Land, energy, and water are among the most treasurable assets of mankind that contribute to climate change, depending on their utilization methods and extensiveness of exploration [
1]. Excessive use of these valuable assets without considering the well-being of the natural environment would harm future generations. After realizing the serious consequences of poor resource management in recent years, the United Nations General Assembly (UNGA) adopted sustainable development goals (SDGs) in 2015. It established a framework for global collaboration to achieve a sustainable future [
1]. The construction industry, being one of the oldest, continues to play a vital role in human development and is also responsible for contributing to this agenda. This industry is the least sustainable globally as it consumes approximately half of the non-renewable resources [
2]. Natural materials such as river sand and crushed fine stone are generally used in concrete as sand. However, there has been an expansion in global urban infrastructure development and a growing demand for environmental protection, particularly in developed countries such as South Korea. The availability of natural resources is diminishing rapidly. In South Korea, the production of sand and gravel was reported to be 661,000 metric tons in 2015, according to the United States Geological Survey [
3]. This massive production has a significant detrimental effect on the environment. Therefore, it would be more beneficial, both economically and environmentally, if industrial waste could be used to partially or completely replace river sand in concrete without compromising the quality of the concrete [
4].
For decades, coal has been the most used fuel in thermal power plants to generate electricity. Raw coal was pulverized to the consistency of flour in coal-fired thermal power plants before being force-fed into a furnace. Clay particles entrapped in coal cracks were separated from the coal during the pulverization process. These clay particles and other non-combustible matter were then produced during the combustion of coal in the furnace, resulting in the production of coal ash. The coal ash content depends on the non-combustible matter present in the coal. The rock detritus that fills the fissures of coal separates from the coal during pulverization. In the furnace, carbon and other combustible matter burn, whereas non-combustible matter results in coal ash. The swirling air carries ash particles out of the hot zone, where it cools. The flue gases extract finer and lighter ash particles from the flue gases. Coal ash produced by electrostatic precipitators is known as fly ash (FA). This accounts for approximately 80 percent of the whole coal ash. Coarser and heavier ash particles settle at the base of the furnace. Some ash particles also settle on the furnace walls and steam pipes. The formed clinkers accumulate and drop to the furnace base when they become too heavy. Before being sluiced at the disposal site, the clinkers were ground to the consistency of the fine aggregate. The ash collected at the base of the furnace is known as coal bottom ash (CBA), which accounts for nearly 20% of the total coal ash. Although a large amount of fly ash has already been used in the construction industry as a partial cement replacement and/or mineral additive in cement production, the use of CBA is limited because of its relatively high unburned carbon content and different structural properties compared to fly ash [
5]. CBA contains coarser and more fused particles than FA, and thus, it has less pozzolanic activity. CBA is generally regarded as an inert material unsuitable for use as a supplementary cementing material. Because the particle size distribution of CBA is similar to that of sand, it has the potential to be used as a sand replacement in various civil engineering applications. CBA is commonly used as a low-cost substitute base material in road construction or blasting grit. According to the American Coal Ash Association (ACCA), the recycling rate of fly ash in concrete and concrete products is 47%, while the recycling rate of CBA is only 5.28% of the total recycling amount, with a total CBA production of around 19.8 million tonnes in 2002. Only 7.6 million tonnes of the output were recycled, with the majority going into structural fill/embankments (26.61%), mining applications (10.43%), and road base/subbase/pavement (19.15%). The same utilization profiles can be provided for the European Commission (EU). Nearly 89% of the produced CBA was recycled, but only 54% was evaluated for reclamation and restoration [
6]. Moreover, the physical appearance of CBA is comparable to that of natural river sand, with particle sizes ranging from fine sand to gravel, as shown in
Figure 1. The grading of CBA requires researchers to investigate its use as a substitute for sand in concrete production. To date, published research reports have shown promising results for the use of CBA as a partial or total substitute material for sand in concrete production. Triches et al. (2006) [
7] indicated that adding CBA to roller compacted concrete (RCC) mixtures may result in low cement content as well as less demand for sand, and the results showed an increase in compressive and flexural strength levels of mixtures with high levels of sand replaced by CBA in a mix with less cement. A cost-benefit analysis revealed that this mixture was beneficial. Cheriaf et al. (1999) [
8] observed that the strength activity index values of low-calcium (0.8%) CBA with ordinary Portland cement increased at all curing ages of hydration. At 28 d of hydration, they noticed the formation of calcium-silicate hydrate (CSH) gel and needles because of the reaction of CBA with portlandite. Shi and Sun (2008) [
9] discovered that at the same W/C ratio, the mechanical properties of concrete indicated that the compressive strength of CBA concrete decreased with increasing CBA content at all ages. This may be the result of the high initial free water content used in the mixes, which resulted in bleeding and weaker interfacial bonding between the aggregate and cement paste. Siddique (2003) [
10] reported that the mechanical properties increased due to the replacement of sand with CBA, which is attributed to the pozzolanic action of CBA. This occurred because coal ash type F (CBA) reacts slowly with calcium hydroxide Ca(OH)
2 released during cement hydration and does not contribute significantly to the densification of the concrete matrix at early ages, according to ASTM C618 [
11]. In addition, the interfacial bond between the paste and aggregates was strengthened. Ghafoori et al. (1997) [
12] investigated a series of laboratory-made roller-compacted concrete (RCC) with high-calcium dry CBA as a sand concrete specimen with six different proportions (cement content of 188–337 kg/m
3 and coarse aggregate content of 1042–1349 kg/m
3) that were prepared and fabricated according to ASTM C1170 Procedure A at their optimum moisture content. The compression, splitting tension, drying shrinkage, abrasion resistance, and rapid freezing and thawing were tested. They concluded that compacted concrete containing CBA achieved good strength, stiffness, drying shrinkage, and abrasion resistance, as well as repeated freezing and thawing cycles. Bakoshi et al. (1998) [
13] used bottom ash in the amount of 10–40% as a replacement for fine aggregate. Test results indicate that the compressive strength and tensile strength of bottom ash concrete generally increase in the replacement ratio of fine aggregate and curing age. The freezing–thawing resistance of concrete using bottom ash is lower than that of ordinary concrete is higher than that of ordinary concrete. Other investigations were conducted by Ghafoori and Cai (1998b) [
14]. When subjected to 300 rapid freezing and thawing cycles, RCC made with 100% high-calcium (22.5%) CBA as sand exhibited 2.3% mass loss and a 91.2% durability factor. The cumulative mass loss of this concrete decreased by 57 and 67% when the cement content increased from 9 to 12 and 15%, respectively. Similarly, the durability factor increased by 21 and 20%, respectively. Furthermore, with an increase in the coarse aggregate quantity in concrete from 50 to 60% and 55 to 60%, the cumulative mass loss decreased by 38 and 25%, respectively.
This study aimed to evaluate the mechanical properties and durability of roller-compacted concrete (RCC) containing CBA as a sand replacement. In this study, the mechanical and durability aspects of RCC, such as compressive and flexural strengths, freeze–thaw resistance, and scaling resistance, were investigated by comparing the effects of standard RCC and RCC with CBA in the laboratory.