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Three-dimensional concrete printing (3DCP) is emerging as an innovative technology and shows promise to revolutionize conventional construction modes. However, the current 3D-printed concrete (3DPC) generally requires higher cement content than conventional concrete to ensure its rheology for printing. From the perspective of cleaner production and reduce carbon emissions, this study explored the feasibility of replacing parts of cement with waste glass powder (WGP, 0%, 20%, 40%, and 60% by mass) and compared the properties of the developed 3DPC, including fluidity (flowable spread), rheology, heat of hydration, buildability, compressive strength, anisotropy, and drying shrinkage. The results showed that less than 40% WGP replacement had limited influence on the initial fluidity and static yield stress, as well as drying shrinkage, of 3DPC. Although the WGP inclusion decreased the compressive strength, it slowed down the fluidity loss and static yield stress increase, which could extend the workable time of the mixture for printing and improve buildability. The 40% WGP replacement was found increase to the buildability of the printing mixture from 150 mm to 155 mm. The printing mixture prepared with 60% WGP reduced the dying shrinkage by 50%. An exponential decay function between the fluidity and static yield stress was established so that the simple fluidity test could be used as an indicator of printability. The findings in this study provided a solution to reduce the consumption of cement in 3DPC, which could contribute to a greener production in the construction industry. [ABSTRACT FROM AUTHOR]

Highlights • An electrochemical specimen of slag-cement-mortar-lined ductile iron (SCMLDI) pipe was designed. • Flowing pure water, NH 4 NO 3 and NH 4 Cl solutions were used in corrosion experiment. • Electrochemical property of SCMLDI pipe was evaluated by TP and EIS. • Microstructure evolution at the interfacial zone and lining of specimen was characterized by SEM/EDS, XRD and XPS. Abstract In this study, a series of experiments were performed to electrochemically and microscopically investigate the corrosion behavior of slag-cement-mortar-lined ductile iron (SCMLDI) pipes in flowing solutions. Firstly, some ductile iron (DI) pipe specimens lined with slag-cement mortar (SCM) were prepared. Secondly, the accelerated corrosion behavior of specimens in flowing pure water, 6M solutions of NH 4 NO 3 and NH 4 Cl were studied by electrochemical measurement, such as Tafel polarization (TP) and EIS. Then, their microscopic morphology, phase composition and chemical valence state of the SCM lining and DI surface were characterized by SEM/EDS, XRD and XPS. Results show that, during the 120-day immersion, the specimen in pure water has basically no leaching of SCM lining and corrosion of DI surface, but in 6M NH 4 NO 3 solution, there produces an obvious leaching of SCM lining and no corrosion of DI surface. However, there produce a severe leaching of SCM lining and a high corrosion of DI surface of the specimen in NH 4 Cl solution. [ABSTRACT FROM AUTHOR]

• 100% content of RFA is proved to be applicable in 3DMP from the perspective of rheology. • The combined use of RFA and sodium gluconate contribute to the continuous printing. • A dimensionless parameter is defined to describe the buildability with higher accuracy. This study explores the feasibility of utilizing recycled fine aggregates (RFA) in 3D mortar printing (3DMP) by evaluating the properties of 3D printing (3DP) mixtures with different contents of RFA (0%, 50%, and 100%) as well as sodium gluconate (0%, 0.6%, and 1.2%) before and after high-rate shearing from the perspective of rheology, failure pattern and buildability. The results showed that, the incorporation of RFA highly increased the 3DP mixture's initial value and time-varying rate of the static yield stress, viscosity and area of thixotropy hoop, while adding sodium gluconate decreased them respectively. Meanwhile, since high-rate shearing were found to highly decrease the mentioned rheological parameters of 3DP mixtures, a feeding system which can provide high-rate and continuous shearing was recommended in practical printing to prolong the open time of printing mixtures. Besides, a dimensionless buildability parameter λ was proposed and defined as the ratio of accumulated offsets (whether compression or tension) to the theoretical height to describe the buildability of 3DP mixtures, which exhibited much higher accuracy than the maximum printing layer, maximum printing height and average vertical strain, respectively. [ABSTRACT FROM AUTHOR]

The huge labour-consumption, construction and demolition (C&D) waste pollution, and shortage of river sand have become increasingly serious problems facing the construction industry. Therefore, the possibility of fully replacing natural fine aggregates (NFA) with recycled fine aggregates (RFA) for fiber-reinforced 3D mortar printing (3DMP) was carefully evaluated by Digital Image Correlation (DIC) technique, mechanical testing, and microscopic analysis in this study. The results show that the 100% replacement of RFA had very limited impact on the failure pattern of the 3D printed (3DP) specimen, while the addition of 1% polyethylene (PE) fibers would change the failure pattern of specimens from brittle to ductile. This study also defines the parameter of anisotropic degree to reflect the anisotropy. It is found that the anisotropic degree of 3DP specimens was different under various loading conditions and obviously affected by both RFA and fibers. The microscopic analysis indicated that the effect of RFA on the 3DP specimens was mainly due to the porous structure and the existence of initial micro-cracks, while the effect of fibers was mainly due to the interfacial bond between PE fibers and matrix. This study also found that after reinforcing with appropriate fibers, the 3DP mortar mixed with 100% RFA has higher mechanical properties and better deformation ability than the fiber-free mortar with 100% NFA. It is believed that the 100% replacement of RFA is applicable in 3DMP, which will bring significant benefits to the cleaner production and sustainable development of the construction industry. [ABSTRACT FROM AUTHOR]

