Synergistic effect of fly ash and blast furnace slag on the mechanical strength of traditional porcelain tiles

Fly ash and blast furnace slag, the by-products of Indian thermal power plant and steel plant, were gradually added to a traditional kaolin—quartz—feldspar based porcelain tile compositions singly and in combination by replacing a part of quartz and feldspar, respectively. The effects of such additions were studied by measuring the linear shrinkage, bulk density, water absorption and flexural strength of the samples heated in the temperature range of 1150–1200 °C. The results reveal that additions of both the by-products have beneficial effects towards mechanical strength. The strength improvement was mainly due to the presence of microcrystalline components such as mullite and quartz in fly ash and alkaline earth oxides in blast furnace slag. Slag containing compositions with or without fly ash vitrified at an early temperature (1175 °C) and achieved highest strength at this temperature compared to 1200 °C for normal and fly ash bearing compositions. Specific combinations of fly ash and slag in the ratio of 1:1 and 1:2 have shown the development of highest flexural strength (>70 MPa) at 1175 °C due to their lowest R2O:R′O (R = K, Na and R′ = CaO, MgO) and moderately higher SiO2:Al2O3 ratio. Beyond this temperature, the strength drastically reduced due to the formation of glassy phases at higher temperatures. XRD studies were also carried out to find out the differences in phase evolution due to addition of fly ash and blast furnace slag to traditional porcelain composition.