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This study aims to improve the stability of alkali-activated products, such as powdered volcanic scoria and metakaolin, by thermal treatment in addition to their partial replacement with aluminium oxy-hydroxide. To this effect, mixtures obtained from a partial replacement of aluminosilicate sources (volcanic scoria or metakaolin) with respectively 0, 10, 20 or 30 % of mass of aluminium oxy-hydroxide were alkali-activated and stored for 28 days at room temperature in the laboratory. The various specimens obtained were heated at 900–1200 °C and both the alkali-aluminosilicates and the heated products were analysed. It appeared that aluminium oxy-hydroxide improves the stability of heated products. Thus, without replacement, alkali-activated specimens of metakaolin showed cracks at 1100 °C while those of volcanic scoria melted at 1150 °C. Conversely, heated alkali-activated specimens of either volcanic scoria or metakaolin with the replacement showed stability up to 1200 °C and an improvement in the residual compressive strength as from 900 °C. Indeed, at 1150 °C, the metakaolin-based geopolymers or the alkali-activated volcanic scoria with 30 % of mass of the replacement showed residual compressive strength of 76.7 and 35.0 MPa, respectively, due to the sintering and to the formation of new crystalline phases (mullite, corundum, carnegieite and nepheline). Yet, residual compressive strength in the alkali-activated aluminosilicate specimens with replacement that were heated at 1200 °C dropped as a result of the partial dissolution of nepheline and carnegieite which may have generated closed pores within their microstructure. Hence, alkali-activation of volcanic scoria or metakaolin partially replaced with aluminium oxy-hydroxide is a remarkable process to get thermally stable products.