The use of sludge as an additive to construction and building materials such as building bricks, lightweight artificial aggregates, and cement-like materials is a win–win strategy because it converts the waste into useful materials and helps reduce the waste disposal problem (Weng, Lin, & Chiang, 2003). The benefits of using sludge as an additive in the fired clay matrix for making brick and tile have been studied by several authors (Jordán, Almendro-Candel, Romero, & Rincón, 2005; Merino, Arévalo, & Romero, 2007; Monteiro, Alexandre, Margem, Sánchez, & Vieria, 2008; Montero, Jordan, Hernandez-Crespo, & Sanfeliu, 2009). These authors agreed that the incorporation of sludge adversely affects the mechanical strength of brick, and thus the right amount of sludge needs to be used in order to meet the relevant standards applied to specific construction materials (Eliche-Quesada et al., 2011). Liew et al. (2004) studied the incorporation of sewage sludge as pore former into clay brick. The physical and mechanical properties of bricks containing 10–40 wt% of dried sludge generally complied with the technical Malaysian Standard MS 7.6:1972 for general wall construction. As shown in Table 6.1, the high amount of 10–40% sludge in clay brick adversely affected the drying shrinkage, but the firing shrinkage was decreased. The water absorption increased to 37% compared to 23.6% of the control brick and the compressive strength decreased to 2 MPa compared to 15.8 MPa of the control brick. The decrease in compressive strength and increase in water absorption with increasing sludge content are due to the increase in pores, gradual pore coarsening, and formation of crack. Thus, bricks with high sludge content have poor surface finishing and may not be suitable for use as facing brick. The incorporation of 10% sludge seems to produce bricks with low shrinkage and water absorption with relatively high compressive strength of 8.9 MPa.

Basegio, Berutti, Bernades, & Bergmann (2002) investigated the utilization of tannery sludge as a raw material for clay products. The results showed that the compressive strength of brick increased with increasing firing temperature but decreased with increasing amount of sludge. The maximum compressive strength of brick with 0–10% sludge addition was 25 MPa at firing temperature of 1180 °C. The same authors suggested that the incorporation of 10% tannery sludge was an appropriate amount to be used, considering the environmental aspects and the minimum requirement for the building industry. Monteiro et al. (2008) used 0, 3, 5, and 10 wt% of sludge from water treatment plants for making red ceramic with firing temperatures of 700, 900, and 1000 °C. The results indicated that the incorporation of up to 10% sludge into clay slightly increases the water absorption and reduces the mechanical strength of the fired ceramic. This is caused by the change in porosity with the high weight loss during the firing stage. The incorporation of sludge into the clay body should be done with care and a small percentage should be used in order not to interrupt the ceramic processing. Rouf & Hossain (2003) used iron and arsenic sludge at dosage levels of 5%, 15%, 25%, and 50% in clay bricks with firing temperatures of 950, 1000, and 1050 °C. The results indicated that 15% by mass is the optimum amount of sludge with firing temperature of 1000 °C. However, the strength of brick can be improved to a value comparable to that of normal clay brick with a slightly higher firing temperature of 1050 °C. Hassan, Fukushi, Turikuzzaman, & Moniruzzaman (2014) studied the use of 3%, 6%, 9%, and 12% arsenic–iron sludge in brick making. Bricks were determined on the basis of laboratory tests (ASTM C67). The durability of clay bricks is largely dependent upon their water absorption. The water absorptions of control clay bricks were between 10.0% and 11.2% and increased to 15.2–19.6% for the prepared clay bricks with sludge ranges of 3–6%. The compressive strength of 3%, 6%, 9%, and 12% sludge bricks were 14.1, 15.1, 9.4, and 7.1 MPa, respectively. The 6% sludge brick met the AASHTO standard's strength requirement of 14.7 MPa for a first-class brick. Moreover, the water absorption capacity of the prepared bricks increased with the gradual increase in sludge content. The optimum amount of clay sludge of 6% by weight is therefore recommended. García, Quesada, Villarejo, Iglesias-Godino, & Corpas-Iglesias (2012) studied the effect of sludge from Jaen (south Spain) on the properties and microstructure of fired clay brick manufacturing. Sludge content in the clay mixture varied from 1% to 15% of dry weight and the bricks were fired at 950 °C. Water absorption was determined in accordance with the standard UNE 67-027. Compressive strength was measured for fired samples in accordance with the standard UNE 67-026. The incorporation of 1–15 wt% sludge resulted in an increase in water absorption from 22.7% to 27.9%. They observed that there was a limit in the incorporation of sludge due to an increase in water absorption and a decrease in mechanical strength as a result of the increase in porosity caused by the decrease in bulk density (Figure 6.2). Results indicated that incorporating up to 5 wt% sludge is beneficial for clay bricks.

