3.1. Characterisation of Raw Materials
Table 2 shows the chemical compositions made by XRF to determine the oxide composition of the raw materials. When Table 2 was examined, it was observed that the major oxides in clay are silicon oxide (SiO2), aluminium oxide (Al2O3), and iron oxide (Fe2O3), representing 51.50%, 17.12%, and 9.91%, respectively. Other important oxides present in the analysed clay sample were calcium oxide (CaO) and magnesium oxide (MgO), which represented 9.46% and 6.86% of the sample, respectively. When the loss on ignition (LOI) value of the clay raw material was considered, it occurred at an approximate rate of 5.35%. The main components of the Karacadağ Scoria waste powder sample were silicon oxide (SiO2), iron oxide (Fe2O3), and aluminium oxide (Al2O3), representing 41.83%, 16.760% and 14.873%, respectively. In addition, Karacadağ Scoria waste powder has rich alkaline content such as calcium and magnesium. When the loss on ignition (LOI) value of the raw material of Karacadağ scoria waste powder was considered, it occurred at an approximate rate of 8.28%. Silica with potassium, calcium, and magnesium oxides were the main constituents of the rice husk ash which was analysed and used in this study. In Table 2, SiO2 + Al2O3 + Fe2O3 can be calculated as 82.639%. Loss on ignition (LOI) of rice husk ash was measured as 4.12%. These rates indicate that RHA was categorised as a Type F pozzolan in accordance with the ASTM C618 standard. LOI of 4.12 indicates that the amount of unburned carbon was low, hence high pozzolanic activity could be achieved.
Table 2
Chemical composition of the raw materials (wt.%)
Compounds | Clay(%) | Karacadağ Scoria(%) | Rice Husk Ash(%) |
SiO2 | 51.50 | 41.83 | 79.165 |
Al2O3 | 17.12 | 14.873 | 1.612 |
Fe2O3 | 9.91 | 16.760 | 1.862 |
MgO | 6.86 | 5.768 | 2.253 |
CaO | 9.46 | 9.80 | 6.404 |
Na2O | 1.01 | 3.878 | 0.220 |
K2O | 2.28 | 1.966 | 9.292 |
TiO2 | 0.997 | 3.203 | 0.205 |
MnO | 0.165 | 0.230 | 0.161 |
SO3 | 0.051 | 0.451 | 2.261 |
P2O5 | 0.182 | 0.779 | 2.985 |
LOI | 5.35 | 8.28 | 4.12 |
The particle size distributions of the raw materials were obtained from the SEM images and are shown in Fig. 4 using the origin program. The average particle sizes of clay, Karacadağ scoria waste powder and waste rice husk ash were approximately 72.8, 87.31 and 77.55µm, respectively. Clay and Karacadağ scoria waste powder contained particles as small as 150 µm, while rice husk ash contained particles smaller than 100 µm. It was observed that the rates of fine clay and fine Karacadağ scoria waste dust particles in the mixture of clay and Karacadağ scoria waste powder was higher than the rate of the waste RHA. The clay and Karacadağ scoria waste powder used in this study contain a high amount of microparticles that will provide an effective bond with RHA particles. The rate of clay particles passing through the 150 µm sieve was 53.74%, the rate of Karacadağ scoria waste powder particles was 51.18%, and the rate of RHA particles passing through the 75µm sieve was 48.87%. This indicates that clay, Karacadağ scoria waste powder and RHA samples were composed of fine particles. Fine Karacadağ scoria waste powder and RHA particles move very easily between clay particles. As a result, the mixing degree of Karacadağ scoria waste powder and RHA with clay is high.
