Since the introduction of cementitious materials over 200 years ago, its use has multiplied many fold to be currently at 3 billion tonnes per year, accounting for about 8% of worldwide carbon emission which is forecast to double within the next 30 years. Upon hydration of cementitious materials, the principle product formed is Calcium Silicate Hydrate (C-S-H) that binds all the aggregates together. Solutions to reduce this Carbon footprint depend on being able to improve the development of early age strength, which is controlled by the self-limiting growth of the C-S-H, particularly when using alternative supplementary cementitious materials (SCMs) to replace Portland cement. The greatest difficulty to overcome this limitation is the lack of knowledge on the growth and structure of C-S-H. It is poorly crystalline and cannot be characterised by classical crystal techniques. Moreover, in the Portland cement system, the effect of different elements on C-S-H growth is unclear. Furthermore, there is a lack of kinetic analysis on C-S-H growth.This thesis work presents a synthetic method to produce uniform C-S-H phases with Ca:Si ratios from 1 to 2, with a ratio above 1.6 being achievable for the first time in any given synthetic system. Higher ratios are of particular importance when compared to a real system, where it exists around 1.75 (Avg.). This was achieved with a novel rapid precipitation method by controlling mixing and reaction chemistry. These synthetic C-S-H precipitates are chemically uniform at the nanoscale and allowed us to determine a better defined atomistic structure with the use of dynamic nuclear polarisation (DNP) NMR techniques and atomistic simulations. The discovered structures reveal the inclusion of a calcium site in the interlayer that bridges chain terminating silicate species. This site is associated with an environment of strong hydrogen bonding that stabilises the structure and, consequently, promotes high Ca:Si ratios in C-S-H. The major result is thus a clear relation between the atomic level defect structure and the high Ca:Si ratio in C-S-H. Further, the C-S-H phases were analysed by Raman micro-spectrometry and Fourier Transform Infrared spectroscopy (FTIR) and revealed a proximity to a defective tobermorite structure and the probability of a single solid solution model for the Ca:Si range from 1.25 to 2. A morphological transformation point is also reported to be near to pH~11, where C-S-H solid phases clearly change from nanoglobules (spherical) to nanofoils (platelets).The developed understanding of the thermodynamic conditions and its control in the formation of desired precipitated synthetic C-S-H phases was applied to the formation of C-S-H in the Portland cement system formed upon the hydration of Alite (an impure Tri-Calcium silicate main phase in cementitious materials). We could predict the calorimetric behaviour of the system based on the knowledge of the formation mechanism of synthetic C-S-H. Also, it allowed us to forecast desirable conditions to get a particular phase or response of the hydrating system. Kinetic data were collected from the synthetic precipitation experiments by following the Ca consumption from solution. Then with help of thermodynamics speciation modelling and Population Balance Modelling (PBM) an empirical growth model for C-S-H was developed. Overall, all these results together have given a much clearer picture about C-S-H formation and of its atomistic structure.