Effects of alkali sulfates in clinker on hydration and hardening performance of Portland cement

In this study, two main alkali sulfates – aphthitalite and calcium langbeinite – fabricated in a laboratory are added to an industrial clinker with low alkali and sulfate content in order to adjust the contents of alkali sulfates in the clinker. Based on measuring the dissolution rate of the alkali sulfates and ion concentration in tricalcium aluminate (C₃A)–alkali sulfate–water (H₂O) systems compared with gypsum, and testing the heat flow, compressive strength, setting time and linear deformation of cement with different alkali sulfate contents, the differences between the alkali sulfates and gypsum are analysed. It is shown that aphthitalite could not act as a retarder, whereas calcium langbeinite can act to achieve almost the same retardation as gypsum. In view of the volume stability and strength development of cement, aphthitalite has a negative action when the equivalent sodium (Na₂O_e) content induced is above 1·42%. The X-ray diffraction analysis patterns and scanning electron microscopy images show that aphthitalite restrains ettringite formation. It is suggested that more attention should be paid to the negative action of aphthitalite when alkali and sulfate contents in clinker appear to be high.