A thermodynamic model of dissolution and precipitation of calcium silicate hydrates

Modelling of Chemical Alteration of Cement Materials in Radioactive Waste Repository Environment

“…This model can predict the equilibria of the incongruent dissolution and precipitation with a continuous change in the Ca/Si ratio of the solid phase by a series of calculations, in which the quantities of the dissolved/precipitated end members are calculated stepwisely, so that the quantities and compositions of the solid and liquid phases and the conditional solubility constants used in the next step can be estimated (Sugiyama & Fujita, 2006). CCT-P contains the geochemical code PHREEQE (Parkhurst et al, 1980) to calculate the chemical equilibrium, and the C-S-H model is employed to calculate the incongruent dissolution and precipitation by iterative calculations, because the simplicity of its numerical description allows its inclusion in chemical equilibrium calculations based on the common approach of using the law of mass action (log K) in PHREEQE (Parkhurst et al, 1980).…”

“…In this model, C-S-H is assumed to be a binary nonideal solid solution of Ca(OH) 2 and SiO 2 , and the log K values of the model end members of the solid solution are given as functions of the Ca/Si ratio of C-S-H (Sugiyama & Fujita, 2006 ' s s0 2 s0 s1 s2 11 11 1 log log log 11 1 1 1 1                        Bond et al, 1997).…”

“…The chemistry of the CaO-SiO 2 -H 2 O (C-S-H) system is a key parameter since it is suggested to be responsible for the high-pH condition in cements and is important in discussing the high-pH chemical condition in the long-term assessment of a repository environment Atkinson, 1985). The author therefore has been developing a series of predictive calculation models based on a discussion of the incongruent dissolution/precipitation of the C-S-H system (Sugiyama & Fujita, 2006;Sugiyama, 2008).…”

“…In this study, a reactive transport computational code, in which a geochemical model including the thermodynamic incongruent dissolution model of C-S-H (Sugiyama & Fujita, 2006) is coupled with the advection-diffusion/dispersion equation including the evolution of the hydraulic properties of the solid cement matrix due to the leaching and precipitation of components, has been developed. A series of experiments on the alteration of hydrated ordinary portland cement (OPC) and low-heat portland cement containing 30 wt% fly ash (FAC) monoliths in deionised water and sodium bicarbonate (NaHCO 3 ) solution were carried out and the model was optimised on the basis of the observations.…”

Changes on the nanostructure of cementitius calcium silicate hydrates (C–S–H) induced by aqueous carbonation

“…The C-S-H sample was synthesized from a solution with a Ca/Si ratio of 1.8 according to the co-precipitation method [26] which consists of adding, under stirring, a solution of Na 2 SiO 3 (Merck, 1l = 1.35 kg) to a calcium saturated solution. This solution was aged for up to 21 days in sealed containers inside an argon-filled glovebox at ambient pressure and temperature.…”