1 Introduction
The field of three-dimensional concrete printing (3DCP) has recently garnered significant global attention and has been successfully applied in many areas of construction owing to its excellent automation ability, high efficiency, unprecedented construction freedom, and environmental friendliness [
1–
5]. Applications of 3DCP include low-cost housing [
6,
7], emergency shelters [
8,
9], military camps, and space settlements [
10], all of which have revealed advantages such as process efficiency, cost-effectiveness, and natural risk mitigation [
11,
12]. Ma et al. [
13] assessed the technology readiness level (TRL) of 3DCP in urban furniture making and house construction, identifying a TRL of 6–7 through analyses. Unlike traditional construction methods, these systems have not yet been practically developed for automation and large-scale construction projects. One of the reasons for this delay is that the TRL value of 3DCP varies with application. However, 3DCP could be a promising economical alternative to manufacturing technologies in extreme environments and remote locations, such as deserts. Adopting the 3DCP technique and exploring printable environmental cementitious materials promotes construction in remote desert regions.
Aeolian sand (AS), which is mainly distributed in north and northwest China, covers approximately 1.3 million km
2 and representing approximately 13% of the land mass of the country. AS resources are abundant in these areas. The exploitation and adaptation of AS for the manufacture of composites for 3DCP onsite applications is beneficial for environmental protection and the mitigation of desertification. Numerous studies on the properties of cement with AS have demonstrated the feasibility of using AS as an alternative to fine aggregate [
14–
16]. Guettala and Mezghiche [
17] discussed the possibility of utilizing AS as a partial substitution for fine aggregate and found that using up to 20% of AS with a fineness of 4000 cm
2/g, as a Portland cement replacement did not adversely affect compressive strength. Luo et al. [
18] found that AS not only has a filler effect but also has a partially active component that can endow it with a pozzolanic reactivity and heterogeneous nucleation effect, resulting in a satisfactory dynamic elastic modulus, splitting tensile strength, and compressive strength. Jiang et al. [
19] discussed the influence of AS on ultrahigh-performance concrete (UHPC), breaking from traditional categories of concrete with ordinary or high-strength. The workability and compressive strength of UHPCs are significantly enhanced when river sand is partially or completely replaced with AS. Because of its small particle size, spherical morphology, and round surface, AS can reduce the friction between particles within a reasonable amount of time, contributing to the workability of fresh UHPC [
20–
22]. Furthermore, Liu et al. [
23] explored the thermostable performance of concrete with different ratios of AS and demonstrated that samples with 40% AS replacement showed the highest compressive strength at elevated temperatures.
However, technical bottlenecks still exist in the manufacturing of 3DCP composites that incorporate 100% AS. This is mainly because of the restrictions imposed by the extrudability and buildability (stacking processes) requirements of 3DCP processes. The poor grading, ultrafine particle size (greater than 90% particle size in the range of 0.074–0.250 mm), large specific surface area, and spherical apparent morphology of AS inevitably have adverse effects on rheology, leading to unsatisfactory static yield stress, dynamic yield stress, and plastic viscosity [
24–
26]. Furthermore, AS is loose and has a low surface activity, a strong collapsibility, a poor water stability, and no aggregation, making the preparation of 3DCP mixed composites considerably difficult. Moreover, studies on the properties of concrete with 100% AS replacement are neither adequate nor systematic. In particular, desert regions are characterized by an extremely dry climate with strong sunlight and high temperatures, which could result in significant drying shrinkage cracking in concrete [
27].
