Compound for sustainable 3D printing of artificial coral reef and preparation method

Abstract

The invention discloses a compound for sustainable 3D printing of artificial coral reefs and a preparation method thereof.The preparation method comprises the steps that 1, soybean meal is soaked in seawater to be prepared into soybean meal solutions with different mass concentrations, standing is conducted, and supernate is taken for standby application; (2) mixing cementing liquids (urea and calcium chloride mixed liquids) with different concentrations with the supernate with the optimal enzymatic activity, and determining the optimal concentration 90g / L of the soybean powder solution according to the yield; 3) mixing biological cement slurry prepared from different cementing liquid concentrations with sodium alginate with different contents, and determining the optimal standing time of 90min, the sodium alginate content of 27.5% and the cementing liquid concentration of 1mol / L according to the standing time, the shore hardness and the static yield stress of the slurry; and (4) the seawater-based bio-carbonate cement / sodium alginate compound prepared under the optimal condition in the step (3) has good extrudability and stability of extruded strips, and then the compound is subjected to 3D printing, so that the coral structure is prepared.

Description

Technical Field

[0001] The present invention relates to a technology for using a seawater-based bio-carbonate cement / sodium alginate composite for sustainable 3D printing of artificial coral reefs, belonging to the technical field of new cement-based materials. Background Art

[0002] Coral reefs are often hailed as the "rainforests of the ocean" and play a crucial role in the marine ecosystem. They provide habitats for numerous marine species and protect coastlines from erosion. The need to protect coral reefs, especially those around remote islands, is more urgent than ever. Remote islands are home to some of the most pristine and unique coral reef ecosystems. However, these coral reefs are facing severe threats from climate change, ocean acidification, overfishing, and pollution. The disappearance of these coral reefs will have a chain reaction on the entire marine ecosystem, leading to a decline in fish populations, loss of biodiversity, and increased vulnerability of coastal communities to storms and sea-level rise. A major challenge in coral reef restoration efforts on remote islands is the lack of suitable cementitious materials. Traditional building materials may not be suitable for the marine environment as they may leach harmful chemicals into the water, which can be detrimental to the delicate coral reef ecosystem. Additionally, finding materials that can withstand harsh marine conditions such as strong waves and seawater corrosion is a daunting task. In many remote islands, fresh water is a scarce resource. In coral reef restoration projects, fresh water is often used in various processes, such as for preparing cement-based materials or for the normal operation of equipment during the restoration process. The lack of fresh water can significantly impede the progress of restoration activities.

[0003] Artificial coral reefs offer a promising solution to the decline of natural coral reefs. They can provide new habitats for marine life, enhance biodiversity, and help restore damaged reef areas. By creating artificial reefs with different pore sizes, a wide variety of marine species can be attracted. In addition, the materials used to build artificial reefs must be non-toxic, durable, and capable of promoting coral growth. 3D printing has emerged as a potential disruptor in the field of artificial coral reef construction. It can precisely control the shape and pore size of the reef structure, enabling the creation of customized habitats for different marine species. This technology can overcome some of the challenges associated with traditional artificial reef construction methods. To fully realize the potential of 3D printing in artificial coral reef construction, a cement-based material suitable for 3D printing is needed. Such a material should have biocompatibility and the ability to be printed into complex structures. One material that shows such ability is bio-cement. Bio-cement, produced by certain bacteria or enzymes through a mineral precipitation process, has the potential to meet these requirements. It is environmentally friendly, non-toxic, and can bond well with other materials, making it an ideal candidate for 3D printing artificial coral reefs. Summary of the Invention

[0004] Technical Problem: The object of the present invention is to provide a composite for sustainable 3D printing of artificial coral reefs and a preparation method thereof, which is different from the method of 3D printing artificial coral reefs using traditional cement-based materials. This method provides an environmentally friendly solution for coral reef restoration, addressing challenges such as environmental compatibility and freshwater scarcity. This provides a basis for innovative strategies for protecting and restoring marine ecosystems.

[0005] Technical Solution: The present invention provides a composite for sustainable 3D printing of artificial coral reefs and a preparation method thereof. The composite is composed of seawater-based bio-carbonate cement and sodium alginate, wherein the sodium alginate accounts for 27% - 28% of the mass of the composite.

