Successful design of cementitious materials for emerging applications like additive manufacturing requires control of rheological properties throughout the lifetime of the material. During processing, a material may experience a wide range of shear histories depending on the type of mixing, pumping, or extrusion operations required. The reacting nature of cementitious materials imposes additional kinetic constraints, defined in part by the initial formulation and environmental conditions like temperature and humidity. Alkali-activated binders (AABs), including geopolymers, are studied for their low carbon footprint and use of naturally occurring aluminosilicate materials or industrial aluminosilicates like fly ash and slag. Valuable formulation guidance for successful AAB processing is achieved by studying model materials, and geopolymers formed from metakaolin are often studied due to their purity and high aluminosilicate content.
The goal of this work is to understand the effects of applied shear on both the rheological property development and final material properties of a model metakaolin geopolymer. A complete characterization of rheological property evolution for the geopolymer is captured via small-amplitude oscillatory shear (SAOS) rheology in addition to the novel application of optimally windowed chirp (OWCh) rheology and steady-shear rheology. The geopolymer is subjected to mechanistically designed shear protocols, varying the magnitude and duration of shear to mimic industrially relevant processing steps. The duration of applied shear relative to the rheological gel point of the material determines the rate of increase in the elastic modulus and network structure of the reacting binder. However, the critical gel time of the geopolymer remains constant regardless of the applied shear protocol. The long-term compressive strength of geopolymers subjected to varying shear protocols is also measured. The novel findings in this work provide a mechanistic understanding of how processing parameters impact the rheological properties of geopolymer binders, facilitating the design of optimized processing routines for emerging applications like additive manufacturing.
