Advancing Low-Magnetic Concrete: Microstructural and Magnetic Analysis of Raw Materials for Residual Magnetization Control

The escalating demand for low-magnetic building solutions in sensitive experimental facilities and high-precision instrumentation sectors underscores the critical limitations of traditional concrete materials. Traditional reinforced concrete structures primarily introduce residual magnetic interference via two main avenues: ferromagnetic reinforcement materials and iron-containing oxides inherent in cement-based binders. While the former can be substituted with non-ferrous alternatives, a comprehensive characterization of the types of iron oxides and the mechanism of residual magnetization in the latter remains elusive. This study pioneers a multidisciplinary research approach, integrating advanced microstructural and magnetic analysis techniques to establish a quantitative correlation between the composition of concrete raw materials and their magnetic properties. The saturation magnetization, crystal structure, iron valence state, and magnetic hysteresis behavior of samples from various cements, ground granulated blast furnace slag (GGBS), and fly ash were thoroughly evaluated. The results reveal that fly ash raw materials exhibited the highest iron content (3.43%) and saturation magnetization (0.71 emu/g), while GGBS raw materials showed the lowest iron content (0.22% Fe) and saturation magnetization (0.16 emu/g). Furthermore, magnetic separation effectively reduce residual magnetic fields, with GGBS samples achieving a low residual magnetic environment (remanence as low as 19.7 nT) after magnetization at 30 mT. Microstructural and magnetic analysis of separated magnetic substances indicate that magnetite (Fe₃O₄) and a small amount of maghemite (α-Fe₂O₃) are the primary contributors to magnetic behavior. Notably, the judicious selection of low-iron content concrete raw materials and/or the application of magnetic separation technology can substantially diminish the residual magnetization characteristics of these materials, offering a promising avenue for the formulation of low-magnetic concrete. This study elucidates the source and removal methodologies for the magnetic properties of concrete raw materials, laying a solid foundation for developing low-magnetic building materials tailored for magnetic control environments.