Effect of Borax on Very High Calcium Geopolymer Concrete

This study examined the effect of borax pentahydrate on alkali-activated very high-calcium fly ash (VHCF)-based geopolymer concrete. The VHCF obtained from the Pangkalan Susu power plant, Langkat, North Sumatra, Indonesia, had 25% CaO and was classified as C-class fly ash according to the new ASTM C618-19. It was activated using an alkali solution produced using Na2SiO3 and NaOH at a ratio of 1.5. Moreover, borax pentahydrate was used due to its high-calcium content, and the setting time, compressive strength, split tensile strength, and flexural strength were investigated. It was discovered from the results that the geopolymer paste had a flash final setting time. The findings showed that the initial setting time was 5 minutes while the final was 25 minutes. The addition of 12% borax pentahydrate was observed to have prolonged the setting time from 25 to 80 minutes. Furthermore, the compressive strength of the concrete after 28 days was 50 MPa using NaOH 8 M and 2% borax pentahydrate, while the split tensile strength was 4.7 MPa and the flexural strength was 4.53 MPa. This implies that the borax pentahydrate can be a retarder to prolong the setting time but can reduce the compressive, flexural, and split tensile strengths.