EFFECT OF A COMBINATION OF FLY ASH AND SHRINKAGE-REDUCING ADDITIVES ON THE PROPERTIES OF ALKALI-ACTIVATED SLAG-BASED MORTARS

This study is aimed at reducing the drying shrinkage of alkali-activated slag (AAS) through the use of low-calcium fly ash (FA) and commercially available shrinkage-reducing additives (SRAs) originally developed for OPC-based binders, since there are no such admixtures tailored for AAS systems. Generally, all the SRAs tested are based on modified alcohols. All the mortars were based on slag activated by waterglass, with the water-to-slag ratio equal to 0.40 and the sand-to-binder ratio 2:1. In the first step, the effect of the partial replacement of slag by FA (25, 50 and 75) % of mass fractions on the drying shrinkage and compressive strength was investigated. On the basis of the obtained results a mortar with 50 % FA in the binder was chosen for subsequent experiments, where the influence of three types of SRAs on the drying-shrinkage behaviour was examined. It was observed that while 25 % of the FA did not affect the drying shrinkage significantly, 50 % and 75 % of FA in the binder decreased the drying shrinkage by 57 % and 78 %, respectively. However, with an increasing content of FA, the compressive strength markedly decreased. All the tested SRAs had a similar effect on the drying shrinkage of the slag/fly ash (50/50) mortar: at a dose of 0.50 % (by mass of slag) the shrinkage was reduced only slightly, whereas 1.0-3.0 % of SRA resulted in a decrease by 49-66 %. Also, the drying shrinkage rate during the first days of drying was modified. However, all the SRAs reduced the compressive strength as compared to the neat slag-FA mortar, especially when the doses were higher than 0.50 %.