3D printable magnesium oxide concrete: towards sustainable modern architecture

Journal Pre-proof 3D printable magnesium oxide concrete: towards sustainable modern architecture Abdullah Khalil (Conceptualization) (Methodology) (Investigation) (Writing - original draft)Writing – review and editing) (Project administration), Xiangyu Wang (Methodology) (Investigation) (Writing - review and editing), Kemal Celik (Writing review and editing) (Supervision) (Project administration) (Funding acquisition)

PII:

S2214-8604(19)32179-7

DOI:

https://doi.org/10.1016/j.addma.2020.101145

Reference:

ADDMA 101145

To appear in:

Additive Manufacturing

Received Date:

12 November 2019

Revised Date:

25 January 2020

Accepted Date:

20 February 2020

Please cite this article as: Khalil A, Wang X, Celik K, 3D printable magnesium oxide concrete: towards sustainable modern architecture, Additive Manufacturing (2020), doi: https://doi.org/10.1016/j.addma.2020.101145

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.

3D printable magnesium oxide concrete: towards sustainable modern architecture Abdullah Khalil, Xiangyu Wang, Kemal Celik* Division of Engineering, New York University Abu Dhabi, Abu Dhabi, P.O. Box 129188, United Arab Emirates *

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Corresponding Author. E-mail address: [email protected]

Abstract

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Reactive magnesium oxide cement (RMC) is gaining increasing attention as a sustainable

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construction material due to its significantly low carbon footprint during the production as well as the operational phase compared to the conventional Portland cement. Whereas several studies have demonstrated the potential of RMC as a suitable and environment-friendly construction material,

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this study reports that RMC can be shaped into complex structures via three-dimensional (3D) printing technology. By adding suitable additives and only 3 wt. % of caustic magnesium oxide to the commercially available RMC, appropriate rheology and buildability were achieved that

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enabled smooth 3D printing of complex structures with precise shape retention. Moreover, the 3D printed RMC exhibited higher densification and nearly twofold the compressive strength as compared to its cast counterpart. Therefore, this work demonstrates the potential of RMC as a 3D printable construction material for sustainable and modern architecture.

Keywords 1

Reactive magnesium oxide cement; caustic magnesium oxide; rheology; 3D printing; sustainability

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Graphical Abstract

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1. Introduction Three-dimensional (3D) printing technology is one of the rapidly emerging technologies that is committed to providing a sound digital framework for the modern production industry with unique

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benefits of unlimited flexibility, reduced cycle time, and labor cost along with minimal waste [1].

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Whereas 3D printing was initially invented and is now well-established for producing complex polymeric components [2], the technology has become equally feasible for producing metallic [3]

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and ceramic [4] parts with intricate geometries. In recent decades, the scope of 3D printing has

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been extended to the construction industry, where this technology has demonstrated excellent potential for generating complex architectural structures that are nearly impossible to produce via

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conventional construction methods. In addition, the key benefits of speed, repeatability and minimal labor cost-justify extensive research and investment in 3D printing technology for the

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construction industry. To date, a wide range types of cement paste and mortar have been 3D printed successfully, having size as big as several meters with excellent shape and strength retention that is required for practical needs [5].

Although the benefits of reduced cycle time and labor cost can be achieved while employing 3D

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printing for construction, the technology remains expensive in terms of material due to its inability to incorporate large aggregates that leads to greater consumption of cement [6]. Ordinary Portland cement (OPC), the most commonly used material for conventional construction as well as 3D printing, involves extensive CO2 emission during its production and does not play any positive role towards environmental protection during its entire life-cycle [7–11]. Therefore, it is vital to

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develop alternate and low-carbon construction materials to make the 3D printing technology holistically sustainable. In this context, several low-carbon construction materials have been developed that require less energy during their production and are capable of permanently sequestering environmental CO2 during their operational life [12,13]. One such material is ‘reactive magnesium oxide cement (RMC)’ that is produced at nearly half the temperature as compared to that of OPC (700 °C vs. 1400 °C) and once hydrated, it permanently absorbs

