Kinetics of geopolymerization: Role of Al2O3 and SiO2

Abstract The early-stage reaction kinetics of metakaolin/sodium silicate/sodium hydroxide geopolymer system have been investigated. The setting and early strength development characteristics, and associated mineral and microstructural phase development of mixtures containing varying SiO 2 /Al 2 O 3 ratios, cured at 40 °C for up to 72 h, were carefully studied. It was observed that setting time of the geopolymer systems was mainly controlled by the alumina content. Essentially, the setting time increased with increasing SiO 2 /Al 2 O 3 ratio of the initial mixture. Up to a certain limit, the SiO 2 /Al 2 O 3 ratio was also found to be responsible for observed high-strength gains at later stages. An increase in the Al 2 O 3 content, i.e. for low SiO 2 /Al 2 O 3 ratio, led to products of low strength, accompanied by microstructures with increased amounts of Na–Al–Si-containing “massive” phases (grains). EDAX analyses showed that the SiO 2 /Al 2 O 3 ratios of geopolymer gel phases were quite similar to those of the starting mixtures, but with an overall lower Na content. Most importantly, this study clearly demonstrates that the properties of resulting geopolymer systems can be drastically affected by minor changes in the available Si and Al concentrations during synthesis.