Sustainable Resource-Efficient Concrete Using Bottom Ash as a Partial Sand Replacement

Abstract

Waste materials are abundant and often act as slow environmental contaminants, creating severe ecological challenges. With rapid industrialization, electricity demand has increased substantially, and in India, coal-based thermal power plants (TPPs) remain the dominant source of power generation. Coal combustion produces two major by-products: fly ash and bottom ash (BA). While fly ash is widely utilized in blended cements due to its pozzolanic nature, BA has received comparatively limited attention despite having similar chemical characteristics. Owing to its coarser particle size, BA shows strong potential as a substitute for natural river sand, the excessive extraction of which has led to severe resource depletion and sustainability concerns. Unlike previous studies that focused on single-source BA or limited performance evaluation, this study investigates the use of BA from multiple sources to develop resource-efficient bottom ash concrete (BAC). Concrete mixes containing 0%, 20%, 35%, and 50% BA as volumetric replacements of river sand were evaluated for their fresh, mechanical, durability, and microstructural properties. The results indicate that BA significantly influences concrete performance due to its porous structure. Among the investigated mixes, 35% river sand replacement with BA showed the most favorable performance for the specific materials and sources used in this study, achieving up to 17.46% higher compressive strength and up to 16.14% higher resistance to transport-related properties at 90 days. Microstructural analysis confirmed the formation of secondary C–S–H gel, which enhanced matrix densification. However, 50% replacement resulted in reduced performance. The findings demonstrate that BA can be effectively utilized in concrete at replacement levels of up to 35% as a sustainable substitute for river sand under the investigated material conditions.

1. Introduction

The search for alternative materials has gained increasing significance and acceptance in the construction industry over recent decades [1]. The term ‘alternative’ extends beyond simple substitution and refers to the effective utilization of industrial by-products or waste materials as replacements for natural resources [2]. Such utilization not only conserves diminishing natural resources but also enhances material performance and broadens future applicability [3]. In this context, considerable research has been conducted on the utilization of industrial by-products and recycled waste materials in concrete to promote sustainable construction and reduce dependence on natural resources. Materials such as fly ash, ground granulated blast furnace slag, and recycled aggregates have been widely investigated as alternative construction materials [4,5,6,7,8,9]. Nevertheless, the exploration of waste-derived aggregate materials continues to gain attention due to the growing scarcity of natural sand resources and the environmental concerns associated with their extraction. Among these underutilized alternatives, coal BA has emerged as a promising material because its granular texture and particle size distribution are comparable to those of river sand, making it suitable as a fine aggregate replacement in concrete [10,11].

Coal remains one of the primary sources of global electricity generation and continues to play a vital role in economic development [12]. Hosseini [13] reported that coal is the most significant fossil fuel, with global proven reserves of approximately 1144 billion tons, nearly 10% of which are located in India. The high ash content of Indian coal (25–50%) has resulted in the generation of enormous quantities of coal ash by-products [14,15]. In India, coal containing nearly 40% ash is predominantly used for thermal power generation [16], leading to significant environmental and disposal challenges [2,15,17]. At present, TPPs in India generates nearly 300 MT of coal ash annually, of which approximately 80% of fly ash, and 20% consists of BA [18,19], corresponding to an estimated 55–60 MT per year. However, only about 40–60% of the generated coal ash is effectively utilized [20]. Currently, BA is utilized in applications such as brick manufacturing [21], road embankments, mine filling, land reclamation, and ground/pulverized bottom ash (PBA) is used in cement production and geopolymer materials [22]. Nevertheless, a significant portion is still disposed of in ash ponds due to handling, transportation, and material quality constraints. Coal ash generation in India is projected to exceed 1000 MT by 2031–2032, and a substantial portion of the generated ash is expected to remain unutilized despite ongoing efforts toward its effective management and utilization [23]. The disposal of BA in ash ponds near TPPs creates significant environmental and land-use concerns, including groundwater contamination, soil degradation, and air pollution [2,10,24]. Owing to its particle size distribution comparable to fine aggregate [25], BA has attracted increasing attention as a potential replacement for river sand in concrete production [26].

