Effect of Borax on Very High Calcium Geopolymer Concrete

Abstract

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 minutes 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 the borax pentahydrate is capable of acting as a retarder to prolong the setting time but has the ability to reduce the compressive, flexural, and split tensile strengths.