Sustainable Application of Blast Furnace Slag in the Field of 3D Printing: Material Configuration and Machine Optimization

Abstract

Blast furnace slag is an industrial waste. Its disposition is generally by means of landfilling or stacking, which goes against the concept of sustainable development. In order to maximize its reuse and abate its adverse effects on the natural environment, this study innovated a solution of using blast furnace slag to produce 3D printing materials. Blast furnace slag was mixed with desulfurization gypsum to adapt to the operation of 3D printers. The mixture has fluidity, viscosity, and hydraulicity. Fluidity allows the mixture to smoothly pass through the transportation pipeline and nozzle of the machine; viscosity ensures that the extruded mixture is gradually stacked and settled; hydraulicity guarantees that the mixture solidifies and forms completely solid objects after dehydration and drying. Fully suitable 3D printers are rare in the market. Therefore, the printing nozzle and reserve device of the 3D printer were designed and improved in this study according to the material characteristics, enhancing the smoothness of the mixture during 3D printing. The sustainable application of blast furnace slag in the field of 3D printing not only favors diminishing environmental pollution and resource consumption but also provides a further sustainable production method for human beings.

1. Introduction

1.1. Background

Blast furnace slag is an industrial waste generated during the smelting process of iron and steel. The chemical compositions of blast furnace slag are similar to those of ordinary Portland cement, including oxides of Si, Ca, Al, Mn, and Mg [1]. The areas with a large usage of blast furnace slag are mainly in the building materials industry and highway construction in low additional forms. Unused slag is usually treated through landfill and accumulation, which not only occupies a large area but also causes serious environmental pollution [2]. Therefore, the effective utilization of blast furnace slag has become a vital issue.

The development of deposition molding in the FDM (fused deposition modeling) 3D printing technology provides new opportunities for the reuse of blast furnace slag. The FDM 3D printing technology has the characteristics of simple technical principles, low equipment and material costs, and safe operation [3]. Compared with other 3D printing technologies such as Selective Laser Sintering (SLS), Selective Laser Melting (SLM), and Adhesive Spray, material extrusion molding has fewer requirements for investment and the printing environment. It can reduce costs, increase efficiency, and enhance the industry’s enthusiasm for the secondary utilization of slag waste. Moreover, this study can provide guidance for the application of blast furnace slag in the field of 3D printing and reference value for similar projects in the future.

1.2. Research Objective and Significance

This study was based on the FDM 3D printing technology, with blast furnace slag as the main raw material and the concept of maximizing environmental protection as the prerequisite to develop new 3D printing materials, expand the feasibility of low-cost application of blast furnace slag in the field of 3D printing, and provide new ideas and methods for the reuse of blast furnace slag. Open-source 3D printing technology was utilized to develop 3D printing of slag through material extrusion printing.

Blast furnace slag, a byproduct of steel smelting, has long been an industrial waste. Depending on the fuels used in plants, for every 1 ton of pig iron produced, slag can be generated between 300 kg and 450 kg, seldom below 300 kg [4]. The output of massive blast furnace slag, if not treated or utilized timely, will induce severe pollution and impact the environment. Adopting blast furnace slag as a 3D printing material, on the one hand, can expand the application field of blast furnace slag and strengthen the reuse of industrial waste. On the other hand, it can alleviate the impact of blast furnace slag on the environment and contribute to sustainable development, possessing significant environmental benefits.

1.3. Research Content

(1)

The potential performance and feasibility of blast furnace slag in 3D printing were analyzed, and effective methods were explored to improve 3D printing performance.

(2)

A 3D-printed raw material with hydraulicity was prepared by mixing blast furnace slag with desulfurization gypsum and water, and the performance and characteristics of the mixtures were verified and evaluated.

(3)

Three-dimensional printing technologies that match the mixtures were investigated. The printing nozzle, extrusion method, and material reservation of 3D printers were designed and improved, and explicit data support was provided.