Concrete, the most widely used construction material, has evolved in response to local material availability [27]. River sand is conventionally employed as fine aggregate; however, excessive extraction has resulted in acute shortages in several Indian states [26] due to regulatory restrictions and environmental concerns. This scarcity is particularly pronounced during monsoon seasons, often leading to the use of inferior-quality materials and subsequent degradation of structural performance [28]. Moreover, the production of manufactured sand is energy-intensive, further increasing environmental burdens. In this context, substituting river sand with BA as a secondary construction material presents a promising and sustainable alternative [10,26,29,30,31,32,33].

Previous studies have demonstrated that the incorporation of industrial waste materials as fine aggregate replacements or supplementary cementitious materials (SCMs) can reduce construction costs while mitigating environmental impacts [34]. In particular, the use of pulverized BA in concrete has attracted considerable attention due to its potential to address disposal issues associated with thermal power plant waste [35,36]. Concrete incorporating pulverized coal ash has also been reported to reduce production cost and environmental impact without compromising mechanical performance [18]. Cheriaf et al. [37] reported that BA exhibits delayed pozzolanic activity, initiating around 28 days and becoming significant at later ages, owing to its high contents of SiO₂, Al₂O₃, and Fe₂O₃, and minimal CaO content (≤1%), classifying it as ASTM Type F ash [38]. Singh and Siddique [17] recommended the use of BA up to 50% as sand replacement in structural concrete with appropriate superplasticizer dosage. However, due to the angular shape, rough texture, and porous nature of BA particles, reduced workability and increased water demand are commonly reported [10,17]. These limitations can be effectively mitigated through optimized mix design and the use of chemical admixtures [11,39].

Durability performance governs the long-term sustainability of concrete structures. Previous studies have generally reported that concrete incorporating BA can exhibit improved durability characteristics, including reduced chloride permeability, enhanced corrosion resistance, and negligible heavy metal leaching due to effective encapsulation within the cementitious matrix [24,40,41,42]. Microstructural investigations further suggest that BA contributes to pore refinement and secondary C–S–H formation, which may enhance long-term durability and hydration characteristics of concrete [43].

Despite extensive research on PBA as SCMs [18,27,43], its application as a partial replacement for fine aggregate remains comparatively underexplored, particularly in terms of durability performance. In addition, most previous studies [2,15,26] have utilized BA obtained from a single thermal power plant and have mainly focused on fresh and mechanical properties, with limited attention given to source-dependent variability arising from differences in coal characteristics and combustion conditions.

Several studies have proposed pretreatment techniques such as grinding, activation, and carbonization coating to improve the performance of BA in concrete applications [18,43,44,45,46]. However, these methods increase energy demand, processing cost, and operational complexity, which may restrict large-scale practical implementation. Therefore, the direct utilization of untreated BA remains an important yet insufficiently explored area of research. To address this gap, the present study investigates untreated BA collected from five geographically distinct coal-based TPPs across India as a partial replacement for river sand. The study focuses on understanding the influence of source-dependent variability on the fresh, mechanical, durability, and microstructural properties of concrete through comprehensive physical, chemical, morphological, and microstructural characterization of BA and its correlation with concrete performance.

Unlike many previous studies that employed processed or chemically treated BA [45], the present study utilized BA in its as-received condition obtained directly from coal-based TPPs to improve practical applicability, economic feasibility, and large-scale implementation. Concrete mixes were designed using the minimum cement content specified for ‘severe’ exposure conditions in IS 456 [47] to develop durable, sustainable, and green concrete while promoting the conservation of natural resources through the utilization of industrial waste materials. Particular emphasis was placed on addressing the source-dependent variability of BA through systematic screening and characterization of BA collected from multiple TPPs, along with careful selection of constituent materials in accordance with relevant standards.

The laboratory experimental program included the evaluation of workability, mechanical properties, durability characteristics, and microstructural evolution of bottom ash concrete (BAC). Durability performance was assessed using multiple complementary indicators, including rapid chloride permeability, rapid chloride migration, electrical resistivity, water absorption, water permeability, and drying shrinkage. Microstructural analyses were also performed to validate the observed behavior and provide a comprehensive understanding of untreated BAC. Although previous studies [29,48] reported negligible heavy metal leaching from BAC, leaching behavior was not investigated in the present study. Since the environmental performance of BA may vary depending on source and combustion conditions, leaching assessment should be considered before recommending large-scale field applications.

