Multiscale study on the mechanical properties of 3D printed mortar using iron tailings

Massive accumulation of iron ore tailings (IOT) threatens environmental sustainability, while 3D printed mortar (3DPM) offers new opportunities for green construction. This study systematically evaluates the multiscale effects of replacing natural sand (NS) with IOT at substitution rates of 0%, 20%, 30%, 40%, and 50% on the mechanical properties of 3D printed mortar. The compressive strength anisotropy was analysed macroscopically at 3 and 28 days; mesoscopically, CT was employed to quantify the pore structure; microscopically, SEM and XRD were utilised to characterise the interfacial transition zone (ITZ) and hydration products. The 20% replacement ratio (T20) yields the best performance: its 28-day compressive strengths in X, Y, and Z directions reach 40.82, 38.59, and 39.83 MPa, 1.3–7.2% above the cast control. The anisotropy coefficient drops from 21.08% (T0) to 7.56% (T20), approaching isotropy. CT shows the T20 printed group has the lowest total porosity, with small pores (0.1–1 mm) accounting for 56.77% and moderate compactness. Microscopically, IOT undergo secondary hydration, consuming calcium hydroxide and generating more C-S-H gel, while fine particles densify the ITZ. Excessive replacement (≥40%) conversely increases porosity and anisotropy, weakening strength. Thus, 20% IOT optimally balances waste utilisation, mechanical isotropy, and microstructural densification.