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atmospheric CO2 by forming stable hydrated magnesium carbonates (HMCs) that densify the

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microstructure and promote strength gain [14]. Several studies have demonstrated RMC as a sustainable replacement due to its impressive ability of permanent CO2 sequestration and

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subsequent strength gain that is comparable to OPC [15–20]. Recently, 3D printing of some low

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carbon and environment-friendly materials such as sulphoaluminate cement [21] and magnesium potassium phosphate cement [22] has been demonstrated. However, the 3D printing potential of

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RMC is yet to be explored, and making RMC and similar low-carbon construction materials 3D printable guarantees a sustainable future for the modern construction industry.

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In a broader sense, good ‘extrudability’, ‘flowability’, and ‘buildability’ are the three major properties that a 3D printable mortar must have [23]. Extrudability refers to the ease with which the mortar can be pushed through the pipes and nozzles, and therefore, this parameter is collectively controlled by the mix composition and the printer head depending upon its maximum

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torque capacity. Flowability, which means the degree of smoothness and continuity in the mortar flow, is primarily the function of mix composition and can be controlled by adding suitable additives such as superplasticizers, viscosity modifying agents, and defoaming agents to maintain the mix homogeneity and rheology during the printing process. Finally, buildability refers to the hardening and setting of the extruded layers at a sufficient rate that ensures shape compliance and

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overall integrity of the 3D printed structure. As evident, buildability is purely a function of mix composition, and it can, therefore, be improved by adding a small proportion of supplementary cementitious materials and reinforcements that impart sufficient yield strength to the freshly printed layers. In the case of RMC, the hydration kinetics are much faster as compared to OPC and similar cements and thus, the extrusion pressure must be high enough to dispense the mortar before its significant hardening. At the same time, suitable additives must be added to maintain good

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flowability throughout the printing process. When it comes to buildability, there are several

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supplementary cementitious materials that can be added to achieve sufficient yield strength of the freshly printed layers [24]; however, they also affect the original microstructure of the matrix and

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hence the physical and mechanical properties of the 3D printed structure. In the case of RMC, the

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primary matrix is reactive magnesium oxide (nearly 93 %), and therefore one suitable choice of supplementary material to impart good buildability is ‘caustic magnesium oxide (cMgO)’ (also

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called ‘light burned magnesia’) that has reactivity at least 10 times higher than that of RMC. Such high reactivity of cMgO is attributed to its very low crystallinity and high surface area as it is

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synthesized at a much lower calcination temperature [25,26]. Therefore, one can predict faster hydration kinetics and hence improved buildability of RMC in the presence of cMgO. In this paper, it is demonstrated that by adding only 3 wt. % of cMgO that has the same chemical composition and stoichiometry as the original matrix, along with small proportions of chemical additives, a 3D

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printable RMC mortar can be produced that possesses excellent extrudability, flowability as well as buildability, enabling prolonged 3D printing of RMC with varying geometry and complexity.

2. Experimental methods 2.1. Materials and processing

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The primary binding matrix used in this study was ‘calcined magnesite 92/200’ RMC (Richard Baker Harrison, UK) while ‘MagChem® 50’ (Martin Marietta Magnesia Specialties, USA) having a purity greater than 98 % was used as the supplementary cementitious material to improve the hydration kinetics and buildability of the RMC mortar. The mortar mixture consists of 6 wt.% of standard sand (ASTM C778), which was added to improve the workability. Table 1 summarizes the properties of these three solid components of the 3D printable RMC based mortar prepared for

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this study. To prepare the mix, 0.1 M magnesium acetate aqueous solution was used as the primary

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liquid media maintaining the water to cement weight (w/c) ratio of 0.67. Magnesium acetate acts as a strong hydration agent in RMC, and therefore its addition is recommended in small

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concentrations to achieve a higher degree of hydration in RMC [27]. Polycarboxylate ether

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(EthacrylG™, Coatex, USA) was added as a ‘superplasticizer’ to reduce the plastic viscosity of the RMC mortar and to maintain the flow consistency over a prolonged period. Hydroxyethyl-

