1. Introduction
Worldwide demand for concrete is significant, with an annual production of approximately 30 billion tons per year [
1]. Cement manufacturing contributes to the global carbon footprint by generating at least 8% of the world’s carbon dioxide (CO
2) emissions [
2]. In the United States, roughly 0.80 kg of CO
2 are released for every kilogram of cement produced, leading to global warming and climate change. Nonetheless, 4.5 GtC was captured between 1930 and 2013 by carbonating cement materials, reducing 43% of the CO
2 emissions from cement production during that period [
3]. Using supplementary cementitious materials (SCMs) is another novel approach that can mitigate the unfavorable consequences of cement production on the environment, and in certain circumstances, its use can lower the cost of the concrete by decreasing the proportion of cement while enhancing the concrete’s properties [
4].
SCMs are mostly residuals of different industries, and their physical and chemical characteristics greatly determine the way they perform in concrete [
5]. Coal combustion products (CCPs), commonly known as coal ash “byproducts,” are termed “products” by the Environmental Protection Agency (EPA) to promote recycling. Coal ash and other industrial byproducts can be used beneficially, and this has been promoted by government regulators and CCP generators as environmental awareness and landfilling prices have increased [
6]. Moreover, the pozzolanic properties of coal combustion products make them qualify as SCMs [
7], where their chemical compositions depend on the coal origin and combustion method [
8]. Pozzolanic components of coal ash products are an excellent substitute for cement, mainly due to iron (III) oxide (Fe
2O
3), silicon dioxide (SiO
2), and aluminum oxide (Al
2O
3) [
9]. The pozzolanic reaction is usually thought to be how SCMs increase concrete durability. This reaction can absorb calcium hydroxide (portlandite) and create calcium silicate hydrate (C-S-H), which is the most significant feature of SCMs that fosters the characteristics of concrete [
10].
The most common SCM in concrete is fly ash, but the increased concerns about climate change have put pressure on many power companies to shut down coal-fired power plants, reducing its availability. Despite the fall in electricity production using coal, the use of reclaimed coal combustion products has increased [
11]. This approach attempts to maintain the availability of valuable materials derived from coal residues for safe and profitable applications. Thus, technological innovations should be welcomed by the construction sector to grow in a fluctuating environment [
12]. According to an American Coal Ash Association (ACAA) study, 46.8 million tons of coal-fired products were used productively in 2022, indicating that the US utilized more than 50% of its coal ash production for beneficial purposes for the 8th year running, as opposed to discarding it [
11]. Adequate identification and classification of the heavy metals and their toxicity levels are necessary to safely dispose of coal ash residues [
13,
14,
15,
16]. One widely used technique in the United States to evaluate the toxicity of environmental contaminants is the toxicity characteristics leaching procedure (TCLP), which is currently an accepted international method for evaluating heavy metal pollution. The US Environmental Protection Agency (USEPA) developed the procedure as the foundation for the land disposal restriction program’s best-proven available technology treatment standards [
17]. Leaching tests have many uses, from classifying industrial wastes for landfill disposal to determining if residues are stable enough to be reused for beneficial purposes [
18]. Therefore, utilizing coal ash that has been kept in landfills is becoming more appealing [
19]. Thus, this study assesses the established procedure that classifies byproducts for safe disposal by determining the concentration of extremely toxic heavy metals and their leachability in coal ash residues, specifically, coal bottom ash and coal boiler slag, subsequently evaluating possibilities as value-added products in the construction industry.
