1. Introduction
The circular economy is an economic development model of continuous recycling and utilization, replacing the linear economic model of manufacturing-production-disposal in the past industrial revolution. This model of material flow forms a cycle of “resources, products, and renewable resources”. It would generate only a small amount of waste within a whole system, or even achieve the goal of zero waste.
However, the current resource cycle is encountering the following problems:
Jurisdiction and responsibilities for waste reuse are not comprehensive;
The quality of recycled resources/products needs to be further improved;
The management and control of the resource-recovery market are out of order;
The quality of basic environmental information needs to be further strengthened;
Disposal and management of reused products.
At present, the resource cycle is not perfect in terms of management and technology. The types of waste that most urgently need effective solutions to include inorganic sludge (D-0902), steelmaking furnace-refining slag (converter slag, desulfurization slag, electric arc furnace-reducing slag), and incinerator bottom slag, etc. There are nearly 5.4 million tons of recycled inorganic sludge resources generated every year. This kind of waste has a large output and various properties. Most of these have the potential to expand, so their reuse is limited. They can only be used for low-strength backfill materials (CLSM) or landfill materials, and even flow into agricultural land and fish farm depressions [
1].
The engineering properties of steelmaking-slag materials are better than natural sand and gravel, but steelmaking slag has stability problems, which makes the reuse rate generally low in actual engineering applications. The traditional treatment methods still cannot convert the free lime and magnesium oxide into calcium hydroxide and magnesium hydroxide, so expansion and deterioration of the concrete after hardening will occur [
2]. Therefore, follow-up stabilization is very important and a key factor in slag reuse requirements. Therefore, for the stabilization of steelmaking slag, in addition to the high-temperature steam curing method—the technology to solve the stabilization of steelmaking slag with low energy consumption—will be a critical factor in improving the reuse of steelmaking slag. In recent years, the production technology of the stainless-steel industry has continued to innovate and develop, and it is now a mature industry. As the output increases year by year, it will inevitably be accompanied by the production of process waste slag. Due to the limited land area of Taiwan, the disposal of slag by traditional burial methods will inevitably face the problem of disposal costs. How to effectively reuse it as construction and engineering materials is an important issue at present.
The high-value product of decoration boards constructed from EAF-reducing slag developed in this study has a wide range of applications. As building materials, cement boards can be applied to indoor partitions, ceilings, etc. Practical applications for the materials could be included in commercial buildings, department stores, restaurants, and residential buildings. There are many potential variations of the materials, and these can be widely used for other special purposes, such as wrapping beams, wrapping columns, decoration, exterior wall decoration, etc. The decoration boards, like fiber-reinforced cement boards, have the advantages of economies of scale for mass production, and can be produced in a standardized or automated manner. Due to the streamlined production process and the reuse of raw materials, it is necessary to coordinate with complete warehousing and logistics to make it a technology-intensive and capital-intensive industry. The high-value products of EAF-reducing slag promoted by this project—fiber-reinforced cement board, through the integration of interdisciplinary technology and the use of diverse materials, can adjust expenses and reduce costs. It can also effectively avoid the risk of a single material source being cut off, while integrating the industry supply chain from the material end to the end user and the market, launching high-value products that are functional, recyclable, low-cost, and comply with regulatory requirements.
The main material components of fiber-reinforced cement board are cement and inorganic mixed materials. From the perspective of the characteristics of the slag, there are two ways to carry out the fiber–cement board trial production. One is to directly replace the inorganic mixed material as the filling material of the board; the other is to replace the amount of cement. The main components of the reduced slag are CaO and SiO
2. The potential of the component as raw materials in cement boards is used to achieve the original role of cement in bonding other materials [
3].
The overall goal of this study was to develop building-material development technology for the stabilization and recycling of EAF-reducing slag. This study was mainly conducted to integrate the relevant raw material screening mechanism, manage the quality of raw materials for the cooperative manufacturer (Walsin Lihwa Co., Ltd., Tainan, Taiwan), and combine controllable density material technology, green building material evaluation, and verification technology to produce fire retardant and soundproof materials. These would then become high-quality green building materials.
