1. Introduction
The annual consumption of cement worldwide is a significant source of CO
2 emissions, posing a primary concern for sustainability. Cement production involves the transformation of limestone and clay into calcium silicates at high temperatures, requiring substantial energy and leading to CO
2 emissions through the combustion of fossil fuels. This sector accounts for approximately 8% of global CO
2 emissions, which can considerably impact climate change [
1,
2,
3].
Table 1 provides an overview of the environmental impacts and CO
2 emissions throughout various stages of cement production.
Cement production typically involves high-temperature calcination, a primary source of CO
2 emissions. Additionally, raw material acquisition, energy usage, transportation, and waste management contribute significantly to environmental pollution and CO
2 emissions. The rising annual demand for cement further deepens these concerns and results in the excessive use of natural resources. However, alternative approaches are being developed to reduce cement consumption for sustainability. One alternative involves using SCMs or other byproducts (FA, SF, GBFS, or volcanic ash) instead of cement in concrete production. These materials can react with cement to enhance the durability of concrete while simultaneously reducing cement usage and lowering CO
2 emissions.
Figure 1 illustrates a schematic emphasizing the significance of utilizing waste materials in cement production to enhance sustainability.
The cement industry and the construction sector are working toward more sustainable production and building processes by implementing these changes in cement consumption. These efforts play a critical role in achieving global targets for reducing CO
2 emissions stemming from cement consumption and aim to establish a greener and more environmentally friendly construction sector by promoting the widespread adoption of sustainable building materials [
4,
5,
6]. Using recycled resources in construction, especially in concrete production, is a successful method for advancing sustainable development. Concrete is the predominant construction material worldwide, and its production significantly contributes to the release of greenhouse gases. Utilizing recycled resources in concrete production can effectively reduce the carbon footprint of the construction sector. Steel waste (SW), waste rubber (WR), waste plastic (WP), waste glass (WG), fly ash (FA), recycled coarse aggregates (RCA), and other industrial and natural byproducts (such as palm oil fuel ash (POFA)) can be used in concrete production as substitutes for cement and natural aggregates [
7,
8]. Therefore, it is necessary to use effective material management strategies such as enhancement, recycling, substitution, and resource optimization to mitigate environmental issues.
Figure 2 illustrates a schematic representation emphasizing the significance of using class-C and class-F FA in enhancing concrete’s strength and durability properties and their relevance to sustainability in concrete mixtures.
SCMs contribute significantly to the sustainability of concrete by displacing cement in its formulation [
10]. This displacement is pivotal in reducing the environmental impact associated with cement production, which is characterized by high energy consumption and substantial CO
2 emissions [
11,
12]. Integrating pozzolans into concrete mixtures mitigates these ecological ramifications and augments the material’s durability, extending the structures’ lifespan [
7,
8,
9].
Table 2 describes the contribution of SCMs in reducing CO
2 emissions and mitigating environmental pollution, considering both economic and technical aspects. This comprehensive table delineates various SCMs’ diverse aspects, including their capacity for reducing CO
2 emissions, environmental contributions such as recycling of resources or waste management, economic considerations in terms of cost-effectiveness, and the technical advantages they offer in terms of enhanced durability, strength, chemical resistance, and thermal performance in concrete production.
Reducing the cement content by incorporating supplementary cementitious materials (SCMs) correlates with a decline in concrete’s overall carbon footprint. Cement production, being energy-intensive, contributes significantly to global CO
2 emissions [
14]. The use of SCMs relies on conventional cement, thus reducing CO
2 emissions and environmental impact. Additionally, including SCMs enhances the concluding properties through the pozzolanic reaction with calcium hydroxide (CH) formed during cement hydration [
15,
16,
17]. This reaction yields additional binding compounds, resulting in denser concrete with improved strength and reduced permeability, enhancing its resistance to various detrimental factors like chemical attacks and cracking. The increased durability of concrete structures incorporating pozzolans prolongs their service life, reducing the need for frequent maintenance or premature replacement [
15,
16,
17]. Consequently, this approach minimizes life cycle costs and mitigates the resource consumption associated with recurrent reconstruction activities, aligning with sustainable construction practices. In essence, utilizing SCMs in concrete offers a multifaceted approach to sustainability by addressing environmental concerns, enhancing structural integrity, and promoting longevity in concrete structures [
18,
19].
