REHYDRATION OF CASTABLE REFRACTORIES

Castable refractories bonded with calcium aluminate cement can be susc eptible to ''rehydration'' if they are initially dehydrated at tempera tures below which substantial sintering can take place (<1200 degrees C) and if they are subsequently exposed to water or steam. Rehydration is known be important in certain castable applications such as in pet rochemical service. Thermal analysis investigations of rehydrated spec imens showed the expected dehydration phenomena with endothermic react ions and weight loss generally in the area of 275-400 degrees C. One c astable specimen exhibited rehydration on cooling in the analyzer in a ir suggesting that some castables may be more sensitive than others to rehydration. In further experiments, castable specimens were dehydrat ed in-situ in a dilatometer by holding at 538 degrees C, and they were exposed to steam on cooling at 450 degrees C producing a very rapid e xpansion of about 0.2% whenever the steam was introduced. Subsequent r ehydration experiments with bar specimens with low pressure steam (0.0 34 MPa or 5 psi) and high pressure steam (1.04 MPa or 150 psi) confirm ed continuing expansion for a period of 5-15 hours with the total line ar expansion of 0.1-0.2%. Rehydration generally did not diminish modul us of rupture of bar specimens for a single rehydration cycle; however , rehydration resulted in a reduction of permeability of specimens. Li mited data suggests that repeated rehydration dramatically reduces the strength of castables. The effect of rehydration on permeability and strength suggests that heating of rehydrated specimens could result in explosive spalling if excessive heating rates are employed.