Determining Optimum Coal Bottom Ash/Slag Content for Sustainable Concrete Infrastructure

Abstract

Concrete usage is increasing rapidly; subsequently, the industry’s carbon footprint is increasing and impacting the environment significantly. Large amounts of fine and coarse aggregate, including cement, are needed to fulfill the increased demand, leading to increased natural aggregate usage. Therefore, finding a fine aggregate replacement in concrete is essential. Coal bottom ash (CBA) and coal boiler slag (CBS), byproducts of coal-burning power plants with pozzolanic properties, can replace fine aggregate in concrete to reduce global natural material depletion, health hazards, and technical and economic problems associated with power plants’ solid waste. This study was conducted to determine the optimum fine aggregate replacement amount of CBA and CBS in concrete while improving concrete performance. The optimum CBA and CBS content is 50%, which reduces fine aggregate usage in a concrete mix by 50% while maintaining equivalent or better concrete strength than the control. The optimum CBA content has a unit weight lower than the control for all mixes tested in this study, which makes the CBA mix lightweight concrete. The optimum CBA concrete has 15%, 43%, and 42% higher compressive strength than the control after 7 days, 28 days, and 56 days of curing, respectively. On the other hand, the optimum CBS concrete has 12%,16%, and 16% increased compressive strength than the control after 7, 28, and 56 days of curing, respectively. The compressive strength of optimum CBA concrete was higher than the optimum CBS, indicating that CBA concrete yields higher compressive strength than CBS, possibly due to the difference in physical properties, water absorption capacity, and bulk density. Nanoclay increased CBA concrete compressive strength at an early stage and increased the optimum content to 80% CBA. Therefore, using CBA and CBS can significantly reduce natural material usage and environmental harm by reducing CBA waste disposal and improving concrete performance.

1. Introduction

The construction industry is growing rapidly due to economic growth, leading to high construction material consumption. Concrete is the construction industry’s primary product, which increases the need for raw materials, such as cement, aggregate, and cement for concrete production [1]. Concrete is a vital construction material; therefore, its consumption increases the demand for Portland cement, which leads to CO₂ emissions and environmental pollution [2]. Cement production accounts for 8% of global CO₂ emissions, with a high global carbon footprint [3]. In the construction industry, a lot of fine aggregates are used with cement. Natural aggregate usage has increased in many countries [4]; therefore, finding a replacement for aggregate materials is essential. Researchers have recently focused on industrial residues or solid waste for concrete applications [5]. Using waste materials, such as coal bottom ash (CBA) and coal boiler slag (CBS), in concrete can reduce the environmental impact caused by using considerable amounts of fine aggregates [6].

CBA and CBS are industrial waste materials accessible worldwide that can be used as a fine aggregate in concrete [7]. CBA and CBS are byproducts of coal-fired power plants collected from the bottom of coal-burning furnaces. CBA and CBS products depend on the types of coal-burning furnaces. A water-filled tanker collects CBA at the bottom of the burning furnace and transfers it to the basin for dewatering using a high-pressure water jet. Approximately 20% of the unburned materials at the dry bottom boiler are bottom ash. Bottom ash has a particle size similar to fine natural aggregate but is lightweight and brittle [8]. The bottom ash gathers at the bottom when it is molten and is transferred to the ash hopper in the presence of water. The molten slag touches the water, and then the bottom ash cracks or breaks, forming CBS, a hard, coarse, glassy material [9].

A recent study indicates a significant increase in CBA and CBS waste; approximately 730 million tons of CBA and CBS are produced worldwide [10]. However, only 61% percent of the CBS and 41% of the CBA produced in the United States were used in 2021 [11]. CBA/CBS disposal in ponds threatens the environment and human health, since the hazardous constituents migrate and can contaminate ground or surface water, soil, and living organisms. Therefore, using CBA as a replacement in concrete mixes reduces its effect on the environment and human health [12].

A test was conducted by Cheriaf et al. [13] to see the effect of CBA on hydration reaction by comparing a paste with a similar amount of calcium hydroxide and CBA at different curing ages and checking the strength and calcium hydroxide consumption. CBA accelerates the pozzolanic reaction of concrete after 28 days of curing. CBA does not consume calcium hydroxide at an early age of the hydration reaction. Pozzolanic materials are used to partially replace Portland cement (OPC) in concrete due to the Pozzolanic properties that depend on the physical and chemical properties of the pozzolanic materials [14]. According to Ganesan et al. [7]’s study, replacing 40% of the fine aggregate with industrial waste, i.e., CBA with a combination of ultra-fine slag material (Alccofine 15%, as a replacement of cement), is a desirable combination to improve concrete performance both in fresh and hardened property. Adding Alccofine has improved workability and strength by reducing the void space caused by the higher water demand of CBA. An 8% cost increase was observed by substituting cement with Alccofine. However, the compressive strength has increased by 58%, which is significant and more advantageous than the cost increase. In addition, the Rapid Chloride Permeability Testing (RCPT) test result revealed low chloride ion penetration due to its dense pore structure and chemical stability, confirming a favorable outcome. On the other hand, the workability of CBA was reported by Kim et al. [15]; substituting natural aggregates with 100% CBA decreases the slump value by up to 100 mm due to its complex shape and rougher surface. Friction is more significant on a rough surface such as that of CBA, delaying the flow characteristics of its fresh properties. Lee et al. [16] also reported that CBA’s slump value decreases with increasing CBA substitution, from 10% up to 40%; however, replacing 100% of the aggregates with CBA significantly reduces the mix’s slump value compared to the control mix. A replacement of 10–40% allows for better workability.

