Evaluating Finely Ground Coal Bottom Ash for Property Self-Compacting Concrete

Abstract

This study investigates the feasibility of utilizing finely ground coal bottom ash (FGCBA) as a supplementary cementitious material in self-compacting concrete (SCC), with an emphasis on its technical performance and environmental implications. Cement was partially replaced by FGCBA and fly ash (FA) at 20%, 40% and 60% substitution rates under water-to-binder (W/B) ratios of 0.4, 0.45 and 0.5. A comprehensive evaluation of the properties of fresh and hardened concrete—including slump flow, setting time, compressive strength, air content, chloride ion permeability and water absorption—was conducted. The results indicate that FA improves workability and enhances long-term strength development, while FGCBA—despite its lower early-age strength—significantly improves durability, particularly in terms of chloride resistance and microstructural densification. These findings underscore the potential of FGCBA as a viable low-carbon alternative in cementitious systems, contributing to resource efficiency and the achievement of circular economy objectives in the construction sector.

1. Introduction

In recent decades, international efforts to mitigate climate change have accelerated, with the Paris Agreement urging nations to limit global warming to well below 2 °C above pre-industrial levels. In response, many countries have pledged to achieve net-zero carbon emissions by the mid-century, driving innovation in renewable energy technologies such as solar, wind and hydroelectric power [1]. Nevertheless, coal remains a dominant component of the global energy mix, particularly in developing countries. As of 2023, coal accounted for approximately 36% of global electricity generation, according to the International Energy Agency (IEA) [2]. Its continued use is attributed to the reliability and affordability of coal-fired power, especially where renewable infrastructure is limited [3]. While clean coal technologies such as carbon capture and storage (CCUS) have been introduced to reduce emissions [4], the environmental challenges associated with coal combustion—including the generation of large volumes of solid waste—remain a pressing concern.

Another key reason for coal’s persistence is its role in ensuring energy security. Compared to variable renewable sources, coal offers a stable baseload supply, which is essential in regions with fluctuating demand or limited renewable integration capacity. Coal is widely available and helps to reduce reliance on imported fuels [5]. Nevertheless, the environmental concerns associated with coal combustion are undeniable. The release of carbon dioxide (CO₂), sulfur dioxide (SO₂), nitrogen oxides (NO_x), and particulate matter contributes to global warming, acid rain, and air pollution, posing severe health risks. Additionally, combustion produces large volumes of solid waste, such as fly ash and bottom ash. Thus, sustainable strategies for coal waste utilization have become a growing area of research, particularly in construction materials such as concrete, where coal combustion byproducts can serve as alternative raw materials.

Coal-fired power generation results in the production of several types of solid waste, each possessing distinct physical and chemical properties. The most common byproducts include fly ash and bottom ash, which constitute a major portion of the coal combustion waste stream, posing disposal challenges but increasingly being reutilized to support circular economy strategies. Fly ash—which constitutes the largest fraction of coal combustion residues—is composed of fine, spherical particles that remain airborne before being captured by electrostatic precipitators or bag filters. Due to its pozzolanic nature, fly ash has been widely incorporated into the construction industry, particularly in cement and concrete production, where it improves mechanical properties and durability [6]. Fly ash has also found use in other sectors, such as soil stabilization and geopolymer production [7]. In contrast to fly ash, bottom ash is a coarser residue that settles at the furnace bottom and has traditionally been used as an aggregate substitute in concrete, asphalt and road base materials [8]. However, its full potential as a cementitious material remains underexplored.

In addition to fly ash and bottom ash, other coal combustion residues have been explored for various industrial uses. These include boiler slag [9], flue gas desulfurization (FGD) gypsum [10], fluidized bed combustion (FBC) ash [11], coal gangue [12] and cenospheres [13], which have found applications in areas such as soil conditioning, brick manufacturing and advanced composites. While these byproducts are diverse in composition and utility, their reuse reflects a broader commitment to circular economy principles. Such reutilization supports global efforts to reduce landfill dependency, enhance resource efficiency and mitigate environmental impacts.

The integration of coal combustion byproducts into concrete has emerged as a sustainable strategy for reducing landfill waste and lowering the environmental footprint of construction materials. Recent advancements in material science have emphasized the need for low-carbon and eco-friendly alternatives in the construction sector to align with global sustainability goals [14]. Fly ash, bottom ash and FGD gypsum are the most widely studied materials due to their potential roles in cementitious systems. One of the primary reasons for incorporating coal combustion byproducts into concrete is their ability to improve its workability, durability and mechanical strength. Fly ash, in particular, acts as a pozzolanic material that reacts with calcium hydroxide to form additional calcium silicate hydrate (C-S-H), leading to increased strength and reduced permeability [15]. Bottom ash, on the other hand, is used as a partial replacement for fine aggregates, contributing to improved resistance against alkali–silica reactions and sulfate attacks [6]. The type of coal combustion byproduct incorporated into concrete determines which component it replaces. Fly ash is commonly substituted for cement, while bottom ash replaces fine aggregates and FGD gypsum is sometimes used in blended cement formulations. This limitation has prompted growing interest in enhancing the pozzolanic behavior of bottom ash through mechanical activation, such as fine grinding, in order to enable its application as a viable supplementary cementitious material. This substitution not only reduces the consumption of natural raw materials but also contributes to lower carbon emissions from cement production.

Among the various coal combustion byproducts, bottom ash (BA) has traditionally been considered a lower-value material due to its coarser particle size and lower pozzolanic reactivity compared to fly ash. As a result, its applications have primarily been limited to partial replacement of fine aggregate in concrete and road base materials, or disposal through landfilling. However, recent advancements in grinding technology have demonstrated that finely ground bottom ash can exhibit pozzolanic properties, making it a viable supplementary cementitious material (SCM) for replacing ordinary Portland cement (OPC) [5]. Grinding improves the reactivity of bottom ash by reducing its particle size and increasing its surface area. This transformation enhances the formation of calcium silicate hydrate (C-S-H) phases, enabling bottom ash to function effectively as a cementitious material that improves concrete strength and durability. SCC, due to its high binder content and reliance on fine materials, provides a suitable medium for incorporating ground bottom ash [15]. This approach is expected to improve both the fresh and hardened properties of SCC while reducing cement usage and promoting bottom ash reuse.

Compared to conventional bottom ash reuse approaches, its application as a cementitious material offers several advantages. First, it helps to mitigate the issue of cement overproduction, a major contributor to global CO₂ emissions. The cement industry contributes approximately 8% of global CO₂ emissions, and each ton of cement replaced with alternative material helps reduce the associated environmental burden [16]. Second, whereas traditional bottom ash used as aggregate contributes little to strength development, finely ground bottom ash exhibits pozzolanic activity, which improves both mechanical performance and durability. Third, grinding bottom ash enables its use in higher-value applications, enhancing its attractiveness for industrial adoption over conventional low-grade or landfill uses. From an environmental perspective, this approach contributes to sustainable waste management and resource conservation. Repurposing bottom ash as a cement substitute reduces reliance on virgin raw materials and mitigates landfill accumulation, thereby lowering the risk of heavy metals leaching into soil and groundwater. Furthermore, using bottom ash in SCC aligns with circular economy principles by transforming an industrial byproduct into a high-value construction material. It provides an environmentally responsible alternative aligned with global net-zero targets.

This study aims to evaluate the feasibility of using finely ground coal bottom ash (FGCBA) as a partial cement replacement in self-compacting concrete (SCC), focusing on its influence on workability, mechanical performance and durability. The objective is to explore FGCBA’s potential as a sustainable supplementary cementitious material and assess its applicability in environmentally responsible construction practices.

