Mechanisms Governing Early-Age Structural Evolution in Alkali-Activated Low-Grade Calcined Clays

Abstract

Expanding alkali-activated clays beyond high-purity metakaolin requires the use of low-grade calcined clays (LCCs), whose variable mineralogy and reactivity will influence fresh-state evolution, setting behavior, and strength development. This work investigates the mechanisms governing early-age structural evolution of three alkali-activated LCCs and one high-purity metakaolin (MK) by linking chemical transformations to rheological development. All systems exhibited a consistent four-stage viscoelastic evolution in small amplitude oscillatory shear (SAOS) tests, indicating a common structural pathway despite differences in precursor composition. The transition at the end of stage III corresponded to the formation of a solid product and aligned closely with measured set times, providing a reliable rheological indicator of setting. Deconvolution of the time-resolved FTIR scans identified the lower-wavenumber, LW band, (900-950 cm^-1) as the most sensitive marker of precursor-specific chemical evolution. Coupling this LW band with SAOS results produced characteristic chemical-to-rheological evolution curves that distinguished the precursors by reaction rate and degree of Si-O-Si/Al peak shift. These trajectories showed the differences in spectral evolution preceding rapid stiffening, and how impurity content and reactive fraction may influence each system as it transitions into a connected network. Total heat from isothermal calorimetry provided a strong comparative indicator for performance of the precursors. This integrated mechanistic framework provides a practical route for interpreting variable LCC feedstocks and designing alkali-activated clay binders with controlled workability, setting, and strength.