In the present work, the effect of different percentages of nano-montmorillonite (NM) powder on the mechanical and microstructural properties of hardened cement paste and mortar were studied. The portland cement was replaced with montmorillonite nanoparticles with 0.5, 1, 1.5, 2, 2.5, and 3 weight percentages of cement. The mechanical properties of cement mortar including compressive strength, tensile strength, and flexural strength according to the ASTM standard and the microstructure of cement paste were investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-powder, field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDX) techniques. The obtained results revealed that adding cement with replaced montmorillonite nanoparticles up to 1.5% will enhance the compressive strength, tensile strength, flexural strength, and improve the microstructure of cement because of filler effect and acceleration of cement hydration through the nucleation effect. Therefore, 1.5% addition of montmorillonite nanoparticles can be considered as an optimum value.
