Fabrication and performance analysis of sustainable municipal solid waste incineration fly ash alkali-activated acoustic barriers

Peng Dong; Hongyan Yuan; Quan Wang

doi:10.1515/rams-2023-0340

Article Open Access

Fabrication and performance analysis of sustainable municipal solid waste incineration fly ash alkali-activated acoustic barriers

Peng Dong , Hongyan Yuan and Quan Wang

Published/Copyright: July 26, 2023

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From the journal REVIEWS ON ADVANCED MATERIALS SCIENCE Volume 62 Issue 1

Abstract

The recycling of hazardous municipal solid waste incineration fly ash (MSWIFA) is drawing more attention, in which the alkali-activation technique may provide great potential to make full use of it as sustainable acoustic materials. The present work evaluated the applicability of alkali-activated MSWIFA-based materials (AAFMs) as porous acoustic barriers. The chemical composition and microtomography of the as-prepared AAFMs were characterized by X-ray diffraction. Fourier-transform infrared spectroscopy, and scanning electron microscopy. With the incorporation of MSWIFA and foaming agents, the dry bulk density and porosity of AAFMs were subsequently examined. Moreover, the compressibility and leachability of AAFMs were also investigated to evaluate their mechanical performance and environmental safety as construction materials. A sound absorption test was eventually conducted to explore the sound absorption performance of AAFMs, considering the main factors such as aluminum addition, MSWIFA dosage, and sample thickness. The results verified the good chemical stability, leachability, and sound absorption performance of porous AAFMs. Specifically, it indicated that the aforementioned factors have a boosting effect on forming highly porous structures that improve sound absorption performance, namely sound absorption coefficient and noise reduction coefficient.

Keywords: noise pollution; MSWIFA; alkali activation; foaming aluminum agent; sound absorption performance

1 Introduction

Municipal solid waste (MSW) is gaining extensive attention because of its exorbitant annual production (billions of tons) [1,2,3]. The heavy burdens of sharply increased waste production have prompted the development of waste treatment technologies [4,5,6] such as landfilling [7] and biodegradation. Therein, incineration has steadily acquired favor as a waste disposal promotion technique in various regions, with approximately 1,179 incineration plants with power generation running around the world by 2015 [8]. As a preferred waste-to-energy technique, the incineration technique can drastically decrease the MSW volume by 90% and simultaneously establish a more sustainable waste management regime [9]. Two typical incineration residuals are primarily generated from a waste incineration process – municipal solid waste incineration fly ash (MSWIFA) and municipal solid waste incineration bottom ash (MSWIBA) [10]. These MSWI ashes are reported to contain a certain amount of pozzolanic minerals, i.e., aluminosilicate, demonstrating their viability as supplementary cementitious materials. As a result, some attempts have been conducted to recycle the MSWI ashes into sustainable construction materials, aiming to achieve high performance and low carbon emission [11,12].

Although the recycling of these residuals shows some promising effects, the conversation and usage of these incineration by-products are still inefficient and cautious. Due to the high contents of dioxin, furan, chlorides, and heavy metals, MSWIFA and MSWIBA are categorized as hazardous waste with environmental risk. The concerns regarding recycling MSWIFA and MSWIBA may also be summarized as low strength, unpredictable durability, and long-term environmental safety [13]. Specifically, there are two major reasons that account for the poor mechanical performance of MSIWA-sourced materials: First, it contains low proportions of active pozzolanic minerals, i.e., CaO, Al₂O₃, and SiO₂, which rarely contribute to the formation of amorphous phases, resulting in uncompacted structures [13,14]. The loose and porous structures of MSWI ash-made samples are also caused by hydrogen release [15,16] during the fabrication process. Because MSW only goes through a brief combustion cycle in the incinerator, around 8–10 s, part of the aluminum in the MSW can be retained in the form of metallic elements. Additionally, the rapid production of dense oxide film on the surface of aluminum may also hinder the further oxidation reaction of Al, thus reacting with the alkali phase in the paste and producing porous and loose structures [17]. Consequently, such aluminum-containing wastes show more potential to be converted into porous cement materials by making full use of inherent aluminum rather than using complicated pre-treatments to remove it, such as thermal process, particle sieving, and water washing.