A study on the unit weight of CBA concrete revealed that the unit weight of the CBA concrete mix decreases when CBA replaces fine aggregate, because of the ash’s lower unit weight and porous structure. The other reason for the decrease in CBA concrete mix density is the higher water demand, which creates several large pores; therefore, the unit weight decreases when the CBA replacement percentage increases [17].

Maliki et al. [18] substituted fine aggregates with CBA, from 0%, as the control, to as much as 100% using regular increments of 10%. The results revealed that the optimum CBA content was 60%, with 7- and 28-day curing periods. Raju et al. [19]’s study shows that the compressive strength of CBA concrete surpasses that of control concrete; however, replacing a durable material with weak materials and the lack of pozzolanic activity at an early age by the CBA reduces CBA concrete’s compressive strength at seven days of curing.

CBA can exist in different forms, such as bituminous, sub-bituminous, anthracite, lignite, etc. [20]. It has various applications and can be used as a fine aggregate in concrete production due to its high shear strength and low compressibility, making it an ideal material for infrastructure design and construction [21]. Design engineers use bottom ash to improve materials using its high permeability and grain size distribution; therefore, it is an economical and robust engineering material [22]. CBA’s properties make it suitable for use as fine aggregates in various types of concrete, asphalt, and brickworks [23]. CBA is used as fine aggregate in hot mix asphalt (HMA); 15% CBA containing HMA has not shown a detrimental effect compared to conventional aggregate [24,25]. Additionally, a case study presented by ACAA indicates the utilization of boiler slag as seal coat aggregate for bituminous surface treatments [26]. Poudel et al. [27] revealed that Ground Coal Bottom Ash (GCBA) can replace up to 10% of the cement in concrete, significantly reducing cement’s carbon footprint. CBA can replace fine aggregate in concrete, aiding in minimizing construction costs and environmental degradation [28].

Many authors have concluded that nanoparticle inclusion increases the hydration process, which increases concrete’s mechanical properties in only three days [29]. These materials increase the pozzolanic reaction because they have a high surface area, acting as a nanofiller to densify the C-S-H (calcium silica hydrate) gel structure [30]. Including nanomaterials in concrete mixtures enhances the concrete structure’s physical, chemical, and durability properties [29]. A study by Kumari et al. [31] demonstrated that a 2% nanoparticle substitution yielded durable concrete in terms of chloride penetration resistance and high pH values. Using nanomaterials in a concrete mix increases the compressive and split tensile strength. The optimum nanoparticle inclusion is 2%, which creates durable and strong concrete.

Mansi et al. [32] reported that nanoclay enhances the concrete’s mechanical properties and microstructure performance, rapidly increasing compressive strength by activating the hydration reaction and serving as a filler to increase the concrete mix’s density. Nanoclay is primarily composed of CaO (20–30 wt.%), SiO₂ (30–40 wt.%), and Al₂O₃ (10–15 wt.%) with traces of other compounds like Fe₂O₃. It enhances the concrete’s mechanical properties early by activating the hydration reaction and serving as a filler to increase the concrete mix’s density, rapidly increasing compressive strength [29]. The same analogy might help CBA concrete initiate early strength; therefore, this research will assess the effects of nanomaterials on the fine aggregate replaced by CBA in Concrete. Considerable research has been conducted on nanomaterial-mixed concrete; however, little or almost no research has been conducted on the optimum CBA content as a fine aggregate replacement with nanomaterials or nanoclay [33]. Therefore, this study used nanoclay to evaluate the concrete’s compressive strength after finding the optimum CBA content. The study of nanomaterials with CBA as partial cement replacements in concrete brings novelty to the research, including applying it to CBA and CBS, which can replace fine aggregate in concrete.

Replacing fine aggregate with CBA will positively impact the environment, reduce the amount of raw materials harvested to produce concrete mixtures, and reduce the carbon footprint of concrete production and the cost of fine aggregate. Therefore, this study’s primary goal was to determine the optimal CBA and CBS content as a replacement for fine aggregate in concrete compared to pure Portland cement-based concrete, used as a control based on compressive strength. Secondly, the effect of nanomaterials on the optimum CBA concrete content was evaluated, especially at early curing age.

2. Materials and Methods

2.1. Experimental Plan

Fine aggregates were replaced with CBA, using 10% increments to reach the optimum threshold based on the compressive strength. The optimum CBA replacement was evaluated with compressive strength comparable to or greater than the control. Once the optimum content of CBA in concrete was obtained from the first mix, two different mixes were used to confirm the finding. A minimum of three tests per sample were carried out for fresh and compressive strength for reliable results. The experimental plan is illustrated in Figure 1.

