High-Volume Cement Replacement with Oil Shale Ash and Metakaolin in Pre-Blended Compositions for 3D Printing

This study presents the development and comprehensive characterisation of a sustainable 3D-printable cementitious composition in which up to 40 wt.% of Portland cement was replaced by a ternary binder containing oil shale ash (OSA) and metakaolin (MK). Following laboratory optimisation, the developed formulations were successfully transferred to industrial production as pre-blended dry mixes at Sakret Latvia Ltd., demonstrating the feasibility of large-scale manufacturing of printable cementitious materials. Attention was devoted to the characterisation of the raw materials and dry mixtures using particle size distribution (PSD), scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS), and X-ray diffraction (XRD). Two compositions—a reference mixture (REF) and the ternary OSA mixture—were evaluated in terms of printability, mechanical performance, durability, and the influence of the type of sample production. The ternary composition (due to the pozzolanic activity of MK and OSA) exhibited strength development resulting in compressive strength (60.6 MPa) exceeding that of the reference mixture after 90 days of curing (55.3 MPa). Mechanical testing of compression and flexural properties indicated direction-dependent differences between printed and cast specimens. Durability assessment, including capillary water absorption and surface freeze–thaw scaling tests performed using two standardised methods, confirmed frost resistance and the suitability of both mixtures for outdoor applications. The results further indicate that the layered manufacturing process governs moisture transport and direction-dependent mechanical behaviour associated with interlayer interfaces. The developed pre-blended OSA–MK composite represents a promising low-carbon material for industrial 3D concrete printing, combining reduced cement consumption with reliable printability, mechanical performance, and durability.