Effects of wetting and drying on alkalinity and strength of fly ash/slag-activated materials

Abstract The alkalinity of alkali-activated materials, so-called geopolymer (GP), is attributed to the alkali activator remained in the pores. For the application of GP in reinforced concrete as binder, it is necessary to investigate the alkalinity change of GP concrete in various environments. In this study, after GP concrete (GPC) and GP mortar (GPM) specimens, which used active fillers of fly ash (FA) and ground granulated blast furnace slag (GGBS), were stored in water, wet-dry repeating, accelerated carbonation (AC), and AC-dry repeating environments for a certain period, we observed the coloring reaction to the spraying phenophthalein indicator, measured pH values in different depths and the compressive strengths, and did micro-structure analysis by SEM. A comparison was also performed between GPM and Portland cement (PC) mortar specimens. It was found that, the alkalinity of GP mortar is much easier to decrease no matter in any of above environments, compared to PC mortar. Heat-cured GP has a higher alkali leaching resistance in the wet-dry repeating environment than ambient air-cured GP. Carbonation only decreases the alkalinity in the outer zone, but drying and wetting redistributes the alkalis and promotes the movement of internal alkalis to the outer zone even to the surface of GP, resulting in an alkalinity drop in both the outer zone and the inner zone. The AC-dry repeating causes a greater alkalinity drop than the wet-dry repeating. The AC results in a faster and greater alkalinity drop in the outer zone, compared to the water immersion. Carbonation and water immersion can raise GP’s strength, but intense drying causes cracks to harm the strength of GP.