Effectiveness of Nano Silica Extracted from Rice Husk Ash (RHA) in Cementitious Media

Abstract

The use of concrete is rapidly increasing worldwide, primarily harming the environment due to its primary component, cement, which accounts for roughly 8% of global CO2 emissions. Hence, exploring sustainable alternatives is crucial. Extracted nano-silica (NS) from rice husk ash is gaining interest as a sustainable construction material. In this study, RHA and extracted SP and NS from RHA were characterized comprehensively at pH 3 and pH 10 to evaluate their suitability for application in cementitious media. By utilizing laser particle sizing, scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF) techniques, the chemical composition, physical characteristics, and microstructure of these binders were examined. The setting time, soundness, autoclave expansion, workability, microstructure, flexural strength, compressive strength, and durability of RHA/SP/NS blended mortar and concrete were assessed. Setting time increased with RHA and SP replacement but was significantly reduced with NS due to early hydration, resulting in early strength development. RHA, SP, and NS blended mortars showed better flexural and compressive strength than the control mix due to higher C-S-H formation. With 2.5% RHA, 10% SP, and 5% NS replacement, maximum strength and better durability were observed in the mortar. At a constant w/b ratio, the compressive strengths found were 59.2 MPa, 60.8 MPa, and 56.2 MPa after 90 days of curing. RHA increased drying shrinkage slightly. However, SP and NS reduced it and improved sulfate resistance at later stages of the curing process. RHA increased the water absorption rate in mortar, whereas adding SP and NS significantly decreased it. On the other hand, in concrete, 7.5% RHA/SP/NS achieved the maximum strength (31.7 MPa, 34.1 MPa, and 33.7 MPa, respectively) and exhibited better durability performance. By increasing the packing density of concrete and reducing porosity, the compressive strength of RHA/SP/NS blended concrete increased significantly, providing better chloride resistance than the control sample. The findings suggest that RHA, SP, and NS have potential as alternative construction materials.