This research conducted a comprehensive assessment regarding the definition of class-C and class-F FA, their significance in the concrete industry for sustainability, and the alterations they bring about in the workability, physical characteristics, strength, and durability of concrete. This research aims to underline the pivotal role of these specific classes of FA in advancing sustainable practices within the concrete sector. The significance lies in elucidating how using class-C and class-F FA contributes to sustainable concrete production. By exploring their distinct properties and impacts on concrete, this inquiry sheds light on their potential to mitigate its environmental impact, enhance its performance, and extend the lifespan of concrete structures. The detailed analysis delves into how these materials, known for their pozzolanic properties, can effectively reduce the reliance on traditional cement, thereby curbing carbon emissions and conserving natural resources. This evaluation examines their effects on the workability, physical attributes such as particle size distribution and water demand, mechanical strength, and long-term durability against freeze–thaw cycles and chemical attacks. This research emphasizes the importance of deliberate material selection and optimized mix designs in achieving more sustainable and resilient concrete formulations by providing a nuanced understanding of these effects. Ultimately, this investigation highlights the pivotal role of class-C and class-F FA in promoting a more sustainable, durable, and environmentally conscious approach within the concrete industry. By emphasizing their multifaceted impact on concrete properties, this research encourages informed decision-making and innovation for enhanced sustainability in construction practices.
2. Exploring FA: Definition and Uses in Concrete Industry
Class-C and class-F fly ash (FA) is a byproduct of coal combustion in thermal power plants. When coal is burned in these facilities, a process that occurs at high temperatures with limited oxygen, some of the organic and mineral constituents of the coal undergo gasification [
10,
11]. In contrast, others remain unburned, dispersing into the atmosphere as fine particles known as FA. Typically captured and separated through filtration systems in thermal power plants, FA’s chemical and physical properties can vary based on factors such as the composition of the coal and combustion conditions [
20,
21].
Table 3 demonstrates that the FA key component is silicon dioxide (SiO
2), enabling its application in concrete production as a partial replacement for cement. Based on the research conducted by Rafieizonooz et al. [
22], the yearly global output of FA corresponds to around 600 to 800 million tonnes. As the electricity demand grows, the issue of disposing of FA is expected to increase. For example, Taiwan relies on imported coal for almost 95% of its energy production. According to Lo et al. [
23], Taiwan imported about 47 million tons of coal in 2018. According to data from the Ministry of Energy and Natural Resources, Turkey’s gross electrical energy consumption stood at 257.2 billion kWh in 2014, experiencing a 2.7% increase compared to the previous year, reaching 264.1 billion kWh in 2015. Concurrently, electricity production, which totaled 252 billion kWh in 2014, witnessed a 3.1% rise in 2015, reaching 259.7 billion kWh. Notably, coal and natural gas are the primary sources of electricity production [
24].
Table 3.
FA chemical composition overview.
Table 3.
FA chemical composition overview.
References | SiO2 | CaO | Fe2O3 | Al2O3 | MgO | SO3 | K2O | Na2O | LOI | Others |
---|
Lin et al. [25] | 29.47 | 35.54 | 3.49 | 19.27 | 1.82 | 7.96 | - | - | – | - |
Subramaniam and Sathiparan [26] | 50.8 | 6.3 | 6.8 | 29.1 | 1.9 | 1.9 | 0.9 | 2.2 | – | 1.2 |
Hwang and Yeon [27] | 30.8 | 5 | 39.6 | 9.9 | 0.4 | 11.4 | 1.0 | 0.9 | 7.6 | 0.6 |
Ni et al. [28] | 42.1 | 11.7 | 3.5 | 19.3 | 1.8 | 7.4 | – | – | – | 3.1 |
Singh et al. [29] | 57.4 | 0.3 | 28.5 | 9.1 | 9.7 | 3.0 | 0.6 | – | 16.0 | 0.9 |
Wang et al. [30] | 56.2 | 3.8 | 22.4 | 14.1 | 2.6 | 1.0 | 2.1 | 0.2 | 1.5 | – |
Saboo et al. [31] | 47.5 | 11.3 | 8.2 | 21.7 | 2.5 | 1.7 | – | – | 2.5 | – |
Opiso et al. [32] | 23.9 | 23.3 | 4.4 | 26.7 | 0.1 | – | – | 1.3 | – | 0.1 |
Liu et al. [33] | 51.3 | 10.7 | 7.2 | 23.2 | 2.9 | 1.9 | – | – | 2.5 | – |
Muthaiyan and Thirumalai [34] | 51.6 | 10.8 | 13.2 | 24.2 | 2.2 | 2.8 | 2.6 | 0.7 | 0.4 | 0.7 |
Swe et al. [35] | 41.7 | 11.1 | 7.0 | 28.0 | 1.2 | 0.4 | 1.9 | 1.5 | – | 1.0 |
Shafabakhsh and Ahmadi [36] | 50.9 | 1.6 | 4.7 | 27.9 | 2.3 | – | – | – | 3.7 | – |
Peng et al. [37] | 50.2 | 12.5 | 7.2 | 23.2 | 2.9 | – | – | – | 2.5 | – |
Soto-P’erez and Hwang [38] | 30.8 | 39.6 | 5.0 | 9.9 | 0.4 | 11.4 | 1.0 | 0.9 | 7.6 | 0.6 |
Mohammed et al. [39] | 36.4 | 17.5 | 18.2 | 17.8 | 0.95 | 2.59 | 2.16 | 0.59 | 1.49 | – |
Ong et al. [40] | 46.0 | 8.4 | 2.1 | 30.5 | 0.1 | 0.4 | – | 1.3 | 1.1 | 1.8 |
Conversely, Australia, Russia, Indonesia, South Africa, and Canada rank among the top five coal-exporting nations, with Australia leading at 59%, followed by Russia at 20%, Indonesia at 11%, South Africa at 6%, and Canada at 2.5%. These coal exports may contain elevated silica, alumina, and iron oxide levels, potentially exhibiting pozzolanic properties.
