Trans. Nonferrous Met. Soc. China 25(2015) 2230-2240

Sintering preparation of porous sound-absorbing materials from steel slag

Peng SUN, Zhan-cheng GUO

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China

Received 22 September 2014; accepted 28 December 2014

Abstract: Porous sound-absorbing materials were prepared from steel slag using waste expanded polystyrene (EPS) particles as pore former. The influences of the experimental conditions such as fly ash content, sintering temperature, sintering time, and pore former addition on the performance of the porous sound-absorbing materials were investigated. The results show that the porosity of the specimens can reach above 50.0%; the compressive strength and average sound-adsorption coefficient of the sintered specimens are above 3.0 MPa and 0.47, respectively. The optimum preparation conditions for the steel slag porous sound-absorbing materials are as follows: mass fraction of fly ash 50%, waste EPS particles 3.6 g, sintering temperature 1100 °C, and sintering time 7.5 h, which are determined by considering the properties of the sound-absorbing materials, energy consumption and cost.

Key words: steel slag; porous sound-absorbing material; noise reduction coefficient; porosity; compressive strength

1 Introduction

Steel slag is a by-product of steel production that makes up about 15% of steel output [1]. The utilization ratio of steel slag in China is less than 22%, and around 60 million additional tons of steel slags are discharged per year. The steel slag dumps occupy large areas of land and result in many serious environmental problems. Therefore, it is of great concern to explore new, efficient ways of using steel slag in order to improve the utilization rateo.

The main chemical compositions of steel slag include calcium oxide (CaO), silicon dioxide (SiO₂), aluminium oxide (Al₂O₃), iron sesquioxide (Fe₂O₃), magnesium oxide (MgO), etc. The common minerals in steel slag are dicalcium silicate (C₂S), tri-calcium silicate (C₃S), dicalcium ferrite (C₂F), RO phase (CaO-FeO– MnO–MgO solid solution), and free CaO [2-7]. Because the steel slag has a high silicate composition, steel slag not only has certain cementitious properties, making it useful as an admixture for cement and concrete [8-13], but it also meets the chemical composition requirements for the preparation of glass–ceramic and ceramics. It can thus be used for preparing slag glass ceramics, ceramics, and other materials [14-17]. As the content of CaO in steel slag is high, generally between 40% and 60%, the siliceous materials need to be added to improve the cementitious properties, stability, sintering properties of steel slag used in building materials [18,19]. Fly ash contains a significant amount of SiO₂, Al₂O₃, Fe₂O₃ and other acidic oxides, a small amount of CaO, and so on. The first three oxides account for more than 75% of fly ash content [20-22]. Consequently, fly ash can be used as siliceous material in the preparation of structural materials by steel slag.

Noise pollution is becoming much severer issues. The use of porous sound absorption material is an important measure to control noise pollution. A sound wave is absorbed when the wave is transmitted into thermal energy, which then disperses into the air, by viscosity resistance, compression-expansion, and/or vibration-transmission of the sound wave through porous sound absorption materials [23]. Therefore, porous sound absorption materials must keep appropriate open porosity and have a lot of fine, uniform, and connected pores [24]. According to the substance, porous sound absorption materials are divided into three categories: organic polymer porous materials, porous metallic materials and inorganic porous materials. Most organic porous materials cannot endure high temperature and the porous metallic materials have a higher cost, while typical inorganic materials are fragile and hazardous to health [25]. New types of inorganic porous materials, such as porous ceramics and foam glass, have been paid more and more attention.

Some studies on the preparation of porous sound-absorbing material from industrial solid wastes such as fly ash, coal gangue, and blast furnace slag, have been done and show that the porous sound-absorbing materials have high porosity but poor mechanical property [26-28]. There are few studies on the preparation of porous sound absorption material by using steel slag besides our group. we have explored sintering preparation of porous sound absorption material [29] using steel slag particles with binder, and the porous sound-absorbing materials have large volume density, poor processing performance because of only particles packing and non-uniform sintering. In order to develop a new method of preparing porous sound absorption material by utilizing steel slag, we prepared porous sound-absorbing material from steel slag powder using fly ash [30,31], waste expanded polystyrene (EPS) particles, and sodium silicate as a modifier, pore former, and binder, respectively.