2. Materials and Methods

2.1. Raw Materials

In this investigation, the materials employed included Type I Portland cement, coal fly ash (FA), finely ground coal bottom ash (FGCBA), natural river sand, 3/8-inch gravel and a liquid water-reducing admixture. The Type I cement was procured from Taiwan Cement Corporation (Taipei, Taiwan). To ensure consistency and reduce experimental variability, both FA and FGCBA were obtained from the same combustion unit at the Taichung Power Plant (Taichung City, Taiwan). The particle size distributions of the powdered materials, determined using an INSITEC laser diffraction analyzer, are presented in Figure 1, with the average particle sizes recorded as 9.82 µm for FA and 11.83 µm for FGCBA. The physical characteristics of the fine and coarse aggregates are provided in

Table 1. The properties of aggregate in this study.

Properties	Fine Aggregate	Coarse Aggregate (3/8″)
Specific gravity	2.6	2.61
Water absorption (%)	2.06	1.08
Fineness modulus	2.35	6.34

, and the results of the sieve analysis are depicted in Figure 2. A Type G liquid water-reducing agent, supplied by Sika Taiwan Ltd. (Taoyuan City, Taiwan), capable of reducing water demand by up to 20%, was utilized.

Table 2. Fineness of bottom ash at different grinding times.

Time (h)	Blaine Specific Surface (cm2/g)
12	2510
24	3100
36	3910
48	4140

summarizes the relationship between grinding duration and the specific surface area of the FGCBA. The chemical compositions of both FA and FGCBA, which are largely similar, are outlined in

Table 3. Chemical properties of the FA produced and FGCBA in this study.

										Unit: %
Material	SiO2	Fe2O3	Al2O3	CaO	MgO	TiO2	K2O	SrO	SO3	LOI
FA	35.53	30.5	4.89	15.95	-	4.97	3.43	1.72	1.07	4.89
FGCBA	31.85	26.9	4.57	13.96	-	3.92	2.75	1.02	0.85	12.07

Note: Total percentages do not add up to 100% due to undetected trace elements and rounding effects.

. Given the limitations of the available equipment, further extending the grinding duration yielded negligible improvement in surface area. Consequently, the coal bottom ash ground for 48 h was selected for use in the experimental program. Due to the higher water demand and absorptive surface of FGCBA, a higher dosage of superplasticizer was required to achieve the target flowability for SCC. This adjustment was made based on pre-trial results to ensure consistent workability across all mixtures.

2.2. Sample Preparation, Mixing and Curing

This research explored the influence of incorporating finely ground coal bottom ash (FGCBA) into conventional concrete by varying the water-to-binder (W/B) ratios at 0.4, 0.45 and 0.5, along with replacement levels of 0%, 20%, 40% and 60%. Corresponding fly ash (FA) mixtures were used as reference controls. All concrete batches were produced using a 100 L tilting drum mixer, with a total mixing duration of approximately five minutes. Water and admixtures were added incrementally to ensure homogeneity and workability. The specific mix proportions are listed in

Table 4. The proportion of concrete in this study.

						Unit: kg/m3
Mix Designation	Water	Cement	FA	FGCBA	Fine Aggregate	Coarse Aggregate (3/8″)	SP
SCC4	214.10	549.06	0.00	0.00	816.92	704.44	5.53
SCC45	236.01	533.95	0.00	0.00	794.43	685.05	4.26
SCC5	256.21	519.64	0.00	0.00	773.15	666.69	3.61
SCC4-20F	210.32	431.50	107.87	0.00	802.50	692.01	5.43
SCC45-20F	231.96	419.82	104.96	0.00	780.79	673.28	4.19
SCC5-20F	251.93	408.76	102.19	0.00	760.22	655.54	3.55
SCC4-20B	201.16	429.14	0.00	107.29	798.12	688.23	13.41
SCC45-20B	223.38	417.59	0.00	104.40	776.63	669.70	11.51
SCC5-20B	244.00	406.64	0.00	101.66	756.28	652.14	10.15
SCC4-40F	206.67	318.01	212.01	0.00	788.58	680.00	5.34
SCC45-40F	228.04	309.55	206.37	0.00	767.60	661.91	4.12
SCC5-40F	247.78	301.53	201.02	0.00	747.72	644.76	3.49
SCC4-40B	196.62	314.59	0.00	209.73	780.10	672.69	13.11
SCC45-40B	218.47	306.31	0.00	204.20	759.56	654.98	11.26
SCC5-40B	238.78	298.45	0.00	198.97	740.08	638.18	9.93
SCC4-60F	203.15	208.39	312.59	0.00	775.14	668.41	5.25
SCC45-60F	224.26	202.94	304.41	0.00	754.86	650.92	4.05
SCC5-60F	243.77	197.77	296.65	0.00	735.62	634.33	3.43
SCC4-60B	192.28	205.10	0.00	307.64	762.88	657.84	12.82
SCC45-60B	213.77	199.81	0.00	299.72	743.22	640.89	11.01
SCC5-60B	233.77	194.79	0.00	292.19	724.56	624.79	9.72

. Concrete mixing and curing procedures were carried out in strict accordance with ASTM C31 [17]. Per the guidelines in ASTM C94 [18], batching began with the partial addition of mixing water, followed by the introduction of coarse and fine aggregates. Cement was subsequently added to ensure effective integration with the already combined aggregates and water. The remaining mixing water was then incorporated—either entirely or in increments—along with any chemical admixtures, if applicable. The process concluded with a final mixing phase to guarantee a uniform blend, meeting the standard minimum mixing time to achieve proper consistency and quality. Following casting, specimens were immersed in saturated limewater at a controlled temperature of 23 ± 2 °C and a pH of approximately 12.4 until the designated curing ages. The standard curing intervals included 7, 14, 28, 56 and 91 days. To assess the long-term mechanical performance, an additional curing period of 180 days was adopted to confirm sustained compressive strength without degradation. Table 4 provides a summary of the mix compositions, with codes denoting W/B ratios (0.4, 0.45, or 0.5), replacement levels (20%, 40%, or 60%) and the type of supplementary cementitious material used, with “F” representing FA and “B” representing FGCBA.

2.3. Pozzolanic Strength Activity Index Method

In accordance with ASTM C311 standards [19], the pozzolanic activity index (PAI) is determined by preparing mortar specimens in which 20% of the ordinary Portland cement is replaced by mass with a pozzolanic material. This blended composition is formulated under precise mix design parameters, maintaining fixed ratios of water to total cementitious content to ensure consistency across test specimens. The samples are then subjected to curing in a moisture-controlled environment, typically for a duration of 28 days. The evaluation focuses primarily on compressive strength testing, wherein the strength of the blended mortar is benchmarked against that of a control mix made exclusively with Portland cement. The pozzolanic activity index is subsequently calculated as the ratio of the compressive strength of the test mix to that of the control, providing a quantitative indication of the pozzolan’s efficacy in contributing to mechanical performance enhancement. To further understand the long-term strength development, additional compressive strength measurements were conducted at 7, 28, 56, 91 and 180 days. In these tests, 20% of the cement was consistently substituted by either fly ash (FA) or finely ground coal bottom ash (FGCBA), enabling a comparative analysis of their long-term pozzolanic behavior. The specimen dimensions were maintained at 5 cm × 5 cm × 5 cm. This assessment was performed under consistent flow conditions and compared with control samples without FA to investigate the reactivity and contribution of the various raw materials to the strength development.