Some attempts have been undertaken to fabricate porous cementitious materials by employing MSWI ashes balancing pore structure, mechanical performance, and leaching behavior. For instance, the expansion effect of alkali-activated MSWIFA-based pastes has been investigated, where the volume expansion ratio increased from 9% to roughly 13.6% with a rise of MSWIFA contents from 50 to 90% [18]. In a slag-based alkali-activated system, the impact of MSWIBA particle size fractions has also been explored, revealing that the smaller particle size of MSWIBA may contribute to the increased porosity. Under 8% of alkalinity, the MSWIBA-aerated geopolymer shows a dry density of 676 kg·m⁻³ with more than 1 MPa of compressive strength [19]. MSWIBA may also have approximately 1/250 gas generation of aluminum powder when used as the gas-foaming agent in a lightweight aerated metakaolin-based geopolymer system, with enhanced strength and thermal conductivity [16].

As a promising potential application of porous cementitious materials, it is considered a feasible solution to noise pollution due to their good sound absorption performance and low initial construction and maintenance costs [20,21]. Currently, construction noise [22], traffic noise [23], and industrial noise [24], mostly in the low-frequency bands of 200–500 Hz, are among the most ubiquitous and readily accessible types of growing urban noise. Nevertheless, noise pollution is generally ignored by traffic/construction regulations, laws, and planning authorities [25]. The sound absorption performance of acoustic cementitious materials mainly depends on their porosity, thickness, additive content, aggregate size [26], and gradations [27]. It is proved that the porosity of concrete is the main contributing factor to the sound absorption performance of cementitious materials attributing to a large part of sound energy dissipation as it passes through a large number of pores or apertures [28,29]. Typically, when porous materials are subjected to incident sound waves, some portions of these sound waves become reflected, whereas the other parts transmit through interconnected pores. Finally, the energy is released in the form of thermal energy due to frictional and viscous losses. The addition of foaming agents, such as aluminum [30], fibers [31,32], rubber [33,34], and polymers, introduces more open voids in acoustic cementitious materials, leading to a high level of porosity ranging from 26.1 to 66.7% with high tortuosity and better acoustic performance. As a result of the above research, the generation of more interconnected pores with adequate tortuosity is the critical challenge for boosting the sound absorption performance of cement-based materials.

Despite the fact that these studies have demonstrated the feasibility and fabrication of MSWIA-based porous cement, their sound absorption properties have been rarely investigated by qualified acoustic measurement techniques. It is obvious that there are two main obvious advantages to explore MSWI ash-based acoustic barriers. On the one hand, its inherent aluminum may react in the alkaline cementitious paste, boosting porous structures due to higher hydrogen release. And the generated aluminum ion may also contribute to the pozzolanic reactions and benefit the strength, producing more structural gels, such as C(N)–A–S–H gels. On the other hand, it would be inspiring if the hazardous MSWI ashes are converted into effective acoustic barriers in dealing with noise pollution. Such conversions may help reduce both environmental risks in a mutually beneficial way. In this way, it is necessary and valuable to explore the feasibility and fabricate high-performance MSWIA-based acoustic barriers.

The purpose of this study was to explore the fabrication processes and sound absorption properties of porous alkali-activated MSWIFA-based materials (AAFMs), with the goal of recycling hazardous MSWIFA into environmentally friendly, acoustic-performing construction materials. Inherent aluminum and additional aluminum powder in MSWIFA boost gas generation in an alkali system, resulting in high porosity. The AAFMs were synthesized using a gradient content of MSWIFA and additional aluminum, and the physical and chemical characteristics (bulk density, morphology, mineral composition) were investigated. To evaluate the mechanical properties, compression tests were performed to assess the loading capacity working as building materials. For the sake of environmental safety, heavy metal leaching tests were also carried out to imitate the solidification performances of AAFMs in diverse ecological situations. The acoustic properties of AAFMs, such as sound absorption coefficient (SAC) and noise reduction coefficient (NRC), were eventually examined by taking into account the influences of MSWIFA content, additional aluminum dosage, and sample thickness.

