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This work explored the way to improve hot modulus of rupture (HMOR) and refractoriness under load ( RUL ) by adding mild-calcined coal gangue ( MCG )in Al203 - SiO2 ultra low cement (ULC) castables,making use of the in-situ effect of the MCG during heating-up.The influence of respective additions of 5％,10％ and 15％ of the MCG powders calcined at 700 ℃was investigated on HMOR at 1 400 ℃ and RUL of the castables.With increased addition of the MCG,HMOR and RUL become significantly enhanced.At 10％ of the MCG addition,HMOR reaches 3 MPa,as compared to 0.3 MPa in the case of no MCG addition.RUL of the specimens dried at 110 ℃ for 24 h can be increased by some 270 ℃ with 10％ of the MCG addition.RUL of the specimens preheated at 1 500 ℃ for 3 h maintaius the growth trend with the MCG addition increasing.The microstructure of the heated castable samples was investigated by means of SEM.The in-situ formed needle-like and interlaced mullite in the matrix is contributive to the improvement.