Uncovering and Optimizing the Chemical Mechanisms in Alkali-activated Materials and Related Engineering Systems

AbstractWith the world facing a climate crisis due to increasing CO2 emissions, there is pressing need to develop and implement sustainable construction/engineering materials across the globe. Alkali-activated materials (AAMs) are one such sustainable alternative to conventional ordinary Portland cement (OPC) concrete; however, questions remain regarding the long-term performance of AAMs which is hampering implementation of this sustainable solution in the construction industry. Furthermore, for OPC-based concrete, the use of extensive clinker substitution to reduce CO2 emissions has led to changes to the underlying chemistry of the main binder gel, where it is uncertain how these novel supplementary cementitious materials augment the long-term properties (e.g., gel stability and pore structure) of the cement paste.

Here, I will outline how fundamental materials research is addressing the long-term performance unknowns of AAMs and certain OPC-based systems, where we are linking key experimental techniques with atomistic and larger length scale simulations. To assess gel/binder stability in Ca-rich AAMs, we have used density functional theory (DFT) and synchrotron-based X-ray pair distribution function (PDF) analysis to investigate the influence of alkali substitution on the structure and thermodynamics of calcium-alumino-silicate-hydrate (C-A-S-H) gel. The results provide important new information on the impact of alkali substitution in this gel system and insight on the structural arrangements present in sodium-substituted C-A-S-H gels (C-(N)-A-S-H). Moreover, we have (i) uncovered a mechanism that mitigates microcracking in alkali-activated slags (i.e., a C-(N)-A-S-H gel system) using nanoparticles and (ii) determined how ZnO slows down the hydration reaction in AAMs and OPC-based systems.

The pore structure of concrete strongly impacts long-term durability, where the pore network and permeability of the gel phase(s) are known to control the rate of degradation. However, the mechanisms controlling the development of the gel/capillary pore network during formation of the binder gel, especially in AAMs, are largely unknown. Here, I will outline our recent progress in the development and implementation of a mesoscale modeling methodology to simulate the formation of the binder gels and pore network in AAMs, along with key experimental data (e.g., small-angle neutron scattering and quasi-elastic neutron scattering) capable of tracking the evolution of the pore network and associated water dynamics immediately after mixing. Given that AAMs can be synthesized using various precursors and activator chemistries, this research, based on a novel computational approach, will enable the prediction of long-term degradation of new cement chemistries in the future.

BioClaire White is an Assistant Professor at Princeton University in the Department of Civil and Environmental Engineering and the Andlinger Center for Energy and the Environment with associated faculty status in the Department of Chemical and Biological Engineering, the Department of Mechanical and Aerospace Engineering, the Princeton Institute for the Science and Technology of Materials, and the Princeton Institute for Computational Science and Engineering. Professor White completed her graduate studies in 2010 at the University of Melbourne supported by an Australian Postgraduate Award from the Australian government. After receiving her PhD, she worked as a postdoc at Los Alamos National Laboratory and was awarded a Director's Postdoctoral Fellowship to research the atomic structure of low-CO2 alkali-activated materials. Her research focuses on understanding and optimizing engineering and environmental materials, with an emphasis on controlling the chemical mechanisms responsible for formation and long-term degradation of low-CO2 cements. This research spans multiple length and time scales, utilizing advanced synchrotron and neutron-based experimental techniques, and simulation methodologies. Professor White is the recipient of a number of awards including an NSF CAREER Award and the Howard B. Wentz Jr. Junior Faculty Award (Princeton University), and has been listed several times on the Princeton Engineering Commendation List for Outstanding Teaching.