High- and low-defect kaolinite for brick making: Comparisons of technological properties, phase evolution and microstructure

Kaolinite is among the most common clay minerals in the starting material for fired brick production. The complex geological origin of kaolinite creates different defects in its structure which may affect the sintering behavior and thus, properties of brick products. In this study, two Source Clays (KGa-1 and KGa-2) from the Clay Minerals Society have been employed as kaolinite sources to create a unique KIQF (kaolinite-illite-quartz-feldspar) system. The impacts of kaolinite defects on the microstructure, phase composition, physical, mechanical and thermal properties of fired brick buttons have been investigated. With increased firing temperature, brick products prepared with high-defect kaolinite exhibit steeper decrease of water absorption and increase of bulk density. When the temperature is below 1050 °C, CHK has higher water absorption than that of CLK. The water absorption and bulk density of brick buttons with different kaolinites are comparable (∼11.30 % and ∼ 1.95 g/cm3, respectively) after 1150 °C. For brick buttons (containing different kaolinites) fired at 850, 1050 and 1150 °C, no significant difference is observed in their color, XRD phase composition, or compressive strength. However, at 950 °C, high-defect kaolinite particles in the brick matrix display lower porosity of 41.52 % and a 2D mean pore size of 9.78 µm2 than that of low-defect kaolinite particles (51.19 % and 10.36 µm2, respectively), and this can be attributed to finer particles and a higher amount of Fe (1.08 %) in the former, which promotes vitrification. Smaller pores and a higher amount of spinel formed using high-defect kaolinite have resulted in a 7.1 % higher compressive strength. In addition, between 850 and 1150 °C, the thermal diffusivity of brick buttons with high-defect kaolinite is consistently higher. Thus, 950 °C is a crucial firing temperature when both higher compressive strength and thermal diffusivity of brick buttons are required. In addition, if lower porosity is required, 1050 °C or higher firing temperature is recommended for high-defect kaolinite.