The Effect of the Si/Al Ratio on the Properties of Water Treatment Residue (WTR)-Based Geopolymers

This research aimed to develop a cementing material from synthetic geopolymers using water treatment residue (WTR) from a water treatment plant in Bangkaen as an aluminosilicate starting material, with the addition of rice husk ash (BHA) from a paddy factory in Thailand as a source of crystalline silica, to achieve Si/Al ratios of 2.00, 3.00, 4.00 and 5.00. The geopolymers were prepared by NaOH activation of the five compositions of WTR and BHA mixtures, followed by curing at ambient temperature (29±2°C). The compressive strengths of the geopolymers were determined after 3, 7, 28 and 60 days. The samples cured for 28 days were examined by XRD, FTIR and SEM. The Si/ Alratio of 2.00 was found to produce the greatest strength at all curing times. After 7 days, all samples were 30% stronger than the 3 day-samples. XRD revealed the presence of sodium aluminium silicate hydrate, while an FTIR vibration band related to the geopolymer product shifted to 1000 cm-1. SEM revealed a homogeneous non-crytalline and dense microstructure in the sample with the highest Si/Al ratio of 5.00, suggesting that a transformation to sodium silicate had occurred in this sample.