Influence of nano-silica on the behaviour of high-performance concrete – multiscale investigation on its mechanical, durability and microstructural properties

In recent years, several studies have been performed with mineral admixtures to enhance the characteristics of high-performance concrete (HPC). However, the long-term studies on the durability characteristics of HPC with nano-silica (NS) and its behaviour with various w/b ratios need to be investigated. Thus, the impact of NS on HPC performance at low and high w/b ratios was investigated in this research. The HPC mixes were prepared with water-to-binder ratios (w/b) of 0.3 and 0.45 with 0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0% NS as substitute for cement. The concrete properties such as workability and strength (compression, tension, flexure) and elastic modulus were investigated with various percentages of NS. The optimized use of NS was observed to be 2.0%, exhibiting enhanced properties in HPC. The durability studies performed on the optimized HPC mixes include shrinkage, resistance to chemical attacks (acid and sulfate), water permeability, and chloride penetration for both w/b ratios. The workability of HPC lowers with an increase in the NS showing a maximum of 4.66% at 0.3w/b and 8.55% at 0.45w/b ratios. The strength of HPC mixes at 0.3w/b and 0.45w/b was 14.32% and 11.95% at 28 days and 15.52% and 13.26% at 90 days. The shrinkage, water penetration and chloride penetration of the optimized HPC mixes were 35.85%, 37.90% and 54.13% lower than control mixes. Microstructural examination using SEM confirm the compactness of the concrete matrix through the formation of additional C–S–H gel subsequent from the substitution of NS. The research inference highlights that the substitution of NS lowers the workability of HPC mixes owing to its finer particle size, however, its optimum utilization improves both the hardened and durability properties. Furthermore, prolonged curing further enhances these properties, even at higher w/b ratios.