2 Experimental

2.1 Raw materials

Steel slag powder with a particle size of less than 0.074 mm was obtained from Laigang Group Taidong Industries Co., Ltd. (Laiwu, Shandong Province, China). Its chemical composition is listed in Table 1. Its mineralogical phases, which were determined by XRD analysis, are given in Fig. 1. The fly ash was low-calcium, grade II and its chemical composition is also shown in Table 1. The fly ash was supplied by Laigang Group Thermal Power Co., Ltd. (Laiwu, Shandong Province, China). The waste EPS particle size was between 1.0 and 2.0 mm and the EPS was laboratory-made. Sodium silicate with a modulus of 2.6-2.9 was obtained from Beijing Jingmian Tongda Trading Co., Ltd. (Fengtai District, Beijing, China). Sodium fluorosilicate, class AR, was purchased from Tianjin Guangfu Fine Chemical Research Institute (Xiqing District, Tianjin, China).

Table 1 Chemical compositions and contents of main materials

Fig. 1 XRD pattern of steel slag

2.2 Preparation of specimens

In accordance with the ratio of the raw materials, the steel slag and fly ash were weighed and mixed together. The mixed material was moistened with a little water for 30 min. The waste EPS was weighed and wetted using 10 mL of 5% sodium hydroxide solution. Then, the aged materials and waste EPS were mixed with 5% sodium silicate. The mixture was then molded. The rough blanks were demolded and dried for 48 h. According to the sintering procedure shown in Fig. 2, the rough blanks were put into a resistance furnace and sintered at 950, 1000, 1050, 1100, and 1150 °C for 4.5-9.0 h, respectively. Then, the steel slag sound- absorbing material samples with dimensions of (90-100) mm × 25 mm were prepared.

Fig. 2 Sintering procedure of steel slag sound-absorbing material

2.3 Characterization of specimens

The density and porosity of the specimens were determined by the Archimedes method, using deionized water as the immersion medium. Microstructural observation and pore distribution were performed using a mineral liberation analyzer (MLA-250). The main mineral phases of the porous sound-absorbing material were measured by a powder X-ray diffractometer (XRD, DMAX-RB 12 kW, Rigaku) at a voltage of 40 kV and a current of 150 mA with Cu K_α radiation, and the detector scanned over a 2θ range of angles from 10° to 100° with a step size of 0.02°. The compressive strength of the specimen was measured by an MTS810 microcomputer control universal material testing machine with reference to test method for crushing strength of porous ceramic (GB/T 1964-1996). In accordance with Acoustics— Determination of sound absorption coefficient and impedance in impedance tubes—Part 1: Method using standing wave ratio, the sound-absorption properties of specimens of cylinders with diameters of 100 mm were tested by the standing-wave tube method, which is the normal incidence sound-absorbing determination.

3 Results and discussion

3.1 Effect of fly ash content on microstructure and properties of sound-absorbing material

The effect of the fly ash content on the structure and properties of the steel slag porous sound-absorbing material was investigated under the conditions of a sintering temperature of 1150 °C, a sintering time of 4.5 h, and 3.0 g of waste EPS. The density, porosity, and compressive strength of the specimens are presented in Table 2. With the increase of fly ash content, the density of the specimens increased from 1.37 to 1.59 g/cm³, and the apparent porosity of the specimens was decreased by 5.4%. The compressive properties of the specimens were improved from 2.29 to 3.92 MPa.

