In contact with water, whether in liquid form or as water vapour, CFBC ashes exhibit considerable chemical activity, which can continue for more than a year. From the point of view of the setting of CFBC ash - water mixtures and their behaviourthe formation of gypsum and of ettringite are important. The crystallization of ettringite is associated with expansion of the set mass and absorption of water into the Solids, but the ash-water system as a whole shrinks. The extent of ettringiteformation is less if carbonation of free lime can take place. There is little carbonation if set solids are covered by water but if contact with air is not restricted, the ettringite formed earlier slowly decomposes, its place being taken by calcite andgypsum. The fate of the Al from decomposed ettrigite is uncertain; no new Al compounds are found. It appears that ultimately most analytical lime present can become carbonated. This may be environmentally beneficial.The main features of the behaviour of CFBC ash - water mixtures can be reproduced in model systems (CaO-Al{sub}2O{sub}3-CaSO{sub}4-H{sub}2O) but that of real ashes is complicated by the presence of silicates and possible interactions between silica andthe strongly alkaline aqueous medium. Also, aluminates present in the ash are probably more active with respect to ettringite formation than pure aluminum hydroxide.
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