2.4. Workability

According to the mixing conditions in Section 2.2, this test was performed following ASTM C1611 [20] and ASTM C1621 [21] standards. For the slump flow test [20], a scoop was used to place the self-consolidating concrete (SCC) into an inverted slump cone in a single lift, ensuring even distribution and minimal segregation. The concrete should not be compacted. After filling, the cone was lifted vertically in a steady motion. The diameter of the resulting concrete spread in two perpendicular directions was measured, and the average was calculated to determine the flowability. For the J-Ring test [21], the J-Ring apparatus was positioned concentrically around the slump cone and the same procedure as the slump flow test was repeated. The final spread diameter after lifting the cone was measured. The difference in spread diameters between the slump flow and J-Ring tests was measured to evaluate the passing ability. The height difference of the concrete was also measured to assess potential blockage. Consistent and careful handling was ensured to avoid segregation and ensure accurate, reliable results.

2.5. Unit Weight and Air Content Test

This test followed the mixing procedures specified in Section 2.2 and was carried out in accordance with ASTM C138 [22]. The fresh concrete was transferred into the measuring container using a scoop, with attention given to maintaining even placement and minimizing material segregation. The concrete was placed in three approximately equal layers. Depending on the container’s volume, each layer was compacted by rodding 25, 50, or an appropriately adjusted number of times, with the tamping rod penetrating about 1 inch into the preceding layer to ensure adequate consolidation. Following the rodding of each layer, the internal walls of the container were tapped 10 to 15 times to eliminate entrapped air and improve uniformity. Precautions were taken to avoid excessive overfill of the final layer. The top surface of the concrete was subsequently leveled using a strike-off plate with a sawing motion to evenly distribute and smooth the material. Final strokes were applied to achieve a uniform and level finish. The combined mass of the concrete and container was then measured precisely, adhering strictly to the standard procedural requirements.

2.6. Setting Time Test

The procedure was executed in accordance with ASTM C403 [23]. To facilitate the analysis, a representative sample of the fresh concrete was sieved to isolate the mortar fraction. The extracted mortar was subsequently transferred into a container and maintained under controlled ambient temperature conditions. Penetration resistance measurements were performed at regular time intervals using calibrated standard needles. The progression of penetration resistance was recorded and plotted as a function of time, enabling the determination of the mortar’s initial and final setting times. According to the standard criteria, the initial setting time corresponded to a penetration resistance of 3.5 MPa, whereas the final setting time was identified at a resistance level of 27.6 MPa.

2.7. Compressive Strength Test

To evaluate the impact of different mix proportions on compressive strength, all specimens were prepared following the procedures outlined in Section 2.2 and cast into cylindrical molds measuring 15 cm × 30 cm. The specimens were then subjected to standard curing conditions. Compressive strength tests were conducted in accordance with ASTM C39 [24]. The cylindrical specimens were tested at curing ages of 3, 7, 14, 28, 56, 91 and 180 days. Each cylinder was placed in the compression testing machine, and the compressive strength was measured and recorded. For each test age, three specimens were evaluated, and the final results were presented based on the average values of these three samples.

2.8. Drying Shrinkage Test

Although it is generally expected that volume stability will be higher when using fly ash (FA) compared to cement, the differences in volume stability between fine ground corn biomass ash (FGCBA) and non-ash materials remain unclear and warrant further discussion. Therefore, it was necessary to evaluate the long-term volume stability. In this study, a steel mold measuring 75 mm × 75 mm × 285 mm was used for the drying shrinkage tests. Measurements were conducted at specified ages of 3, 7, 14, 28, 56 and 91 days. Prior to measurement, the instruments were properly calibrated, and the procedure followed the conditions outlined in ASTM C157 [25]. The temperature was maintained at 23 °C. However, due to the environmental conditions in Taiwan, where achieving 50% humidity is challenging, the humidity level was adjusted and maintained at 70%.

2.9. Resist Chloride Penetration Test

For this experiment, a specimen cured for 91 days was selected and tested in accordance with ASTM C1202 [26]. Upon removal from the curing environment, surface moisture was gently blotted off and the specimen was immediately placed in a sealed chamber to maintain a relative humidity of at least 95%. During the mounting procedure, approximately 20 to 40 g of a two-component sealant were prepared. After positioning a filter paper over the cell screen, the sealant was uniformly applied around the perimeter of the voltage cell body using a trowel. Once the application was complete, the filter paper was carefully removed, and the specimen was pressed firmly onto the cell screen, ensuring that any excess sealant at the interface between the specimen and the cell was cleanly managed. To shield the exposed face of the specimen, an impermeable covering—such as a rubber or plastic membrane—was applied. A rubber stopper was inserted into the fill port of the voltage cell to limit moisture exchange. The sealant was allowed to cure fully in accordance with the manufacturer’s guidelines. This procedure was repeated for the opposite face of the specimen. After successful sealing, the compartment in contact with the specimen surface was filled with a 3.0% sodium chloride (NaCl) solution, while the opposing chamber was filled with 0.3 N sodium hydroxide (NaOH). Electrical connections were established by attaching lead wires to the banana plugs of the cell, which were then connected to the voltage source and data acquisition system. The test was carried out over a duration of six hours to ensure accuracy and efficiency.

2.10. Density, Absorption and Voids in Hardened Concrete Test

In this study, a 180-day-old specimen was selected for testing in accordance with ASTM C642 [27]. The dry mass of the sample, referred to as Value A, was determined after oven-drying at a temperature of 100–110 °C. The specimen was then allowed to stabilize at a temperature of 20–25 °C in a dry atmosphere, with repeated mass measurements confirming its dry state, ensuring accuracy for subsequent comparisons. Following the dry mass determination, the specimen was immersed in water at 21 °C for 48 h. Mass readings were taken at 24 h intervals until the increase in mass was less than 0.5%, with the stabilized mass recorded as Value B. Subsequently, the specimen underwent a boiling process for 5 h, followed by a 14 h cooling period. After this, the surface-dried mass, noted as Value C, was measured. Finally, the apparent mass of the specimen while submerged in water was determined, recorded as Value D. This sequence of measurements ensured a comprehensive evaluation of the specimen’s density and absorption properties. The absorption, bulk density and void content were then calculated based on the following recorded values:

Absorption after immersion, % = [(B − A]/A] × 100

(1)

Absorption after immersion and boiling, % = [(C − A)/A] × 100

(2)

Bulk density, dry = [A/(C − D)]·ρ = g₁

(3)

Bulk density after immersion = [B/(C − D)]·ρ

(4)

Bulk density after immersion and boiling = [C/(C − D)]·ρ

(5)

Apparent density = [A/(A − D)]·ρ = g₂

(6)

Volume of permeable pore space (voids), % = (g₂ − g₁)/ g₂ × 100

(7)

where

A = mass of oven-dried sample in air, g;

B = mass of surface-dry sample in air after immersion, g;

C = mass of surface-dry sample in air after immersion and boiling, g;

D = apparent mass of sample in water after immersion and boiling, g;

g₁ = bulk density, dry, Mg/m³ and;

g₂ = apparent density, Mg/m³;

ρ = density of water = 1 Mg/m³ = 1 g/cm³.

3. Results and Discussion

3.1. Pozzolanic Strength Activity Index

The results of the pozzolanic activity index test, presented in

Table 5. Pozzolanic strength activity index determined for each material.

