Alkali-activated binders linking chemistry, structure and performance

Full text: Alkali-activated binders are finding increasing utilization worldwide as a versatile, low-CO₂ construction material in many applications. These materials have been studied in the laboratory, and deployed in practice, through user confidence which has been built on an ever-increasing fundamental knowledge base. Here, we describe key recent advances in the understanding of the chemistry and materials science of alkali-activated binders. In particular, the application of high-resolution solid state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy to these materials, in conjunction with advances in structural and thermodynamic modelling, has provided essential new insights into the fundamental nature of alkali-activated binder systems. The decomposition of the complex spectral line shapes observed in both ²⁹Si and ²⁷Al MAS NMR spectra of alkali-activated binder systems -particularly those based on blast furnace slag - can be accurately conducted via consideration of the structural constraints of the gel structure, and the presence of residual unreacted precursor grains. Peaks in the spectra of the quadrupolar ²⁷Al nucleus must also be quantified through description as asymmetric features, defined according to the quadrupolar coupling constant and isotropic chemical shift of each site identified. A thermodynamic model for the chemistry of (alkali,aluminium)-rich calcium silicate hydrate gels, making use of the new insight developed through these techniques as well as validation against solubility data, provides a rigorous description of the phase assemblages in alkali-activated slag binders as a function of reaction degree and mix composition. This information can then be used to give confidence that the phase assemblages formed through an alkali-activation reaction are going to be stable (chemically and dimensionally) through a very extended service period, and provides guidance regarding binder design for optimal chemical and physical durability. (author)