Abstract
With the growing population, it is also expected that the municipal solid waste (MSW) generation would be increasing in the coming years. Management of MSW is already an identified issue and the incineration process is widely adopted as a disposal solution for non-compostable wastes. Incineration of MSW results in secondary waste residues such as almost 20% of fly ash and 80% of bottom ash. These are rich in silica and alumina content, naturally has the potential to be an aluminosilicate precursor to produce alkali-activated materials. Several issues are pointed out for the application of MSWI residues as cementing materials, either a Supplementary Cement Material (SCM) or an Alkali-Activated Material (AAM). One such issue concerning AAM application is the presence of metallic aluminium. Heavy metals from the MSWI residues are normally extracted before disposal, however metallic aluminium is not effectively removed in most of the cases. Metallic aluminium acts as a pore-forming agent by generating H2 gas in the alkali environment, thereby reducing the strength properties. In the present study, an attempt is made to understand the influence of the metallic aluminium present in the MSWI fly ash of different size fractions, fine (0–0.5 mm) and coarse (0.5–1.0 mm). MSWI fly ash is activated with solid/liquid alkalis, such as sodium hydroxide (NaOH), sodium silicate (Na2SiO3) and combination of both. With the Na2O content maintained constant across the activators, the sample activated with NaOH is the weakest and the one with sodium silicate is the strongest with small, well-distributed pore structure, irrespective of the size fraction of MSWI fly ash used. Solid activator performed equally well as that of the liquid one.
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1 Introduction
Waste to energy treatment is preferred for municipal wastes in places where recycling is not an option, owing to its various advantages including volume reduction and energy recovery [1]. In 2019, 502 kg of municipal waste per capita were generated in EU, of which 30% is incinerated which is double the capacity compared to 1995 [2]. However, it also comes with the problem of producing other residues such as fly ash, bottom ash, and air pollution control (APC) residues [3]. Such residues were studied for their use as secondary raw materials in construction sector [4, 5]. Utilization of municipal solid waste incinerated (MSWI) fly ash was examined for applications such as supplementary cementitious materials [6,7,8,9]. Though MSWI bottom ash falls under non-hazardous category and considered for various applications, the finer fractions (MSWI fly ash) are hazardous with heavy metals and organics content [10]. Hence, stabilization of MSWI fly ash became an important and interesting point of consideration before even considering them for secondary utilization. In this context, alkali activation of this material is regarded as an promising method to immobilize heavy metals while also serving as an alternative binder material [11, 12].
In the present work, MSWI fly ash was considered for its potential to be used as precursor in one-part alkali activated materials (AAM). One-part AAM are “just add water” material like ordinary Portland cements unlike the traditional two-part alkali activated material where the aluminosilicate precursors to be added with alkali activator solution [13]. Sodium silicate and sodium hydroxide solids were mixed with MSWI fly ash acting as binder material. Compressive strength results were compared between one-part and two-part AAMs with similar Na2O content. Influence of milling of MSWI fly ash at different time (1 and 5 min) was also focused.
2 Materials and Methodology
2.1 Materials
The MSWI fly ash used in this study was obtained from municipal waste incineration plant in Oulu, Finland. Major elements of MSWI fly ash was analyzed using X-ray fluorescence spectrometer (XRF), and the results are presented in Table 1. The fly ash has higher silica and CaO content, but much lower alumina content. There are also high amount of lead and titanium (Table 1). 3.2% of chlorides are seen in this fly ash which is also mentioned in many literature as one of the deleterious element [14]. MSWI fly ash was initially sieved through 1 and 0.5 mm sieve, which resulted in 4 and 13% residues, respectively. The residues mostly comprised of unburnt carbon (Fig. 1). Samples passing 1 mm were taken into consideration in this study, so as to utilize maximum of the material. Anhydrous sodium metasilicate (silica modulus, SiO2/Na2O = 0.9) was procured from Alfa Aesar, Germany. Liquid sodium silicate with silica modulus 3.2 and 55% water content was supplied by VMR chemicals. Reagent grade sodium hydroxide (>97%) powder was used to adjust the Na2O content to 5% across different activators.
