Effect of Spherical Electric Arc Slag on Solid Waste-Based 3D-Printed Concrete

Three-dimensional-printed concrete (3DPC) is an additive manufacturing technology that forms 3D solids via layer-by-layer printing based on 3D model data, but it consumes large amounts of river sand (RS) and has poor frost resistance. To address these issues, this study used industrial waste electric arc furnace slag (EAFS) as an aggregate at 0–100% replacement ratios to test the workability, mechanical properties, frost resistance, and microstructures of 3DPC specimens. The results show that EAFS improves mortar flowability and extends the printing window, but full replacement increases slump and reduces constructability. The stress dispersion and dense packing effects of EAFS ensure excellent mechanical properties of specimens before and after freeze–thaw cycles. At an 80% EAFS replacement ratio, compressive and flexural strengths increase by 2.52%/13.8% and 10.6%/18.2%, respectively; after freeze–thaw cycles, the specimens exhibit the best frost resistance. The interfacial transition zone between EAFS and cement matrix is only 2 μm, with 1.8% lower porosity and 20.14% fewer harmful pores than the 100% RS specimen after freeze–thaw cycles. In conclusion, 80% EAFS replacement balances 3DPC performance and solid waste utilization, providing important references for EAFS’s safe application in 3DPC and its performance improvement mechanism.