Kinetic mechanisms and activation energies for hydration of standard and highly reactive forms of β-dicalcium silicate (C2S)

The activation energy for hydration of beta-C2S paste was measured as a function of hydration time using a calorimetric method and was found to depend on the surface area and reactivity of the powder as well as on the addition of sodium silicate. For neat paste made with standard beta-C2S (similar to that found in portland cement), the activation energy is approximately 32 kJ/mol and is constant with time. For neat paste made with reactive beta-C2S (calcined at lower temperature and with high surface area), the activation energy is about 55 kJ/mol and is also constant with time. This large difference in activation energy reflects a difference in the rate-controlling step for hydration. After investigating the effects of sodium silicate and synthetic calcium-silicate-hydrate on the kinetics, we hypothesize that the lower activation energy represents C2S dissolution, while the higher value represents nucleation and growth of hydration product.