2 Methods

2.1 Materials

In the designed alkali-activated system, the predominant binders generally include MSWIFA, coal fly ash (CFA), and ground-granulated blast furnace slag (GGBS). Therein, MSWIFA was derived at an incineration plant in Shenzhen, Guangdong Province, China, where the MSWIFA processing mostly consists of trash sorting, calcium hydroxide spraying, combustion, filtering, and packing [35]. The chemical composition of MSWIFA was determined by X-ray fluorescence spectrometry (Table 1). The MSWIFA particle size (calculated by a laser particle sizer; HELOS-RODOS, Sympatec Gmbh, Germany) ranged from 1.10 to 82.26 μm with an average size of 8.36 μm (Figure 1). The low calcium CFA (class F) was sourced from a coal power plant (Wuhan, China), where the powder was collected after the combustion of pulverized coal. GGBS was also introduced in binder systems as a crucial aluminosilicate source to boost polymerization, which is acquired from Wuxin Mater. LLC. The Al/Si ratios of CFA and GGBS were 0.97 and 0.58, respectively. As the principal foaming admixture, high-purity aluminum powder (supplied by HANGYU Chem. LLC, China) was added with a purity of 99.9% for spherical aluminum powder and an average size of 200 μm.

Table 1

Chemical composition of MSWIFA (wt%)

Composition	Weight	Composition	Weight
CaO	49.275	ZnO	0.4173
Cl	19.303	P₂O₅	0.2692
Na₂O	9.678	TiO₂	0.1637
SO₃	8.247	Br	0.0946
K₂O	7.301	PbO	0.0937
SiO₂	2.263	CuO	0.0405
MgO	1.118	MnO	0.0374
Fe₂O₃	0.782	SrO	0.0284
Al₂O₃	0.645	BaO	0.0216

Figure 1

Particle size distribution of MSWIFA.

The activators for AAFMs were adapted as the combination of the analytically pure sodium hydroxide (NaOH, 99 wt%) and sodium metasilicate (Na₂SiO₃·5H₂O, 99 wt%). Both chemicals were provided by ZHIYUAN Chem. LLC, Tianjin, China. Deionized water was utilized to avoid excessive interfering ions in water during the sample preparation.

2.2 Mixing design

The AAFM mixtures were designed using different combinations of procurers and alkali activators in this system, as illustrated in Table 2. A total of four mixing schemes (AAFM-M0, M10, M30, M50) were proposed with different MSWIFA dosages (0–50 wt%) aiming to explore the effect of MSWIFA dosage on the acoustic performance of AAFMs. Similarly, a series of three mixing schemes (AAFM-A01, A006, A002) were set with different foaming aluminum contents (0.1, 0.06, and 0.02 wt% of total binder contents, respectively) to explore the impact of aluminum powder dosage on the acoustic performance of AAFMs. The alkali activators were prepared by adjusting the relative amounts of sodium hydroxide, sodium metasilicate, and deionized water. The alkali solutions for all mixtures had the same Na/Si molar ratio of 0.86 with constant alkalinity of 7%. Furthermore, the water/solid (W/S) ratio remained constant (0.37) in all samples.

Table 2

Mixing proportions of AAFMs (mass ratio)

No.	Binder			Alkali activators		Additional aluminum
	MSWIFA	CFA	Slag	NaOH	Na₂SiO₃·5H₂O
AAFM-M0	0	0.5	0.5	0.0127	0.2060	0
AAFM-M10	0.1	0.4	0.5	0.0127	0.2060	0
AAFM-M30	0.2	0.3	0.5	0.0127	0.2060	0
AAFM-M50	0.5	0	0.5	0.0127	0.2060	0
AAFM-A01	0.1	0.4	0.5	0.0127	0.2060	0.1
AAFM-A006	0.1	0.4	0.5	0.0127	0.2060	0.06
AAFM-A002	0.1	0.4	0.5	0.0127	0.2060	0.02

The Si/Na molar ratio in the alkali activators was 0.86. The water/binder ratio in the AAFM pastes was 0.37. The alkalinity of AAFMs was 7%.

