Partial substitution of silica fume with fine glass powder in UHPC: Filling the micro gap

The design of ultra-high-performance concrete (UHPC) depends basically on the packing density and particle-size distribution (PSD) of its ingredients. The PSD of cement exhibits a gap at the micro scale that needs to be filled with finer materials such as silica fume (SF). Filling this gap solely with SF requires a high amount of SF (25% to 30% by wt. of cement) due to its extreme fineness. The use of high SF volume negatively effects concrete rheology. In addition, the limited resources and high cost of SF impede the wide use of UHPC in concrete market. This paper reports on a study to determine the possibility of producing and using fine glass powder (FGP) as a SF partial replacement in UHPC. The results show that FGP with a mean particle size (d50) of 3.8 µm could be recommended as an optimal PSD to fill the gap between cement and SF particles. The results demonstrate that compressive strength values of 235 and 220 MPa under 2 days of steam curing can be achieved, respectively, when replacing 30% and 50% of SF with FGP, compared to 204 MPa for the reference UHPC containing 100% SF. The rheology of fresh UHPC is improved by replacing SF particles with nonabsorptive glass particles.