Figure 5a-c show the X-ray diffraction (XRD) analysis of clay, Karacadağ Scoria waste powder, and rice husk ash, respectively. According to the XRD analysis, the clay raw material mainly contains the quartz (SiO2), muscovite, albite, zussmanite, and calcite (CaCO3) phases. Karacadağ Scoria waste powder predominantly contains hematite, muscovite, anorthite, cristobalite, diopside and amorphous material from high to low counting values. The organic part of the rice husk is removed during combustion and the residue is rich in terms of silica. The soluble fraction of silica for the waste RHA sample collected from the oven was 83.2% and this shows that SiO2 in waste RHA was more amorphous and reactive in nature. The XRD appearance of RHA (Fig. 5) shows a broad band between 20˚ and 40˚ and a diffraction angle of 2θ, which does not correspond to a defined peak, thus indicating that the material was amorphous and reactive. Quartz peaks were observed as impurities at 20˚72' and 30˚92.
TGA curves of clay, Karacadağ scoria waste dust and RHA are shown in Fig. 6. According to the TGA analysis of the clay, a total weight loss of approximately 15% (red curve) was observed when heated to 1100°C. From this curve, it is understood that 3 basic reactions occurred. The first reaction was the evaporation of physical water up to 100°C, the second was the dehydration of hydroxyl-containing clay compounds between 350–500°C, and the third was the decomposition of the small amount of calcium carbonate observed in the XRD analysis of the clay after 600°C and the removal of CO2 from the structure. According to the TGA curve of Karacadağ scoria shown in the figure, the total weight loss is about 11%. The sample appears to lose only 2 wt.% up to 100°C, which was attributed to the physically absorbed water contained in the slag. According to the XRD analysis, it can be asserted that the muscovite phase in the sample content began to decompose at 760°C and the hydroxyl structure in its structure was decomposed. The weight decrease was observed with the removal of the chemically bound hydroxyl structure from the structure. It can be asserted that the decomposition reaction (weight decrease) in the TGA curve after 760 oC was very close to the thermal properties of an ideal muscovite mineral Wide temperature range for dehydroxylation (760–1000 oC) may be associated with the wide distribution of the thermal response of hydroxyls. According to the TGA analysis of the RHA, a total weight loss of approximately 25.1% was observed when heated to 1150°C. In the DTA curve, there were mainly two major exothermic reactions between 500 and 1000°C, corresponding to the burning of the rice husk.
Figure 7 (a), (b), and (c) show SEM images of the particle morphologies of the raw materials. As can be seen from Fig. 7 (a), the clay raw material had a micron-size stratified particle structure and it was agglomerated. Sub-micron and micron-sized particles were observed in the morphology of agglomerated powders. Micron-sized porous structures with varying diameters are observed when compared to Fig. 7 (b) and (c). Waste RHA was nested and agglomerated.
3.2. Characterisation of brick samples after firing
3.2.1. Physical Properties
In order to determine the effect of the conventional sintering process on the produced samples, it is crucial to perform the characterisation of the physical properties through loss of linear shrinkage (LS) and bulk density (BD) [37]. In order to determine the loss of LS, unidimensional measurements were performed directly on the diameter of the cylindrical samples with the manual calliper. The loss of LS was calculated by determining the pre-sintering diameter of the compacted sample (L0) and the post-sintering diameter of the compacted sample (L1) and using Eq. 1 below. The pre-sintering mass (m) and volume (V) of the produced samples were found and their densities were calculated using Eq. 2 below. The post-sintering densities were then determined according to ASTM C20. The loss of BD was calculated by determining the pre-sintering density of the compacted sample (p0) and the post-sintering density of the compacted sample (p1) and using Eq. 3 below. Figure 8 shows the LS and BD loss of the bricks produced.
