Geopolymers are inorganic, typically ceramic, materials that form long-range, covalently bonded, non-crystalline (amorphous) networks. Ideally, they are synthesized by mixing a source of aluminosilicates with a highly caustic alkaline and silica-rich solution. This unique composite exhibits many appealing properties such as: early strength development, low carbon dioxide footprint, high strength-to-weight ratio, high fracture toughness, high flexural strength, and high compressive strength. As of now, the influence of microstructure and heterogeneity on the constitutive behavior of geopolymer binders reinforced with aggregates—particulates or fibers, is not yet fully understood. Therefore, mechanistic models are needed to fully understand the impact of a heterogeneous and multiscale microstructure on the stress-strain response for geopolymer composites. Previous studies showed that several phenomenological approaches have been developed to correlate the mechanical response to the chemistry of sodium-based metakaolin and fly-ash geopolymer binders either based on linear regression or neural network models. Moreover, it is evident from literature that finite element approaches have also been formulated at the macroscopic scale to investigate the response of geopolymer concrete reinforced with steel rebars. Nonetheless, there is still a missing link between the mechanical response of geopolymer composites and their chemistry and mix design.
On this account, Professor Ange Therese Akono from the Northwestern University in collaboration with Professor Seid Koric and Professor Waltraud M. Kriven at the University of Illinois at Urbana-Champaign proposed a study whose main objective was to formulate a rigorous physics-based and multiscale micromechanics-based theory that could connect the elasto-plastic response of geopolymer composites to the heterogeneity and to the pore structure. In other words, they formulated a nonlinear physics-based mechanistic model that described the constitutive response of geopolymer composites and bridged the molecular and macroscopic length-scales. Their work is currently published in the research journal, Cement and Concrete Composites.
In their work, the research team employed a physics-based energetic approach to explore the influence of the nanogranular and particulate nature of geopolymer composites on the elasto-plastic constants. To this end, they formulated a multiscale model for geopolymer composites spanning 12 length-scales, from the molecular level to the macroscopic length-scale to capture the influence of chemistry and structure on the mechanical response. Next, they described the evolution of the elastic and strength properties across multiple length scales by application of micromechanics theory. They then validated the theoretical model on unreinforced potassium geopolymer as well as particulate geopolymer composites. Finally, the model was applied to investigate the influence of porosity, processing, and chemistry on the inelastic behavior of geopolymer composites.
The researchers found out that the strength behavior of geopolymer composites is significantly influenced by local contacts between geopolymer nanoparticles at the nanoscale. They also realized that air voids, depending on their size, affect the mechanical response differently. For instance, nanoscale airvoids are dictated by the chemistry of the geopolymer precursor and promote strength development through local rearrangement. In contrast, micron-scale air voids are a product of the mixing procedure and they play a detrimental role as they act as stress concentrators, thereby initiating premature failure. Meanwhile particulate and fiber reinforcement enhance the strength response in different manners. Fibers provide a bridging effect whereas microparticles local dissipate mechanical energy through particle-to-particle contacts as well as matrix-to-particle friction.
“We provide the missing link between strength response of metakaolin-based geopolymer composites and pore structure. We found that the effective strength is controlled by local contacts between geopolymer nanoparticles at the nanoscale and, at the microscale, by the presence of micropores resulting from the mixing procedure. Our methodology is important to inform the design of high-performance lightweight smart materials with enhanced stiffness and strength.” said Professor Ange T. Akono to Advances in Engineering.
In summary, the study shed light on the influence of heterogeneity and porosity on the constitutive behavior of geopolymer composites. Remarkably, the presented theoretical approach enabled the researchers to rank processing methods according to their effectiveness. An important takeaway is that four major factors contribute to the strength of geopolymer composites: the chemistry of the geopolymer precursor at the molecular scale, the nature and the shape of the reinforcement (usually at the microscale), and the mixing procedure. In a statement to Advances in Engineering, Professor Ange T. Akono further mentioned that their research work earmarked an important step in the mechanistic modeling of the behavior of geopolymer composites so as to advance the science and technology of geopolymer composites.
Ange Therese Akono, Seid Koric, Waltraud M. Kriven. Influence of pore structure on the strength behavior of particle- and fiber reinforced metakaolin-based geopolymer composites. Cement and Concrete Composites volume 104 (2019) 103361.