Enhancement of mechanical performance in metakaolin-based geopolymer by cyclic loading and creep process in relation with microstructural evolution

Geopolymers are more and more widely used in engineering applications as low carbon construction materials. Their mechanical properties under monotonic loading have been widely investigated. However, there is still a crucial need to investigate their mechanical responses under cyclic loading and during creep deformation. In this study, a new series of triaxial compression tests are performed, including both monotonic and cyclic as well creep loading paths on a metakaolin-based geopolymer paste (MKGP). Deformation behavior, shrinkage process, failure strength and pattern are investigated. Various values of confining stress are considered. The experimental results indicate that both cyclic loading and creep process induce a clear enhancement of failure strength compared with monotonic loading mainly due to accumulated plastic hardening. In particular, the enhancement of cohesion coefficient is significant while the change of internal friction angle is minor. The tested MKGP samples undergo continuous volumetric compaction throughout loading path without transition to dilatancy, which is further emphasized by cyclic and creep loading. At the microscopic scale, for all loading paths considered, the pore volume and size progressively decrease while the effective contact area increases, leading to enhanced compactness and mechanical performance. Three-dimensional computed tomography reconstructions further confirm that increasing confining stress effectively suppresses damage development and reduces crack volume. Shrinkage measurements demonstrate that both autogenous and drying shrinkage increase with curing age at a decreasing rate, with most shrinkage occurring at early ages.