Role of Modifiers on the Properties of One-Part Alkali-Activated Rapid Hardening Repair Mortar

Rapid-repair mortars require high early-age strength, dimensional stability, and reliable substrate bond simultaneously, yet conventional alkali-activated materials (AAMs) have only partially met these requirements, and AAM feedstock base itself is contracting as blast furnace slag and fly ash availability declines under steel and power sector decarbonization. Thus, in this study, the systematic production of a one-part, rapid hardening repair mortar based on calcined clay (CC) and basic oxygen furnace slag (BOFS) is presented for the first time. Concurrently, a direct comparison of three distinct modifier families is conducted within this underutilized binding system. These modifiers consist of a soluble anion-active accelerator (sodium fluoride, NF), an Fe- and Na-bearing mineral residue (red mud, RM), and a reactive oxide (calcined alumina, CAL). Finally, the mechanistic connections between the modifier-induced phases and the macroscale mortar properties are analyzed and evaluated according to ASTM C928, ASTM C1600, and EN 1504-3 standards. The precursors were activated using solid sodium metasilicate, and the setting behavior, compressive strength development, drying shrinkage, substrate bond strength, and microstructural properties were determined for each mortar system. The results indicate that all formulations satisfied the R2 strength class for rapid hardening repair mortars as per ASTM C928 and the incorporation of NF markedly promoted the early-age reactions, reducing the setting time by up to 73% and increasing the 3 h compressive strength by up to 81% (12.8 MPa at 3 h) compared to the control mortar. Furthermore, RM, CAL, and NF effectively mitigated the drying shrinkage of the control mortar from approximately 4252 µƐ down to 162 µƐ. The bond strength of the repair mortars substantially improved through the addition of CAL and NF, fulfilling the R3 and R4 structural repair mortar requirements specified in EN 1504-3.