2.3 AAFM preparation

The fabrication route of porous AAFM specimens is illustrated in Figure 2. The solid precursors and corresponding solid alkaline pellets (NaOH + Na₂SiO₃·5H₂O) were added to a mixer and dry-mixed for nearly 2 min to obtain a homogeneous solid mixture. Afterward, deionized water was added to the aluminum powder and stirred to obtain a suspension, which was subsequently poured into the solid mixture and mixed at a slow speed for 30 s, then at a high speed for 1 min, and finally at a slow speed for 30 s. Afterward, the as-obtained paste was cast in a mold and vibrated to eliminate any bubbles. The diameter of the mold was 120 mm; however, its thickness varied. After demolding, the dense sample was finally cured under laboratory conditions for 7 and 28 days.

Figure 2

The preparation route of porous AAFMs.

2.4 Experimental tests

2.4.1 Porosity and bulk density

The porosity of AAFMs was evaluated by the underwater gravity method, and their dry bulk density was quantified by the oven drying technique. First, the specimens were placed in an oven at 60°C and weighed every 3 h to a tolerance of 0.1 g between two measured values. These samples were then submerged in water for 48 h to achieve saturation. After draining and cleaning their surfaces, the specimens were measured in the air, with the saturated mass of each sample weighed three times to avoid inadvertent error. The porosity of AAFMs was determined as

(1) ρ = M sat − M dry ρ w ⋅ V bulk ,

where ρ is the porosity of AAFMs, M sat is the saturated mass of AAFMs, M dry is the dry mass of AAFMs, ρ w is the density of water, and V bulk is the bulk volume of the AAFM specimens. The bulk density of AAFMs was calculated as

(2) ρ db = M dry V bulk ,

where ρ db is the bulk density of AAFMs.

2.4.2 Compression tests

A compression test is conducted referring to the ASTM Standard (ASTM C109, 2012 [36]) with sample dimensions of 50 mm × 50 mm × 50 mm after being cured at 7 and 28 days. Three cubic specimens were tested for each test, with the average value adapted to avoid accidental inaccuracy. Thus, a multi-function mechanical testing apparatus (WANCE ETM-104B, China) was used at a loading rate of 50 N·s⁻¹ throughout all tests.

2.4.3 Microtopography

The microtopographies of AAFMs were revealed using a Zeiss Merlin scanning electron microscope (SEM) at an acceleration voltage of 5 keV and a current of 5 mA. To improve the imaging quality, the crushed samples were thoroughly heated at 60°C in an oven and then coated with Pt by a coater.

2.4.4 Mineralogy

A high-resolution powered X-ray diffractometer (XRD; Bruker D8 Powder) was used to illustrate the phase composition of the AAFM samples. The samples were first crushed into powders and sieved through 45 mm-sized sieves before being equally paved on slides for the XRD analysis. Herein, the diffraction angle range was set from 6.25° and 90° with a 0.02° increment.

2.4.5 Fourier transform infrared spectroscopy (FTIR)

FTIR was performed on a Perkin Elmer Frontier spectrometer. In the attenuated total reflection mode, the spectrum resolution was set to 1 cm⁻¹ with a scanning cycle of 20, and FTIR spectra were collected in a wavenumber range of 2,000–400 cm⁻¹.

2.4.6 Leachability

The leaching performances of the samples were assessed by the toxicity characteristic leaching procedure (TCLP) according to the Chinese Standard HJ/T 557-2010. The weighed samples (<5 mm) were initially dissolved into 5% glacial acetic acid (solid/liquid ratio = 1:20), horizontally oscillated for 8 h, and finally settled for another 16 h. The leachate was collected and further analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES; Perkin Elmer) after being filtered with a 0.45 μm filter paper. Special attention was paid to the leaching performances of typical heavy metals, such as Zn, Cr, Pb, Ni, and Cd. The leachability of the samples was further determined according to the GB 16889-2008 standard.

