Effect of nano-silica on sodium hydroxide activated blended millet husk ash-calcium carbide residue concrete

Abstract

The production of Portland cement is associated with high energy consumption and significant carbon dioxide (CO₂) emissions, necessitating sustainable alternative binders. This study investigates the effect of nano-silica (NS) on the fresh and mechanical properties of sodium hydroxide (NaOH)-activated blended Millet Husk Ash-- Calcium Carbide Residue (MHA-CCR) concrete. Nano-silica was extracted in-house from MHA via a sol-gel chemical precipitation method. A four-factor Central Composite Design (CCD) was employed, varying MHA content (55-65%), CCR content (30-45%), NS dosage (3-6%), and NaOH molarity (8-12M) across 30 experimental runs. Slump, setting time, density, compressive strength, and splitting tensile strength were evaluated at 7, 14, 28, and 56 days. Increasing NaOH molarity and NS dosage reduced workability and accelerated setting, although slump remained above 45 mm for most activated mixes. NS incorporation improved density, compressive strength, and splitting tensile strength through enhanced matrix densification and cohesion. At 28 and 56 days, the MHA-CCR-NS system achieved compressive strengths of 41 N/mm² and 50 N/mm², respectively, equivalent to approximately 87% and 82% of the OPC control at the same ages. CCD optimization identified an optimal mix of 61.5% MHA, 34% CCR, 4% NS, and 11 M NaOH, with a predicted 28-day compressive strength of 41.86 N/mm². A desirability value of 1.0 indicates that the imposed response constraints are optimally satisfied within the experimental design space. These findings demonstrate the technical feasibility of producing structurally viable, low-carbon alkali-activated concrete from locally available agro-industrial wastes. Further studies are recommended to evaluate long-term durability and field performance.