[0006] The preparation method of the composite for sustainable 3D printing of artificial coral reefs includes:

[0007] Step a. Soak soybean powder in seawater to make 6 portions of soybean powder solutions with different mass concentrations, let it stand, and take the supernatant for standby;

[0008] Step b. Mix 6 portions of different concentrations of cementing liquid, namely the mixture of urea and calcium chloride, with the supernatants of the multiple portions of soybean powder solutions obtained in step a respectively to prepare multiple portions of seawater-based bio-carbonate cement. Determine the soybean powder solution with the optimal mass concentration according to the highest production amount among the multiple portions of seawater-based bio-carbonate cement prepared;

[0009] Step c. Use the multiple portions of seawater-based bio-carbonate cement prepared in step b and mix them with multiple portions of sodium alginate with different contents respectively to obtain multiple portions of seawater-based bio-carbonate cement / sodium alginate composite slurries. Select the optimal content of sodium alginate and the optimal concentration of the cementing liquid according to the standing time, Shore hardness, and static yield stress of the multiple portions of seawater-based bio-carbonate cement / sodium alginate composite slurries;

[0010] Step d. Uniformly mix the supernatant of the soybean powder solution with the optimal mass concentration and the cementing liquid with the optimal concentration of 1 mol / L in equal volume, and let it stand to obtain the optimal seawater-based bio-carbonate cement slurry;

[0011] Step e. Add the optimal content of sodium alginate to the optimal seawater-based bio-carbonate cement slurry obtained in step d to obtain the optimal seawater-based bio-carbonate cement / sodium alginate composite, and then perform 3D printing to prepare an artificial coral reef.

[0012] In step a, the soybean powder is used to prepare multiple soybean powder solutions with different mass concentrations by drying the soybean powder at a high temperature greater than 60° C., weighing 7, 9, 11, 13, 15 and 17 g of the soybean powder, respectively, fully mixing with 100 mL of seawater, and refrigerating in an environment of 5° C. to prepare soybean powder solutions with mass concentrations of 70, 90, 110, 130, 150 and 170 g / L, respectively, and taking the supernatant for later use.

[0013] The step b is specifically as follows: 0.5-3 mol of 6 concentrations of urea are added to 1L of seawater respectively, and then added to 0.5-3 mol of 6 concentrations of calcium chloride solution respectively, to prepare cementing solutions with concentrations of 0.5, 1, 1.5, 2, 2.5 and 3 mol / L respectively; 1 mol / L of cementing solution is correspondingly mixed with 6 mass concentrations of soybean powder supernatant of 70, 90, 110, 130, 150 and 170 g / L obtained in step a to prepare seawater-based biocarbonate cement, and the average yields thereof are 98.1%, 99.7%, 97.9%, 99.7%, 95.4% and 92.7% respectively; according to the average production amount of the above seawater-based biocarbonate cement, the optimal soybean powder solution concentration is determined to be 90 g / L.

[0014] The electrical conductivities of the soybean powder supernatants at six mass concentrations of 70, 90, 110, 130, 150 and 170 g / L are 14.7, 26.9, 22.9, 24.1, 24.1 and 19.1 μs / cm / min, respectively.

[0015] In the step c, the standing time of the seawater-based biocarbonate cement / sodium alginate composite slurry is within 150 minutes; the Shore hardness is 5.3-52.4 HA; the static yield stress is 14-460 Pa; the optimal sodium alginate content is selected to be 27.5% of the total mass of the seawater-based biocarbonate cement / sodium alginate composite slurry, and the optimal binder concentration is 1 mol / L.

[0016] The method for selecting the optimal sodium alginate content is: select the optimal seawater-based biocarbonate cement slurry and mix it with sodium alginate accounting for 5%, 10%, 15%, 20%, 25%, 26.5%, 27%, 27.5%, 28% and 30% of the mass of the composite, evaluate the stacking state during extrusion, and determine that the optimal printability is obtained when the sodium alginate accounts for 27.5% of the mass of the composite.

[0017] In the step d, the optimal mass concentration of the soybean powder solution is 90 g / L, the optimal binder concentration is 1 mol / L, and the standing time is 90 min, thereby obtaining the optimal seawater-based biocarbonate cement slurry.

[0018] In step e, the optimal seawater-based bio-carbonate cement slurry is added with the optimal content of sodium alginate, and the standing times are 0, 30, 60, 90, 120, and 150 min respectively to obtain the optimal seawater-based bio-carbonate cement / sodium alginate composite. After curing for 7 days at room temperature, the average Shore hardness of the composite is 5.3, 30.4, 38.6, 52.4, 40.3, and 37.4 HA respectively. According to the Shore hardness, the optimal standing time is determined to be 90 min.

