Employing Discrete Solid Phases to Represent C-S-H Solid Solutions in the Cemdata07 Thermodynamic Database to Model Cement Hydration Using the PHREEQC Geochemical Software

Abstract

This paper presents a cement hydration model over time using the cemdata07 thermodynamic database and a series of derived discrete solid phases (DSPs) to represent calcium silicate hydrate (C-S-H) as a binary solid solution with two end-members. C-S-H in cement is amorphous and poorly crystalline with a range of molar Ca/Si ratios from 0.6 to 1.7. It 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 cemdata07 thermodynamic database as a number of solid solutions using different end-members with differing levels of complexity. While solid solutions can be included in most modern geochemical software programs, they often lead to a significant increase in computation time. This paper presents how an incongruent solid solution between two C-S-H end-members may be represented as a number of DSPs to model cement hydration over time using the PHREEQC geochemical software. By using DSPs rather than modelling C-S-H as a nonideal solid solution, this gives the user full control of the input for the model, reducing the computational demand and analysis time with no loss in accuracy in predicting stable-phase assemblages and their associated pore chemistry over time.

Predicting Chemical Shrinkage in Hydrating Cements

“…The CEM I cement has ~2.9% limestone powder content by mass, compared to ~11.23% in the CEM II/A-L, which are both within the limits of EN197-1 [22]. Thermodynamic modelling was performed using PHREEQC [23] using input data to describe the normative clinker content, its dissolution rate, and accounting for minor constituents, oversaturation and alkalis binding to the C-S-H [24][25][26]. Anhydrite (CaSO 4 ) is used here as gypsum.…”

“…Previous work by Holmes et al [26] has demonstrated how the C-S-H gel phase can be modelled as a series of discrete solid phases (DSPs) using suitable end members in the cemdata18 database. Dissolution reactions and solubility products for the DSP for the CSH-3T model can be calculated on a spreadsheet and suitably formatted such that the user simply needs to copy and paste directly into PHREEQC.…”

Comparing the Measured and Thermodynamically Predicted AFm Phases in a Hydrating Cement

“…The thermodynamic modelling was undertaken using the PHREEQC geochemistry software [7] which, using appropriate input for the normative cement content, calculating the clinker dissolution rate while accounting for accessory phases, oversaturation and alkalis binding to the C-S-H, determines the equilibrium phase assemblages over time, as described in detail in [5,6,19,20]. Anhydrite (CaSO 4 ) is referred to here as gypsum.…”

“…Holmes et al [20] showed how C-S-H gel solubility can be modelled as a series of discrete solid phases (DSPs) derived from the CSH3T and CSHQ end-members in the cemdata18 database. It was shown that for OPC hydration, solid solution models are not required, which only requires portlandite and a suitable C-S-H gel phase of fixed Ca:Si and H (water):Si molar ratio of between 1.6-1.8 and 2.0-2.1, respectively.…”

“…As may be seen in Figure 6, the three methods show excellent comparisons. The PHREEQC input file also accounts for the kinetic dissolution of the clinker phases as a function of time, oversaturation of the precipitating hydrates from 0-12 hrs and the release and uptake of alkalis by the C-S-H gel, as described in previous publications [5,6,19,20].…”