The study focused on the fiber-reinforced cement board as the product of EAF-reducing slag reuse. The study of the product development process was carried out in two parts. The first was conducted to establish the screening mechanism of inorganic material characteristics and high-value management procedures. The second part was conducted to match the verification standards of environmental compatibility and slag-modification technology. The flow chart of the study is shown in
Figure 1. The study presents a feasible solution for a low-energy consumption silicon-based modification method to solve the expansion problem of EAF-reducing slag, and the production of high-performance products for building materials based on the EAF-reducing slag.
3. Results
From the results of the evaluation for EAF-reducing slags as building decoration boards, the characteristics would compare to the demands of performances from standards CNS 13777. The performances would include dry unit weights, bending strength, water absorption length change rate, flame resistance class, and sound transmission loss. According to the comparison of specimens, the basic mechanic characteristics have been shown higher strength than the same types of cement boards. With regard to environmental compatibility, the results of the toxicity characteristic leaching procedure were made available to the application of building materials. In fire retardant and soundproof properties, the results were also shown to be 1.2 to 1.5 times the values in performance compared to normal cement boards. An advanced description for several parts is presented below.
3.1. Application of Inorganic Polymerization Technology to Controllable Density Materials
On the basic mechanical properties of pouring products, the study mainly discussing the mechanical properties of cementing materials at 28 days. The measurement results would be the reference for the proportion adjustment of the second part of high-pressure building material products. The second part of the high-pressure product part mainly establishes the proportion design and mechanical test of the two groups of concrete fiber boards with a bulk density of 1.1 and a density of 1.3. The high-pressure production method of specimens would relate to the actual process of fiber-reinforced cement boards. In this part, the first part would show the results from the 28-day mechanical test of the pouring products as
Table 4. The second part would show the results from the mechanical tests related to the specific gravity of water absorption, water absorption expansion rate, and bending test of high-pressure products. The results are described below as
Table 5 and
Table 6.
For the part of high-pressure products, mainly based on the results of the 28-day compressive strength of the irrigation products, two groups of C/S ratios of 0.35 and 0.45 were selected, with one proportion of 70% reducing slag and 30% fly ash, and the other proportion of 60% reducing slag and 40% fly ash. The ratio was carried out to establish the proportioning design of two groups of fiber–cement boards with dry unit weights of 1.2 and 1.5. The specimen was initially produced by pressing molding and high-temperature steam-curing. The density control can reach the design of two groups of ratios of 1.2 and 1.5. For the mechanical test part of high-pressure products, bending strength tests were mainly carried out according to CNS 13777, and the test results are shown in
Table 6. The results show that when the S/F ratio is 1:1, the dry unit weight is 1.33–1.40 g/cm
3 after drying at 60 °C for 24 h. According to the results of the bending strength test, as the ratio of C/S is 0.35, the proportion of 60% EAF-reducing slag and 40% fly ash shows the best strength, which is 9.97 N/mm
2. The second is 9.39 N/mm
2 with C/S ratio of 0.35, the proportion of 70% EAF-reducing slag and 30% fly ash. Furthermore, when S/F is 1:2, the dry unit weight is 1.11–1.15 g/cm
3 when dried at 60 °C for 24 h. With the C/S ratio is 0.45, the results of the bending test showed that the proportion of 70% EAF-reducing slag and 30% fly ash is the best strength, which is 5.97 N/mm
2. The second is 5.66 N/mm
2 with C/S ratio of 0.35, the proportion of 70% EAF-reducing slag and 30% fly ash.
For the water absorption, dry unit weights, and water absorption length change rate tests of high-pressure products (fiber–cement boards), the tests are shown in
Table 6 and
Table 7. According to the requirements of CNS13777 cement board, when the dry unit weight is 1.1 g/cm
3, its water absorption rate and water-absorption length change rate are not required in the specification [
12].
The dry unit weight is 1.3 g/cm3, the water absorption rate is below 33%, and there is no requirement for the change rate of water absorption length. As for the dry unit weight of 1.5 g/cm3, the water absorption rate is below 28%, and the change rate of water absorption length needs to be 0.25%. The results show that in the group with S/F as 1:1, the dry unit weights are 1.28–1.32 g/cm3, the water absorption rate is 32.43–34.18%, and the change rate of water absorption length of the test body is 0.157–0.181%. As for the weight of S/F as 1:2, the dry unit weights are 1.01–1.14 g/cm3, the water absorption rate is 40.91–49.67%, and the change rate of water absorption length of the test body is 0.158–0.184%. According to the results of the proportion design of this project, the proportion of dry unit weights at 1.3 g/cm3, and its water absorption is just around the standard value. The rate is lower than 0.25%.
