Rheokinetic modeling of N-A-S–H gel formation related to alkali-activated aluminosilicate materials.

Alkali-activated aluminosilicate (N-A-S–H) gels are models for understanding structure formation in sustainable cement alternatives known as geopolymers. Temporal measurements of elastic shear moduli development of these aluminosilicate gels show systematic trends in kinetics with respect to silicon to aluminum (Si/Al) ratio. This behavior can be reduced to a single master curve when nondimensionalized with the Si/Al-dependent gel time and equilibrium modulus. An autocatalytic reaction model and Avrami model fit the master curve well. The Avrami exponent is 1.5, consistent with the nucleation and growth steps separately modeled in the autocatalytic reaction model, and further is within the expected values for diffusion-limited growth suggested from independent measurements of the structural fractal dimension by neutron scattering. These results unifying rheokinetic behavior over a range of gel compositions suggest a common underlying microstructure development in such gels and the possibility of developing predictive models for property development in geopolymers more broadly. An autocatalytic reaction kinetic model describing nucleation and growth is fit to the evolution of the storage modulus, G', of alkali-activated aluminosilicate gels with varying concentration of aluminum ( S i / A l / N a = 2 / n Al / 1.5 ). A master curve is constructed by nondimensionalizing the timescale and storage modulus of each dataset. [ABSTRACT FROM AUTHOR]