Effect of silica modulus on the geopolymerization activity of natural pozzolans

The present study evaluates the effect of silica modulus on the reaction kinetics, degree of reaction and phase composition of natural pozzolan-based geopolymers. Six natural pozzolan samples have been subjected to geopolymer synthesis in the alkaline medium using sodium hydroxide and sodium silicate solutions at ambient conditions. Reaction kinetics studied by isothermal conduction calorimetry show that the early age dissolution of aluminate and silicate species depends on the SiO2 and Na2O molar content of the alkaline solution. The degree of reaction increases with the reduction of silica modulus. However, the reduction of silica modulus beyond certain limit has a negative influence on the mechanical properties of resultant geopolymers. The optimum silica modulus requirement depends on the mineralogical composition of the raw sample used. The rate of partial dissolution of semi-crystalline and crystalline mineral phases present in natural pozzolan depends on the silica modulus of the alkaline solution used. (C) 2019 Elsevier Ltd. All rights reserved.