The volume and morphology of calcium hydroxide (CH) precipitating from supersaturated solutions is monitored as a function of the concentration of either of two types of nanoparticles dispersed in the solution. The CH precipitated in the presence of sulfonated graphene nanosheets (SGN) had well-developed hexagonal platelet shapes, while that forming in the presence of cationic polyurethane nanospheres (PUC) tended to aggregate around the PUC and developed as spherulitic masses. The terminal CH platelet size in SGN suspensions was 8 μm; with increasing SGN dosage, the mean size increased to 23 μm. Taking into consideration complementary experimental measurements of isothermal adsorption and zeta potential, we speculate that calcium from the solution adsorbs on the surfaces of both SGN and PUC prior to nucleation of CH. Furthermore, molecular-scale mechanism indicated the interaction of Ca-Ocoo from PUC is stronger than Ca-Oso3 from SGN. Simultaneously the number of adsorbed calcium by PUC is roughly 3 times greater than for SGN, which is perfectly matched with the measured adsorption isotherm. Hopefully, this work can provide scientific guidance for hydration mechanism of cementitious materials in the presence of nanoparticles suspensions.