated with the increase in apparent density, which in turn leads to a decrease in the total porosity of the samples as observed in the first graph of conductivity versus pore volume. In the particular case of bricks with the incorporation of glass waste, the formation of a glassy phase helps to reduce porosity and therefore there is an increase in density and thermal conductivity (19XinY, RobertD, MohajeraniA, TranP, PramanikBK. 2023. Transformation of waste-contaminated glass dust in sustainable fired clay bricks. Case Stud. Constr. Mater.18:e01717. 10.1016/j.cscm.2022.e01717). It is important to highlight that the thermal conductivity of brick products is influenced by the characteristics of the pore system, the mineralogical composition, and the microstructure because certain crystalline phases demonstrate higher thermal conductivity compared to some amorphous or glassy phases (1Eliche-QuesadaD, Corpas-IglesiasFA. 2014. Utilisation of spent filtration earth or spent bleaching earth from the oil refinery industry in clay products. Ceram. Int.40(10, B):16677–16687. 10.1016/j.ceramint.2014.08.030, 39WangS, GaineyL, MackinnonIDR, AllenC, GuY, XiY. 2023. Thermal behaviors of clay minerals as key components and additives for fired brick properties: A review. J. Build. Eng.66:105802. 10.1016/j.jobe.2022.105802).

In contrast to the physical and mechanical properties, the thermal conductivity and diffusivity of fired clay bricks have traditionally garnered minimal attention, but, with the rising global emphasis on energy saving, these thermal properties have gained increasing recognition in recent years (39WangS, GaineyL, MackinnonIDR, AllenC, GuY, XiY. 2023. Thermal behaviors of clay minerals as key components and additives for fired brick properties: A review. J. Build. Eng.66:105802. 10.1016/j.jobe.2022.105802). Therefore, reducing the thermal conductivity of bricks is pivotal for realizing potential energy savings in buildings, as its improvement facilitates the efficient storage and absorption of heat, resulting in cooler interiors during summer and warmer spaces in winter. Consequently, occupants benefit from reduced energy consumption for air conditioning (19XinY, RobertD, MohajeraniA, TranP, PramanikBK. 2023. Transformation of waste-contaminated glass dust in sustainable fired clay bricks. Case Stud. Constr. Mater.18:e01717. 10.1016/j.cscm.2022.e01717). One of the most effective ways to reduce the thermal conductivity in traditional ceramics is by increasing their porosity, which is possible to achieve by incorporating different pore-forming organic fuels. However, one of the resulting disadvantages is a deterioration in the mechanical properties (1Eliche-QuesadaD, Corpas-IglesiasFA. 2014. Utilisation of spent filtration earth or spent bleaching earth from the oil refinery industry in clay products. Ceram. Int.40(10, B):16677–16687. 10.1016/j.ceramint.2014.08.030). The thermal conductivity of the standard brick (C), and those that had an incorporation of up to 30% of SBE, as reported by (19XinY, RobertD, MohajeraniA, TranP, PramanikBK. 2023. Transformation of waste-contaminated glass dust in sustainable fired clay bricks. Case Stud. Constr. Mater.18:e01717. 10.1016/j.cscm.2022.e01717), are within the range established for bricks of fired clay: 0.4 W/mK to 0.7 W/mK.

With the use of waste such as SBE, not only a considerable decrease in thermal conductivity would be achieved, but also the valorization of waste for the manufacture of environmentally friendly ceramic products with a decrease in their manufacturing costs. Another important factor in using waste materials for manufacturing fired clay masonry elements is specific heat, since due to the high organic matter content in SBE waste releases a substantial amount of heat during combustion, significantly aiding the ceramic firing process (27MonteiroSN, VieiraCMF. 2014. On the production of fired clay bricks from waste materials: A critical update. Constr. Build. Mater.68:599-610. 10.1016/j.conbuildmat.2014.07.006).

3.5. Development of an eco-product prototype

The CSBC1GS2 mix had the highest compressive strengths and lowest water absorption, complying with the standards established in the Colombian regulations for fired clay masonry. The good mechanical behavior was associated with the contribution of the glass sediment in the densification of the clay matrix, in addition to the reinforcing action of this residue as particulate material in the matrix. It was therefore selected as the optimal composition for the manufacture of a larger-scale eco-product prototype in the form of a decorative brick (facade). The manufacturing process was similar to that used for the 20x20x20 mm experimental units. The final product is seen in Figure 8. Its properties are: Compressive strength 37.59 MPa, Absorption 12.82%, Thermal conductivity 0.75 W/ mK.

Figure 8 Prototype obtained with CSBC1GS2.