This study was designed to determine the effect of temperature on the mechanical strength (in both in-situ and post-exposure trials) of Portland cement (CEM I 42.5 (OPC)), a pre-industrial hybrid cement (CEM HYC = 30% clinker + 32.5 % BFS + 32.55 % FA + 5 % alkaline activator) and a geopolymer (fly ash-activated with NaOH solution). The Portland cement was studied as reference material. Two types of mechanical tests were conducted on samples made of each of these three cementitious systems: i) high-temperature in-situ mechanical tests, including bending and compressive strength, fracture toughness and modulus of elasticity ; ii) post-thermal treatment tests , which evaluate residual strength after two hours of exposure to temperatures ranging from 400 °C to 1000 °C. Differential thermo-gravimetric (DTA)/TG, X-ray diffraction, MAS NMR studies and Hg porosimetry were applied to track the mineralogical and microstructural variations taking place in the materials. The results from the three binders have provided new ideas about the relationship between the elemental composition of materials and high-temperature performance.