Early hydration of cementitious systems

Underground job sites such as tunnels and mines rely on the quality of recently sprayed concrete to secure freshly excavated zones. Rapid setting and early high strength development are required for safety as well as for the rapid progression of the construction site. This is ensured by the addition of accelerating admixtures. These admixtures are of various chemical formulations and have different impacts on the early hydration kinetics. It is therefore important to understand their impact on the mechanism of hydration. However, knowledge on the early hydration mechanisms, though extensively studied, is still incomplete. The present investigation studied the mechanisms taking place at the solid-liquid interface during the early hydration of cementitious compounds. Crystal dissolution theory developed in the field of geochemistry was applied to alite in order to explain the rapid slow-down of the reactions taking place after the first rapid dissolution. To assess the validity of this theory, alite was synthesised in laboratory. Annealing of alite powders of narrow particle size distributions were carried out to assess the influence of the density of crystallographic defect on the duration of the induction period. Morphological observations of alite surfaces dipped in solutions of different saturation states were also performed in order to observe the rate controlling mechanism of dissolution. On the basis of these experimental results as well as an extensive literature review, a new model is proposed for the early hydration of alite in which dissolution is the rate controlling factor up to the onset of the acceleration period. Parameters affecting the kinetics of hydration such as the influence of mixing speed or the accelerating effect of calcium chloride are discussed bearing in mind the concept of crystal dissolution. Finally, the impact of alkali-free accelerators on the kinetics of hydration was also studied. The reactions of hydration were monitored by isothermal calorimetry and in-situ XRD. It is found that these admixtures lead to a massive precipitation of ettringite which might impair the ability of gypsum to control the C3A hydration leading to a competition between the silicate and the aluminate reactions to fill up the available pore space