2. Materials and Methods

2.1. Materials

The selection of materials was based on local availability to ensure cost optimization and timely procurement. Among the constituent materials, river sand was of primary importance, as it was partially replaced with BA to evaluate the influence of this replacement on concrete properties. The physical properties of river sand and coarse aggregates are summarized in

Table 1. Physical properties of river sand and coarse aggregates.

Properties	River Sand	Coarse Aggregates
		20 mm	10 mm
Specific gravity	2.66	2.78	2.75
Fineness modulus (FM)	2.84	–	–
Water absorption (%)	1.2	0.8	1.0
Flakiness and elongation index (%)	–	18	–

.

River sand (RS) was collected from the Ajay River in West Bengal. Its grading and particle size distribution were evaluated through sieve analysis because these characteristics improve particle packing, reduce voids within the concrete matrix, and significantly influence workability and density. Sieve analysis was conducted in accordance with IS 2386 (Part 1) [49]. The percentage passing values and particle size distribution discussed in Section 2.1.4 confirmed that the sand satisfied the Zone II grading requirements specified in IS 383 [50]. The specific gravity and water absorption of the river sand were also determined in accordance with IS 2386 (Part 3) [51], as these properties affect mix proportioning, moisture correction, and durability performance of concrete.

Coarse aggregates (CA) of 20 mm and 10 mm sizes were procured from a stone quarry in the Panchami region of West Bengal. The aggregates consisted of crushed particles with rounded-to-angular geometry and a dark, polished surface texture. As the primary load-bearing component of the concrete matrix, the grading and shape characteristics of the aggregates were evaluated due to their significant influence on aggregate interlocking, packing density, workability, and strength performance of concrete. Sieve analysis was conducted in accordance with IS 2386 (Part 1) [49], which confirmed the grading of the single-size 20 mm and 10 mm aggregates. A 50:50 blend of the two aggregate fractions satisfied the requirements of a combined graded nominal 20 mm CA as specified in IS 383 [50]. The specific gravity, water absorption, flakiness index, and elongation index of the aggregates were also determined following IS 2386 (Part 3 and Part 1) [49,51], as these properties affect the strength, durability, and deformation behavior of concrete.

Ordinary Portland Cement (OPC) of 53 grade was used as the primary binding material in the concrete mixes. Upon hydration with water, the cement formed C–S–H gel, which contributed significantly to the strength development of concrete. The physical properties of cement, including specific gravity, consistency, setting time, and compressive strength, were evaluated because these parameters influence hydration, workability, setting behavior, and strength development. The specific gravity of cement was found to be 3.15, determined using the Le Chatelier flask in accordance with IS 4031 (Part 11) [52]. Standard consistency, initial setting time, and final setting time were determined using the Vicat apparatus as per IS 4031 (Part 4), and IS 4031 (Part 5) [53,54], and were found to be 28%, 120 min, and 215 min, respectively. The compressive strength of cement was determined using 70.6 mm mortar cubes prepared with a cement-to-sand ratio of 1:3 and water content of $(P/4+3.0)\%$ by weight of total material in accordance with IS 4031 (Part 6) [55]. The mortar cubes achieved compressive strengths of 28.00 MPa, 39.50 MPa, and 56.00 MPa at 3, 7, and 28 days, respectively, confirming compliance with IS 269 [56].

Water used in the concrete mix was sourced from groundwater with a pH value of 7.4, satisfying the requirement of IS 456 [47]. The quality of mixing water influences cement hydration, setting behavior, strength development, and durability of concrete. Water provided the required workability and facilitated hydration; however, excessive water increases porosity and reduces strength.

A polycarboxylate ether (PCE)-based superplasticizer (SP), modified with a viscosity-modifying agent (VMA) and conforming to IS 9103 [57], was used to improve workability at a lower water-to-cement ratio. The admixture had a specific gravity of 1.09, a pH of 7.09, and appeared dark reddish-brown.

Bottom ash was the central focus of this study. A thorough evaluation was carried out to characterize its properties and assess its suitability as a partial replacement for river sand in concrete. Since the characteristics of BA vary significantly depending on coal source and combustion conditions, BA samples from different TPPs were considered to evaluate source-dependent variations in concrete performance. BA samples were collected from five major TPPs across India: (a) Anpara-B Thermal Power Station (A-BTPS), Sonebhadra, Uttar Pradesh; (b) Koradi Thermal Power Station (KTPS), Nagpur, Maharashtra; (c) Mettur Thermal Power Station (MTPS), Tamil Nadu; (d) Talcher Super Thermal Power Station (TSTPS), Odisha; and (e) Ukai Thermal Power Station (UTPS), Gujarat. A detailed collection process and the physical, chemical, and microstructural properties of BA are discussed in subsequent sections.

