Effects of Temperature and of the Addition of Accelerating and Retarding Agents on the Kinetics of Hydration of Tricalcium Silicate

The formation of calcium silicate hydrates (C−S−H) during the hydration of tricalcium silicate (C 3 S) in pure water and in water solutions containing 1% CaCl 2 (accelerator) and 0.01% saccharose (retarder) was studied by small-angle X-ray scattering (SAXS). SAXS measurements were performed under isothermal conditions within the temperature range 25 °C < T < 52 °C. The experimental results indicate that the time variation of the mass fraction of the C−S−H product phase, α( t ), can be fitted, under all conditions of paste setting, by Avrami equation, α(t) = 1 − exp(−( kt ) n ), k being a rate parameter and n an exponent depending on the characteristics of the transformation. The parameter n is approximately equal to 2 for hydration of C 3 S in pure water. Depending on temperature, n varies from 2 to 2.65 for hydration in the presence of CaCl 2 and saccharose. The value n = 2 is theoretically expected for lateral growth of thin C−S−H plates of constant thickness. The time dependence of SAXS intensity indicates that the transformed phase (C−S−H) consists of colloidal particles in early stages of hydration, evolving by two-dimensional growth toward a disordered lamellar structure composed of very thin plates. The activation energy Δ E for the growth of C−S−H phase was determined from the time dependence of X-ray scattering intensity. These data were obtained by “in situ” measurements at different temperatures of hydration. The values of Δ E are 37.7, 49.4, and 44.3 kJ/mol for hydration in pure water and in water solutions containing CaCl 2 and saccharose, respectively.