There are many magnesite and salt-lake resources in the arid regions of China that are ideal raw materials for the production of magnesium oxychloride cement (MOC). MOC is an air-hardening cementitious material formed by mixing mainly magnesium chloride (MgCl
2) and light-burnt magnesium oxide (MgO) in a certain ratio. Compared to ordinary Portland cement (OPC), MOC has the advantages of low carbon emissions, low energy consumption, rapid setting, good thermal insulation performance, and remarkable adhesion to various fillers and aggregates [
28–
31]. Generally, MOC exhibits outstanding compressive strength (≥ 69 MPa), high flexural strength (≥ 4 MPa), and a remarkable elastic modulus (70–85 GPa) [
32,
33]. These properties can be attributed to specific composition phases below the 100 °C curing temperature, which include phase 3 (3Mg(OH)
2·MgCl
2·8H
2O) and phase 5 (5Mg(OH)
2·MgCl
2·8H
2O). The crystalline phases of the products are mainly dependent on the molar ratios of magnesia-to-magnesium chloride (MgO/MgCl
2) and water-to-magnesium chloride (H
2O/MgCl
2) [
34–
36], resulting in different mechanical properties of the specimen. Karimi and Monshi [
37] reported that an MOC with magnesium chloride hexahydrate (MgCl
2·6H
2O), magnesite, and water at 1.5, 13, and 12 mol, respectively, has the best compressive strength of 77.86 MPa. Li and Chau [
38] also discussed the influence of the molar ratios of raw materials on the properties of the MOC and concluded that the most critical factor in the design process is the molar ratio of MgO/MgCl
2. However, its application is limited by poor water tolerance, which is attributed to the hydrolysis of hydration crystals. The formation of flaky brucite could lead to destructive porosity and a loose structure, resulting in the worsening of the mechanical properties of the MOC [
39,
40]. Nonetheless, MOC has potential for application in arid and semiarid regions. This type of construction material can harden in air, avoiding specific curing requirements, and are suitable for harsh-field construction conditions. Owing to its advantages of low energy consumption and facile production processes, MOC is more beneficial for environmental protection and sustainable development than OPC [
41,
42]. Therefore, MOC exhibits promising potential for the development of three-dimensional (3D)-printed construction in isolated and expeditionary environments.
Based on these studies, the introduction of the 3DCP technology and development of printable MOC with AS in construction in desert regions could yield a cost-effective and environmentally friendly solution. In this study, a printable cementitious material composed of MOC and fine aggregates with 100% AS was developed. The effect of the molar ratio of MgO/MgCl2 on the mechanical properties and water resistance of the MOC with AS is discussed. Moreover, based on the optimal MOC proportion, the influence of the sand/binder (S/B) ratio on the compressive and flexural strengths, drying shrinkage, printability, buildability, and specimen cost were evaluated to explore the performance of MOC when exposed to a desert environment.
2 Materials and methods
2.1 Raw materials
The Xinjiang Uygur Autonomous Region (located in northwest China) is rich in magnesite and saline lake resources, which are natural sources of light-burnt MgO and MgCl2·6H2O. MgO with a purity of 70% and MgCl2·6H2O crystals were purchased from Urumqi Tenai Building Materials Co., Ltd. (Xinjiang, China) (Fig.1). The chemical compositions of MgO and MgCl2·6H2O , characterized using X-ray fluorescence (XRF), are shown in Tab.1 and Tab.2, respectively. In this study, AS, from the Gurbantunggut Desert in the Xinjiang Uygur Autonomous Region was utilized as the only fine aggregate in the concrete mixing process. River sand was purchased from Lingshou Shuokai Mineral Products Co., Ltd. (Hebei, China). Compared with river and mechanism sand, AS showed a distinct morphology with a rounded surface and uniform particle size distribution (PSD) (Fig.2). As displayed in the sieve analysis results in Fig.3, the PSD of AS was the most concentrated, with over 85% of grain sizes in the range of 50 to 200 μm. In contrast, the PSD ranges of river sand and mechanism sand exhibited broader distributions of 15 to 4950 μm and 25 to 1000 μm, respectively. The distribution of river sand was more uniform, whereas that of mechanism sand was more concentrated, with 80% concentrated in the 150 to 550 μm range. Hydroxypropyl methyl cellulose (HPMC, 200000 MPa·s, Shijiazhuang Ansen Chemical Co., Ltd. (Hebei, China)) was used to adjust the viscosity and buildability of the fresh cementitious composite and as a retarder. Moreover, silica fume (SF) with 2000 mesh and Class F (ASTM C618) fly ash (FA) were utilized in this research, and the PSD of SF and FA have been provided in Tab.3.
2.2 Mix proportion
Two experimental steps were performed to optimize MOC composites. To obtain the optimal mixture, the effects of AS on the mechanical behavior, water resistance, and drying shrinkage of 3D printing specimens were investigated.
Step-1: Since the crystalline phases of the products and their strength are dependent on the molar ratio of MgO/MgCl2·6H2O, four mixtures with different mole ratios were designed as 4:1, 6:1, 8:1, and 10:1 and marked as M4, M6, M8, and M10, respectively. To this end, the molar ratio of H2O/MgCl2 and AS/binder (S/B) ratio were fixed at 18 and 1.54, respectively.
