**1. Introduction**

Coal ash is produced by drying and pulverizing bituminous coal in a pulverizer at a power plant and burning the pulverized coal in a boiler. About 15 to 45% of the pulverized coal is mainly collected from a dust collection system or the bottom of the boiler. Bottom ash refers to the ash that has fallen into the clinker hopper at the bottom of the boiler. After it falls to the bottom of the boiler and becomes accumulated in the hopper, it is ground by a grinder. The amount of the bottom ash produced accounts for around 10 to 25% of the total coal ash [1,2].

Approximately 40% of the bottom ash produced from Korea is recycled, while the remaining 60% is landfilled. Most of the bottom ash is recycled into landfill aggregates, which have low added value. Bottom ash has a high chloride concentration, a high unburned carbon content, and a high moisture content due to the use of seawater in the cooling process, and the chloride contained in the bottom ash causes corrosion of the embedded reinforcing bars. Moreover, the non-constant condition causes technical difficulties when using bottom ash and acts as an impediment to adding value as it cannot be used in a dry state [3,4].

The unburned carbon in bottom ash is partially used as a low value-added material such as a filling material or a subsidiary material for cement, whereas its uses in high value-added products such as ready-mixed concrete (unburned carbon content of less than 5%) are only marginal. As for domestic technologies related to bottom ash, there have mostly been studies on its uses as a raw material for cement, binder, or construction

**Citation:** Kim, J.; Kim, H.; Shin, S. An Evaluation of the Physical and Chemical Stability of Dry Bottom Ash as a Concrete Light Weight Aggregate. *Materials* **2021**, *14*, 5291. https:// doi.org/10.3390/ma14185291

Academic Editors: Carlos Morón Fernández and Daniel Ferrández Vega

Received: 23 August 2021 Accepted: 6 September 2021 Published: 14 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

material, the pretreatment of bottom ash, and its catalytic potential and other applications. Due to the characteristics of bottom ash, there are limits to its use as a cement, binder, an aggregate or a lightweight aggregate (LWA) material. Accordingly, it is mainly used as an aggregate (low value-added use) for filling and covering sports fields and drainage areas. In order to promote the recycling of bottom ash, research has been carried out to determine its dissolution potential based on the content of harmful substances and dissolution characteristics so that it could be used as a fill and cover material, but a negative perception of bottom ash is prevalent due to the uncertainty of the potential environmental impact. For the purpose of resolving the issue of unstable quality, domestic power plants have switched from the wet process to the dry process when it comes to bottom ash treatment so as to lower the content of moisture, salt and unburned coal, and are striving to boost the recycling rate and expand the scope of recycling. Dry bottom ash (dBA) is an industrial byproduct emitted from thermal power plants, and a detailed description is provided in Section 2.1.

As of 2018, the amount of bottom ash is 1.35 million tons in Korea, of which dBA is estimated to be 30%, which is about 400,000 tons. The outer section of dry bottom ash is characterized by an open-cell porous structure, so it is lightweight with a high absorption rate, high hardness, and excellent physical properties, making it suitable as a permeable aggregate [5,6].

In this study, the physical and chemical properties of dry bottom ash produced in Korea using the dry method were analyzed, and the possibility of using dry bottom ash as LWA for lightweight concrete was experimentally examined. 

#### **2. Preceding Research on the dBA**

*2.1. The Discharging Process of dBA*

Dry bottom ash (hereinafter referred to as dBA) is an industrial by-product discharged from thermal power plants in Korea, as shown in Figure 1. dBA discharged from a dry process is moved without coming into contact with water by the primary clinker conveyor. After passing through the primary conveyor, it is pulverized to a certain size and then finally cooled on the secondary conveyor and discharged after the secondary crushing process. The passage time of the primary and secondary conveyors is 40 to 60 min each, and most of the unburned coal is burned during residence on the conveyor. The particle size of the discharged dBA varies greatly in range, from more than 100 mm to less than 1 mm. It is quite irregular in shape, as shown in Figure 2 [7].

**Figure 1.** Bottom ash discharging system by the air-cooling process.

**Figure 2.** Shape of dBA [7].

### *2.2. Pore Properties of dBA*

Figure 3 is a photograph showing the surface and internal pore properties of dBA observed with a scanning electron microscope (SEM). It shows that the dBA surface is composed of pores on a thin film and that the outside and the inside are connected due to partial destruction. On the inside, there are pores of various sizes that exist independently or continuously. Figure 4 is a graph showing the porosity according to the particle size. While the total porosity varies depending on the size of dBA, it has been found to be 50 to 60%, and there was a positive correlation between the size of the aggregate and the number of closed pores.

**Figure 3.** Pore properties of dBA; (**a**) Surface porosity of dBA; (**b**) Internal porosity of dBA.

**Figure 4.** Pore distribution of dBA.

### *2.3. Physical Properties of dBA*

Table 1 shows the density, absorption rate, unit volume mass, and the percentage of absolute volume as an examination of the physical properties by dBA size. The density and absorption rate of dBA were measured using the standard test method for the density and absorption of a coarse aggregate (KS F 2503) and the standard test method for the density and absorption of a fine aggregate (KS F 2504). The unit volume mass and percentage of the absolute volume of dBA were measured by using the standard test method for the bulk density and the percentage of absolute volume in an aggregate (KS F 2505) [8–10].


