Influence of pore structure on the strength behavior of particle- and fiber-reinforced metakaolin-based geopolymer composites

Geopolymers are X-ray amorphous materials with appealing properties such as high flexural strength and high compressive strength. Yet, the influence of the heterogeneity and porosity on the constitutive behavior is not fully understood. We formulate a multiscale physics-based mechanistic model to describe the strength behavior of geopolymer composites. Using an energy-based approach, we derive novel solutions to describe the effective yield criterion of fiber-reinforced and particle-reinforced metakaolin geopolymers. We calibrate our theoretical model using nanoindentation tests and validate our theoretical framework via flexural strength tests on metakaolin-based geopolymer composites. Geopolymer composites are found to exhibit a pressure-dependent granular behavior. We subdivide the porosity into nanoporosity and microporosity. Our results indicate that the nanoporosity is solely influenced by the chemistry and is not influenced by the processing and the presence of reinforcement. Due tothe presence of nanoporosity, the strength-total porosity relationships are not unique. However, an approximate one-to-one correspondence exists between the strength and the microporosity. The nanogranular structure and the chemical composition at the nanometer scale have a profound influence on the effective mechanical response.Our conceptual framework is an important step in the mechanistic modeling of the behavior of geopolymer composites.