Step-2: The optimal MgO/MgCl2 molar ratio obtained in stage 1 was fixed. In stage 2, the mechanical property, printability, and shrinkage performance of concrete with various S/B ratios (from 1.54 to 3.46) were investigated. The specimens with S/B ratios of 1.25, 1.5, 1.75, 2, and 2.25 times that of M8H were named M8H1.25, M8H1.5, M8H1.75, M8H2, and M8H2.25, respectively. OPC with a 1.54 S/B ratio was used as the control group in the drying shrinkage test.
The molar ratios were converted to mass ratios for easy operation and are listed in Tab.4. The MOC with AS was prepared using the following procedure. First, the MgO powder, AS, admixtures (SF and FA), and HPMC were mixed by stirring for three minutes. Secondly, MgCl2·6H2O crystals were dissolved in water completely and poured into the above mixture. After an extra three minutes of stirring, a mixture was obtained.
2.3 3D printing procedure
A robotic printer was used in this experiment for the evaluation of printability and specimen manufacturing, which could achieve a maximum print size of 5100 mm × 1200 mm × 2200 mm with a fabrication accuracy of ±1 mm. During the printing process, the linear and vertical printing speeds were fixed at 80 and 50 mm/s, respectively. The printing nozzle diameter was 20 mm.
2.4 Testing methodology
2.4.1 Measurement of mechanical properties
The mechanical properties of 3D printing composites were measured using a hydraulic testing machine according to the Chinese National Standard, GB/T 17671. Prismatic specimens with dimensions of 40 mm × 40 mm × 160 mm were cut from 3D-printed block specimens for compressive and flexural experiments. The loading rates were 2400–200 and 50–10 N/s for compression and flexural tests, respectively. Three samples were tested for compressive and flexural strengths at the ages of 3, 7, 14, and 28 d. At least three samples were tested for each group.
2.4.2 Measurement of water resistance
Water resistance is an important indicator for evaluating the performance of the prepared mixtures. The softening coefficient (Rn) was used to evaluate the water resistance of the MOC prepared in step-1, and it was calculated using Eq. (1). The larger the Rn, the better the water resistance. After being cured for 28 d, the compressive and flexural strengths of the specimens were measured, and then they were soaked in water for 3 d to obtain the softening coefficient R3.
where Cn refers to the strength after immersion in water for n d and C0 is the strength before immersion.
2.4.3 Measurement of rheology
The rheological property of the MOC with AS was measured using a Viskomat NT rheometer (Schleibinger, Germany) with a torque range of 0–250 N·mm. To fully reflect and characterize the printability of the composites, a rheological test procedure that simulates the 3DCP process is shown in Fig.4. The test procedure is divided into three stages.
Stage 1 (0–150 s): A static test was performed first, during which the material is subjected to shear at a relatively low constant speed of 0.03 r/min for 2 min, followed by stopping the rotation to 0 r/min and allowing the material to rest for 30 s. The static yield stress τs1 can be obtained at this stage.
Stage 2 (150–330 s): Then, a dynamic test is conducted by increasing the rotor speed from 0 to 60 r/min within 1 min; followed by shearing the material at a high constant speed of 60 r/min for 1 min, and then decreasing the speed to 0 r/min within 1 min. This stage yields the dynamic shear stress τd and plastic viscosity η can be obtained.
Stage 3 (330–480 s): After a rest period of 30 s, the static test steps from stage 1 are repeated to obtain the static yield stress τs3.
Two parameters, namely rheology recovery (Rr) and rheology stability (Rs), were obtained. These parameters reflect the change in the properties of the mixes after shearing and during the extrusion process, as well as the stability of the composites after deposition.
where τs1 and τs3 represent the static yield stress of the fresh concrete in stages 1 and 3, respectively; τA and τB represent the dynamic shear stress at 210 and 270 s, respectively, during stage 2.
2.4.4 Measurement of drying shrinkage
The early age shrinkage characteristics of the MOC mixture were investigated and compared with those of the OPC mixture to highlight the advantages of the MOC for early age anti-shrinkage performance. The OPC sample was prepared by directly replacing the MgO powder and MgCl2·6H2O crystals with 42.5 OPC. The early age shrinkage properties of each mixture were investigated using digital image correlation (DIC, VIC-3D). The specimens used in the experiment were cuboids with sizes of 140 mm × 70 mm × 30 mm (L × W × H), and the top surface of the cuboid was tested by two tungsten lamps at room temperature (25 ± 1 °C) and 40% humidity, as shown in Fig.5. To prevent any potential impact of the fresh cementation material on the quality of the DIC spot calibration, the tests were completed within 0.5 h after the specimen was printed and lasted for 8 h.
