Study On Mechanical Pronerties And Microscopic Mechanisms Of The Metakaolin-based Geopolymer Concrete

Geopolymer concrete is a kind of new concrete materials.Because of its low energy consumption and carbon dioxide emissions.it has attracted worldwide attention of researchers.The binder phase in geopolymer concrete(geopolymer)and that in ordinary concrete(Portland cement)have essential differences in chemical composition and structures,which also cause inherent differences in mechanical properties between the two concrete types.There are plenty of researches that contributed to the understanding of mechanical properties of geopolymers and geopolymer concretes.Based on these previous researches,this thesis studied several mechanical properties,which have important impact on engineering applications.The object of studies was focused on metakaolin-based geopolymer(MKG)and MKG concrete.It was expected that these studies would reveal some underlying micro-mechanisms of the chosen mechanical properties and guide the application of geopolymer concrete.First,aiming at the creep properties of MKG,the nanoindentation creep test was conducted in this paper.The elastic modulus,hardness,creep properties of MKG were studied and the effect of Si/AI ratio on these properties was also discussed.Combined with the characterization of microstructure,it is found that the mechanical properties of the gel phase in MKG have obvious size effect with the characteristic pore size.The influences of the Si/Al ratio on these properties is majorly due to this size effect.Based on micromechanical analysis,a shell-core model was proposed to explain the size effect.Secondly,in view of the drying shrinkage of MKG,the water loss-shrinkage curves of MKG with different water-solid ratio were measured by drying experiments.It is found that there is a two-staged relation between the drying shrinkage and water loss.The pore structure plays a controlling role in the drying shrinkage behavior of MKG.Based on the characterization of microstructure,a physical model which considering capillary pressure,surface energy change and gel consolidation was established and the shrinkage behavior of MKG was quantitatively estimated.Moreover,the key role of microstructure in controlling the shrinkage of geopolymers was emphasized.Thirdly,aiming at the bond behavior between MKG concrete and steel bar,the influence of bond size and loading rate on the bond behavior between MKG concrete and steel bar was studied by static and dynamic pull-out tests.It is found that there is a higher cohesive force between geopolymer concrete and steel bar than ordinary concrete.The required cover thickness and bond length of steel bars in MKG concrete are smaller than those in ordinary concrete when the same load is transmitted.At the same time,the effect of loading rate on the bond performance of MKG concrete-steel bar is higher than that of ordinary concrete.Through an analytical mechanical model,the decisive influence of the interface between geopolymer and reinforcement on the dynamic sensitivity of bond strength was revealed.Finally,the mechanical-electrical coupling problem in the process of MKG piezoresistive functionalization was discussed.The design method of introducing conductive polymer into natural cracks in geopolymer to realize piezoresistive functionalization was put forward in this work.The feasibility of this design method was proved by piezoresistive test.Combining with the characterization of crack surface morphology,a coupled mechanical-electrical contact model for analyzing piezoresistive behaveor of materials was also presented,which lays a foundation for further optimization of this material.