# Comparing the Measured and Thermodynamically Predicted AFm Phases in a Hydrating Cement
###### Abstract  
In hydrating Portland cements, more than one of the AFm family of calcium aluminates may exist. Depending on the amount of carbonate and sulfate present in the cement, the most common phase to precipitate is monosulfate, monocarbonate and/or hemicarbonate. It has been reported in the literature that hemicarbonate often appears in measurements such as XRD but not predicted to form/equilibrate in thermodynamic models. With the ongoing use of commercial cements such as CEM I and CEM II containing more and more limestone, it is important to understand which hydrate solids physically precipitate and numerically predict over time. Using 27 cement samples with three w/c ratios analysed at 1, 3 and 28 days, this paper shows that although hemicarbonate was observed in a hydrating commercial Portland cement, as well as being predicted based on its carbonate (CO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)) and sulfate (SO\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\)) ratios, thermodynamic analysis did not predict it to equilibrate and form as a solid hydrate. Regardless of the w/c ratio, thermodynamic analysis did predict hemicarbonate to form for calcite contents < 2 wt.%. It appears that the dominant stability of monocarbonate in thermodynamic models leads to it precipitating and remaining as a persistent phase.  
cement; hydration; thermodynamics; PHREEQC; mineralogy +
Footnote †: journal: Applied sciences  
thereof. [PERSON] et al. [10] showed that only 3.9 and 7.7 wt.% of carbonate are sufficient to form hemicarbonate or monocarbonate, respectively. Carbonate AFm phases are reported to be more thermodynamically stable than sulfate phases, such as monosulfate. With modern commercial cements containing at least 4-5% calcite, it has a dual function of being reactive with cement and active during hydration and as an inert filler [10].  
Subsequent research by [PERSON] et al. [8] and [12] found that, through XRD measurements, the initial formation of hemicarbonate was slowly transformed into monocarbonate. In aqueous systems, supported by thermodynamic calculations in Portland cement, hemicarbonate is unstable in the presence of calcite [13] even though there is little difference in solubility between the two carbonate phases. However, the difference in the solubility of hemi- and monocarbonate within the pore solution is rather small. Due to little difference in precipitation kinetics between hemi- and monocarbonate [13; 14] and the slower dissolution of calcite above pH 9 [15], [PERSON] et al. [8] suggest the intermediate formation of hemicarbonate is most likely caused by the slow dissolution of limestone as insufficient dissolved carbonate is available. However, over time, with increasing calcite being available, hemicarbonate is converted to monocarbonate.  
[PERSON] et al. [11] demonstrated that due to foreign anions in its structure, hemiconate is stabilised and forms AFm carbonate-sulfate solid solutions. As a result, AFm phases will be stabilized with respect to ettrinigite and monocarbonate and these solid solutions could explain the differences between experiments and current thermodynamic models in long-term samples of cement blended with limestone.  
In the current study, PHREEQC was used to undertake a full thermodynamic analysis of the hydration of CEM I cement in terms of predicting changes in anhydrous and hydrated phase assemblage and pore solution chemistry using suitable input parameters. Cement hydration is complicated by the need to provide an account of several features, including empirical rate equations to describe the dissolution of OPC clinker phases, oxide components dissolved in the OPC clinker phases, oversaturation of pore solution with respect to specific phases during the first 12 h of hydration, and the release and uptake of the alkali elements K and Na by the C-S-H. All these features were addressed in the current study and have been successfully implemented into PHREEQC to provide live graphical output and comparisons with experimental data from the literature.