$$\text{L}\text{S}=\frac{{L}_{0}-{L}_{1}}{{L}_{0}}x100\%$$
1
ρ = m/V (2)
$${\rho }_{1}=\frac{{\rho }_{0}-{\rho }_{1}}{{\rho }_{0}}x100\%$$
3
LS and loss of BD increased depending on the increase in the amount of RHA and KS in the samples. While linear shrinkage varied between 2.3% and 4.7%, loss of bulk density varied between 21% and 38%. This was caused by the SiO2 crystals in the RHA and KS, and possibly the cristobalite crystal, which leads to higher shrinkage and loss of densification due to the higher RHA, KS content. Also, higher RHA and KS contents result in more gaps, which causes the radius of the sintered samples to have higher linear shrinkage with increasing RHA and KS contents. Denser bricks limit the absorbing capacity, thus reduces water absorption, and results in a better mechanical performance. It was observed that the samples produced were affected by the amounts of alkali activators (NaOH and Na2SiO3). In Fig. 2, the more sintering additives added to each of the produced samples, the better they sintered. Additionally, light clay bricks were obtained by adding fixed rate of KS waste powder and various rates of RHA. Therefore, while the sample C4 was the lightest, the sample A1 was the heaviest sample. Thermal, mechanical, and physical properties of composite structures were affected by the density of the materials. Considering the density of the brick, it was affected by many factors such as the volume, density, mixing rates, firing temperatures of its components. Bulk densities of fired clay bricks are listed in Table 3 and shown in Fig. 9. The comparison of the bulk density of the bricks containing KS waste powder and RHA with the control brick (A0) is shown in percentage on the bars in the figure. Bulk density values vary between 1.29 and 1.91 g/cm3. As the rates of RHA and AA increased with KS waste powder at fixed rate (by weight), a significant decrease was observed in the densities. When the rate of RHA in the samples in the G1 group reached 20 wt.%, their bulk density showed a decrease of 20.9% and a decrease of 4.71% with the addition of 5 wt.% RHA when compared to the control brick. When the amount of RHA in the samples in the G2 group reached 20 wt.%, their bulk density showed a decrease of 24.08% and a decrease of 8.38% with the addition of 5 wt.% RHA when compared to the control brick. When the amount of RHA in the samples in the G3 group reached 20 wt.%, their bulk density showed a decrease of 32.46% and a decrease of 12.57% with the addition of 5 wt.% RHA when compared to the control brick. The fired clay bricks produced have adequate density values (a density less than 2.0 g/cm3) of the relevant standards (ASTM C469, ACI 213 and BS EN 206-1) that are recommended for structural applications. It is beneficial to reduce the bulk density in structural applications. Another point to be considered regarding the results obtained is that the bricks produced had a competitive lightness when compared to the unit weight values of other bricks in the literature (Table 4).
Table 3
The test results of the bricks.
Grup | Code | Clay (%) | RHA (%) | KS (%) | NaOH + Na2O3 (%) | Bulk Density (g/cm3) | Porosity (%) | TC (W/mK) | WA (%) | CS (Mpa) | Loss on Ignition (wt%) |
G1 | A0 | 100% | 0% | 0% | 0% | 1.91 | 30.6 | 1.043 | 12.1 | 32.5 | 5.1 |
A1 | 65% | 5% | 30% | 5%+5% | 1.82 | 35.9 | 0.735 | 16.7 | 18.3 | 6.3 |
A2 | 60% | 10% | 30% | 5%+5% | 1.71 | 38.7 | 0.687 | 20.3 | 13.7 | 7.6 |
A3 | 55% | 15% | 30% | 5%+5% | 1.65 | 41.3 | 0.558 | 24.5 | 11.2 | 8.4 |
A4 | 50% | 20% | 30% | 5%+5% | 1.53 | 43.6 | 0.443 | 26.4 | 9.2 | 9.1 |
G2 | B1 | 65% | 5% | 30% | 10%+10% | 1.75 | 37.5 | 0.602 | 19.7 | 17.4 | 7.4 |
B2 | 60% | 10% | 30% | 10%+10% | 1.62 | 39.8 | 0.514 | 22.2 | 12.8 | 8.2 |
B3 | 55% | 15% | 30% | 10%+10% | 1.51 | 43.4 | 0.421 | 25.8 | 9.7 | 9.3 |
B4 | 50% | 20% | 30% | 10%+10% | 1.45 | 45.2 | 0.356 | 28.3 | 8.1 | 10.2 |
G3 | C1 | 65% | 5% | 30% | 15%+15% | 1.67 | 38.7 | 0.523 | 21.8 | 13.3 | 8.3 |
C2 | 60% | 10% | 30% | 15%+15% | 1.49 | 41.6 | 0.401 | 24.2 | 10.6 | 9.7 |
C3 | 55% | 15% | 30% | 15%+15% | 1.41 | 45.9 | 0.322 | 26.9 | 8.3 | 10.5 |
C4 | 50% | 20% | 30% | 15%+15% | 1.29 | 47.8 | 0.263 | 28.4 | 7.2 | 11.7 |
Table 4
A summary of the literature on clay bricks and their findings in contrast to this study.
