Sustainable 3D mortar printing: material optimization and ultrasonic characterization for enhanced construction performance

This study investigates the optimization of 3D mortar printing mixtures and processes to enhance sustainability and efficiency in construction. Utilizing a novel 3D mortar printer equipped with a 14-bar pumping system and interchangeable nozzles of 1.5, 2, and 3 cm diameters, the research incorporates sustainable materials such as fly ash, silica fume, and polypropylene fibers. These materials are selected to improve the mechanical properties of the mortar while reducing the environmental impact of construction practices. The study analyzes the effects of key parameters, including binder type (Fast-Setting Cement (FSC) vs. Ordinary Portland Cement (OPC)), water-to-powder (W/P) ratio, and nozzle diameter, on the fresh properties and mechanical performance of printed elements. The research evaluates the printability, rheological properties, and hardened properties, such as flow table tests and flexural strengths, of various mixtures. Early-age performance is monitored by both traditional mechanical tests and advanced ultrasonic techniques. Flexural strength tests at 4h, 6h, and 24h are complemented by ultrasonic pulse velocity (UPV) measurements, hybrid ultrasonic indices (Sideband Peak Count Index (SPC-I), and Spectral Dissipation Index (SDI)). The optimized FSC mixtures (W/P ratios 0.34 and 0.37) exhibited excellent workability (flow 142–150%) and rapid strength gain, reaching flexural strengths of 3.06 and 1.83 MPa at 4h and about 4.3 MPa at 24h. Ultrasonic monitoring revealed that UPV values increased to 3.1 km/s by 24h, while hybrid ultrasonic indices peaked at 6h (SPC-I 1.1–1.6 and SDI 1.1–1.4 relative to the initial values) before dropping by more than 50% at 24h, correlating with transient microstructural adjustments observed via scanning electron microscopy (SEM). By establishing correlations between printing parameters, mix designs, and material properties, the study aims to develop a nonproprietary optimized 3D mortar (printing) mixtures and processes. The results demonstrate that carefully selected mix designs and printing parameters can significantly enhance the sustainability, efficiency, and performance of 3D concrete printing (3DCP), potentially leading to its wider adoption in the construction industry. Ultrasonic evaluation provided novel insights into early-age material behavior, strengthening the understanding of printability versus buildability. Specifically, this work is one of the first to integrate hybrid ultrasonic indices into 3DCP monitoring, a novel approach that reveals early microstructural evolution beyond standard tests. This comprehensive approach offers valuable guidance for improving construction practices, reducing environmental impact, and increasing the viability of additive manufacturing in the built environment. This research contributes to the advancement of 3D mortar printing technology, promoting sustainable and efficient construction practices.