Moisture-Expansion Characteristics of a Fired Kaolinite-Hydrous Mica-Quartz Clay

The natural and autoclave moisture expansions of a fired clay containing kaolinite, hydrous mica, and quartz were studied. A relation was established between the two expansions, which, together with a moisture‐expansion peak at about 1000°C, is explained in terms of the poorly crystalline and noncrystalline phases developed during firing. When low‐fired specimens that were exposed to moisture were desorbed by re‐firing, the shrinkage that accompanied the loss of moisture was interrupted by an expansion at about 5OOOC. This expansion depended on the firing temperature and probably was caused by the dehydroxylation of hydrous mica regenerated from mica anhydride by moisture sorbed after firing. In the case of specimens fired above the moisture‐expansion peak, the moisture responsible for expansion must be held in much the same way in the natural and in the autoclaved specimens. A study of the kinetics of moisture expansion showed a linear relation between the magnitude of expansion and the log of time for the period during which the specimens were exposed to moisture in the air. A similar relation existed for specimens fired from 1000° to 1150°C and autoclaved from 2 to 200 hours. It is further shown that it was possible, using preexisting data, to estimate, from the magnitude of expansion after a short period, the expansion to be expected at any time within the period to which the preexisting data referred.