Influence of partial replacement of fly ash by metakaolin on mechanical properties and microstructure of fly ash geopolymer paste exposed to sulfate attack

This work aims to investigate compressive strength changes, Vickers-hardness of exposed surface, microstructure evolution and pore structure of geopolymer prepared using fly ash as resource material and activated by sodium silicate and sodium hydroxide solutions after exposed to sulfate attack for 28, 90 and 180 days when fly ash was partially replaced by metakaolin at levels ranging from 0% to 20% with an interval of 5%, by weight. The experimental results uncover that geopolymer suffers strength loss after sulfate attack exposure but gains strength with increasing replacement level of fly ash by metakaolin from 5% to 20% and obvious increasing in compressive strength could be observed when the replacement percentage exceeds 15%. The sulfate attack exposure shifts the peak value of pore size to a higher value and leads to the development of a macropore system with high porosity and deduction of surface Vickers-hardness. A denser microstructure with higher surface Vickers-hardness of metakaolin containing geopolymer can be obtained compared to geopolymer without metakaolin. A good linear relationship has been found between compressive strength loss and Vickers-hardness values of exposed surface. Compressive strength loss in geopolymer after sulfate attack exposure relates to the microcrack development and high porosity. Longer exposure leads to higher deduction in compressive strength and acceleration in deterioration degree which lead to expansion stress and cracks. Partial replacement of fly ash by metakaolin improves the mechanical properties, optimizes the microstructure and reduces the level of damage from sulfate attack. (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.