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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 (Na2Oe) 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.

Knowing and promoting the strength development of concrete at an earlier age is essential for accelerating formwork circulation of the on-site construction and precast product manufacture. The strength development rate at earlier ages of less than the initial 24 h was investigated. The effect of measures of adding silica fume, calcium sulfoaluminate cement, and early strength agent on the strength development of earlier concrete at ambient temperatures of 10, 15, 20, 25, and 30 °C was studied. The microstructure and long-term properties were further tested. It is shown that the strength increases exponentially first and then logarithmically, different from what is commonly recognized. Increasing cement content exhibited a certain effect only above 25 °C. When the cement content increased from 420 to 460 kg/m3, the strength only increased from 6.2 to 6.7 MPa after 12 h at 25 °C. The early strength agent could increase the strength significantly, the strength could be increased from 6.4 to 10.8 MPa after 20 h at 10 °C and from 7.2 to 20.6 MPa after 14 h at 20 °C. All measures for promoting earlier strength did not have an evident negative effect. The results could be potentially referred for the formwork removal at a suitable moment.