183 Journal of Sustainable Architecture and Civil Engineering 2022/1/30 Porous ceramics as sustainable material can become and, in some industries, already are used to achieve environmental applications considering their properties. Research on important composition properties helps to identify the best product to be used in construction industry with a priority of a minimum number of raw materials and simplest and energy-efficient technology during production process. The elaborated research identifies compositions with glass waste, clay, and soot with minimum volume density within the range of chosen product line. Water absorption, thermal conductivity, and compressive strength were researched to identify possible applications of elaborated material in construction industry. Keywords: porous ceramics, glass, clay, waste materials. Porous Ceramics Produced From Glass and Clay *Corresponding author: aleksandrs.korjakins@rtu.lv Porous Ceramics Produced From Glass and Clay Received 2020/09/05 Accepted after revision 2021/09/26 Journal of Sustainable Architecture and Civil Engineering Vol. 1 / No. 30 / 2022 pp. 183-191 DOI 10.5755/j01.sace.30.1.27604 JSACE 1/30 http://dx.doi.org/10.5755/j01.sace.30.1.27604 Pavels Tihomirovs, Aleksandrs Korjakins Riga Technical University, Department of Building Materials and Products, Institute of Materials and Structures, Faculty of Civil Engineering, Kalku street 1, Riga Abstract Introduction Today, improving energy and environmental performance is a key goal, of the construction indus- try and it is strategically important for achieving sustainability of construction materials. The de- velopment of new ecological insulation materials produced from municipal waste, ground glass waste, combustion waste, and raw materials is a topical problem in Latvia. The effective properties of porous ceramics are depending on the microstructure as the result of fabrication technology. Exploration of the relation between properties of material and microstruc- ture can be used to quantify the influence of microstructure on properties of porous ceramic. One of the basic and important microstructural information is the pore volume fraction (porosity). A very important property of porous materials is their mechanical strength, which is mainly due to their composition and structure, preparation and treatment of raw materials as well as the tech- nology of production. The second important property of porous materials is thermal conductivity. These two important properties are being evaluated in the research about porous ceramics. The utilization of waste glass in a porous material for construction industry is an important issue to be researched. “The versatility of the glass-ceramic production process is manifested by the many wastes that have been used as raw materials for glass-ceramics, which include coal fly ash…, mud from zinc hydrometallurgy …, slag from steel production …, ash and slag from waste incinerators …, red mud from alumina production …, waste glass from lamp and other glass products … as well as electric-arc furnace dust and foundry sands …. “. (Rawlings, 2006). Journal of Sustainable Architecture and Civil Engineering 2022/1/30 184 Following raw materials glass waste, clay and a gasifier have been applied for obtaining porous ceramic, and sintering technology was the subject of this research. Virtually all ceramic materials are constructed systems consisting of crystalline, glass, and gas phases (Дятлова, 2012) In general, the temperature of the ceramic sintering process is between 800°C and 1200°C. Fabrication temperature, which is used in the production of foam glass is in the range 790°C and 860°C (Можегов, 2000), and in the range 700°C and 920°C (Rautanen, 2020). The temperature of sintering depends on the gasifier used (Dorokhova, 2017), (Yakub, 2012). According to previously made researches the basic principle of porous material manufacture on a basis of glass waste using sintering methods of powder is to generate a gas in