The effect of Al2O3 and SiO2 on setting and hardening of Na2O-Al2O3 -SiO2-H2O geopolymer systems

Abstract

This study investigates the effect of Al2O3 and SiO2 on setting behaviour and early strength development characteristics of low-temperature cured Na2O-Al2 O3-SiO2-H2O systems. The study also explores associated mineral and micro-structural phase development. Geopolymer mixtures with varying amounts of SiO2 and Al2O3 but constant H2O/Na2O molar ratios prepared using alkali-activated metakaolin and cured at 40° up to 72 hrs were examined by XRD and SEM-EDAX techniques to characterise microstructures of these systems. It was revealed that the setting time of the geopolymer systems is mainly controlled by Al2O3 and that the setting time increased with increasing SiO2/Al2O3 ratio of the initial mixture. However, the amount of Si seems to play an important role in subsequent strength development. Increasing SiO2/Al2O3 molar ratios up to 3.4-3.8 is largely responsible for the high strength gains observed at later stages. A corresponding increase in Al2O3 (low SiO2/Al2O3) leads to products of low strengths, accompanied by microstructures with increased Na-Al-Si "massive" structures rather than the regular amorphous Na-Al-Si-containing matrix. The underlying chemistry of above changes is discussed with respect to basic dissolution-hydrolysis-condensation processes that occur in Na2O-Al2 O3-SiO2-H2O systems