To investigate the feasibility of geopolymer mortar as a repair material for strengthening concrete structures in which fire resistance is one of the primary requirements, tensile strength and bond strength of geopolymer mortar on Portland cement mortar and concrete substrate were tested at ambient temperature and after exposure to elevated temperatures. Comparative tests were conducted on cement mortar specimens. Thermogravimetry and differential scanning calorimetry (TG-DSC) analysis were conducted on geopolymer paste to explore the strength degradation mechanism of geopolymer mortar at high temperatures. The results show that the bond strength of geopolymer mortar at ambient temperature on cement mortar and cement concrete substrate is 2.15 MPa and 1.7 MPa respectively, and the residual bond strength is about 1.5 MPa after exposure to 300 ℃, which is much higher than that of Portland cement mortar. When temperatures exceed 300 ℃, geopolymer mortar exhibits significant strength degradation, due to the microstructural damage induced by the dehydration of geopolymers at high temperatures. Based on the above test results, geopolymer mortar can be used as a repair material for concrete structures in high temperature environment below 300 ℃.