Material design for additive manufacturing (AM) applications requires appropriate rheology for mixing, pumping, extrusion, and deposition with high shape fidelity after extrusion. AM of reacting materials, particularly cementitious materials for construction applications, imposes additional kinetic constraints on processing parameters and material formulation. The colloidal physics of early-age cementitious materials play a critical role in defining final material properties, requiring a complimentary understanding of time-dependent structure and material properties. Alkali-activated aluminosilicate binders (AAB’s) are an important class of alternative cement materials (e.g., geopolymers) with the potential to reduce carbon emissions in cement construction by up to 80%. This type of binder can be produced from naturally occurring aluminosilicates and waste products including fly ash and slag. To reduce complexity in understanding the proposed AAB reaction mechanism, an aluminosilicate colloidal gel can be synthesized without the presence of larger scale, heterogeneously composed particles. When sodium is used as the alkali cation, this gel is referred to as the sodium aluminosilicate hydrate (N-A-S-H) gel. The early age properties and gelation kinetics of the model gel system offer insight into the complicated structural evolution in the complete AAB system.
The goals of this work are to (1) connect gelation kinetics of N-A-S-H gels measured via rheology to structural evolution measured via stopped-flow SAXS and (2) understand how relevant compositional changes affect the early-age gel structure. A complete characterization of material property evolution during gelation is captured via small amplitude oscillatory shear rheology. Stopped-flow SAXS (CHESS) and USAXS (ESRF) enable rapid mixing of gel precursor solutions in the beamline with ~100 ms time resolution, capturing the colloidal dynamics of the rapidly evolving gel structure at early times. Gel structural parameters including primary particle radius of gyration, fractal dimension, and aggregate cluster size are characterized over the full gel lifetime and are correlated with rheological data. The Shih scaling model is applied to both the gel volume fraction calculated from SAXS data and macroscopic rheological properties of the gel. Understanding the impact of gel composition on the structure and kinetics of material property development will allow for optimized design of sustainable AAB construction materials.
