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In this study, effects of steel slag on the hydration and strength development of calcium sulfoaluminate (CSA) cement were investigated. Pastes with 0%, 10%, 20%, and 40% (by mass) of steel slag replacements for CSA cement were tested for setting time, chemical shrinkage, electrical resistivity, and compressive strength. The hydration products, bound water content, pore structure, and pH value of the pore solutions of these pastes were also analyzed. The results indicate that at early ages (from 2 h to 28 d), steel slag worked like a filler and had a dilution effect on CSA cement hydration. Therefore, setting time increased and chemical shrinkage, electrical resistivity, chemically bound water content, and compressive strength of the pastes reduced with increase of steel slag replacements. But, at late ages (≥90 days), these properties in pastes with steel slag replacements surpassed those of the plain CSA cement paste due to hydration of steel slag, which increased pH values of pore solutions and subsequently stabilized ettringite in the pastes. Steel slag replacement for CSA cement also accelerated stratlingite formation. Both ettringite stabilization and stratlingite formation appeared to be the primary mechanisms for strength compensation of CSA cement paste at late ages.

The electrical resistivities of the calcium sulfoaluminate (CSA) cement pastes at different water to cement (W/C) ratios, over a period of 1440 min (or 24 h) were measured by a non-contacting electrical resistivity apparatus. The electrical resistivity–time curve decreased slightly after mixing, and then increased sharply around the setting time, followed by two peaks, and finally developed at a very low rate up to 1440 min. The porosity plays a dominant role in the electrical resistivity development of the CSA cement paste in the earlier period of hydration (before the initial setting time), while the ion concentration plays a major role at the later period (after the second peak). The electrical resistivity–time curve demonstrates the process of ettringite formation and transformation to monosulfate in the CSA hydration system, and this is confirmed by the SEM and XRD observations. The high temperature and the insufficiency of gypsum in the CSA cement system during hydration result in the decomposition of ettringite at the first peak, while at the second peak the decomposition of ettringite is due to the lack of gypsum. The relationship between W/C and electrical resistivity at 1 h follows a negative trend but a positive trend is observed at 24 h. The relationship between the compressive strength and the electrical resistivity at 24 h for the different W/C ratios follows a negative relationship.