Development of 300 MPa ultra-high-strength mortar through a special curing regime

The goal of this study was to develop 300 MPa ultra-high-strength mortar (UHSM) based on optimization of ingredients, mix proportion, and curing regime. To fabricate eco-friendly UHSM, two types of supplementary cementitious materials, i.e., silica fume (SF) and ground granulated blast furnace slag (BFS), were also incorporated. An optimized mixture was finally determined based on numerous compressive strength tests and mercury intrusion porosimetry (MIP) analysis, and several important findings were obtained as follows. Using coarse particle cement (CPC) was beneficial in delaying a decrease of the fluidity of fresh mortar and enhancing strength due to a higher packing density than ordinary Portland cement (OPC). The use of zirconium (Zr) SF was effective in improving the strength of UHSM compared to that of regular SF, and its optimum replacement ratio was 20%. The optimum replacement ratios of BFS, CPC, and anhydrous gypsum (Gy) were found to be 20–25%, 30–70%, and 5%, respectively, for making UHSM with higher compressive strength. The curing regime significantly influenced the strength development of UHSM (i.e., longer curing under heat and high relative humidity led to the higher strength), and the optimum curing regime was suggested to consist of 48-h normal curing, 120-h steam curing, and 24-h heat curing. We developed UHSM with a compressive strength of 341 MPa based on the optimized mix proportion and curing regime.