2.2. Material

2.2.1. Fine and Coarse Aggregate, CBA, and CBS

The effect of CBA/CBS on concrete was studied using three mixes with CBA and CBS. Great River Energy (Coal Creek Station), based in Maple Grove, MN, USA, and Minnkota Power Cooperative (Milton R. Young Power Station), based in Grand Forks, ND, USA, provided CBA and CBS, respectively. Strata Corporation, Aggregate Industries (AI), and Kost Materials (Grand Forks, ND, USA) provided fine aggregate, coarse aggregate, and physical properties of the aggregates, following the American Society for Testing Materials (ASTM) and American Association of State Highway and Transportation Officials (AASHTO) specifications, including the chemical properties for CBA and CBS. The lab’s measured properties were similar to those of the companies provided, as illustrated in

Table 1. Physical Properties of Fine and Coarse Aggregate, CBA, and CBS.

	Physical Properties	Bulk Specific Gravity		Bulk SSD Specific Gravity	Absorption (%)	Fineness Modulus
Strata Corporation	Fine Aggregate	Lab	2.62	2.641	0.36	2.85
		Supplier	2.67	2.678	0.36	2.86
	Coarse Aggregate	Lab	2.61	2.634	0.91	-
		Supplier	2.66	2.69	0.91	-
Aggregate Industries	Fine Aggregate	Lab	2.64	2.657	0.542	2.9
		Supplier	2.66	2.671	0.558	2.5
	Coarse Aggregate	Lab	2.63	2.663	1.21	-
		Supplier	2.69	2.715	0.86	-
Kost Materials	Fine Aggregate	Lab	2.65	2.65	0.38	2.74
		Supplier	2.67	2.678	0.36	2.86
	Coarse Aggregate	Lab	2.64	2.688	0.86	-
		Supplier	2.69	2.709	Not Received	-
CBA	Coal Creek	Lab	2.23	2.26	2.31	2.55
		Supplier	NA	NA	NA	NA
CBS	MR Young	Lab	2.23	2.26	2.31	2.55
		Supplier	NA	NA	NA	NA

. The first mix was Strata, followed by AI and Kost. Nanoclay was used to replace 2.5% cement at the optimum content CBA to evaluate the nanomaterial effect on the compressive strength of concrete. The following standards were used to determine the properties of the material: Specific Gravity and Absorption of Fine Aggregate—AASHTO T 84-13 [34]; Specific Gravity and Absorption of Coarse Aggregate—AASHTO T 85-14 [35]; Fineness Modulus of fine aggregate was obtained following ASTM C136 [36]; and bulk density (unit weight) according to the ASTMC138 [37].

2.2.2. Physical Properties of CBA and CBS

The physical properties of CBA and CBS were determined using the standards for fine aggregate, and the physical properties of the CBA and CBS are similar to the fine aggregate, as shown in Table 1. The CBA’s Fineness Modulus and bulk density values were lower than the fine aggregate; however, the CBA’s absorption capacity was higher than the CBS and fine aggregate. Figure 2 depicts fine aggregate, CBA, and CBS.

CBA has an angular, irregular, porous, and rough surface texture, and its particle size distribution ranges from fine gravel to fine sand (Figure 3). CBA is more brittle and lighter than natural sand, and its specific gravity was lower than that of fine aggregate and CBS (see Table 1).

The CBA was sieved using the number 4 sieve size (4.75 mm) to avoid the larger size of CBA in the mix. The particle size distribution of CBA, CBS, and fine aggregate was obtained following AASHTO T 27-20 [38] and is similar, as shown in Figure 3.

2.2.3. Chemical Properties of CBA and CBS

The suppliers have provided the chemical properties of CBA and CBS. The primary constituents of CBA and CBS are silica, alumina, and iron, including a small amount of calcium, magnesium, sulfate, and other compounds shown in

Table 2. Chemical Properties of CBA and CBS.

Constituents Percentage by Weight (%)	CBA (GRE)	CBS (Minnkota)
${\mathrm{S}\mathrm{i}\mathrm{O}}_2$	51.87	47.9
${\mathrm{A}\mathrm{I}}_2{\mathrm{O}}_3$	13.98	14.87
${\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3$	7.20	12.55
Sum of Oxides	73.05	75.32
${\mathrm{T}\mathrm{i}\mathrm{O}}_2$	0.62	0.62
CaO	15.05	12.34
MgO	4.63	4.48
${\mathrm{S}\mathrm{O}}_3$	0.66	0.21
${\mathrm{N}\mathrm{a}}_2\mathrm{O}$	1.83	3.33
${\mathrm{K}}_2\mathrm{O}$	1.7	1.71
${\mathrm{P}}_2{\mathrm{O}}_5$	0.2	0.09
Total	97.75	98.1
Sro	0.31	0.29
BaO	0.39	0.53
Sum	98.44	98.92

. Both CBA and CBS have similar chemical properties. Silica, alumina, and iron have the same chemical properties found in cement clinkers. Therefore, the CBA and CBS silicates react with calcium hydroxide and form C-S-H, the main strength-gaining component in concrete. The more the C-S-H, the more the concrete gains strength over time.

