Densification of C-S-H is mainly driven by available precipitation space, as quantified through an analytical cement hydration model based on NMR data

NMR relaxometry (Muller et al., 2012, 2013) quantifies the mass fractions of differently bound water in cement paste, as functions of hydration degree and water-to-cement ratio. We here reduce these findings to a single explanation: the density of C-S-H gel is solely governed by the specific precipitation space, across three “hydration regimes”: In regime I, solid C-S-H platelets (without gel porosity) precipitate on clinker surfaces. In regime II, C-S-H starts to include gel porosity. The forming gel densifies linearly with decreasing precipitation space. Thereby, regime III starts once the C-S-H gel has completely filled the capillary porosity. These findings allow for developing an analytical hydration model. It provides expressions for C-S-H gel densification and for the evolution of the volume fractions of clinker, of solid hydrates, as well as of the gel and capillary porosities, as functions of the water-to-cement ratio and the hydration degree; consistently representing published experimental data.