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cellulose (Sigma-Aldrich, USA) was added as a ‘suspension-aid additive’ to maintain the mix homogeneity during extrusion. Finally, a non-ionic surfactant, also termed as ‘defoamer’,

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(Rhealis™ Dfoam, Coatex, USA), was added to avoid air entrapment in the modified mortar. To produce a homogenous 3D printable mix, the solid components were first dry mixed in a planetary ball mill (XQM-2A, Tencan, China) for 3 min to obtain a uniform powder mix while the liquid components were separately mixed in a beaker using a magnetic stirrer to obtain a homogenous

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liquid solution. This solution was then poured into the jars containing the mixed powders followed by 10 min of mixing. During each mixing, agate jars and balls were used, maintaining the speed of 350 rpm and balls to paste weight ratio of nearly 5:1. By applying this procedure, a homogenous 3D printable RMC based mortar was obtained. Table 2 summarizes the mix composition that was subsequently 3D printed.

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Table 1 Properties of RMC, cMgO and graded sand

Bulk density (g/cm3) Surface area (m2/g) Reactivity (s)

3.02 16.3 520

cMgO MgO 98.2, CaO 0.8, SiO2 0.35, Fe2O3 0.15, Al2O3 0.1, Cl 0.3, SO3 0.05, LOI 3.5 3.58 60 9

Graded sand SiO2 100 (approx.)

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RMC MgO 93.2, CaO 0.87, SiO2 2.25, Fe2O3 0.53, Al2O3 0.22, LOI 4.0

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Property Composition (%)

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1.47 0.81 Non-reactive

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Table 2 Composition for 3D printable RMC mortar

RMC cMgO

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Sand

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Component

Weight % 54 3 6 36

Superplasticizer

0.4

Defoamer

0.3

Suspension aid additive

0.3

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0.1 M magnesium acetate aqueous solution

2.2. Isothermal calorimetry The reaction kinetics of the RMC paste was studied using an isothermal calorimeter (I-Cal Ultra, Calmetrix, USA) at 25 °C. For calorimetric studies, pure RMC and RMC + 3 wt. % cMgO mixture

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were uniformly mixed with 0.1 M magnesium acetate aqueous solution in low-density polyethylene vials, maintaining the w/c ratio of 0.67 by weight. The vials containing the paste were immediately transferred into the calorimeter, and the rate of energy release and the cumulative energy release during the hydration were monitored for 10 hr. 2.3. Rheology Rheological properties of the freshly prepared 3D printable RMC mortar were measured using a

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rotational rheometer (RheolabQC, Anton Paar GmbH, Austria) equipped with a 4-vane measuring

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system having a diameter of 22 mm and length of 16 mm. A glass beaker with an internal diameter of 50 mm was used as a container for rheological measurements that was gripped externally via

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fixture to avoid any rotational displacement of the beaker during the measurement. Before starting

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the experiment, the vane was carefully immersed, such that the vane axis was almost aligned with the beaker axis while maintaining nearly 15 mm of mortar spacing above and below the vane. To

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estimate the yield strength and plastic viscosity of RMC mortar, the shear rate was increased logarithmically from 0.1 s-1 to 100 s-1, whereas to determine the time-dependent behavior, a

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constant shear rate of 2.5 s-1 was applied, and the corresponding shear stress was recorded as a function of time.

2.4. 3D printing

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3D printing of RMC mortar was carried out using a gantry type 3D printer (Hydra16A, Hyrel 3D, USA) equipped with a syringe dispensing system head, which is designed to hold a 150 ml syringe. The G-code understandable by the printer was generated through Slic3r software by importing the STL format of the target object while setting a layer height, nozzle diameter, and a printing speed of 1.5 mm, 2 mm, and 4 mm/s, respectively. The infill percentage and perimeter values were set

