This article assesses the effect of mix design parameters on the compressive strength and thermal performance of alkali silicate-activated blends of metakaolin (MK) and granulated blast furnace slag (GBFS). A strong interrelationship between the effects of activator composition and the GBFS/(GBFS + MK) ratio is identified through statistical analysis of compressive strength data. Pastes formulated with higher SiO(2)/Al(2)O(3) molar ratios show improvements in mechanical strength with increasing GBFS addition, associated with the formation of a structure comprising coexisting aluminosilicate 'geopolymer' gel and Ca-rich Al-substituted silicate hydrate (C-(A)-S-H) reaction products. The inclusion of GBFS in MK-based geopolymers seems also to improve their performance when exposed to high temperatures, as higher residual compressive strengths are reported for these mixtures compared to solely MK-based systems. Only slight differences in shrinkage behaviour are observed at temperatures of up to 600 A degrees C with the inclusion of GBFS; however, slag-blended pastes exhibit enhanced stability at temperatures exceeding 800 A degrees C, as no variation in the compressive strength and no additional shrinkage are identified. These results suggest that nanostructural modifications are induced in the gel by the inclusion of GBFS into MK-based geopolymers, improving the overall performance of these materials.