**Table 1.** Physical properties of dBA according to size.

\* OD: Oven Dry condition of dBA. \*\* SSD: Saturated Surface-dry condition of dBA.

An analysis of the correlation between density and absorption rate shows that a decrease in particle size is accompanied by an increase in density and absorption rate, as shown in Figure 5. This is opposite to the inverse relationship between density and absorption rate in general. Figure 6 shows the correlation between the density and absorption rate of general aggregates. The difference between dBA and general aggregates is deemed to be due to the large number of pores present in the matrix in the former. In the case of density, the larger the particle, the lower the number of pores and the higher the absorption rate, and this is judged to be because there are a relatively larger number of continuous pores when there are smaller dBA particles, resulting in an increased amount of moisture penetrating the dBA. Moreover, while the unit mass increases, the percentage of the absolute volume tends to decrease. It is thought that this is because the number of open pores increases as the particle size decreases, as shown in Figure 4. Furthermore, it has been indicated that the rate of sphericity or flatness of the aggregate does not improve even when the aggregates become smaller [11,12].

**Figure 5.** Relationship between the density and the absorption rate of dBA.

**Figure 6.** Relationship between the density and the absorption rate of a general aggregate (natural river aggregate or crushed aggregate). Source: Kim, H.S. A Study on the Quality Improvement of Recycled Fine Aggregate using Neutralization and Low Speed Wet Abraser. Notification on 2011-02; Kongju National University: Cheonan, South Korea, 2011.

#### *2.4. Feasibility of dBA as a Construction Material in Relation to its Physical Properties*

In a preceding study, the following conclusions were obtained as a result of examining the physical properties of dBA, a by-product in the electric power industry, as a LWA (Light Weight Aggregate) for construction [13].

As for the shape of dBA, it is structurally weak due to the sharp and angular edges and the flat and elongated shape, but unlike the surface, where there are open cells, the inside features closed cells and is relatively high in hardness.

In addition, although the particle density decreases along with a decrease in the particle size, the apparent density tends to increase slightly. This is because large cells are destroyed as the particles become smaller. Since closed cells that are larger than 100 μm are destroyed and turn into open cells as a result of the reduced particle size, there is an increase in the open porosity, a decrease in the closed porosity and total porosity, and an increase in the absorption rate.

Therefore, based on the findings of the previous study, it appears that dBA can be used as a LWA and has excellent properties in terms of weight and thermal insulation if the relatively weak surface is removed and it is processed into a near-spherical shape [14].

Table 2 shows the advantages and drawbacks of various aggregates including dBA used in this study. The wBA discharged from the wet process is difficult to recycle due to problems such as high unburned carbon, high chloride, and moisture contents. Natural aggregates are relatively high-quality aggregates that have been used for a long time, but they are related to environmental problems and resource depletion. In the case of the artificial lightweight aggregate, it is a product with a constant quality with a spherical shape and a uniform particle size because it is produced in a factory; however, it has the disadvantage of high manufacturing costs and greenhouse gas emissions due to the calcination process. In addition, in the case of Korea, the cost is high because most of it depends on imports. On the other hand, dBA has the potential to be applied as an alternative material for lightweight aggregates, owing to the low content of unburned carbon, chloride and SO3. Nevertheless, the structural weakness caused by the irregular shape and high absorption is a problem to be overcome.


**Table 2.** Comparison of various aggregates.

#### **3. Experimental Plan and Method**

*3.1. Experimental Plan*

Table 3 shows the experimental plan of this research. This study was conducted with the aim of determining the possibility of using dBA, which has been confirmed to have appropriate physical properties as LWA, for concrete manufacturing by evaluating its chemical properties.



\* XRF: X-ray fluorescence, \*\* XRD: X-ray Diffraction. \*\*\* ICP: Inductively Coupled Plasma, \*\*\*\* DT-TGA: Thermogravimetry-Differential thermal analysis.

> The chemical properties of dBA were analyzed based on X-ray fluorescence (XRF) to check the oxide composition and SO3 content, X-ray diffractometry (XRD) to examine the mineralogical properties, loss on ignition to measure the amount of unburned coal as well as chloride content and the pH, and the heavy metal leaching test.

> The dBA evaluated in this study was bottom ash discharged from a dry process at the B Thermal Power Plant operated by J Power in Korea. For a comparison, fly ash (FA), wet bottom ash (wBA), and four domestic and foreign artificial LWAs were examined as well. The experimental plan is shown in Table 2.

*3.2. Shapes of Various Artificial Aggregates*

Figure 7 shows digital camera and SEM images of the samples used in this study.

**Figure 7.** Shape of the test samples.

As described above, dBA has sharp edges and is flat, elongated, and irregular in shape. In the case of wBA, it has the appearance of a crushed aggregate (crushed stone), and the corners are relatively round compared to dBA. On the other hand, FA has a near-perfect spherical shape.

LWA-1 is an artificial LWA produced in Korea that has a generally round shape, while LWA-2 is a product from the United States and has a relatively angular shape compared to LWA-1. In the case of LWA-3, it is a product made in Japan and is an intermediate between LWA-1 and 2 in terms of shape, while LWA-4 is a product made in China and its shape is closest to a sphere.
