SI-29 MAS NMR-STUDY ON STRUCTURAL-CHANGE OF SILICATE ANIONS WITH CARBONATION OF SYNTHETIC 11-ANGSTROM TOBERMORITE

Structural change of synthetic 11 angstrom tobermorite with the carbonation was examined by IR spectroscopy and XRD method. The synthetic 11 angstrom tobermorite decomposed into CaCO3 (calcite) and silica gel with the carbonation. In such a decomposition process, CaCO3 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 11 angstrom tobermorite was studied by Si-29 MAS NMR method. Three Si-29 NMR peaks assigned to Q1, Q2 and Q3 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 Ql peak and appearance of peaks due to Q4. In the carbonation more than 80%, only Q4 peaks were observed at -101 and -111 ppm, which were assigned to the silicon atoms of (HO)Si*(OSi less than or similar to)3 and Si*(OSi less than or similar to)4 in silica gel. Si-29 NMR data revealed that the silicate anions in synthetic 11 angstrom 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.