Mixture design optimization of millet husk ash, calcium carbide residue, and nano silica for sustainable concrete

Abstract

This study investigates and optimizes a ternary binder system incorporating millet husk ash (MHA), calcium carbide residue (CCR), and nano-silica (NS) using a statistically driven mixture design approach. A constrained optimal design was employed to evaluate the individual and interactive effects of the components on setting time and compressive strength at 7, 28, 56, and 120 days. Results show that setting behaviour is governed by competing mechanisms, with CCR inducing retardation and NS accelerating early hydration, while MHA–NS interactions enable controlled modification of setting characteristics. Compressive strength increased progressively with curing age, reaching a maximum of 53.47 N/mm² at 120 days, demonstrating the complementary roles of NS in early-age reactivity and MHA in long-term pozzolanic activity. The developed regression models exhibited good agreement with experimental data and were validated through confirmatory experiments with prediction errors within ±5.2%. Multi-response optimization identified an optimal composition of MHA ≈ 60.8%, CCR ≈ 35.8%, and NS ≈ 3.3%, achieving a balanced combination of strength and setting performance. The findings demonstrate the feasibility of producing structural-grade concrete using high volumes of waste-derived binders, offering a sustainable alternative to conventional Portland cement. However, applicability is limited to the defined compositional ranges, and further studies are required to assess durability and long-term performance.