2.1.1. Bottom Ash Handling System and Collection of Samples

Pulverized coal is widely used for power generation; however, complete burnout is rarely achieved despite advances in combustion technology. Consequently, approximately 46% of the coal remains unburned [15], resulting in the generation of large amounts of coal combustion by-products, mainly fly ash and BA [18]. The disposal of these by-products poses significant environmental concerns, including land occupation, dust emission [2], and the potential leaching of hazardous components into soil and groundwater [58]. According to standard definitions, the ash collected from the bottom of the boiler is classified as BA [2,17,59].

BA is generally collected using a water-filled hopper installed beneath the boiler bed, into which fused residues of partially combusted coal descend. As shown in Figure 1, large ash clinkers are mechanically crushed by a bottom ash handling grinder (BAHG) located below the hopper outlet to facilitate hydraulic transport through pipelines [31] using high-pressure pumps. In the wet ash handling system adopted in this study, BA, together with a minor fraction of uncollected fly ash, is first conveyed to a sump pit. The resulting slurry is then discharged into ash ponds or lagoons for disposal. When BA mixes with fly ash under slurry conditions, the material is commonly referred to as pond ash [59], which exhibits increased fineness due to the presence of fly ash. Such disposal practices raise concerns related to land consumption, and long-term environmental sustainability [15].

To enable value-added utilization and reduce environmental impacts, the BA used in the present study was collected before its mixing with fly ash in the sump pit, thereby preserving its distinct physical characteristics. Approximately 500 kg of BA was collected from each source using three different batches collected within a single day. After collection, the material, which had already undergone mechanical crushing during handling, was dried to remove free moisture associated with wet collection systems. The dried BA is then hand-mixed to ensure uniformity of the sample, subsequently sieved through a 10 mm sieve to remove any remaining oversized particles. This procedure allowed the evaluation of BA in a condition representative of its as-received state at TPPs, without additional grinding or chemical treatment.

2.1.2. Physical Properties of Bottom Ash

BA is a relatively lightweight material [17], with a specific gravity ranging from 2.00 to 2.24. Sieve analysis indicates that the particle size distribution of BA is comparable to that of river sand. SEM observations confirm that BA particles possess an uneven surface texture, with shapes varying from angular to sub-rounded and exhibiting an interlocking, porous, popcorn-like morphology [15] having dark gray colour [15,27]. The bulk density of BA ranges from 1320 to 1450 kg/m³. The detailed physical properties of BA are summarized in

Table 2. Physical properties of bottom ash samples.

Properties	A-BTPS BA	KTPS BA	MTPS BA	TSTPS BA	UTPS BA
Zeta potential (mV)	$-19.6$	$-22.4$	$-31.7$	$-29.3$	$-17.4$
Specific gravity	2.12	2.12	2.00	2.01	2.24
Fineness modulus (FM)	1.83	1.00	1.70	1.74	1.37
Water absorption (%)	1.65	2.25	2.40	1.80	1.90
Bulk density (kg/m3)	1430	1420	1320	1380	1450

.

Zeta potential (ZP) is a key physicochemical parameter that characterizes the surface charge of particles suspended in a fluid and governs their dispersion, flocculation, and coagulation behavior [60]. In cementitious systems, ZP provides insight into particle stability and interparticle interactions, which influence fresh-state properties.

In this study, the ZP of BA was measured by a dynamic light scattering (DLS) test to assess its surface charge characteristics. The measured ZP values ranged from −17.4 mV to −31.7 mV, indicating a predominantly negative surface charge. Such values suggest moderate electrostatic repulsion and a tendency toward controlled flocculation in the cement pore solution. The measured ZP values are summarized in Table 2, and the corresponding ZP distribution is presented in Figure 2.

