Multi-scale study on synergistic effect of cement replacement by metakaolin and typical supplementary cementitious materials on properties of ultra-high performance concrete

The synergistic effect of cement substituted by metakaolin (MK) and several typical supplementary cementitious materials (SCMs), including fly ash (FA), granulated blast furnace slag (BS), glass powder (GP), and steel slag powder (SP), on the behaviors of ultra-high performance concrete (UHPC) was investigated in multiple scales. Replacement of cement by 10% MK increased the compressive and flexural strengths, the elastic modulus, the resistance to water and chloride permeability, and decreased the drying shrinkage of UHPC. Compared with other SCMs, an additional 10% replacement by BS showed the highest mechanical properties and resistance to permeation at later ages while not compromising the dimensional stability and the early age strengths. Results from low-field nuclear magnetic resonance and X-ray diffraction first demonstrated that BS helped to promote the hydration of cement in initial stage. Meanwhile, as indicated by mercury intrusion porosimetry, nanoindentation, and scanning electron microscopy with back scattered electron imaging, the additional incorporation of BS resulted in a pronounced reduction of porosity, an increased amount of high density C-S-H gel, and a refinement of steel fiber/paste interfacial transition zone, owing to the medium particle size varying between cement and MK, and the high pozzolanic activity of BS. In short, the synergistic effect of cement replaced by MK coupled with BS is reflected by a combination of the micro-aggregate filling effect, the hydration promotion effect, and the pozzolanic effect and is highly beneficial to the improvement of the properties of UHPC.