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While the 3D printing technology for cementitious composites has developed rapidly, a combination of 3DP technology and lightweight engineered cementitious composites (LWECCs) could improve many aspects of the construction industry. In this study, a fibre-reinforced high-performance LWECC for extrusion-based printing was proposed. First, six LWECCs were prepared, incorporating two kinds of hollow glass microspheres (HGMs) in varying replacement ratios of fly ash (FA) at 60 wt%, 80 wt%, and 100 wt%. In addition, polyvinyl alcohol (PVA) fibre was introduced given its shrinkage resistance and improvement in printability performance. Thereafter, fresh property (slump loss and setting time), unconfined compression strength (UCS), and flexural strength experiments thoroughly investigated the optimised LWECC design, which was later calibrated for the printing procedure via a printability assessment, including extrudability and buildability. The UCS, flexural strength, and densities of the printed and cast specimens were compared. Lastly, a microstructural investigation using a scanning electron microscope described the reinforcement mechanism of PVA fibre upon the performance of the printed structures and HGMs. The addition of HGMs significantly improve the lightweight property that reaches a value at 1384 kg/m3 but inevitably negates mechanical properties. The printed LWECC obtains 33.6 MPa for UCS and 9.29 MPa for flexural strength. When the printed filament was perpendicular to loading direction, superior toughness was observed, creating a 63% and 40% increase for UCS and flexural strength, respectively. [ABSTRACT FROM AUTHOR]

• EMW-absorbing composite containing 6 wt% copper powder and 25 wt% copper slag. • EMW superstructures using 3D-printed oriented copper fiber upon microwave reflectivity. • Equivalent waveguide attenuator model and mechanism is proposed. • The copper fiber-reinforced element through crossed printing is the optimized configuration. • The superstructure creates a −20.43 dB reflectivity peak with an 11.79 GHz bandwidth. Electromagnetic wave (EMW) pollution negatively impacts information, equipment security, and the human body. However, neither powder phase absorbent nor ferrite fiber using traditional casting methodology cannot form electromagnetic superstructure to flexibly enhance EMW absorbing capacity. Thereby, ferrite fiber absorbent in well-oriented manner by 3D printing can be a feasible solution in EMW absorption due to its structural EMW anisotropy property. In this study, the influence of EMW superstructures using 3D-printed oriented copper fiber upon microwave reflectivity is investigated, and an equivalent waveguide attenuator model is proposed. First, an EMW-absorbing composite containing 6 wt% copper powder and 25 wt% copper slag is prepared. In copper slag cementitious composite, copper fiber (CF) and steel fiber (SF) are incorporated at 0.5 wt% separately. In each group, EMW-absorbing elements are manufactured through casting, laminar parallel printing, and cross printing. The superstructure anisotropy and EMW absorption performance are investigated through a network analyzer. The cross superstructure is outstanding in low-frequency absorption, while the parallel superstructure demonstrates desirable reflectivity at a high-frequency range. The copper fiber-reinforced element produced through crossed printing is determined as the optimized configuration, reaching a −20.43 dB reflectivity peak with an 11.79 GHz bandwidth. Lastly, an equivalent waveguide attenuator model is proposed to explain the superposition enhancement of the dielectric property, space impedance matching, and multiple scattering. [ABSTRACT FROM AUTHOR]

A multitude of antimony tailings (AT) solid wastes haphazardly accumulated in the open environment resulted in significant land resource occupation and serious ecological environment damage. AT as the fine aggregate additive in concrete is an effective approach for achieving solid waste resource utilization. Currently, 3D-printed AT concrete offers notable benefits in design flexibility, precision, material efficiency, productivity, and sustainability compared to conventional concrete. However, traditional AT-concrete is plagued by drawbacks including uneven strength distribution, protracted construction periods, and considerable resource consumption. Furthermore, the low reactivity and non-degradability of AT necessitate implementing modified activation techniques in AT concrete. This paper employs four modification methods (physical grinding, alkali excitation, hydrothermal activation, and combined activation) to activate AT. The modified AT is incorporated into concrete at five ratios (0 %,10 %, 20 %, 30 %, and 40 %) and integrated into a 3D printing system, manufacturing a total of 450 samples for mechanical property experiments. The result demonstrates that the combined modification method emerges as the optimal choice for enhancing the unconfined compressive strength (65.7 MPa) and flexural strength (12.9 MPa) in 3D printing. The SEM results revealed that the combined modification, alkali excitation, and hydrothermal activation led to AT depolymerization and sediment adsorption. Abundant C-S-H gel and a handful of Ca(OH) 2 crystals were observed on the physically modified AT (30 μm) surface, indicating sufficient pozzolanic reaction on 3D printed fine AT. • The 3D-printed method generally illustrates excellent impacts in strength enhancement. • AT physical modification achieves a substantial 20 % increase in mechanical properties. • Alkali-excitation modification notably increased the strength of low-content AT mortars by 35 %. • Hydrothermal modification boosts mix properties by 28% within a replacement ratio of 0 % to 30 %. [ABSTRACT FROM AUTHOR]

