Early-age hydration behavior of calcium sulfoaluminate (CSA) cement/ordinary Portland cement-blended ultra-high performance concrete

This study investigated the early-age hydration behavior of calcium sulfoaluminate cement (CSA)/ordinary Portland cement (OPC) blended ultra-high performance concrete (UHPC) to control its setting behavior and enhance its early-age strength. In-situ X-ray diffraction coupled with quantitative analysis, time-domain nuclear magnetic resonance spectroscopy, and isothermal calorimetry were conducted immediately after mixing. Compared with plain UHPC, the CSA/OPC-blended UHPC showed a faster hydration reaction within 3 h owing to ettringite formation, whereas the C₃S hydration in that system was delayed to 3 d. Unlike plain UHPC, CH was not found in the CSA/OPC blended UHPC as AH₃ produced from the hydration of ye'elimite reacted with CH to form the AFt or AFm phases. The compressive strengths of the CSA/OPC-blended UHPC containing gypsum at 3 h and 6 h were 17 MPa and 38 MPa, respectively. The main strength-giving phase in the CSA/OPC-blended UHPC pastes within 1 d is ettringite, which can be enhanced by the addition of gypsum, and the subsequent age strength is contributed by the hydration of C₃S.