2.2.4. Water Reducer and Superplasticizer

Concrete workability is very important during the mixing and placing of concrete. A water reducer and high-range plasticizer were used to maintain the desired slump value of 3 to 4 inches. The water reducer EUCON WR-91 was supplied by AI and was manufactured in Cleveland, Ohio, USA, and used to increase the workability of the CBA mix. However, the increasing workability of this water reducer was insufficient due to CBA’s high water absorption capacity. Therefore, we decided to use a high-range water reducer/superplasticizer EUCON SPJ manufactured in Cleveland, Ohio, USA, which produces concrete with very low water-to-cement ratios and obtains the desired workability.

2.2.5. Nanoclay

The main components of nanoclay are carbon, aluminum, silicon, and oxygen. It has a layer thickness structure of about 1 nm and a 100 nm lateral dimension. Nanoclay functions as a filler in a concrete matrix, reducing the holes and increasing the surface area of the reaction. Consequentially, it increases concrete’s resistance to cracking, strength, and stiffness [39].

2.3. Mix Design

Strata Corporation and Aggregate Industries (AI) are materials supplier companies based in North Dakota, USA, and provide the mix design. However, Kost Materials, based in North Dakota, USA, did not provide a mixed design; therefore, a mixed design was prepared in the lab. The control mix proportion has coarse aggregates, a second coarse aggregate for Strata mix, fine aggregates, cement, water, and an air-entraining admixture, including a high-range water reducer for AI control, and the proportion was modified based on various mix types as shown in

Table 3. Strata control and % CBA of concrete mixes.

Mix Design	Control	10%	20%	30%	40%	50%	60%	70%	80%
Material (Kg/m3)		CBA	CBA	CBA	CBA	CBA	CBA	CBA	CBA
Cement	335	335	335	335	335	335	335	335	335
Coarse Aggregate	973	973	973	973	973	973	973	973	973
Coarse Aggregate # 2	74	74	74	74	74	74	74	74	74
Fine Aggregate	819	737	655	573	491	409	327	246	164
CBA (GRE)	-	68	135	203	270	338	406	473	541
Water	151	151	151	151	151	151	151	151	151
Air Content (mL/m3)	115	115	115	115	115	115	115	115	115
Water Reducer (mL/m3)					706	1236	1766	2649	3178
W/C	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45

and

Table 4. Strata control and % CBS concrete mixes.

Mix Design	Control	50% CBS	60% CBS
$\mathbf{Material} (\mathbf{Kg}/{\mathbf{m}}^3$)
Cement	335	335	335
Coarse Aggregate # 1	973	973	973
Coarse Aggregate # 2	74	74	74
Fine Aggregate	819	407	324
CBS (Minnkota)	-	411	494
Water Reducer (mL/${\mathrm{m}}^3$)	-	177	177
Water	151	151	151
Air content (mL/${\mathrm{m}}^3$)	115	115	115
W/C	0.45	0.45	0.45

. The water-to-cement ratio (W/C) was 0.45 for Strata and IA and 0.42 for Kost. All aggregate and CBA/CBS used in the experiment were oven-dried or had the least moisture content based on the mix design provided by Strata and AI. A moisture correction method was used in the mix design to consider the amount of water loss of the aggregate, including the equivalent volume method for the CBA weight-obtaining method.

The mix design’s air-entrained value was 6%; however, after many trial batches, the content of the air-entrained value became extremely high, going up to 12%. Therefore, it was decided to reduce the amount of air entrained to 115 mL/${\mathrm{m}}^3$ to obtain a desirable air-entrained value of 5–7%. This discrepancy made it challenging to maintain an air content and slump value. Therefore, the companies’ provided mix design was modified for Strata and AI before substituting % CBA.

The Strata project was used to determine the optimum CBA content; therefore, it was not necessary to start mixing from 10% CBA for the other projects. The test began with 50% CBA and CBS replacement for AI and Kost to confirm the finding of optimum content; see

Table 5. AI, Kost and AI control, 2.5% nanoclay, % CBA concrete mixes.

Mix Design	Kost				AI					AI
Material (kg/m3)	Control	50%	60%	70%	Control	50%	60%	70%	80%	2.5% NanoClay	2.5% NanoClay
		CBA	CBA	CBA		CBA	CBA	CBA	CBA	70% CBA	80% CBA
Cement	367	367	367	367	335	335	335	335	335	328.3	328.3
Coarse Aggregate	1133	1133	1133	1133	1048	1048	1048	1048	1048	1048	1048
Fine Aggregate	583	291	233	175	794	397	317	238	158	238	158
CBA (GRE)	-	241	289	337	-	329	395	461	527	461	527
Water	154	154	154	154	151	151	151	151	151	151	151
Air Content (mL/m3)	239	239	239	239	124	124	124	124	124	124	124
High Range Water Reducer (mL/m3)	-	-	-	-	311	706	1413	2119	2471	2119	2471
W/C	0.42	0.42	0.42	0.42	0.45	0.45	0.45	0.45	0.45	0.45	0.45

.