Materials Content | Usage Ratio (%) | Bulk Density (g/cm3) | Apparent Porosity(%) | Thermal Conductivity (W/mK) | Water Absorption (%) | Compressive Strength (MPa) | References |
SWW | 5-17.5 | 1.68–1.86 | 56–74 | 0.42–0.54 | 16–20 | 13–27 | [38] |
SWTP/RSA/SBA/WSA | 5–15 | 1.44–1.63 | 22–29 | – | 35–40 | 12–15 | [39] |
SS/FA/OS | 5–10 | 1.62–1.75 | 21–27 | 0.7 (approx.) | 11–13 | – | [40] |
SD | 2.5–10 | 1.41–1.83 | 22–32 | 0.47 − 0.22 | 12–22 | 4–18 | [41] |
TW/SD/WS | 2–4/2–6/1–2 | 1.36–1.68 | 29–37 | 0.38–0.48 | 17–24 | 6–13 | [42] |
BA | 5–20 | 1.59–1.62 | 31–35 | – | 20 (approx.) | 7–46 | [43] |
HCWPAN/WCC/YCC | 10–40 | 1.61–2.32 | 15–33 | 0.155–0.34 | 8.9–18.2 | 2–4 | [44] |
SW/SS/WA/BA | 5–20 | 1.43–1.73 | 15–27 | – | 12–17 | 3–11 | [45] |
WMS | 5–25 | – | 17–42 | 0.47–0.56 | 15–23 | 4–9 | [46] |
MCW | 5–25 | 1.92–1.97 | 23–32 | 0.77-1.00 | 11–13 | 7–28 | [47] |
OPBA | 5–20 | 1.43–1.52 | 18–34 | 0.33–0.41 | 16–28 | 9–11 | [48] |
VS | 5-11-17 | 1.12–1.68 | 28–41 | 0.28–0.738 | 16–36 | 1.5–38 | [49] |
CC/WG | 10/5–20 | 1.44–2.18 | 16–34 | 0.57–0.81 | 10–20 | 5–19 | [50] |
HP/MP | 2.5–15 | 1.42–1.91 | 38–41 | 0.29–0.37 | 14–33 | 3–21 | [51] |
KS/RHA | 30/5–20 | 1.29–1.91 | 30.6–47.8 | 0.263–0.735 | 12.1–28.4 | 7.2–32.5 | This Study |
Bulk densities (ρb) of the fired clay bricks were determined according to Eq. 3. At first, the dried weight (W1) of the samples, then the soaked weight (W2) by allowing to wait in water for 3 hours, then the weight suspended in water (W3) using a spring suspension balance were calculated. In Eq. 3, ρw is the density of water (g/cm3).
ρ b = W1 × ρw / W2 − W3 (3)
The apparent pores (P) of the produced samples were determined according to Eq. 4.
P = W2-W1/W2-W3 (4)
The water absorption test was conducted on the produced porous samples by boiling the fired test pieces in water at 1000C for 2 hours, followed by an additional soaking in water for 4 hours. Water absorption values was calculated by Eq. (5). Wa = Percentage absorption of water by the sample; Ws= soaked weight of the sample after boiling at 1000C for 2 hours; and Wd= dry weight of the sample.
