Swelling, shrinkage and creep: a mechanical approach to cement hydration

Recent advances in physical understanding of creep and shrinkage of cement paste suggest a novel approach to setting and hardening processes. In high-strength concrete, due to a low water-cement ratio, self-desiccation occurs immediately after setting, and capillary pressure produces compaction of the assembly of hydrating cement grains. For higher watercement ratio, water possibly can withstand cavitation, but then the volumetric balance of hydration requires that water move through the solid skeleton. According to Darcy's law, water only can be displaced by a gradient of hydraulic pressure, as long as the water pressure remains lower than its cavitation threshold. This water pressure only can be equilibrated by a compaction of the solid matrix, with compressive contacts particularly between the C-S-H layers. In both cases, with or without liquid-gas menisci, hydration acts like a distributed pumping of water, the liquid phase is under tension, and the solid skeleton is compressed. The high affinity of C-S-H for water and the viscoplastic behavior of the cement gel can explain the evolution of the contact between two hydrating cement grains and the formation of a continuous and cohesive bridge. Swelling and shrinkage then can be analyzed as two co-existing mechanisms, whose balance is controlled by the permeability of the solid matrix.