The accelerative carbonation of autoclaved light-weight concrete (ALC) was studied by XRD, IR, microscope, etc. And the carbonation shrinkage was studied. The carbonation products were vaterite, aragonite and calcite. Under the microscope, the crystal growth of carbonate minerals was observed by the heat treatment under 40°C after CO
2 reaction. The heat treatment was named “crystallization treatment”. The linear shrinkage rate after CO
2 reaction was approximately 3×10
-3. And the linear shrinkage rate after crystallization treatment was approximately 1-2×10
-2. This paper sugests the carbonation mechanism of ALC defined by two process, 1) chemical process (production of carbonate by Tobermorite-CO
2 reaction), and 2) physical process (crystal growth of carbonate).
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