Ultra-early performance and hydration characteristics of sulfoaluminate cement paste under negative temperatures

The fast hardening of sulfoaluminate cement provides a way for rapid repairing of transportation concrete infrastructures in cold regions. This study investigated the ultra-early (i.e., 4, 8 and 24 h) mechanical performance, hydration characteristics and pore properties of sulfoaluminate cement paste with multiple water-to-cement (w/c) ratios under several negative temperatures. Results showed that negative temperatures had a more significant impact on 4-h performance. Compared with the paste cured at 0 °C, reducing the temperature to − 20 °C led to a decrease of up to 41.50% and 63.85% in the 4-h compressive and flexural strength, respectively. Given a longer curing age of 8 and 24 h, the difference in the strength gap was lessened. This is because negative curing temperatures mainly affected the formation of AFt during the first 4 h of hydration. The lower the curing temperature, the less the amount of AFt. Therefore, an increased porosity as well as a deteriorated pore structure with less pores smaller than 100 nm can be expected. In addition, at negative temperatures, increasing the w/c ratio from 0.30 to 0.34 caused mechanical properties to increase first and then decrease. With a higher w/c ratio, the excessive free water will freeze in negative temperatures to adversely affect cement hydration and pore structure of sulfoaluminate cement pastes. The results presented in this study provide insights into improving rapid repair materials for cold climates.