The effect of mixing molar ratios and sand particles on microstructure and mechanical properties of metakaolin-based geopolymers

Abstract

In this study the influence of the molar ratios such as SiO2:Al2O3:Na2O:H2O, as well as the sand particles on the mechanical performance, shrinkage and microstructure of metakaolin based geopolymers was studied. Considering different content of the reactive silica and alumina in metakaolin, forty eight different compositions were prepared. The proper condition for achieving the highest mechanical performance as well as less structural defects by tailoring the curing condition, molar ratios and addition of sand particles are presented. Special attention was paid to the incorporation of sand particles up to 70 wt% on the microstructure, shrinkage and mechanical properties of metakaolin based geopolymers. Results from mechanical testing and microstructural observations revealed that replacing metakaolin with 30 wt% sand particles can significantly reduce drying shrinkage and improve compressive and flexural strength. It was suggested that this strengthening is probably due to a partly reactive sand/matrix interface as well as stacking of filler particles and their ability to carry load and limit crack growth. Comparison of the compressive and flexural strength between geopolymer and cement mortars containing up to 50 wt% sand, revealed that the strength of geopolymer specimens were at least three times more than that of cement.