Wa = Ws−Wd / Wd × 100 (5)
Apparent pores of fired clay bricks are listed in Table 3 and shown in Fig. 9. The porosity changes of the bricks containing waste RHA and KS when compared to the control brick (A0) were submitted as percentages on the bars in the figure. Values varied between 30.6% and 47.8%. As seen in Fig. 9, the pores of the bricks showed a significant increase with the addition of RH, KS, and AA. Compared to control brick (A0), maximum increase (approximately 56.21%) in apparent porosity of the samples was obtained at 20 wt.%, 30 wt.% KS and 15 wt. %AA (C4); whereas, minimum increase (approximately 17.32%) was obtained at 5 wt.% RHA, 30 wt.% KS, and 5 wt.% AA (A1). As known, porosity significantly affects the physical, mechanical, and thermal characteristics of construction materials such as compressive strength, unit weight, and thermal properties. As the RHA, KS, and AA contents added increased, the pores of the fired clay bricks showed a continuously increasing trend. It is thought that such an increase may be due to the porosity of RHA, the size of its specific surface, its amorphous structure, and the increase in the amount of unreacted material in the brick. Figure 10 shows the correlations between experiments and waste contents. As can be seen in the figure, a positive linear correlation was observed between apparent porosity and waste content. The range of apparent porosity values obtained from the current study was close to the average when compared to other values obtained from the literature (Table 4). Porosity had an important effect on numerous properties of the bricks. Figure 11 shows the relations between the properties of the bricks. As shown in Figs. 9 and 11, an increase in apparent porosity resulted in an increase in water absorption capacity and a decrease in bulk density, compressive strength, and thermal conductivity.
Water absorption values of the samples are listed in Table 3 and shown in Fig. 9. The changes in water absorption of the bricks containing waste RHA and KS when compared to the control brick (A0) were submitted as percentages on the bars in the figure. As the values increased, they varied between 12.1% and 29.4%. As the amounts of RHA, KS and alkali activator added increased, a significant increasing trend was observed in the water absorption capacity of the bricks. As seen in Fig. 9, there was a positive linear correlation between water absorption and waste contents. A maximum increase of about 134.71% was obtained in water absorption capacity of the clay brick at 20 wt.% RHA, 30 wt.% KS, and 15 wt.% AA; whereas, a minimum increase of 38.02% was obtained with addition of 5 wt.% RHA, 30 wt.% KS, and 5 wt.% AA.
Porosity and density have a great effect on the water absorption capacity. A more porous structure means more water absorption and it reduces the durability of clay bricks [38]. As can be clearly seen from the porosity values and waste rates (Fig. 9 and Table 3), the addition of RH and AA resulted in a stable increase in porosity. Also, the water absorption range obtained from the study was lower than most of other water absorption ranges obtained from the literature (Table 4). If higher rates of RHA, KS, or AA are added during the production of clay bricks, water absorption rates will increase more. In this case, the bricks produced will be useless. The rates used for the present study are sufficient to produce this type of brick. In brick standards, the water absorption of first, second, and third class load-bearing wall bricks is max. 15%, 19%, and 23%, respectively [39,40]. Brick A1 was within the water absorption limits of the second class brick group, while bricks A2, B1, B2 and C1 met the water absorption limits of the third class brick group. These mixtures may be preferred for load bearing applications, while others may be recommended for use in closed environments that are not exposed to water.
3.2.2. Loss on Ignition
Loss on ignition (LOI) values of fired clay bricks are listed in Table 3 and shown in Fig. 11. Values varied between 5.1 wt.% and 11.7 wt.%. As shown in 11, a significant increase was observed in the loss on ignition when the amount of clay reduced and waste RHA and KS were added to the brick structure at different rates. This may be due to the fact that the waste RHA and KS had a higher LOI (12.4%) than the raw clay (5.35%) as shown in Table 1, or due to the release of physically absorbed water during firing from the void forming due to the amount of unreacted material before firing, dehydroxylation reactions of minerals and combustion reaction of RHA.