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to 0 and 1, respectively, to print only the outer walls of the target object. This resulted in the material extrusion rate that corresponds to the shear rate of nearly 2.5 s -1 during the printing process. Once the G-code was generated, the 3D printable RMC mortar was immediately transferred to a 150 ml syringe and mounted to a 3D printer head for subsequent printing while the printer bed was covered with an aluminum foil on which the structures were printed. The dimensions of the 3D printed structures varied from 50 to 150 mm resulting in a typical printing

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duration of about 30 to 60 min, during which the RMC mortar maintained its flow consistency and

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workability. Once the 3D printing cycle was over, the structures were left undisturbed on the printer bed for about 2 to 3 hr after which they became strong enough for manual handling. The

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structures were then removed from the printer and stored under ambient conditions (temperature

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= 22 – 23 °C, relative humidity = 56 – 59%) for 3 days, during which they were frequently sprayed with 0.1M aqueous solution of magnesium acetate to achieve maximum hydration of RMC.

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Finally, to densify the microstructure and promote strength gain, the 3D printed RMC structures were carbonated for 7 days in an environmental chamber under the CO2 concentration of 20%

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while the temperature and relative humidity were maintained at 30 °C and 80%, respectively.

2.5. Compressive strength

For estimating the compressive strength of 3D printed RMC mortar, cylindrical samples with an

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average diameter and height of 25 mm were 3D printed with the same printing settings as that of complex geometries but with an infill percentage of 50. To compare the strength of the cast and 3D printed samples with similar geometry, cylindrical samples having the same dimensions were also cast from the 3D printable mortar using the plastic molds. Prior to the strength test, the cast and 3D printed samples were cured in a similar fashion, as stated in the previous section. In the

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case of 3D printed cylinders, the cross-sectional area was estimated by averaging the diameter (measured through Vernier calipers within 0.01 mm accuracy) across the cylinder length and multiplying it by a factor of 0.7 as at least 30 percent of the area in each layer was empty due to the selected values of infill percentage and layer height during the printing process. The empty areas were measured by importing several cross-sectional images into an image analysis software where the area of randomly shaped vacant regions was calculated and subtracted from the diameter

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based cross-sectional area of the 3D printed cylindrical samples. The compression tests were

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performed using a 500 kN hydraulic system (Matest, Italy), ensuring at least 3 repetitions for both

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the cast and 3D printed samples.

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2.6. Morphology and microstructure

The porosity and the actual volume of the samples obtained after 3 days of ambient curing and 7

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days of carbonation were measured using a helium-based pycnometer (AccuPyc II, Micromeritics, USA), which was then subsequently used to determine the density of the cast and 3D printed

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samples. Bulk surface morphology of the cast and 3D printed samples was observed using an optical microscope (Nikon LV100). To observe the microstructural features, the surface (outer surface exposed to atmosphere) and the interior (3 to 4 mm deep inside the sample from the surface) morphology was examined using a scanning electron microscope (SEM) (Scios, FEI,

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Netherlands) operating at an accelerating voltage and current of 5 kV and 25 pA, respectively. To avoid the charging issues during imaging, the surface of the samples was coated with a thin layer of gold prior to the SEM analysis. Powder X-ray diffraction (PXRD) analysis (Empyrean, PANalytical, UK) was used for the qualitative phase analysis of cast and 3D printed samples. The spectra were collected using Cu-Kα radiation (λ = 1.54 Å) in the 2θ range of 5 to 80° with a step

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size of 0.02° and the data collection time of 200 s per step. To compare the carbonation efficiency of cast and 3D printed samples, thermogravimetric analysis and derivative thermogravimetric (TGA/DTG) were conducted using a simultaneous thermal analyzer (SDT Q600, TA Instruments, USA) in the temperature range of 25 – 1000 °C maintaining the heating rate of 10 °C/min. Peak deconvolution of the DTG data was performed to estimate the relative carbonation percentage of cast and 3D printed samples. The PXRD and the TGA/DTG analysis were conducted for the finely

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ground powders that were obtained by grinding only the outer 3 to 4 mm segments of the cast and

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3D printed samples.