2. Feasibility of the Application of Blast Furnace Slag in 3D Printing

2.1. Analysis of 3D Printing Adaptation Technologies for Blast Furnace Slag

In recent years, 3D printing technologies have shown a trend of diversification and extensive application. Some common 3D printing technologies include Fused Deposition Modeling (FDM), Stereolithography (SLA), Selective Laser Sintering (SLS), Digital Light Processing (DLP), Binder Jetting, and Electron Beam Melting (EBM).

According to

Table 1. Analysis of 3D printing technologies.

3D Printing Method	Technical Difficulty	Requirement for Printing Environment	Equipment Size and Capital Investment.	Pollution
FDM	low	Relatively low, and a regular office environment is sufficient.	Most equipment is small and medium-sized; the investment in small equipment is low.	Almost no pollution.
SLA	medium	A relatively clean and enclosed environment is required to avert dust and impurities from entering the liquid-photosensitive resin.	Mostly is small equipment with low investment.	Uncured resin residues and wastewater are generated. The printing environment should be controlled for collection and treatment.
SLS	high	A relatively enclosed environment is required to prevent the leakage of powder materials and the entry of impurities.	Usually needs large-scale equipment and investment.	Uncured powder materials are generated. The printing environment should be controlled for collection and treatment.
DLP	medium	A relatively enclosed environment is required to inhibit the leakage of photosensitive materials and the entry of impurities.	Usually is medium-sized equipment with moderate investment.	Uncured resin residues and wastewater are generated. The printing environment should be controlled for collection and treatment.
Binder Jetting	medium	A relatively enclosed environment is required to prevent the leakage of powder materials and the entry of impurities.	Mostly is small equipment with low capital investment.	Uncured adhesive residues and wastewater are generated. The printing environment should be controlled for collection and treatment.
EBM	high	A vacuum or low-pressure environment is required to ensure the normal operation of the electron beam.	Usually needs large-scale equipment and investment.	Uncured metal powders are generated. The printing environment should be controlled for collection and treatment.

, the FDM technology builds 3D objects by stacking molten thermoplastic materials layer by layer. This approach is relatively simple and has low technical difficulty, providing a promising possibility for the application of slag.

Compared to other methods, the FDM technology has fewer requirements for the printing environment. Moreover, the investment in small FDM equipment is low. This can reduce investment, increase efficiency, and enhance the industry’s enthusiasm for the secondary utilization of slag waste.

Additionally, compared to other printing means, the FDM 3D printing technology has the advantages of high material utilization and low emissions. The printed objects are formed by gradual stacking, and the solidification process is through natural cooling, without cumbersome material collection and sorting in the later stage. Moreover, the entire printing process only produces minimal material waste and pollution emissions, conforming to the development concept of green environmental protection.

2.2. Feasibility Analysis of Preparing 3D Printing Materials Using Blast Furnace Slag

Rui Song et al. [5] discovered that rock mimics composed of silica sand (SS) and gypsum powder (GP) could be used to print and manufacture samples with specified heterogeneity using bonding inkjet technology or selective laser solidification technology. This can enlighten the exploration of blast furnace slag using 3D printing technology. In order to achieve efficient and high-quality 3D printing, the materials must comply with specific printable ranges. In 1997, American scholar Joseph Pegna [6] first proposed a 3D printing construction method applied to cement materials. Through layer-by-layer accumulation and selective solidification, it can manufacture free-form components. This technology has many similarities with the FDM technology, both based on digitalization and automation to produce designed 3D models or structures by adding materials layer by layer. Therefore, the characteristics and preparation methods of 3D-printed cement materials are critically significant for the development of blast furnace slag 3D printing materials.

The slurry formed by mixing powdery cement with water has fluidity, which can satisfy the pumping conditions of pipelines and the extrusion conditions of nozzles for 3D printing. The viscosity of the cement slurry can ensure that the material extruded with solid agglomeration has good adhesion. It can harden in the air and ultimately form a solid shape. By referring to the preparation process of 3D-printed cement materials, the slag powder can be modified and treated to meet the needs of 3D printing.

