Application of a sub-lattice model to predictions of cement hydrate chemistry

The incongruous dissolution of C-S-H gel is central to the performance of the chemical barrier in a deep geological disposal repository for nuclear wastes. Numerous thermodynamic models have been developed with which the dissolution of C-S-H gel may be simulated. One of the limitations in many of these models is their inflexibility in terms of incorporating additional chemical elements into the C-S-H gel structure. This chapter reports the application of a sub-lattice model for C-S-H gel, allowing for example, substitution of alumina, sulphate or heavy metals into the structure. Comparisons are drawn between the sub-lattice representation and other models, illustrating the inherent flexibility of this approach.Examples are presented comparing the solubility of arsenic phases in the solid and aqueous solutions as calculated using the sub-lattice method. The partitioning of arsenic between solid and aqueous phases is explored over a range of activities and temperatures, ultimately bounded by the appearance of solubility limiting phases.Extending this approach to more realistic cement mineral assemblages introduces both stoichiometric hydrates and an additional solid solution representing hydrogarnet. Two cement types are used for the final examples: an ordinary Portland cement and a blended Portland-blast furnace slag, typical of a UK encapsulation grout. Simulations of their dissolution by percolating groundwater illustrate the influence of these cements in controlling the local chemical environment through their service life.