2.1.3. Chemical Properties of Bottom Ash

The compound obtained from a chemical reaction primarily depends on the source of coal [31], and nature and quantity of the reactants involved. The chemical composition of BA was determined using XRF analysis. The results indicate that silica (SiO₂), alumina (Al₂O₃), and iron oxide (Fe₂O₃) are the predominant constituents, while calcium oxide, magnesium oxide, sulfate, and other oxides are present in minor quantities. These chemical characteristics significantly influence the strength development of concrete incorporating BA.

As summarized in

Table 3. Chemical composition of bottom ash samples.

Content (%)	A-BTPS BA	KTPS BA	MTPS BA	TSTPS BA	UTPS BA
Silicon dioxide (SiO2)	70.42	58.36	70.19	63.89	66.75
Aluminum oxide (Al2O3)	17.41	23.75	17.77	21.74	19.65
Iron oxide (Fe2O3)	6.11	11.84	5.53	5.77	8.70
SiO2 + Al2O3 + Fe2O3	93.94	93.95	93.49	91.40	95.10
Calcium oxide (CaO)	0.53	2.01	0.88	0.74	0.67
Magnesium oxide (MgO)	0.30	0.51	0.46	0.39	0.45
Potassium oxide (K2O)	1.00	0.72	1.38	0.78	1.15
Sodium oxide (Na2O)	0.11	0.17	0.17	0.08	0.14
Titanium dioxide (TiO2)	1.27	1.52	1.25	1.76	1.47
Loss on ignition (LOI)	1.90	0.43	1.50	3.79	0.28

, the high combined content of SiO₂, Al₂O₃, and Fe₂O₃ confirms the pozzolanic nature of the material, enabling secondary hydration reactions. Based on IS 3812 [59] classification and consistent with ASTM C 618-03 [38], the BA qualifies as siliceous (ASTM class F) ash [37]. The total oxide content (SiO₂ + Al₂O₃ + Fe₂O₃) for A-BTPS, KTPS, MTPS, TSTPS, and UTPS ranges from 91.40–95.10%, exceeding the ASTM minimum requirement of 70%. Except for KTPS, all samples exhibited negligible CaO content (<1%), indicating limited cementitious behavior and minimal participation in early-age hydration reactions responsible for the formation of primary C–S–H gel as the initial binding phase. However, the high reactive content of SiO₂ promotes pozzolanic reactions of later age with Ca(OH)₂ released during cement hydration, leading to the formation of a secondary hydrate, i.e., C–S-H gel [5,10] and the consequent densification of the matrix. Therefore, the observed strength enhancement is primarily attributed to pozzolanic activity at later curing stages.

2.1.4. Particle Size Distribution of River Sand and Bottom Ash

Based on IS 383 [50], sieve analysis indicated that the BA gradation corresponds to Zone III and Zone IV. When blended with Zone II river sand, partial replacement levels of 20% and 35% shifted the combined gradation to a range between Zone II and Zone III, whereas a 50% replacement resulted in a combined gradation falling within Zone IV. The percentage passing values for river sand and BA are provided in

Table 4. Percentage passing values of river sand and bottom ash obtained from sieve analysis.

Sieve Size	Percentage Passing
	River Sand	A-BTPS BA	KTPS BA	MTPS BA	TSTPS BA	UTPS BA
10.00 mm	100.00	100.00	100.00	100.00	100.00	100.00
4.75 mm	95.82	98.14	99.32	95.98	93.64	94.61
2.36 mm	88.77	95.56	98.18	94.19	90.30	91.07
1.18 mm	77.44	91.28	96.53	90.97	85.15	88.30
0.600 mm	44.01	84.07	93.03	82.79	78.65	84.38
0.300 mm	8.29	26.22	77.21	37.89	44.67	69.90
0.150 mm	1.97	21.63	38.06	28.32	33.32	34.77
Zone	II	IV	IV	IV	III	IV

. The particle size distribution curves are shown in Figure 3. The finer BA fractions, reflected in the lower portion of the curve, exhibit pozzolanic activity and contribute to pore structure refinement within the concrete matrix [10].

2.1.5. Microstructural Properties of Bottom Ash

Scanning electron microscopy (SEM) was performed by mounting the BA samples on a glass plate using an adhesive. Figure 4C(i)–C(v),S(i)–S(v) present SEM images of clustered and individual/single BA particles collected from five different TPPs, illustrating their morphology and surface texture. The microstructural observations provide a clear understanding of the physical form and textural characteristics of the material. The SEM images reveal that the BA particles are predominantly angular and rough-textured, exhibiting a porous, popcorn-like morphology, while some particles show partial sphericity.