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3. Results and discussion

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3.1. Reaction kinetics and rheology

As stated previously, the mortar consisting of only cement and water irrespective of w/c ratio can

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hardly be 3D printed due to poor extrudability, flowability, and buildability, and the same case was observed for RMC in the initial experiments. The primary issue faced was poor buildability

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because only the first two to three layers were deposited, after which the successive layers began to fall, leading to structural collapse. Similar behavior was observed even after adding 6 wt. % of graded sand that is expected to improve the workability and buildability of cement mortar. Therefore, to accelerate layer hardening and strength development in RMC mortar, 3 wt. % of

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cMgO was added to the RMC mix. The motivation behind adding cMgO is its very high reactivity (~ 50 times as compared to RMC) that is expected to induce early hydration and hence, a faster setting of RMC mortar. Also, both the MgO present in RMC and the cMgO are essentially the same in terms of composition and stoichiometry except for the lower crystallinity and much higher surface area of the later that makes it highly reactive. Fig. 1 presents the rate of heat dissipation

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and the cumulative energy released by the pure RMC paste and when mixed with 3 wt. % cMgO. As evident from Fig. 1(a), the initial exothermic peak corresponding to the dissolution of MgO remains unaffected in terms of position and intensity upon addition of cMgO. However, the second exothermic peak representing the hydration reaction of MgO shows a significant difference in terms of both the intensity as well as position. In the case of pure RMC paste, the hydration of MgO saturates in nearly 3 hr; however, in the presence of 3 wt. % cMgO, the hydration saturates

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in less than 1.5 hr with a higher peak intensity. This shows that the addition of only 3 wt. % cMgO

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does not only induce early hydration and setting in RMC but also improves its total degree of hydration. As shown in Fig. 1(b), the cumulative energy release remains higher throughout the

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reaction for RMC added with 3 wt. % cMgO confirming the positive effect on the hydration degree

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and hence the buildability of RMC. Such improved hydration degree and kinetics upon addition of cMgO can be attributed to its much smaller particle size, bulk density, and crystallinity, which

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will collectively provide a higher density of active nucleation sites [28] within the RMC matrix even when added in a very low concentration.

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After achieving good buildability, the next objective was to achieve good extrudability and flowability to make RMC mortar fully 3D printable. For this, commercially available additives were selected to impart suitable rheological properties to the RMC mortar. These include a superplasticizer, a suspension-aid additive, and a defoamer that helped in reducing the plastic

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viscosity, improving the paste homogeneity, and reducing the air entrapment, respectively, during the 3D printing process. Before 3D printing, the rheological characteristics of 3D printable RMC mortar were determined that are presented in Fig. 2. The flow curve obtained under the shear rate of 0.01 to 100 s-1, Fig. 2(a), follows the well-known ‘Bingham model’ as observed previously for a variety of 3D printable mortars [29]. The yield stress reached a maximum value of 1.98 kPa

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under the shear rate of 0.42 s-1 followed by a sharp decline to nearly 1 kPa upon a further increase in the shear rate representing a shift from ‘solid’ to ‘liquid’ like behavior. Upon further increase in shear rate to 10 s-1, the yield stress remained constant at about 1 kPa, as depicted in the inset of Fig. 2(a), representing steady flow of the RMC mortar within the shear rate range of 0.42 to 10 s1

. A continued increase in the shear rate resulted in a slight and steady increase in the yield stress,

which reached a maximum value of 1.36 kPa under the maximum applied shear rate of 100 s -1.

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The Bingham fit of the experimental flow curve data resulted in the shear yield stress and the

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plastic viscosity of 0.95 kPa and 4.45 Pa⋅s, respectively. These values fall well within the range that is reported for various other 3D printable cement mortars [30]. In a separate experiment, time-

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dependent yield stress of the freshly prepared 3D printable RMC mortar was recorded for 90 min

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at a constant shear rate of 2.5 s-1 (the value that prevailed during the 3D printing experiments based on the nozzle diameter of 2 mm) to analyze the mortar consistency with time, and the results are

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presented in Fig. 2(b). During first 20 min, the yield stress remains at around 2 kPa that increases to about 5 kPa after 60 min. Within next 30 min, the value reaches as high as 14 kPa representing

