According to the capillary tension theory, the elastic strain energy of porous materials will be released when their capillary water losses completely in the drying precess. This explains the reason of discontinous shringkage of porous materials, and is confirmed for the first time at drying of fly-ash autoclaved aerated comcrete (F-AAC). It is also applicable to the swelling deformation process of F-AAC. Further studies indicate that the F-AAC after carbonation displays continuous shrinkage or swelling due to change of the ingredients and destruction of porous structure and cement gel structure. By the studies of the various properties of F-AAC, the mechanism and conditions of discontinuous shrinkage (or swelling) have been clarified theoretically, and the macroscopic properties of the materials may be controlled by controlling their microscopic structure.
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