Effect of Ultrafine Slag Powder on the Rheological Behaviour, Compressive Strength, and Microstructure of Ultra-High Performance Concrete

Abstract

The effect of the addition of ultrafine slag powder (USL) on the rheology, strength, and porosity of Ultra-High Performance Concrete (UHPC) was investigated, whose microstructure was analysed by XRD, NMR, XPS, and SEM. The results show that whether the USL was incorporated or not, the paste keeps its original shear thickening behaviour. However, the yield stress first increased and then decreased with the addition of the USL content. The change in the compressive strength of the UHPC kept close to that of the yield stress. Incorporating 20 % USL, the UHPC reached a maximum compressive strength of 147.8 MPa, which was increased by 12.2 % compared to the reference sample, and its porosity was 6.72 %, which was decreased by 14% compared to the reference sample. In addition to the formation of more C-S-H gels, the pozzolanic reaction of USL increased the chain length of C-S-H and the binding energy of Ca2p, Si2p, O1s, and Al2p. Furthermore, it reduced the Al/Si ratio and Ca2P-Si2P value, making the paste microstructure more compact.