3.2. Environmental Compatibility Verification
The total element analysis is mainly based on the standard method NIEA M301.00B & NIEA M104.02C of the Environmental Inspection Institute. Microwave digestion equipment and an inductively coupled plasma emission spectrometer were used for testing, and an X-ray fluorescence spectrometer (XRF) was used to assist in confirming the correctness of the data. The EAF-reducing slag raw materials were subjected to microwave-assisted acid digestion, and the concentration of cations in the solution was analyzed by an inductively coupled plasma emission spectrometer, and the oxide content was converted to the analysis results of the X-ray fluorescence spectrometer (XRF) listed in
Table 8 and
Table 9.
As shown by the analysis results, the main components are CaO, MgO, SiO2, Al2O3, etc. The content of other main components such as Fe2O3, MnO2, TiO2, etc. is relatively low. Overall, the proportions of elements in the main components presented by the two analysis methods are similar. In the secondary composition, XRF found that the content of Cr2O3 was higher than the results of microwave digestion and ICP-OES analysis, which may be due to the difference in the sensitivity of the instrument to specific elements.
EAF-reducing slag was analyzed for heavy metal concentration after the standard method of the Environmental Inspection Agency—Toxic Characteristic Dissolution Procedure (R201.15C) [
13]. The results are shown in
Table 10. It can be seen from the table that the results of the dissolution concentration of heavy metals in stainless steel reducing slag are all lower than the TCLP dissolution standard value in the hazardous industrial waste identification standard.
To confirm the safety of the EAF-reducing slag used in watering products, the heavy metals concentrations of the finished product produced by using the EAF-reducing slag as a raw material were analyzed by the standard method of the Environmental Inspection Administration-toxicity characteristic dissolution procedure (R201.15C). The results are shown in
Table 11. It can be seen from the table that most of the heavy metals were below the detection limit in the samples containing the proportion of the reduced ballast, and only the heavy metals barium, chromium, and selenium were eluted, but the eluted concentrations all met the current national standards.
3.3. Fire Retardant Properties of EAF Reducing Slags Used in Building Decoration Boards
Based on the formula design of fiber–cement boards reused by EAF-reducing slag in this study, the C/S ratio was set at 0.35 and 0.45, the design strength is 280 kgf/cm
2 and 350 kgf/cm
2, and the production methods included the pouring method (cement boards of EAF-reducing slag base without fiber) and high-pressure method (fiber-reinforced cement board of EAF-reducing slag). The high-pressure method was divided into density ≈ 1.3 and 1.1, and its thermal conductivity (k) value is in the following order: pouring method > high-pressure method (density ≈ 1.3) > high-pressure method (density ≈ 1.1) (as shown in
Figure 5). Under the four ratios of C/S ratio as 0.45, the trend of the thermal conductivity (k) value is similar to that of the series of C/S ratio as 0.35 series, and slightly higher than that of the series of C/S ratio as 0.35 (see
Figure 5). Above all, the fiber-reinforced cement board of EAF-reducing slag (density ≈ 1.1) had good thermal insulation properties when the C/S ratio is 0.35 and 0.45.
