Influence mechanisms of silica nanoparticles' property enhancement in cementitious materials and their green synthesis: A critical review

Green synthesis of silica nanoparticles (SiO 2 -NPs) from agricultural residues is a promising avenue for cementitious materials application. However, very few studies are dedicated to reviewing the influence mechanisms underlying the green synthesis methods and the mechanisms governing their effectiveness in property enhancement in cementitious materials. Our review established that the influence mechanisms of green synthesis of SiO 2 -NPs include fracturing, polymerization, solubilization, and oxidoreductase. The highest silica purity of over 99 % was mainly reported in chemical sol -gel synthesis, meanwhile, the biggest surface area was reported with mechanical milling of up to 1441 m 2 /g from rice straw ash. SiO 2 -NPs were found to enhance the cementitious matrix mainly through nucleation seeding and pozzolanicity. Our review critically discusses other influence mechanisms of the hydration kinetics and interfacial transition zone, pore structure, and matrix densification optimization. SiO 2 -NPs were reported to enhance the early compressive strength of cementitious materials, sulfate attack resistance, and chloride permeability by up to 100.6 %, 79 %, and 86.2 % respectively. Optimum dosages based on compressed strength improvement varied mainly between 1.0 % and 4.0 %. Existing research seems not to address how the synthesis influencing parameters, such as pH, gelation time, temperature, and milling parameters could be optimized, hence an avenue for future research.