1. Introduction
Municipal solid waste incineration fly ash (MSWIFA) is recognized as a byproduct generated during the incineration process of municipal solid waste (MSW) in waste-to-energy power plants [
1]. The incineration process can be regarded as a sustainable MSW management regime. The generation of MSWI fly ash has rapidly grown and approximately more than 5.8 million tons of MSWIFA was produced in China in 2021. Currently, the most extensively used procedure of MSWIFA disposal is landfilling, which occupies a large area of land. Moreover, MSWIFA contains a variety of soluble salts and heavy metals that are leachable. In extreme conditions, MSWIFA may contaminate soil or groundwater and cause serious environmental threats [
2]. Therefore, solidification has been developed as an efficient treatment technology for MSWIFA, and ordinary Portland cement (OPC) is commonly used for the solidification of MSWIFA. The toxic elements in MSWIFA can be effectively immobilized in a binder made with OPC by the actions of physical encapsulation and chemical fixing [
1]. For instance, arsenic (As) and lead (Pb), categorized as heavy metals, were successfully immobilized by the formation of Ca
3(AsO
4)24H
2O and Pb
3(NO
3)(OH)
5 through the reaction with hydrated Ca(OH)
2 existing in cementitious materials [
2].
However, using OPC-based solidification technology is associated with high CO
2 emissions. OPC manufacturing is responsible for a high carbon footprint (0.66–0.82 t CO
2 per tonne) and high energy consumption [
3,
4,
5]. Geopolymer has been introduced as an alternative low-carbon binder purported to OPC. The benefit of geopolymers is mainly based on their ability to bring high-volume industrial wastes into construction products, leading to a significant reduction in CO
2 emissions. Geopolymer is manufactured by activation of aluminosilicate byproducts through alkaline additives where the polymerization process can transform them into reaction products [
6]. Recently, geopolymer science has been applied to immobilize heavy metals presented in MSWIFA. It has been found that heavy metals were successfully immobilized in geopolymer binders made with various raw materials, including metakaolin [
3], red mud [
7], coal gangue [
8], and pulverized fly ash (PFA) [
9]. Among these materials, PFA (a byproduct generated during the burning of coal in power stations) is one of the most commonly used raw materials for manufacturing geopolymers. Zhan and Kirkelund (2021) investigated the solidification of MSWI fly ash by incorporating it into a PFA-based geopolymer binder. The mixture of 80% PFA and 20% MSWIFA activated with 8 M NaOH solution was found optimal under the heat curing condition (24 h at 80 °C temperature). At the age of 28 days, the mixture developed a compressive strength of 15.3 MPa and showed a similar immobilization capacity for heavy metals as compared to the OPC binder. However, the heat-cured regime can be used for the construction of precast members but is regarded as limitated for cast-in-situ applications. Moreover, the previous studies [
3,
8,
9] on MSWIFA/PFA blended geopolymers developed a strength of less than 16 MPa, which does not meet the requirement for structural applications.
Masonry mortar is an efficient bonding material to connect building blocks (including bricks and stones) and seal the gaps between them. The strength required for masonry mortar is not as high as for structural applications. The twenty-eight-day compressive strength of masonry mortar is generally in a range of 5 MPa to 30 MPa. Therefore, masonry mortar manufacturing is likely to constitute an alternative option for recycling MSWI fly ash, although technical and scientific studies on this topic have not been reported in the literature. Masonry mortar is composed of binders and natural sand that have become depleted due to overexploitation as a result of the development of civilization. In the current study, the natural sand was replaced by recycling glass (RG) at different replacement percentages. RG can be categorized as a byproduct generated by crushing mixed-colour bottles and other glass products collected from both municipal and industrial waste streams. Waste glasses are nonbiodegradable and noncombustible. Although the glass recycling rate has reached 50% in China, about 10 million/year of waste glass is still disposed of in landfills. Replacing natural sand with RG could increase the glass recycling rate and reduce the consumption of natural resources in the construction industry.
Overall, it can be concluded that a majority of the previous literature studies on alkali-activated MSWI fly ash have focused on heat-cured geopolymers whereas the engineering properties and matrix formation mechanism (including the reaction products) of ambient-cured MSWI fly ash/PFA-based geopolymer have not been investigated in detail. On the other hand, to our best knowledge, studies on using waste glass in MSWI fly ash/PFA-based geopolymer systems have not been reported in the literature. Thus, the current knowledge of alkali-activated MSWI fly ash is still limited, restricting the use of MSWI fly ash in construction materials.
