Synergistic Utilization of Recycled Fine Aggregates and Air-Entraining Agents in Alkali-Activated Concrete: Early-Age Strength Development under Subzero Temperatures and Freeze-Thaw Resistance

Abstract To overcome dual technical constraints—setting inhibition and freeze-thaw damage—in cold-region concrete construction while maximizing performance-cost efficiency, this study integrates alkali-activated slag concrete (AASC) and recycled fine aggregate (RFA) two green technologies, conducting comprehensive low-temperature performance investigations. Research examines early-age strength development under subzero conditions and long-term frost resistance at the strength stabilization stage. Air-entraining agents (AEA) were selected to counteract RFA-induced negative impacts through pore structure optimization, demonstrating the RFA-AEA synergy's dual influences on air-entrainment mechanisms and concrete strength. Key metrics—setting time, entrained air content, compressive strength, and relative dynamic elastic modulus—undergo systematic analysis. Critical findings reveal that:(1) RFA significantly shortens setting time and impairs air-entraining efficacy, substantially reducing AASC workability. (2) Under low-temperature curing, AASC hydration depends primarily on alkali-activator dosage, exhibiting superior setting capability versus OPC based concrete. Early strength development correlates with initial raw material temperature; preheated and thermal protected 4% alkali-activator AASC rapidly achieves 5 MPa critical freezing damage threshold. (3) RFA promotes early strength via alkaline release-induced hydration compensation at low temperatures, but this effect diminishes with increasing alkali-activator dosage. Traditional air-entraining technology provides no significant early strength enhancement at -15°C. (4) While RFA-incorporated AASC still maintains excellent freeze-thaw resistance, excessive replacement ratios impair air-entrainers' pore-structure optimization capacity and weaken frost resistance improvements. At 50% replacement, RFA and air-entrainers demonstrate optimal synergistic compatibility in AASC, enabling concurrent early strength development and frost resistance, providing a sustainable low-carbon solution for winter concrete construction.