This study finds that only mixing slag powder with water has the characteristics of low viscosity and strong fluidity. In order to be suitable for 3D printing, slag powder can be mixed with appropriate bonding materials to have viscosity, fluidity, and hydraulicity similar to cement. This can meet the fundamental conditions for the application of blast furnace slag in 3D printing, providing possibilities for the application of blast furnace slag in manufacturing complex shapes and customized structural components.

3. Material Analysis and Experiment

3.1. Composition and Harmful Analysis of Blast Furnace Slag Powder

As a recycled material, blast furnace slag may pose threats to human health. During the modulation and filling of 3D printing materials, as well as the observation process of model printing, people have direct contact with the material. Therefore, identifying the detailed chemical composition of the material and analyzing its potential threat to the human body is essential.

Table 2. Composition analysis of blast furnace slag.

Element	Wt%	Wt% Sigma	Atomic %
C	1.89	0.22	3.67
O	34.89	0.25	50.98
Na	0.36	0.05	0.36
Mg	4.96	0.08	4.77
Al	8.58	0.10	7.44
Si	16.67	0.14	13.87
S	1.01	0.06	0.74
K	0.35	0.07	0.21
Ca	29.54	0.23	17.23
Ti	0.69	0.13	0.34
Cu	1.05	0.14	0.39
Total:	100.00		100.00

lists the content of chemical elements in blast furnace slag powder, among which that of oxygen, calcium, and silicon is the highest, accounting for 81.1% of the total amount. These three elements are common in nature. Blast furnace slag is a residue composed of impurities in the ore and non-ferrous substances in the burden during the smelting process of the blast furnace. Its chemical composition and physical properties make it highly resistant to chemical reactions. Therefore, it possesses a certain degree of chemical inertness. In most cases, it does not pose a direct threat to human health. However, the material also contains trace amounts of aluminum and copper, and direct contact with these two elements may cause skin inflammation or discomfort for people with metal element allergy symptoms. Therefore, laboratory safety gloves should be worn during operation to avoid excessive direct contact with the material.

This study involved the hardening reaction of blast furnace slag, gypsum powder, and water. Ettringite and ferric hydroxide were produced in this process. The related chemical reactions are as follows:

Gypsum powder (CaSO4·2H2OCaSO4·2H2O) reacts with water to form calcium sulfate dihydrate, as shown in Formula (1):

CaSO₄·2H₂O + 2H₂O→CaSO₄·2H₂O

(1)

Calcium sulfate dihydrate reacts with the components in blast furnace slag (such as CaO) and generates alumina (Ca₆(AlO₃)(SO₄)₃·32H₂OCa₆(AlO₃)(SO₄)₃·32H₂O), as expressed in Formula (2):

3CaO·Al₂O₃ + 3CaSO₄·2H₂O + 32H₂O→Ca₆(AlO₃)(SO₄)₃·32H₂O

(2)

The iron in blast furnace slag can react with water and form ferric hydroxide, as shown in Formula (3):

Fe + 3H₂O→Fe(OH)₃

(3)

Iron hydroxide generally does not have direct toxicity to the human body. It is a common chemical substance that can be found in nature and is used as a raw material in industrial production. The ettringite produced in the reaction is a compound containing calcium, aluminum, and sulfur. It does not have apparent toxicity to the human body at room temperature, but ingestion and inhalation should still be avoided.

In terms of physical properties, the delicacy of the material can significantly impact the smoothness of 3D printing. Due to the small size of the material particles, dust masks should be equipped during the mixing treatment to hinder these particles from entering the lungs and causing other adverse effects.

3.2. Experimental Study on the Hydraulicity of Blast Furnace Slag Powder

The S105 blast furnace slag powder is a finely ground powder of granulated blast furnace slag produced in the iron-smelting process. It is mainly composed of molten substances, such as silicates and aluminates. These substances are quenched to form a granular shape and then subjected to grinding, ultimately generating the required blast furnace slag powder. S105 blast furnace slag powder has high performance, characterized by fine particles and high activity. Its raw material is shredded blast furnace slag [7]. The fineness and chemical composition after processing are key indicators affecting the activity of granulated blast furnace slag, directly reflecting the strength of the solidification of blast furnace slag [8]. These characteristics endow blast furnace slag powder with the characteristic of hydraulicity.

