alpha-Cristobalite formation in ceramic tile and sewage pipe bodies derived from Westerwald ball clay and its effect on elastic-properties

The formation of α-cristobalite and its effect on the elastic modulus (E modulus) was investigated in sewage pipe (SP) and tile bodies derived from Westerwald ball clay (WBC), employing the results of previous research on WBC as a baseline. In this study, the SP and tile bodies derived from WBC are compared with the bodies derived from two residual Turkish kaolin and one Turkish ball clay, resulting in four sources of α-cristobalite formation: tile, SP, residual kaolin and ball clay. The crystallization of α-cristobalite in the fired WBC results from two primary pathways. The first pathway occurs as a devitrification product from the glass phase saturated with excess silica, which is typically present in ball clay after ≥1100 °C; the second pathway results from a minor amount of transformation of peripheries of quartz crystals, firing at over ≥1200°. In addition to these two sources, other α-cristobalite sources for sewage-pipe bodies include smectitic clay and chamotte (prefired clay) addition. Conversely, no α-cristobalite occurred in tile mixture bodies containing ≈2 wt% potassium oxide (K 2O). The α-cristobalite phase only forms in kaolinite-rich ball clay when K 2O content is less than ≈2 wt% because devitrification of excess silica phase of tile bodies is primarily governed by K 2O content. The ≈2 wt% K 2 O ratio inhibited α-cristobalite formation in tile bodies, while the same percentage or greater of K 2O content in SP bodies did not inhibit the α-cristobalite formation due to the presence of chamotte and smectitic clay in the SP mixture. The measured minimum E moduli were attributed to cristobalite formations because of chamotte and smectitic clay additions to SP bodies. In the bodies derived from Turkish residual kaolin at over 1000 °C, the opaline and chalcedonic silica phase are transformed to α-cristobalite at a lower temperature than those ob- served for rock crystal quartz and Si-rich glassy phase in WBC bodies. Thus, determining the different α-cris- tobalite sources in clay-based ceramic bodies derived from either the ball clay, the residual kaolin or the bodies containing smectitic clay together with chamotte provides the essential pre-condition for the optimum admixing ratio in industrial SP and tile bodies