3.2.3. Compressive Strength
Compressive strength is one of the most important mechanical properties of building bricks and is a distinguishing feature in selecting the brick materials for building applications. Compressive strengths of fired clay bricks are listed in Table 3 and shown in Fig. 11. The changes in the compressive strength of the bricks containing RHA, KS and AA compared to the regular fired clay brick (A0) were presented as percentages on the bars in the figure. Values varied between 7.2 and 32.5 MPa. As seen in Fig. 11, the compressive strength of the bricks shows a remarkable decrease with the addition of RH and AA. Figure 12 shows a negative correlation between compressive strength and waste content. Among the fired bricks, the sample A1 had the highest compressive strength at about 18.3 MPa with a reduction of 43.69% compared to the control brick (A0), while the samples B4 and C4 have the lowest compressive strength at 8.1 and 7.2 MPa with drops of 75.08% and 77.85%, respectively. This resulted in a lower degree of geopolymerisation depending on the increase in the SiO2/Al2O3 ratio as the content of RHA increased. This increased the amount of unreacted material which resulted in a looser brick structure, and the resulting microstructure provided greater porosity and lower compressive strength. This caused the formation of various weak points within the brick structure as stress concentration centres. The range of compressive strength obtained from the present study was found to be higher than most of the studies in the literature (Table 4). Bricks with low compressive strength are a problem for structural applications, especially in seismic zones; however, they provide an advantage where thermal insulation is required. Pursuant to the brick standards, the compressive strength of first and second class load-bearing wall bricks was min. 15 MPa and 10 Mpa, respectively [41]. Thus, control brick (A0), A1 and B1 bricks met the first-class brick strength limits and A2, A3, B2, C1, and C2 samples met the second-class brick strength limits. In addition, bricks should have a compressive strength of at least 7MPa according to European and Turkish standards (TS EN 771-1) [41]. Therefore, even though the compressive strength of all samples produced decreased, they met the required standards.
3.2.4. Thermal Conductivity
Thermal conductivities of the fired clay bricks are listed in Table 3 and shown in Fig. 11. The changes in thermal conductivity of the bricks containing RHA, KS, and AA when compared to the control brick (A0) were submitted as percentages on the bars in the figure. Values varied between 0.263 W/mK (C4) and 1.043 W/mK (A0). The addition of waste RHA to the brick structure at the lowest rate (A1) caused an extreme decrease in the thermal conductivity of the fired clay brick (about 29.53%). When the amounts added reached RHA 20 wt.%, KS 30 wt.%, and AA 15 wt.%, the lowest thermal conductivity (0.263 W/mK) showed an approximate reduction of 77.78% when compared to the control brick (A0). As the waste RH content increased in the brick, the thermal conductivity decreased consistently. As shown in Fig. 12, this was supported by a negative correlation between thermal conductivity and waste content. Thus, the contribution of waste RHA into the brick improved the thermal performance of the fired clay brick. This can be referred to as the porous structure of the bricks containing waste RHA and it traps the air inside, and reduces thermal conduction. The thermal conductivity results obtained from the study appeared to be under the average when compared to the thermal conductivity ranges of other bricks obtained from the literature (Table 4). As seen in Fig. 11, thermal conductivity values of the samples decreased with increasing RHA and AA addition in all cases. Decreasing thermal conductivity values corresponded to increasing porosity as well as decreasing density values in all cases. It was clearly seen that the decrease in the thermal conductivity of the samples produced in the study improved the building performance in terms of energy saving by providing thermal insulation in the buildings. If hollow bricks are produced with the compositions in the present study or additional pore-forming agents are added to the brick, thermal conductivities may be more satisfactory. There are no studies performed regarding the performance of hollow and solid pumice bricks with RHA, KS, and AA etc. Thus, RHA, KS, and AA-mixed samples were produced and the performance of the bricks was examined in the present study. Reuse of these construction materials used to make clay-based bricks as secondary raw materials affected the technological properties of the bricks as shown in Table 5.
Table 5
Summary of the characterization results of the bricks.