This work investigates the utilization of waste glass as a partial replacement of natural sand in MSWI fly ash-blended PFA geopolymer binders for the production of masonry mortars. The main aim is to increase the recycling rate of MSWI fly ash, PFA, and waste glass, leading to the promotion of sustainable construction. The specific aims are (i) investigation of effective MSWIFA treatment for reducing volume expansion; (ii) investigation of the effects of mixing parameters on the reaction products and compressive strength (of MSWIFA/PFA geopolymers) via varying the MSWIFA dosages, alkali activators’ compositions, and the ratios of alkali activator to solid; (iii) evaluation of the properties of fresh and hardened MSWIFA/PFA geopolymer mortars with a replacement of up to 30% of natural sand by recycling glass; and (iv) study of the capacity of MSWIFA/PFA geopolymer mortars for immobilization of toxic elements.
The investigation programme was divided into two parts, and the research framework is demonstrated in
Figure 1. In the first part, the chemical and physical characteristics of pure MSWIFA, including particle size, morphology, corrosivity, and mineral composites, were characterized. A water immersion method was employed to pretreat MSWI fly ash for solving the volume expansion of MSWIFA/PFA blended geopolymers, which does not require the expensive electrodialytic equipment used in the previous study [
9]. Then, the feasibility of preparing the binder with treated MSWIFA and PFA under an ambient curing regime was explored. Next, the effect of the activator nature and MSWIFA content on the compressive strength was analysed. The second part was dedicated to the study of the fresh and hardened characteristics of MSWIFA/PFA geopolymer masonry mortar replacing various dosages of natural sand with recycling glasses. The main concern regarding using glass in cementitious materials is the probable chemical reaction between the silica-rich glass and the alkali present in the pore solution, i.e., alkali–silica reaction (ASR) [
10]. The expansion characteristics of MSWIFA/PFA geopolymer mortar bars containing RG were evaluated through the accelerated mortar bar as presented in ASTM C 1260 Standards.
2. Experimental Program
2.1. Raw Materials
Municipal solid waste incineration fly ash (MSWIFA) was supplied from a waste incineration plant in Chongqing. An ASTM class F fly ash (named PFA) provided by a Henan steam power plant was utilized as another raw material for making geopolymer. The XRD patterns and chemical compositions (detected by X-ray fluorescence, Hitachi High-Tech Analytical Science, Tokyo, Japan) of these two ashes are presented in
Figure 2 and
Table 1, respectively. It can be seen from
Table 1 that CaO and Cl are the main constituents of MSWIFA while Al
2O
3 and SiO
2 are the main constituents of PFA.
Figure 2 shows that crystalline phases present in the MSWIFA are CaClOH, NaCl, KCl, and CaCO
3. The XRD patterns of PFA indicate the reflection peaks of quartz and mullite as well as a small amount of crystalline phase with iron oxides, whose structure is similar to maghemite. As shown in
Figure 3, a laser particle size analyser was employed for measuring the particle size distribution of the materials. The average size of MSWIFA and PFA was 57.39 μm and 6.01 μm, respectively.
After cleaning, the collected green beer bottles were ground by a YX-3/100A crusher. The maximum particle size was around 0.3 mm. It is obvious from
Figure 2 and
Table 1 that the main phase of the recycling glass was amorphous silica with 63.88% SiO
2 content.
The sand was supplied from Xiamen Aisiou Standard Sand Co., Ltd. (Xiamen, China)
Figure 4 illustrates the particle size distribution of the sand, which mets requirements established by the standard of GB178-77 “Standard sand for cement strength test”.
The alkali activator solution used in the experiments was prepared by mixing sodium silicate solution with sodium hydroxide solution. The modulus of sodium silicate solution is 2.2.
2.2. Pretreatment of MSWI Fly Ash
When MSWIFA was used as a partial replacement for PFA, the MSWIFA/PFA geopolymer expanded in volume at the early stage of reaction (as shown in
Figure 5). The expansion grew with the increment of MSWIFA dosage. The same phenomenon was also observed in the alkali-activated slag system [
11]. Previous studies [
11,
12] have reported that volume instability was often associated with hydrogen produced by the reaction of metallic aluminium according to Equation (1).
According to the above equation, 1/9 g hydrogen gas is produced when 1 g Al is consumed. At ambient temperature, the volume of 1/9 g hydrogen gas is around 3.69 times higher than that of 1 g Al. Moreover, the soluble salts may lead to the deterioration of the durability of cementitious materials containing MSWIFA.
