Mechanical performance and hydration behavior of anhydrite blended concrete under different steam curing regimes

Steam-cured concrete was prepared with anhydrite and the effects of steam curing temperature and duration times on the mechanical performance, hydration products and microstructure of the concrete were investigated. The study revealed that extending curing time enhanced compressive strength for concrete cured at 60 ℃, but led to strength reduction at higher temperatures (70 ℃ and 80 ℃). Steam curing accelerated hydration but rapid product formation coarsened pores, reducing the compactness of the materials. XRD analysis revealed that anhydrite participation under steam curing promoted ettringite (AFt) formation in the concrete. At 3 d hydration age, the AFt phase remained stable at lower curing temperature (60 °C and 70 °C), whereas 80 °C curing induced its decomposition. TG-DTG analysis demonstrated that extending the curing time to 7 or 8 h significantly enhanced the hydration reaction of the concrete cured at 60 °C. SEM analysis revealed that thermal damage induced microcrack formation in concrete. In specimens cured at 60 °C, hydration products partially filled these microcracks. However, as the steam curing temperature increased to 70 °C, thermal damage became more severe, producing microcracks that could not be effectively repaired. At 80 °C steam curing, the combined presence of microcracks and pores facilitated secondary delayed ettringite formation (SEF), which induced cracking and reduced compressive strength. These microcracks further served as preferential pathways for chloride ion penetration, adversely affecting concrete durability.