| Abstract
| - The glass-forming reactions between sodium carbonate (Na2CO3) and silica (SiO2) have been investigated by23Na, 29Si, and 13C magic-angle spinning (MAS) NMR spectroscopy. The multinuclear MAS NMR approachidentifies and quantifies reaction products and intermediates, both glassy and crystalline. A series of powderedbatches of initial composition Na2CO3·xSiO2 (x = 1, 2) corresponding to a sodium metasilicate (Na2SiO3)and sodium disilicate (Na2Si2O5) stoichiometry were investigated after periods of isothermal and nonisothermalheat treatments at different temperatures. Analysis of the 23Na quadrupolar coupling parameters has identifiedthe early reaction product in all cases as crystalline Na2SiO3. In the nonisothermal experiment, this reactionis preceded by an early silica-rich melt phase formed around 850 °C. The early reactions are controlled bysolid-state Na+ diffusion across the reaction zone in the grain interface layer. Crystalline Na2SiO3 precipitatesin the interface layer, increasing its thickness between the Na2CO3 and the SiO2 grains and slowing down therate of Na+ migration. This creates a secondary phase, which is temperature dependent. At low temperatures,where Na+ migration is impaired, the production of Na2SiO3 ceases and silica-richer phases are precipitated.In the case of the sodium disilicate batch, where excess SiO2 is present, a secondary reaction of Na2SiO3 withSiO2 forming a glassy phase is observed. A transient carbon-bearing phase has been identified by 13C NMRas a NaCO3- complex loosely bound to bridging oxygens in the silicate network at the SiO2 grain surface.
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