High Strength Concrete (HSC) has become an essential material in contemporary architecture due to its ability to deliver excellent strength, durability, workability, and resistance to heavy loads and aggressive environmental conditions. HSC offers higher structural efficiency, longer service life, and many other advantages compared to conventional concrete, making it an eminent material option for high-rise buildings, bridges, industrial facilities, etc. This paper deals with the effect of using nano-silica as a partial replacement for cement on the mechanical properties and durability of high-strength concrete.

Nano-silica is an ultrafine, high-surface-area pozzolanic material that promotes the hydration of cement. It combines with calcium hydroxide produced during cement hydration to create more calcium silicate hydrate (C-S-H) gel, the main compound that gives concrete its strength. It enhances the pore structure, reduces micro-cracks, and produces a denser, stronger concrete matrix. This study investigated the effect of nano-silica as a partial replacement of cement at 0%, 1%, 2%, 3%, 4% ands5% by weight of cement to find its optimum dosage.

An extensive experimental program was conducted to evaluate the behaviour of the developed mixes. Manipulating fresh concrete properties, slump and flow tests were performed to characterise workability. Mechanical properties were evaluated in hardened concrete to obtain compressive strength, split tensile strength, flexural strength and modulus of elasticity. Specimens were subjected to long-term performance tests: water absorption, rapid chloride penetration, sulphate resistance, carbonation resistance and alkali-silica reaction. Moreover, SEM and XRD tests were conducted to study the microstructure and determine the type of hydration products in light of the use of nano-silica.

Experimental results indicated that the incorporation of nano-silica significantly enhances both strength and durability properties of high-strength concrete. From these mixtures, however, the concrete with 2% of nano-silica provided the most overall performance ultimately. The 28-day compressive strength obtained at this optimal replacement level was around 82.4 MPa, which is nearly about 28% higher than the mix control. Splitting tensile strength and flexural strength were similarly improved by about 28%, further confirming the beneficial effects of nano-silica on the overall mechanical properties of concrete. Conversely, for mixes with more than 3% nano-silica content, the workability decreased significantly, after which strength enhancement also appeared to lag due to particle agglomeration that impeded the proper dispersion of nano-sized particles within the cement matrix.

The durability assessment showed that nano-silica improved the concrete's resistance to all vectors of deterioration mechanisms. The presence of nano-silica resulted in lower water absorption and less permeability through concrete, thus limiting the penetration of aggressive substances (chloride ions and sulphate), leading to better environmental durability. In addition, the developed concrete exhibited a good resistance to carbonation and alkali-aggregate reaction; therefore, showing its applicability in aggressive environmental conditions. The SEM and XRD analyses supported the improvements in mechanical as well as durability properties, which can be attributed to a denser microstructure with more C-S-H gel and fewer internal voids.

According to the results of this research, it can be concluded that nano-silica could appropriately be used as a supplementary cementitious material to improve high-performance concrete's properties. The optimal replacement level was determined to be equal to 2% of the weight of cement, considering a good balance between workability, strength and durability. The enhanced merits obtained at this dosage imply that nano-silica modified high-strength concrete can be effectively used in challenging engineering applications, including but not limited to high-rise buildings, bridges, marine structures, industrial constructions and other vital infrastructure projects where superior mechanical performance and long-term durability are required.