Inorganic polymer binders (IPBs) are attractive as low-CO2 alternatives to conventional cementitious structural materials and consist of networked (alumino)silica particles which partially dissolve and crosslink under alkaline conditions. For example, colloidal alumina and silica particles in aqueous alkali slurries (e.g., 6 M NaOH) partially dissolve over reaction times of days to weeks. Solution phase species condense and reprecipitate, forming aluminosilica networks at and between particle surfaces that yield macroscopic strength which evolves over time. Tracking the evolution of such aluminosilica networks and the corresponding development of interparticle adhesion and macroscopic material strength has been challenging due to the heterogeneous surface compositions and small (~nm) particle-particle distances under realistic reaction conditions. Recently developed solid-state NMR techniques and advances in the surfaces forces apparatus (SFA) enable detection and correlation of atomic bonding and hydration environments in aluminosilica networks formed at different reaction times (hours to weeks) as well as the nanoscopic interparticle adhesion energies and strength properties. Specifically, ex-situ solid-state dynamic nuclear polarization (DNP) enhanced NMR techniques yield drastic improvements in NMR signal intensity from particle surfaces. The signal enhancements provided by DNP-NMR enable acquisition of two-dimensional (2D) heteronuclear DNP-NMR correlation spectra that establish the types, nanoscale proximities, and interactions of 1H, 23Na, 27Al, and 29Si species in the aluminosilica networks formed at different reaction times. Complementary in-situ surfaces forces measurements between alumina and silica surfaces under analogous reaction conditions track the surface morphologies, interparticle layer thicknesses, and adhesion forces and energies of aluminosilica networks. The combined DNP-NMR and SFA analyses yield new insights into the development of strength in IPBs, with implications for improved control of the mechanical properties of low-CO2 cements.

Understanding network formation and strength development in alkali-activated aluminosilica cements

255th National Meeting (2018)

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