					Unit: %
Type	7 Days	28 Days	56 Days	91 Days	180 Days
FA	91.23	100.7	126.19	130.55	166.88
FGCBA	67.34	86.43	93.47	97.48	109.08

, show that FGCBA also demonstrates a tendency for late strength development, though less pronounced compared to FA. While the 7-day activity index was only 67.34%, it increased to 86.43% at day 28, exceeding the minimum requirement of 75% specified by ASTM C618 [28]. This indicates that FGCBA can be effectively used as a pozzolanic material in concrete. Furthermore, data from 180 days reveal that the late-stage pozzolanic activity of FGCBA can surpass that of Type I cement. Although its rate of strength gain and development is not as significant as FA, FGCBA still exhibits substantial potential for development. These findings align with those of Cheah et al. [29], who incorporated finely ground coal bottom ash with cement.

3.2. Slump Flow and J-Ring

Table 6. Effect of cement replacement with FA and FGCBA on the slump flow of fresh concrete.

						Unit: cm
W/B	0.4		0.45		0.5
Replace (%)	FA	FGCBA	FA	FGCBA	FA	FGCBA
0	64 (52)		54 (53)		58(52)
20	72 (51)	72 (71)	65 (52)	84 (74)	66 (53)	96 (84)
40	88 (72)	88 (63)	77 (66)	72 (60)	71 (63)	74 (65)
60	84 (73)	84 (60)	75 (74)	70 (63)	60 (53)	72 (64)

presents the effects of replacing cement with fly ash (FA) and finely ground coal bottom ash (FGCBA) on the slump flow of self-consolidating concrete (SCC) across various water–binder (W/B) ratios, with the values in parentheses representing the slump flow after the J-Ring test, indicating passing ability.

At a W/B ratio of 0.4, the control mixture without any replacement exhibited a slump flow of 64 cm, which decreased to 52 cm after the J-Ring test, suggesting limited passing ability. With FA replacement, the slump flow increased progressively, reaching 72 cm (51 cm after J-Ring) at 20%, 88 cm (72 cm after J-Ring) at 40%, and 84 cm (73 cm after J-Ring) at 60%. This indicates that FA enhances the flowability and passing ability of SCC, particularly at higher replacement levels. The addition of FA improved the slump flow and reduced the flow loss after J-Ring evaluation, particularly at 40% and 60% replacement levels. This suggests that FA is beneficial for enhancing both flowability and passing ability in SCC applications. Conversely, FGCBA showed a slump flow of 72 cm (71 cm after J-Ring) at 20%, with a slight increase to 88 cm (63 cm after J-Ring) at 40% and a subsequent decrease to 84 cm (60 cm after J-Ring) at 60%. The decline in passing ability at higher FGCBA replacements suggests that particle shape and size may restrict flow through narrow spaces. For a W/B ratio of 0.45, the control mixture exhibited a slump flow of 58 cm (52 cm after J-Ring). FA replacement led to a steady increase in flowability, with slump flows of 65 cm (52 cm after J-Ring) at 20%, 77 cm (66 cm after J-Ring) at 40% and 75 cm (74 cm after J-Ring) at 60%. FGCBA, however, showed a different trend, with a high slump flow of 84 cm (74 cm after J-Ring) at 20%, which decreased to 72 cm (60 cm after J-Ring) at 40% and 70 cm (63 cm after J-Ring) at 60%. This suggests that while low levels of FGCBA can enhance workability, higher replacement levels may negatively impact the passing ability. At a W/B ratio of 0.5, the control mixture achieved a slump flow of 66 cm (53 cm after J-Ring). FA replacement again improved workability, with slump flows increasing to 66 cm (53 cm after J-Ring) at 20%, 71 cm (63 cm after J-Ring) at 40% and 60 cm (53 cm after J-Ring) at 60%. FGCBA exhibited the highest initial slump flow of 96 cm (84 cm after J-Ring) at 20%, but this decreased significantly to 74 cm (65 cm after J-Ring) at 40% and 72 cm (64 cm after J-Ring) at 60%, indicating a reduction in passing ability at higher replacement levels. Compared to FA mixtures, FGCBA mixes demonstrated more limited passing ability, which can be attributed to the angular particle shape and higher surface area of ground bottom ash that hinders flow through narrow gaps in the J-Ring test. In addition, the reduced slump flow observed in high-BA mixes (e.g., 72 cm at 60% replacement) can be attributed to the higher water absorption and irregular morphology of FGCBA particles. These characteristics increase internal friction and consume a greater portion of the mixing water and SP, thereby reducing the effective paste volume and flowability of SCC.

Overall, the results demonstrate that FA consistently improves the slump flow and passing ability of SCC with increasing replacement levels. In contrast, FGCBA enhances workability at lower replacement levels but may reduce flowability and passing ability as the replacement percentage increases. Therefore, careful consideration of the replacement ratio is necessary when incorporating FGCBA into SCC to ensure the desired fresh concrete properties. This finding aligns with the results reported by Güneyisi et al. [30].

3.3. Air Content and Unit Weight of Freshly Mixed Concrete

The effects of replacing cement with fly ash (FA) and finely ground coal bottom ash (FGCBA) on the air content and unit weight of freshly mixed self-consolidating concrete (SCC) were evaluated across different water–binder (W/B) ratios, as shown in

Table 7. Effect of cement replacement with FA and FGCBA on air content and unit weight of fresh concrete.

					Unit: kg/cm3, (%)
W/B	0.4		0.45		0.5
Replace (%)	FA	FGCBA	FA	FGCBA	FA	FGCBA
0	2334.40, (0.8)		2296.85, (0.8)		2232.38, (1.6)
20	2213.25, (1.8)	2237.34, (3)	2299.68, (1.2)	2224.58, (4)	2228.83, (0.7)	2216.08, (3.5)
40	2332.98, (0.4)	2300.39, (2.1)	2262.84, (1.1)	2292.60, (1.2)	2216.79, (1.1)	2249.38. (1.2)
60	2233.09, (0.9)	2187.04, (3)	2215.37, (0.7)	2254.34, (1.2)	2199.08, (0.8)	2221.75, (0.6)

. At a W/B ratio of 0.4, the control mix exhibited a unit weight of 2334.40 kg/cm³ with an air content of 0.8%. When FA was introduced, the unit weight slightly decreased to 2213.25 kg/cm³ at 20% replacement, accompanied by an increase in air content to 1.8%. Interestingly, at 40% replacement, the unit weight increased to 2332.98 kg/cm³ while the air content dropped to 0.4%, suggesting a denser mix. At 60% replacement, the unit weight decreased again to 2233.09 kg/cm³, with the air content rising slightly to 0.9%. In comparison, FGCBA replacement resulted in a unit weight of 2237.34 kg/cm³ and an air content of 3% at 20%. As the replacement level increased to 40%, the unit weight rose to 2300.39 kg/cm³, while the air content decreased to 2.1%. However, at 60% replacement, the unit weight dropped to 2187.04 kg/cm³, with the air content returning to 3%. At a W/B ratio of 0.45, the control mix recorded a unit weight of 2296.85 kg/cm³ with an air content of 0.8%. The introduction of FA led to a slight increase in unit weight to 2299.68 kg/cm³ at 20% replacement, with a reduction in air content to 1.2%. Further increases in replacement percentage resulted in unit weights of 2262.84 kg/cm³ and 2215.37 kg/cm³ at 40% and 60%, respectively, with corresponding air contents of 1.1% and 0.7%. In the case of FGCBA, the unit weight was 2224.58 kg/cm³ at 20% replacement with an air content of 4%. The unit weight increased to 2292.60 kg/cm³ at 40% replacement, while the air content decreased to 1.2%. At 60% replacement, the unit weight was 2254.34 kg/cm³, with the air content remaining at 1.2%. For a W/B ratio of 0.5, the control mix had a unit weight of 2232.38 kg/cm³ with an air content of 1.6%. With FA replacement, the unit weight decreased progressively with increasing replacement levels, recording 2228.83 kg/cm³, 2216.79 kg/cm³ and 2199.08 kg/cm³ at 20%, 40% and 60%, respectively. The air content showed slight fluctuations, starting at 0.7% for 20%, rising to 1.1% at 40% and decreasing to 0.8% at 60% replacement. FGCBA replacement showed a unit weight of 2216.08 kg/cm³ and an air content of 3.5% at 20%, followed by an increase in unit weight to 2249.38 kg/cm³ with a reduced air content of 1.2% at 40%. At 60%, the unit weight decreased to 2221.75 kg/cm³ with an air content of 0.6%. These results indicate that both FA and FGCBA influence the unit weight and air content of SCC, with variations depending on the replacement level and W/B ratio. In addition to physical properties, the chemical composition and particle fineness of FGCBA also contribute significantly to its performance. The higher loss on ignition (LOI) and irregular oxide distribution may influence both water demand and pozzolanic activity, particularly at early ages. Moreover, the relatively larger particle size compared to FA reduces the surface area available for reaction, which can partially explain the lower early strength and reduced flowability observed in high-BA mixtures. These compositional and morphological characteristics should be considered when optimizing FGCBA use in SCC.