To reduce the soluble salts and metallic aluminium content, commonly used treatment processes include water washing/water immersion treatment [
11], acid-base treatment, and chemical reagent treatment [
12]. The pH of the MSWIFA leaching solution was as high as 12.06, indicating relatively strong alkalinity. According to Equation (1), a pH value of 12 is sufficient for the initiation of the reaction. Therefore, tape water was used and the immersion treatment method was adopted for removing metallic Al in MSWIFA. The treatment procedures were selected according to Ref. [
11]. Water and raw MSWIFA (RMSWIFA) were mixed with a mass ratio of 2:3, and RMSWIFA was fully immersed in water for 72 h. Then, the treated MSWIFA (TMSWIFA) was dried at 105 °C for 48 h.
2.3. Sample Preparation
The sample preparation was divided into two stages. The paste samples were prepared in the first stage with an aim to optimize the mix proportions by considering the various mixing parameters. The investigated parameters included MSWIFA dosages, ratios of sodium silicate (SS) solution to sodium hydroxide (SH) solution, concentrations of SH solution, and alkali liquid to solid (L/S) ratio. The MSWIFA dosages were 10%, 20%, 30%, and 40%. The SS/SH ratios were 1.5, 2.0, 2.5, and 3.0. The SH concentrations were 8 M, 10 M, 12 M, and 14 M. The L/S ratios were 0.45, 0.50, 0.55, and 0.60. The second stage was dedicated to preparing mortar samples to evaluate the impact of the inclusion of RG as a sand replacement (0%, 10%, 20%, 30%) on the performance of mortars. The preliminary test results of the current research have shown that the inclusion of RG has little effect on the compressive strength of samples. Therefore, RG is considered a filler rather than a cementitious material in the current study, although 80% of its particle size is less than 75 μm. The proportions of all pastes are summarized in
Table 2. The proportions of mortars will be presented in
Section 3.5.
The dry materials (MSWIFA, PFA, RG, and sand) and alkali liquid were mixed in a 40 L Hobart mixer. The dry materials were thoroughly mixed before adding alkali activators. The alkali liquid was then gradually added and mixed continuously for 4 min until a glossy and consistent mixture was reached. A vibration table was used to remove air bubbles during the casting and compaction processes. Before demolding, the samples were kept in the mould for 24 h. The specimens were then cured at ambient temperature under a controlled condition until testing. The paste and mortar were cast into 45 × 45 × 45 mm and 70.7 × 70.7 × 70.7 mm steel moulds to measure compressive strength, respectively. The block of 25 mm × 25 mm × 285 mm was used for assessing ASR potential.
2.4. Test Method
- (1)
Work performance test
Testing of setting in accordance with GB/T1346-2011 “Test methods for water requirement of normal consistency, setting time, and soundness of the Portland cement” was carried out (for paste samples) using the Vicat needle method. The initial and final setting times are reached when the needle is 3~5 mm and 0.5 mm away from the bottom plate, respectively. Mortar water retention and consistency tests were done after mixing, following JCJ/T2009 “Standard for test method of performance on masonry mortar”. The consistency of the mortar was evaluated by the flow table test. All these experiments were performed at a controlled temperature of 23 °C.
- (2)
Compressive strength test
This test was accomplished on both paste and mortar samples by a hydraulic universal testing machine. For paste samples, the strength was measured at the early age of 3 days while the strength of mortars was measured at 3, 7, and 28 days. JCJ/T2009 standard was employed to conduct this test by adopting a loading rate of 0.25 MPa/s. The average value of three identical specimens was reported as the compressive strength testing result. The calculated standard deviation values were used as error bars length in the corresponding figures. As a percentage, the coefficient of variation (for 90% of results) is less than 10%.
- (3)
Heavy metal leaching
The mortar samples cured for 28 days were tested for heavy metal leaching concentration according to HJ/T300-2007 “Solid Waste-Extraction procedure for leaching toxicity-Acetic acid buffer solution method”. The crushed particles after strength testing were collected, and the particles were then passed through an 8 mesh sieve. The particles were mixed with deionized water at a ratio of liquid–solid (L/kg) of 20:1. The suspension was then vibrated using a horizontal shaker with the operation at a speed of 30 ± 2 r/min. The shaking was conducted at 23 ± 2 °C for 18 h, and then the leachate was filtered through a 0.45 μm filter. Inductively Coupled Plasma Mass Spectrometer (ICP-MS, Perkin Elmer) was employed to measure the concentration of heavy metal ions in the leaching solution. Whether the leaching concentration exceeded the limitation was judged according to requirements presented in GB16889-2008 “Standard for pollution control on the landfill site of municipal solid waste for hazardous wastes”.
