Predicting the evolution of stiffness in cement paste requires linking morphological properties of the microstructure to the evolution of mechanical properties. Measures are typically difficult to obtain with sufficient time resolution to capture the transitions as the morphology evolves. In this work, we show the combined result H-1 NMR and EMM-ARM, a real-time measure of the modulus, with hydration and finite element modelling. Together, these reveal the different mechanisms through which the cementitious matrix gains stiffness, and further hints at links to the morphology of hydrates. A quantitative measure of the roles of percolation, hydrate growth, and space filling is thus provided. A practical outcome of this research is an empirical law linking the stiffness of C-S-H foam to its porosity from the start of the hydration.