Synergistic effect of nano silica and metakaolin on mechanical and microstructural properties of concrete: An approach of response surface methodology

Nano and micro materials are both effective individually in enhancing the mechanical and microstructural properties of concrete. The combination illustrates an iconic shift in material engineering to produce concrete with distinctive characteristics. In this work, the combined effect of Nano Silica (NS) and Metakaolin (MK) on the mechanical and microstructure properties were examined by conducting compressive strength, split tensile strength, and Field Emission Scanning Election Microscope (FE-SEM) with Energy-dispersive X-ray (EDX) analysis, X-ray Diffraction (XRD) spectroscopy, Fourier Transform Infra-Red (FTIR) spectroscopy, respectively. The concrete matrix was investigated by varying the concentration of NS and MK with different Water-Binder Ratio (WBR). Further, the Response Surface Methodology (RSM) technique was used to identify the complex interaction between NS (1–3 %) and MK (5–20 %) at different WBR (0.35, 0.40 and 0.45). The synergistic effect between NS and MK resulted in enhanced compressive and split tensile strengths of the concrete. The optimum mix proportion of the concrete was found to be 2 % NS and 12.5 % MK at a WBR of 0.4. The compressive strength enhancements for the optimum mix proportion were 35.75 %, 34.4 %, 33.1 %, and 32.5 %, while the split tensile strength enhancements were 26.56 %, 25.92 %, 20.95 %, and 17.8 %, respectively on the 7th, 28th, 56th, and 90th day. The RSM technique developed for all the mixes has been statistically validated to have less than 5 % of mean error. Moreover, the microstructural analysis revealed early hydration, formation of additional Calcium Silicate Hydrate (C-S-H) products, and presence of dense microstructure resulting in improved microstructural properties. The incorporation of NS and MK in the concrete changes the concrete’s configuration at the nano and micro levels, thus paving the way for the development of novel and sustainable construction materials. [Display omitted] •Blended effect of nano silica and metakaolin with different water-binder ratios is investigated.•Significant strength improvement is observed in nano silica - metakaolin modified concrete.•The mechanical properties of nano silica-metakaolin modified concrete is predicted using response surface methodology.•The hybrid effect resulted in a dense microstructure of the modified concrete.