Evidence of a Fractal Percolating Network During Geopolymerization

This study focuses on the transition from the paste state to the solid state which occurs during the hardening of metakaolin-based Na-geopolymers. The occurrence of primary interactions and the structural properties were investigated using classical oscillatory rheology (OR) methods. A time-frequency-resolved rheology (TF2R) method was used in order to obtain more detailed information about the percolation and aggregation mechanisms. The results obtained show the occurrence of the following process: (i) Elastic behavior predominates initially over the viscous behavior at low frequency. (ii) A percolating process then takes place when the viscoelastic parameters become parallel over more than two frequency decades. At the gel point, a mass-fractal dimension of approximately 2 was determined. (iii) During the formation of the porous network, the viscous behavior predominates over the elastic one due to the occurrence of polycondensation reactions. (iv) Lastly, a solid state is reached, where the elastic modulus shows a plateau and the viscous modulus decreases. These macroscopic mechanical results are compared using the small angle x-ray scattering technique. Scattering experiments were in complete agreement with the rheological measurements and showed the presence of aggregated oligomers and a mass fractal dimension equal to 2.1 at the gel point, increasing up to a surface fractal dimension reflecting the formation of the mesoporous network.