Heterogeneous dynamics in aging phosphate-based geopolymer

In phosphate-based geopolymers the dissolution of the aluminosilicate source in acidic environment is thought to lead to distinct Al−O−P and Si−O−Si amorphous products; however, information about the reaction mechanisms and structural details of the material is still incomplete. The impact of concentration and amount of acid phosphate solution on the reaction mechanisms has been gained by following the time-evolution of the dynamics, microstructure and mechanical response using synchrotron X-ray photon correlation spectroscopy in small-angle scattering mode and by rheological tests. The results pointed to networks with surface fractal dimensions forming very early with the accumulation of products near and at the surface of the aluminosilicate source, as observed in Mg-phosphate cements and alkaline geopolymers. The system ages by entanglement and branching of small Si polymeric units and concomitantly, growth of components of the aluminophosphate hydrogel network by gathering together in solution PO4 3- and Al3+ ions, in a ‘densification’ process characterized by progressively more localized dynamics, as the two intermixed gel-like reaction products form a percolated network possessing surface fractal scaling of progressively shorter average correlation length. The nominal Al/P molar ratio of the mix, as an index of the network-forming ability, is positively correlated with the dynamic viscosity and the overall kinetics, whereas the contrary occurs for the fraction of water.