Calcium alumino-titanate (CAT) can be used as a raw material owing to its excellent refractoriness and thermal expansion. CAT-containing taphole clay (THC) with different particle sizes was prepared and characterized. The results indicated that THC with CAT aggregate has better thermo-mechanical performance than that with a fine CAT powder because of different sintering processes. The CAT aggregate and SiO2 reacted and formed liquid phase covering their surface, enhancing the bonding of the particle–matrix and promoting the internal transformation of the substance. Further, the mullite was formed in aggregate with the interwoven network structure. Fine CAT powder has a higher specific surface area, and its surface energy decreases with the particle size. This could cause a reaction between CAT and SiO2 at relatively low temperature, making it unable to form a core–shell structure similar to that of the aggregate, with the excessive liquid phase affecting the performance of the samples. Meanwhile, the liquid phase was formed to close the pores and cover the SiC surface, and this was found to enhance the oxidation resistance. A corrosion test confirmed that the THC refractories with CAT aggregates showed better corrosion resistance than other samples. Scanning electron microscopy and energy-dispersive spectrometry results indicated that Ti(C,N) transformed by TiO2 in CAT was deposited at the interface between the molten slag and the refractory, and it efficiently prevented the penetration and reaction owing to its high melting point and small wetting angle with the slag.