Table 2 Effect of fly ash content on density, apparent porosity, and compressive strength of specimens

Figure 3 shows the morphology and distribution of the pores in the steel slag sound-absorbing material. The specimen with 30% fly ash (Fig. 3(a)) was relatively loose and had many more and larger pores. The specimen with 50% fly ash (Fig. 3(b)) was more compact, and had somewhat fewer and smaller pores. It was found from Figs. 3(c) and (d), which showed the micromorphology of the pores, that the addition of fly ash impacted the sintering of the base material under the same sintering conditions. The specimen with 30% fly ash had a more extensive crystal phase, with a visible crystal boundary and crystals of different sizes and shapes. A pore network was formed and the sintering neck grew to a certain extent, but the specimen had a relatively low level of densification. For the specimen with 50% fly ash, the boundary of the crystal phase was not obvious and more glassy material was formed. The pore network was retained but the pore size decreased. The size of the sintering neck increased. Figure 4 compares the XRD patterns of the specimens with 30% fly ash and 50% fly ash. It was determined from Fig. 4 that the mineral phases of the specimens changed with increasing amount of fly ash. When the fly ash content increased from 30% to 50%, the main mineral phases of the specimens changed from gehlenite, kirschsteinite, merwinite, and small amounts of magnesium iron oxide and C₂S to gehlenite, kirschsteinite, and a small amount of magnesium iron oxide. It was found by comparing Fig. 1 with Fig. 4 that the main mineral phases of C₃S, C₂S, C₂F, etc. in the steel slag participated in the sintering reaction at high temperatures, and their diffraction peaks disappeared, whereas the diffraction peak of magnesium iron oxide decreased significantly. As the amount of fly ash increased, the ratio of calcium to silicon increased and the diffraction peaks of gehlenite and kirschsteinite were enhanced. These results accorded with the results reported in Ref. [31]. Therefore, by increasing the amount of fly ash in order to increase the ratio of silicon to calcium, the sintering properties of the base material were improved and the densification degree of the specimen was enhanced. The density of the specimen increased and the apparent porosity decreased. The compressive strength of the specimen was enhanced accordingly.

Fig. 3 SEM morphologies and distribution of pores in specimens with different fly ash contents

Fig. 4 XRD patterns of specimens with different fly ash contents

Figure 5 shows the test results of the sound-absorption properties of specimens with different fly ash contents. The sound-absorption coefficients of the specimens in the low-frequency range changed significantly and were between 0.05 and 0.85, whereas the sound-absorption coefficients in the high-frequency range were between 0.30 and 0.55. The sound- absorption performance thus initially increased and then decreased with the increase in the amount of fly ash. The noise reduction coefficient (NRC) was between 0.29 and 0.41. The sintering reaction of the porous materials was accelerated with increasing fly ash content in the base material. The density, apparent porosity, and pore size of the porous sound-absorbing material changed. The apparent porosity did not determine the sound-absorption performance of the specimens from the variation of porosity with fly ash addition here; but the aperture in particular, which has more obvious influence on the sound-absorption properties of materials, became finer. Some studies [32-35] found that the sound-absorbing performance would be improved with the decrease of the aperture; as the pore diameter reduces further, the ends of the some pores are closed to form the lateral cavities or enclosed pores and result in the gradual decrease of the sound absorption. The decrease of the pore diameter size increases the friction of sound waves passing through the pores, and more sound energy is converted into heat, so the sound absorption shows an increasing trend with adding fly ash. The NRC of the specimen with 40% fly ash reached 0.40. However, the pore size is too small and sound waves are reflected on the material surface instead of diffraction in the pores, which makes the sound absorption performance of the porous material decrease. Therefore, the NRC of the specimen with 50% fly ash decreased to 0.29. The porous sound-absorption material can be as a resonance sound absorption structure. In a certain range, the porosity decreases and pore size correspondingly is reduced; the speed of sound decreases in the pores and the resonance frequency will be reduced, which can effectively improve the sound-absorption performance at medium and low frequencies. As a result, the sound absorption coefficient of sample in 500-800 Hz range increased significantly when fly ash was added to 30% or 40% [36-38].

Fig. 5 Effect of fly ash content on sound-absorption properties of specimens

3.2 Effect of sintering temperature on microstructure and properties of sound-absorbing material

The effect of the sintering temperature on the structure and properties of the steel slag porous sound- absorbing material was investigated under conditions of sintering time 9.0 h, 50% fly ash, and 3.0 g waste EPS. Table 3 lists the density, porosity, and compressive strength of the specimens at different sintering temperatures. It was apparent from Table 3 that the sintering temperature had a significant impact on the density, apparent porosity, and compressive strength of the specimens. As the sintering temperature increased, the density of the specimens increased from 1.09 g/cm³ to 1.86 g/cm³ and the apparent porosity of the specimens decreased by 20.5%. Correspondingly, the compressive strength of the specimens was improved from 0.83 MPa to 5.03 MPa.

