Calcium hydroxide (CH) is well known as one of the main products of cement hydration.The growth and final morphology of CH may have important effect on the hydration kinetics of cementitious systems.Consequently a better understanding of CH growth may lead to a better control of the hydration kinetics.The morphology, distribution and amount of CH in various cementitious systems and coprecipitation systems (CaCl2+NaOH) were investigated to determine the key factors for CH nucleation and growth.Morphological image analysis of SEM images was used to characterize the microstructure of CH particles.XRD and TG-DSC were used to quantitative analysis of CH in cementitious system.It was observed that both silicate and sulfate ions could poison CH nucleation.In coprecipitation systems, the addition of sulfate had been observed to promote hexagonal platelet morphology of CH.The addition of silicate led to the formation of rod-like crystal.When addition of sulfate and silicate simultaneously, short hexagonal-prism shape emerged.The reduction of supersaturation led to less particles with larger aspect ratio.In pure C3S system, without addition of sulfate,hexagonal platelet morphology of CH also emerged, which seemed to indicate that the concentration of calcium ion in C3S pastes had significant effect on the growth habit of CH.