Currently, 3D concrete printing attracts worldwide attention due to its digital-construction advantages, meanwhile, the problems of pollution caused by construction and demolition waste as well as the shortage of river sand become more and more serious. Based on it, this research tries to verify the printability of 3D printing mortar with 100% recycled sand by assessing its fluidity, extrudability, printability window, and green strength. Result shows that, when additional and suitable sodium gluconate is added into the printing mortar with 100% recycled sand, this mortar can not only meet the printability requirements of 3D printing, but also obtain a longer printability window and a higher green strength than the mortar with 100% natural sand, which is beneficial for the practical application of 3D concrete printing. [ABSTRACT FROM AUTHOR]

Green recycled hybrid powder (RHP) was proved to partly replace cement to prepare 3D printable mortar, which can recycle the demolition waste and reduce carbon emissions. In this study, different contents of RHP and polyethylene fiber (PE fiber) were selected to prepare 3D printable RHP mortar (3DPRM). The initial rheological parameters, including dynamic yield stress, plastic viscosity, static yield stress and thixotropy, as well as the rheological parameters varying with time were investigated. Besides, the early-age hydration of RHP paste was studied to reveal the influence mechanism on rheological properties. The experimental results show that the dynamic yield stress, plastic viscosity and thixotropy as well as their time-varying properties were all increased with RHP replacement. The static yield stress of 3DPRM with 30 % RHP had the most significant enhancement, which was 416.5 % increased after resting for 20 min, showing the great buildability enhancement potential of RHP. The addition of PE fiber (6 mm and 12 mm) increased the rheological parameters of 3DPRM, among which the increase ratio of initial dynamic yield stress was 60 %-195 %. However, the increase ratio with time for dynamic/static yield stress of 3DPRM with PE fiber was lower than that without PE fiber, indicating that PE fiber had great effect on the extrudability but small effect on buildability of 3DPRM. The energy intensity, carbon emission and cost of RHP were 65.27 %, 66.57 % and 12.5 % lower than that of cement, illustrating the sustainable contribution of RHP for construction industry. The results can be used to guide the printability design of 3D printable mortar containing RHP and PE fiber. • Rheology of 3DPRM with various amounts of RHP and PE fiber was investigated. • RHP enhanced the rheological properties and their time varied properties of 3DPRM. • RHP had the greatest enhancement effect on the static yield stress of 3DPRM. • The carbon emission and cost of RHP were 66.6 % and 12.5 % lower than cement. • The initial dynamic yield stress was 60–195 % increased as 6/12 mm PE fibers used. [ABSTRACT FROM AUTHOR]

• 3DPM with RP had the highest compressive strength under X loading direction. • Splitting tensile and flexural strengths of 3DPM-RP were higher than cast samples. • The addition of RP increased the anisotropy coefficient of 3DPM. • The porosity of 3DPM-RP20 increased 40% (from X-CT) considering the interlayers. • RP had limited effect on pore structure of 3D printed filament without interlayer. This study investigated the hardened mechanical properties of 3D printed mortar (3DPM) with different contents (0–30% by mass of cement) of recycled powder (RP) by anisotropic compressive, splitting tensile and flexural strengths. The pore characteristics tested by X-ray computed tomography and mercury intrusion porosimeter as well as hydration products of 3DPM were analyzed to reveal the influence mechanism of RP on 3DPM. The results showed that the highest compressive strength of the 3D printed specimens was obtained on the loading direction paralleled with the length of printing filament (X direction). When the RP replacement ratio was 20%, the X-direction compressive strength of 3DPM was 24.11% lower than that of cast samples, while the splitting tensile strengths of 3DPM were closed to and the flexural strengths of 3DPM were higher than that of cast samples regardless of the loading directions. This can be attributed to the high fluidity loss rate of 3DPM with RP, leading to the reduction of the surface moisture of interlayer. Besides, more pores at the interlayer and interstrips led to the decrease of mechanical properties. In contrast, the addition of RP had a limited effect on the micro pore structure of the single printed filament without considering the interlayer and interstrip, owing to the secondary hydration and filling effect of RP. [ABSTRACT FROM AUTHOR]

• Drying shrinkage of cast mortar and 3D printed mortar (3DPM) is compared. • Mechanism behind the shrinkage behavior of 3DPM with different printing paths is explored. • Effects of recycled fines on the flow loss and drying shrinkage of 3DPM are focused on. 3D printing process poses great impacts on the properties of mortar, especially on the shrinkage behavior of 3D printed mortar (3DPM) prepared with recycled fine aggregate (RFA) and recycled powder (RP). This study focuses on the effects of printing paths on the flowability and the drying shrinkage of 3DPM. RFA and RP, together with their contents, are used as variables in 3DPM. The results show that the flow loss of mortar during the first 30 mins increased with the incorporation of recycled fines due to the high water-absorption of RFA and the low hydration reactivity of RP. The drying shrinkage development of 3DPM was higher than that of cast mortar during the first 28 days, but exhibited a deceleration in shrinkage increasing rate after 28 days. Printing path showed a limited effect on the mass loss of 3DPM during the test period, but had significant impacts on the drying shrinkage and the shrinkage development trend. An increase in drying shrinkage of 3DPM was caused by 50 ∼ 100% RFA replacement (29.1 ∼ 57.9% in 60 days), while 10 ∼ 20% RP content decreased the drying shrinkage of 3DPM (3.3 ∼ 12.4% in 60 days). Additionally, the incorporation of recycled fines delayed the development of 3DPM drying shrinkage. [ABSTRACT FROM AUTHOR]