[0019] Beneficial effects: Compared with the existing cement technology, the present invention has the following advantages:

[0020] 1) The carbon emission is relatively low compared with the traditional cement production method, and the seawater-based bio-carbonate cement also omits the links of using fresh water and culturing microorganisms, so its carbon emission and cost are relatively lower;

[0021] 2) The bio-carbonate cement product does not contain calcium hydroxide and calcium aluminate hydrate, and has seawater erosion resistance, non-combustibility, and low alkalinity. Description of the drawings

[0022] Figure 1 X-ray diffraction pattern of the seawater-based bio-carbonate cement / sodium alginate composite;

[0023] Figure 2 Scanning electron microscope images of the samples: (a) Bio-carbonate cement (1 mol / L), (b) Bio-carbonate cement (1 mol / L) + sodium alginate, (c) Bio-carbonate cement (1.5 mol / L), (d) Bio-carbonate cement (1.5 mol / L) + sodium alginate;

[0024] Figure 3 Effect of soybean powder content on conductivity;

[0025] Figure 4 Effect of soybean powder content on the yield of bio-carbonate cement;

[0026] Figure 5 Effect of the concentration of the seawater-based cementing liquid on the yield of bio-carbonate cement;

[0027] Figure 6 Printability of the seawater-based bio-carbonate cement / sodium alginate composite;

[0028] Figure 7 Effect of the standing time of bio-carbonate cement on the Shore hardness of the bio-carbonate cement / sodium alginate composite;

[0029] Figure 8 Printability of the composite of sodium alginate and seawater-based bio-carbonate cement at different dosages;

[0030] Figure 9 Creep recovery behavior of the composite prepared with 1 mol / L seawater-based cementing fluid when the shear stress is (a) 50 Pa, (b) 159 Pa, and (c) 160 Pa;

[0031] Figure 10 Creep angular velocity of the composite prepared with 1 mol / L seawater-based cementing fluid at different shear stress values;

[0032] Figure 11 Static yield stress of the composite prepared at different seawater-based cementing fluid concentrations;

[0033] Figure 12 Shore hardness of the sodium alginate and biocarbonate cement composite prepared at different molar concentrations;

[0034] Figure 13 3D printed coral reef-like structure. Specific implementation manner

[0035] The composite for sustainable 3D printing of artificial coral reefs and its preparation method of the present invention are specifically as follows:

[0036] a. Preparation of soybean powder filtrate: Raw soybean powder is yellow. Weigh 7, 9, 11, 13, 15, and 17 g of soybean powder respectively, mix them fully with 100 mL of seawater, and then refrigerate them in a refrigerator at 5 °C for 24 h to obtain soybean powder solutions with mass concentrations of 70, 90, 110, 130, 150, and 170 g / L. Take the supernatant for use;

[0037] b. Preparation of cementing fluid: Add 0 - 3 mol of urea to 1 L of seawater to prepare calcium chloride solutions with different concentrations (0 - 3 mol / L), and obtain seawater-based cementing fluids with concentrations of 0, 0.5, 1, 1.5, 2, 2.5, and 3 mol / L;

[0038] c. Determination of the optimal soybean powder filtrate concentration: Mix 100 mL of soybean powder supernatant with different concentrations with 100 mL of seawater-based cementing fluid (1 mol / L) evenly, let it stand for 12 - 24 h to obtain seawater-based biocarbonate cement. Then wash the obtained cement 3 times with tap water and dry it in an oven at 80 °C for 3 d to obtain the cement powder, calculate the yield, and determine the optimal soybean powder filtrate concentration;

[0039] d. Determination of the optimal cementing fluid concentration: Mix 100 mL of seawater-based cementing fluid (0, 0.5, 1, 1.5, 2, 2.5, and 3 mol / L) with 90 g / L of soybean powder supernatant (100 mL) evenly, let it stand for 24 h to obtain biocarbonate cement. Wash it 3 times with tap water and then dry it in an oven at 80 °C for 3 d, and finally calculate its yield;

[0040] e. Determination of the optimal sodium alginate content in the composite: After thoroughly mixing 100 mL of the supernatant of 90 g / L soy powder with 100 mL of the seawater-based cementing solution (1 mol / L), let it stand for 0, 30, 60, 90, 120, and 150 min, then add sodium alginate accounting for 27.5% of the total slurry mass to obtain the seawater bio-carbonate cement / sodium alginate composite; mix the bio-carbonate cement standing for 0 and 90 min with sodium alginate accounting for 5%, 10%, 15%, 20%, 25%, 26.5%, 27%, 27.5%, 28%, and 30% of the slurry mass to determine that the extrusion and stacking performance is optimal when sodium alginate accounts for 27.5% of the total slurry mass; measure the Shore hardness and microscopic morphology after curing at room temperature for 7 days;