2.4.5 Microtopography
To observe the microstructure of the hydration products more clearly, AS was removed from the matrix before testing by scanning electron microscope (SEM, JEOL-6700F). The morphology and chemical composition of the samples were characterized using SEM and energy dispersive spectrometer, respectively, after 28 d.
3 Results and discussion
3.1 Optimization of the magnesium oxychloride cement composites
3.1.1 Mechanical strength of magnesium oxychloride cement composites
As shown in Fig.6(a), with an increase in the MgO/MgCl2 mole ratio, the compressive stress of the MOC with the addition of AS showed a slight variation within 3 and 7 d, which was mostly concentrated between 15 and 20 MPa, and between 25 and 32 MPa, respectively. Meanwhile, the flexural strength also exhibited a similar trend, achieving 3–4 MPa after 3 d of curing and 4–5 MPa after 7 d of curing. After 14 d of curing, the compressive and flexural strengths show a rising trend that first increased and then decreased with an increase in the MgO/MgCl2 mole ratio. Among the four mixtures, M8 exhibited the best mechanical properties, with compressive strength and flexural strengths of 39.7 and 10.5 MPa, respectively, at 14 d. Additionally, the compressive strength of M8 at 28 d could reach 57.8 MPa, which is a 36.7% increase from the lowest strength observed in M4 (42.3 MPa). In contrast, M8 also demonstrated the highest flexural strength of 13.1 MPa, which is a 33.7% increase from the strength of M4 of 9.8 MPa (Fig.6(b)).
As the molar ratio of MgO/MgCl
2 was further increased from 8 to 10, both the compressive and flexural properties of M10 exhibited an obvious decrease to 15.7% and 20.6%, respectively, at 28 d. This decrease is due to the different crystalline phases of the hydration products, which are dominated by the molar ratio of MgO/MgCl
2. With the rise of the MgO content, MgCl
2 could react to form the stable phase 5 product (5Mg(OH)
2·MgCl
2·8H
2O), which is the main source of strength. When the MgO/MgCl
2 ratio exceeds 8, excessive MgO can form large amounts of brucite, further resulting in the loss of strength [
38]. Test results of the mixture M8 exhibited the desired mechanical properties.
3.1.2 Water resistance of magnesium oxychloride cement composites
Owing to the Ionic lattice structure of the main hydration products composed of phases 3 and 5, the mechanical properties of the MOC can be reduced by the decomposition of the hydrate phases containing chlorine. Simultaneously, excess MgO and MgCl
2 in the cementitious composites continues to react with water, resulting in volume expansion, accelerating the destruction of the concrete structure [
43–
45]. Therefore, the investigation of the water resistance of MOC concrete with AS is critical for the evaluation of application feasibility. In this study, the softening coefficients of four types of specimens (M4–M10) were determined through compressive and flexural tests. As shown in Fig.7, after immersion in water for 3 d, both the compressive and blending strengths of the specimen were significantly impaired and first increased and then decreased. For compression, M8 displayed a relatively high softening coefficient of 0.64, whereas the values for the remaining three groups were below 0.60. M8 also exhibited a maximum flexural softening coefficient of 0.43, which was approximately 39% higher than that of M10. Compared with the compressive behavior, water action had a greater impact on the flexural strength, leading to a decline of approximately 60%. Combined with the mechanical test results, the mixture M8 exhibited the optimal mechanical and water resistance properties. The poor water resistance can be attributed to the decomposition of the MOC hydration products. The phase 5 (5Mg(OH)
2·MgCl
2·8H
2O) and phase 3 (3Mg(OH)
2·MgCl
2·8H
2O) which are the main strength sources of MOC will decompose to Mg(OH)
2, which has a much lower strength than phases 5 and 3. Therefore, the mechanical capacity of MOC decreases when immersed in water [
46].