Based on the features of S105 blast furnace slag powder and the preparation characteristics of 3D printing materials, a mixing experiment was performed on mineral powder and water to verify the hydraulic reaction of using blast furnace slag powder alone and its performance characteristics under various ratios, as shown in

Table 3. Hydraulicity experiment of blast furnace slag powder.

Number	Proportions	After 30 s	After 30 min	After 1 h	The Moisture Was Removed	After 22 h
1	1:9
2	2:8
3	3:7
4	4:6
5	5:5
6	6:4
7	7:3
8	8:2
9	9:1

. The S105 blast furnace slag powder used in the experiment was produced by Dehang Mining Products Co., Ltd. in Lingshou County, Shijiazhuang City, Hebei Province, China.

The powder-wetting method was adopted in this study. A total of 100 g of slag powder and water with different proportions were mixed and stirred for 30 s to fully blend. Through testing, under the ratios of slag powder to water of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, and 7:3, the slag powder can be completely moistened and achieve a certain fluidity. When the ratios of slag powder to water are 8:2 and 9:1, particle aggregation will occur after stirring, the mixture does not have fluidity, and water cannot fully wet the bottom slag powder in the cup.

In the hydraulic reaction, under the ratios of slag powder to water of 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, and 7:3, there are significant layering changes after 30 min of standing, and the changes are more pronounced after one hour of standing. After removing the upper layer of moisture and standing for 22 h, the mixtures all harden and become solids. Due to the low strength of some materials, a Japanese Sanliang digital push-pull force tester with a maximum load of 100 N was selected for testing by manually applying pressure, as shown in Figure 1.

In

Table 4. Results of strength testing of blast furnace slag in hardening reaction.

Number	Ratio of Slag Powder to Water	Material Strength (N)
1	1:9	5
2	2:8	8
3	3:7	17.3
4	4:6	26.5
5	5:5	41
6	6:4	68.2
7	7:3	91.3

, based on the pressure tests on seven groups of solid materials, it can be inferred that the material strength increases with the proportion of slag powder. When the proportion is 7:3, the strength reaches the maximum of 91.3 N.

3.3. Analysis of the Effect of Adding Gypsum and Hydraulicity

The experiment exhibits that the time of water hardening reaction for using slag powder alone is long, and there is an evident layering phenomenon in the process of standing. In order to lessen the water hardening reaction time of the slag powder, absorb excess water, and cut down operation steps, desulfurization gypsum powder was selected as the auxiliary gel material for the preparation of 3D printing materials. Thermal power generation forms a large amount of sulfur dioxide and fly ash while providing enormous energy [9,10,11,12]. In flue gas desulfurization, the limestone–gypsum wet desulfurization process will generate a large amount of industrial waste desulfurization gypsum [13,14,15,16,17]. Mixing desulfurization gypsum with blast furnace slag, which is also an industrial by-product, can not only compensate for the shortcomings of slag powder but also realize the green maximization of the materials. Moreover, the addition of gypsum powder is beneficial for stimulating the activity of slag powder. The priming effect of gypsum on slag powder and the formation of ettringite also promote material strength [18].

Gypsum hardens quickly when it comes into contact with water. Therefore, after the preparation of 3D printing materials, it is necessary to meet certain storage conditions before machine extrusion. Identifying the hardening time and reaction temperature can effectively promote the utilization rate of the material and abate machine blockage. The reference [19] reported that the unit heating time could be controlled by adjusting the temperature of injected water. This finding throws insights into experiments on altering the solidification time of gypsum. According to another experiment by the author, mixing gypsum powder and water totaling 100 g, the hardening rate accelerates with the ratio of gypsum.

Meanwhile, the temperature changes significantly in the water-hardening reaction of gypsum. In order to further explore the influence of temperature on the hardening rate of gypsum, a quantitative ratio of 7:3 of gypsum to water was selected, and then water at 10 °C, 20 °C, 30 °C, 40 °C, 50 °C, and 60 °C was added for mixing. After sufficient stirring for 30 s, the effects of water temperature changes on the reaction temperature and speed were observed using a temperature tester and a mortar flow tester, as shown in Figure 2.

