Abstract Data from small-angle X-ray scattering experiments are used to determine the fractal types and dimensions of hydrated portland cement pastes over a range of length scales. The larger-scale, 200–1500 Å, geometry at most degrees of water saturation is found to be that of a rough surface fractal. The smaller-scale, 30–200 Å, geometry is found to be a mass fractal in saturated pastes. This geometry changes gradually with drying, and becomes a rough surface fractal at saturations less than about 50%. Completely oven dried pastes are found to have a considerably altered geometry.