Utilization of iron powder as a partial replacement for silica sand in heavy-weight UHPC

This study investigates the feasibility of utilizing iron powder (IP) as a partial to full replacement for silica sand in Ultra-High Performance Concrete (UHPC), examining its effects on density, mechanical strength, durability, and microstructure. Six mixtures were prepared with IP replacement levels ranging from 0% to 100% by volume of fine aggregate. The results reveal a non-monotonic relationship between IP content and performance. Density increased consistently with IP content, reaching 3,006 kg/m³ at 100% replacement—a 23.7% increase over the control. However, mechanical testing established that a 50% replacement level served as the critical optimum, delivering peak compressive (121 MPa), splitting tensile (7.5 MPa), and flexural (10.1 MPa) strengths, representing enhancements of 14%, 19%, and 25% over the control, respectively. Scanning electron microscopy confirmed that at 50% IP, the particles acted as micro-fillers and nucleation sites for C-S-H gel, densifying the interfacial transition zone and producing a homogeneous matrix. Conversely, exceeding 50% IP induced severe degradation; at 100% replacement, compressive and tensile strengths dropped by 13% and 31% below the control, while water absorption rose dramatically from 1.9% to 4.3%, indicating significant capillary porosity. This deterioration is attributed to disrupted packing density and insufficient cement paste to coat the excessive metallic particles. It is concluded that 50% iron powder replacement offers an optimal balance, producing high-density, high-strength UHPC suitable for radiation-shielding and heavyweight applications.