Improving the structural consistency of C-S-H solid solution thermodynamic models

Simple aqueous-solid solution models of C-S-H (calcium silicate hydrate) are widely used in studies of cement hydration and waste-cement interactions. Even without a clear structural/mechanistic basis, such thermodynamic models yield a good description of solubility data in [Ca]-C/S space, while only satisfactory in [Si]-C/S, H2O-C/S, [Ca]-[Si] spaces and in the 0.1 < C/S < 0.8 range. Here, using a multi-site (sublattice) concept, the ideal solid solution model of C-S-H is revised to make it consistent with the Richardson-Groves structural model of C-S-H and with the modern interpretation of spectroscopic (29Si MAS NMR) and solubility data. Consideration of two site substitutions, (1) coupled H2OCa2+ for SiO2H22+ replacement in bridging tetrahedral and adjacent interlayer sites, and (2) substitution of interstitial Ca(OH)2 for a vacancy, leads to a new CSHQ model of (A,B) (C,D)X type composed of two tobermorite-like and two jennite-like end members. Because this ideal sublattice SS model cannot fit solubility data well at 0.8 < C/S < 1.1, a simpler CSH3T model is constructed from a polymeric TobH (CaO)2(SiO2)3(H2O)5, a dimeric T2C (CaO)3(SiO2)2(H2O)5, and an ordered pentameric T5C (CaO)2.5(SiO2)2.5(H2O)5 tobermorite-like end members. This solid solution model, limited to the range 0.67 < C/S < 1.5, has a correct built-in dependence of the mean silicate chain length on C/S, yields quite realistic fits to the solubility data, and provides a basis for extensions with foreign cations whose sites in the defect-tobermorite structure of C-S-H are known. To account for C-S-H compositions with C/S > 1.5, CSHQ end members were downscaled to one tetrahedral site and used within the simple mixing model. Despite some loss of structural consistency, the solubility and mean silicate chain length data can be reproduced well with this downscaled CSHQ model, capable of temperature corrections and dependencies of density and water content in fully-hydrated C-S-H on C/S ratio. Most literature solubility data sets can be modeled at the cost of moderate adjustments of CSHQ end-member solubility products within 0.2 to 0.6 pK units.