The compressive strength and the setting time of partial replacement of fly ash by palm oil fuel ash on geopolymer paste and mortar

Abstract

This study replaced fly ash with palm oil fuel ash (POFA) in geopolymer paste and mortar. The alkaline activator was a mixture of sodium silicate solution and sodium hydroxide solution with a ratio of 2.5. Three variables were evaluated: the raw material combination (100% fly ash and 25% POFA substituted to fly ash), the mass ratio of raw material to alkali activator (60/40, 65/35, and 70/30), and NaOH molarity (8 M and 10 M). The geopolymer mortar was utilized based on the highest compressive strength of geopolymer paste. The ratio of fine aggregate to binder was 65 to 35. The result showed that the compressive strength of geopolymer paste increased as NaOH molarity increased. 25% substitution of POFA for fly ash increased the compressive strength of geopolymer paste from 12.0% to 27% at 28 days. The raw material/ alkali activator ratio of 65/35 indicated the highest compressive strength. The POFA replacement for fly ash had a longer setting time than 100% fly ash. The compressive strength ratio of mortar to the geopolymer paste was 0.66 and 0.59 for the 65/35 and 70/30 ratios of raw material to alkali activator, respectively.