Effect of delay time and Na2SiO3 concentrations on compressive strength development of geopolymer mortar synthesized from TPOFA

•TPOFA-based geopolymer mortars have been synthesized.•Ca(OH)2, Al(OH)3 and silica fume served as mineral additives.•Effects of delay time and Na2SiO3 concentration were evaluated.•Influence on compressive strength development was quantified.•Phase composition and chemical bond characteristics were identified. The aim of this study was to investigate the main factors affecting the compressive strength (σf) development of geopolymer mortar synthesized from treated palm oil fuel ash (TPOFA), with curing period of up to 120days. The geopolymer mortar was prepared using mixtures of solid material and an alkaline activator. The solid materials used were TPOFA (source material) and mineral additives (Ca(OH)2, Al(OH)3, and silica fume). The alkaline activator was prepared from NaOH and Na2SiO3. The two main factors considered are delay time (i.e., 1 and 24h) and variation of Na2SiO3 concentrations (i.e., high, medium, and low). The changes in the σf development were examined using X-ray diffraction, and confirmed by FTIR analyses. The results for σf were in agreement with the phase changes and quantitative X-ray diffraction (QXRD) analysis, and also matched the FTIR analysis. It was found that mixtures with 24h delay time and having high Na2SiO3 concentration produced the highest development of σf at 120days (78.0% with respect to 28days). It was evident that the concomitantly developed C–S–H and N–A–S–H binding gels observed at 28days transformed to a more stable C–A–S–H type gel at prolonged curing period.