Abstract
The results obtained are summarized as follows:
(1) Syn-Tob is decomposed by reacting with CO2 into CaCO3 and silica gel.
(2) In the decomposition process, CaCO3 is present as three crystal forms (vaterite, aragonite and calcite) in the low carbonated Syn-Tobs. With the progress of carbonation, the fraction of calcite increases and in the complete carbonation all CaCO3 is transformed into calcite.
(3) Silicate phase in Syn-Tob is composed of the double chain structure with some Si-O-Si bridges between the chains. This double chain structure is destroyed with the carbonation and finally silica gel with the three-dimensional cross-linked framework structure is formed.
(4) The double chain structure is kept even at a considerably high degree of carbonation (around 70%). This may cause the pseudomorphism phenomenon.
(5) In the completely carbonated Syn-Tob, two 29Si NMR peaks are observed at around -111 and -102ppm, which are assigned to the silicon atoms of Si*(OSi_??_)4 and (HO)Si*(OSi_??_)3 structures in silica gel.
