Cold‐cured alkali‐activated slag/Portland cement composites: The role of cement content and alkali dosage on mechanical performance, microstructure, and sulfate durability

Abstract In cold‐weather concreting, energy demand and CO 2 emissions are significantly heightened, largely due to frost‐protection practices and constraints on employing standard cementitious materials. Portland cement (PC) undergoes delayed hydration under cold weather conditions, slowing strength development and posing significant challenges for winter applications. Alkali‐activated slag (AAS), specifically derived from ground granulated blast furnace slag (GBFS), emerges as a viable alternative, providing continuous hydration at low temperatures. This study investigates the influence of PC content (0%, 10%, and 20%) and alkali activator dosage (AD) (6%, 8%, and 10%) on mechanical performance, microstructural properties, and sulfate resistance of AAS mortars cured at 0°C. Compressive strength and ultrasonic pulse velocity (UPV) tests were conducted after 7, 28, and 90 days. Resistance to sulfate attack was evaluated by exposing the mortar samples to 5% magnesium sulfate solution for 90 days. A mixture containing 20% PC and 8% activator dosage (AD) achieved the highest compressive strength of 66.9 MPa after 90 days of curing. Increasing the PC content enhanced compressive strength at all curing ages. UPV results confirmed improved matrix densification in mixtures incorporating PC, with higher values correlating strongly with compressive strength at early curing stages. Mixtures formulated with 6% alkali dosage and 100% GBFS demonstrated exceptional resistance to sulfate ingress and degradation; following sulfate exposure, their strength increased by 38.7% relative to the unexposed control specimens. Scanning electron microscopy revealed that increasing the PC content resulted in a denser internal structure, although this microstructure was disrupted by sulfate attack in certain mixtures. Considering the balance between strength and durability, mixtures containing 10% PC demonstrate the potential of alkali activation in cold temperatures. Overall, the results indicate that AAS shows significant promise for 0°C applications when the mixture design is optimized.