Engineering properties of fiber-reinforced alkali-activated blast furnace slag/portland cement mortars cured at low temperatures

This study investigates the mechanical performance and acid resistance of cold-cured hybrid alkali-activated mortars composed of granulated blast furnace slag (GBFS) and partially substituted Portland cement (PC). Two alkali activator dosages (8% and 12% Na₂O equivalent by binder mass) and three basalt fiber (BF) contents (0%, 0.3%, and 0.6%) were examined to evaluate their effects on compressive strength, ultrasonic pulse velocity (UPV), and mass changes after 90 days of sulfuric acid exposure. All mixtures were initially cured at 0°C for 90 days before acid immersion. Results revealed that increasing the activator dosage improved early densification but led to microstructural vulnerabilities in the absence of fiber reinforcement. The optimal mix with 8% alkali, 15% Portland cement, and 0.6% BF achieved the highest 90-day strength (61.72 MPa) under cold curing, while higher alkali dosage (12%) improved early strength but reduced long-term performance in fiber-reinforced mixes. The highest post-acid compressive strength (69.33 MPa) was observed in the mixture containing 8% AD, 100% GBFS, and 0.6% BF, while the lowest strength values were associated with fiber-free and PC-containing compositions. SEM analyses confirmed that acid exposure promoted secondary densification around fibers but also revealed localized degradation in mixtures rich in PC. The findings demonstrate that cold curing combined with optimized alkali activator dosage and fiber reinforcement can effectively enhance the mechanical and acid resistance of hybrid alkali-activated mortars. However, careful control of PC content is critical to minimizing acid-susceptible phases and maximizing long-term performance under aggressive environments.