Test | Standard | Requirement | Clay Brick (A1, A2, B1, B2, C1) |
Shrikange | ASTM | < 8% | 3.1–3.5–3.4–3.6–3.7 |
Apparent Porosity | ASTM C373 | [20%-55%] < 40% | 35.9–38.7–37.5–39.8–38.7 |
Water Absorption | ASTM C62 | < 22% | 16.7–20.3–19.7–22.2–21.8 |
Bulk Density | NF P 94 − 093 | [1.5–1.8] g/cm3 | 1.82–1.71–1.75–1.62–1.67 |
Flexural Strength | EN 771-1 | > 7 Mpa | 18.3–13.7–17.4–12.8–13.3 |
Using of waste RHA at different rates (by weight) and KS powder at a fixed rate as secondary raw material (A1, A2, B1, B2, C1) can be a new method to produce a new material that can be used as a construction material that meets the physical and mechanical properties.
3.2.5. Microstructure Examination
Figure 13 shows SEM images of the fractured surfaces of the control brick, bricks containing 5 wt.% RHA, 30 wt.% KS, 5 wt.% AA (A1), 10 wt.% RHA, 30 wt.% KS, 10 wt.% AA (B2), 15 wt.% RHA, 30 wt.% KS, 15 wt.% AA (C3), and 20 wt.% RHA, 30 wt.% KS, 15 wt.% AA (C4). Brick containing 5 wt.% RHA, 30 wt.% KS, 5 wt.% AA can be selected as the sample with the highest compressive strength. Brick containing 20 wt.% RHA, 30 wt.% KS, 15 wt.% AA can be selected as the sample with the best thermal performance. The SEM images show the porosity remained by the unreacted materials in the brick samples as a result of the firing reaction.
The fractured surface image of the control brick sample was dense, while the reinforced samples were more porous. Since the porosity of the samples increased depending on the amount of additive, this can be noticed from the fractured surface of the samples. It was observed that the clay and quartz particles, which formed a durable structure due to the effect of high temperature as a result of firing, were partially bonded to each other diffusely.
As shown in Fig. 13, the brick samples with additives had more and larger porous structure than the control brick, which is attributed to the release of AA from the reaction during firing, the porosity, specific surface size, amorphous structure and increased amount of unreacted material of RHA. Irregular gaps in the form of micropores were observed in the fired brick structure. The number and size of pores increased with the increase in the amount of waste additive added. Therefore, bricks containing 20 wt.% RHA, 30 wt.% KS, 15 wt.% AA had more and larger pores than bricks with 5 wt.% RHA, 30 wt.% KS, and 5 wt.% AA. This was also supported by the results of compressive strength and thermal conductivity tests. The results of the superficial EDS analysis of the fractured surfaces of bricks A1, B2, C3, C4 are shown in Fig. 14 and listed in Table 6. Si, Al, Fe, Ca, and Mg oxides were observed as main components. EDS analysis showed that the clay had a content compatible with chemical composition of RHA, and KS raw materials.
Table 6
EDS results of the fired clay bricks (wt%).
Element | Sample Code |
| A1 | B2 | C3 | C4 |
Si | 39.1 | 45.3 | 46.5 | 50.9 |
Ca | 21.8 | 16.8 | 14.6 | 12.6 |
Al | 14.4 | 11.5 | 15.0 | 13.1 |
Fe | 14.2 | 13.7 | 10.7 | 11.4 |
Mg | 6.6 | 6.2 | 6.3 | 6.2 |
Na | 1.7 | 3.4 | 3.3 | 2.4 |
K | 1.2 | 2.0 | 2.6 | 2.6 |
Ti | 1.1 | 1.0 | 1.0 | 0.9 |
Figure 15 shows the results of XRD analysis of waste RHA, KS, and AA brick samples to explain the crystalline phases forming in the brick samples after firing. XRD results show that the samples fired at 9500C with 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.% waste RHA, 30 wt.% KS and 5 wt.%, 10 wt.%, 15 wt.% AA reinforced samples were mainly composed of quartz, albite, hematite, cristobalite, and muscovite crystal phases. The natural mineral components forming the content of the clay raw material were transformed into new crystalline phases via high temperature reactions during firing. The quartz phase in the clay content remained undissolved as the main phase in the fired brick structure and provided the dimensional stability of the brick by acting as a filler.