Feasibility evaluation of coal mine overburden for one-part alkali-activated 3D printable concrete

The integration of 3D printing technology with one-part alkali-activated concrete provides a promising pathway toward sustainable construction. This study investigates the feasibility of utilizing coal mine overburden (CMO), a mining by-product, as a replacement for natural fine aggregate in one-part alkali-activated 3D printable concrete. The research was conducted in three stages: material characterization, mix design optimization, and performance evaluation of printed elements. Coal mine overburden was characterized through physical, chemical, mineralogical, and morphological analyses, confirming its suitability as a sustainable alternative to natural sand. A one-part alkali-activated binder comprising fly ash, ground granulated blast furnace slag, and sodium metasilicate was developed. Mix optimization was performed by varying the superplasticizer dosage and water-to-binder ratio (0.43–0.70), while maintaining a constant binder-to-filler ratio of 1:2 and a precursor-to-activator ratio of 90:10. Fresh-state performance was evaluated through extrudability, open time, buildability, and early-age stability. The optimized polypropylene fiber-reinforced mix exhibited smooth and continuous extrusion, an effective open time of 46 min, buildability of 10 layers without collapse, and a green strength of 32.74 kPa at 60 min, demonstrating excellent printability. The optimum mix achieved a compressive strength of 38.68 MPa, while the printed specimens exhibited compressive strengths of 29.04 MPa parallel to the print layers and 3.63 MPa perpendicular to the print layers, highlighting the influence of printing-induced anisotropy and interlayer bonding. The findings establish coal mine overburden as a viable circular resource for one-part alkali-activated 3D printable concrete and demonstrate the potential of one-part alkali-activated systems for sustainable, structurally reliable, and scalable additive manufacturing in construction.