glass at the tem- perature between 700°C and 900°C (Дятлова, 2012), (Можегов, 2000), (Rautanen, 2020), (Dor- okhova, 2017). The gas expands thus forming a structure of cells and producing a porous body. During the process of fabrication from green body to end product the material expands. The aim of the research is to elaborate the optimal composition of porous ceramics based on waste glass as raw materials with minimum number kind of raw materials with additives and low sintering temperature. Materials Compositions has been made according to previous researches and technologies elaborated by other researchers (Dorokhova, 2017), (Chen, 2021),. (Капустинский, 2010), (Погребенский, 2001), (Апкарьян, 2009), (Дамдинова, 2014), (Rincon, 2018). Following raw materials have been taken for preparing mixes for manufacturing porous ceram- ics: waste glass, clay, charcoal, sawdust, soot, and anthracite. Part Ingredient volume from the total raw material volume, % SiO2 45.67 Al2O3 12.27 Fe2O3 5.34 TiO2 0.67 CaO 11.60 CO2 11.22 K2O+Na2O 4.56 Part Ingredient volume from the total raw material volume, % SiO2 from 69 till 74% Na2O from 10 till 16% CaO from 5 till 14% MgO from 0 till 6% Al2O3 from 0 till 3% Other parts from 0 till 5% Table 1 Clay chemical composition Table 2 Glass chemical composition The ability of the clay to expand is very im- portant in the porosity process. Clays with good superficial properties contain illite, montmorillonite, iron oxide, aluminum oxide and calcium compounds, feldspar, dolomite, and quartz. Clays are aluminosilicate mate- rials with high SiO2 (in our case 45.67%) and low Al2O3 (12.27%) content, which participate in the formation of the ceramic partition. The low Al2O3 content helps to reduce the soften- ing temperature to 380°C and the resulting inflation temperature to 530°C. The high con- tent of Fe2O3 (5.34%) and CaO - 11.6% reduc- es the crystallization rate and temperature of SiO2. SiO2 and Fe2O3 affect the viscosity, melt surface tension. The clays from the produc- tion plant of Ltd. “Ceplis” have been used as raw material in the mixes.. Chemical compo- sition of the clay is shown in Table 1. Chem- ical composition of used soot is C (94.84%), H (0.88%), S (0.01%), O (4.25%, other parts (0.02%). Glass waste has been obtained from window glass. The chemical content of this one is shown in Table 2. 185 Journal of Sustainable Architecture and Civil Engineering 2022/1/30 Sawdust obtained from the mixed hardwood and conifers was dried in an oven at 60° C for 48 hours before applying. Glass waste was ground to a powder substance to a powder substance in planetary ball mill Retsch PM400. Glass was sieved through a 500 mkm sieve. Based on the former researches three mixtures have been designed for investigation of the influ- ence of different kinds of raw materials on the properties of porous ceramic. Mixtures content by weight is shown in Table 3. Methods Experiment Nr. Molten glass, % Burning temperature, °C Time of burning, minutes Soot, % Clay, % Water, % X1 X2 X3 1 2 3 4 5 6 2.3.1. experiment 89.500 850 60 0.179 10.321 10.000 2.3.2. experiment 89.500 850 120 0.179 10.321 10.000 2.3.3. experiment 89.500 850 180 0.179 10.321 10.000 2.3.4. experiment 89.500 800 60 0.179 10.321 10.000 2.3.5. experiment 90.320 800 120 0.200 9.480 10.000 2.3.6. experiment 90.320 800 180 0.200 9.480 10.000 2.3.7. experiment 89.500 900 60 0.179 10.321 10.000 2.3.8. experiment 90.320 900 120 0.200 9.480 10.000 2.3.9. experiment 89.500 900 180 0.179 10.321 10.000 2.3.10. experiment 90.000 800 180 0.179 9.821 10.000 2.3.11. experiment 90.000 800 60 0.179 9.821 10.000 2.3.12. experiment 90.000 800 120 0.179 9.821 10.000 2.3.13. experiment 90.000 850 180 0.179 9.821 10.000 2.3.14. experiment 90.000 850 60 0.179 9.821 10.000 2.3.15. experiment 90.000 850 120 0.179 9.821 10.000 2.3.16. experiment 90.000 900 180 0.179 9.821 10.000 2.3.17. experiment 90.000 900 60 0.179 9.821 10.000 2.3.18. experiment 90.000 900 120 0.179 9.821 10.000 2.3.19. experiment 89.000 800 180 