• The industrial by-products, solid wastes, carbon materials are discussed. • The steel fiber and ferrite exhibit satisfied EMW absorbing mechanical resistance. • The resonance materials improve EMW absorbing capacity with inferior strength. • The multi-phase and multi-layer composites and macrostructure are prospective. • The combination of 3D printing and EMW absorbing brings advantages. The ever increasing electromagnetic energy and multiple reflections of electromagnetic wave (EMW) impair human health and information security. The EMW absorbing cementitious material paves an innovative way to improve EMW absorbing performance of concrete structures. This study will elaborate on the EMW absorbing mechanism including energy exhaustion and impedance matching. A comprehensive review on the raw absorbent cementitious materials and the relevant preparation methods to be integrated into EMW absorbing concrete. Two new theoretical models are proposed including optimization zone model and equivalent electromagnetic parameter model to improve the current transform line model. Subsequently, the measurement and evaluation methodologies for EMW absorbing performance of cementitious materials are summarized. The expected research topics and applications of EMW absorbing cementitious materials are suggested. Through this critical review on the research status of EMW absorbing concrete, suggestions for optimizing the performance and workability of cementitious material are made to promote large-scale 3D printing technology. [ABSTRACT FROM AUTHOR]

• Four nano-sized graded absorbents were explored. • The 5 wt% coupling agent and 10 wt% dispersant was validated for graded absorbent. • The double-layer structure was made using spraying and extruding methodology. • The EMW mechanism for the graded double-layer structure was firstly summarized. • Theoretical calculation optimized a 15 mm absorbing layer and 15 mm impedance layer. The ever-increasing electromagnetic wave (EMW) energy and its reflections may impair human health and information security. An ordinary EMW-absorbing structure usually exhibits limited EMW-reflecting capacity given its ungraded structure in both the micro and macro scales. In this study, four nano-sized graded absorbents are explored by wrapping carbonyl-iron powder with nano-sized silicon dioxide. The micro-scale wrapping effects are assessed via a scanning electron microscopy experiment to identify the best sample, including a 5 wt% coupling agent and a 10 wt% dispersant in CIP ethanol. The 10 mm thick concrete matching layer containing the graded absorbent is built using spraying methodology, and the 15 mm thick absorption layer is fabricated using extruding methodology. The mercury intrusion porosimetry experiments show that the sprayed matching layer incurs maximum porosity at 19.8% while the 3D extruded layer ensures the minimum porosity at 11.13%, strengthening the laminar grading effect to improve the EMW impedance matching. The Naval Research Laboratory experiments corroborate that double-layer structures improve the EMW-reflecting capacity compared to their single-absorption-layer counterparts and that the double-layer element containing the best wrapping effect absorbent yields the best reflection loss value, with a minimum of −14.7 dB and a 9.72 GHz effective bandwidth. The EMW-reflecting mechanism for the novel graded double-layer structure containing the graded nano-sized absorbent is then summarized. Theoretical calculations using the transform line theory suggest that a 15 mm absorbing layer and a 15 mm impedance matching layer is specified as the most optimized configuration, with −10.47 dB in average reflection, −15.18 dB in maximum RL, and 9.56 GHz in effective bandwidth. [ABSTRACT FROM AUTHOR]

