On the interaction of Class C fly ash with Portland cement-calcium sulfoaluminate cement binder

Shrinkage cracking in concrete is a widespread problem, especially in concrete structures with high surface-to-volume ratio such as bridge decks. Expansive cements based on calcium sulfoaluminate phase were developed to mitigate the shrinkage cracking of concrete. The compressive stress induced due to restrained expansion of concrete has been shown to counteract the tensile stress generated during drying shrinkage. This research attempts to address the differential behavior of fly ash type (i.e., Class C vs. Class F) on early-age expansion and hydration characteristics of ordinary Portland cement (OPC)-calcium sulfoaluminate (CSA) cement blend. It was observed earlier that the presence of Class C fly ash (CFA), unlike Class F fly ash, shortened the expansion duration of OPC-CSA cement blend, which was hypothesized to be correlated to early depletion of gypsum. This paper presents a detailed verification of the hypothesis. Addition of external gypsum to OPC-CSA-CFA blend led to simultaneous increase in expansion and disappearance of a shoulder peak in the calorimetric curve. Thermodynamic calculations using a geochemical modeling program (GEMS-PSI) revealed higher saturation levels of ettringite in presence of external gypsum, which led to higher crystallization stress, and thereby increased expansion.