0.179 10.821 10.000 2.3.20. experiment 89.000 800 60 0.179 10.821 10.000 2.3.21. experiment 89.000 800 120 0.179 10.821 10.000 2.3.22. experiment 89.000 850 180 0.179 10.821 10.000 2.3.23. experiment 89.000 850 60 0.179 10.821 10.000 2.3.24. experiment 89.000 850 120 0.179 10.821 10.000 2.3.25. experiment 89.000 900 180 0.179 10.821 10.000 2.3.26. experiment 89.000 900 60 0.179 10.821 10.000 2.3.27. experiment 89.000 900 120 0.179 10.821 10.000 Table 3 Research compositions Journal of Sustainable Architecture and Civil Engineering 2022/1/30 186 After weighing the raw materials were mixed by Food mixer 1340 in dry conditions for 10 minutes. Water was added continuously mixing during additional 10 minutes. The mix was molded in cube forms 50 * 50 * 50 mm and plate forms 300 * 300 * 30 mm , preparing samples for conductivity tests. Molded compositions were left curing for 24 hours at room temperature. Dried samples were fired in an oven for sintering. The operating temperature of each gasifier must be 50-70°C higher than the initial temperature of the melting glass powder. The softening temperature and the rate of increase of the partial pressure must be low. There were elaborated nine burning schemes (see Fig. 1…Fig. 9) Fig. 1 Burning scheme with maximum temperature 800°C during 60 minutes Fig. 2 Burning scheme with maximum temperature 800°C during 120 minutes Fig. 1 Burning scheme with maximum temperature 800°C during 60 minutes Fig. 2 Burning scheme with maximum temperature 800°C during 120 minutes Fig. 3 Burning scheme with maximum temperature 800°C during 180 minutes Fig. 4 Burning scheme with maximum temperature 850°C during 60 minutes Fig. 5 Burning scheme with maximum temperature 800°C during 120 minutes Fig. 3 Burning scheme with maximum temperature 800°C during 180 minutes Fig. 4 Burning scheme with maximum temperature 850°C during 60 minutes Fig. 5 Burning scheme with maximum temperature 800°C during 120 minutes Fig. 3 Burning scheme with maximum temperature 800°C during 180 minutes Fig. 4 Burning scheme with maximum temperature 850°C during 60 minutes Fig. 5 Burning scheme with maximum temperature 800°C during 120 minutes Fig. 6 Burning scheme with maximum temperature 850°C during 180 minutes Fig. 7 Burning scheme with maximum temperature 900°C during 60 minutes Fig. 8 Burning scheme with maximum temperature 900°C during 120 minutes Fig. 6 Burning scheme with maximum temperature 850°C during 180 minutes Fig. 7 Burning scheme with maximum temperature 900°C during 60 minutes Fig. 8 Burning scheme with maximum temperature 900°C during 120 minutes Fig. 6 Burning scheme with maximum temperature 850°C during 180 minutes Fig. 7 Burning scheme with maximum temperature 900°C during 60 minutes Fig. 8 Burning scheme with maximum temperature 900°C during 120 minutes Three factor experiment was performed taking into estimation three parameters: X1- ground glass % in the whole composition, X2- burning temperature °C; X3 – time of burning in minutes. After weighing the raw materials were mixed by Food mixer 1340 in dry conditions for 10 minutes. Water was added continuously mixing during additional 10 minutes. The mix was molded in cube forms 50 * 50 * 50 mm and plate forms 300 * 300 * 30 mm , preparing samples for conductivity tests. Molded compositions were left curing for 24 hours at room temperature. Dried samples were fired in an oven for sintering. The operating temperature of each gasifier must be 50-70°C higher than the initial temperature of the melting glass powder. The softening temperature and the rate of increase of the partial pressure must be low. There were elaborated nine burning schemes (see Fig. 1…Fig. 9) Fig. 1 Burning scheme with maximum temperature 800°C during 60 minutes Fig. 2 Burning scheme with maximum temperature 800°C during 120 minutes Fig. 3 Burning scheme with maximum temperature 800°C during 180 minutes Fig. 4 Burning scheme with maximum temperature 850°C during 60 minutes Fig. 5 Burning scheme with maximum temperature 800°C during 