2.4.7 Acoustic properties

The normal incidence acoustic properties of geopolymers were characterized by the two-microphone transfer-function method according to the ASTM E1050-12-2012 [37] standard using an impedance tube (Brüel & Kjær 4206, Denmark). As illustrated in Figure 3(a), a loudspeaker was installed at one end of the impedance tube as a sound source. When the test starts, broadband waves (frequency range = 50–2,000 Hz) propagated along the tube and struck the sample fixed at the opposite end of the tube (Figure 3(b)). A standing-wave interference pattern was then super-positioned due to the propagation, contact, and reflection of deuterogenic waves. The wave information was then recorded by two microphones positioned at a certain location and transformed into a complex pattern using a two-channel digital frequency analyzer. Subsequently, the parameters such as the NRC and SAC ( α ) can be calculated as follows:

(3) NRC = ( α 250 + α 500 + α 1 , 000 + α 2,000 ) 4 ,

and

(4) α = 1 − | γ 2 | ,

where γ is the measured reflection coefficient; and α ₂₅₀, α ₅₀₀, α _1,000, and α _2,000 are the SACs at 250, 500, 1,500, and 2,000 Hz, respectively.

Figure 3

Impedance tube: (a) Brüel & Kjær 4206 and (b) working schematic.

3 Results

3.1 Bulk density and porosity

The dry bulk density and porosity of AAFMs are presented in Figure 4. It is noticeable that the porosity of AAFMs was relatively proportional to the MSWIFA dosage in the binder and the amount of additional foaming aluminum. Herein, the AAFM-A01 sample had the lowest bulk density of ∼1.11 g·cm⁻³ and the highest porosity of 43.2%. The porosity of AAFMs decreased with the addition of foaming aluminum; thus, the porosity of AAFM-A002 decreased to ∼35.36%. Similarly, the higher the MSWIFA dosage in the binder, the better the porosity of AAFMs; hence, AAFM-M50 had higher porosity (∼34.1%) than AAFM-M0. Furthermore, the result of bulk density also verifies the fact that the addition of both MSWIFA and foaming aluminum greatly decreased the compactness of AAFMs. The decreased compactness of AAFMs is mainly due to the promotion of gas-generation reaction in the alkali system, from which a growing amount of metallic aluminum sourced from underburned MSWIFA may react with the alkali [18,19]. The generated bubbles during paste preparation may directly lead to the high porosity of hardened AAFMs [17,38], further causing a decent bulk density.

Figure 4

Bulk density and porosity of AAFMs.

3.2 Compressive strength

Figure 5 presents the compressive strengths of AAFMs after 7 and 28 days of curing. It is evident that the compressive strength of AAFMs (AAFM-M0, M10, M30, M50) dropped with the increasing MSWIFA dosage. The maximum compressive strength of ∼41.2 MPa was obtained when no MSWIFA was incorporated into the binder (sample AAFM-M0). After the incorporation of 10 wt% (sample AAFM-M10) and 50 wt% (sample AAFM-M50) of MSWIFA, the maximum compressive strength decreased to ∼22.96 and ∼6.76 MPa, respectively, indicating that even a modest MSWIFA dosage had a significant impact on the compressive strength of AAFMs. It can be speculated that as the MSWIFA dosage increased, more aluminum in MSWIFA promoted the gas generation reaction in the alkali system. The release of hydrogen generated more cellular AAFMs with lower bulk density and strength.

Figure 5

Compressive strength of AAFMs.

Likewise, the quantity of additional foaming aluminum may have an inverse effect on the compressive strength of AAFMs. Compared with AAFM-A002 cast by a less foaming agent (0.2 wt%), the incorporation of 0.1 wt% aluminum decreased the compressive strength of AAFMs from 10.95 to 5.54 MPa after 28 days of curing. In this way, the compressive strength of AAFMs was inversely proportional to the contents of both MSWIFA and the additional foaming aluminum.