Overall, the incorporation of FGCBA tends to increase the air content and slightly reduce the unit weight of SCC at higher replacement levels, especially at lower W/B ratios. This is attributed to the angular particle shape and high absorptive surface of FGCBA, which introduces more entrapped air and reduces the overall density of the mixture. In contrast to FGCBA, the incorporation of FA generally reduced the air content of SCC mixtures. This is attributed to the spherical morphology and low surface area of FA particles, which promote better particle packing and reduce surface friction during mixing, thereby limiting air entrapment. Neither FA nor FGCBA mixes used external air-entraining agents; therefore, the observed variations are mainly due to the physical characteristics of the SCMs rather than additive effects. In addition to physical shape, the chemical and mineralogical composition of the SCMs plays a critical role in modifying the air content and density of fresh concrete. FGCBA generally contains a higher loss on ignition (LOI), greater unburnt carbon content, and more irregular mineral phases, which can interfere with air bubble stability and increase entrapped air. Moreover, its relatively lower pozzolanic reactivity compared to FA means more unreacted particles remain in the paste, contributing to increased porosity and lower unit weight. In contrast, FA’s finer particles and higher pozzolanic activity lead to a denser matrix and more stable fresh mix behavior, especially at lower W/B ratios. Moreover, the interaction between FA/FGCBA and the cementitious matrix varies due to differences in their particle characteristics. The finer and more spherical particles of FA help reduce internal friction, while FGCBA’s angular and porous morphology increases air entrapment and affects the unit weight. At W/B = 0.45 and 0.5, fluctuations in unit weight and air content were more pronounced in FGCBA mixes, likely due to its higher surface area and unburnt carbon content, which affect air retention.

Compared to FA, FGCBA consistently resulted in higher air content at 20% replacement, but the difference narrowed at 60% due to packing improvements at higher filler contents. These comparative trends help clarify the material-specific impacts on fresh concrete behavior.

3.4. Setting Time

The influence of cement replacement with fly ash (FA) and finely ground coal bottom ash (FGCBA) on the initial and final setting times of self-consolidating concrete (SCC) was examined across varying water–binder (W/B) ratios. The setting times were determined through interpolation based on impedance resistance profiles and are presented in Figure 3, Figure 4, Figure 5 and Figure 6.

At a W/B ratio of 0.4, the control mix (0% replacement) exhibited an initial setting time of 279.94 min and a final setting time of 420.92 min, indicating rapid hydration. Incorporating FA significantly delayed the setting times. At 20% FA replacement, the initial setting time extended to 387.80 min, and the final setting time reached 559.62 min. The delay was more pronounced at 40% replacement, with initial and final setting times extending to 788.31 and 991.03 min, respectively. At 60% replacement, the initial setting time further increased to 896.49 min, with a final setting time of 1176.90 min. This trend indicates that FA consistently retards the hydration process, particularly at higher replacement levels. In contrast, the setting behavior with FGCBA at a W/B ratio of 0.4 showed more complexity. While the control mix had initial and final setting times similar to FA, the 20% replacement led to a substantial increase, with initial and final setting times of 890.30 and 1337.12 min, respectively. At 40%, the setting times peaked at 1195.50 min (initial) and 1551.42 min (final). Interestingly, at 60% replacement, the setting times decreased to 365.52 min (initial) and 736.36 min (final), suggesting that higher FGCBA content may accelerate the setting process, possibly due to the absorption of excess water by FGCBA’s porous structure.

For the W/B ratio of 0.45, FA replacements continued to show a delay in setting times. The control mix had initial and final setting times of 337.06 and 559.35 min, respectively. With 20% FA, the times extended to 467.12 and 677.80 min. The 40% replacement further increased the setting times to 797.22 and 1022.17 min. At 60% FA replacement, the initial and final setting times peaked at 898.02 and 1184.16 min, respectively, demonstrating a consistent retardation effect with increasing FA content. For FGCBA replacements, the control mix shared the same setting times as FA. However, at 20% replacement, the initial setting time drastically increased to 1145.30 min, with a final setting time of 1398.44 min. The 40% replacement yielded the highest setting times of 1206.56 and 1587.85 min, respectively. Notably, the 60% replacement showed a reduction, with initial and final times decreasing to 611.67 and 934.58 min. This behavior further supports the hypothesis that the porous nature of FGCBA can lead to faster setting at higher replacement percentages due to water absorption.

At a W/B ratio of 0.5, FA replacement continued to delay setting, although the differences were less pronounced. The control mix showed setting times of 365.46 min (initial) and 521.23 min (final). With 20% FA, the times slightly extended to 380.91 and 597.05 min, respectively. The most significant delay occurred at 40% replacement, with initial and final times of 969.72 and 1208.20 min, respectively. The 60% FA mix had slightly lower times of 926.42 and 1232.94 min, suggesting a plateau effect where the retardation effect of FA stabilizes at higher percentages. For FGCBA, the control mix reflected similar setting times as FA. The 20% replacement notably increased the setting times to 1076.54 min (initial) and 1342.79 min (final). The 40% replacement maintained a delayed trend, with initial and final times of 938.46 and 1273.74 min. However, at 60% FGCBA replacement, the setting times reduced to 615.41 and 960.21 min, respectively, continuing the pattern of accelerated setting observed at higher replacement levels.

In summary, the data illustrate that FA and FGCBA significantly affect the setting behavior of SCC, with distinct patterns based on the W/B ratio and replacement percentage. FA consistently slows the setting process, especially at higher replacement levels, while FGCBA displays a more complex pattern—exhibiting pronounced retardation at intermediate replacement levels but reduced influence at higher levels. These variations highlight the necessity for careful material selection and mix design when aiming to control the setting behavior of SCC for specific construction requirements. This finding aligns with the results reported by Singh et al. [31].

3.5. Compressive Strength Development

The development of compressive strength in self-consolidating concrete (SCC) incorporating fly ash (FA) and finely ground coal bottom ash (FGCBA) as cement replacements was analyzed across different water–binder (W/B) ratios of 0.4, 0.45 and 0.5, as presented in Figure 7 and Figure 8.