The Kost project was used to evaluate the effect of the water reducer on CBA concrete and confirm the optimum CBA and CBS content in concrete. Therefore, the concrete mix has no water reducer (Table 5 and

Table 6. Kost control and % CBS concrete mixes.

Mix Design	Kost
Material (kg/m3)	Control	50% CBS	60% CBS
Cement	367	367	367
Coarse Aggregate	1133	1133	1133
Fine Aggregate	583	291	233
CBS (Minnkota)	-	290	347
Water	154	154	154
Air Content (mL/m3)	239	239	239
W/C	0.42	0.42	0.42

). Nanoclay, at 2.5%, was substituted for cement to evaluate the effect of nanomaterials on CBA concrete in AI (Table 5). Only one mechanical mixer was used to reduce the variation caused by mixer rotation, and the concrete was mixed according to the AASHTO R39 [40] standard.

2.4. Testing

Figure 4 shows the fresh properties, including slump value, air content, and unit weight, measured immediately after mixing. The slump was determined following ASTM C143/C143M-20 [41], and ASTMC138 [37] was followed to obtain air content and measure and calculate fresh concrete density.

The compressive strength of concrete describes its ability to withstand an applied load that tends to crash or compress it. A 10 cm diameter by 20 cm length cylinder was used to test the compressive strength of the concrete following the AASHTO T-22 [42] standard using the Universal Testing Machine (Figure 5). The compressive strength was the main parameter used to determine the optimum content of CBA in this study. At least three samples were tested for each mix.

3. Results and Discussions

3.1. Strata Mix

3.1.1. Fresh Properties of % CBA and Strata Control

CBA is porous and has a high absorption capacity. The workability of the fine aggregate mix with CBA replacement decreased as the CBA content increased. For example, the slump value of the concrete mix decreased with an increased CBA content, from 10% to 50%. A slump value of 2 cm for the 30% CBA concrete mix significantly decreased the mix’s workability, and a further increase in CBA content yielded a 0 cm slump value; therefore, the research team decided to use a water reducer to increase the mix’s workability as the CBA content increased. About 706 mL/m³, 1236 mL/m³, and 1766 mL/m³ of the water reducer were used to obtain 2 cm, 6.4 cm, and 5 cm slump values for the 40%, 50%, and 60% CBA replacement values, respectively (Figure 6); however, a further increase of up to 70% with 2649 mL/m³ and 80% with 3178 mL/m³ led to a zero-slump value, which is unacceptable. Therefore, it was decided that a high-range water reducer or Superplasticizer should be used for the next project to obtain the desired workability.

The slump value decreased with an increased % of CBA until it reached 50% CBA, possibly due to CBA’s high-water demand. The compressive strength increased with increased CBA and decreased slump value until 50% CBA (Figure 6) was reached. The air content decreased with increased CBA content compared to the control and increased compressive strength until 50% CBA was reached; further, increased air content led to a decrease in compressive strength after 50% CBA; therefore, at 50% CBA replacement, all the fresh and compressive strength changed in the reverse direction (Figure 6). The air content of the % CBA concrete mix decreased with an increase in CBA content up to 40% CBA compared to the control, then increased, which may have been caused by the spacing factor of the voids in the mix and CBA’s porous behavior. The air content has a significant effect until 60% replacement had occurred; however, with decreased slump values to 0 cm for 70% and 80% CBA concrete, the air content increased significantly (see

Table 7. Fresh properties of % CBA and Strata control.

Fresh Properties	Strata Control	10% CBA	20% CBA	30% CBA	40% CBA	50% CBA	60% CBA	70% CBA	80% CBA
Measured Slump (cm)	9.5	5.1	2.5	1.9	1.9	6.4	5.1	0.0	0.0
Measured Air Content (%)	7.4	5.6	5.9	5.4	5.9	7.9	9.6	9.9	11.9
Measured Unit Weight (kg/${\mathrm{m}}^3$)	2291	2322	2322	2322	2315	2258	2207	2149	2111

).

The unit weight of the Strata control mix and 50% CBA replacement were comparable; however, the rest of the CBA-containing concrete had a higher unit weight than the control, up to 40%, possibly due to CBA’s fineness properties. Increasing the % CBA decreased the unit weight of the concrete, possibly due to CBA’s higher water demand, which leads to pore formation and larger pore sizes. The % CBA concrete unit weight increased with decreased slump value from 10% to 40% CBA replacement and decreased from 50% to 60% CBA replacement with increased slump value (see Table 5). CBA concrete at 70% and 80% were disintegrated concrete samples with 0 cm slump value and no concrete workability.

