## 5 Results
Adding the urease activity or the multiplier to the pKa results in a higher residual for the predicted precipitated calcite volume fractions; see Table 3. Note that the calcite volume fractions were not used as observations during the calibration procedure. In general, similar trends can be seen when using the four parameter sets, as calibrated to the concentration measurements of column #1 (0.33 M), to predict the concentrations of urea and calcium over time and the final distribution of precipitated calcite for the second column experiment (0.66 M); see Figures 5 and 6. All fitted parameters were the same as in the simulations for column #1, except for the parameter set including the injected concentrations of the mineralization medium, for which the fitted values for column #1 (0.33 M) were doubled. This was done assuming that whatever process might have been responsible for the change in concentrations acted proportionally to the concentrations and that the prepared solutions were of double the concentration for column #2 \begin{table}
\begin{tabular}{l l c c c c c c} \hline  & & \(k_{\text{a}}\) & \(k_{\text{d}}\) & \(C_{\text{Ca}^{2+},\text{inj}}\) & \(C_{\text{urea},\text{inj}}\) & \(c_{\text{u},0}^{\text{e}}\) & \(f_{\text{pKa}}\) \\ \hline Sorption & \(k_{\text{a}}\) & 1.0 & 0.156 & - & - & - & - \\ coefficients only & \(k_{\text{d}}\) & 0.156 & 1.0 & - & - & - & - \\ \hline Sorption & \(k_{\text{a}}\) & 1.0 & 0.172 & \(-0.352\) & \(-0.218\) & - & - \\ coefficients and & \(k_{\text{d}}\) & 0.172 & 1.0 & 0.105 & \(8.01\times 10^{-2}\) & - & - \\ injection & \(C_{\text{Ca}^{2+},\text{inj}}\) & \(-0.352\) & 0.105 & 1.0 & 0.424 & - & - \\ concentrations & \(C_{\text{urea},\text{inj}}\) & \(-0.218\) & \(8.01\times 10^{-2}\) & 0.424 & 1.0 & - & - \\ \hline Sorption & \(k_{\text{a}}\) & 1.0 & \(9.76\times 10^{-3}\) & - & - & 0.743 & - \\ coefficients and & \(k_{\text{d}}\) & \(9.76\times 10^{-3}\) & 1.0 & - & - & 0.135 & - \\ urease activity & \(c_{\text{u},0}^{\text{e}}\) & 0.743 & 0.135 & - & - & 1.0 & - \\ \hline Sorption & \(k_{\text{a}}\) & 1.0 & \(-0.129\) & - & - & - & \(4.64\times 10^{-2}\) \\ coefficients and & \(k_{\text{d}}\) & \(-0.129\) & 1.0 & - & - & - & \(6.07\times 10^{-2}\) \\ pKa multiplier & \(f_{\text{pKa}}\) & \(4.64\times 10^{-2}\) & \(6.07\times 10^{-2}\) & - & - & - & 1.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Parameter correlations as given by the PEST protocol [58] for the four investigated sets of calibration parameters calibrating to the concentration measurements of column #1 (0.33 M). (0.66 M). None of the data of column #2 (0.66 M) were previously used during the calibration. The most important observation is that all parameter sets can qualitatively predict the measurements, even for the changed concentrations and injection strategy, and that the four parameter sets do not result in drastically different predictions. For all parameter sets and every sub-data set, the residuals for column #2 (0.66 M) are much larger, approximately one order of magnitude compared to those of column #1 (0.33 M); see Table 3. The increased residuals are expected, as the data were not used for calibration. The injected urea and calcium concentrations were doubled for column #2 (0.66 M), increasing the range of concentrations and thus the potential discrepancies between model predictions and measurements. This doubling of the injection concentrations might be responsible for an increase in the residuals be a factor of four when assuming a linear scaling of the difference between measured and predicted concentrations; see Equation (29). In addition, urease was added only before every odd-numbered mineralization-medium injection for column #2 (0.66 M), which may have contributed to the increased residuals. As a consequence, the model-predicted ureolysis and precipitation reactions behave differently for the odd- and the even-numbered injections, which is not for all injections the case in the experiment (Figure 6).