Figure 4S(i),S(ii),S(iv) show that the BA particles from A-BTPS, KTPS, and TSTPS are rounded to sub-rounded in shape with rough surface textures. In contrast, the MTPS BA exhibits a popcorn-like morphology with hollow internal structures (Figure 4C(iii)), which may influence the strength and durability characteristics of concrete. The UTPS BA particles exhibit angular and irregular shapes, as shown in Figure 4S(v).

Energy-dispersive spectroscopy (EDS), coupled with SEM, was used to examine the elemental composition and morphology of the BA particles simultaneously. The EDS analysis confirmed the presence of the major elemental constituents in the BA, as shown in Figure 4E(i)–E(v), including Si, Al, Fe, O, Ca, and Mg. The high intensities of the Si and Al peaks are generally consistent with the chemical composition results presented in Table 3. The highest Si peak intensity was observed for KTPS, followed by A-BTPS, UTPS, MTPS, and TSTPS. However, the EDS results indicate a relatively higher Si content in KTPS BA, which is inconsistent with the XRF-based chemical analysis showing lower SiO₂ content for KTPS BA. This variation may be attributed to the localized nature of EDS analysis, which reflects the elemental composition at a specific spot or region of the sample surface, whereas XRF provides the bulk chemical composition of the material.

A comparatively high Al peak intensity was observed in the MTPS BA. The elevated alumina content may influence the hydration characteristics of the cementitious system and alter the formation of hydration products in the concrete matrix [61].

Furthermore, the elemental distribution obtained from the EDS analysis corroborates the phase identification derived from the XRD results. The XRD patterns reveal dominant quartz (Q) peaks corresponding to crystalline SiO₂, along with other mineral phases such as mullite (M), Al₂O₃ (A), Fe₂O₃ (F), and CaO (C), as shown in Figure 5a–e, which is consistent with previous studies [26]. The highest intensity quartz peak observed at $2\theta \approx 26.6°$ confirms the silica-rich and highly crystalline nature of the BA samples. This observation is further supported by the prominent Si peaks identified in the EDS spectra. Since crystalline quartz is relatively inert under normal cement hydration conditions, it mainly contributes through micro-filler and particle packing effects rather than significant pozzolanic reactivity [62]. However, excessive crystalline quartz content may reduce the overall reactivity of the ash due to the lower proportion of reactive amorphous silica.

2.2. Mixing and Sampling

In this study, targeting a M 30 grade concrete was prepared with a water–cement ratio of 0.45. Normal concrete (NC) was produced with 0% BA, while BAC was prepared with 20%, 35%, and 50% replacement of river sand using BA sourced from five coal-based TPPs. The choice of replacement levels was primarily governed by the combined particle size distribution and percentage passing characteristics of river sand and BA. Cement content, water content, and water–cement ratio were kept constant in this study. The targeted slump value (100–150 mm) was achieved by using SP dosages of 0.5%, 1.0%, 1.2%, and 1.3% by mass of cement for 0%, 20%, 35%, and 50% sand replacement levels with BA, respectively. Mix proportions were determined in accordance with IS 10262 [63], and the quantities of cement, water, fine aggregate, CA (20 mm and 10 mm), and SP admixture are summarized in

Table 5. Mix proportions.

Materials	0% BA	20% BA					35% BA					50% BA
	NC	BAC-AB20	BAC-KT20	BAC-MT20	BAC-TS20	BAC-UT20	BAC-AB35	BAC-KT35	BAC-MT35	BAC-TS35	BAC-UT35	BAC-AB50	BAC-KT50	BAC-MT50	BAC-TS50	BAC-UT50
Cement, OPC-53 (kg/m3)	324	324	324	324	324	324	324	324	324	324	324	324	324	324	324	324
Water (kg/m3)	146	146	146	146	146	146	146	146	146	146	146	146	146	146	146	146
w/c	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45	0.45
River sand (kg/m3)	748	597	597	597	597	597	485	485	485	485	485	373	373	373	373	373
Bottom ash (kg/m3)	–	119	119	112	113	126	208	208	196	197	220	297	297	280	282	314
CA, 20 mm (kg/m3)	642	641	641	641	641	641	640	640	640	640	640	640	640	640	640	640
CA, 10 mm (kg/m3)	631	629	629	629	629	629	629	629	629	629	629	629	629	629	629	629
Admixture (kg/m3)	1.62	3.24	3.24	3.24	3.24	3.24	3.88	3.88	3.88	3.88	3.88	4.21	4.21	4.21	4.21	4.21
Theoretical density (kg/m3)	2492.62	2459.24	2459.24	2452.24	2453.24	2466.24	2435.89	2435.89	2423.89	2424.89	2447.89	2413.21	2413.21	2396.21	2398.21	2430.21