Weng et al. (2003) studied the utilization of sludge collected from an industrial wastewater treatment plant. The results indicated that the amount of sludge and firing temperature are the two key factors determining the quality of brick, as shown in Figure 6.3. With firing temperature of 1000 °C, first-class brick is obtained with the use of up to 25% sludge and second-class brick is obtained with up to 33% sludge. For a firing temperature of 960 °C, first-class brick is obtained with the use of up to 12% sludge and second-class brick is obtained with up to 20% sludge. It was suggested that the waste offered economic benefits while maintaining the properties of the manufacturing clay brick (Zani, Tenaglia, & Panigada, 1990).

Baskar, Meera Sheriffa Begum, & Sundaram (2006) studied the production of fired clay bricks with 0–30% of sludge from textile water treatment plants. The fired clay bricks were tested to evaluate their shrinkage, weight loss, and compressive strength. The results indicated that the increase in amount of waste results in a loss in compressive strength. The maximum amount of sludge that can be added is in the range of 6–9% to produce brick with acceptable compressive strength between 3.5 and 4.2 MPa satisfying the Bureau of Indian Standards.

Jahagirdar, Shrihari, & Manu (2013) also studied the use of textile sludge in making burnt clay bricks and reported a slightly higher amount of sludge addition of 15% to produce brick with compressive strength greater than 3.5 MPa (tested in accordance with IS 3495 (Part-I)-1992). Durability of the bricks can be judged by water absorption test (IS 3495 (Part-II)-1992). The density, compressive strength, and ringing sound of bricks reduce, whereas water absorption and efflorescence increase, as the sludge content in brick increases. High firing temperature and firing period (i.e., 800 °C and 24 h) give good results in terms of compressive strength.

Herek et al. (2012) used textile laundry wastewater sludge to produce bricks and reported that sludge can be incorporated up to 20% (by mass) in producing bricks with suitable mechanical properties. The incorporation of more than 20% sludge results in significant reduction in the flexural rupture strength of brick, as shown in Figure 6.4. The incorporation of sludge decreased the flexural strength of brick, but it is still higher than the 1.5 MPa limit specified by the Brazilian standard. An applied leaching test and a solubilization test were also performed on the produced bricks and indicated that they are sufficiently safe and inert. The scanning electron microscope images of the ceramic bricks with and without sludge incorporation are shown in Figure 6.5. The sample with sludge shows increases in pore sizes that are in line with the results of the water absorption test.