3.3 Mineral analysis

Figure 6 reveals the mineral compositions of AAFMs with different MSWIFA dosages. The principal crystal phases of AAFMs within each group were almost identical, including calcite, portlandite, quartz, hydrocalumite, ettringite, and NaCl. In contrast to the sample without MSWIFA (AAFM-M0), new crystals of NaCl and KCl appeared in the samples with MSWIFA (Figure 6(a)). Furthermore, the peak of calcium carbonate (CaCO₃) increased due to the carbonation of calcium hydroxide (Ca(OH)₂) during the sample preparation. Moreover, a typical hump from 20° to 36° was noticed because the peak intensity of amorphous N(C)–A–S–H gel gradually diminished with the increasing MSWIFA content. As MSWIFA had lower active aluminosilicate sites than CFA, fewer N(C)–A–S–H gels were generated during geopolymerization [13,39], leading to falling humps. The mineral analysis reveals the relatively steady chemical compositions of AAFMs in different groups.

Figure 6

Chemical compositions of MSWIFA and AAFMs.

3.4 Microtopography

The morphologies of AAFMs and MSWIFA are displayed in Figure 7. It is noticed that MSWIFA presents an irregular shape and a loose structure due to internal impurities. A limited number of needle-like, flake-shaped, and fluffy crystals were detected in AAFMs [10,13,40] (Figure 7(b) and (c)). Therein, the needle-like crystals were further recognized as ettringite, which was also verified in the study of Liu et al. [41]. The presence of ettringite, as seen in Figure 7(b), induced fractures due to its volume expansion and deteriorated the compactness and strength of AAFMs. Moreover, amorphous N(C)–A–S–H gels with a dense structure and a smooth appearance were also detected (Figure 7(d)), contributing to the strength and compactness of AAFMs.

Figure 7

Microtomographies of MSWIFA and AAFMs: (a) MSWIFA, (b) fluffy crystals, (c) needle-like crystals and flakes, and (d) N–(C)–A–S–H gel.

3.5 FTIR spectra

The FTIR spectra of MSWIFA and synthesized AAFMs are exhibited in Figure 8. According to Figure 8, the geopolymers (AAFM-M0, AAFM-M10, and AAFM-M50) exhibited comparable spectral patterns, indicating that the reaction products were stable despite different MSWIFA fractions. The absorption peaks at 1,637 and 1,410 cm⁻¹ could be assigned to the stretching of O–H bonds in free water and the stretching vibration of C–O bonds in CaCO₃ [41]. Additionally, asymmetric stretching vibrations of Si–O–X may be responsible for these bands at around 952 cm⁻¹, where X is tetrahedral Si or Al [18]. Therein, the higher transmittance in AAFMs than in raw MSWIFA implies that more N–(C)–A–S–H gels were synthesized after polymerization [42]. These spectral results are consistent with the XRD patterns, which clearly detected humps of N(C)–A–S–H gel, as presented in Figure 6.

Figure 8

FTIR spectra of MSWIFA and AAFMs.

3.6 Leaching performance

Table 3 presents the toxicity-leaching performances of raw MSWIFA and AAFMs. The concentration of hazardous heavy metals in AAFMs was lower than that in MSWIFA. Moreover, the AAFM samples exhibited a stronger solidification effect in comparison to raw MSWIFA, implying that alkali activation is an effective strategy for heavy metal solidification [43]. The improved solidification efficiency of alkali-activated technologies can expand the possible applications of AAFMs and lower carbon emissions and the economic cost of structural materials.