XRD and SEM images of class-C and class-F FA are shown in
Figure 4 and
Figure 5, respectively. The phase analysis of the class-C FA was investigated by XRD (Model: Rigaku, D/MAX, Ultima III, Japan) at a scan speed of 10°/minute and step size of 0.02 for a scan range of from 10° to 80° [
43]. XRD patterns of class-F FA samples were performed on a MiniFlex600 diffractometer (Japan Rigaku Co., Ltd., Akishima, Japan) with Cu Kα radiation (50 KV, 200 mA) [
44]. The combined Fe
2O
3, Al
2O
3, and SiO
2 concentrations in both FA classes must exceed 50%. The quantity must fall between 50% and 70% for class-C FA, but it should exceed 70% for class-F FA. Furthermore, the calcium oxide (CaO) concentration in class-C FA must exceed 18%, but it should be below 18% for class-F FA.
Class-C FA, derived from the combustion of low-rank coals, has a significant quantity of calcium and exhibits more excellent cementitious characteristics than class-F FA. As a result, concrete made with class-C FA demonstrates superior early strength compared to conventional concrete. Its pozzolanic characteristics stem from its ability to react with cement-based binding materials.
Figure 5 shows SEM images of class-C and class-F FA. The class-C and class-F FA microstructural investigation was carried out using a TESCAN Vega 3 SEM. When used in concrete production, FA interacts with the cement during hydration. This reaction leads to the formation of hydration products, contributing additional binding components within the concrete matrix. As a result, it enhances the concrete’s mechanical properties, durability, and chemical resistance. FA finds application in producing sustainable building materials and in eco-friendly construction practices. However, carefully assessing FA’s properties is essential before its use, and it should be incorporated into concrete mixtures at optimal ratios. The composition and attributes of FA can significantly impact concrete’s mechanical performance and workability [
46].
Consequently, understanding the role of FA in concrete production remains a crucial area of research concerning its properties, advantages, and appropriate utilization methods. Olarewaju [
47] reveals that India is the leading global producer of coal fly ash (CFA), with a yearly production of 112 million tonnes and a utilization rate of 38%. China follows closely with a production of around 100 million tonnes and a utilization rate of 45%. The National Development and Reform Commission (NDRC) annual report of China provides data on the production and utilization of FA, indicating a significant production of coal fly ash (CFA) in China and its many applications, as shown in
Figure 6. China is the leading producer of cement and concrete using CFA as a source. In addition, according to the latest Central Electricity Authority (CEA) data available for 2016–2017, thermal power plants in India generated almost 169 million tones (mt) of FA.
Figure 7 displays the FA utilization and generation patterns from 2012 to 2017 in India.
On the other hand, class-F FA results from burning bituminous coal, which generally contains lower amounts of calcium oxide [
49,
50,
51,
52,
53,
54]. It does not exhibit self-cementing properties to the same extent as class-C FA. Class-F FA relies on external sources of CH to initiate the pozzolanic reaction. It tends to have finer particles and may require more time to develop strength than class-C FA [
12,
13,
14]. The differences between class-C and class-F FA primarily stem from their chemical compositions, specifically regarding the calcium oxide content and reactivity. These variations affect their behavior in concrete mixtures, influencing factors like the setting time, early strength development, and overall performance. Understanding these distinctions is crucial for selecting the appropriate class of FA based on the desired concrete properties and performance criteria in construction applications.