120 minutes Fig. 6 Burning scheme with maximum temperature 850°C during 180 minutes Fig. 7 Burning scheme with maximum temperature 900°C during 60 minutes Fig. 8 Burning scheme with maximum temperature 900°C during 120 minutes 1 2 3 4 5 6 7 8 187 Journal of Sustainable Architecture and Civil Engineering 2022/1/30 Fig. 9 Burning scheme with maximum temperature 900°C during 180 minutes After burning finishing and forms cooling, the samples were demolded and prepared for testing. The compression strength tests and thermal conductivity tests have been performed by universal testing system Zwick 100 and FOX600. Density and water absorption have been measured in the framework of research. The porosity of the samples has been evaluated by Microscope Veho DX-3. Two kinds of forms were used for burning the samples: metal for 50*50*50 mm samples (Fig.10-11) and metal for 300*300*30 mm samples. Fig. 9 Burning scheme with maximum temperature 900°C during 180 minutes After burning finishing and forms cooling, the samples were demolded and prepared for testing. The compression strength tests and thermal conductivity tests have been per- formed by universal testing system Zwick 100 and FOX600. Density and water absorption have been measured in the framework of re- search. The porosity of the samples has been evaluated by Microscope Veho DX-3. 9 Fig. 10 Metal form Fig. 11 Burning form made from metal10 11 Two kinds of forms were used for burning the samples: metal for 50*50*50 mm samples (Fig.10- 11) and metal for 300*300*30 mm samples. Foil for 50*50*50 mm samples was used to let samples be taken out. Forms contain holes to evenly heat the samples. Some of the samples after burning can be seen in Fig. 12-14. Fig. 12 Burning temperature 850°C 120 min Fig. 13 Burning temperature 850°C 120 min Fig. 14 Burning temperature 850°C 60 min Fig. 15 Sample under microscope (850°C 120 min.) (sample from the plate form) 12 13 14 15 Journal of Sustainable Architecture and Civil Engineering 2022/1/30 188 Results of water absorption tests of porous ceramic samples are shown in Table 4. Table 4 Water absorption test results Experiment sample Nr. Weight after sintering, g Volume density, kg/m3 Weight after 6 days, g Water absorption, % 3-1 46.00 494.19 77.80 69.13% 3-1-1 31.10 259.60 89.60 188.10% 3-2 32.40 345.97 62.50 92.90% 3-2-1 46.90 358.20 85.80 82.94% 3-3 40.20 378.75 68.80 71.14% 3-3-1 33.30 376.70 59.50 78.68% 3-4 47.60 364.94 77.20 62.18% 3-4-1 37.00 341.44 70.10 89.46% The increase of samples volume and volume density of the ceramic samples were measured. Prepared samples were tested for compressive strength. The results of the experiment are shown in Table 5. Results Table 5 Experiment results Experiment Nr. Volume increase after sintering, times Compressive strength, MPa Volume density, kg/m3 2.3.1. experiment 4.69 1.06 306.00 2.3.2. experiment 4.18 0.45 232.13 2.3.3. experiment 4.68 1.19 329.53 2.3.4. experiment 5.04 0.69 299.70 2.3.5. experiment 4.66 1.31 318.86 2.3.6. experiment 4.26 1.38 296.47 2.3.7. experiment 4.73 0.81 317.94 2.3.8. experiment 4.37 0.70 291.70 2.3.9. experiment 4.32 0.80 271.67 2.3.10. experiment 4.68 1.40 326.59 2.3.11. experiment 4.54 1.25 311.64 2.3.12. experiment 5.05 1.31 295.37 2.3.13. experiment 4.17 0.81 274.37 2.3.14. experiment 4.91 0.84 271.36 2.3.15. experiment 4.42 1.19 264.64 2.3.16. experiment 4.31 1.31 322.08 2.3.17. experiment 5.18 0.74 328.04 2.3.18. experiment 4.84 0.51 285.39 2.3.19. experiment N/A 1.35 291.85 189 Journal of Sustainable Architecture and Civil Engineering 2022/1/30 Volume density of the obtained samples is in the range from 232.13 to 329.64 kg/m3 (Table 5) with minimal value for experiment Nr. 2.3.2. Volume density of the obtained ceramic depends on content of the raw materials– more volume of clay increases the density in the final product. The density depends on the sintering temperature as well. The compressive strength of the obtained samples is in the range from 0.45 MPa to 1.41 MPa depending on the burning temperature