Partial substitution of feldspar by B.F. slag in triaxial porcelain: Phase and microstructural evolution

Feldspar was gradually substituted by B.F. slag (a glassy by-product of Indian steel plant) to the extent of 5–20 mass% in a triaxial porcelain composition consisting of 45 mass% kaolinitic clay, 30 mass% feldspar, 25 mass% quartz. The green compacts were heated at 1200 °C for a period of 120 min. The microstructure and phase changes as they evolve on heating were studied using SEM and XRD techniques. The results reveal that quartz level was reduced from 26 to 9 mass% by the addition of 5% slag, while mullite level was reduced from 20 to 2 mass% by the addition of 10% slag. Beyond this, further addition of slag did not alter the quartz and mullite level much. Slag used in this study was enriched with alumina and contributed towards development of anorthite (CaO·Al2O3·2SiO2) phase by crystallization of melted glassy phase and its quantity increased with increase in slag content. Sudden increase in flexural strength of 20% slag containing body is attributed to stronger pre-stress caused by the difference in thermal expansion coefficient between glassy matrix, quartz and anorthite during cooling process. This also caused circumferential cracking around quartz grains. The paper further discusses the variation in their physico-mechanical characteristics with respect to slag content.