Abstract: A reduction in CO₂ emission from Portland cement production can be achieved by energy savings associated with a lowering of the temperature at which the high temperature alite (Ca₃SiO₅) and belite (Ca₂SiO₄) silicates form. This can be accomplished by fluoride mineralization where a small amount of fluorine (e.g., CaF₂) is added to the raw mix of starting materials. This work provides the mechanism for incorporation of fluoride ions in the calcium silicate phases of Portland cements which is important in the optimization of the fluoride mineralization. It is demonstrated by double-resonance ²⁹Si{¹⁹F} CP/MAS NMR experiments that the fluoride ions are exclusively incorporated into the alite phase of the two calcium silicates. The fluoride ions substitute for oxygen by a coupled mechanism that also involves replacement of Si⁴⁺ by Al³⁺ to achieve charge balance. Most importantly, ²⁹Si{¹⁹F} REDOR NMR experiments reveal that the fluoride ions are incorporated in alite with a site preference for the “interstitial” oxygen sites and thus not the covalently bonded oxygens of the SiO₄ units. This implies that only one-fifth of the oxygen sites in alite are available for substitution by fluoride ions which limits the gain in entropy of mixing that is a key factor for the reduction in upper temperature of the cement kiln.

*Abstract of Site Preferences of Fluoride Guest Ions in the Calcium Silicate Phases of Portland Cement from ²⁹Si{¹⁹F} CP-REDOR NMR Spectroscopy, online at http://pubs.acs.org/doi/abs/10.1021/ja905223d

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