Effect of alkali sulfates in clinker on hydration and hardening properties of cement incorporating SCMs

Abstract

In practically fly ash and slag have been widely used in cement production. The effect of alkali sulfates in clinker on hydration and hardening of cement blended with fly ash and slag was studied. The results of hydration heat evolution, hydrates analysis and mechanical properties indicate arcanite (K2SO4), thenardite (Na2SO4), aphthitalite (3K2SO4·Na2SO4, K3NS4) and Ca-langbeinite (K2SO4·2CaSO4, KC2S3) in clinker could activate the pozzolanic activity of fly ash and slag facilitating the appearance of the 2nd hydration peak at initial hydration period and promote the hydration and hardening properties of blended cement. The adverse effects of alkali sulfates in clinker are alleviated with the addition of fly ash and slag, which can be attributed to the dilution effect and filler effect of SCMs as well as the effect of alkali sulfates activating fly ash and slag. K2SO4, Na2SO4 and K3NS4 present a better activating effect on mortar with slag than fly ash mortar to increase compressive strength and reduce drying shrinkage while the content of alkali sulfates is less than 0.6% Na2Oeq.