Hardening Properties and Enhancement Mechanisms of Modified Alumina Cement at Minus Temperature

Because the hydration reaction of cement is unable to proceed below the freezing point and hinders cementing, a research on the hardening properties optimization and enhancement mechanism for early strength of modified alumina cement was conducted. Changes in setting time, fluidity and compressive strength of cement at different temperatures of 0, −10, and −18 °C were compared, and the relationship between these changes at different temperatures and the setting time modifier SCEG was studied. Based on the analysis of phase compositions and microstructures of hydration products, the mechanism of hardening of cement at minus temperature was studied, and gypsum was added to promote the formation of AFt so as to enhance the compressive strength of hardened cement paste. The experiment demonstrated that the cement can be hardened within 0.3–6.0 hour when SCEG about 0–3% cement mass is added to the modified alumina cement from 0 °C to −18 °C. In that case, its compressive strength can reach 9.7–11.2 MPa within 24 hours and even rise 30% after gypsum about 10%–15% cement mass is added. X-ray diffraction (XRD) analysis indicates that the main hydrated mineral in the cement is aluminate clinker instead of silicate clinker at minus temperature, with Ca₂Al(Al,Si)₂O₇, AFm, AFt, and a few C－S－H as the main products. To be specific, a higher content of AFt can bring higher compressive strength of hardened cement paste. The results show that modified alumina cement possesses optimized hardening properties at minus temperatures, and appropriate amount of gypsum can increase the content of AFt in hydration products, improving the compressive strength of hardened cement paste.