The Influence of Fluorine, Chlorine and Water on the Rheology and Structure of Na2O-CaO-Al2O3-SiO2 Melts

This study investigates the influence of 1.6 to 14.5 mol% water, 1.1 to 18.3 mol% fluorine (F) and 0.5 to 1.4 mol% chlorine (Cl), alone and combined, on the structure and viscosity of peralkaline and peraluminous Na2O CaO Al2O3 SiO2 glasses and melts with ~ 66 mol% SiO2 (on a volatile free basis). The composition of the peralkaline samples is intended as a model system for phonolitic melts. The anhydrous samples were produced in 1 atm furnaces using oxide, carbonate and halide compounds. The hydrous peralkaline samples were synthesised in an internally heated pressure vessel and the hydrous peraluminous samples in a piston cylinder. The viscosity of the melts was measured with the micropenetration technique (108.5 1013 Pa s) and the parallel plate method (105.5 Pa s 109 Pa s). The structure of the glasses was analysed with magic angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR) of the nuclides 19F, 23Na, 27Al, 29Si and 35Cl. Fluorine and water, alone and combined, decrease melt viscosity, with the effect of water on the viscosity being stronger than that of F. Both volatiles decrease viscosity more strongly in the peraluminous melts than in the peralkaline melts. In peralkaline melts F decreases viscosity up to a concentration of 1.9 mol% F, but for a concentration of 6.2 mol% F no further decrease was observed. In peraluminous melts, up to a concentration of 18.3 mol% F, further addition of F leads to further decrease in viscosity. The combined effect of water and fluorine on viscosity is less than could be expected from the sum of their single effects, indicating that their effects on viscosity are not independent from each other. The composition and concentration-dependent effect of F on melt viscosity is consistent with differences in the incorporation mechanism of F in the glasses. 19F MAS NMR spectra showed that in the peralkaline glasses F exists in “salt like” F Ca(n) and F Na(n) as well as non bridging Si F Na(n), Al F Ca(n), Al F Na(n) and bridging Al F Al environments (“n” indicates that the number of atoms is variable or unclear). F Ca(n) is the most abundant environment, although Ca is the least common cation. In the peraluminous glasses F only exists in the Si F and Al F environments described above, with Al F Na(n) being the most abundant environment. The formation of salt like F Ca(n) and F Na(n) environments should increase viscosity by reducing the amount of network modifying cations. The formation of Si F and Al F environments should decrease viscosity by either reducing the number of bridging oxygens, in the case of the non bridging F environments, or due to the replacement of bridging oxygens by bridging fluorines, which are assumed to have lower bond strength. It can be concluded that the decrease in viscosity due to F is less in the peralkaline melts than in the peraluminous melts, because in the peralkaline melts F exists in environments that could increase and decrease viscosity, while in the peraluminous melts F exists only in environments that decrease viscosity. The concentration dependent effect of F on viscosity in the peralkaline samples seems to be related to a change in F speciation: The relative fraction of the F Ca(n) environments, which are assumed to cause an increase in viscosity, increases from 42 to 53% for an increase in F concentration from 1.2 to 6.2 mol%. Changes in F speciation also appear to be responsible for the fact that F and water combined reduce viscosity less than expected. 19F MAS NMR spectra of hydrous F bearing peralkaline samples showed that the relative fraction of Al F environments, which should decrease viscosity, is reduced with increasing water content and that instead the relative fraction of F Ca(n) increases. No difference in the OH/H2O ratio for the same total water contents due to the presence of F was observed with IR spectroscopy. In contrast, there is evidence that in the peraluminous samples F reduces the OH/H2O ratio for the same total water contents, which would explain why in the peraluminous melts F and water combined reduce viscosity less than expected from their single effects. The influence of Cl on viscosity and structure of the melts and glasses is different to that of F. Chlorine increases the viscosity in peralkaline melts and decreases viscosity in peraluminous melts. Viscosity measurements of hydrous Cl free and Cl bearing peralkaline melts showed that the effect of Cl on viscosity is not affected by the presence of water. The observed 35Cl NMR signal shows, that Cl exist in Na Ca Cl environments dominated by Na, as expected from the Ca/Na ratio of 1/5. The Cl environment is similar in the peralkaline and peraluminous glasses, though there is more Ca incorporated in the Cl environment in the peraluminous glasses. In the 35Cl MAS NMR spectra only a part of the 35Cl NMR signal of the peralkaline and peraluminous samples were observed compared to a sodium silicate glass. The missing signal indicates that a part of the Cl anions exists in asymmetric or disordered environments with a line width which is too broad to be detected with the applied NMR methods. The increase in viscosity due to Cl in the peralkaline melts can be explained by Cl reducing the amount of network modifying cations, while several possibilities remain to be dicussed, how Cl reduces viscosity in the peraluminous melts. The effects of F and Cl on viscosity are independent of each other and add together. No evidence was found from NMR spectroscopy that the presence of F has an influence on the incorporation mechanism of Cl.