Figure 7.
Temporal analysis of FA: utilization and generation patterns over the years in India [
55].
Figure 7.
Temporal analysis of FA: utilization and generation patterns over the years in India [
55].
The specific gravity of fly ashes (FAs) typically ranges from 1.3 to 4.8, a variance attributed to particle morphology, chemical composition, and color. For instance, American-produced FAs exhibit specific gravities of between 2.14 and 2.69, whereas those from Turkey range from 1.83 to 2.99. Various parameters, including the content of amorphous phases, fineness, chemical composition, and residual carbon content, govern the pozzolanic reactivity of FAs [
56,
57,
58]. While many studies affirm that fine grinding significantly enhances the pozzolanic activity, it is noted that increases beyond 6000 cm
2/g yield marginal improvements [
59,
60]. In Turkey, FA is predominantly generated as a byproduct of coal combustion in thermal power plants. The country possesses significant coal reserves, leading to a considerable annual FA production. This byproduct is actively used in the construction sector, particularly in the concrete industry. FA is an SCM in concrete output due to its pozzolanic properties, enhancing various characteristics of concrete mixtures. Its utilization helps to improve concrete structures’ workability, durability, and long-term strength [
15,
16,
17].
Furthermore, it reduces the demand for cement, thereby diminishing carbon emissions and promoting sustainability in the construction industry. However, despite its advantageous uses, challenges persist regarding the consistent quality and classification of FA, which may vary based on its source and composition. Additionally, efforts are ongoing to optimize the incorporation of FA into concrete mixtures, ensuring its compatibility and effectiveness in diverse construction applications. Turkey actively utilizes FA in the concrete industry as a sustainable solution, although continued research and standardization efforts are essential to maximize its benefits and address associated challenges [
18,
19,
20].
4. Environmental Impact Analysis: Concrete Incorporating FA—A Life Cycle Assessment
The use of class-C and class-F FA in concrete involves a comprehensive assessment throughout its life cycle, encompassing extraction, production, utilization, and end-of-life phases. Class-C and class-F FA, derived from coal combustion, offer substantial potential as SCMs, impacting the environmental footprint and performance of concrete structures.
Figure 10 illustrates the influence of SCM incorporation in concrete through a comprehensive LCA.
In the extraction phase, the acquisition of FA involves processes related to coal combustion in thermal power plants. While this phase emits pollutants during coal combustion, using FA as a byproduct mitigates environmental burdens by diverting waste from landfills and reducing the need for virgin materials in concrete production. However, differences in class-C and class-F FA production, such as combustion temperatures and coal classes, result in distinct chemical compositions and reactivity, influencing their environmental impact. Using FA in concrete mixtures reduces cement consumption, thus decreasing greenhouse gas emissions associated with cement production. This substitution offers benefits in terms of enhanced durability, reduced energy consumption, and improved mechanical properties of concrete structures. However, the transportation of FA to concrete plants and the varying geographical availability of specific FA classes impact the overall environmental footprint and sustainability of its use in concrete. Throughout the service life of concrete structures, the performance and durability benefits provided by class-C and class-F FA reduce the need for maintenance, extending the lifespan of the structures. However, the end-of-life phase entails considering the recycling or disposal of demolished concrete containing FA. Conclusively, incorporating class-C and class-F FA in concrete presents a promising strategy for reducing environmental impacts throughout the life cycle of concrete structures [
28,
29,
30]. The ecological benefits of reduced cement consumption and the improved performance must be weighed against factors like transportation emissions and variations in FA properties. The aforementioned factors necessitate a comprehensive life cycle analysis (LCA) for informed decision-making in sustainable concrete construction practices. The utilization of SCMs in concrete production delineates various stages, and their environmental impacts are shown in
Table 4.
Table 5 outlines various aspects demonstrating the importance of using SCMs from the perspective of sustainability and life cycle analysis. It highlights how these materials contribute to reducing CO
2 emissions, conserving natural resources, enhancing durability, managing waste, recycling possibilities, and increasing energy efficiency in concrete production. For instance, substituting cement with class-C and class-F FA may yield energy savings and reduce CO
2 emissions during raw material acquisition. However, transportation activities might contribute to a higher carbon footprint. Incorporating class-C and class-F FA in concrete, enhancing its durability, and potentially reducing long-term maintenance needs are critical factors influencing the effectiveness of the undertaken activities. This table evaluates the environmental footprint and resource utilization throughout the concrete life cycle, emphasizing FA’s role in sustainable concrete production.