3.1.2. Compressive Strength of % CBA and Strata Control

The compressive strength of the Strata control mix was 25 MPa and 30 MPa after 7 and 28 days of curing, respectively, and the values for the 10% CBA replacement after seven days of curing were similar to the control’s strength after 28 days of curing. The compressive strength decreased when the CBA replacement was increased from 10% to 30%; however, it still had higher compressive strength than the control after seven days of curing. The research team decided to use a water reducer after 30% CBA replacement to achieve the desired workability, which led to an increase in compressive strength with 40%, 50%, and 60% CBA replacement, reaching an optimum content of 60% at seven days of curing. The compressive strength of the control vs. the CBA concrete at seven days reveals that increased CBA replacement increases the concrete strength when the replacement is up to 40% and starts to decline after 50% CBA replacement. The same trends were obtained after 28 days and 56 days of curing (see Figure 7). The compressive strength increased from 30% to 40% and 50%, possibly due to adding a water reducer to increase workability. Another mix of 30% CBA concrete with a water reducer was created to determine the effect on compressive strength. The results indicated that a mix containing 30% replacement with an added water reducer had a compressive strength of 31 MPa compared to 30% CBA without a water reducer at 29 MPa. On the other hand, 60% and 70% CBA concrete have gained or maintained strength, probably due to the higher amount of water reducer used in the mix see mix design in Table 3. These results must be researched further to conclude if CBA with a water reducer yields a higher compressive strength.

The 40% CBA mix yielded a 26% increase in compressive strength, and the 50% CBA mix yielded a 10.7% increase after 28 days of curing. The CBA concrete’s compressive strength was higher than the Strata control’s at 7 and 28 days of curing, indicating that this replacement technique yielded acceptable results, and that the mix exhibited better performance.

The compressive strength of the CBA mix continued to increase after 28 days of curing, with replacement amounts of up to 40% CBA. The 30% CBA mix’s performance surpassed the control concrete by 7 MPa after 56 days of curing, or an increase of 21.2%, indicating that the CBA concrete mix’s strength does not decrease with time. A rise of 30.9% was observed in the 40% CBA mix after 56 days of curing. The mix with compressive strength is higher or comparable to the control at 50%. The optimum CBA content, 50%, had compressive strength values of 29, 34, and 31 MPa after 7, 28, and 56 days of curing, higher performance than the Strata control. Therefore, the results indicate that CBA increases the concrete’s compressive strength after 7, 28, and 56 days of curing up to 50% CBA optimum content. Additionally, the optimum content, 50% CBA, had a lower unit weight compared to the control, which is lightweight concrete compared to concrete without CBA, as illustrated in Figure 7.

3.2. Aggregate Industries Mix

3.2.1. Fresh Properties of % CBA and AI Control

The workability of the CBA mixes decreased as the % CBA increased; similar observations were obtained from the Strata mix, possibly due to CBA’s higher water demand. To obtain the desired workability, 20, 40, 60, and 80 mL/m³ Superplasticizer were used for 50%, 60%, 70%, and 80% CBA concrete mix, respectively.

The unit weight of the 50% CBA was comparable to the control, which was the same as the Strata project. As the unit weight decreased with increasing % CBA (

Table 8. Fresh properties based on % CBA and AI control.

Fresh Properties	AI Control	50% CBA	60% CBA	70% CBA	80% CBA
Measured Slump (cm)	10.2	1.3	3.2	2.0	2.5
Measured Air Content (%)	8.8	4.4	7.9	8.3	11.9
Measured Unit Weight (kg/${\mathrm{m}}^3$)	2276	2273	2258	2243	2162

), possibly due to CBA’s higher water demand, leading to pore formation and larger pore sizes, a similar finding was obtained in the Strata project.

Air content and slump value decreased with an increase in CBA from the control to 50% CBA, with an increase from 50% CBA to 60% CBA, and 60% CBA being the optimum content. This result confirms the strata’s findings (Figure 8). The slump value increased from 1.27 cm to 3.18 cm as CBA increased from 50% to 60%, and the compressive strength decreased from 43 MPa to 34 MPa, respectively (see Figure 8). The two projects showed similar trend results: when the slump value increases, the compressive strength decreases, and vice versa.

3.2.2. Compressive Strength of % CBA and AI Control

Compressive strength increased with decreased slump and increased CBA content up to 50% CBA, then decreased with increased air content and slump value, leading to an optimum content of 60% CBA after 28 days of curing, possibly due to CBA’s fineness and porous properties (Figure 8). The compressive strength of the AI mixes with % CBA replacement compared to the control is illustrated in Figure 9, and the optimum CBA content was 60% at 28 days of curing. The 50% CBA has 25.8% higher compressive strength than the control after 28 days of curing, and the optimum content of 60% has 8.3% higher compressive strength than the control concrete without CBA (Figure 9). Therefore, CBA enhances the compressive strength of concrete, as the two projects confirm the same results. The results for the mix with 70% and 80% replacement after seven days of curing were not comparable to the control; therefore, we only considered the 50% and 60% mixes at 28 days of curing (Figure 9). The two projects had similar fresh and compressive strength properties, which confirms the results.

3.3. Kost Mix

3.3.1. Fresh Properties of % CBA and Kost Control

A third project was needed to confirm the two projects’ findings with a lab-based mix design without a supplier mix design. Only 10 mL/${\mathrm{m}}^3$ and 20 mL/${\mathrm{m}}^3$ Superplasticizer was used to obtain the desired workability for 60% and 70% CBA mix, respectively. The slump value has increased from 50% CBA to 60% CBA, and the air content increased from 50% CBA to 60% CBA, indicating that 60% is the optimum content for the Kost project. A similar trend was observed for both Strata and AI projects shown in Table 7, Table 8 and

Table 9. Fresh Properties of % CBA and Kost Control.

