An experimental study was conducted to evaluate the mechanical properties of mortar containing flyash subjected to both thermal and chemical activation. Mortar specimens containing class F flyash and various activators were prepared. Up to 50% by weight of cement was replaced with flyash and the results were compared with the control mixture. In order to activate the hydration reactions, additives such as sodium hydroxide (NaOH) and potassium hydroxide (KOH) were used at a rate of 2.5% of total binder weight. Thermal activation was achieved using an autoclave curing process. Both the strengthening and toughening mechanisms were studied in compression, flexure. The fracture results are analyzed using the fracture energy method, the two-parameter facture model of Jenq and Shah, and also the R-curve approach. Experimental data indicate that increasing the flyash content from 20% to 50% results in a favorable influence on compressive strength whereas the flexural and fracture properties remain virtually at the same level. R-Curves provide a more descriptive measure of fracture response. Autoclave curing of high flyash mortar samples results in a marked increase in the strength but a marginal reduction in ductility.