^<29>Si MAS NMR法による合成11Åトバモライトの炭酸化に伴うケイ酸イオンの形態変化に関する研究 ^<29>Si MAS NMR Study on Structural Change of Silicate Anions with Carbonation of Synthetic 11Å Tobermorite

Structural change of synthetic 11Å tobermorite with the carbonation was examined by IR spectroscopy and XRD method. The synthetic 11Å tobermorite decomposed into CaCO_3 (calcite) and silica gel with the carbonation. In such a decomposition process, CaCO_3 existed in three crystal forms (calcite, vaterite and aragonite) in the low carbonated tobermorites, all of which were transformed into calcite with the progress of carbonation. In addition, the structural change of silicate anions with the carbonation of synthetic 11Å tobermorite was studied by ^<29>Si MAS NMR method. Three ^<29>Si NMR peaks assigned to Q^1, Q^2 and Q^3 were observed at -79.7, -85.9 and -95.9 ppm in the low carbonated tobermorites and further carbonation (>40%) led to the disappearance of Q^1 peak and appearance of peaks due to Q^4. In the carbonation more than 80%, only Q^4 peaks were observed at -101 and -111 ppm, which were assigned to the silicon atoms of (HO)Si^*(OSi〓)_3 and Si^*(OSi〓)_4 in silica gel. ^<29>Si NMR data revealed that the silicate anions in synthetic 11Å tobermorite change from a double chain structure with some Si-O-Si bridges to the three dimensional cross-linked framework structure with the progress of carbonation.