Slag Content Governs Compressive Strength of Ambient-Cured Alkali-Activated Mortars: An Experimental and Ensemble Machine Learning Study

Abstract

Alkali-activated materials are a sustainable alternative to Portland cement, yet the relative importance of activator and precursor parameters under ambient curing is unquantified, and literature-trained models are rarely validated against independent mixtures. Twelve fly ash–GGBS mortars were prepared in a 2 × 2 × 3 factorial design varying Na2O dosage (4% and 5%), silica modulus (1.0 and 1.5) and fly ash/GGBS ratio (70:30, 50:50 and 30:70) at constant water to binder (0.50) and binder to sand (0.33) ratios, then characterized by flow, compressive strength at 7, 14 and 28 days, water absorption and scanning electron microscopy. Four ensemble models trained on 361 published records were tested on the withheld mixtures. Ambient cured strengths of 31.83–57.91 MPa were obtained, 95% developing by 14 days. Factorial analysis ranked the fly ash/GGBS ratio first (∆ = 18.83 MPa), followed by Na2O dosage (∆ = 3.83 MPa) and silica modulus (∆ = 0.31 MPa), higher GGBS fractions giving lower water absorption and denser matrices under SEM. LightGBM gave the highest cross-validated accuracy (R2 = 0.793; range 0.738–0.793). Precursor calcium content governs strength in ambient cured systems, allowing sodium silicate to be reduced without mechanical penalty and establishing data-driven design as a screening tool that condenses and improves the selection of experimental work towards the application level.