- (4)
Alkali-silicic reaction expansion
ASR expansion was assessed by the mortar bar method according to ASTM C1260. Just after casting, the moulded geopolymer mortars were cured in a standard curing room for 24 h. Demolding the samples and measuring the length of the samples as the initial length was then carried out. The change in length of mortar bars was measured after alkaline immersion (1 M NaOH solution at 80 °C) at 3, 7, 10, and 14 days. The potential of the aggregate undergoing deterioration (due to ASR) was analysed according to the GB/T 14684-2011 “Sand for construction”.
- (5)
Spectroscopic/microscopic analysis
After compressive strength test, the crushed specimens were immediately immersed in ethanol to cease the reaction. Before the analysis, the specimens were dried in an oven at 30 °C to achieve a constant weight (moisture content <1%). The spectroscopic analysis was conducted using a high-resolution X-ray diffractometer (Rigaku SmartLab, Tokyo, Japan). The fragments of crushed samples were ground to pass through a 200 mesh sieve. The XRD scan was operated over a range from 10 to 80° 2θ with tube setting to 45 KV and 200 mA at a 0.03° step size. In terms of microscopic analysis, a scanning electron with energy-dispersive X-ray spectroscopy (SEM-EDX, HITACHI S-3700N) was employed to investigate the morphology and elemental composition of the geopolymer. Small fragments of crushed samples were coated using gold and fixed on the sample holder with conductive glue.
4. Conclusions
This study proves the water immersion method is a promising way to mitigate volume instability, thus promoting an amount of MSWI fly ash to be used in construction materials. In this investigation, the influence of the MSWIFA dosage, SS/SH ratio, L/S ratio, and SH concentration on the compressive strength of MSWIFA/PFA geopolymers were systematically determined. The XRD and SEM analyses were performed to understand the strength development mechanism based on the reaction products and micromorphology. Moreover, this study also evaluates the feasibility of partially replacing natural sand with RG in geopolymers for making masonry mortars. According to the results of this study, the main conclusions are as follows:
This study analyses the feasibility of partially replacing PVA with treated MWSIFA in geopolymers for making masonry mortars. Four mortars were prepared with various sand replacement ratios using recycling glass. According to the results of the study, the main conclusions are as follows:
- (1)
MSWIFA has a high pH of 12.06 which provides sufficient OH− in the water immersion process for removing the metallic Al existing in the ash. The volume expansion of the specimens prepared with the treated MSWIFA was considerably mitigated compared to the specimens prepared with the raw MSWIFA.
- (2)
The initial setting time of mixes with RMSWIFA was much shorter than that of mixes with TMSWIFA. This is due to the fact that Ca2+ from CaClOH may react with silicate (from sodium silicate) to accelerate the reaction at an early age. The water immersion method could remove the CaClOH existing in MSWIFA.
- (3)
SEM images of the MSWIFA blended PFA geopolymer mostly illustrated an amorphous geopolymeric gel and calcium-containing hydration product. The calcium-containing hydration product filled the voids within the geopolymeric matrix, resulting in the reasonable strength development of specimens without heat curing.
- (4)
The inclusion of up to 30% fine recycled glass in masonry mortar production did not remarkably affect the mortars’ properties in the fresh and hardened states except for the ASR potential. When the RG content was higher than 20%, the expansion strain increased obviously. The expansion of the mortar with 30% RG could reach 0.028% after 14 days of alkaline immersion. This value was still lower than the limitation proposed by GB/T 14684-2011 standard.
- (5)
The MSWIFA blended PFA geopolymer mortar with an A/B ratio of 0.5, SS/SH ratio of 3, and SH concentration of 12 M reported the highest twenty-eight-day compressive strength (24.3 MPa) at ambient curing conditions.
- (6)
The concentration of leachable heavy metals of MSWIFA blended PFA geopolymer mortar significantly plummeted to less than 1%. For all curing days, including 7 and 28 days, the concentrations of all six metals were within the limitations presented in the relevant standard.
This research illustrates that using multistream wastes in masonry mortar manufacturing could be a viable alternative that would help increase the recycling rate of MSWIFA, PFA, and RG. However, the chemical compositions and physical properties of MSWIFA and PFA vary from place to place, and the ubiquity of mixing parameters still needs further verification and support. In addition, more attention should be focused on long-term performance in future research. From the authors’ point of view, future research interest in MSWIFA/PFA geopolymer mortars should focus on the two aspects: (1) further investigating the mechanical properties, including flexural and bonding strength; (2) further exploring the durability issues, including shrinkage, efflorescence, permeability, and resistance to freezing.