Site Preferences of Fluoride Guest Ions in the Calcium Silicate Phases of Portland Cement from 29Si{19F} CP-REDOR NMR Spectroscopy

A reduction in CO2 emission from Portland cement production can be achieved by energy savings associated with a towering 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 Si-29{F-19} 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, Si-29{F-19} 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.