Fresh Properties	Kost Control	50% CBA	60% CBA	70% CBA
Measured Slump (cm)	12.1	7.6	7.9	15.2
Measured Air Content (%)	8.1	6.2	6.3	13.1
Measured Unit Weight (kg/${\mathrm{m}}^3$)	2281	2303	2278	2102

and Lee et al. obtained similar findings [16]. The authors reported that CBA’s slump value decreases with increasing CBA substitution, from 10% to 40%.

All the fresh properties for the Kost project, 50% CBA and 60% CBA concrete, had similar trends. For the Kost projects, samples were tested only for 50% CBA, 60% CBA, and 70% CBA since the optimum content is 50% for Strata and 60% for AI projects. Figure 10 illustrates the compressive strength, slump value, and air content with % CBA replacement compared to the control.

Unit weight decreased with increases in CBA content from 2303 kg/m³ to 2278 kg/m³, for 50% CBA and 60% CBA concrete, respectively, possibly due to CBA’s higher water demand leading to pore formation and larger pore sizes (see Figure 10). However, the optimum content of 60% CBA has almost a similar unit weight to the control. The other two projects’ optimum CBA concretes have similar findings, having either equivalent or lower unit weights than the control. Therefore, CBA-based concrete is lightweight compared to the control; a similar finding was reported by Singh and Siddique [17].

3.3.2. Compressive Strength of % CBA and Kost Control

Compressive strength decreases after 60% CBA, the air content increases afterward, and the optimum CBA content is 60%. The optimum content of 60% CBA concrete has 15%, 43%, and 42% higher compressive strength than the control after 7 days, 28 days, and 56 days, respectively (See Figure 11). The compressive strength, similar to the Strata and AI projects, increased over time, possibly due to CBA’s pozzolanic properties. Therefore, it can be concluded that CBA increases the compressive strength of concrete over time. The third project, Kost, confirms the results of the other two projects, as shown in Figure 11. The optimum content is 60%, and all the fresh properties for 50% and 60% CBA have similar trends for all three projects. Maliki et al. [18] reported similar findings, substituting fine aggregates with CBA using regular increments of 10%. The results revealed that the optimum CBA content was 60% after 7 and 28 days of curing.

3.4. CBS at Kost and Strata Projects

3.4.1. Fresh Properties of Kost and Strata CBS Concrete

CBA and CBS are byproducts of power plant stations with different steps and similar properties, as explained in the literature review and material properties sections. A test was conducted to see if CBA and CBS have differences in fresh properties and compressive strength and if the optimum content of CBS was the same as CBA.

CBS has significantly less water absorption than CBA. Consequently, 177 mL/${\mathrm{m}}^3$ Superplasticizer was used for both 50% and 60% CBS in the Strata mix to obtain slump values of 1.91 and 6.35 cm, respectively, which is less than the control. When the slump value increases, the air content increases, as seen in all three projects with CBA; see

Table 10. Fresh Properties of Kost and Strata % CBS and Control.

Fresh Properties	Strata Control	Strata CBS		Kost Control	Kost CBS
		50% CBS	60% CBS		50% CBS	60% CBS
Measured Slump (cm)	9.53	1.91	6.35	12.07	8.26	7.87
Measured Air Content (%)	7.40	4.60	7.70	8.10	7.80	7.50
Measured Unit Weight (kg/${\mathrm{m}}^3$)	2291	2371	2287	2281	2307	2313

. However, the unit weight for 50% Strata CBS is higher than the control and comparable to the control at 60% Strata CBS. On the other hand, 50% and 60% Kost CBS concrete have similar fresh properties as Strata 50% and 60% CBS concrete; both the slump and air content decreased from 50% to 60% CBS, and the unit weight increased. This trend is similar for Strata, AI, and Kost with CBA. However, the unit weight has increased compared to the control (see Table 10), possibly due to CBS’s higher bulk density, crystalline or glassy properties, and less water absorption capacity than CBA. Therefore, the higher unit weight for CBS concrete indicates that CBA is lighter than CBS, which might be due to the nature of their processing method and the higher specific gravity of CBS, as seen in Table 1 in the Materials Section.

3.4.2. Compressive Strength of the Strata and Kost, % CBS and Relative to Controls

The optimum CBS content is 50% after 56 days of curing, as shown in Figure 12, for both Kost and Strata projects. This confirms the finding for CBA optimum content in the previous section, which means both CBA and CBS have similar fine aggregate replacing capacities. The compressive strength of 50% Strata CBS is 16.6% higher than that of the control after 28 days of curing. On the other hand, Kost 50% CBS, after 90 days of curing, had 17% increased compressive strength compared to the control, as illustrated in Figure 12. Furthermore, the compressive strength of 50% CBA concrete was higher than the 50% CBS, indicating that CBA concrete yields higher compressive strength than CBS (

Table 11. Compressive Strength of Kost Control, 50% CBS and 50% CBA.

