Properties of green ultra-high performance sulfoaluminate cement concrete incorporating recycled sand under sustained low temperature curing

Abstract

In this paper, green ultra-high performance sulfoaluminate cement concrete incorporating recycled sand (GS-UHPC) was developed, and its low temperature properties were studied. The setting time, hydration phases, internal temperature, pore structure, micromorphology, compressive strength, and flexural performance of GS-UHPC cured at 20 °C, 10 °C, 2 °C, −5 °C, and −10 °C were comprehensively evaluated. Meanwhile, digital image correlation and acoustic emission techniques were used for analysis. Results suggested that the curing temperature had a limited effect on the rapid setting characteristic of GS-UHPC. However, as the temperature dropped from 20 °C to −10 °C, the hydration rate and hydration degree progressively declined. For GS-UHPC cured at 2 °C, the microstructure deteriorated relative to that cured at 20 °C and 10 °C, forming more less-harmful and harmful pores, which slightly increased the total porosity. At −5 °C and −10 °C, a substantial amount of unhydrated phases remained in GS-UHPC, and the proportion of more-harmful pores increased obviously, but the hydration reaction continued with prolonged curing, gradually improving the microstructure. Therefore, although the mechanical strength, toughness, and crack resistance of GS-UHPC were reduced at low temperatures, these properties still showed rapid enhancement within the first 4 hours and 3 days of curing, and steadily improved over time. For example, the compressive and flexural strengths at 28 d decreased from 121.1 MPa and 19.36 MPa at 20 °C to 62.2 MPa and 12.14 MPa at −10 °C. Notably, at −10 °C, the compressive and flexural strengths reached 15.1 MPa and 5.89 MPa at 4 h, and 31.4 MPa and 8.32 MPa at 3 d, respectively. These findings reflect the distinctive low temperature performances of GS-UHPC, indicating its good application prospects for engineering construction and repair in cold regions.