Table 3 Effect of sintering temperature on density, apparent porosity, and compressive strength of specimens

As indicated in Fig. 6, the specimen sintered at 1100 °C (Fig. 6(a)) developed porosity; the aperture size was uniform and the pore shape was irregular. The specimen obtained at 1150 °C (Fig. 6(b)) had fewer pores; the pore size was non-uniform and the pore shape was somewhat round. Also, some closed pores were formed. Figures 6(c) and (d) show the micromorphologies of the pores in the specimens. Sintering temperature had a significant impact on the micromorphologies of the specimens under the same sintering conditions. The specimen sintered at low temperature (1100 °C) had an extensive crystal phase with a clear boundary and crystals of different sizes and shapes. Growth trend of the sintering neck was clear. The pores were larger and had better connectivity. At 1150 °C, the boundary of the crystal phase was no longer apparent and a glassy material gradually formed in the sintered specimen. A sintering neck gradually formed and increased in size. The pore size decreased and the trend of closed-pore formation was apparent. The specimen had a relatively high level of densification. Figure 7 shows the XRD patterns of the specimens prepared at 1000 and 1050 °C. The main mineral phases in the specimens sintered at low temperature included gehlenite, kirschsteinite, merwinite, magnesium iron oxide, and a small amount of C₂S. With the increase of sintering temperature, the diffraction peaks of gehlenite and kirschsteinite were enhanced; the diffraction peaks of merwinite and magnesium iron oxide obviously decreased and the diffraction peak of C₂S disappeared. Thus, with increasing sintering temperature, the liquid was added to the system to promote the sintering reaction and the densification of porous sound-absorbing materials was accelerated. The pore shape in the specimen was changed, and some pores were closed or disappeared. The density of the specimens increased continuously with the reduction of the porosity and the compressive strength of the specimens was improved.

Fig. 6 SEM morphologies and distribution of pores in specimens prepared at different temperatures

Fig. 7 XRD patterns of specimens prepared at different temperatures

Figure 8 shows the sound-absorption coefficients of the specimens prepared at different sintering temperatures. The sound-absorption coefficients of the specimens prepared at different sintering temperatures were between 0.05 and 0.55 at low frequencies and between 0.20 and 0.70 at high frequencies. The sound-absorption performance initially increased and then decreased with increasing sintering temperature. As the sintering temperature increased from 950 °C to 1100 °C, the NRC was improved from 0.13 to 0.42. But the NRC of the specimen prepared at 1150 °C declined to 0.15. With increasing sintering temperature, the density of the porous materials increased and the porosity decreased. As the apparent porosity of specimens changed little before 1100 °C, the sound-absorption performance was mainly affected by pore structure. The pore size was reduced with declining the apparent porosity of specimens, and the sound-absorption performance of the specimens is significantly improved. Many research results of various materials supported this [39-41]. The linear flow resistance increased with the decrease of the pore diameter and air friction increased, which improved the sound-absorption effect [42]. The effect of apparent porosity on the sound- absorption performance became the main factor after 1150 °C. The apparent porosity decreased rapidly, the flow resistance increased to reduce the air permeability, which caused the sound-absorption coefficient of the specimen sintered at 1150 °C to decrease sharply. The specimen sintered at 1100 °C had higher porosity; the pores were more irregular, more uniform and more tortuous, so its sound-absorption coefficient was good and reached 0.42.