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a rapid hardening and setting of the RMC mortar. This observation is consistent with the 3D printing experiments where smooth extrudability and flowability of the RMC mortar for 50 to 60 min was observed. However, for further prolonged durations, the flow from the nozzle became discontinuous and fragmented, representing a hardening of the RMC mortar beyond the torque

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capacity of the 3D printer head. Therefore, in the case of RMC mortar, 60 min can be considered as an approximate ‘open time’ which is defined as the time during which the mortar maintains its extrudability and flowability for smooth 3D printing. On average, the reported open times during the 3D printing of various cement mortars are in the range of 30 to 60 min [31], which shows that the designed mix of 3D printable RMC mortar has a strong potential for prolonged 3D printing at

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larger scales. As evident from Fig. 2(b), the overall yield stress vs. time data follows an exponential function with 2nd order polynomial (R2 = 0.98). Therefore, the yield stress of the 3D printable RMC mortar at any given time can be predicted using the exponential function with 2 nd order

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polynomial for the given value of shear rate.

Fig. 1. Reaction kinetics of RMC paste with and without cMgO. (a) Rate of energy dissipation and (b) cumulative energy released over the period of 10 hr.

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Fig. 2. Rheology of freshly prepared 3D printable RMC. (a) Flow curve with the corresponding

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Bingham fit and (b) shear stress development with time.

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3.2. 3D printability, morphology, and strength Computer-aided designed (CAD) models of three different structures with a varying aspect ratio and complexity were created and are shown in Fig. 3(top row). These models were exported in the STL format to the slicing software to generate their layered geometry, Fig. 3(middle row), and the corresponding g-code that is understandable and executable by the 3D printer. The g-code resulting from the selected printing settings in the slicing software provided smooth and repeatable 3D

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printing of the selected geometries. An example demonstration of a complete 3D printing cycle

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(51 min long) of the RMC mortar is shown in the supplementary movie S1, where it can be seen that the 3D printing process was carried out smoothly without flow or structural discontinuity. The

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photographs of the 3D printed structures after 3 days of ambient curing and 7 days of carbonation

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are presented in Fig. 3(bottom row). As evident, the structures maintained their shape integrity and uniformity during the 3D printing as well as the curing process without any noticeable cracking or

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layer detachment.

The surface morphology of the cast and 3D printed samples, as revealed by an optical microscope,

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are presented in Fig. 4. As evident, the cast samples are characterized by smaller pores, Fig. 4(a), and a smooth overall surface as shown by the zoomed-in image, Fig. 4(b). On the contrary, the 3D printed samples exhibit much larger pores, Fig. 4(c), and a rough surface as depicted by the zoomed-in image, Fig. 4(d). The smooth morphology and smaller pores of cast samples are

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expected due to their consolidation inside a plastic mold. On the other hand, the consolidation of 3D printed samples in open air leads to a rough surface morphology with much larger pores. Although higher porosity generally leads to a lower strength, this characteristic may lead to increased strength in RMC samples due to greater penetration of CO2 and hence a greater formation of HMCs.

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Fig. 5(a) shows the photograph of the cylindrical cast and 3D printed samples (length and diameter = 25 mm) that were used for compression tests, and the results are presented in Fig. 5(b). Whereas the cast samples achieved a maximum strength of about 18 MPa, the 3D printed samples reached a compressive strength of nearly 30 MPa, which reveals that the 3D printed RMC can be significantly stronger as compared to its cast counterpart. Also interesting to observe is the difference in the failure mechanism of the cast and 3D printed samples. While the cast sample

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displays an almost steady increase in strength till the failure point, the 3D printed sample undergoes

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significant fluctuation (highlighted in grey) before reaching the maximum strength. This fluctuation in strength was understood by close examination of the sample state after the

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completion of the compression test, which is shown in Fig. 5(c). Even though the cast samples

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underwent homogenous cracking and failure throughout its length, the 3D printed samples exhibit ‘delamination’ of the outer wall which is formed by the successively deposited perimeters (outer

