Using CaO as a modifying agent greatly optimized the microstructure and physical properties of foamed ceramics that were prepared using granite scrap as a raw material and SiC as a foaming agent. The results of this work show that the appropriate amount of CaO can reduce the viscosity of the melt to balance the expansion drive of the pores and the surface tension of the melt and can also promote phase separation and crystallization. Wollastonite crystals, when separated from the pore wall, can stabilize the gas‒liquid interface, thereby helping to obtain a full, independent, uniform pore structure. Homogeneous pore structure and crystallization can also improve the compressive strength so that the foamed ceramics have a relatively high compressive strength while having high porosity. Adjusting the sintering temperature and heating rate can synergistically assist in improving the pore structure. The uniform porous structures and the most favourable comprehensive properties were achieved by adding 8 wt% CaO, sintering at 1130 °C with a heating rate of 5 °C/min, and including a high closed porosity of 76.58%, bulk density of 580.31 kg/m³, compressive strength of 4.33 MPa, water absorption of 0.41%, and thermal conductivity of 0.241 W/(m·k).