Optimization of Fonio Husk Ash and Calcined Waste Treatment Sludge for Age-Dependent Compressive Strength of Portland Limestone Cement Concrete

The sustainable application of agro-industrial and treatment facility waste-based materials in concrete requires the collective assessment of material reactivity, substitution amount and strength development. This study examined the combined effects of fonio husk ash (FHA), calcined waste-treatment sludge (CWTS) and curing age on the compressive strength of Portland limestone cement (PLC) concrete via a face-centered central composite design (FCCD) with response surface methodology (RSM). Physicochemical characterization revealed high surface areas for FHA and CWTS, whereas X-ray fluorescence analysis revealed combined SiO2 + Al2O3 + Fe2O3 contents of 82.24% and 73.20%, respectively. The compressive strength was evaluated at 7, 28, 56 and 90 days. The strength increased with increasing curing age, whereas excessive combined replacement reduced the strength, indicating that clinker loss becomes dominant beyond a threshold. The RSM models demonstrated strong predictive ability, with R2 values ranging from 0.9969 to 0.9998. Numerical optimization produced optimal FHA and CWTS contents of 7.63% and 10.21%, respectively, at a desirability of 0.728. The validation results revealed errors of less than 4% across all curing ages. Thus, FCCD-RSM reliably captured composition- and age-dependent strength trends, providing a basis for valorizing agro-industrial and treatment wastes in lower-clinker PLC concrete.