[0041] f. Comparison of the extrusion and stacking of seawater-based bio-carbonate cement slurries and sodium alginate composites prepared with different concentrations of cementing solutions: Take equal volumes of the supernatant of 90 g / L soy powder and mix it thoroughly with seawater-based cementing solutions of 0, 0.5, 1, and 1.5 mol / L. After standing for 90 min, add sodium alginate accounting for 27.5% of the total solution mass to make the seawater-based bio-carbonate cement / sodium alginate composite, add it to a syringe and extrude and stack the composite to preliminarily judge its printability;

[0042] g. Printing of coral structures with the composite having the optimal printability: Mix equal volumes of the supernatant of 90 g / L soy powder and 1 mol / L seawater-based cementing solution to prepare a seawater-based bio-carbonate cement slurry. After standing for 90 min, add 27.5% of sodium alginate to the slurry and perform 3D printing of coral structures after mixing.

[0043] The acquisition of the soy powder, the preparation of solutions with different mass concentrations, and the method for obtaining the optimal mass concentration are as follows: Soybeans and soybean powder are dried at high temperature (above 60 °C), and the shelf life of soybean powder is 1 day to 90 days; Weigh 7, 9, 11, 13, 15, and 17 g of soybean powder respectively, mix them thoroughly with 100 mL of seawater, and place them in a refrigerator at 5 °C for refrigeration for 24 h to prepare soybean powder solutions with mass concentrations of 70, 90, 110, 130, 150, and 170 g / L. The conductivity change values of their supernatants are 14.7, 26.9, 22.9, 24.1, 24.1, and 19.1 μs / cm / min respectively. Among them, the change value of 90 g / L is the largest and the enzyme activity is the highest. Take its supernatant for standby.

[0044] The method for obtaining the cementing liquids (urea-calcium chloride mixture) with different concentrations is as follows: Add 0 - 3 mol of urea to a calcium chloride solution containing 0 - 3 mol prepared from 1 L of seawater to prepare a seawater-based cementing liquid, and its concentrations are 0, 0.5, 1, 1.5, 2, 2.5, and 3 mol / L respectively; The average yields of bio-carbonate cement prepared by reacting the supernatants of soybean powder solutions with concentrations of 70, 90, 110, 130, 150, and 170 g / L with the 1 mol / L cementing liquid through mineralization are 98.1%, 99.7%, 97.9%, 99.7%, 95.4%, and 92.7% respectively; According to the production amount of bio-carbonate cement, the optimal concentration of the soybean powder solution is determined to be 90 g / L again.

[0045] The standing time, Shore hardness, and static yield stress range of the said slurry include: The standing time of the slurry is 0 - 150 min; The Shore hardness is 5.3 - 52.4 HA; The static yield stress is 14 - 460 Pa.

[0046] The method for obtaining the standing for 90 min and sodium alginate (accounting for 27.5% of the total mass of the slurry) is as follows: The supernatant of the 90 g / L soybean powder solution is evenly mixed with the 1 mol / L seawater-based cementing liquid in equal volume. The standing times of the mixed liquid are 0, 30, 60, 90, 120, and 150 min respectively. The obtained bio-carbonate cement slurry is mixed with 27.5% of sodium alginate to prepare a composite. After curing at room temperature for 7 d, the average Shore hardness of the composite is 5.3, 30.4, 38.6, 52.4, 40.3, and 37.4 HA respectively. According to the Shore hardness, the standing time is determined to be 90 min. The bio-carbonate cement standing for 90 min is mixed with sodium alginate accounting for 5%, 10%, 15%, 20%, 25%, 26.5%, 27%, 27.5%, 28%, and 30% of the slurry mass, and the stacking state during syringe extrusion is evaluated. When the sodium alginate accounts for 27.5% of the total slurry mass, the optimal printability is obtained.

[0047] Example:

[0048] The main chemical components of the bio-carbonate cement synthesized by 1 and 1.5 mol / L seawater-based cementing liquid are calcite and magnesian calcite ((Mg 0.03 Ca 0.97 )(CO 3 )) as Figure 1 shown. In a seawater environment, magnesian calcite can be formed by the deposition of carbonate mediated by soybean urease or urease-secreting bacteria. When sodium alginate is added to the bio-carbonate cement slurry, its components are magnesian calcite and sodium alginate. Among them, sodium alginate has an amorphous structure, and its XRD diffraction peak is in the shape of a bread roll. The particle size of the bio-carbonate cement after molding for 90 min is smaller, and its crystallinity is lower than that after molding for 24 h.

