# Comparing the Measured and Thermodynamically Predicted AFm Phases in a Hydrating Cement
## 5 Analysis Output
### Phase Assemblages  
A typical phase assemblage over time for the three w/c ratios is shown in Figure 8. The main AFm phase predicted to precipitate is monocarbonate. In Portland cements, sulfates will react with aluminates to form ettringite. The addition of calcite in the CEM I cement promotes the formation of carbooluminates. Along with sufficient SO\({}_{4}\) available for reaction, ettringite is stabilised as seen in the XRD patterns. Thus, the formation of \begin{table}
\begin{tabular}{c c c c c c} \hline  & & \multicolumn{2}{c}{**Portlandite**} & \multicolumn{2}{c}{**Calcite**} \\ \cline{3-6}
**w/c** & **Time (Days)** & **Measured** & **Predicted** & **Measured** & **Predicted** \\
**Ratio** & & & **(\%)** & **(\%)** & **(\%)** & **(\%)** \\ \hline \multirow{3}{*}{0.4} & 1 day & 10.41 & 6.44 & 4.16 & 3.19 \\  & 7 days & 14.39 & 12.34 & 3.95 & 2.67 \\  & 28 days & 15.34 & 13.41 & 4.02 & 2.65 \\ \hline \multirow{3}{*}{0.5} & 1 day & 11.35 & 5.59 & 4.07 & 2.78 \\  & 7 days & 15.62 & 10.77 & 3.75 & 2.32 \\  & 28 days & 16.65 & 11.85 & 3.27 & 3.27 \\ \hline \multirow{3}{*}{0.6} & 1 day & 10.68 & 4.94 & 3.59 & 2.46 \\  & 7 days & 15.42 & 9.52 & 3.23 & 2.05 \\ \cline{1-1}  & 28 days & 16.61 & 10.50 & 3.21 & 1.95 \\ \hline \end{tabular}
\end{table} 
Table 4: Comparison between measured and predicted portlandite and calcite contents over time. Figure 7: Comparison between measured and predicted portlandite and calcite contents over time. hemicarbonate is observed in measured data but not always predicted by thermodynamic modelling, reflecting its kinetic persistence experimentally. Additionally, it has been demonstrated above that the dissolution of calcite is much higher than that measured, which would further support the precipitation of monocarbonate over hemicarbonate due to the higher CaCO\({}_{3}\) content. The calculated saturation indices of hemicarbonate and monocarbonate are shown in Figure 9a-c. As shown, monocarbonate is predicted to form after 2-3 days while hemicarbonate is undersaturated and would not precipitate as it is not thermodynamically stable. [PERSON] et al. [8] developed predicted phase assemblages that will typically occur in hydrated OPC, as shown in Figure 10, and the dependence on initial sulfate (SO\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\)) and carbonate CO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)) contents to determine which AFm phase(s) will form. The total molar amount of Al\({}_{2}\)O\({}_{3}\) reacted was taken as cumulative reacted gypsum, initial C\({}_{3}\)A, C\({}_{4}\)AF and SO\({}_{3}\) contained in each of the 47 PHREEQC solutions. CO\({}_{2}\) was taken as the cumulative amount of reacted calcite. As shown, the cement here falls within Zone I, with the AFm phase predicted to form as hemicarbonate (Hs) and monosulfate (Ms) along with AFt. As shown, the 'turning point' is at \(\sim\)2 days, which is the same time as monocarbonate forms and calcite begins to dissolve into the solution, as shown in the phase assemblage in Figure 8. The full change in carbonate and sulfate over the 1250 days of hydration is shown in Figure 11, where significant changes in the latter during the initial hour of hydration, particularly as the accessory clinker phases (time, calcite, gypsum, periclase, arcanite and thenardite) reach equilibrium with the pore solution in the first time-step and are immediately available to contribute to the formation of hydrate phases. Indeed, Figure 12 shows how the immediate release of sulfate from Na\({}_{2}\)SO\({}_{4}\), K\({}_{2}\)SO\({}_{4}\) and SO\({}_{3}\) has partly precipitated as gypsum, as the volume of gypsum is higher than expected in the first hour of hydration. Figure 8: Typical phase assemblages from three hydrating CEM I cements. [PERSON] et al. [8] found that OPC with very low carbonate contents will form hemicarbonate during hydration but it was not stable at 25 \({}^{\circ}\)C in the presence of excess calcite.