The incorporation of waste from marble industry in the matrix of fired clay brick has been studied by several authors and proved to be very efficient for improving the brick’s mechanical properties (Balint & Mattyasovszky-Zsolnay, 1982; Bozadgiev, 1996; Freire & Mota, 1997; Saboya et al., 2007; Vincenzini & Fiori, 1976). Saboya et al. (2007) showed that the incorporation of 15–20% marble waste is appropriate for the manufacturing of ceramic brick. As water absorption is one of the most critical properties for ceramic brick, the incorporation of 20% or higher content of marble waste produced bricks with higher water absorption than that allowed for construction purposes. The marble residue is also used in conjunction with recycled sawdust, spent earth from oil filtration, and compost for brick manufacturing (Eliche-Quesada et al., 2012). The optimum sintering temperature was shown to be 1050 °C. At a low firing temperature of 950 °C, the bricks contained a large amount of open pores and this adversely affected the compressive strength of the bricks. Based on the results obtained, the same authors suggested the optimum proportion of 5 wt% sawdust, 10 wt% compost, and 15 wt% spent earth from oil filtration and 70% marble waste.

Demir et al. (2005) showed that kraft pulp fiber residue could be used as pore former in fired brick. The thickness of the cellulose fiber is around 5–20 μm with a small amount of inorganic content deposited at its surfaces. The main chemical components are 52% SiO₂, 21% A₂O₃, 9% Fe₂O₃, and 9% loss on ignition (LOI). Up to 5% pulp residue could be effectively used as pore former in producing clay brick at a firing temperature of 900 °C. The increase in the dosage beyond this level was not effective in decreasing the bulk density of the clay body. The compressive strength of the samples decreased with the increase in amount of residue added. It has also been shown that the residues can be easily utilized as pore-forming additives into bricks bodies to facilitate production of vertically perforated insulation bricks.

Muñoz, Juárez, Morales, & Mendívil (2013) studied the use of paper pulp as a lightening additive and its influence on the thermal and mechanical properties of fired clay brick. The results indicated that the use of paper pulp increases the porosity of bricks and has a more significant impact in terms of reducing compressive strength than on the thermal conductivity. The incorporation of 15% paper pulp has improved the thermal conductivity at 10 °C by 39.7% compared to a conductivity value of clay brick without additive of 0.45 W/m K. The thermal conductivity behavior is not linear and decreases appreciably only with addition of more than 5% paper pulp.

Porous and lightweight anorthite ceramics from the mixtures of fired clay and paper waste with sawdust addition were also studied and successfully produced (Sutcu & Akkurt, 2010). Recycled paper processing residues (PPR) and clay of different sources for production of porous anorthite ceramics were fired at 1100–1300 °C and PPR contents of 20–50 wt%. Results from this study demonstrated that clay mixture with 30–40 wt% of PPR fired at 1200–1400 °C contained anorthite as a major phase and some minor secondary phases such as mullite or gehlenite phase in the mixtures. The compressive strength of the samples ranged from 8 to 43 MPa. It was claimed that upon completion of this study, porous thermally insulating anorthite ceramics were successfully produced (Sutcu, Akkurt, Bayramb, & Uluca, 2012). An anorthite refractory insulating brick was produced from clay, recycled paper waste, and sawdust with firing temperature of 1200 °C. Pores in the brick were created from the burning of cellulose fiber and the decomposition of calcium carbonate. Depending on the porosity of bricks, the bulk densities ranged from 1.12 to 0.64 g/cm³, and the thermal conductivities varied from 0.25 to 0.13 W/m K. The same authors also indicated that the strengths were sufficiently high for use as an insulating fired brick. In conclusion, paper residues can potentially be used in the production of lighter and economical new brick material and in this way it can be utilized in an environmentally safe way (Demir et al., 2005; Raut, Sedmake, Dhunde, Ralegaonkar, & Mandavgane, 2012).

The other waste from paper industry contains high amounts of CaO and can also be used in making fired clay porous brick with reduced thermal conductivity (Sutcu & Akkurt, 2009). Inorganic content of this residue is largely calcite. The ingredients were powdered, mixed, and hydraulic pressed under a pressure of 10 MPa, and then fired at 1100 °C. The results indicated the normal trend that the increase in the residue addition resulted in the increase in porosity content and the decrease in the compressive strength of samples, but compressive strength was still higher than the strength values required by standard. It has also been shown that the thermal conductivity values of the produced porous brick (<0.4 W/m K) showed more than 50% reduction compared to local brick of the same composition (0.8 W/m K). Preliminary trials were successfully conducted on industrial-scale-product bricks with this high CaO waste from paper industry.