The experiment used an SC-145 mortar fluidity tester to detect the setting time of gypsum, as shown in Figure 3. The lever was vertically pressed downwards by hand, and the test needle was inserted into the mixture within five seconds. The first measurement value was recorded and repeated every minute. When the resistance reached 0.3 MPa, the value was detected every 20 s until the resistance was up to 0.6 MPa, so as to determine the approximate setting time period of gypsum.

The results of this experiment indicate that gypsum releases heat when it reacts with water. With a water temperature of below 40 °C, the highest temperature usually occurs during solidification; that is, when the hydration reaction reaches the maximum extent, the released heat is the most. After solidification, the temperature will remain for a period and then gradually drop. When the water temperature is above 40 °C, the overall temperature of the mixture will first decrease, and then the hardening reaction occurs.

The change in water temperature can affect the hardening rate of gypsum. With 40 °C as the boundary, when the water temperature is below 40 °C, the solidification time will shorten with the rise of temperature. Under a water temperature of above 40 °C, the solidification time will increase with the water temperature. When the water temperature is 40 °C, the hardening reaction is the fastest. Regarding changes in solidification temperature, when the water temperature is below 40 °C, the solidification temperature approaches 37 °C. When the water temperature is above 40 °C, the solidification temperature elevates with the water temperature.

Therefore, during the preparation of 3D printing materials, adding desulfurization gypsum powder may delay or accelerate the hardening reaction.

4. Exploration of the Optimal Ratio and Characteristics of Mixed Materials

4.1. Experiment on the Optimal Ratio of Mixed Materials

The orthogonal experiment was used in the material mixing experiment in this study. Based on the results of experiments on blast furnace slag and gypsum powder conducted by the author, three proportions of slag, 60 g, 75 g, and 90 g, were selected. The experiment aimed to consume blast furnace slag as much as possible. Therefore, the proportion of blast furnace slag in the three materials was the largest. Desulfurized gypsum served as an auxiliary material, and three proportions were adopted: 30 g, 45 g, and 60 g. The experiments presented in Table 3 showed that excessive water can diminish the adhesiveness of blast furnace slag powder, decreasing material strength. Water had the least proportion in material mixing, and three ratios of 34 g, 37 g, and 40 g were adopted in the experiment.

The experiment was conducted nine times. Different proportions of materials were poured into containers and thoroughly mixed using a stirrer. The bubbles generated during mixing were discharged through 30 s of vibration with a vibrator. Then, the materials were placed stably until completely set. In order to explore whether the material has the potential for long-term shaping and use after molding, the formed experimental samples were stored under natural indoor conditions for 45 days and then subjected to pressure testing using the TIRA test 28,100 machine, as shown in Figure 4.

The pressure testing presents the diversity in strength when materials are mixed with different ratios, as shown in

Table 5. The results of pressure testing.

Number	Proportion of Blast Furnace Slag Powder (g)	Proportion of Desulfurized Gypsum (g)	Proportion of Water (g)	Material Strength (N)
1	60	30	34	87,000
2	60	45	37	37,900
3	60	60	40	92,500
4	75	30	37	84,200
5	75	45	40	above 100,000
6	75	60	34	97,500
7	90	30	40	above 100,000
8	90	45	34	60,000
9	90	60	37	78,000

. The ratios of (75 g, 45 g, and 40 g) and (90 g, 30 g, and 40 g) all achieve the maximum measurement strength of the machine above 100,000 N.

The damage of the material after the experiment in Figure 5 shows that although the main body of the material with proportions of 90 g, 30 g, and 40 g withstands the maximum pressure strength, the edges are severely damaged. In Figure 5a, when the proportions are 75 g, 45 g, and 40 g, the material not only bears the maximum pressure strength but also suffers less damage at the edges than the material in the seventh experiment.

