Deriving discrete solid phases from CSH-3T and CSHQ end-members to model cement hydration in PHREEQC

This paper presents a cement hydration model over time using the CEMDATA thermodynamic database and a series of discrete solid phases (DSP) to represent calcium silicate hydrate (C-S-H) as a ternary (CSH-3T) and quaternary (CSHQ) solid solution. C-S-H in cement is amorphous and poorly crystalline with a range of molar Ca/Si ratios = 0.6-1.7 and displays strongly incongruent dissolution behaviour where the release of calcium into solution is several orders of magnitude greater than silicon. It is therefore important that any cement hydration model provides a credible account of this behaviour. C-S-H has been described in the CEMDATA thermodynamic database as a number of binary, ternary and quaternary solid solutions using different end-members with differing levels of complexity. While solid solutions can be included in most modern geochemical software programs, it often leads to a significant increase in computation time. This paper presents how the two of the more complex C-S-H solid solutions, CSH-3T and CSHQ, available in the CEMDATA database, can be represented by DSP to model cement hydration over time using the PHREEQC geochemical software. By using DSP in place of solid solutions, analysis time is much improved with no loss in accuracy in producing stable phase assemblages and reasonable predictions of pH over time.