Figure 6,
Figure 7 and
Figure 8 show the results of the flame resistance test, demonstrating that the EAF-reducing slag foundation fiber-free cement board, the control group (C100) series test pieces of the pouring method, and the EAF-reducing slag fiber–cement board of the pressing method have densities ≈1.3, 1.1, the control group (C100 1:1), and (C100 1:2) series test pieces. Before the heat test, the board must go undergo pre-treatment procedures such as drying procedures [
14]. After drying, the surface of the specimen has a nearly off-white cement surface with no cracks on the surface. After being subjected to the heat test, a large area of the surface of the heated surface exposed to the flame appears dark gray, while the back surface of the heated surface has no burning, penetration, cracks, or holes on the surface. The surface condition of the specimens with the pouring method, the EAF-reducing slag with fiber-free cement board and the control group (C100) series test pieces showed the same off-white color after drying. The cement board prepared with the high-pressure method, the density ≈1.3, 1.1, and the control group (C100 1:1 and C100 1:2) series of test pieces are gray-black, and the wall around the test piece is black-brown, and dark gray. The layer is caused by the gradient of the ceramic fiber board used to fix the test piece to black-brown and dark gray after being heated, and it is not caused by the characteristics of the test piece itself. The temperature-change image of the test piece taken by the infrared thermal imager and the whole heating test process were taken with a high-quality camera.
This study was based on the thermal characteristics, compressive strength, and bending strength, including tests on samples made using the pouring method (cement boards of the EAF-reducing slag base without fiber) and high-pressure method (fiber-reinforced cement board of EAF-reducing slag). The high-pressure method was divided into density ≈ 1.3 and 1.1, from which seven formulas were selected, including pouring method CS0.35-70-30, pouring method CS0.35-80-20, pouring method CS0.45-70-30, and high-pressure method CS0.35-70-30-1:1, high-pressure method CS0.45-70-30-1:1, high-pressure method CS0.35-70-30-1:2, and high-pressure method CS0.45-70-30-1:2 for the cone calorimeter test. As a result, it passed the flame-resistance level 1 test, which will help to replace natural materials in the future and reduce manufacturing costs [
15]. According to the test results of the cone calorimeter, the high-pressure method CS0.35-70-30-1:2 and the high-pressure method CS0.45-70-30-1:2 in the flame resistance level 1 test all exceed the total heat release of the first point of the item: the total heat release of flame-resistant grade 1 materials was below 8 MJ/m
2, but in the second point, the total heat release is below 15 MJ/m
2, and the b parameter calculated according to A.2 of CNS14705-3 was below −0.4 [
16], confirming that the calculation results meet the first level of flame resistance.
3.4. Soundproof Properties of EAF-Reducing Slags Used in Building Materials
According to the results of the transmission loss of each specimen, as shown in
Figure 9 and
Table 12, the effects provided by the boards of the building materials are mainly rigidity and strength. Therefore, reflecting the influence of its specific gravity and thickness on the transmission-loss performance, the middle and low frequencies are mostly the main contribution range of the board’s sound-insulation performance. Since most of the fiber-reinforced cement boards used in this study are used for the main body of the boards for building construction, 80~1600 Hz was used as the comparison range of transmission-loss performance. In addition, based on the above-mentioned mechanical test results, the test was carried out with the optimal proportion the performance requirements of fiber-reinforced cement board, and the blending ratio of the fiber and fly ash was adjusted to confirm its benefits. The following section analyzes the transmission-loss characteristic values of various specimens and uses 12 mm gypsum board as a control group to make a comprehensive comparison of each material to confirm the benefits of different application methods.
The transmission-loss characteristics of the heavy board prepared with EAF-reducing slag and fly ash have good benefits in the low-frequency range, showing the advantage of higher density. In this ratio, the original low-frequency disturbance is effectively eliminated, its overall performance can reach more than 30 dB, and it can only reach more than 35 dB above 1000 Hz. The existing 10 mm thick board has the characteristics of high transmission loss in the middle and low frequencies. In the future, if it is further combined with the internal filling material matching the partition wall system, it will be feasible to subsequently develop heavy-duty sound insulation systems.
With the addition of 1:1 fiber content, the mechanical properties of the board can be improved to meet the requirements of standard specifications. It also has a good effect on the transmission-loss characteristics of low and medium frequencies. Under the influence of a slight decrease in specific gravity, it is difficult to increase the thickness above 500 Hz. However, at a density of 1.3, it can still achieve an average sound transmission loss of more than 30 dB. Compared with the same grade of 12 mm gypsum board, it can achieve more than 10 dB sound insulation advantage. In addition, under the existing frame-type compartment system, the board can be combined with different filling materials and board-assembly methods to improve the sound insulation effect for medium and high frequencies.
