To enhance the substitution of SCMs in cements, a better understanding of the formation of hydration products is necessary.Calcium Silicate Hydrates (C-S-H) being the main product of hydrated Portland cements is the main focus.A better understanding of the kinetics, growth mechanism and atomic structure will provide us with knowledge in assessing the new alternative material for substitution on a more scientific basis rather than trial and error.Real cement systems are too complex to perform such kind of studies due to the presence of many phases.Hence we have adopted a synthetic approach to precipitate C-S-H and gain more insight into the kinetics and growth mechanism of C-S-H.According to the state of art in synthetic systems, it has been very difficult to achieve higher Ca/Si ratio (1.7~ 1.9) with the highest reported ratio of Ca/Si ratio around 1.6 whereas in real systems it is often > 1.6.The aim of this project is to synthesis C-S-H via precipitation route at Ca/Si ratio varying from 0.7 to 2.All the physical parameters such as pH, Ca2+ ion coucentration, reactant addition are monitored.Reaction times between 1 and 24 hrs.producing a solid precipitate of C-S-H.The solid is separated,washed and filtered over the 200nm filter.The collected solid and the aqueous phase has been characterized by XRD, TGA, ICP, SEM-TEM, BET and 29Si NMR.The characterization results showed that we could achieve higher Ca/Si: 2, with Ca(OH)2 being present at less than 1% in the solid.It is the first time anyone could produce such a high Ca/Si ratio in a synthetic system.Also, on comparison of experimental results with thermodynamics modeling software (i.e.) Gibbs energy minimization software (GEMS) a good match ids found when the formation of Ca(OH)2 is prohibited in the simulations.