# Modelling Leaching Using C-S-H Solid Solutions
## 2 Materials and Methods

Table 1: Previous studies using PHREEQC for modelling cementitious material leaching. The solubility data in Figure 2 were used to compare the predicted pH, Ca, and Si against the C-S-H Ca/Si ratio. These data were deemed to be representative of C-S-H gel near equilibrium near room temperature (17-30 \({}^{\circ}\)C), with a curing time of one week or more. The PHREEQC [24] analysis was undertaken using the chemical and physical properties of a commercially available CEM I cement, as described in Table 1. The chemical compositions of the materials were determined by X-ray fluorescence (XRF). The phase composition was calculated using normalisation. Figure 1: C-S-H gel solubility data from 777 C-S-H gel equilibrated solutions for pH, Ca (mmol/L), and Si (mmol/L) taken from [PERSON] et al. [16]. Thermodynamic Modelling  
Thermodynamic calculations were carried out using the PHREEQ geochemical software along with the cemdata18 thermodynamic database to simulate leaching and provide predictions of the Ca, Si, and pH in the leachate at 25 \({}^{\circ}\)C. The CSHQ, CSH3T, and tobermorite-jennite C-S-H solution models were analysed using their derived DSPs [22,23] and the SOLID_SOLUTIONS data block in PHREEQC. The thermodynamic model was based on the image shown in Figure 3, where 100 g of cement was placed in 1000 mL of pure water (Solution 1) with a water/solid (w/s) ratio of 10. Every fourteen days, Solution 1 was replaced with 1000 mL of pure water (Solution 0) until all the C-S-H and portlandite was dissolved. Separate PHREEQC models, depending on the C-S-H solid solution employed, provided simulations of the changing pH, calcium (mmol/L), and silica (mmol/L) in the replaced leachate (Solution 1) as well as the Ca/Si ratio of the C-S-H solid phase. The analysis included 266 changes, which was equivalent to 10 years. Figure 2: Selected C-S-H gel solubility data for comparison with PHREEQC predictions. The PHREEQC inputs for Solution 0 and Solution 1 are shown and described in Table 2. The CEM I data input for the model within the EQUILIBRIUM_PHASES data block is shown in Table 3. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**Oxide (g/100 g)**} & \multicolumn{2}{c}{**Phase (g/100 g)**} \\ \hline SiO\({}_{2}\) & 19.52 & C\({}_{3}\)S & 54.53 \\ Al\({}_{2}\)O\({}_{3}\) & 4.41 & C\({}_{2}\)S & 15.51 \\ Fe\({}_{2}\)O\({}_{3}\) & 2.63 & C\({}_{3}\)A & 7.33 \\ CaO & 60.57 & C\({}_{4}\)AF & 8.10 \\ MgO & 2.84 & Lime & 1.01 \\ K\({}_{2}\)O & 0.90 & Calcite & 3.45 \\ Na\({}_{2}\)O & 0.31 & Gypsum & 2.46 \\ SO\({}_{3}\) & 2.00 & Periclase & 0.61 \\ CaO (free) & 1.00 & K\({}_{2}\)SO\({}_{4}\) & 1.52 \\ CO\({}_{2}\) & 1.50 & Na\({}_{2}\)SO\({}_{4}\) & 0.32 \\ Periclase & 0.60 & K\({}_{2}\)O & 0.09 \\ LOI & 3.20 & Na\({}_{2}\)O & 0.17 \\ Blaine fineness (m\({}^{2}\)/kg) & 395 & MgO & 2.87 \\  & & SO\({}_{3}\) & 2.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Composition of CEM I cement. Figure 3: PHREEQC leaching conceptualisation model. \begin{table}
\begin{tabular}{l l l} \hline \hline \multicolumn{2}{c}{**PHREEQC Input**} & \multicolumn{2}{c}{**Description**} \\ \hline Solution & 0 & \multicolumn{2}{c}{Defines the aqueous concentration, the amount of water, and the temperature and pressure for Solution 0 [24]} \\ -Temp & 25 & Sets temperature to 25 \({}^{\circ}\)C \\ pH & 7 charge & Sets initial pH to 7 and allow charge balance \\ -Water & 1 & Sets water to 1 kg \\ -Units & mmol/kgw & Sets units for initial Ca and Si in solution \\ Ca & \(1.00\times 10^{-12}\) & Adds initial Ca to aid calculations \\ Si & \(1.00\times 10^{-12}\) & Adds Si to aid calculations \\ Save Solution & 0 & Saves solution \\ END & & \\ Solution & 1 & \multicolumn{2}{c}{Defines the aqueous concentration, the amount of water, and the temperature and pressure for Solution 1 [24]} \\ -Temp & 25 & Sets temperature to 25 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 3: PHREEQC input for solution data.