Table 4 describes the LCA and ecological impact of using class-C and class-F fly ash as a substitute for cement in concrete.
Table 5 comprehensively overviews each stage’s environmental impacts and contributions. Using class-C and class-F fly ash in concrete production presents potential for resource reduction, energy efficiency, transportation impacts, enhanced durability, recycling prospects, and waste management strategies.
Table 6 provides a comprehensive insight into the profound influence of SCMs within concrete, emphasizing their substantive impact through quantifiable metrics. It delineates their pivotal role in facilitating several critical facets of concrete production. Specifically, it elucidates the significant contributions of SCMs in curbing CO
2 emissions, meticulously conserving finite natural resources, bolstering structural longevity by enhancing durability, orchestrating effective waste management strategies, and optimizing energy efficiency throughout concrete production processes. It includes quantifiable data in percentages is an illustrative representation, showcasing SCMs’ tangible effects and efficacy across each distinct aspect and underlining their multifaceted contributions to sustainable practices within concrete engineering.
5. Conclusions
Incorporating class-C and class-F FA as viable cement substitutes in concrete represents a promising avenue for mitigating CO2 emissions, significantly contributing to environmental sustainability and human health enhancement. The culmination of our findings, particularly highlighting the reduction in porosity and improvements in mechanical strengths associated with these FA classes, paints a vivid picture of their multifaceted impact on concrete performance. The significant achievement in reducing CO2 emissions is attributed to the decreased reliance on cement within concrete formulations. As cement production is a notorious contributor to global carbon emissions, the diminished usage of cement through the integration of FA, a byproduct of industrial processes, not only curtails CO2 emissions but also alleviates the environmental strain associated with cement manufacturing. FA’s multifaceted advantages extend beyond its mere cement-replacement role. The improvements in concrete properties, such as reduced porosity and enhanced durability, signify a paradigm shift towards longer-lasting structures. This reduction in the need for frequent repairs or replacements minimizes resource consumption and waste generation, contributing to a more sustainable construction ecosystem.
Moreover, this approach has ancillary benefits for human health. The resilience and longevity of concrete structures fortified by fly ash (FA) translate into safer and more stable infrastructures, reducing the potential hazards associated with deteriorating constructions. The overall enhancement in structural integrity and reliability safeguards public safety and potentially reduces the economic burden of maintenance and repair activities. Class-C and class-F FA in concrete reverberate with profound environmental and societal implications. Beyond the reduction in CO2 emissions, these materials represent a transformative force in constructing more resilient, durable, and sustainable infrastructures, aligning with global sustainability initiatives while promoting the well-being of present and future generations. In the future, the utilization of FA is poised to bolster sustainability across various industries, primarily in construction and infrastructure development. FA’s multifaceted contributions towards mitigating environmental impacts, optimizing resource utilization, and fostering resilient, eco-friendly practices are significant. Firstly, FA’s use as a supplementary cementitious material in concrete holds immense promise. As sustainable construction practices gain prominence, reducing cement consumption for its substantial carbon footprint becomes increasingly vital. FA presents a compelling alternative to industrial processes like coal combustion. Its integration into concrete formulations effectively reduces the demand for cement, thus curtailing CO2 emissions and lessening the environmental strain associated with traditional concrete production.
Furthermore, FA’s role extends beyond emissions reduction. Its incorporation enhances concrete’s properties, fostering durable and longer-lasting infrastructures. This durability reduces maintenance requirements and extends structure lifespan, aligning with sustainability goals by curbing resource depletion and minimizing waste generation. Moreover, the circular economy concept underscores FA’s significance. FA exemplifies resource optimization and waste minimization by repurposing a byproduct of industrial processes into a valuable construction material. Its integration into construction aligns with the principles of sustainability by closing the loop on material utilization, reducing reliance on finite resources, and diverting waste from landfills. In sustainable development, FA’s versatility as a material extends beyond concrete. It finds applications in various sectors, including agriculture, geotechnical engineering, and waste stabilization, showcasing its potential to drive sustainability in diverse domains.
In conclusion, FA’s imminent importance in promoting sustainability is multifaceted. Its role in reducing CO2 emissions, enhancing infrastructure durability, optimizing resource utilization, and contributing to a circular economy aligns seamlessly with global sustainability agendas. As industries increasingly pivot towards eco-conscious practices, using FA is a beacon of sustainable innovation, promising a future where industrial byproducts become catalysts for a more sustainable, resilient, and environmentally conscious world.