Mix design optimization and performance enhancement of high-volume limestone powder concrete with nano-silica

Reducing cement consumption through the incorporation of supplementary cementitious materials (SCMs) is an effective strategy for developing low-carbon concrete. This study develops a practical mix-design framework for low-carbon concrete by optimizing a high-volume limestone powder (LP) binder system modified with fly ash (FA), silica fume (SF) and nano-silica (NS). A four-factor three-level Box–Behnken Design was adopted (29 mixtures). The investigated factors were LP replacement (10–35%), FA (10–15%), SF (0–10%) and NS (1–3%), and the responses were compressive strength at 7, 14 and 28 days (Y1–Y3) and 28-day flexural strength (Y4). Quadratic response surface models were established with high goodness-of-fit. Microstructural analysis revealed that the incorporation of NS effectively enhanced the reactivity of LP-dominated matrices by promoting C–S–H formation and reducing CH content, thereby improving the packing density and structural compactness. Based on multi-performance objectives, the optimized mixture was determined as 29.03% LP, 5.43% FA, 1.56% SF, and 1.36% NS, with deviations between predicted and experimental strengths controlled within 5%. This work provides a practical design framework for formulating low-carbon concrete using synergistic SCM combinations and demonstrates that high-volume LP concrete can achieve both sustainability and mechanical reliability through rational mix proportioning.