Fig. 8 Effect of sintering temperature on sound-absorption property of specimens

3.3 Effect of sintering time on microstructure and properties of sound-absorbing material

The effect of the sintering time on the structure and properties of the steel slag porous sound-absorbing material was studied under conditions of sintering temperature of 1100 °C, 50% fly ash, and 3.0 g waste EPS. Table 4 shows that as the sintering time increased, the density of the specimens increased from 1.35 g/cm³ to 1.42 g/cm³ and the apparent porosity of the specimen decreased by 4.1%. The compressive strength of specimens increased correspondingly from 2.54 MPa to 3.51 MPa. When the sintering time exceeded 7.5 h, the changes of the density, apparent porosity, and compressive strength diminished.

Table 4 Effect of sintering time on density, apparent porosity, and compressive strength of specimens

Fig. 9 SEM morphologies and distribution of pores in specimens at different sintering time

The SEM images shown in Fig. 9 indicate that the specimen sintered for 4.5 h (Fig. 9(a)) had much higher porosity than that sintered for 9.0 h (Fig. 9(b)); its pores were larger than those of the specimens sintered for 9.0 h, which were fine, uniform, and had an irregular shape. Figures 9(c) and (d) show that the sintering time impacted the morphology of the specimens to some degree, the specimen with the short sintering time (4.5 h) had many crystal phases with visible crystal boundaries. The phases appeared as stripes and sheets and had different sizes. The pore size was bigger and the size of the sintering neck increased, but the specimen had a low level of densification. The crystal phases grew continuously with increasing sintering time (9.0 h), and the boundaries of some crystal phases diminished. More glassy materials formed. The sintering neck grew continually and the pore size decreased. Figure 10 shows the XRD patterns of the specimens sintered for 4.5 and 9.0 h. The main mineral phases in the specimen sintered for 4.5 h included gehlenite, kirschsteinite, merwinite, a small amount of magnesium iron oxide, and C₂S. With the extension of sintering time, the diffraction peaks of gehlenite and kirschsteinite were enhanced and the diffraction peak of magnesium iron oxide decreased, but the diffraction peak of merwinite and C₂S disappeared. Thus, increasing the sintering time promoted the formation of a mineral phase and completed the sintering reaction. So, the densification of the porous sound-absorbing materials was improved. A few closed pores were formed and the apparent porosity decreased slightly. The density of the specimens also increased and the compressive strength of the specimens was enhanced accordingly.

Fig. 10 XRD patterns of specimens at different sintering time

It is shown from Fig. 11 that the sound-absorption coefficients of the specimens are between 0.05 and 0.65 at high frequencies and between 0.40 and 0.60 at low frequencies. The sound-absorption performance was therefore increased with increasing sintering time. The NRC of the specimens sintered for 4.5 and 9.0 h improved from 0.36 to 0.41. The increase of sintering time had little effect on the density and porosity of the specimens but significantly affected the pore morphology. It is seen from Figs. 9(c) and (d) that the pore size decreased, the pore became more rough and irregular. The main influence factor of sintering time on the sound-absorption performance was similar to that of sintering temperature, and the sound-absorption coefficient of porous materials increased with the decrease aperture. Moreover, irregular pore and rough pore wall also had effect on the sound absorbing performance. The research by YU et al [43] showed that the sound-absorption coefficient of polygonal holes was higher than that of the circular hole at a frequency of 1.6 kHz with the same pore size and thickness of the sample. The specimen after sintering for 7.5 h had irregular, small, and uniform pores with many complicated pore channels. So, its sound-absorption performance was better and its NRC was 0.41.

Fig. 11 Effect of sintering time on sound-absorption property of specimens

3.4 Effect of porosity regulation on microstructure and properties of sound-absorbing materials

The influence of the porosity regulation method on the properties of the steel slag porous sound-absorbing material was investigated under conditions of sintering temperature of 1100 °C, sintering time of 7.5 h, and 50% fly ash. Certain generalizations can be derived from the data in Table 5. With the increase in the amount of waste EPS added, the density of the specimen decreased from 1.38 to 1.31 g/cm³, and the apparent porosity increased by 9.5%; however, the density and porosity did not significantly change when the amount of waste EPS added exceeded 3.6 g. The compressive strength of the specimens decreased from 3.59 to 2.06 MPa.

