This paper discusses the results of tests conducted to ascertain at high temperatures the mechanical behavior of cements whose most prominent characteristic is the absence or near absence of Portland cement in their composition. More specifically, it reports on the study of three binders: (a) Portland cement (control); (b) 100% sodium silicate-activated fly ash; and (c) a blend of 70% ash+30% clinker activated with solid-state activators (solid sodium silicate+Na(2)CO(3)). Two types of tests were conducted: (i) tests to determine the mechanical strength and fracture toughness of the three materials between 25 degrees and 600 degrees C and (ii) postthermal treatment tests to evaluate the residual strength after 1 h of exposure to temperatures ranging from 200 degrees to 1000 degrees C. XRD and SEM techniques were also deployed to track the mineralogical and microstructural variations in the materials as a result of such an exposure. The findings showed that a pseudo-viscous phase is generated in alkaline cements at around 600 degrees C that would explain the decline in strength observed when specimens were loaded at increasing temperatures. This phase and its subsequent recrystallization into new phases (nepheline, albite, and gehlenite) were also responsible for the high-residual mechanical strength values observed in these materials after exposure to high temperatures and subsequent cooling but absent in OPC.