2.2 Methodology
MSWI fly ash was dried in 100 ℃ for 24 h and ground in vibratory disc mill for 1 and 5 min. Disc mill Retsch RS 200 was employed at 1500 rpm in batches of 100g of MSWI fly ash. Three different activators were used, precisely, combination of solid sodium silicate and sodium hydroxide, liquid sodium silicate and liquid sodium hydroxide. Na2O content was maintained as 5% in all the activators. Water to solids ratio was maintained constant as 0.45. To study one-part AAM, MSWI fly ash was mixed with solid sodium silicate and sodium hydroxide combination prior to introducing the water. In two-part, the alkali activator solution was mixed to the MSWI fly ash to make paste samples. Cylindrical specimens (20 × 25 mm) were cast using the one-part and two-part samples to observe their physical appearance and study the strength properties. AAM specimens were subjected to two different curing regimes; (1) 60 ℃ for 24 h, continued by room temperature curing, (2) room temperature curing until further testing.
3 Results and Discussion
3.1 Physical Appearance
Cylindrical specimens were demolded after 24 h and observed for physical form. Specimens cured at 60 ℃ for 24 h is shown in Figs. 2, 3 and 4. Unmilled MSWI fly ash appears disintegrated with solid activators and liquid NaOH, whereas, comparatively solidified with liquid sodium silicate (Fig. 2). This is also the case with room temperature cured specimens.
After milling, one-part activators performed better in MSWI fly ash pastes with solid specimens as that of two-part specimens made of liquid sodium silicate (Fig. 3). However, liquid NaOH does not produce stronger specimens though Na2O content was maintained constant.
With increasing milling time to 5 min, one-part seems to be a better option as the pore size increased in two-part AAM specimens (Fig. 4). In all the three cases, Two-part specimens with liquid NaOH were very weak due to their high porosity and hence, were not considered for strength testing.
3.2 Compressive Strength
Solid activator performs equally good as that of liquid activator in the activation of MSWI fly ash as represented by compressive strength results in Figs. 5 and 6. Combination of Na2SiO3 and NaOH acted as solid activator making one-part AAM and liquid Na2SiO3 was used as activator for producing two-part AAM for the specimens used for strength testing.
It is to be noted that only two-part activation worked for unmilled MSWI fly ash samples, leading to a conclusion that one-part activation need milling as a pre-treatment. However, increasing the milling from 1 to 5 min was not favorable for alkali activation as this either does not impact (Fig. 5) or reduced the strength (Fig. 6) in case of solid activators. With the use of liquid activators, milling reduced the compressive strength, irrespective of the type of curing adopted. This can be related to the increasing amount of metallic aluminium with milling time and hence, swelling, porosity and strength reduction [15].
Type of curing shows noticeable impact on the strength development with age in one/two-part alkali activated MSWI fly ash (Figs. 5 and 6). Specimens cured at room temperature shows gradual increase in compressive strength with one-part activation (Fig. 6). It reaches a maximum strength of 4 MPa at 56 days of curing age which is similar to that of two-part AAM specimens. However, early age strength seems to be higher with two-part activation though strength improvement at later ages is not noticeable.
4 Conclusions
MSWI fly ash was studied as a source of aluminosilicate precursor for alkali activation. The possibility to use solid activators (one-part) was examined and compared with conventional two-part alkali activation method. The study concludes that milling helps in activation of MSWI fly ash in one-part method, however, it also can result in the increased release of metallic elements which can negatively affect the properties. Heat curing helps in achieving high early strength in one-part activation, however, does not contribute to the strength increase at later ages.
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Acknowledgement
This work was supported by the funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska Curie grant agreement No [839848].
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Perumal, P., Illikainen, M. (2023). Feasibility Study of One-Part Alkali Activated Material with MSWI Fly Ash. In: Escalante-Garcia, J.I., Castro Borges, P., Duran-Herrera, A. (eds) Proceedings of the 75th RILEM Annual Week 2021. RW 2021. RILEM Bookseries, vol 40. Springer, Cham. https://doi.org/10.1007/978-3-031-21735-7_63
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