Compressive strength of treated palm oil fuel ash based geopolymer mortar containing calcium hydroxide, aluminum hydroxide and silica fume as mineral additives

In this work, the compressive strength of treated palm oil fuel ash (TPOFA) based geopolymer mortar was studied. The geopolymer mortar was prepared using mixtures of TPOFA and mineral additives ((Ca(OH)2, Al(OH)3, and silica fume (SF)). The alkaline activator was prepared from NaOH and Na2SiO3. The samples with different total oxide molar ratios (Al2O3:SiO2, CaO:SiO2, Al2O3:Na2O, and SiO2:H2O) were tested for compressive strength. Moreover, the influence of unreacted mineral additives in the final geopolymer products was also evaluated. The results showed that the early age compressive strength of TPOFA-based geopolymer mortar improved significantly when the source material (TPOFA) was substituted with Ca(OH)2, exhibiting a relative compressive strength of 188.45% at 3 days. Furthermore, the modification of TPOFA with Al(OH)3 and SF caused significant improvement, demonstrating a relative compressive strength of 200.90% and 175.22% at 3 and 28 days, respectively. The C–S–H and N–A–S–H binding gels formed at 28 days were transformed over time to C–A–S–H type gel at 120 days which was the most stable product.