[0049] SEM images show that the biocarbonate cement particles synthesized from seawater-based cementing fluids of 1 and 1.5 mol / L mainly exhibit an aragonite-type structure, and their shapes are mainly spherical, as shown in Figure 2 (a) and Figure 2 (c). However, when the seawater-based biocarbonate cement is allowed to stand for 90 min and then mixed with sodium alginate, its morphology mainly shows calcite and aragonite-type structures, and its shapes are mainly square and spherical, as shown in Figure 2 (b) and Figure 2 (d). It should be noted that sodium alginate has an amorphous structure. In addition, the seawater-based biocarbonate cement is evenly distributed on the sodium alginate structure.

[0050] Figure 3 Show the relationship between the soybean powder content and the change in seawater conductivity. When the seawater contains 70, 90, 110, 130, 150, and 170 g / L of soybean powder respectively, the recorded average conductivity change values are 14.7, 26.9, 22.9, 24.1, 24.1, and 19.1 μs / cm / min respectively. Higher conductivity changes correspond to higher urease activities. When the soybean powder content is 90 g / L, the maximum average conductivity change is 26.9 μs / cm / min. These results indicate that a soybean powder content of 90 g / L obtains the best urease activity, thereby promoting the efficient hydrolysis of urea, the generation of carbonate ions, and the increase in the yield of seawater-based biocarbonate cement.

[0051] Figure 4 Show that the soybean powder content also affects the yield of biocarbonate cement. When 70, 90, 110, 130, 150, and 170 g of soybean powder are added to each liter of seawater respectively, the corresponding average yields of biocarbonate cement are 98.1%, 99.7%, 97.9%, 99.7%, 95.4%, and 92.7% respectively. This shows that when the soybean powder dosage is between 70 and 130 g / L, both the conductivity and the soybean powder dosage have an impact on the yield of biocarbonate cement, but when its dosage increases to 150 - 170 g / L, the soybean powder dosage has a greater impact on the yield of biocarbonate cement. Considering the production cost and the difficulty of extracting the supernatant of soybean powder comprehensively, 90 g / L is selected as the optimal soybean powder dosage, and the corresponding urease activity and yield are both relatively excellent at this time.

[0052] The concentration of the seawater-based cementing fluid has a significant impact on the yield of biocarbonate cement, as shown in Figure 5As shown in the figure. When the concentration of the seawater-based cementing liquid is 0, 0.5, 1, 1.5, 2, 2.5, and 3 mol / L, the average yields of the biocarbonate cement are 0, 67%, 99.7%, 99.3%, 82.5%, 56.8%, and 48.7% respectively. It can be seen that a seawater-based cementing liquid with too high a concentration (>1.5 mol / L) inhibits the urease activity in the supernatant of soybean powder, hinders its effective hydrolysis of urea, resulting in a decrease in the yield of biocarbonate cement. The unreacted calcium ions react with sodium alginate immediately to form a hydrogel, affecting the printability. Therefore, the concentration of the seawater-based cementing liquid should be less than or equal to 1.5 mol / L.

[0053] Set the standing time of the biocarbonate cement to 0 min, prepare sodium alginate / biocarbonate cement composites with different contents, and evaluate their printability through the extrusion stacking state of the slurry in the syringe. The relationship between the extrusion stacking state of the biocarbonate cement / sodium alginate composite and the mass fraction of sodium alginate is as Figure 6 shown. When the mass fraction of sodium alginate is 5%, 10%, 15%, 20%, and 25%, the prepared biocarbonate cement / sodium alginate composite slurry is in a solution / slurry state and cannot be extruded and stacked through a syringe. When the mass fraction of sodium alginate is 26.5%, 27%, 28%, and 30%, the extrusion stacking effect decreases compared with that at 27.5%. When the mass fraction of sodium alginate is 26.5% and 27%, the composite slurry is easier to extrude through the syringe compared with 27.5%, but the stacking effect is still slightly worse. When the mass fraction of sodium alginate is 28% and 30%, the biocarbonate cement / sodium alginate composite slurry is thicker compared with 27.5%, and it is more difficult to extrude and stack, and the extrusion stacking effect is not good. When the mass fraction of sodium alginate is 27.5%, the extrusion stacking effect of the composite slurry is the best.

