Improved ettringite stabilization by calcium carbonate and calcium nitrate additions in ternary PC-CSA-C$ systems

Mixing Portland cement (PC) and calcium sulfoaluminate (CSA) cement can provide several advantages for specific applications. However, the stability of ettringite has a significant impact on ettringite-rich cements properties. This study aims to investigate the impact of calcium nitrate (Ca(NO3)2 = CN) and calcium carbonate (CaCO3 = CC) on the performance evolution and stabilization mechanism of PC-CSA-C$ systems. The experimental results demonstrate that the incorporation of CC and CN has a significant impact on the early hydration kinetics. A second exothermic peak appears indicates the depletion of the calcium sulfate, and its appearance time is related to the amount of CC and CN added. The blended system containing CN exhibited the highest early and late compressive strength, as well as excellent volume stability. Compared with CC, CN was confirmed to be more effective in stabilizing ettringite in the PC-CSA-C$ system and preventing its conversion into monosulfate. Furthermore, the analysis of the pore structure reveals a significant increase in the proportion of pores with diameters <= 50 nm in the systems containing CN. In particular, CN displays a stronger ability to stabilize ettringite to reduce chemical shrinkage and increase the content of C-(A-)S-H gel, and the 28-day compressive strength was approximately 10 MPa higher compared to the control group.