## 3 Result and Discussion
### Effect of LDPE particles on Compressive Strength
A graph of compressive strength versus percent metacalay (MC) or percent metakaolin (MK) is shown in Figure 8a,b. Compressive strength was found to depend on the geopolymer material, the particle size of the reinforcement material and the composition of the geopolymer [42]. The compressive strength of the geopolymer/LDPE composite formed with 0.6 mm particle size was higher than that of 1.18 mm particle size. This was true for geopolymer/LDPE composites cured at 30 \({}^{\circ}\)C and 50 \({}^{\circ}\)C (Figure 8a,b). The higher compressive strength of 0.6 mm LDPE addition is the result of smaller LDPE particles, which fill the interstitial site and absorb the stress for a longer time before crack propagation occurs. The 5% LDPE showed the highest strength for both 0.6 mm and 1.18 mm particle size; this could be due to the fact that the 5% LDPE addition spread through the geopolymer ceramics matrix while higher LDPE percentage addition (10% and 15%) may cause clusters in the matrix, and hence lower compressive strength. The compressive strength of geopolymer ceramics from metaclay (MCGC) was higher than that of geopolymer ceramics from metakaolin  
Figure 8: **(a) A plot of compressive strength versus % MC. (b) A plot of compressive strength versus % MK. **  
(MKGC). The higher percentage of CaO and Fe\({}_{2}\)O\({}_{3}\) in MC (10.1% CaO/Fe\({}_{2}\)O\({}_{3}\)) resulted in higher compressive strength (Figure 8a). This result is in agreement with the study by [PERSON] al. (2016) who evaluated the role of CaO and Fe\({}_{2}\)O\({}_{3}\) on the compressive strength of geopolymer concrete [39]. SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio contributed to the increase in compressive strength. The presence of more Si-O-Si and Al-O bonds, as seen from FTIR, caused an increase in the geopolymerization reaction. A higher SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio in MC suggests higher Si-O-Si stronger bonds than Si-O-Al bonds in MK (Figure 8b), and hence higher compressive strength [49,53]. A higher curing temperature also affects the compressive strength of geopolymer ceramics. The curing temperature at 50 \({}^{\circ}\)C for both 0.6 mm and 1.18 mm LDPE was higher than curing temperature of 30 \({}^{\circ}\)C for both 0.6 mm and 1.18 mm LDPE addition. The increase in compressive strength at a higher curing temperature is due to the increase in geopolymerization reaction at a higher curing temperature [44]. Figure 8b is a plot of compressive strength of geopolymer ceramics formed with metakaolin. Similar to the observation made for MC samples, the addition of LDPE particles increased the compressive strength of the geopolymer ceramic. The largest increase was observed for 5% LDPE powder; an LDPE percentage greater than 5 weight percent decreased the compressive strength. Similar to the observation made for the MC samples, MK compressive strength corresponding to LDPE particle size 0.6 mm was higher compared to that of 1.18 mm. These results are in agreement with previous work [54]. Research carried out on the El-Hiswa kaolinite deposits showed a compressive strength of 44.4 MPa with 16% NaOH at 80 \({}^{\circ}\)C for 14 h [55]. The measured compressive strength (56.8 MPa) for geopolymer ceramics formed with NaOH, sodium silicate/NaOH after being cured at 60 \({}^{\circ}\)C for 24 h in this work was higher than that of [PERSON] et al. (2011). The higher compressive strength in his study was due to of LDPE particles added to the geopolymer. The compressive strength of LDPE in the geopolymer ceramics can be estimated by the rule of mixture from Equation (4)  
\[\sigma_{c}=V_{m}\sigma_{m}+V_{p}\sigma_{p} \tag{4}\]  
\[\frac{\sigma_{c}-V_{m}\sigma_{m}}{V_{p}}=\sigma_{p} \tag{5}\]  
where  
\(\sigma_{c}\) is the composite strength;  
\(V_{m}\) is the matrix volume fraction;  
\(\sigma_{m}\) is the matrix strength;  
\(V_{p}\) is particle volume fraction;  
\(\sigma_{p}\) is the particle strength.