[0054] Figure 7 shows the influence of the standing time of the seawater-based biocarbonate cement on the Shore hardness of the biocarbonate cement / sodium alginate composite. The average Shore hardness of the composites with standing times of 0, 30, 60, 90, 120, and 150 min after curing at room temperature for 7 d are 5.3, 30.4, 38.6, 52.4, 40.3, and 37.4 HA respectively. Obviously, when the standing time is 90 min, the Shore hardness is the best. This indicates that there is a significant correlation between the standing time of the biocarbonate cement and the Shore hardness of the composite material prepared therefrom. The difference in the standing time will cause differences in the size and morphology of the carbonate particles in the slurry, thereby affecting the Shore hardness of the composite material.

[0055] When sodium alginate interacts with the supernatant of seawater-based soy powder, gel particles form rapidly. These particles are neither extrudable nor stackable. To evaluate the effect of mixing sodium alginate at different mass fractions with bio-carbonate cement (standing time 90 min) on printability, the stacking state during syringe extrusion was evaluated, as Figure 8 shown. For sodium alginate at 5%, 10%, 15%, 20% and 25% of the total slurry mass fraction, the bio-carbonate cement / sodium alginate composite was in a solution / thin mud state and could not be extruded or stacked with a syringe. For sodium alginate at 26.5%, 27%, 28% and 30% mass fraction, the extrusion stacking mass ratio was worse than the optimal mass fraction of 27.5%. Among them, for sodium alginate at 26.5% and 27% mass fraction, the extrudability through the syringe was slightly better than 27.5%, but the stacking mass was slightly inferior to 27.5%; for sodium alginate at 28% and 30% mass fraction, the bio-carbonate cement / sodium alginate composite became thicker, was more difficult to extrude and stack, and was worse than 27.5%. Therefore, when sodium alginate accounted for 27.5% of the total slurry mass fraction, it had the best printability.

[0056] The static yield stress is usually defined as the minimum shear stress that initiates material flow. Additionally, it is an important indicator for evaluating the structural construction of 3D printing slurries, corresponding to structural stability and an undisturbed, well-connected internal microstructure. In the creep recovery protocol, a constant shear stress is continuously applied and maintained for a certain time, and then the shear force is removed. The evolution of the resulting strain and strain rate is recorded. Taking the composite prepared with 1 mol / L seawater-based cementing liquid as an example: within the specified shear stress range, i.e., below 50 and 159 Pa respectively, the angular deformation finally stops during the creep stage, and the material shows a certain degree of recovery, as Figure 9 (a) and (b) shown. In addition, when the applied shear stress exceeds the critical threshold, i.e., 160 Pa, the material will continuously deform during creep without showing any recovery, as Figure 9 (c) shown.

[0057] Figure 10Shows the development of angular velocity during creep of seawater-based cementitious liquid composites with different molar concentrations. A bifurcation phenomenon was observed in all mixtures. Taking the composite material with 1 mol / L seawater-based cementitious liquid as an example: at shear stresses below 159 Pa, such as 80 and 120 Pa, the angular velocity finally decreased to zero. On the contrary, at slightly higher shear stresses, such as 160 and 200 Pa, the angular velocity finally increased to a constant higher value. This bifurcation of angular velocity implies the existence of a critical shear stress. Based on the discussion of the bifurcation behavior, the critical shear stress is considered to be the static yield stress. Therefore, the static yield stress of the composite material with 1 mol / L seawater-based cementitious liquid is 159 Pa. For seawater-based cementitious liquid mixtures with other molar concentrations, the measurement process of the static yield stress is similar to that of the 1 mol / L seawater-based cementitious liquid mixture.

[0058] Figure 11 Summarizes the static yield stress values of different mixtures. It can be seen that as the content of seawater-based cementitious liquid increases from 0 to 1.5 mol / L, the static yield stress significantly decreases from 460 Pa to 14 Pa. Specifically, compared with the mixture without seawater-based cementitious liquid, when 0.5 mol / L seawater-based cementitious liquid is added, the static yield stress is significantly reduced by 53%, indicating that the introduction of seawater-based cementitious liquid is effective and significantly improves the fluidity of the composite. When the content of seawater-based cementitious liquid increases to 1 mol / L, the reduction in static yield stress is less, about 26.4%. When the content of seawater-based cementitious liquid further increases to 1.5 mol / L, the static yield stress significantly decreases to 14 Pa, a sharp drop of about 91.2%.

