AbstractIn this study, the effects of zero-valent iron (ZVI) nanoparticles (NPs) on the mechanical performance of cementitious composites were investigated. For this purpose, the use of a novel nanomaterial containing ZVI NPs blended with carboxymethyl cellulose (CMC) and SiO2 was introduced. CMC/SiO2-ZVI NPs were prepared by a green and low-cost sonochemical method at room temperature and characterized using scanning electron microscopy/energy dispersive X-ray spectroscopy, thermal gravimetry analysis, Brunauer–Emmett–Teller, and X-ray diffraction techniques. The values of intrinsic viscosities of CMC/SiO2-ZVI NPs were examined to determine the optimal miscible condition as affected by the different mass ratios of CMC/SiO2 and sonication time. Then, eight different cement mortar specimens containing CMC/SiO2 and CMC/SiO2-ZVI NPs were prepared in ratios of 0.2%, 0.5%, 1%, and 2% by weight, and flexural and compressive strengths of the specimens were determined at the ages of 7 and 28 days. The microstructure of the specimens was also evaluated. Comprehensive results proving the improvement in the mechanical properties have been obtained in the specimens containing 1% and 2% CMC/SiO2-ZVI NPs.