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Short carbon fiber preform reinforced geopolymer composites containing different contents of α-Al_2O_3 filler (C_f(α-Al_2O_3)/geopolymer composites) were fabricated, and the effects of heat treatment temperatures up to 1 200 ℃ on the thermal-mechanical properties were studied. The results show that the thermal shrinkage in the direction perpendicular to the lamination of the composites gradually increases with the increase of the heat treatment temperatures from room temperature (25 ℃ ) to 1 000 ℃ . However, the composites in the direction parallel to the lamination show an expansion behavior. Beyond 1 000 ℃ , in the two directions the composites exhibit a larger degree of shrinkage due to the densification and crystallization. The mechanical properties of the composites show the minimum values in the temperature range from 600 to 800 ℃ as the hydration water of geopolymer matrix is lost. The addition of α-Al_2O_3 particle filler into the composites clearly increases the onset crystalline temperature of leucite (KAlSi_2O_6) from the amorphous geopolymer matrix. In addition, the addition of α-Al_2O_3 particles into the composites can not only help to keep volume stable at high temperatures but also effectively improve the mechanical properties of the composites subjected to thermal load to a certain extent. The main toughening mechanisms of the composites subjected to thermal load are attributed to fiber pulling-out.