[0059] A lower static yield stress facilitates the flow and extrusion of the material. A higher static yield stress will ensure that the material maintains a stable shape after extrusion and does not collapse or deform. The comparison of the extruded strips by the syringe clearly reflects the difference in the static yield stress. Without the seawater-based cementing liquid, a static yield stress as high as 460 Pa results in a greater force required for extruding the strip. The mixture before extrusion is relatively dry, dispersed, and in large granular form. The overall shape of the extruded strip remains regular, but a large number of defects and discontinuities can be observed. After adding 0.5 mol / L seawater-based cementing liquid, the static yield stress is significantly reduced to 216 Pa. The shape and continuity of the extruded strip are significantly improved, and there are no obvious defects on the surface. However, a large thrust is still required during the extrusion process. When adding 1 mol / L seawater-based cementing liquid, the mixture before extrusion is wet, viscous, and in paste form. At a low static yield stress of 159 Pa, the extrusion of the strip is significantly smoother and easier. In addition, the shape and continuity of the extruded strip still remain good. When adding 1.5 mol / L seawater-based cementing liquid, the significant reduction of the static yield stress leads to a significant increase in fluidity. At a low static yield stress of 14 Pa, the mixture is in a slurry form. Although it can be easily extruded with a little force, the extruded strip will spread out instead of piling up into a stable height and regular structure. Therefore, determining the molar concentration of the seawater-based cementing liquid is crucial for obtaining an appropriate static yield stress to achieve smooth extrusion and stable multi-layer stacking.

[0060] Figure 12 The bio-carbonate cement and sodium alginate composites prepared with 0, 0.5, 1, and 1.5 mol / L seawater-based cementing liquid are shown. After curing at room temperature for 7 d, the average Shore hardness values are 0, 16.5, 52.4, and 40.3 HA respectively. Among them, the composite prepared with 1 mol / L seawater-based cementing liquid has relatively excellent Shore hardness. It can be seen that under the same conditions, the amount of the bio-carbonate cement slurry does not affect the Shore hardness of the composite. The syringe extrusion printing experiment shows that the bio-carbonate cement and sodium alginate composite prepared with 1 mol / L seawater-based cementing liquid can successfully print the expected stacked shape, while the composite prepared with 1.5 mol / L seawater-based cementing liquid cannot obtain the expected shape.

[0061] After the rheological test and the preliminary manual printing test, 1 mol / L seawater-based cementing liquid is selected to prepare the mixture of bio-carbonate cement and sodium alginate for the 3D printing test. The freshly mixed seawater-based bio-carbonate cement / sodium alginate composite is smoothly extruded through the nozzle without phenomena such as blockage, tearing, and discontinuity. In order to better demonstrate the 3D printing performance of the composite in complex and special structures, a twisted and complex trough-shaped structure similar to a brain coral reef is designed in the present invention. Figure 13Shows the top and detailed views of a 3D-printed coral reef-like structure. Although the printing path is complex with many sharp turns, no signs of vertical deformation, layer disconnection, or excessive deformation were observed in the deposited layers. In recent years, coral bleaching and death have become increasingly severe, putting many species that rely on corals for food, shelter, and breeding grounds at risk of extinction. The 3D-printed coral reef in this patent can be placed on the seabed, and the narrow and secluded space will protect the polyps and symbiotic algae living in it and provide habitats for many marine organisms. Compared with traditional concrete and metal materials, plant-based carbonate cement is more environmentally friendly and does not emit greenhouse gases and toxic substances into the seawater.

[0062] Overall, the seawater-based biocarbonate cement / sodium alginate composite shows good extrudability, printability, and constructability during the 3D printing process. From the perspective of ecological protection, it offers more possibilities and hopes for manufacturing components with complex shapes through 3D printing technology.

Claims

1. A composite material for sustainable 3D printing of artificial coral reefs, characterized in that: The composite material is composed of seawater-based biocarbonate cement and sodium alginate, wherein the sodium alginate accounts for 27% to 28% of the mass of the composite material.

2. A method for preparing a composite material for sustainable 3D printing of artificial coral reefs as claimed in claim 1, characterized in that The method includes: Step a. Soak the soybean powder in seawater to prepare 6 soybean powder solutions of different mass concentrations, let stand and take the supernatant for later use; Step b. 6 portions of cementing liquids of different concentrations, i.e., a mixture of urea and calcium chloride, are mixed with the supernatants of the multiple portions of soybean powder solutions obtained in step a to prepare multiple portions of seawater-based biocarbonate cement, and the optimal mass concentration of the soybean powder solution is determined according to the highest yield of the multiple portions of seawater-based biocarbonate cement prepared; Step c. Prepare multiple portions of seawater-based biocarbonate cement in step b, respectively mix them with multiple portions of sodium alginate having different contents, to obtain multiple portions of seawater-based biocarbonate cement / sodium alginate composite slurries, and select the optimal content of sodium alginate and the optimal concentration of the cementing fluid according to the standing time, Shore hardness, and static yield stress of the multiple portions of seawater-based biocarbonate cement / sodium alginate composite slurries; Step d. The supernatant of the optimal mass concentration of soybean powder solution and the optimal concentration of 1 mol / L cementing liquid are uniformly mixed in equal volumes, and the optimal seawater-based biocarbonate cement slurry is obtained after standing; Step e. Adding the optimal seawater-based biocarbonate cement slurry obtained in step d with an optimal content of sodium alginate to obtain an optimal seawater-based biocarbonate cement / sodium alginate composite, followed by 3D printing to prepare an artificial coral reef.