Figure 6.9 shows that the total porosity of fired clay brick increased with the increasing dosage of rice husk and decreased with the increasing sintering temperature. With 1100 °C sintering temperature, the total porosity of brick increased from 5% to 38% with a corresponding increase in rice husk from 0% to 20%. As sintering progresses, pores become rounded and smaller than those obtained with no rice husk addition. The pores are completely isolated from the surface at the end of sintering, and then closed. Sintering is accelerated at firing temperatures over 950 °C because of the vitreous phase formations. This phase penetrates into pores and closes them, and then separates them from neighboring pores (Milheiro, Ferire, Silva, & Holanda, 2005). The mechanism can explain the reduction in water absorption with increasing sintering temperature.

Görhan & Şimşek (2013) investigated the effects of rice husk addition on the porosity and thermal conductivity properties of fired clay bricks. The dosages of 0–15% rice husk and firing temperature 700–1000 °C were employed. Figure 6.10 shows the variation of water absorption rate for the firing temperatures of 700, 800, 900, and 1000 °C. The effect of rice husk content on water absorption is more significant than the effect of firing temperature. For the BG series, the incorporation of 0%, 5%, 10%, and 15% rice husk resulted in samples with water absorption rates in the order of 15%, 24%, 27%, and 32%. The rate of water absorption for the BC series also increased in a similar fashion with the increase in rice husk content. The water absorption for bricks with 10% rice husk is slightly higher than 25%, the boundary value for water absorption. The incorporation of 5% and 10% of rice husk resulted in clay bricks with slightly lower compressive strengths of 7–10 MPa, which are lower than the reference clay bricks but nevertheless satisfy the requirements of TS EN 772-1 for disaster regulation for a building material to be used in indoor structural applications. The use of 10% of rice husk is the optimum composition for production of clay bricks. Rice husk could thus be used as a good pore former additive in clay body.

Souza, Teixeira, Santos, Costa, & Longo (2011) indicate that the SCBA is rich in crystalline silica and thus is possible for use as ceramic raw material. Also, with a sufficient amount of amorphous silica and with proper grinding, it has been used as a pozzolan in cement, mortar, and concrete mixtures (Frías & Villar-Cocina, 2007; Ganesan et al., 2007; Payá, Monzó, Borrachero, Diaz-Pinzón, & Ordónez, 2002). The use of SCBA waste in clay ceramics has also been recently suggested (Borlini et al., 2006; Teixeira, Souza, Santos, & Peńa, 2008; Vieira, Soares, Sánchez, & Monteiro, 2004). Faria, Gurgel, & Holanda (2012) reported that the incorporation of SCBA into clay bricks resulted in an increase in water absorption and a decrease in strength of the fired clay brick. However, with a sufficiently high firing temperature of 1000 °C, the water absorption for clay bricks still conforms to the specifications. The same authors recommended the use of SCBA up to 10 wt% into clay brick body to limit the adverse effect on the strength of fired bricks, thus ensuring the safe and sustainable use of SCBA in the brick industry.

Souza et al. (2011) use SCBA as an additive in ceramic materials. The raw material used in roof tile production was mixed with 0%, 20%, 40%, and 60% ash by weight. The samples were fired at 800, 900, 1000, 1100, and 1200 °C. The incorporation of 0–60% SCBA in the samples decreased the water absorption. Water absorption did not change much at the firing temperatures below 1000 °C, but was pronounced at 1100 °C and 1200 °C. The formation of the liquid phase resulted in a decrease in the open pores at the high firing temperatures. The addition of SCBA also decreased the flexural strength; however, the flexural strength of samples increased with increasing firing temperature due to a decrease in porosity and an increase in bulk density with an increasing temperature. The result showed that the 20–60% weight of SCBA can be incorporated in clay bricks and roof tiles at firing temperatures up to 1000 °C. Conclusively, the results revealed that in terms of environmental protection, waste management practices, and saving of raw materials, SCBA can be regarded as a potential raw material as a pore former in the manufacturing of clay bricks with improved thermal conductivity.

