Quantifying the Impact of Magnesium on the Stability and Water Binding Energy of Hydrated Calcium Carbonates by ab initio Thermodynamics
Determining conditions that drive carbonate formation is important for establishing paleoindicators for mineral deposition, understanding the carbon cycle and biomineralization, and designing industrial applications such as scale inhibition, manufacturing of cement and concrete, and environmental remediation. Mg and incorporated water have been observed to play critical roles in nonclassical crystallization pathways of calcium carbonate, the dominant carbonate found in nature, through promoting formation of low energy metastable intermediates such as monohydrocalcite (CaCO3H2O), ikaite (CaCO36H2O), and amorphous calcium carbonate (CaCO3H2O). The impact of Mg on the thermodynamics and water binding ability of these hydrated intermediates is challenging to measure and is not understood at the molecular level. In this work density-functional theory and ab initio thermodynamics are used to quantify the impact of Mg on structure, thermodynamics and water binding energies of the crystalline hydrated Ca carbonates as a function of temperature in aqueous and ultra-high vacuum conditions, as well as CO2-rich environments relevant to carbon sequestration. For monohydrocalcite, Mg incorporation is found to destabilize the structure despite a dramatic increase in the water-binding energy, thus confirming that Mg promotes monohydrocalcite formation kinetically rather than thermodynamically. For ikaite, however, Mg promotes its formation both kinetically and thermodynamically, expanding the stability region for ikaite in cold water. For portlandite Ca(OH)2, Mg significantly destabilizes the structure which cannot shift atomic positions to accommodate the smaller cation, unlike the positional flexibility afforded by water molecules and carbonate groups in monohydrocalcite and ikaite. Mg does result in a decreased lattice volume for ikaite and monohydrocalcite, but only a very small amount and hence cannot be used to quantify the amount of Mg incorporation in natural samples.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1525109
- Report Number(s):
- PNNL-SA-139647
- Journal Information:
- Journal of Physical Chemistry A, Vol. 123, Issue 13
- Country of Publication:
- United States
- Language:
- English
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