Evolution of microstructure, mineralogy and properties during firing of clay-based ceramics with borates

The effect of sieve boron waste (SBW) and borate-containing Evansite® on the thermal behaviour, microstructure and properties of a clay-based body was investigated. SBW and Evansite® were introduced in quantities that correspond to 0.6 wt.% B2O3 addition in the dry body for both cases. Cylindrical samples were extruded and fired at three different peak temperatures 900, 950 and 1000 °C. The reference body, R, and the body with SBW, RB, demonstrate a comparable dilatometric behaviour whereas the densification for the body with Evansite®, RV, initiated 50 °C approximately lower and resulted in higher firing shrinkage. After firing at 900 °C, the physico-mechanical properties as well as the microstructure are comparable. Nonetheless, åkermanite is formed in RB, whereas hercynite and mullite, the latter at 1000 °C, are formed in RV. For firing at 1000 °C, the role of borates is intensified. Water absorption is reduced by 16.1% and 18.0%, whereas bending strength increased by 27.6% and 40.8%, for RB and RV respectively, compared to the reference formulation. This is attributed predominantly to the enhanced vitrification that took place in the boron-containing bodies.