# Modelling Leaching Using C-S-H Solid Solutions
## 3 Results  
Using the input data above to analyse the cement described in Table 1, Figure 4 shows a comparison of the changes in Ca, Si, and pH during leaching against the C-S-H Ca/Si ratio using (1) the CSHQ gel model employing the DSP [22, 23] and (2) the solid-solution option in PHREEQC. As may be seen, both approaches gave similar predictions, with the DSP showing kinks across the compositional range. This was a result of the non-ideal nature of the CSHQ DSP gel model, where imperfect mixing and incomplete solubility led to the 'jagged' output shown. While both provided clear predictions, the SOLID_SOLUTIONS data block was employed here throughout.  
The results from the predicted leaching of the cement described in Table 1, undergoing the water changes shown in Figure 3, are shown in Figure 5 for the three C-S-H solid-solution gel models in Table 4. As may be seen, all three models provided reasonable predictions of the selected C-S-H gel solubility data for the Ca/Si ratios from 2.7 to 0.5 shown. While all three C-S-H gel models provided good comparisons with the measured data from the literature, CSH3T performed slightly better. In terms of pH, for example, the tobermorite-jennite model underestimated at Ca/Si ratios greater than 1.0. It also appeared to overestimate the Ca and Si concentrations at higher Ca/Si ratios.  
\begin{table}
\begin{tabular}{l l l} \hline \multicolumn{2}{c}{**PHREEQC Input**} & \multicolumn{1}{c}{**Description**} \\ \hline SOLID\_SOLUTIONS 1 & & \\ CSHQ\_model & & Name of solid solution \\ -comp & CSHQ-TobH & 0 & Name of end-members (from \\ -comp & CSHQ-TobD & 0 & cemdata18 database [15]; \\ -comp & CSHQ-JenH & 0 & Table 4) and initial \\ -comp & CSHQ-JenD & 0 & concentration (moles) \\ \hline \end{tabular}
\end{table}
Table 6: PHREEQC input for the solid-solution calculations.  
\begin{table}
\begin{tabular}{c c c c c} \hline \multicolumn{2}{c}{**Solid**} & \multicolumn{2}{c}{**End-Member**} & \multicolumn{1}{c}{**log\_k**} \\ \hline \multirow{4}{*}{CSHQ} & CSHQ-TobH & (CaO)\({}_{0.67}\)(SiO\({}_{2}\))\({}_{1}\)(H\({}_{2}\)O)\({}_{1.5}\) + 1.33H\({}^{+}\) = 0.67 Ca\({}^{2+}\) + 2.17H\({}_{2}\)O + SiO\({}_{2(\text{aq})}\) & 8.286 \\  & CSHQ-TobD & ((CaO)\({}_{1.25}\)(SiO\({}_{2}\))\({}_{1}\)(H\({}_{2}\)O)\({}_{2.75}\))\({}_{0.67}\) + 1.67H\({}^{+}\) = 0.83 Ca\({}^{2+}\) + 2.67H\({}_{2}\)O + & 13.655 \\  & CSHQ-JenH & (CaO)\({}_{1.33}\)(SiO\({}_{2}\))\({}_{1}\)(H\({}_{2}\)O)\({}_{2.17}\) + 2.67H\({}^{+}\) = 1.33 Ca\({}^{2+}\) + 3.5H\({}_{2}\)O + SiO\({}_{2(\text{aq})}\) & 22.179 \\  & CSHQ-JenD & (CaO)\({}_{1.5}\)(SiO\({}_{2}\))\({}_{0.67}\)(H\({}_{2}\)O)\({}_{2.5}\) + 3H\({}^{+}\) = 1.5 Ca\({}^{+2}\) + 4H\({}_{2}\)O + 0.67 SiO\({}_{2}\) & 28.730 \\ \hline \multirow{4}{*}{CSH3T} & CSH3T-T2C & ((CaO)\({}_{0.75}\)(SiO\({}_{2}\))\({}_{0.5}\)(H\({}_{2}\)O)\({}_{1.25}\))\({}_{2}\) + 3H\({}^{+}\) = 1.5 Ca\({}^{2+}\) + 4H\({}_{2}\)O + SiO\({}_{2(\text{aq})}\) & 25.271 \\  & CSH3T-TobH & (CaO)\({}_{1}\)(SiO\({}_{2}\))\({}_{1.5}\)(H\({}_{2}\)O)\({}_{2.5}\) + 2H\({}^{+}\) = Ca\({}^{2+}\) + 3.5H\({}_{2}\)O + 1.5 SiO\({}_{2(\text{aq})}\) & 12.529 \\  & CSH3T-T3C & ((CaO)\({}_{1}\)(SiO\({}_{2}\))\({}_{1}\)(H\({}_{2}\)O)\({}_{2}\))\({}_{1.25}\) + 2.5H\({}^{+}\) = 1.25 Ca\({}^{2+}\) + 3.75H\({}_{2}\)O + & 18.137 \\  & 1.25 SiO\({}_{2(\text{aq})}\) & & 11.144 \\ \hline \end{tabular}
\end{table}
Table 5: Solid-solution end-members (taken from [15]).  
Figure 4: Comparison between DSP and solid-solution solver in PHREEQC.