Kadir & Maasom (2013) studied the use of sugarcane bagasse (SB) in fired clay bricks. Fired clay bricks were tested for thermal conductivity and compressive strength. The densities of 1%, 2%, and 3% of SB bricks decreased compared to control brick to 1790, 1640, and 1520 kg/m³ with corresponding decrease in the thermal conductivities to 0.0117, 0.0111, and 0.0107 W/m K and compressive strengths to 22.8, 14.2, and 5.8 MPa. The strength significantly reduces with the increase in the content of the bagasse ash, but with the 3% addition the strength is still sufficient.

Okunade (2008) used wood ash in conjunction with sawdust as admixtures in laterite clay bricks. Sawdust (for burning out) and wood ash admixtures at a ratio of 70:30 by weight was incorporated into laterite clay. The testing was done in accordance with ASTM C67. The admixtures were added in various combinations of proportions by volume (from 0% to 10%). The results indicated that the dry and wet compressive strength of bricks made from the control mix were 18.40 and 15.20 MPa, respectively. The average water absorption of bricks made from the control mix was 13.0–15.0%. Conclusively, the results revealed that wood ash on its own would also result in production of lightweight and more porous products.

Phonphuak & Thiansem (2011) tested the physical and mechanical properties of fired clay briquettes containing 0%, 2.5%, 5%, 7.5%, and 10% of charcoal by weight. Charcoal particles of 2–3 mm (size 1), 1–2 mm (size 2), and less than 0.5 mm (size 3) and a firing temperature of 950 °C were used. The charcoal additives in the specimens were burnt out through the process of firing, leaving abundant pores in the clay bricks. The water absorption of the briquette specimens was in the range of 18–40% and was directly proportional to apparent porosity (Table 6.2). The highest apparent porosity was 53% (10% of charcoal size 1) and the lowest was 31% (2.5% of charcoal size 3), suggesting that the high percentage of charcoal in the specimens caused an increase in porosity. In addition, compressive strength of clay briquette specimens decreased with an increase in the amount of charcoal (Table 6.2).

Figure 6.13(a)–(d) shows the surface texture of fired clay specimens with charcoal. The fired briquette specimens with the addition of fine charcoal exhibited the fine pore structure and those with coarser-size charcoal exhibited coarser pores; the brick with fine pores also showed a low water absorption capacity. Charcoal is thus regarded as an appropriate additive to raw materials used in producing lightweight fired clay bricks.

The effect of firing temperature of fired clay brick with addition of charcoal was also studied by Phonphuak & Thiansem (2012). In order to determine the extent of the pore-forming effects of charcoal, additive was added into raw brick clay and divided into five different batches of specimens mixed with five different percentages of charcoal additives: 0%, 2.5%, 5.0%, 7.5%, and 10% by weight. Specimens were fired at four different temperatures: 900, 950, 1000, and 1100 °C. The water absorption was found to increase with the increase in the amount of charcoal, as shown in Table 6.3. The highest porosity was 48.0% with 10% charcoal addition, and the lowest was 20.3% with 2.5% charcoal addition. The strengths of the fired briquettes were reduced with the increase in the charcoal content due to the increased porosity. However, the strength was also found to increase with firing temperature. The increase in firing temperature from 900 to 950 °C substantially increases the strength of briquettes.

The authors suggested that the briquettes consisting of 2.5% charcoal with sizes less than 0.5 mm fired at 950 °C are desirable in terms of mechanical and physical properties. The briquettes were more porous and stronger compared to the commercial bricks.