4.2. Verification of the Influence of Water Temperature on Material Solidification Time

When heated in the air, gypsum loses moisture and transforms into calcined gypsum first (semi-hydrated calcium sulfate). If further heated, it converts into anhydrite (anhydrous calcium sulfate) [20]. After being mixed with gypsum powder, it has hydraulicity. Therefore, based on previous experiments, the solidification time of the material can be controlled by adjusting the temperature. The extrusion of 3D printing requires the material to maintain a fluid state as much as possible so that it can be transported and printed through pipelines.

The experiment was based on the proportions in the fifth experiment, as shown in Figure 6. After adding water at 60 °C, 50 °C, 40 °C, 30 °C, 20 °C, and 10 °C, the temperature of the material during the reaction was measured to determine the effect of water temperature on the solidification time. The results show that the water temperature of 10 °C can effectively prolong the time of material solidification, and the material hardening time is the shortest at 40 °C.

Therefore, cooling the material reserve device can lessen the solidification time of the material and maintain its flow state. After the material is extruded, the hardening time can be shortened by controlling the temperature of the bottom plate or printing environment, making the material easier to pile up and less prone to collapse.

5. Improvement of Printing Equipment

5.1. Printer Selection

The Prusa i3 printer is the third generation of the Reprap printer Prusa Mendel, as shown in Figure 7. Reprap is a 3D printer designed and produced by Adrian Bowyer et al. from the School of Mechanical Engineering at the University of Bath in the UK. When the printer was released, designers also opened the program and structural design of Reprap, allowing users to freely access design information and program source codes related to the printer on their website. The system can also be modified and upgraded according to users’ needs. Due to the openness of Reprap, it is easier to learn and upgrade. Moreover, the simple structural design of the Prusa i3 model makes it convenient to assemble and maintain, and it is relatively cost-effective for DIY printer users. The main body of i3 includes a rectangular gantry responsible for the movement of the printing head in the z-axis and y-axis directions. The other part is a printing platform, which undertakes the movement in the x-axis direction simultaneously. The renovation of this printing equipment was based on the frame structure of this printer. The nozzle and material reserve device of the printer were designed and improved according to the characteristics of the materials.

5.2. Improvement in the 3D Printing Nozzle

As a core component in the 3D printing process, the design and performance of the 3D printing nozzle play a vital role in printing speed, quality, material selection, and the types of objects that can be printed. The ordinary FDM printing nozzle outputs through hot-melt extrusion. In order to smooth the output of blast furnace slag, a spiral structure was added inside the nozzle to drive the flow and output of materials inside the nozzle. In response to the characteristics of high viscosity and easy accumulation of materials, small slopes were incorporated into the thread design of the spiral structure, making the material less prone to accumulation and blockage during output. The residual material will solidify in the nozzle, which is adverse to the next use. A side feeding port was set on the nozzle, facilitating pipeline replacement and flushing the inside of the nozzle after printing, as shown in Figure 8.

In terms of structural design involving internal spiral supply, in the case of a rectangular cross-section, there exists a material retention area, as shown in Figure 9a, resulting in material accumulation and directly affecting the stability of the material flow. To address this problem, the side slope can be elevated to make the cross-section present a trapezoidal structure, as shown in Figure 9b.

According to the comparison of end face sectional profile by Xu Tai et al. [21], the spiral sleeve with a trapezoidal cross-section is characterized by small flow fluctuations. The end and axial cross-sections of the 3D printing nozzle designed in this paper are shown in Figure 10. Where p is the pitch of the spiral groove, 8.5 mm; W is the width of the top of the thread, 1.9 mm; H is the height of the thread, 1.75 mm; α is the inclination angle of the two-wedge structure, 25°; D is the maximum diameter of the drill bit, 17.6 mm.

When the motor rotates one circle, the single spiral groove through which the material flows can be cut into a cross-section, forming a trapezoidal structure, as shown in Figure 11.

