Compatibility and Performance Evaluation of Early-Strength Admixtures in Repair Mortar Containing Recycled Concrete Powder and GGBFS

Repair mortars containing recycled concrete powder (RCP) and ground granulated blast-furnace slag (GGBFS) are promising low-carbon materials for the rapid repair of concrete structures and pavements. However, their practical use is often limited by slow early hydration, insufficient early strength, and weak bonding with existing concrete substrates. In this study, four early-strength admixtures, namely calcium formate, anhydrous sodium sulfate, calcium acetate, and triethanolamine, were incorporated into a P·I 42.5 cement-based repair mortar containing RCP and a low dosage of GGBFS. Their effects on fluidity, flexural and compressive strength, tensile bond strength, drying shrinkage, and hydration characteristics were investigated. The results showed that the suitable dosages of calcium formate, anhydrous sodium sulfate, calcium acetate, and triethanolamine were 1.5%, 1.0%, 0.8%, and 0.05% by mass of total cementitious materials, respectively. Among the four admixtures, calcium formate provided the best balance among strength enhancement, bond performance, workability retention, and dosage tolerance. Compared with the control group, the 3 d and 28 d flexural strengths of the 1.5% calcium formate group increased by 37.0% and 20.3%, respectively. Anhydrous sodium sulfate gave the highest tensile bond strength, with the 14 d value increasing by 33.15% to 1.052 MPa, but its effective dosage range was relatively narrow. Calcium acetate was more effective in reducing drying shrinkage, with a 28 d shrinkage value of 695.14 × 10−6. SEM and XRD results suggested that the admixtures mainly accelerated early hydration, while no new major crystalline phases were detected. Excessive dosages caused strength loss, bond deterioration, or increased drying shrinkage. These findings are applicable to the specific RCP–GGBFS repair mortar formulation and dosage ranges investigated here. They provide a practical basis for selecting early-strength admixtures for RCP-containing repair mortars used in concrete structure and pavement repair.