Kinetics of the Hydration of Tricalcium Silicate

The hydration of tricalcium silicate was followed at a water/C3S ratio of 0.7 between 5° and 50°C by determining free lime and combined water. Free lime was estimated by the o-cresol method, whereas combined water was calculated from the amount of free water remaining in the paste as determined by extraction with methyl ethyl ketone. The ratio of free lime to combined water was constant throughout the hydration; this ratio indicated that CSH(II) may be represented as 1.68CaO · SiO2· 2.58H2O. When maximum supersaturation of the solution with Ca2+] is attained, the induction period terminates and the reaction proceeds rapidly, probably as the result of propagative surface nucleation-growth of CSH(II). Kinetic equations were derived for these reactions. When the surface of C3S is entirely covered by CSH(II), the reaction becomes slow and is controlled by diffusion of water. Constants involved in the kinetic equations are evaluated and discussed.