Abstract
The aim of this paper is to elucidate the role of hydrated silicate monomer on the hydration of alite. A combination of XRD Rietveld method and 29Si NMR analysis was used for quantifying hydrated silicate monomer. The thickness of hydrated silicate monomer layer was the same as the thickness of silicate gel layer in hydrated alite, which was calculated from the depth-direction distribution of hydrogen atom that was determined by nuclear resonance reaction analysis, which was performed by Livingston. The discussion based on the thermodynamic phase equilibrium theory indicated that the hydrated silicate monomer was existed in the form of ion in the solution. This implies that the silicate gel consisted of the calcium leached layer including the hydrated silicate monomer. In addition, it showed that the divergence point from the basic curve in relation to the produced amount of hydrated silicate monomer and the hydration reaction degree was consistent with the starting point of the accelerated period in hydration of alite with the addition of CaCl2 as accelerator. These result obtained in this study could support the metastable barrier hypothesis that was proposed to explain the mechanism of induction period and acceleration period during alite：the metastable CSH produces the hydrated initial stage, and this plays as selective membrane that permeate calcium ion but not silicate ion. As proceeded the hydration of alite, the ionic concentration in liquid phase was changed, the equilibrium was shifted from metastable C-S-H to stable CSH, resulting in transport of silicate ion toward the out of alite particles. This is the trigger of starting the accelerated period of alite.