Table 5 Effect of addition of EPS on density, porosity and compressive strength of specimens

The SEM images in Fig. 12 show that the porosity of the specimens obviously increased with increasing amount of waste EPS. The specimen with 3.6 g waste EPS had better developed and better connected pores with non-uniform apertures and irregular shapes. Thus, the addition of EPS significantly improved the porosity and allowed the pores to connect. Although density of the specimen would decrease slightly with the increase of porosity, the compressive strength of the specimen would obviously decrease.

Fig. 12 SEM images showing effect of waste EPS addition on density, porosity, and compressive strength of specimens

Figure 13 shows the experimental results of porosity regulation. The sound-absorption properties of the specimens prepared using waste EPS as a pore former at low frequencies were better than those at high frequencies; the sound-absorption coefficients were 0.10-0.85 and 0.35-0.65, respectively. The absorption coefficient was thus improved with increasing amount of waste EPS and the NRC was between 0.40 and 0.48. However, the sound-absorption properties of the specimens were not obviously improved when the amount of waste EPS exceeded 3.6 g. With increasing waste EPS, the porosity changed very obviously, which had an important influence on the sound-absorption properties of materials. Some previous studies [44,45] reported that it was adverse to improve the sound- absorbing performance that the material had too high or too little porosity; the porosity should have an optimal value. In a certain range, the sound-absorption performance of the sample with high porosity is much better than that of low porosity, and the sound absorption coefficient of the peak moves to the low frequency. Similar study results were obtained in Refs. [46-49]. Increasing the porosity, diffuse reflection or refraction is increased and the vibration of pore wall is enhanced after sound waves enter into the porous material, and the sound waves are attenuated; at the same time, the micro-pores and cracks cause friction and air viscous consumption to increase and the sound absorption performance of the porous materials is improved. Therefore, increasing the amount of waste EPS not only improved the porosity of the specimen, but also enabled the pores to connect and improved the sound-absorption properties of the porous materials at low frequencies, so the NRC could reach about 0.47.

Fig. 13 Effect of addition of EPS on sound-absorption properties of specimens

4 Conclusions

1) Apparent porosity and the NRC of the steel slag porous sound-absorbing material prepared by this method could reach more than 50.0% and 0.47, respectively. The compressive strength of the sintered specimens was above 3.0 MPa.

2) The sound-absorption performance of the materials initially increased and then decreased by increasing fly ash content and sintering temperature, and was improved by extending the sintering time which caused the pore structure of the specimens to change. By adding waste EPS, the apparent porosity and pore connectivity of the specimens were improved, and the sound-absorption performance at low frequencies increased.

3) The optimum preparation conditions for steel slag porous sound-absorbing material were 50% fly ash, 3.6 g waste EPS, sintering temperature 1100 °C, and sintering time 7.5 h, which were determined by analyzing the properties of the sound-absorbing material. The utilization percent of the steel slag could reach higher than 65.0% by this method.

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利用钢渣烧结制备多孔吸声材料

孙 朋，郭占成

北京科技大学 钢铁冶金新技术国家重点实验室，北京 100083

摘  要：以废聚苯乙烯颗粒为造孔剂制备钢渣多孔吸声材料，研究粉煤灰添加量、烧结温度、烧结时间和造孔剂掺加量对钢渣多孔材料性能的影响。实验结果显示，该方法制备的多孔吸声材料的显气孔率达到50.0%以上；材料抗压强度和平均吸声系数分别达到3.0 MPa和0.47以上。综合考虑材料性能、能源消耗和生产成本，较优的制备条件如下：粉煤灰添加量50%、废聚苯乙烯颗粒质量3.6 g、烧结温度1100 °C和烧结时间7.5 h。

关键词：钢渣；多孔吸声材料；降噪系数；孔隙率；抗压强度

(Edited by Wei-ping CHEN)

Foundation item: Project (2011AA06A105) supported by the National High-tech Research and Development Program of China

Corresponding author: Zhan-cheng GUO; Tel/Fax: +86-10-82375042; E-mail: zcguo@metall.ustb.edu.cn

DOI: 10.1016/S1003-6326(15)63865-1