Effects of retarders on the rheological properties of coal fly ash/superfine iron tailings-based 3D printing geopolymer: Insight into the early retarding mechanism

To increase the utilization rate of superfine iron tailings (SIT), a novel coal fly ash (CFA)/SIT-based three-dimensional printing geopolymers (CS-3DPG) were synthesized for high-value resource application of SIT. The optimal preparation parameters of CS-3DPG were sodium citrate of 0.4%, water-solid ratio of 38%, SiO 2 /Na 2 O molar ratio of 1 and alkali activator of 6%, with the compressive strength of 32.14 MPa. Besides, the yield stress and plastic viscosity (rheological properties) of CS-3DPG reached 284 Pa and 18.8 Pa·S, respectively. The sodium aluminum silicate hydrate (N-A-S-H) gels were identified as dominant hydration products by the selective chemical extractions. Microstructural analysis also successfully indicated that the retarder caused some CFA and SIT to be unreacted, resulting in the decrease of early compressive strength compared with CS-3DPG. Based on the full width at half-maximum (FWHM) values, kinetics of hydration and hydration kinetic contribution degree results, retarders effectually inhibited nucleation and growth (NG) and diffusion (D). The phase-boundary interaction (I) reaction played a major role after adding retarders. This work successfully reveals the early retarding mechanism of CS-3DPG, and provides the possibility of large-scale resource application of SIT-based 3DPG in 3D printing construction.