The micro-facial characteristics and mechanical behavior of metakaolin and steel fibers modified concrete at high fluidity

The pumped concrete, which being used in driven cast-in-place piles, demand lower cement usage, higher fluidity, preferable mechanical properties at pre-cracking stage and higher capacity of deformation resistance at the post-cracking stage. To develop the above properties, the metakaolin (MK) modified steel fiber (SF) reinforced concrete at water/binder = 0.6 were used in this case, and its microstructure and mechanical behavior after 7 and 28 days curing were investigated. The compression and splitting tests, Scanning Electron Microscope (SEM), X-ray Energy Dispersive Spectroscopy (EDS), X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM) tests were performed. The results showed that the coupling effects of SF and MK on the fluidness reduction were evident. Without incorporation of SF, the compressive strengths, the tensile strengths, the deformation modulus E50 and elastic modulus E were improved by 140–162%, 171–178%, around 150% and 130–140% respectively by MK. At post-cracking stage, SF and MK combination promoted residual strengths and prevented the further development and connection of the crack. The portlandite (CH) were massively consumed by MK, and more stable Hydrated Calcium Silicate (CSH) gel and ettringite phase matters were produced to fill the porosity and defects. A large amount of hydration products in the form of acicular and fibrous particles enhanced the bonding strength on interfacial transition zone of MK concrete. The steel fibers were significantly stretched during pulling out with MK incorporation. Based on AFM tests, the average roughness Ra and mean square roughness Rq of microstructural faces both decreased with curing age and MK contents. The conclusions would be used to proportioning design of MK-SF concrete in an economic way, and understand the mechanisms of micro cracking between geopolymer and inert matters.