We investigate the mechanisms by which titanium dioxide nanoparticles (nano-TiO2) interact with cement hydration products. To this end, we synthesize nanomodified cement samples with 1 wt% and 5 wt% of TiO2. We investigate the physical properties using depth-sensing-based methods such as statistical nanoindentation and microscopic scratch testing. Fourier transform infrared spectroscopy yields the chemistry, whereas micromechanics modeling provides insights into the nanostructure. The macroscopic plane strain modulus increases by 16% and 83%, respectively, and the macroscopic indentation hardness increases by 37% and 40%, respectively. The fracture toughness rises by 3% and 11%, respectively. Environmental scanning electron microscopy reveals a 30% reduction in crack width for TiO(2)cement nanocomposites compared to plain cement. Meanwhile, Fourier transform infrared spectroscopy and statistical deconvolution show an increase in the fraction of high-density calcium silicate hydrates (by 22% and 12%, respectively), and in the fraction of calcium hydroxide (by 101% and 251%, respectively). Within the framework of the colloidal and granular models of C-S-H, the increase in stiffness and strength after nano-TiO(2)modification of cement paste is due to the closely packed structure and the high atomic coordination number of high-density C-S-H. Similarly, due to the high dimensional stability of high-density C-S-H and calcium hydroxide, our results explain the reported improvements in drying shrinkage and creep properties following cement modification with nano-TiO2.