By increasing the amount of regenerated fiber to achieve light weight and maintain the mechanical strength of the heavy board, the overall frequency band value is reduced by 1–3 dB due to the impact of light weight. However, it can still be maintained above 20 dB, and it still has excellent transmission-loss characteristics below 500 Hz. Above 500 Hz, it is maintained below 30 dB, and it is difficult to increase it further. Same as the existing frame-type compartment system, it can be combined with different filling materials and boards assembly methods to improve the sound insulation effect of medium and high frequencies.
Based on the comparison of the above three groups, the feasibility of using slag as a fiber-reinforced cement board is confirmed by using the existing 12 mm gypsum board as the control group and comparing the transmission-loss performance. The board prepared with EAF-reducing slag and fly ash can be used as a heavy decoration board for subsequent applications. The performance of each frequency band is significantly higher than that of the control group. There is a difference of more than 10 dB in the middle and low frequencies. Still with 1–3 dB higher performance. The bending-resistant boards prepared with fiber material can correspond to the decorative boards for building construction in subsequent applications. There is still a difference of more than 8 dB in the performance of the middle- and low-frequency bands, and the high frequency has the same performance. Boards with improved fiber-reinforced flexural properties have a higher performance of 3–7 dB below 400 Hz, while the mid- and high-frequency parts are relatively insufficient.
4. Discussion
This plan was mainly developed with EAF-reducing slag as the material source. The target product was the fiber-reinforced cement boards used for partition wall and ceiling decoration. The developed high-pressure fiber–cement board underwent a process adjustment to improve the mechanical strength of the board, so that the product specification can be upgraded from CNS 3802 to the requirements of CNS 13777, and its performance can be compared with the general commercially available calcium silicate board.
Based on the doubts and risks in the use of heavy metals in the finished products of EAF-reducing slag, this study used different environmental conditions and contact times to explore the long-term release of harmful heavy metals in the environment, so as to fully evaluate the environment of resource recycling-product compatibility [
17,
18]. Through the environmental compatibility assessment and analysis, the building materials containing the reduced ballast were determined to not be impactful or harmful when used in the environment. The average values of heavy metals dissolution in different accumulation periods were all within the monitoring and control standard values. From this, it can be confirmed that the water quality of the water body contacting the building materials made of reduced ballast is non-polluting and harmful. The use of materials containing the reduced ballast in the environment can meet environmental safety standards [
19,
20].
For the thermal characteristics test, the thermal conductivity of these test pieces was monitored at length during the proportioning process, and it was found that the thermal conductivity (k) of the test pieces maintained a good consistency during the proportioning process, and maintained good heat-resistance and heat-insulation effects. According to the measurement results of thermal conductivity (k), under the test of CNS 14705-1 “Test Method for Combustion Heat Release Rate of Building Materials—Part 1: Cone Calorimeter Method”, the specimens with the ratio of EAF-reducing slag and fly ash for any series at 70%: 30% could all pass the first class of flame resistance.
Based on the improvement of soundproof performance, a total of seven groups of soundproof performance tests of the fiber–cement board samples were completed. It was confirmed by the test results that the transmission loss of its sound insulation performance is more than 3–8 dB better than that of the same grade board (12 mm gypsum board), and the optimal ratio can reach more than 30 dB in the overall frequency band.
Above the results of measurement, the performances meet the standards of CNS 13777 as shown in
Table 13. The high-value products developed through this study and those meeting the standards account for at least 12.1 billion wall and ceiling applications of 37 billion annual interior decoration projects. It is expected that use of the materials will reduce the use of natural resources by about 35%, and at the same time reduce, by about 28–32%, the production cost of building materials. According to the existing national market demand for decoration boards, it is estimated that the decorative panels made by reusing waste can effectively reduce processing costs by nearly 500 million per year.
Comprehensive development results showed that for effective use of the established stainless-steel slag for cement products technology, the low-energy consumption silicon-based modification method solved the expansion problem of stainless-steel slag, while reducing the amount of cement and improving the bending strength of the specimens [
21,
22]. Combined with the testing technology for thermal characteristics, it was found that the modifier has a significant effect on improving sound insulation and heat insulation. The results are conducive to the products developed in this study meeting the mechanical strength, sound insulation performance, and thermal performance specifications required for market access.