The nano-montmorillonite, which has characteristics of high aspect ratio and interaction between polymer chains and dispersed nanolayers, has been widely used in the development of new reinforced nanocomposite polymers to improve their mechanical properties. Since a potential pozzolanic reaction may occur between Portland cement paste and high amount of silicon dioxide (SiO2) in nano-montmorillonite, the effects of introduction of montmorillonite to Portland cement-based material on the improvement of matrix properties of cement paste is of great interest in the construction industry. In this study, a liquid-form of nano-montmorillonite particle with a planar diameter of about 100 nm were incorporated into the Portland cement paste at five different dosages and analyzed at four different ages to identify the nanosizing effects on material properties of such cement-based composite. Experimental results show that the composite with 0.60% and 0.40% of added nano-montmorillonite by weight of cement have the optimum compressive strength and permeability coefficient, respectively, in which the increase of compressive strength is about 13.24%, and the decrease of permeability coefficient about 49.95%. Microstructural properties through the analyses of XRD, DSC, NMR, and MIP also indicate that the microstructures of cement paste with nano-montmorillonite contain more dense solid material and more stable bonding framework.