Abstract
For prediction of physical adsorption of chlorides, the surface complexation model is usually used；chloride ions adsorb on the SiOH sites on the surface of C-S-H. However, Pointeau et. al suggested that there were two types of SiOH sites：silanol and silandiol sites. Silanol sites are present in the pairing tetrahedron position of silicate chain, while silandiol sites are in the bridging tetrahedron position. Both sites generated surface charge by the deprotonation and ionization. Most of past studies considered only one type of SiOHsite, and could not reproduct the chloride adsorption isotherms. This implies that when considered only one site, one cannot predict exactly the physical adsorption of chloride ion. In this study, we consider two different SiOH sites for chloride adsorption and enhance prediction model of physical adsorption of chloride ions. 29Si and 27Al MAS-NMR studies indicated that the density of pairing and bridging tetrahedron position in C-S-H may change at the Ca/Si ratio and the substitution of Al atom for Si atom in bridging position. Thus, the effect of Ca/Si and Al substitution on surface potential was examined and then the density of silanol and silandiolsites was quantified. Next the thermodynamic equilibrium constant of surface complexation was calculated using zeta potential data in NaOHand Ca（OH）2 solutions and chlorides adsorption. It can be confirmed that our refined model can predict more exactly the physical adsorption of chloride ions on C-S-H as compared with previous model.
