The microstructure of calcium–silicate–hydrate (C−S−H) gel, a major hydrated phase of Ordinary Portland Cement, with and without polycarboxylic ether (PCE) additives is investigated by combined analyses of small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) data. The results show that these comb-shaped polymers tend to increase the size of the disk-like globules but have little influence on the thickness of the water and calcium silicate layers within the globules. As a result, the fractal packing of the globules becomes more open in the range of a few hundred nanometers, in the sense that the mass fractal dimension diminishes, since the PCE adsorption on the globules increases the repulsive force between and polydispersity of the C−S−H units. Moreover, scanning electron microscope (SEM) study of the synthesized C−S−H gels in the micrometer range shows that the PCEs depress the formation of fibrils while enhancing the foil-like morphology.