3. The method for preparing a composite material for sustainable 3D printing of artificial coral reefs according to claim 2, characterized in that: In step a, the soybean powder is used to prepare multiple soybean powder solutions with different mass concentrations by drying the soybean powder at a high temperature greater than 60° C., weighing 7, 9, 11, 13, 15 and 17 g of the soybean powder, respectively, fully mixing with 100 mL of seawater, and refrigerating in an environment of 5° C. to prepare soybean powder solutions with mass concentrations of 70, 90, 110, 130, 150 and 170 g / L, respectively, and taking the supernatant for later use.

4. The method for preparing a composite material for sustainable 3D printing of artificial coral reefs according to claim 2 or 3, characterized in that: The step b is specifically as follows: 0.5 to 3 mol of urea in six concentrations are added to 1 L of seawater respectively, and then added to 0.5 to 3 mol of calcium chloride solution in six concentrations respectively, to prepare cementing solutions with concentrations of 0.5, 1, 1.5, 2, 2.5 and 3 mol / L respectively; 1 mol / L of the binder liquid was mixed with 6 kinds of soybean powder supernatant concentrations of 70, 90, 110, 130, 150 and 170 g / L obtained in step a to prepare seawater-based biocarbonate cement, and the average yields thereof were 98.1%, 99.7%, 97.9%, 99.7%, 95.4% and 92.7%, respectively. According to the average production amount of the above seawater-based biocarbonate cement, the optimal soybean powder solution concentration was determined to be 90 g / L.

5. The method for preparing a composite material for sustainable 3D printing of artificial coral reefs according to claim 4, characterized in that: The electrical conductivities of the soybean powder supernatants at six mass concentrations of 70, 90, 110, 130, 150 and 170 g / L are 14.7, 26.9, 22.9, 24.1, 24.1 and 19.1 μs / cm / min, respectively.

6. The method for preparing a composite material for sustainable 3D printing of artificial coral reefs according to claim 2, characterized in that: In the step C, the standing time of the seawater-based biocarbonate cement / sodium alginate composite slurry is within 150 minutes; the Shore hardness is 5.3-52.4 HA; the static yield stress is 14-460 Pa; the optimal sodium alginate content is selected to be 27.5% of the total mass of the seawater-based biocarbonate cement / sodium alginate composite slurry, and the optimal binder concentration is 1 mol / L.

7. The method for preparing a composite material for sustainable 3D printing of artificial coral reefs according to claim 6, characterized in that: The method for selecting the optimal sodium alginate content is: select the optimal seawater-based biocarbonate cement slurry and mix it with sodium alginate accounting for 5%, 10%, 15%, 20%, 25%, 26.5%, 27%, 27.5%, 28% and 30% of the mass of the composite, evaluate the stacking state during extrusion, and determine that the optimal printability is obtained when the sodium alginate accounts for 27.5% of the mass of the composite.

8. The method for preparing a composite material for sustainable 3D printing of artificial coral reefs according to claim 2, characterized in that: In the step d, the optimal mass concentration of the soybean powder solution is 90 g / L, the optimal binder concentration is 1 mol / L, and the standing time is 90 min, thereby obtaining the optimal seawater-based biocarbonate cement slurry.

9. The method for preparing a composite material for sustainable 3D printing of artificial coral reefs according to claim 2, characterized in that: In the step e, the optimal seawater-based biocarbonate cement slurry is added with an optimal content of sodium alginate, and the standing time is 0, 30, 60, 90, 120 and 150 minutes respectively to obtain an optimal seawater-based biocarbonate cement / sodium alginate composite. After curing at room temperature for 7 days, the average Shore hardness of the composite is 5.3, 30.4, 38.6, 52.4, 40.3 and 37.4 HA respectively; according to the Shore hardness, the optimal standing time is determined to be 90 minutes.

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