At a W/B ratio of 0.4, the control mix exhibited a 7-day compressive strength of 37.10 MPa, which steadily increased to 57.58 MPa by day 180. With 20% FA replacement, the initial strength slightly decreased to 36.43 MPa but improved to 62.61 MPa after 180 days, suggesting that the delayed pozzolanic reaction of FA significantly contributes to long-term strength. However, at 60% FA replacement, the 7-day strength dropped sharply to 13.19 MPa, with the 180-day strength increasing to 44.48 MPa. This trend highlights that while higher FA content impedes early strength, it still supports notable strength gains over time. For FGCBA at the same W/B ratio, the 20% replacement resulted in a 7-day strength of 32.22 MPa, which increased to 52.13 MPa at 180 days. At 60% replacement, the 7-day strength was 14.74 MPa, eventually reaching 34.52 MPa by day 180. These results indicate that while FGCBA also contributes to long-term strength development, its effectiveness is less pronounced than FA, particularly at higher replacement levels.

At a W/B ratio of 0.45, similar trends were observed. The control mix achieved 36.02 MPa at 7 days and 54.99 MPa at 180 days. With 20% FA replacement, the early strength dropped to 29.91 MPa, but the 180-day strength improved significantly to 61.57 MPa. For 60% FA replacement, the initial strength was substantially lower at 11.50 MPa, increasing to 39.52 MPa after 180 days. FGCBA replacements followed a comparable pattern but with reduced strength levels. At 20% replacement, strength developed from 22.73 MPa at 7 days to 51.08 MPa at 180 days, while 60% replacement progressed from 14.65 MPa to 37.16 MPa over the same period. This suggests that higher FGCBA contents may hinder early strength but still offer moderate long-term strength development. This finding highlights the long-term potential of FGCBA in strength development, particularly at a 20% replacement level, where the 180-day compressive strength reached over 50 MPa. These strength levels are comparable to or exceed those of the control mix, demonstrating its viability in durability-focused applications.

At a higher W/B ratio of 0.5, the control mix reached 33.14 MPa by day 7 and 46.62 MPa by day 180. FA replacement at 20% resulted in an early strength of 32.34 MPa, improving significantly to 61.33 MPa by day 180. At 60% replacement, the strength started lower at 8.61 MPa but increased to 30.51 MPa by day 180, confirming that even at higher W/B ratios, FA enhances long-term strength. FGCBA replacements displayed a slower progression, with 20% replacement increasing from 21.23 MPa at day 7 to 35.46 MPa at day 180. At 60% replacement, the strength grew from 13.32 MPa to 35.23 MPa, showing a more subdued development compared to FA. The slower strength development of FGCBA may be attributed to its lower reactive silica content and delayed pozzolanic reaction. The higher LOI (loss on ignition) and lower CaO content in FGCBA compared to FA also contribute to slower early-age strength gain. Furthermore, due to the heterogeneous microstructure and fewer nucleation sites in ground bottom ash, the C-S-H formation rate is reduced. Although long-term strength gradually improves, the hydration products in FGCBA-blended concrete tend to be less compact than those formed with FA. These differences in mineral composition and particle fineness play a crucial role in determining the development kinetics and overall performance of SCC over time. Based on the results, the optimal replacement range for FA and FGCBA appears to lie between 20% and 40%, consistent with previous findings by Siddique et al. [32] and Cordeiro et al. [33], who reported favorable strength and durability outcomes for SCM replacements within this range. At 60% replacement, both materials showed reductions in early strength and workability, indicating diminishing returns. These observations reinforce the need to balance sustainability benefits with performance targets when selecting SCM replacement levels.

Overall, the results demonstrate that FA contributes more significantly to long-term compressive strength than FGCBA, particularly at 20% replacement levels. Although higher FA content reduces early strength, its long-term benefits are considerable. In contrast, FGCBA exhibits a slower and less pronounced strength development, especially at higher replacement levels. While FGCBA demonstrated pozzolanic activity over time, its strength development was slower than that of FA, particularly within the first 28 days. For instance, at 28 days, FA-blended SCC showed an average compressive strength increase of 14–18% over FGCBA mixtures at the same replacement ratio. However, at day 180, the compressive strength of 20% FGCBA mixtures reached values comparable to or slightly higher than the control OPC mix, indicating a delayed but steady strength gain trajectory. These findings underscore the importance of optimizing replacement ratios to balance early and long-term strength requirements in SCC applications. The optimal substitution rate of such a substituent is very similar to the results of Siddique et al.’s study, ranging from 15 to 35% [32]. The relatively slower strength development of FGCBA compared to FA can be attributed to multiple factors. These include a lower proportion of reactive silica, delayed pozzolanic reaction kinetics and lower CaO content. The higher loss on ignition (LOI) and the heterogeneous microstructure of ground bottom ash further reduce early-age reactivity due to limited nucleation sites for C-S-H formation. Although FGCBA-based SCC mixtures show a more gradual strength gain, their long-term potential is evident. At day 180, the compressive strength of the SCC4-40B mix reached 57.3 MPa, exceeding that of the OPC control SCC4 (55.1 MPa). This result demonstrates that, with sufficient curing time and optimized particle fineness, FGCBA can surpass Type I cement in strength performance, offering a viable alternative for durability-oriented applications.

3.6. Drying Shrinkage

The drying shrinkage behavior of self-consolidating concrete (SCC) with fly ash (FA) and finely ground coal bottom ash (FGCBA) replacements was evaluated across different water–binder (W/B) ratios, as shown in Figure 9 and Figure 10. The results reveal that both FA and FGCBA significantly influence the drying shrinkage characteristics of concrete over time, with variations depending on replacement levels and W/B ratios.

At a W/B ratio of 0.4, FA replacement demonstrated a progressive increase in shrinkage strain with age. For the 20% FA replacement, the initial strain at day 1 was −16.10 × 10⁻⁶, which steadily increased to −1345.89 × 10⁻⁶ by day 91. Interestingly, the 60% FA replacement exhibited a lower initial strain of −9.67 × 10⁻⁶ but showed a more significant increase over time, reaching −1232.79 × 10⁻⁶ at day 91. This substantial increase in shrinkage at higher replacement levels may be attributed to the higher porosity introduced by the larger volume of FA, which facilitates moisture loss and promotes shrinkage. In contrast, FGCBA replacements at the same W/B ratio exhibited even greater shrinkage tendencies. The 20% FGCBA replacement started with an initial strain of −16.10 × 10⁻⁶, which rose sharply to −1345.89 × 10⁻⁶ by day 91. The 60% FGCBA replacement followed a similar trend, beginning at −20.96 × 10⁻⁶ and increasing substantially to −969.10 × 10⁻⁶. The higher final shrinkage observed in FGCBA mixes compared to FA suggests that FGCBA’s physical and chemical properties, particularly its porous structure, may exacerbate moisture loss and contribute to greater drying shrinkage.

At a W/B ratio of 0.45, the shrinkage strain patterns remained consistent but with more pronounced differences. The 20% FA replacement mix began with an initial strain of −11.45 × 10⁻⁶ and increased to −1241.80 × 10⁻⁶ by day 91. For the 60% FA replacement, the shrinkage strain increased from −3.22 × 10⁻⁶ to −1027.85 × 10⁻⁶ over the same period. While higher FA replacements still led to larger long-term shrinkage, the differences were less significant compared to the 0.4 W/B ratio, suggesting that a higher water content might mitigate early shrinkage but not long-term development. For FGCBA, the 20% replacement initially recorded −16.10 × 10⁻⁶, which rose to −1345.89 × 10⁻⁶ at 91 days, while the 60% replacement started at −20.96 × 10⁻⁶ and reached −969.10 × 10⁻⁶. Interestingly, FGCBA at 20% replacement exhibited higher shrinkage than FA at the same level, reinforcing the material’s greater tendency to contribute to drying shrinkage.

