Influence of alkali sulfates in clinker on the hydration and hardening of Portland cement

Alkali sulfates originate from cement clinker, occurring as arcanite (K2SO4), thenardite (Na2SO4), aphthitalite (K3NS¯4) and Ca-langbeinite (KC2S¯3). The impacts of alkali sulfates, prepared through high temperature heating in the laboratory, on cement hydration and hardening properties were studied. Results show that the initial setting time would not significantly shorten or extend with the increase of alkali sulfates/SO3 molar ratio. Alkali sulfates cannot promote 1 d compressive strength, and result in significantly linear decrease of 28 d compressive strength and increase of drying shrinkage with increasing of the molar ratio of Na2Oeq/SO3. The adverse effect of alkali sulfate on cement hydration and hardening properties is significant, and the content is better less than 0.5 of Na2Oeq/SO3 molar ratio. Ca-langbeinite in cement slightly decreases compressive strength and increases drying shrinkage, but can replace gypsum to play a good role in cement. Besides, it is found that alkali sulfates would promote the growth of C-S-H and portlandite during cement hydration.