When the spiral structure rotates one round, the volume V₁ of the conveyed material can be obtained from Formula (4):

$${\mathrm{V}}_1=\pi \mathrm{DN}[\frac{\left(\mathrm{W}1+\mathrm{W}2\right)\mathrm{H}}{2}]$$

(4)

Among them, W1 and W2 are the upper width and bottom width of of the trapezoidal groove, respectively, in mm; H is the groove depth, in mm; N is the number of threads. The upper and lower bottom widths of the trapezoid are 6.2 mm and 9.12 mm, respectively. By inputting numerical values, the calculated volume output of rotating this printing nozzle one circle is approximately 2222.442 mm³. The total usage time of the machine can be regarded as the total supply time of the material. By calculating the total supply time and the time it takes for the spiral sleeve to rotate for one cycle, the total amount of V₂ of the material can be calculated, as shown in Formula (5):

V₂ = V₁t₁t₂

(5)

where V₁ is the volume output of the spiral structure rotating one round, in g; t₁ is the total supply time, in seconds; t₂ is the rotation time of the spiral structure for one round, in seconds.

5.3. Printing Speed of 3D Printers

The speed of 3D printing directly affects multiple factors during the printing process, which needs to be carefully balanced. Appropriate printing speeds are conducive to solid interlayer adhesion, making the final printed object more sturdy. For smooth material printing, the speed of the 3D printer needs to be adjusted.

By regulating the material temperature and the rotation speed inside the nozzle, the material exhibits Newtonian fluid characteristics in the nozzle. The shear rate of the material flowing through the nozzle can be calculated using Formula (6):

$$\dot{\gamma }=\frac{8\mathrm{v}}{\mathrm{d}}$$

(6)

where $\dot{\gamma }$ is the shear rate, v is the fluid velocity, and d is the inner diameter of the pipeline. The velocity v can be calculated by the volumetric flow rate V₁ and the cross-sectional area of the pipeline. The formula can be transformed into Equation (7):

$$\dot{\gamma }=\frac{8\mathrm{V}1}{\pi {dr}^2}$$

(7)

The inner radius of the nozzle pipeline is 9.6 mm. The calculated shear rate of the material passing through the nozzle is about 3.2 s⁻¹.

Referring to the paper “3D printing of robotic soft actuators with programmable bioinspired architectures” [22], it can be concluded that the maximum shear rate flowing through the nozzle is mainly related to the volumetric flow rate of the printing material and nozzle size. The printing speed can be calculated using Formula (8) as follows:

$${\dot{\gamma }}_{\max }=\frac{4\mathrm{Q}}{\pi r^3}$$

(8)

where r is the radius of the printing nozzle; Q is the volumetric flow rate: Q = vr², where v is the printing speed. By inputting the shear rate of the previous material flowing through the nozzle and the nozzle radius of 3 mm, the printing speed of the printer should be around 7.529 mm·s⁻¹ to achieve the optimal printing effect.

In the future, in order to adapt to more printing scenarios, the ratio of the radius of the printing nozzle to the printing speed can be used to determine whether to replace the printing head with a larger nozzle radius or alter the printing speed based on the printing head radius.

5.4. Design of the 3D Printing Material Reserve Device

The material reserve methods of ordinary FDM printers cannot satisfy the requirements for material storage and output. This study proposed a design of the material reserve device based on the characteristics of the material. It includes four major parts: a storage cylinder, a telescopic shaft structure, a refrigerator, and a placement frame, as shown in Figure 12.

There are 3D printers with similar output methods in the market. Gas compression devices are mainly used to assist material transportation. However, gas compressors are bulky and inflexible, constraining the usage environment of the printer. In order to make the usage scenario of the printer more flexible, this study created a material conveying assistance structure with a telescopic shaft. As shown in Figure 13, the rotation of the transmission gear drives the up-and-down movement of the telescopic shaft, thereby heightening or lowering the pressure on the material inside the cylinder and facilitating material output. Compared to gas compressors, the telescopic shaft structure is more compact, enriching the usage environments of the printer.

According to previous material experiments, the hardening time of the material is relatively short. In order to extend the solidification duration and the storage time, it is necessary to lower the temperature of the material storage environment and postpone the hardening reaction of the material. Therefore, a refrigerator was incorporated into the reserve device. A DC 24V minimum compressor r290 small refrigeration equipment produced by China HVAC&R Pioneer Company was selected as the refrigeration machine, as shown in Figure 14. With a height of 78 mm, it is nimble and small and can reach a minimum cooling capacity of 7.2 °C. The speed ranges between 2000 and 5000 rpm/min, which can meet the cooling needs of previous experiments. This small refrigerator can cool the material storage cylinder and slow down the hardening of the material.