Electromagnetic wave (EMW) pollution impairs the performance of equipment, facilities, information security, and human health. Ordinary, plain cementitious composite has limited EMW-reflecting capacity. To correct this, a macrostructure EMW absorbing element has been developed to reduce EMW pollution. As conventional casting of macrostructures is not cost-effective and lacks quality because of the demand for special formwork, 3D printing technology offers greater design flexibility, higher efficiency, and improved mechanical properties. Moreover, the surface texture of 3D-printed material exhibits natural corrugation to enhance EMW reflection further. In this study, novel wave-shaped EMW absorbing elements were fabricated using 3D cementitious printing technology. The EMW absorbing performance was analyzed by the naval research laboratory (NRL) system in the range of 1 GHz–18 GHz. Significant improvement of reflectivity was observed, especially in the low-frequency range. The optimum configuration was identified through effectiveness evaluation to be a K 3 sample with 45 mm of heave height and 2/3 tan(θ), exhibiting a minimum reflectivity −43.7 dB, mean reflectivity −25 dB, and an absorbing bandwidth of 18 GHz. The experimental reflectivity was also simulated by the finite element method, which agrees with the experimental results. The synthetic vector electromagnetic field distribution was also studied to explore space impedance characteristics. Moreover, the finite element method attempted to correct drawbacks in experiments, including multiple direction reflection measurement and wave-number interruption. Based on simulation calibration, the influence of wave-shaped parameters (heave height and tangent value) upon EMW absorbing capacity was induced accurately. [ABSTRACT FROM AUTHOR]

In the manufacturing process of 3D Concrete Printing (3DCP), defects and anomalies have a significant impact on both the success rate and the quality of the final products, underscoring the need for real-time monitoring. Currently, monitoring is primarily based on manual observation and existing automated methods are limited in real-time performance and accuracy. This study introduced a real-time and highly accurate defect detection and measurement system for using deep learning (DL) and computer vision (CV) techniques. A range of improvement methods were applied in YOLOv7, showing better capacities of accuracy and speed for detecting defects in 3DCP than current cutting-edge detectors such as YOLOv8. Notably, the virtual high-fidelity data were produced by DL based data augmentation strategy and their effects were assessed. Replacing real data as the training dataset, the generated virtual data were used in the models to improve measurement accuracy. Applying the proposed method, the comprehensive insights into 3DCP defects were obtained. Consequently, the relationship formula between defect frequency and printer parameters was investigated by the proposed method, guiding operators in effectively controlling printer parameters and preventing breakpoint defects during the printing process. • An innovative model based on dual deep learning technologies. • A range of improved methods used in the detector. • High-precision detection and measurement of 3DCP anomalies. • The relationship formula between defect frequency and printer parameters. [ABSTRACT FROM AUTHOR]

• A novel fibre-jacket-reinforced mould is developed for 3D printing application. • Effect of hybrid fibre on tensile and flexural properties is investigated. • A properly designed printing path can improve the strength of 3D-printed mortar. • A model is developed to predict the bending capacities of the reinforced specimens. Recently, 3D printing technology has been rapidly developing in the construction sector around the world. However, the insufficient tensile and flexural strengths of the printed material is a crucial problem due to the difficulty of applying steel reinforcement. As a printable material, fibres can be added to the mixture to improve the post-cracking behaviour and toughness of reinforced mortar. Therefore, the present study proposes a 3D-printed fibre-reinforced mortar frame with basalt and carbon fibres to develop a fibre-jacket-reinforced specimen. Specimens with different numbers of reinforcing layers (two, four and six layers) and four different printing paths were fabricated to investigate the strengthening effect on both flexural and tensile capacities. Results from both tests show that a properly designed printing path can significantly improve the tensile and flexural strengths of 3D-printed mortar due to the fibre alignment effect. The strength of fibre-jacket-reinforced specimens is enhanced to a different extent compared with the strength of cast specimens without fibre (with maximum flexural strengths of up to 167% and splitting tensile strengths of up to 128% compared with the respective strengths of the control groups). Meanwhile, both strengths of the fibre-jacket-reinforced specimens with six reinforcement layers almost reach the same strengths of a fully printed fibre-reinforced specimen, but the amount of fibre is reduced by half. In addition, an electromechanical impedance (EMI) technique is applied to monitor the damage during the loading process and an analytical model is developed to predict the enhanced bending capacities of the proposed fibre-jacket-reinforced specimens. [ABSTRACT FROM AUTHOR]