Micromechanical multiscale model for alkali activation of fly ash and metakaolin

The process of alkali activation of fly ash and metakaolin is examined in the view of micromechanics. Elasticity is predicted via semi-analytical homogenization methods, using a combination of intrinsic elastic properties obtained from nanoindentation, evolving volume fractions and percolation theory. A new quantitative model for volume fraction is formulated, distinguishing the evolution of unreacted aluminosilicate material, solid gel particles of N-A-S-H gel, and open porosity, which is partially filled with the activator. The stiffening of N-A-S-H gel is modeled by increasing the fraction of solid gel particles. Their packing density and intrinsic elasticity differ in N-A-S-H gels synthesized from both activated materials. Percolation theory helps to address the quasi-solid transition at early ages and explains a long setting time and the beneficial effect of thermal curing. The low ability of N-A-S-H gel to bind water chemically explains the high porosity of Ca-deficient activated materials. Micromechanical analysis matches well the elastic experimental data during the activation and elucidates important stages in the formation of the microstructure.