Coal bottom ash (CBA) and coal boiler slag (CBS) are coarse, granular residues removed from the bottom of furnaces, and their properties vary depending on the kind of furnace used to combust the coal. The porous, dark gray bottom ash is collected in dry bottom boilers where pulverized coal is burned. Meanwhile, when molten ash from a wet-bottom boiler encounters water that quenches, it rapidly ruptures to form boiler slag [
20]. When usage was considered, the manufacture of CBS grew to more than 1.6 million tons in 2022 from 1.2 million tons in 2021 [
11]. Approximately two-thirds of lignite deposits in the United States are found in North Dakota, which could play a vital role in the state’s construction sector and economy. Studies have investigated the potential of effectively replacing cement with 20% CBA in concrete mixtures, promising sustainability benefits in terms of compressive strength. The optimal substitution range for CBA in cement replacement to attain high compressive strength is 10–20% [
21]. Chuang et al. [
22] studied the use of finely ground coal bottom ash (FGCBA) as a Portland cement substitute in concrete, and the results indicated that the maximum strength attained by FGCBA was 97.7% of the control group, with cement only, at an optimal 20% replacement rate on the 91-day compressive strength test. According to Jaturapitakkul and Cheerarot [
23], ground bottom ash concrete exhibited a higher development rate of compressive strength when 20% by weight was replaced. Furthermore, Bajare et al. [
24] found that ground coal combustion bottom ash (CCBA) can successfully substitute up to 20% cement without lowering the concrete’s compressive strength, comparable to the reference mix. Additionally, it has been noted that concrete’s compressive strength considerably decreases when 40% of the cement is replaced by coal combustion bottom ash (CCBA). Yet, they failed to provide the tensile, flexural, and durability properties, which could serve as key CBA- and CBS-based concrete performance indicators. Poudel et al. [
25] reported increased compressive, tensile, and flexural strength when cement was substituted with 10% ground coal bottom ash; improved durability was also observed in the later curing days compared with conventional concrete. However, the CBA and CBS replacement was limited to 15%. Increasing the amount of replacement while maintaining improved concrete performance will result in significant economic and social savings. Thus, this study aims to evaluate the impact of 20% substitution of the CBA- and CBS-based concrete properties in fresh and hardened phases, including compressive strength, flexural strength, tensile strength, and durability at different curing periods.
This research paper attempts to employ CBA and CBS, which are not currently recycled in North Dakota but could be excellent resources for the concrete sector. As a result, a substantial amount of residue could be utilized as a construction material rather than contaminating the environment and potentially jeopardizing public health due to their leachable elements seeping into water.
4. Conclusions
In this article, the objectives were to explore coal combustion residues—in particular, coal bottom ash (CBA) and coal boiler slag (CBS)—as supplementary cementitious materials (SCMs) to reduce the environmental impact of cement manufacturing by lowering the carbon footprint as well as the amount of cement used while improving concrete performance. In some cases, SCMs can also reduce the cost of concrete. The scope of the experiment covered the chemical, fresh, and hardened properties as well as the effects of replacing 20% of the CBA- and CBS-based concrete characteristics. Moreover, this study evaluated the established procedure that classifies byproducts for safe disposal and then examined their potential as a value-added product in the construction industry by measuring heavy metals concentration and leachability.
It was found that concrete with 20% CBA could be considered a potential cement replacement, as it enhances the compressive, flexural, and splitting tensile strength, and the modulus of elasticity, owing to a pozzolanic reaction that gives better strength development than CBS-based concrete. Replacing 20% cement with CBA and CBS improved the concrete’s ability to withstand the ingression of chloride ions due to the amelioration of chloride-binding capacity, which is advantageous for structures in harsh environments. However, more durability tests need to be conducted to ensure consistent concrete performance. Furthermore, the TCLP test regarding the leachability concentration of extremely toxic heavy metals indicates that none of the residues exceed EPA regulatory limits for hazardous waste. The adoption of sustainable concrete could be further promoted by more research on the freeze–thaw resistance of CBA- and CBS-based concretes and by using natural fibers so their tensile and flexural strength could be enhanced.
Overall, though more standardization is required to fully realize their benefits, the incorporation of 20% cement replacement of CBA and CBS into the concrete mixture offers a promising path toward sustainable construction practices by promoting the use of byproducts and attaining almost equivalent and enhanced concrete performance in comparison with conventional concrete. Furthermore, by adhering to EPA guidelines, CBA and CBS can be used as SCMs without posing significant environmental risks.