Full length articleUtilization of contaminated air pollution control residues generated from sewage sludge incinerator for the preparation of alkali-activated materials
Graphical abstract
Introduction
Alkali-activated materials (AAMs), as a product of aluminum-silicon precursors reacting with strong alkali solutions, are considered as potential alternatives to ordinary Portland cement due to their comparable mechanical properties and durability, low CO2 emissions and energy consumption during manufacturing (Davidovits, 2013; Duxson et al., 2007; Provis and Deventer, 2009). Many solid wastes with rich aluminium-silicon contents (e.g., fly ash, ferrous slag, non-ferrous metallic slag, etc.,) could be large-scale utilized as raw materials to produce sustainable AAMs via their good alkali-activation potentials (Shi and Qian, 2000; Sun et al., 2021; Van Sande et al., 2020). However, in some regions, the availability of high-quality precursor materials is limited due to growing environmental concerns and diminishing heavy industries. For instance, in Hong Kong, coal-fired power plants will be phased out soon due to pressure on reducing CO2. Therefore, using locally available alternative raw materials as the precursors of the AAMs is promoted to reduce the accumulation of industrial waste and CO2 emissions.
In Hong Kong, waste glass contributes to a significant proportion of municipal solid waste (291 tonnes/day in 2017), but only 20% of waste glass could be recycled due to the lack of a glass manufacturing industry (Ling et al., 2013). As a potential alkali-siliceous material, waste glass powder (GP) had been investigated extensively as potential raw material to produce the AAMs (Pascual et al., 2021). Due to GP's lower activation and reactive alumina content, a significant strength reduction of the alkali-activated GP was reported compared to alkali-activated slag or fly ash (Redden and Neithalath, 2014). In previous studies, it was demonstrated that the combined use of glass powder and slag could provide sufficient Ca and Al to form stable C-(N)-A-S-H gels and compensate for the strength loss of such AAMs (He et al., 2020a; He et al., 2021b; Zhang et al., 2020). Therefore, the beneficial reuse of waste glass powders in AAMs manufacturing provides significant economic and environmental benefits, and there is still increasing demand for such sustainable materials.
Moreover, approximately 1,200 tonnes of sewage sludge are produced daily in Hong Kong (Swann et al., 2017). The Hong Kong government has constructed the world's largest sewage sludge incinerator to incinerate a maximum of 2,000 tons of dewatered sewage sludge a day, reducing the waste volume by almost 90% and waste mass by 70%. But this disposal solution to the sewage sludge still generates a new problem, namely disposing the incineration sewage sludge ash and air pollution control (APC) residues. Landfill disposal of these solid wastes is inadvisable due to occupying the valuable land resources and wasting the recoverable resources. Research on reusing incineration sewage sludge ash has provided some feasible solutions for the recycling of the incineration fly ash. An approach is to make the AAMs by co-disposing incineration sewage sludge ash, with pozzolanic waste, such as metakaolin, granulated ground blast furnace slag or fly ash (Chen et al., 2018; Hadas, 2021; Longhi et al., 2022). Additionally, depending on the characteristics of the sewage sludge, which is incinerated, the APC residues usually contain a high content of chlorides, sulfates, and heavy metals (Quina et al., 2010a; Rani et al., 2008a). Recent studies have investigated the possible use of the sulfate from the solid waste fly ash as mineral feed for cement manufacture or as activators for slag (Lampris et al., 2009; Ren and Ling, 2021; Sun et al., 2022a), but the high contents of sulfate and chloride in the incineration sewage sludge fly ash would have negative impacts on the durability of cementitious materials, such as causing steel corrosion and expansion due to the formation of ettringite (Cai et al., 2021a; Cai et al., 2022b; Gunasekara et al., 2019; Sun et al., 2022b). In addition, as a potentially hazardous waste, improper disposal of the APC residues would cause adverse effects on human health and environmental concerns. Several studies have reported AAMs as construction materials can be used to solidify/stabilize the heavy metals of the contaminated sewage sludge fly ash (Wang et al., 2010; Ren and Ling, 2021). Therefore, recycling of the APC residues in the AAMs not only reduced the burden of landfills, but also eliminated the potential environmental risks due to the leaching of heavy metals. To reduce the detrimental effects of soluble salts and heavy metals of the APC residues, a pre-treatment washing step of these residues with water was commonly adopted (Bayuseno and Schmahl, 2011; Bogush et al., 2019a; Quina et al., 2008b). However, little information on the fundamental characteristics of the washed and unwashed APC residues, such as the physical and chemical characteristics and the leaching behavior of heavy metals, is available.
To stabilize the heavy metals and soluble salts of the APC residues and maximize the recycling rate of the APC residues, a solidification/stabilization process using one-part AAMs is proposed. The current study aimed to determine the effect of the liquid/solid ratio during the washing process on the chemical composition of the APC residues before final stabilization/solidification. The washed and unwashed APC residues, recycled waste GP, and slag were used as the precursors of the AAMs, and a ternary contour diagram of the mixing design was used to determine its optimal mixing design ratio. In addition, the effect of the washed and unwashed APC residues on the mechanical and microstructure properties of the AAMs and the leaching behavior of heavy metals were assessed. This research finding would facilitate the recycling of the APC residues and waste GP as precursors in AAMs, which would directly reduce the demand for cement and concrete in virgin construction materials.
Section snippets
Raw materials
A combination of ground granulated blast-furnace slag, waste glass powder (GP), and air pollution control (APC) residues are employed as precursors to prepare the one-part alkali-activated materials (AAMs), and a commercial solid sodium silicate composed of 35.8% Na2O and 62.9% SiO2 was used as the alkali activator. As-received APC residues were collected from a local sewage sludge incinerator (Sludge Treatment Facilities, T-park, Hong Kong), and the waste glass cullet was collected from a
Effect of L/S ratios on the APC residues
Table 1 shows the chemical compositions of the as-received APC residues and washed APC residues. From Table 1, the as-received APC residues were mainly composed of the 35.1% Na2O, 33.0% SO2, and 4.11% Cl. The high sodium sulfate content in the as-received APC residues resulted from the removal of gaseous SO2 accomplished by using sodium bicarbonate in the flue gas treatment process. Besides, the amount of 4.11 wt.% Cl and 5.12% of Fe2O3 in the as-received APC residue were attributed to using
Conclusions
The APC residues generated from the sewage sludge incineration process were detrimental to the environment due to its rich soluble sulfate and chloride salts (e.g., NaCl and Na2SO4) and toxic heavy metal contents. This study explored recycling and reusing the washed and unwashed APC residues as precursors and weak alkaline activators for producing AAMs as sustainable and low carbon footprint construction materials. The effect of the washed and unwashed APC residues on the physical properties
CRediT authorship contribution statement
Keke Sun: Conceptualization, Methodology, Visualization, Writing – original draft, Writing – review & editing. Hafiz Asad Ali: Conceptualization, Visualization, Writing – review & editing. Weiyi Ji: Visualization. Jiaxing Ban: Visualization. Chi Sun Poon: Conceptualization, Resources, Project administration, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study work was supported by a grant from the Research Grants Council (NO. P0033406 & B-Q80K) and The Hong Kong Polytechnic University.
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