Thermomechanical behavior of high-alumina refractory castables with synthetic spinel additions

The thermomechanical behavior of a high-alumina refractory that contains 10 wt% of synthetic spinel has been studied. Results have been correlated wit h the microstructural and mineralogical evolutions using X-ray diffraction and scanning electron microscopy, as function of the firing temperature. Dr y, castable, raw-material mixtures were sieved to <125 mu m, to separate th e fraction that is considered as the matrix of this castable. Matrix sample s were obtained for thermomechanical characterization, and the results were compared with those corresponding to the entire refractory, The thermal ex pansion mismatch between coarse grains and the matrix was observed to be re sponsible for the degradation in mechanical strength that was observed at l ow firing temperatures. From room temperature to similar to 1000 degrees C, the material exhibited a quasi-brittle behavior, with diffuse and localize d damage. The diffuse damage was analyzed by comparison of the load-strain bend curves to a smeared crack model that was calculated using a finite-ele ment method, A good fit was obtained, thereby allowing the parameters of th e model to be determined. The localized damage regime was evaluated by reco rding the strain-energy release rate. Because of the specimen size, an incr ease in energy was observed, relative to increasing crack size. At temperat ures of >1000 degrees C, viscoelasticity-plasticity dominated the mechanica l behavior. In this case, the description was initiated by conducting load- relaxation tests, which allowed the behavior to be related to the viscous-p hase content.