and composition (see Table 5). The obtained thermal conductivity for samples from experiment Nr. 2.3.2. is 0.088 W/(m K) with a sample thickness 51 mm. Volume of expanded samples increase from 3,68 till 5,18 times from the initial volume. Measured water absorption of the porous ceramic samples is in the range from 62.18% till 188,10%, which may be evaluated as high. Sizes of the pores are in the range 0.12 -3.40 mm (see Fig. 15). By analysis of obtained experimental results of elaborated compositions by software Statistica the optimal parameters of porous ceramics, such as volume density of 249 kg/m3 can be achieved by temperature 852°C with a sintering period of maximum temperature 124 minutes. Discussion During previous researches according to made experiments, it was found that the range of the melting temperatures is from 850°C to 1200°C depending on the composition of the raw materials. After the specified temperature limit, the solid phase increases, and the gas phase in the material decreases, as a result of which the sample becomes dense and ineffective as insulation porous ceramic material. The process of forming the ceramic structure is created in the following stages of coagulation-> sintering -> pore formation. The coagulation structure is formed by the action of intermolecular forces between the clay particles at the interlayers of the liquid medium, forcing the filler from the substance onto the surface layer during granulation. During drying, a condensation structure forms after the removal of the liquid phase. As a result of heating, the amorphous structure is formed by burning the glass-ceramics with intermediate pores forming a solid uniform system. In the general case, as the volume density increases, the compressive strength also increases. The density and size of the pores depend on the sintering temperature and the amount of the gas- ifier. As the sintering temperature increases, the volume density decreases, the number of pores increases, and the mechanical strength decrease. The strength of a ceramic product is affected by various modified additives that are used to form the structure of the ceramic product. The duration of foaming also affects the pore structure and accordingly also to the compressive strength. At the longer foaming time, the gas pressure in the pores increases, in result the pore’s size increases and makes the pore partitions more fragile, which leads to a decrease in compressive strength. Experiment Nr. Volume increase after sintering, times Compressive strength, MPa Volume density, kg/m3 2.3.20. experiment 5.13 1.20 310.13 2.3.21. experiment 3.99 1.22 320.78 2.3.22. experiment 4.45 0.73 285.24 2.3.23. experiment 4.06 1.41 310.62 2.3.24. experiment 4.26 0.72 255.24 2.3.25. experiment 4.09 0.90 310.17 2.3.26. experiment 4.90 1.11 326.71 2.3.27. experiment 3.68 1.31 329.64 Journal of Sustainable Architecture and Civil Engineering 2022/1/30 190 Achieved thermal conductivity of 0.088 W/(m K), which is similar to the performance of porous construction products in this group of construction materials. Water absorption depends on the shape of the pores, the amount of opened pores and sizes of the pores. The shape and size of the pores depend on the composition, amount of gasifier, tem- perature, and time of burning. To achieve the lowest water absorption results, it is necessary to elaborate compositions with a larger number of closed pores. References Chen Y., Wang N., Ola O., Xia Y., Zhu Y. Porous ce- ramics: Light in weight but heavy in energy and en- vironment technologies. Mater. Sci. Eng. R Reports, vol. 143, p. 65, Jan. 2021, https://doi.org/10.1016/j. mser.2020.100589 Dorokhova E. S., Zhernovaya N. F., Bessmertnyi V. S., Zhernovoi F. E., Tarasova E. E. Control of the Structure of Porous Glass-Ceramic Material. Glas. Ceram., vol. 74, no. 3, pp. 95-98, 2017, https://doi. org/10.1007/s10717-017-9936-x Luckham P. F., Rossi S. Colloidal and rheological properties of bentonite suspensions. Adv. Colloid Interface