Table 3

Heavy metal concentrations in TCLP leachates after 28 days

	TCLP (mg·L⁻¹)
	Zn	Pb	Cu	Ba	Cr	Cd
GB16889-2008	100	0.25	40	25	4.5	0.15
AAFM-M0	0.071	0.027	0.045	1.814	0.037	0.002
AAFM-M10	0.504	0.198	0.042	1.511	0.044	0.009
AAFM-M30	2.543	0.512	0.070	0.778	0.069	0.011
AAFM-M50	8.004	0.985	0.195	0.872	0.058	0.132
MSWIFA	13.179	1.659	1.164	1.108	0.105	0.808

In particular, the concentrations of Zn, Cu, Ba, and Cr in AAFMs and MSWIFA were lower than the values specified in the Chinese Standard [44] standard. In contrast, the concentration of Pb in AAFM-M10 (0.198 mg·L⁻¹) totally complied with the standard value, whereas the concentration of Pd in AAFM-M50 (0.25 mg·L⁻¹) surpassed the limit. The toxicity leaching performance indicates that AAFM-M10 can be used as an ideal sustainable construction material.

3.7 Sound performance

3.7.1 Effects of foaming aluminum dosage

The effects of foaming aluminum dosage on the sound absorption performance of AAFMs are illustrated in Figure 9. It is noticeable from Figure 9 that the SACs of the porous AAFMs sample with 50 mm thickness exhibited first a pattern of growing, then dropped, and finally, was steady. Specifically, the AAFM-A01 sample had a superior SAC at low frequencies (200–500 Hz), with a peak of roughly 0.782 at 286 Hz, indicating that it exhibited an outstanding sound absorption capacity for low-frequency noises. On the contrary, AAFM-A01 displayed poor sound absorption performance in mid-frequency (500–1,000 Hz) and high-frequency (1,000–2,000 Hz) ranges. When the additional foaming aluminum dosage dropped to 0, the peaks noticeably shifted to lower intensities and higher frequencies. Similarly, the NRC of AAFMs was proportional to the foaming aluminum dosage (Figure 10(a)). It was found that the NRC of AAFM-A01 was ∼0.253, and the value decreased to 0.184 for AAFM-A002. Similar trends were also noticed in the samples with 80 and 120 mm thicknesses.

Figure 9

Sound absorption performance of proposed AAFMs: SACs of AAFMs with thicknesses of (a) 50 mm, (b) 80 mm, and (c) 120 mm. NRCs of AAFMs with thicknesses of (d) 50 mm, (e) 80 mm, and (f) 120 mm.

Figure 10

Effect of MSWIFA contents on the acoustic performance of AAFMs: (a) SAC and (b) NRC.

The aforesaid peak shift could be attributed to the improved porosity of highly connected and zigzag cellular structures [45], which hindered the propagation of sound waves because more sound energy was consumed due to the diffuse refraction of sound waves in pores [46]. In addition, more interfaces between the solid and inner pores in the bulks may boost the direct reflection of sound waves and thus enhance the sound absorption performance of AAFMs [30]. The continuous accumulation of pores created a network of interconnected pores, resulting in a rugged and complicated porous structure within AAFMs. Hence, it became more difficult for sound waves to refract and reflect through this porous network, which undoubtedly consumed the energy of sound waves and dissipated more energy as heat [47]. The enhanced acoustic energy dissipation caused by the altered porous structure optimizes the sound absorption performance of the AAFM samples.

3.7.2 Effects of sample thickness

Sample thickness also affected the sound absorption performance of AAFMs. In comparison to the AAFM-A01 sample with 50 mm thickness (Figure 9(a)), the thicker AAFM-A01 specimen (with 120 mm thickness) achieved the highest SAC value of 0.873 at around 141 Hz (Figure 9(c)), indicating that the SACs of AAFMs grow with the increasing sample thickness. An obvious trend is also tracked, in which the dominant sound absorption frequency shifted to the low-frequency range with the increase in sample thickness. For instance, the dominant sound absorption frequency of AAFM-A002 shifted from 376 to 242 Hz as the thickness increased from 50 to 120 mm. It might happen because the change in the sample thickness had different effects on the absorption and reflection of sound waves at different wavelengths [48]. It is worth mentioning that some extra dominant sound absorption peaks may be also observed among samples with increased sample thicknesses [49]; however, this phenomenon was rarely tracked in this research. The presence of extra SAC peaks further verifies the change in the reflection and absorption properties of sound waves during their propagation through thicker AAFM samples. Similarly, the thicker AAFM samples had higher NRCs (Figure 9(d)–(f)), indicating their better sound absorption performance. Consequently, the effect of sample thickness on NRCs was consistent with that on SACs.

