The ratio of alkali modulus in strength development of class c fly ash geopolymer mortar

Abstract

Fly ash as cement replacement material has been widely used due to the environmentally unfriendly production process of cement. The combination of fly ash and alkaline activator produce geopolymer. However, it requires a specific ratio of alkali modulus which can be achieved by determining the correct sodium silicate and NaOH ratio. This research aim is to identify the role of alkali modulus ratio in strength development of fly ash geopolymer mortar cured at room temperature. The specimen was developed using high calcium class C fly ash combined with sodium silicate and NaOH 12 Molar with a water to solid ration of 0.35. The ratio of sodium silicate to NaOH was vary from 0.5 to 4.0 with the increased interval of 0.5. The alkali modulus was determined based on the ratio of total SiO2 to total Na2O from all precursor materials. The strength development was measured at 7, 14, and 28 days. The results show that the highest strength of 55.14 MPa was achieved by fly ash geopolymer mortar with sodium silicate and NaOH ratio of 1.5. This result shows that the highest alkali modulus that can be achieved is 2.18 with Na2O content of 11.89%. Increasing the alkali modulus beyond this value tend to reduce the geopolymer strength due to the decrease of Na2O content. Moreover, it also can be considered that high calcium class C fly ash can be used as cement replacement materials.