Effect of the reactivity of alkaline solution and metakaolin on geopolymer formation

•The amount of low order species controls the reactivity of the alkaline solution.•The polycondensation rate depends on the metakaolin and the alkaline solution.•The metakaolin reactivity is responsible for generating one or several networks.•A highly reactive alkaline solution governs the geopolymerization reaction.

This paper focuses on the effects of alkaline solution reactivity and metakaolin properties on geopolymer formation. To examine these effects, several geopolymer samples were synthesized from two alkaline solutions and four metakaolins. The structural evolution of the formed geopolymers was investigated using FTIR spectroscopy and pH values during the material formation. The mechanical properties were measured using compression tests. The results show that the type and amount of siliceous species and non-bridging oxygen atoms control the alkaline solution reactivity. The effect of the metakaolin reactivity is more significant when it is activated with a poorly reactive alkaline solution. However, the alkaline solution governs the reaction when it is highly reactive. Therefore, the extent of depolymerization of the alkaline solution and the reactivity of metakaolin are crucial parameters that control the rate of polycondensation and the compressive strengths of geopolymer materials.