Performance and microstructure of ultra-high-performance concrete (UHPC) with silica fume replaced by inert mineral powders

Ultra-high-performance concrete possesses extraordinary strength and durability. However, the high preparation cost of UHPC limits its large-scale engineering application. Recently, some studies have been done on the incorporation of granite powder (GP) in UHPC. It is reported that the granite powder possesses low pozzolanic activity and could replace part of quartz powder (QP) under autoclave curing. However, the silica fume (SF) would preferentially consume the Ca(OH)2 generated from the hydration to some extent, where the inert powder like granite powder or quartz powder could hardly participate in the pozzolanic activity due to low Ca(OH)2 content. In this research, the properties of UHPC incorporated by the silica fume and the inert powders like granite and quartz powder were studied, which are SF20, GP20A, GP20B, QP20A and QP20B respectively. Then the hydration products and microstructure of them were analyzed by the X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), Energy Dispersive Spectrometer (EDS), 29Si nuclear magnetic resonance (NMR), and Mercury intrusion porosimetry (MIP). Results demonstrated that the replacement of inert powders to SF greatly lowers the rheological parameters like shear stress and apparent viscosity, and improvs the flowability. And the sample SF20 possesses the highest mechanical strength due to its dense pore structure, of which the compact tobermorite was found in the products but few Ca(OH)2 was found. Besides, the compressive strengths of samples QP20A and QP20B were close to SF20 because their similar hydration products, pore size distribution and mean chain lengths (MCL). Due to the lower reaction activity of GP-A, the sample GP20A possessed the lowest strength but the highest pore diameter and porosity. While the grounded granite powder showed the higher reaction activity, resulting in the formation of tobermorite and a finer pore size distribution and the higher strength in GP20B. Hence, it can be concluded that the pozzolanic activity of inert powders, the quartz powder and the granite powder, could be greatly improved by autoclave curing and reducing the particle size. This study provides the theoretical basis for replacement for silica fume with inert powder and the possibility to reduce the preparation cost of UHPC and to enlarge its application.