The sulfate resi̇stance of mortar made from lowtemperature cured alkali̇ne acti̇vated hi̇gh furnace slag / portland cement

Cold weather concreting is associated with high energy demands and CO₂ emissions, driven by frost-prevention measures and limitations in using traditional cementitious materials. This study presents hybrid alkali-activated slag/Portland cement (PC) mortars as a sustainable solution for low-temperature construction. The effects of PC substitution (0%, 10%, and 20%) and varying alkali activator dosages (6%, 8%, and 10%) on mechanical performance, microstructural properties, and durability were evaluated at a curing temperature of 0°C. Hybrid mixtures demonstrated significant improvements in strength development and sulfate resistance compared to pure alkali-activated slag systems. A mixture containing 20% PC and 8% activator dosage achieved the highest compressive strength of 66.88 MPa at 90 days. At lower PC contents (10%), the blend exhibited superior durability under sulfate exposure, with only a 4.19% strength reduction after 90 days, compared to pure slag systems which experienced up to 10.6% strength loss. Pure granulated blast furnace slag (GBFS) mixtures activated with 8% alkali dosage reached 58.37 MPa at 90 days, demonstrating the potential of alkali activation in cold climates. Ultrasonic pulse velocity (UPV) results confirmed improved matrix densification in hybrid mixtures, with higher values correlating strongly with compressive strength at early curing stages. Scanning electron microscopy revealed a dense network of calcium silicate hydrate (C-S-H) and calcium-aluminosilicate-hydrate (C-A-S-H) gels, crucial for strength and durability, particularly in systems with 10–20% PC. These gels mitigated voids and reduced micro cracks at 0°C. However, excess alkali dosage (10%) led to the formation of alkali-rich phases, which reduced long-term strength. This research highlights the feasibility of hybrid alkali-activated mortars for energy-efficient and durable cold-weather construction, leveraging the synergistic effects of slag activation and PC hydration while reducing the reliance on external heating and chemical additives.