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circular boundary of each layer), followed by the failure of inner core which is formed by the successively deposited infills of each layer. This highlights the fact that, although the 3D printed

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RMC can be stronger than its cast counterpart, its strength is very much limited by the perimeter/infill adhesion, which has also been one of the strength limiting factors in 3D printed polymeric parts [32]. Therefore, the 3D printed RMC (and other mortars) can be further strengthened by optimizing the infill patterns (such as concentric, rectilinear, hexagonal, etc.) and

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percentage to achieve maximum adhesion area between the perimeters and the infills. The ‘interlayer adhesion’ has also been identified as an important strength limiting parameter especially in 3D printed concrete structures [33]; however, in the current case, the interlayer adhesion was observed to be strong enough as the continuity and integrity of layers can be observed even in the delaminated perimeter walls, Fig. 5(c). Table 3 summarizes the average percentage

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porosity (including the closed pore volume), average true density (based on the volume including the closed pores), and the average compressive strength along with the standard deviation measured for three cast and three 3D printed samples. Despite higher porosity, the 3D printed samples exhibit higher density and compressive strength which confirms that the pores and the systematic bulk voids created within the RMC during 3D printing provide pathways for carbonation leading to greater sequestration of atmospheric CO2 and hence increased

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microstructural densification and strength. On the contrary, the compact nature of cast RMC allows

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limited penetration of atmospheric CO2 inside the sample, leading to its reduced carbonation, density and strength. This also highlights the significance of optimizing the printing parameters

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and geometry of 3D printed RMC and other CO2 capturing mortars so as to maximize the

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carbonation pathways and hence the microstructural densification.

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Fig. 3. 3D printing of RMC in 3 different geometries. CAD models (top row), sliced models (middle row), and the corresponding 3D printed structures with dimensions in mm (bottom row).

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Fig. 4. Optical microscope images of the cast (top row) and 3D printed (bottom row) RMC.

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Panels (b) and (d) are the zoom-in views of panels (a) and (c), respectively. Scale bar in panels

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(a, c) is 1 mm, and in panels (b, d) is 0.1 mm.

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Fig. 5. Compressive strength and failure mechanism of cast and 3D printed RMC (a)

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samples.

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showing

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cylindrical samples that are nearly 25 mm in both the diameter and height. (b)

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Compressive strength curves and (c) state of samples after the compression test.

Table 3 Properties of the cast and 3D Printed RMC Cast RMC

3D Printed RMC

Porosity (%)

57.842 ± 0.063

59.123 ± 0.054

Density (g/cm3)

2.371 ± 0.013

2.463 ± 0.011

Compressive strength (MPa)

16.398 ± 1.318

30.988 ± 5.827

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Property

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3.3. Microstructure and composition The surface and the interior morphology of the cast and 3D printed specimens, as revealed by SEM, are presented in Fig. 6. The surface of the cast specimens, Fig. 6(a), is mainly composed of loose and large flakey structures, which are the characteristic of ‘dypingite’ [34], while the interior consisted of a large amount of uncarbonated brucite (clustered particles) with some localized growth of dypingite, Fig. 6(b). On the contrary, the surface of 3D printed specimens, Fig. 6(c),

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primarily consisted of smaller and densely packed flakey structures, which are the characteristic

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of ‘hydromagnesite’ [35], while the interior consisted of both the dypingite and hydromagnesite with no noticeable brucite, Fig. 6(d). Some hydromagnesite in the interior of cast specimens was

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also observed but only at few locations and in small proportions as compared to brucite. The denser

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microstructure of 3D printed RMC due to a higher concentration of HMCs, primarily in the form of hydromagnesite, explains its superior strength. This was further supported by PXRD patterns

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shown in Fig. 7(a) where the 3D printed specimens provided significantly intense peaks of hydromagnesite and diffuse peaks of brucite as compared to that of cast specimens. Moreover, it

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can be seen that the brucite peaks are much more intense in case of cast specimens which confirms its lower degree of carbonation and limited conversion to HMCs. Also, the casting process in an enclosed plastic mold will allow greater retention of water within the RMC mix leading to prolonged reaction of MgO with water and hence the formation of brucite with higher crystallinity.

