The lamellar hydrates of CAC were designed with the introduction of nano CaCO3 or Mg-Al hydrotalcite (M-A-H), and the effects on the green strength, pore structures, and high-temperature fracture behavior of alumina-spinel castables were investigated. The results show that nano CaCO3 or M-A-H stimulates rapidly the hydration of CAC and the formation of lamellar C4AcH11 or coexistence of C2AH8 and C4AcH11 at 25 °C. The formation of lamellar hydrates can contribute to a more complicated pore structure, especially in the range of 400-2 000 nm. Meanwhile, the incorporation of well-distributed CaO or MgO sources from nano CaCO3 or M-A-H also regulates the distribution of CA6 and spinel (pre-formed and in-situ). Consequently, the optimized microstructure and complicated pore structure can induce the deflection and bridging of cracks, thus facilitating the consumption of fracture energy and enhancing the resistance to thermal stress damage.