Heterogeneous dynamics in aging phosphate-based geopolymer

Abstract

The time-evolution of dynamics as well as microstructure and mechanical response of phosphate-based geopolymers was probed using x-ray photon correlation spectroscopy and rheological tests. The analyzed relaxation processes in the freshly prepared geopolymer mixes evidenced a q-independent mode of the autocorrelation function, ascribed to density fluctuations of the already established molecular network, undergoing reconfiguration without significant mass transport. Upon curing, the detected motions are localized and depict a system evolving toward structural arrest dominated by slower hyperdiffusive dynamics, characterized by a compressed exponential regime, pointing to a structural relaxation process subjected to internal stresses, in a context of marked dynamical and structural heterogeneity. The system ages through a “densification” process producing declining small angle scattered intensity, as two finely intermixed gel-like reaction products, namely, one hydrated aluminophosphate and one hydrated silica, form a percolated network possessing surface fractal scaling of progressively shorter average correlation length. In this scenario, the nominal Al/P molar ratio of the mix, being an index of network-forming ability, is positively correlated with the dynamic viscosity and the overall kinetics, whereas the contrary occurs for the fraction of water.