Synthesis of volcanic ash-based geopolymer with calcium oxide (CaO) addition for building material application

Volcanic ash is a natural pozzolanic material with high mol ratio SiO₂/Al₂O₃ but low in CaO. This research aims to study the volcanic ash geopolymer characteristics related to CaO addition. Geopolymers are prepared by mixing volcanic ash, alkali activator solution, sodium silicate, and calcium oxide. Calcium oxide added varies 5-135 g with each interval is 5 g in Solid/Liquid (S/L=1.83) ratio. Geopolymer characteristics investigated include setting time, compressive strength, resistance to sulfuric acid and mineral phase. The results showed that the more CaO was added, the shorter the setting time and the greater the compressive strength until it reached optimum. The amount of optimum CaO that added is 30 g, with the setting time 2.61 hours and the compressive strength 36.26 MP better than the volcanic ash geopolymer without CaO as 103 hours in setting time and 20.34 in compressive strength. Volcanic ash-based geopolymer at the optimum CaO addition proved to be better than cement mortar, which has a shorter setting time, greater compressive strength and greater resistance to sulfuric acid. Mineral phase analysis by XRD shows that the optimum geopolymer containing an amorphous phase with a major mineral is quartz and sodalite.