Assessment of elevated temperature curing on compressive strength and setting time of alkali activated slag mortar and paste, associated with silica fume

Use alkali activated slag as a replacement of OPC, in spite of preference for purpose of some properties, like mechanical strength and durability of concrete or mortar against aggressive environmental conditions, has some problems like short setting time, high contraction and inconstant chemical composition. To improve the setting time and compressive strength of alkali activated slag (AAS) based paste and mortar, this research carried out to investigate the synergic effect of using Silica Fume (SF) and elevated temperature on setting time and compressive strength of AAS paste and mortar by substitution of SF at 0 ‌w%, 5w% and 10w% of slag and elevated temperature curing at 40˚C, 60˚C and 80˚C. Combination of sodium silicate and sodium hydroxide was applied for activation of slag. The concentration of sodium hydroxide solution was 4M and the ratio of sodium hydroxide to sodium silicate was 3 and the ratio of alkaline solution to slag was 0.45. Curing temperature at 80˚C had the highest compressive strength, and the lowest setting time among the made mortars and pastes. On the other hand, 5w% replacement of silica fume, was the best choice for compressive strength aspect, and 10w% replacement represented the longest setting time. In terms of the synergic effect of substitution of slag with silica fume and heat treatment, with desirability of longer setting time, the substitution of 5w% silica fume, conjoint with 40˚C heat curing, yielded the optimum results. The obtained resultants, was verified by means of Scanning Electron Microscope (SEM) pictures.