The last generation of cement hydration accelerators relies on the seeding effect induced by amorphous calcium silicate hydrate particles. However, little is known about the effect of variables, such as the degree of crystallinity and morphology, on their performance. In this work, particles with a Ca/(Si+Al) molar ratio of 0.83 and different proportions of aluminum substitution for silicon were synthesized under hydrothermal conditions. X-ray diffractograms, nuclear magnetic resonance spectra, and scanning electron microscopy images showed that this altered the degree of crystallinity, structure, and morphology of the particles. Nevertheless, the addition of the synthesis products to cement pastes and their subsequent study by isothermal calorimetry and mechanical tests showed that the variables with the largest impact on cement hydration are the dosage and the proper dispersion of the particles. In fact, the use of a dispersing agent is highly recommended while drying of the particles should be avoided to prevent irreversible agglomeration processes from taking place.