In order to further fix the reserve device and ensure its better operation, this study constructed a storage rack for the reserve device, as shown in Figure 15. It can fix the storage cylinder in an inverted state, which is instrumental to the flow and output of materials. The storage rack was also equipped with an adjustable fixed device, ensuring the stability of the equipment during compressing. In addition, a platform was installed on the storage rack to accommodate the refrigerator, guaranteeing the stable output of the refrigerator to the storage cylinder.

6. Summary and Feasibility Analysis

6.1. Summary

This study probed into the feasibility of applying blast furnace slag in the field of 3D printing. With the concept of maximizing environmental protection, two industrial by-products, blast furnace slag and desulfurization gypsum, were selected as the mixing materials for 3D printing. Based on the experiments, when the ratio of slag, gypsum, and water is 75 g, 45 g, and 40 g, the material has the best compressive strength after hardening. Moreover, the temperature experiment exhibits that at the water temperature of 10 °C, the hardening of the material can be effectively suspended, and at 40 °C, the hardening reaction can be accelerated.

Depending on the material characteristics, the 3D printer components were designed and improved. A 3D printing nozzle with an internal spiral supply structure and a trapezoidal cross-section was built. The material volume output of rotating the nozzle one circle was calculated to be 2222.42 mm³, the shear rate of the material passing through the printing nozzle was about 3.2 s⁻¹, and the optimal printing speed was controlled at around 7.529 mm·s⁻¹. According to the experiment, the material reserve device was updated by incorporating a telescopic shaft to provide pressure for the material output and a refrigerator to retard the hardening of the material, retaining its optimal flow state.

6.2. Feasibility Analysis

This study is an interdisciplinary integrated design research. It provides a feasible solution for the application of blast furnace slag in 3D printing by optimizing the mixing ratio of materials and the design of printing equipment, thus achieving sustainable development with environmentally friendly materials and processes.

Due to the dependence of the infrastructure industry on steel, it is impossible to completely eliminate the production of blast furnace slag. Three-dimensional printing technologies can efficiently consume blast furnace slag and promptly prepare the required models or items. This approach can avoid the generation of large amounts of waste like traditional manufacturing methods while efficiently utilizing industrial waste. In terms of material selection, priority should be given to reusable resources, such as blast furnace slag and desulfurization gypsum. These industrial wastes can be recycled and treated as raw materials for 3D printing, diminishing the consumption of new resources. FDM printing technology can accurately control material usage and avoid wasting, thus achieving maximum utilization of materials.

The combination of gradual-stacking 3D printing technology and blast furnace slag can fulfill sustainable utilization of industrial waste and generate more added value for blast furnace slag. In terms of economic value, blast furnace slag is highly cost-effective. Regarding material preparation, blast furnace slag has wide sources. It can be used as a raw material for 3D printing after simple grinding treatment. According to the Centennial Architecture Annual Report [23], the average price of slag powder in China in 2023 is about 0.24 RMB per Kg (237 RMB per ton), and desulfurization gypsum is generally between 0.5 and 0.7 RMB. According to the proportions in this paper, the cost price of one Kg of printing materials is 0.34 to 0.41 RMB, which is much lower than 3D printing materials currently in the market. Therefore, it has significant potential profitability.

High-quality materials can enhance the market competitiveness of products. Experiments have confirmed that this material can withstand the pressure of over 100,000 N, which has the characteristic of maintaining structural integrity under high stress. This provides the feasibility for future application in load-bearing structural buildings or related directions.

Optimizing FDM process parameters can promote production efficiency and material utilization, decline energy consumption, and alleviate environmental burden, thus achieving sustainable development.

Blast furnace slag is barely applied in the field of 3D printing. Its advantages of low cost, high quality, and customizable 3D printing can largely expand its application scope in the future market. Meanwhile, in the context of increasing environmental awareness, the sustainability of the material can further heighten its market potential.