Performance of sodium silicate free geopolymers from metakaolin (MK) and Rice Husk Ash (RHA): Effect on tensile strength and microstructure

The objective of the present work is to study the effect of RHA on the performance of fresh and hardened metakaolin geopolymer pastes with focus on tensile strength and microstructure. The metakaolin was partially substituted by 12.5-50% RHA in the solid mixes and reacted with 10 M sodium hydroxide solution at ambient conditions of 20 ± 3 °C and 70 ± 5% relative humidity using a liquid/solid ratio of 0.95. Physical, chemical and mineralogical characteristics of raw materials were determined. Fresh geopolymer pastes were tested for consistency and setting. Hardened products at 7 and 28 days of curing underwent tensile strength measurements according to ASTM C 307-03 (2012). SEM/EDX and thermogravimetric analyses were done on samples at 28 days of curing. Geopolymer pastes in which MK was substituted with up to 25% RHA showed improvement in workability and tensile strength compared to the mixture of MK with no RHA as substitution. Microstructural observations and SEM/EDX analyses of hardened pastes suggested that, the presence of RHA affected Si/Al ratios of geopolymers, with consequences on their polycondensation reactions products. Enhancement of tensile strength resulted from the densification of the matrix and the reduction of the negative effect of drying shrinkage and associated micro-cracks.