The effect of different Na2O and K2O ratios of alkali activator on compressive strength of fly ash based-geopolymer

•Sarawak fly ash was used to produce geopolymers with various mixture proportions.•Higher oxide molar ratios do not necessarily lead to higher compressive strengths.•Na2O was postulated as the major alkali oxide required for the strength development. Fly ash sourced from Sarawak, Borneo was alkali-activated by a combination of alkali hydroxides (i.e. NaOH, KOH or Ca(OH)2) and sodium silicate (i.e. industrial or commercial grade Na2SiO3) in this research. A comprehensive overview of the compressive strength of geopolymer due to different alkali activator/fly ash and Na2SiO3/NaOH (Na2SiO3/KOH or Na2SiO3/Ca(OH)2) were studied. The results show that geopolymer made using Sarawak fly ash can be a potential construction material depending on the requirements of workability and compressive strength. The core study of this research was the investigation on the variations of compressive strength due to the oxide molar ratios. The results revealed that higher oxide molar ratios do not necessarily lead to higher compressive strengths. SiO2/Na2O, Al2O3/Na2O, SiO2/K2O and Al2O3/K2O weight ratios had dissimilar effects on the compressive strength of NaOH- and KOH-based geopolymer. The reduction of SiO2/Na2O and Al2O3/Na2O in NaOH- and KOH-based geopolymer using commercial grade Na2SiO3 intensively increased the strength capability. The ratios of SiO2/Na2O and Al2O3/Na2O in Ca(OH)2-based geopolymer were the highest, nevertheless, the compressive strength and workability of Ca(OH)2-based geopolymer were the weakest in comparison to NaOH- and KOH-based geopolymers. Na2O was postulated as the major alkali oxide required for the strength development rather than K2O in both NaOH- and KOH-based geopolymer. However, K2O in KOH-based geopolymer may govern the strength development when Na2O is low in the mixture.