Experimental Study of Sulfoaluminate Cement-Based Rapid Repair Mortar Undergoing Hot/Wet Harsh Conditions: Mechanical Strengths, Hydration Products, and Ettringite Evolution Mechanism

Rapid repair mortar (RRM) is widely used to repair infrastructures, which must achieve high early strength and long-term service. This study investigated the influence of hot/wet harsh environmental conditions on RRM prepared using solid-waste-based sulfoaluminate cement (WSAC). Orthogonal experiments were conducted to obtain the optimized WSAC-based RRM. Then, the impact of simulated harsh conditions (cross-combination of 20/60 °C and 10/40/90%RH humidity) on RRM and synthetic ettringite was studied. High temperature and low humidity restrained the volume deformation and promoted the growth of the later-age compressive strength of the RRM, but reduced the flexural strength. In this case, ettringite lost some water molecules in the Ca–H2O groups and transformed into amorphous meta-ettringite, which retained its needle-rod morphology. A small amount of ettringite decomposed into gypsum, AH3, and Ca(OH)2 under high-humidity conditions, which caused a slight expansion and reduced the compressive strength of the RRM. Fortunately, most ettringite was well protected by the dense RRM matrix, and harsh temperature and humidity conditions could only slowly promote the transformation of ettringite in RRM over a long period. Therefore, short-term rain or hot weather has little effect on the mechanical properties of WSAC-based RRM. This study provides a theoretical basis and data support for the promotion of industrialization of WSAC-based RRM, which is in line with the concept of green and sustainable development.