Alkaline activation of metakaolin.: Effect of the soluble silicate addition and curing temperature

This study has been undertaken to determine the effect that the soluble silica content in the activating solution (molar ratio SiO2/Na2O between 0 and 0.69) and curing temperature (85 degrees C, 150 degrees C, and 200 degrees C) have on the physico-mechanical properties and mineralogical and microstructural characteristics of products formed by alkaline activation of metakaolin. The compression strength of the materials obtained was determined and a mineralogical and microstructural study of these materials was conducted by XRD, SEM-EDX, Si-29 MAS-NMR, and mercury porosimetry. The results show that the mechanical strength of the material is closely related to the nature, microstructure, and chemical composition of the reaction products, as well as to the curing conditions. In all cases, the main reaction product that forms is an inorganic alkaline polymer with cementing properties (N-A-S-H gel), which is largely responsible for the mechanical properties of the material. Some zeolites form as by-products, whose quantity and type (socialite, zeolite A, faujasite, etc.) depend on the nature of the activator and the curing conditions. The presence of soluble silica in the activating solution leads to the formation of gels with a larger Si/At ratio and slower zeolitisation, these both being factors with a positive effect on mechanical strength. The curing temperature exhibits a threshold value that depends on the composition of the material, beyond which an increase in curing temperature ceases to positively affect the evolution of the material's mechanical strength.