Durability-Oriented Design of Alkali-Activated Mortars for Low-Carbon Construction: Single, Binary, and Ternary Precursor Combinations

The construction industry needs durable low-carbon binders whose engineering behaviour is reproducible across precursors, yet published comparisons of alkali-activated mortars rarely place single, binary and ternary systems side by side under the same workability constraint. This study tests seven mortars built from fly ash (FA), ground granulated blast furnace slag (GGBFS) and metakaolin (MK) in single, binary and ternary combinations, with the alkali activator-to-binder ratio adjusted between 0.40 and 1.00 to obtain a consistency suitable for mixing and casting, so that activator dose, water content and precursor composition are reported together. Water absorption, bulk density, sorptivity, compressive strength, residual strength after 25 freeze–thaw cycles and after 75 days in 5% sulfuric acid, and gel chemistry from scanning electron microscopy with energy-dispersive X-ray spectroscopy are reported. The slag mortar (U-S) formed the densest, strongest system (2.36% absorption, 2366.5 kg/m3, 97 MPa) and retained 97% of its strength after freeze–thaw cycling and 78% after acid exposure. Fly ash alone (U-F) retained about 63% of its strength under acid attack, second only to U-S, reflecting the intrinsic acid stability of its low-calcium N-A-S-H gel, while the fly ash-metakaolin blend (BI-FM) and the ternary blend lost the most strength under acid and frost respectively. The results identify two distinct controls on durability, matrix density and gel chemistry, and provide a quantitative basis for selecting precursor combinations in low-carbon construction.