Compressive Strength (MPa)	Kost Control	50% CBA	50% CBS
7 Days	21	29	21
28 Days	23	39	25
56 Days	25	40	25
90 Days	24	44	28

), which might be again due to the difference in physical properties, water abortion capacity, and bulk density.

3.5. Effect of Nanoclay on Optimum Content for AI Project

Based on different studies in the literature review section, 2% nanoclay replacing cement in concrete yields higher concrete performance [31]. The effect of 2.5% nanoclay by cement weight on CBA-based concrete was investigated.

3.5.1. Fresh Properties of % CBA, AI and Nanocaly

The air content decreases when the slump value decreases, similar to the other findings. However, the unit weight had increased compared to the control, from 70% to 80%, as shown in

Table 12. Fresh Properties and Compressive Strength of % CBA Nanoclay and Control.

	AI Control	Nanoclay AI 2.5%
Mix Design		70% CBA	80% CBA
Measured Slump (cm)	12.1	7.6	2.5
Measured Air Content (%)	8.8	5.8	3.8
Measured Unit Weight (kg/${\mathrm{m}}^3$)	2278	2294	2332
Compressive Strength (MPa)
7 Days	29	35	39
28 Days	38	40	44
56 Days	37	45

, which might be due to the fineness properties of nanoclay that filled the void in the cement matrix and increased the unit weight of the concrete.

3.5.2. Compressive Strength

The compressive strength of 80% CBA was higher than the control after 28 days of curing. This result advanced the finding of this study from 50% CBA content being optimum to 80% CBA with nanoclay, as shown in Table 12. Nanoclay enhances the performance of concrete by consuming Ca (OH)₂ in hydration reaction at an early age, and CBA does this at a later age. Therefore, this combination provided a better performance concrete with 80% CBA. The 80% CBA nanoclay-mixed concrete had 4.8% higher compressive strength after 7 days of curing than the AI control after 56 days, and 70% CBA nanoclay mixed concrete had 5.2% higher compressive strength than the control after 28 days of curing. The 80% CBA with nanoclay mixed concrete had 15% higher compressive strength than the control after 28 days of curing (Table 12), possibly due to the decreased slump value, increased compressive strength, and the effect of nanoclay. This result indicates that nanoclay initiates the hydration reaction at an earlier age, and increases strength earlier than CBA, which increases strength after 28 days or at a later age. Combining the two materials provided a better-performing concrete, and increased the optimum CBA content to 80% after 28 days of curing. A similar result was obtained by Kumari et al. [31]. The authors investigated and concluded that replacing 2% cement with nanomaterials like nano-${\mathrm{T}\mathrm{i}\mathrm{O}}_2$ and nano-${\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3$ enhance the mechanical and durability properties of concrete.

The effects of nanoclay on CBA concrete are significant, as illustrated in

Table 13. 70% CBA With and Without Nanoclay Compared to the Control.

	Compressive Strength (MPa)
Mix Design	70% CBA Without Nanoclay	AI Control	70% CBA with 2.5% Nanoclay
7 Days	27	29	35
28 Days	32	38	40
56 Days		37	45

; the compressive strength of 70% CBA nanoclay mixed concrete was 30% higher than the 70% CBA concrete without nanoclay after only 7 days of curing. Nanoclay increased the concrete’s compressive strength at an early age. 70% CBA with 2.5% nanoclay has increased compressive strength by 21% after 56 days of curing compared to the control. Nanoclay enhances the strength of concrete at an early age due to its high specific surface area and the fineness property, which fills the cement matrix.

4. Conclusions and Recommendations

The following conclusions can be made based on this study:

• The optimum content of CBA and CBS in concrete is at least 50%, which indicates that both materials could replace a minimum of 50% fine aggregate in a concrete mix.
• The 50% CBA concrete yielded a comparable unit weight to the controls. However, 50% CBS concrete has a slightly higher unit weight than the control and the 50% CBA concrete.
• Using a high-range water reducer minimizes the decrease in workability by replacing fine aggregate with CBA.
• After 7 days, 28 days, and 56 days of curing, the 50% CBA concrete had 15%, 43%, and 42% higher compressive strength than the control, respectively, and compressive strength higher than that of the 50% CBS concrete, indicating that CBA concrete yields higher compressive strength than CBS. However, both 50% CBA and CBS concrete have higher compressive strengths than the control, which is a promising result in terms of sustainability.
• Nanoclay increases CBA concrete compressive strength at an early age of curing, and adding 2.5% nanoclay in CBA concrete increases the optimum CBA content from 50% to 80%.
• The materials used in this experiment have no water content; both CBA/CBS are oven-dried. A moisture correction calculation was used to replace fine aggregate with CBA/CBS. Therefore, minor impreciseness is expected during calculation errors, and the materials’ moisture content and properties vary.

Based on this study, the following recommendations can be made:

• Further study is needed to see how a water reducer affects the compressive strength of CBA concrete.
• The mechanical and durability properties of CBA concrete should be assessed from various perspectives and compared to the same control.