3.1.3 Microtopography analysis of magnesium oxychloride cement composites
The typical morphologies of the MOC mixtures after AS removal are shown in Fig.8. With the MgO/MgCl
2 equal to 4, the hydration products of the MOC were unstable and progressively decomposed into flake-like Mg(OH)
2 (Fig.8(a)). This resulted in dimensional expansion and low mechanical strength [
17,
23,
27]. As the ratio of MgO/MgCl
2 increased, a small quantity of phase 5 hydrate (red boxes) accompanied by the MgCl
2·6H
2O (as circled in red) was observed (Fig.8(b)). The presence of MgCl
2·6H
2O was the main reason for the poor water resistance. Along with a further increase in MgO, a denser microstructure containing numerous needle-shaped phase 5 products was obtained in M8, which guaranteed good mechanical behavior, higher softening coefficient, and water resistance (Fig.8(c)) [
37,
47,
48]. However, for a MOC with a MgO/MgCl
2 ratio of 10, a gel-like phase (red circles) and unreacted granular MgO (red boxes) were interspersed among the phase 5 hydrates, leading to a loose microstructure and noticeable worsening of physical properties and water tolerance (Fig.8(d)) [
28,
31]. Because M8 produced the most phase 5 compounds and compact structure, the mechanical properties and water resistance of the four groups of mixtures were the best, thus validating the aforementioned results.
3.2 Influence of sand/binder on the properties of magnesium oxychloride cement composites
3.2.1 Mechanical properties of magnesium oxychloride cement composites with different sand/binder
Based on the results of stage 1, M8 was identified as the optimal proportion for the second research stage. According to a previous research, mineral admixtures and HPMC can improve the printability and buildability. Therefore, 12 wt.% SF, 12 wt.% FA, and 0.1 wt.% HPMC were applied to optimize the proportion. In this part of the study, the influence of the S/B ratio on the mechanical strength of the composites was measured at four curing ages of 3, 7, 14, and 28 d. As the S/B ratio increased from 1.54 to 3.46, the compressive and flexural strengths of the specimens decreased significantly. The mechanical properties exhibited an increasing trend with increasing curing age, as shown in Fig.9. When the S/B ratio increased from 1.54 to 3.46, the compressive strength decreased from 16.5 to 7.8 MPa and from 57.8 to 37.5 MPa at 7 and 28 d, respectively. The flexural strength of M8H reached approximately 13.1 MPa, which was almost 2 times that of M8H2.25 (6.7 MPa). Owing to the discontinuous gradation of AS and its smooth surface, a larger S/B ratio has a negative impact on the mechanical properties. Furthermore, with an increase in the S/B ratio, there are insufficient binders to coat the admixtures and AS particles, which can result in a decline in the mechanical performance [
23,
49].
3.2.2 Early-age shrinkage of magnesium oxychloride cement composites with different sand/binder
1) Test results of shrinkage
Three types of specimens with S/B ratios of 1.54, 2.31, and 3.08 were tested. As shown in Fig.10(a), the deformation nephogram visualized by DIC shows the drying shrinkage of MOC concrete with various AS contents after 8 h of irradiation. The violet areas implied more serious shrinkage, and the resulting MOC composite exhibited satisfactory shrinkage resistance. Increasing AS content could contribute to shrinkage reduction. This could be attributed to the decline in the effective water/binder (W/B) ratio with the increase in AS, which effectively mitigated the pores caused by free water evaporation. After the 8 h drying shrinkage test, OPC with the same mixing proportion as the reference sample exhibited a shrinkage stain of −0.01101. In contrast, the shrinkage strain of M8H is only −0.00402, which is a 66.6% drop in the OPC. Moreover, the shrinkage stain of M8H1.5 and M8H2 with higher S/B ratios decreased further to −0.00365 and −0.00269, respectively. According to the DIC cloud atlas, the shrinkage of the OPC was concentrated in the middle area, and the shrinkage of the MOC composites was distributed at the boundary. This can be attributed to the different pore water contents of the OPC and MOC under various curing conditions. The hydraulic OPC samples were subjected to standard curing, whereas the air-hardened MOC specimens were treated with air at room temperature. The water in the fresh MOC mixtures reacts and evaporates, resulting in a denser microstructure. In contrast, after standard curing, the OPC formed capillary channels, and the pore water content was still higher. When the OPC samples were exposed to high temperatures, the free water inside continued to evaporate outward through the capillary channels, resulting in large shrinkage in the middle area. In addition, as shown in Fig.10(b), there were few apparent fractures on the surface of M8H, whereas contrasting multi-fractures were observed in the OPC system. The deceleration of moisture loss in concrete through the cooperation between MOC and AS is a significant concern considering the severe internal drying shrinkage of the OPC. Therefore, the results indicate that the MOC exhibits significantly lower evidence of shrinkage cracking compared to the OPC, which is of great significance in preventing the early cracking of 3DCP components.