3.7.3 Effects of MSWIFA dosage

Figure 10 depicts the effects of MSWIFA dosage on the acoustic performance of AAFM samples with 50 mm thickness. All specimens exhibited a similar SAC pattern, decreasing from the low- to the high-frequency range (Figure 10(a)). Furthermore, the higher the MSWIFA content, the greater the SAC. For example, the AAFM sample containing 50 wt% MSWIFA had the highest SAC of 0.541. Peak SACs of AAFMs demonstrated a general reduction and shifted from the low- to the high-frequency range with the decrease in MSWIFA dosage. It was because more MSWIFA generated more pores in AAFMs and resulted in better sound absorption performance. The cellular structure of AAFMs with high MSWIFA contents was driven by unburned aluminum-alkali agent reactions, resulting in decreased bulk density and a large number of pores (Figure 4). The effect of MSWIFA dosage on NRC was also consistent with that on SRC (Figure 10(b)). The higher the MSWIFA content, the greater the NRC of AAFMs, demonstrating better sound absorption performance. However, the NRCs of AAFMs with high MSWIFA dosage contents were lower than those of the sample with low foaming aluminum contents (Figure 9(a)), indicating that the performance enhancement is limited when compared to the use of foaming aluminum. In conclusion, the use of MSWIFA in binders improves the sound absorption capacity of the produced AAFMs owing to their cellular structures, although at low efficiency.

4 Conclusions

In this work, novel AAFMs have been fabricated and used as sound absorbers, with good mechanical performance and environmental safety. The acoustic performance of AAFMs was enhanced by incorporating additional foaming aluminum and more MSWIFA in the binder. The sound absorption performance of AAFMs was investigated by considering the effects of foaming aluminum dosage, MSWIFA content, and sample thickness. The main observations of this work are as follows:

MSWIFA was transferred as a reliable and sustainable construction material by the alkali-activation method. The addition of 10 wt.% MSWIFA in the binder resulted in promising heavy metal solidification and relatively good compressive strength.
The introduction of additional foaming aluminum greatly enhanced the sound absorption performance of AAFMs, even at low dosages. The incorporation of additional foaming aluminum increased the SAC and shifted the peak of SAC to lower frequencies because the porosity and tortuosity of AAFMs were improved.
The sound absorption performance of AAFMs was boosted with the increasing MSWIFA content. The maximum SAC of the AAFM-M50 reached around 0.541 with 50 wt% MSWIFA used.
The sound absorption performance of AAFMs was also dependent on the specimen thickness. The greater the sample thickness, the better the sound energy dissipation and the sound absorption performance.

The feasibility of MSWIFA recycling into sustainable construction materials was verified under laboratory conditions. However, further investigations under field conditions are necessary to analyze the durability and long-term leachability of AAFMs. Furthermore, the effects of different aggregates and humidity on the sound absorption performance of cementitious materials also need to be studied. The distinctive smart sensor [50] may also be useful in identifying future performance degradation of the proposed AAFMs.

Acknowledgments

The authors are grateful for the reviewers’ valuable comments that improved the manuscript.

Funding information: The authors would like to acknowledge the funding support from the Stable Support Plan Program of Shenzhen Natural Science Fund (grant no. 20200925155345003), and the Science, Technology, and Innovation Commission of Shenzhen Municipality (grant no. ZDSYS20210623092005017).
Author contributions: Peng Dong: methodology, formal analysis, and manuscript writing. Hongyan Yuan: editing, funding acquisition, and investigation. Quan Wang: conceptualization, funding acquisition, supervision, and project administration. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.
Data availability statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Received: 2023-03-17

Revised: 2023-05-23

Accepted: 2023-06-14

Published Online: 2023-07-26

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/rams-2023-0340

Keywords for this article

noise pollution; MSWIFA; alkali activation; foaming aluminum agent; sound absorption performance

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