Structural Evolution during geopolymerization from an early age to consolidated material

Time-resolved rheology, small angle X-ray scattering (SAXS), and electron paramagnetic resonance (EPR) techniques were used to study the polymerization of geopolymers. These polymers are inorganically synthesized by the alkaline activation of an aluminosilicate source (metakaolin) in aqueous solution. The influence of the alkali activator (Na(+), K(+), and Cs(+)) was investigated at room temperature. As observed through the variation of the viscoelastic moduli (G', G"), curing proceeds in steps that are well pronounced when NaOH is used. These steps correspond to a specific dissolution/polycondensation mechanism and are smoothed when the size of the alkali cations increases. This size effect also has an impact on the gelation time (maximum of tan δ). Structural analysis through SAXS experiments allows us to characterize these mechanisms on the nanoscale and to show that the growth of the geopolymer is due to the aggregation of oligomers with a size that is even smaller than the cation is chaotropic. Finally, water behavior during geopolymerization was assessed by using a spin probe. The results show that the spin-probe signal progressively disappears during the first moment of the reaction and reappears when the solid polymeric gel is formed, highlighting the role of water molecules in the different chemical reactions during the process. The EPR signal is in fact increasingly masked as the ion size decreases (because of the strength of the hydration shell). At the end of the reaction, some water molecules were released within the pores, restoring the visibility of the isotropic spin-probe signal.