Sci., vol. 82, no. 1, pp. 43-92, 1999, https:// doi.org/10.1016/S0001-8686(99)00005-6 Monich P. R., Romero A. R., Höllen D., Bernardo E. Porous glass-ceramics from alkali activation and sinter-crystallization of mixtures of waste glass and residues from plasma processing of municipal sol- id waste. J. Clean. Prod., vol. 188, pp. 871-878, Jul. 2018, https://doi.org/10.1016/j.jclepro.2018.03.167 Rautanen J., Jääskeläinen T. Process for producing foam glass. EP 3 656 747 A1, 2020. Rawlings R. D., Wu J. P., Boccaccini A. R. Glass-ceram- ics: their production from wastes. A review. Springer Science and Business Media LLC, 2006; [Online]. Avail- able: https://core.ac.uk/download/pdf/294576.pdf. Rincón A., Desideri D., Bernardo E. Functional glass-ceramic foams from ‚inorganic gel cast- ing‘ and sintering of glass/slag mixtures. J. Clean. Prod., vol. 187, pp. 250-256, 2018, https://doi. org/10.1016/j.jclepro.2018.03.065 Yakub I., Du J., Soboyejo W. O. Mechanical proper- ties, modeling and design of porous clay ceramics. Mater. Sci. Eng., vol. A 558, pp. 21-29, 2012. https:// doi.org/10.1016/j.msea.2012.07.038 Апкарьян А.С., Абакумов Е. П., Христюков В. Г. Способ изготовления пеностеклокерамики [Method of making foam glass ceramics]. РФ 2374191, 2009. Дамдинова Д. Р., Анчилоев Н. Н., Павлов В. Е. Пеностекла системы стеклобой - глина - гидроксид натрия:составы, структура и свойства. Строительные материалы, no. 8, 2014, Accessed: May 05, 2021. [Online]. Available: https://cyberlenin- ka.ru/article/n/penostekla-sistemy-stekloboy-gli- na-gidroksid-natriya-sostavy-struktura-i-svoystva. Дятлова Е. М. Химическая технология керамики и огнеупоров [Chemical technology of ceramics and refractories]. 2012. Капустинский Н. Н., Кетов П. А,. Кетов Ю. А. Способ получения пеностеклянных изделий [Method of obtaining foam glass products]. RU 2453510, 2010. Можегов В. С., Ли Чи-Тай, Суворов С.А., Шевчик А.П., Способ получения пеностекла [Method of ob- taining foam glass]. RU2187473C2, 2000. Погребенский Г. М., Искоренко Г.И., Канев В.П. Спо- соб изготовления гранулированного пеностекла из стеклобоя [Method of manufacturing granulated foam glass from cullet]. РФ 2162825, 2001. Conclusions It was obtained experimentally that the sintering temperature of a porous ceramic material can be reduced to 800-850°C by creating a ceramic composition with a glass content of 89% -90% and using soot as a gas generating agent in amount 0.179-0.200%. By analysing water absorption results (from 62.33% to 188.10%) it can be concluded that all the tested samples have a large number of open pores, therefore the water absorption is high. The obtained thermal conductivity result is 0.088 W/(m K) for composition 850°C during 120 minutes indicates that the obtained tested samples are close to diatomite thermal insulation ma- terials (0.115 - 0.162 W/(m K)). The obtained increase in volume during the sintering process in comparison to the initial volume is from 3 to 5 times, which corresponds to other researches (Rautanen, 2020). 191 Journal of Sustainable Architecture and Civil Engineering 2022/1/30 PAVELS TIHOMIROVS Position at the organization PhD student Main research area Building materials, porous ceramics, recycling of glass. Address Riga Technical University, Department of Building Materials and Products Kalku street 1, Riga LV1658 Tel. +371 67089248 E-mail: mans.pasts@gmail.com About the Authors This article is an Open Access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 (CC BY 4.0) License (http://creativecommons.org/licenses/by/4.0/). ALEKSANDRS KORJAKINS Position at the organization Professor, head of Department of Building Materials and Products Main research area Building materials, porous ceramics, UHPC and light concrete, recycling, insulation materials, ecological materials applying waste from agricultural industry. Address Riga Technical University, Department of Building Materials and Products Kalku street 1, Riga LV1658 Tel. +371 67089248, +371 26422442 E-mail: aleksandrs.korjakins@rtu.lv