Effects of grain size on the reactivity of limestone temper in a kaolinitic clay

Carbonates in clay based ceramics produces higher sintering at lower firing temperatures, but may cause lime spalling, affecting the physical and mechanical behaviour of the ceramic body. The present study investigated the mineralogical and microstructural changes that occur in a kaolinitic clay tempered with different contents of limestone sand with two skewed grain size distributions, after firing. The firing temperatures were set at 500, 750 and 1000 °C. The mineralogy of the fired bodies was analyzed by XRPD and quantitative phase analysis was performed using Rietveld method. SEM-EDS analyses were carried out to investigate the changes in microstructures and the clay/limestone reactivity. The use of sand-sized limestone temper and short firing times induced the formation of non-stoichiometric phases at the clay/limestone boundary, ruled by the lateral variation of CaO activity. The structure and composition of the spinel-type phase (e.g. γ-Al2O3), as typical firing product of kaolinite clays, were investigated. Different Ca-silicates and -aluminosilicates (gehlenite, rankinite and larnite) in ceramics fired at 1000 °C are found according to the limestone grain size. Lime spalling already occurs in ceramics fired at 750 °C; it is triggered by coarse calcined grains (σspalling > σmatrix failure) and then fractures propagates through finer calcined limestone grains.