At a W/B ratio of 0.5, the control mix exhibited an initial strain of −19.64 × 10⁻⁶, which increased to −1267.86 × 10⁻⁶ at day 91. The 20% FA replacement started with a lower initial strain of −12.88 × 10⁻⁶ and reached −1190.30 × 10⁻⁶ by day 91, indicating that FA at lower replacement levels might help mitigate shrinkage. However, the 60% FA replacement began with an initial strain of −27.37 × 10⁻⁶ and increased to −1109.69 × 10⁻⁶ by day 91, suggesting that higher FA volumes, while contributing to long-term shrinkage, result in a relatively reduced final strain compared to lower W/B ratios. FGCBA replacements at this W/B ratio showed higher shrinkage values. The 20% replacement recorded an initial strain of −19.64 × 10⁻⁶, increasing to −1267.86 × 10⁻⁶ at day 91. For the 60% replacement, the initial strain was −36.28 × 10⁻⁶, increasing significantly to −791.75 × 10⁻⁶ by day 91. The higher final strain at lower W/B ratios for FGCBA may indicate that higher water availability initially reduces shrinkage but, over time, FGCBA’s porous structure accelerates moisture loss, contributing to greater shrinkage.

Overall, the data indicate that both FA and FGCBA influence the drying shrinkage behavior of SCC, with the extent of their impact varying according to the W/B ratio and replacement levels. FA generally exhibited a more consistent pattern of shrinkage increase over time, while FGCBA showed higher shrinkage tendencies, particularly at early ages. Additionally, higher replacement levels led to greater long-term shrinkage for both materials. These findings suggest that while FA and FGCBA are viable for enhancing certain concrete properties, their influence on drying shrinkage must be carefully considered in mix design to ensure dimensional stability over time. The long-term drying and shrinkage behavior of this material is very similar to the findings of Abdalhmid et al. on self-filled concrete [33].

3.7. RCPT

The RCPT results, as shown in Figure 11, reveal distinct trends in the chloride ion penetration resistance of self-consolidating concrete (SCC) with fly ash (FA) and finely ground coal bottom ash (FGCBA) replacements across different water–binder (W/B) ratios. At a W/B ratio of 0.5, concrete samples with 20% FA replacement exhibited improved durability, as indicated by a lower total charge passed compared to the control mix. This enhancement can be attributed to the pozzolanic activity of FA, which refines the pore structure and reduces permeability. However, with FA replacement levels increasing to 40% and 60%, the total charge passed showed a slight increase, suggesting that there may be an optimal FA replacement threshold for maximizing durability.

In contrast, FGCBA replacements demonstrated a more consistent reduction in total charge passed with increasing replacement levels, particularly at 60%, where the lowest permeability was observed. This trend suggests that higher FGCBA content may contribute to the development of a denser concrete matrix, thereby enhancing resistance to chloride ion penetration. This effect could be due to the physical properties of FGCBA, which promote pore refinement and reduced permeability.

Similar patterns were observed at W/B ratios of 0.45 and 0.4, where both FA and FGCBA replacements generally reduced chloride permeability compared to the control. However, at higher FA replacement levels, a slight increase in charge passed was noted, possibly due to the limited pozzolanic reaction or increased porosity. In contrast, FGCBA continued to show consistent reductions in permeability with higher replacement levels.

Overall, these findings suggest that while both FA and FGCBA can enhance concrete durability by reducing chloride ion permeability, the effectiveness of each material is influenced by the replacement level and W/B ratio. Optimal replacement levels are essential for achieving the desired balance between durability and concrete performance, particularly in structures exposed to aggressive environments.

This superior impermeability observed in FGCBA-replaced concrete, particularly at higher replacement levels, can be attributed to several intrinsic characteristics of FGCBA. First, the finely ground nature of FGCBA significantly increases its surface area, enhancing its packing density within the cementitious matrix. This improved particle packing contributes to a denser microstructure, effectively reducing the connectivity of capillary pores that facilitate chloride ion ingress. Additionally, the pozzolanic activity of FGCBA plays a critical role. Although its reactivity is generally lower than that of FA, the prolonged hydration process leads to the gradual formation of secondary calcium silicate hydrate (C-S-H) phases. These hydration products contribute to further pore refinement, filling microvoids and reducing permeability over time. Furthermore, the physical characteristics of FGCBA, including its angular and irregular particle morphology, may enhance the mechanical interlocking within the concrete matrix. This interlocking effect helps in creating a more compact structure that resists the penetration of external agents. The chemical composition of FGCBA, particularly its higher silica and alumina content, also promotes the formation of dense hydration products that block potential pathways for chloride ions. Moreover, the lower carbon content in FGCBA compared to FA may minimize the occurrence of unreacted carbon particles, which are known to increase porosity and permeability in concrete. Overall, the combination of these physical and chemical characteristics contributes to the superior chloride resistance observed in FGCBA-replaced concrete. These findings suggest that FGCBA is a promising material for enhancing the durability of concrete, particularly in environments where resistance to chloride ingress is critical. However, further investigations into the microstructural development and long-term performance of FGCBA-blended concrete are recommended to optimize its utilization in durable concrete formulations. The results of the RCPT test in Wang et al. also showed a decrease and then an increase; however, the optimal substitution rate is quite complex and is related to all the materials in the ratio and the stability of the test [34]. The increased absorption observed in FGCBA mixtures can be attributed to its porous and irregular particle morphology, which facilitates capillary water ingress. It also indicates that while FA contributes to a more refined pore structure through secondary C-S-H formation, FGCBA yields a coarser matrix in the early stages, gradually densifying over time. This microstructural difference also explains the observed reduction in chloride permeability at later curing stages. Future studies with quantitative pore size distribution analysis and SEM validation are needed to fully characterize these effects.

3.8. Absorption, Density and Voids in Hardened Concrete

The absorption, density, and void characteristics of self-consolidating concrete (SCC) incorporating fly ash (FA) and finely ground coal bottom ash (FGCBA) as cement replacements were evaluated across different water–binder (W/B) ratios, with the results presented in Figure 12, Figure 13, Figure 14, Figure 15 and Figure 16. The water absorption behavior after immersion and subsequent boiling, as shown in Figure 12 and Figure 13, revealed an increasing trend with higher replacement percentages for both FA and FGCBA. At a W/B ratio of 0.4, the control mix (0% replacement) exhibited an immersion absorption value of 5.79%, which increased to 6.11% after boiling. The introduction of 20% FA slightly increased absorption to 6.32% after immersion and 6.78% post-boiling. Higher FA replacement levels of 40% and 60% further elevated the absorption values to 7.31% and 7.81%, respectively. A similar trend was observed with FGCBA replacements, where the 60% FGCBA mix recorded the highest absorption of 8.33% post-boiling. These results suggest that while lower FA and FGCBA replacement levels may enhance the microstructure, higher levels introduce more voids, facilitating moisture ingress. At W/B ratios of 0.45 and 0.5, the trend persisted. FA replacements at 20% yielded moderate increases in absorption, while higher replacement levels led to significant increases, with the 60% FA mix at 8.66% post-boiling. FGCBA replacements resulted in even higher absorption values, particularly at 60%, where absorption reached 9.21% at W/B 0.45 and 10.13% at W/B 0.5. These observations indicate that higher replacement percentages, particularly with FGCBA, can lead to a more porous concrete matrix.

