Early hydration of calcium sulfoaluminate cement through electrical resistivity measurement and microstructure investigations

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.

Research highlights
► Monitor the hydration characteristics of CSA cement by electrical resistivity measurement.
► Analyze the roles of the ions and the porosity in the earlier period and the later period in a CSA cement hydration system.
► Investigate the mechanism of the partial transformation of phase ettringite to phase monosulfate in CSA cement.
► Propose a negative relationship between the compressive strength and the electrical resistivity at 24 hours for different W/C ratios.