2) Shrinkage resistance mechanism
When considering the shrinkage resistance mechanism of MOC composites, the vapor pressure of the solvent cannot be ignored. The early plastic shrinkage was mainly attributed to the loss of water from the matrix. According to Raoult’s law of thermodynamics, the vapor pressure of a solution is equal to that of the solvent multiplied by the molar fraction of the solvent in the solution. The higher the vapor pressure, the greater the evaporation, as shown in Eq. (4); therefore, the evaporation of the MgCl2 solution per unit time was less than that of pure water.
where P is the vapor pressure of the solution; PA is the vapor pressure of the solvent; NA is the mole fraction of the solvent; NB is the mole fraction of the solute.
The early plastic shrinkage was mainly attributed to the loss of water from the matrix. The OPC mixture was made by directly mixing water with the OPC powder, and the MOC mixture was made by mixing the MgCl2 solution with MgO powder; therefore, according to Eq. (4), the vapor pressure of the MgCl2 solution was less than that of water. Consequently, the evaporation rate of the MOC mixtures should be lower than that of the OPC mixtures. To confirm this hypothesis, the evaporation rate of the solution and mass loss of the 3DCP components were measured. The evaporation rate was determined by placing 500 mL solution of MgCl2 into a beaker with a diameter of 7.5 cm. The concentration of the solution was the same as that used for the preparation of the 3DCP mixture, and an equal volume of water was used as the control. The two were heated in an oven at 40 °C, and the mass was measured at every hour. The evaporation rate was calculated as the amount of evaporation per hour per unit area. The sizes of the specimens used in the mass-loss test were the same as those used in the shrinkage test, and the mixes tested were OPC, M8H, and M8H2. The three groups of specimens were tested in a laboratory at room temperature (25 °C) and 65% humidity and weighed at intervals. The difference between the residual and initial masses was regarded as the mass loss. The evaporation test results in Fig.11(a) show that at a temperature of 40 °C, the evaporation rate of the pure water (H2O) is approximately 2.5 times faster than that of the MgCl2 solution. In the mass loss test (Fig.11(b)), the mass loss of M8H is only about 36% that of the OPC at room temperature (25 °C). The mass loss of M8H2, with a larger S/B ratio, was only 27% of that observed in the OPC. This finding partly explains the much lower the shrinkage value of MOS composites compared to OPC composites at early ages.
3.2.3 Rheology of magnesium oxychloride cement composites with different sand/binder ratios
Six types of MOC with various S/B ratios (1.54 and 3.46) were measured using a rheometer, and the test results are shown in Fig.12. According to previous studies, the optimum static yield stress was 0.8–2.4 kPa, which is suitable for continuous printing and stable stacking. The static yield stress tended to increase with an increase in the S/B ratio. Apart from M8H2.25 with a static yield stress τs1 equal to 2.66 kPa, the static yield stresses of the other specimens were all within the acceptable printable range (Fig.12(a)). During the stage 3 of the static rheological test, the τs3 of M8H2.25 did not meet the requirement of 3D printing, although the value dropped to 2.51 kPa after a high-speed dynamic shear test (Fig.12(b)). Fig.12(d) shows that the dynamic yield stress τd increases with the addition AS, ranging from 0.56 kPa for M8H to 1.33 kPa for M8H2.25. Meanwhile, the increase in AS content resulted in an increased the plastic viscosity η (the slope of linear fitting equations in Fig.12(d)), which ranged from 9.0 Pa·s for M8H to 6.8 Pa·s for M8H2.25. The plastic viscosity is nearly stable at 6.0 Pa·s when the S/B ratio is up to 2.96 (M8H1.75) (Fig.12(d)). Furthermore, based on Eq. (3), the Rs of the MOC composites with S/B ratios from 1.54 to 3.46 were 0.92, 0.92, 0.97, 0.96, and 0.93, and the corresponding Rr were 0.98, 0.95, 0.97, 0.95, and 0.94. Although all the values were lower than those of M8H, they were all greater than 0.90, and the increase in the AS content had no significant influence on these parameters. Therefore, satisfactory rheological properties of MOC with AS can be achieved when the S/B ratio is within 3.0.
