Assessment of diffusive and mechanical properties of hardened cement pastes using a multi-coated sphere assemblage model

This paper addresses the problem of the estimation of the diffusive and mechanical properties of hardened cement pastes by means of a simplified multi-coated sphere assemblage model. This model relies on a microstructure description involving the main hydrated products (portlandite, C–S–H, and aluminous compounds) and accounts for the physical properties of each phase as well as their volume fraction and to a certain extent their size and relative disposition. The basic idea is that any initial cement grain hydrates partly to form an external (outer) layer of C–S–H and partly an internal (inner) layer of C–S–H, both having different properties. The multi-coated sphere model, due to Hashin for its 2-phase version, is then used and applied to calculate the diffusion coefficient and Biot coefficient of such an assemblage. The evolutions of the diffusive properties when the initial material is subjected to lixiviation and calcite formation are estimated on the basis of simplified description of these degradation mechanisms; the issue of expansion due to internal pressure generated by crystal growing (crystallization pressure) in the context of sulfate attack is also analyzed.