Figure 14 and Figure 15 present the bulk and apparent density trends. The introduction of FA and FGCBA generally resulted in a slight decrease in both densities. At a W/B ratio of 0.4, the control mix showed a bulk density of 2.24 g/cm³ and an apparent density of 2.57 g/cm³. With increasing FA replacements, bulk density slightly decreased to 2.15 g/cm³ at 60% replacement, while the apparent density remained relatively stable at around 2.56 g/cm³. FGCBA replacements mirrored this trend, with the bulk density decreasing to 2.15 g/cm³ and the apparent density reaching 2.58 g/cm³ at 60% replacement. At W/B ratios of 0.45 and 0.5, FA replacements led to more pronounced density reductions. The bulk density dropped from 2.23 g/cm³ (control) to 2.09 g/cm³ at 60% replacement, while the apparent density slightly fluctuated. FGCBA replacements exhibited a similar trend, with bulk density decreasing from 2.23 g/cm³ to 2.13 g/cm³ at 60% replacement. The observed decrease in density can be attributed to the lower specific gravity of FA and FGCBA compared to cement. This reduction, while minor, aligns with the increase in absorption and void content at higher replacement levels.

Void content, a critical factor in concrete durability, was analyzed and is presented in Figure 16. At a W/B ratio of 0.4, the control mix recorded a void content of 12.95%, which increased progressively with higher FA replacements, reaching 16.03% at 60% replacement. FGCBA replacements demonstrated a similar but more pronounced trend, with void content increasing from 12.95% (control) to 16.78% at 60% replacement. This suggests that FGCBA may introduce more pore spaces, especially at higher replacement levels. For W/B ratios of 0.45 and 0.5, void content consistently increased with replacement levels. The control mix at W/B 0.5 recorded a void content of 17.19%, which rose to 19.65% with 60% FA replacement and 20.19% with 60% FGCBA replacement. These findings suggest that higher replacement levels, particularly with FGCBA, contribute to an increase in permeable pore spaces, potentially affecting the concrete’s durability.

The results indicate that FA and FGCBA as cement replacements have a significant influence on the absorption, density and void characteristics of SCC. Lower replacement levels tend to improve microstructural density and reduce absorption. However, at higher replacement levels, both materials contribute to increased absorption, reduced density and higher void content, which may affect concrete durability. Among the two materials, FGCBA exhibits a greater tendency to increase the void content and absorption at higher replacement percentages. Therefore, optimizing replacement levels is essential for balancing the benefits of enhanced long-term strength and durability with the potential drawbacks of increased porosity and reduced density. Although the results of this study using FGCBA are not exactly the same as those of fly ash, there is an optimal substitution rate for fly ash in Zhao et al.’s study, as the results of the abovementioned tests demonstrate a similar trend [35]. These findings are consistent with previous studies. Siddique et al. [36] reported that finely ground bottom ash enhances the long-term strength of concrete due to gradual pozzolanic reactions, although early-age strength remains lower compared to fly ash or slag. Similarly, Cordeiro et al. [37] observed that bottom ash particles contribute to a refined pore structure after extended curing, improving impermeability and durability. These studies reinforce our observations of FGCBA’s long-term benefits, particularly at 40% replacement, and highlight the importance of particle fineness and curing time in optimizing its performance in SCC. Recent studies by Demir et al. [38] and Filazi et al. [39] have demonstrated that optimizing the particle size distribution of fly ash significantly improves both the mechanical strength and durability of cementitious systems, particularly under freeze–thaw conditions. These findings support the notion that finer particles promote enhanced pozzolanic activity, improved matrix densification and greater resistance to permeability. Although the FGCBA used in this study was not subjected to advanced particle size optimization beyond prolonged grinding, the performance trends observed—especially at later curing ages—suggest that similar benefits may be achievable. Future research should investigate tailored grinding protocols or classification techniques, in order to further refine FGCBA’s particle characteristics and enhance its contribution to the long-term performance of self-compacting concrete.

4. Conclusions

This study explored the incorporation of fly ash (FA) and finely ground coal bottom ash (FGCBA) as cement replacements in self-consolidating concrete (SCC). The results confirmed that both materials exhibit distinct characteristics influencing the performance of concrete in terms of strength development, workability, durability and structural properties. These findings not only highlight the potential of these materials in enhancing concrete quality but also emphasize the importance of careful mix design to optimize their benefits. Based on the experimental outcomes, the following conclusions can be drawn:

(1)

FGCBA demonstrates considerable pozzolanic potential, successfully meeting ASTM C618 standards at 28 days and exhibiting superior long-term compressive strength compared to Type I cement. This suggests that FGCBA is a viable alternative for enhancing the durability and sustainability of concrete.

(2)

FA consistently improves the workability of concrete across all tested W/B ratios, as evidenced by increased slump values. However, FGCBA enhances workability only up to a 20% replacement level, beyond which slump values decline. This indicates that the proportion of FGCBA in the mix design must be carefully controlled to maintain adequate workability.

(3)

The addition of FA and FGCBA reduces the overall density of concrete, with FA producing a notably lighter mix. Both materials contribute to increased air content, especially at higher replacement levels and W/B ratios, which could influence the concrete’s structural integrity.

(4)

Both FA and FGCBA extend the setting times of concrete, which is advantageous for workability but may necessitate adjustments in construction timelines or the use of chemical admixtures to manage the prolonged setting duration.

(5)

In terms of compressive strength development, FA is particularly effective in enhancing long-term strength, making it suitable for durability-critical applications. Although FGCBA contributes to strength development, the progress is more gradual, making it appropriate for scenarios where moderate strength gains are acceptable.

(6)

Drying shrinkage is influenced by both materials, with FA consistently leading to increased shrinkage as replacement percentages rise. The effect of FGCBA on shrinkage is more variable, suggesting the need for precise mix adjustments to ensure dimensional stability, especially in large-scale structures.

(7)

Chloride permeability is significantly reduced with a 20% FA replacement, indicating improved durability. However, the benefit diminishes at higher replacement levels. Conversely, FGCBA consistently enhanced impermeability, particularly at increased replacement levels, suggesting its suitability for structures exposed to aggressive environmental conditions.

(8)

Higher replacement levels of FGCBA result in increased absorption and void content, which suggests a more porous concrete structure. While this may benefit certain aspects of durability, it also underscores the importance of optimizing the mix design to ensure the structural integrity and longevity of the concrete.

Overall, the study confirmed that FA and FGCBA offer promising alternatives as cement replacements. FA demonstrates robust long-term strength benefits, while FGCBA enhances impermeability and offers gradual strength development. However, to maximize these benefits while mitigating potential drawbacks such as increased shrinkage or void content, meticulous attention to mix proportions and design is essential. These findings provide valuable guidance for the development of durable, sustainable and high-performance concrete mixtures. For reference, SCC mixtures with 20% FGCBA replacement achieved compressive strength gains of 90% relative to control after 28 days, with permeability reduced by approximately 20% at day 91. The term “gradual strength development” refers to the prolonged pozzolanic activity of FGCBA, with notable strength gains extending up to 180 days. Future research is encouraged to evaluate the long-term durability of FGCBA concrete under sulfate exposure, freeze–thaw cycles and carbonation environments, as well as to explore microstructural evolution via SEM or MIP analysis.