3.2.4 Extrudability of magnesium oxychloride cement composites with different sand/binder ratios
With the cooperation of the admixture and HPMC, the MOC concrete with AS exhibited suitable printability, as demonstrated by the extrusion test shown in Fig.13(a). When the S/B ratio was increased to 2.31, the printed stripe of the mixture began to narrow; however, the printing process was still extruded continuously and smoothly, displaying a favorable 3DCP printing situation. As expected, a fracture occurred at the junction between the first two lines of the filament, where the S/B ratio increased further to 3.08. Furthermore, although the rheology of M8H2.25 was within the required printable range, high viscosity posed a significant challenge to consistency during the extrusion process, resulting in apparent multiple interruptions (Fig.13(d)). This phenomenon is derived from the fineness and large specific surface area of AS, which requires a higher water content to maintain the desired workability. The extrudability test results showed that the mixed composite material achieved good extrudability when the S/B ratio was equal to or greater than 3.0.
3.2.5 Printing case
Finally, the M8H1.5 mixture was determined to be the optimal composite for large-scale 3DCP. A Mongolian yurt model with a bottom diameter and height of 1.2 and 1.4 m, respectively, was designed and printed, as shown in Fig.14. The print nozzle outlet was 20 mm, the layer thickness was 10 mm, the printing speed was 80 mm/s, and the extrusion speed was 0.35–0.50 m3/h. The entire 3DCP process is divided into three sections, and the sidewall is divided into two sections for printing owing to the limited of volume capacity of the concrete mixer. The top of the “yurt” is printed separately. The entire printing process lasts for 85 min in a laboratory at a temperature of 21–23 °C and a relative humidity of 75%. The printing of the “yurt” model demonstrates that MOC-based composite with FA with 100% AS has good printability and buildability, and provides an example for onsite 3DCP construction. 3D-printed concrete using desert sand as a fine aggregate cannot only meet basic construction functions but also has environmental advantages. Research on the reliability of 3D printing desert sand concrete is a good starting point for the successful application of 3D printing technology in remote areas. In addition to exploring new 3D printing materials and their properties, future research should also consider the environmental impact of traditional and new materials, traditional cast technology, and 3D printing technology. Identifying the break-even point between these options can help make a rational choice of construction technology, maximize the advantages of 3D printing technology, and promote its practical application in remote areas.
4 Conclusions
3D printing AS is a promising and environmentally friendly construction method for remote deserts. An economic evaluation was conducted to verify the feasibility of the 3D printing of MOC concrete with AS. The following conclusions were drawn.
1) In the first research stage, M8 with a MgO/MgCl2 molar ratio of 8:1 had the best mechanical properties and water resistance with compressive and flexural strengths of 57.8 and 13.1 MPa, respectively, and a softening coefficient of 0.64. The optimal MgO/MgCl2 molar ratio was determined, such that the mixing proportion could guarantee the development of printable MOC composites.
2) An increase in the S/B ratio had a negative effect on the mechanical performance, especially the flexural strength, which declined by 42% after 28 d. However, the early age drying shrinkage strain of MOC specimens was mitigated dramatically with the rise of the S/B ratio, which decreased from −0.004 at M8H to −0.003 at M8H2. The improved shrinkage mitigation was attributed to the decrease in the effective W/B ratio and lower vapor pressure of the solvent.
3) By incorporating SF, FA, and HPMC, the static yield stress of MOC concrete with AS was modified to be within 0.8 to 2.4 kPa to meet the printability requirements. Meanwhile, with the increase in S/B ratio, the dynamic yield stress increased from 0.56 to 1.33 kPa, while the plastic viscosity gradually decreased to 6.8 Pa·s.
4) Moreover, both the rheological recovery and stability showed only minor fluctuations above 0.9, indicating that the change in the S/B ratio had no significant influence. Excessive AS content could lead to poor extrusion, especially over 3.08; therefore, the S/B ratio must be maintained within a suitable range.
5) The nonhydraulic chemical property of the MOC composite breaks through the adverse impact of extreme environments on on-site curing, indicating its great application potential for projects in remote deserts.
PDF
(7940KB)