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The TGA/DTG results presented in Fig. 7(b) provided nearly 5% greater weight loss with a greater intensity of endothermic peaks located in the range of 350 to 420 °C (dehydroxylation of HMCs and decomposition of uncarbonated brucite) [36] and 530 to 680 °C (decarbonation of HMCs) for the 3D printed specimens [37]. In addition, the deconvolution of the peaks located at 570 °C and 635 °C in the DTG data provided 35% higher weight loss due to decarbonation in the 3D printed

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specimens as compared to the cast ones. This confirms a greater degree of carbonation for the 3D printed RMC due to its porous nature leading to greater microstructural densification and hence, an increased compressive strength. In both specimens, nesquehonite (needle-like morphology) was not observed which is the densest HMC and is, therefore, the major strength contributor in RMC [38,39]. This can be attributed to the mix composition and curing conditions that were employed in the current study. As reported in literature [40], by optimizing w/c ratio in the RMC mix as well

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as the curing conditions such as the temperature and CO2 concentration, nucleation, and growth of

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nesquehonite can be promoted that is expected to provide even higher strength values for 3D printed RMC. It is also important to investigate the role of polymeric additives, which are used to

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improve the extrudability and flowability during 3D printing, on the hydration and strength

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development of RMC. Although, in the current case, these additives comprise of less than 1 wt. % of the mortar and their role can be safely neglected, future studies are required to systematically

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address this aspect because in a relatively higher proportion, the polymeric additives can have a significant effect on the cement chemistry and subsequent strength development [41].

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Nevertheless, the limited availability and hence a higher cost of RMC remains as a barrier towards its commercial use in construction. However, the utilization of RMC extracted from waste brine [42] and developments in the use of aggregates for 3D printable mortar [43] can lead to significant reduction in cost that may justify the use of RMC for construction as well as 3D printing in future.

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Finally, despite sophisticated mix design, a large scale 3D printing of RMC can be envisioned due to some recent advancements in the design of pumping and extrusion systems that provide in-situ and optimized mixing procedures for smooth and prolonged 3D printing of concrete [44].

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Fig. 6. SEM analysis of cast (top row) and 3D printed (bottom row) RMC. Panels (a, c) and (b, d) represent the surface and interior microstructure, respectively. The scale bar in all panels is 10

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µm.

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Fig. 7. Carbonation efficiency of the

through (a) XRD and (b) TGA/DTG analysis.

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cast and 3D printed samples revealed

Conclusions

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In summary, this work demonstrates that by adding only 3 wt. % cMgO and suitable additives in small proportion, excellent extrudability, flowability, and buildability can be achieved for RMC, making it suitable for 3D printing. Structures with moderately complex geometries were successfully 3D printed for durations as long as 60 min without any flow interruption or structural collapse, and they displayed excellent shape retention and overall integrity even after accelerated

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carbonation. Moreover, the 3D printed RMC was found to be nearly twice as strong compared to its cast counterpart, which is attributed to the higher degree of carbonation promoted by the porous nature of 3D printed RMC. This study warrants further research and development on the 3D printing of RMC to uncover several aspects related to mix composition, printing parameters, and curing conditions to achieve an even higher degree of carbonation and strength. This study also highlights a strong potential of RMC as a 3D printable construction material for holistically

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sustainable and modern infrastructure.

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Author_statement

Abdullah Khalil: Conceptualization, Methodology, Investigation, Writing - Original Draft & Editing, Project administration

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Xiangyu Wang: Methodology, Investigation, Writing - Review & Editing

Declaration of interests

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Kemal Celik: Writing - Review & Editing, Supervision, Project administration, Funding acquisition

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

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This work was sponsored by New York University Abu Dhabi (NYUAD) and was carried out using the research facilities in Advanced Materials and Building Efficiency Research Laboratory (AMBER Lab) and Core Technology Platform (CTP) at NYUAD.

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