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In this paper, based on the multiple effects of pore-filling, pore wall supporting, and low-density buffering, lightweight porous aggregate (i.e., expanded perlite-EP) is used to develop the spray-based 3D printed foam concrete with the dual performance of lightweight and high-strength for a tunnel-yielding support layer. The synergistic optimization mechanism of EP on the density and strength of printed foam concrete is systematically analyzed. The test results show that EP can effectively optimize the printability of foam concrete. It attributes that EP can greatly decrease the disturbing external forces on the foam during the spray-based 3D printing process, i.e., F c , F e , and F z , and then improve the stability of the foam. Due to the low-density buffering effect, EP enhances the pore stability and optimizes the pore structure of printed foam concrete with the total porosity of S1-EP1 increased by 142.0 % to that of S1-EP0. Combined with the synergistic effect of density reduction and strength enhancement by EP, the spray-based 3D printed optimal lightweight and high-strength foam concrete (S2-EP1) achieves the maximum compressive strength of 16.9 MPa at 28 d with a density of 956 kg/m3. The S-3D printed S2-EP1 foam concrete layer increases the compressive strength of C30 concrete by 36.36 % under stress concentration loading conditions, which further demonstrates the effect of pressure-yielding support for tunnel lining structure. [Display omitted] • EP can optimize the S-3D printability of foam concrete. • EP improves the pore stability of foam concrete during the S-3D printing process. • Optimal printed foam concrete achieves 16.9 MPa compressive strength with 956 kg/m3. • S-3D printed foam concrete can work as a tunnel-yielding support layer. [ABSTRACT FROM AUTHOR]

Highlights • An optimal mixture for spray-based 3D printing has been selected. • Dimensional accuracy of spray-based 3D printing was improved. • Effect of rheological properties on material distribution was discussed. • Material design criteria for spray-based 3D printing were proposed. Abstract 3D printing is a novel construction method, which utilizes sequential deposition of printable material to build structures. It contributes to the automation in civil engineering and offers advantages of design, greenness and efficiency. Similarities between conventional spray technology and 3D printing indicate the feasibility of spray-based 3D printing, which could enhance the automation in vertical and overhead construction. However, low dimensional accuracy of sprayed profiles with conventional materials greatly affects the quality of spray-based 3D printing. This study contributes to the development of cementitious material to improve the dimensional accuracy of spray-based 3D printing. In this study, fly ash cenosphere and air entraining agent were introduced to obtain the optimal mixture design considering the delivery and deposition requirements. Subsequent spray tests show that the optimal mixture has the smallest splash width and most uniform material distribution among all the designed mixtures. Analysis of deposition process reveals that the distribution of sprayed material is closely related with its rheological properties, which could guide the future research work on spray-based printing of cementitious materials. [ABSTRACT FROM AUTHOR]

3D concrete printing technology greatly facilitates automation in construction which enhances efficiency, productivity and sustainability. This study develops a slag-based mixture as a cementless material for sustainable spray-based 3D printing. Effects of MgO and fly ash cenosphere (FAC) addition on setting, hydration and rheological properties of fresh mixtures are investigated to obtain the optimal mixture. Results show that inclusion of MgO effectively reduces initial setting time of the fresh mixtures. With 40 wt% of GGBS replaced by MgO, initial setting time is greatly reduced from 305 min to 67 min (78% reduction). Fourier-Transformed Infrared (FTIR) spectra suggest the acceleration is plausibly due to the physical aspects. Furthermore, the FTIR spectra show that MgO can effectively activate the slag and also improve water retention. Rheological tests reveal that FAC addition generally reduces dynamic yield stress and plastic viscosity while increases static yield stress of the fresh mixtures, resulting in lower pumping pressures and higher critical ratios. The mixture with 20 wt%/40 wt% FAC addition has 29%/31% lower pumping pressure and 78%/68% higher critical ratio compared with plain MgO-activated slag material, respectively. Hence, the material with tailored rheology leads to better delivery and deposition performance of the mixture and overall spray-printing quality. An optimal mixture was finally selected based on setting, hydration, rheological properties and spray performance. The developed cementless mixture was successfully applied in the vertical spray-based 3D printing of filament and profile, which confirmed its feasibility in engineering applications. Image 102134 • A cementless mixture has been designed with MgO-activated slag. • Fly ash cenosphere was introduced for rheological tailoring. • Setting and hydration behaviors of MgO/slag-based mixtures were analyzed. • Feasibility of developed mixture for spray-based printing was confirmed. [ABSTRACT FROM AUTHOR]