Effects of a mineral-based viscosity-reducing admixture on the properties and microstructure of C60 manufactured sand concrete

This study systematically investigated the effects of a self-developed mineral-based viscosity-reducing admixture (VRA) on the macro-properties and microstructure of C60 high-strength manufactured sand concrete. The effectiveness and underlying mechanisms of VRA were evaluated by examining variations in rheological properties, mechanical strength, durability, hydration behavior, and pore structure across a VRA dosage range of 0%–12%. Results indicate that the VRA significantly improved the workability of fresh concrete through the micro-filling effect of ultrafine particles and the morphological lubrication from spherical particles, with improvements increasing progressively with dosage. At 8% VRA, the yield stress and plastic viscosity of mortar decreased by 27.1% and 29.2%, respectively, while the T 500 time and inverted slump cone drainage time of concrete were reduced by 26.2% and 49.8%, respectively. An optimal dosage of 8% VRA was identified, leading to increases of 3.5% in 28-day compressive strength, 6.5% in splitting tensile strength, and 3.4% in elastic modulus. Mechanical properties began to decline beyond 10% VRA owing to cement dilution. The VRA also improved chloride ion penetration resistance and increased autogenous shrinkage, although it moderately suppressed drying shrinkage. Microstructural and hydration analyses revealed that 6% VRA promoted later-age hydration, enhanced the formation of hydration products, reduced total pore volume, and refined the pore size distribution. In contrast, 12% VRA increased total pore volume due to insufficient hydration products. This study offers theoretical and experimental support for the rational application of mineral-based viscosity-reducing admixtures in high-strength manufactured sand concrete. • Developed a novel mineral-based VRA specifically for high-strength manufactured sand concrete. • 8% VRA dosage optimally enhances workability, strength, and chloride resistance. • Multi-scale analysis establishes structure-property relationships for practical guidance.