Note: NC = normal concrete; BAC = bottom ash concrete; CA = coarse aggregate; OPC = ordinary Portland cement; w/c = water-to-cement ratio.

. All mixes were designed to satisfy the minimum cement content and maximum water–cement ratio requirements specified for ’severe’ exposure conditions as per IS 456 [47], so that the widespread use of BAC, including coastal areas and structures immersed in seawater.

River sand and CA were used in a saturated surface-dry (SSD) condition, whereas BA was pre-conditioned to a moisture content of 13–15% to minimize rapid water absorption during mixing. The preliminary trial mixes indicated that the porous BA absorbed and retained water within its internal pore structure, resulting in a dry and harsh mix with reduced workability [17] and prolonged mixing time. Therefore, pre-moisture conditioning was adopted to improve the mixing characteristics and stabilize workability. During batching, the free surface moisture associated with the pre-conditioned BA was accounted for through wet aggregate water adjustment by deducting the corresponding quantity from the mixing water, in accordance with the SSD-based mix design approach recommended in IS 10262 [63], thereby maintaining the targeted effective water–cement ratio.

All specimens were cast in accordance with IS 1199 (Part 2) [64]. Fresh concrete was placed into the molds and compacted manually using a standard tamping rod of 16 mm diameter and 600 mm length. After casting, the specimens were cured under water at 27 ± 2 °C until the respective testing ages. The dimensions of the cube, beam, and cylindrical specimens, along with the corresponding testing procedures, are described in the subsequent ‘methods’ section.

2.3. Methods

The workability and consistency of fresh concrete were evaluated using the slump test, which provides a practical measure of ease of placement, compaction, and finishing without segregation or bleeding [61]. The test was performed using a slump cone conforming to IS 1199 (Part 2) [64]. The cone had a height of 300 mm, a top diameter of 100 mm, and a bottom diameter of 200 mm, and the slump value was recorded to the nearest 5 mm.

The compressive strength ($f_{ck}$) tests were carried out on all specimens under uniaxial compression until failure, following IS 516 (Part 1/Sec 1) [65]. Cube specimens of size $150\times 150\times 150\mathrm{mm}$ were tested at curing ages of 7, 28, 56, and 90 days using a compression testing machine. The compressive strength was calculated using the following expression,

$$f_{ck}={\displaystyle \frac{P}{A_c}}$$

where P is the maximum load in N, and $A_c$ is the loaded area in mm².

The flexural strength ($f_{cr}$), representing the tensile behavior of concrete, was determined using prism specimens of dimensions $100\times 100\times 500\mathrm{mm}$. The specimens were tested under third-point loading as per IS 516 (Part 1/Sec 1) [65], and the results were expressed in terms of the modulus of rupture. When the distance between the fracture location and the nearest support exceeded 133 mm for a 100 mm specimen, the modulus of rupture was calculated as,

$$f_{cr}={\displaystyle \frac{PL}{b{d}^2}}$$

If failure occurred at a distance less than 133 mm but greater than 110 mm, the modulus of rupture was calculated using the following expression,

$$f_{cr}={\displaystyle \frac{3Pa}{b{d}^2}}$$

where P is the maximum load in N; a is the distance between the line of fracture and the nearest support in mm; b and d are the breadth and height, respectively, in mm; and L is the supported span length in mm.

The splitting tensile strength ($f_{ct}$) tests were conducted in accordance with IS 516 (Part 1/Sec 1) [65] using cylindrical specimens of 150 mm diameter and 300 mm height. The load was applied along the longitudinal diameter through the loading platens. The splitting tensile strength was computed using the following expression,

$$f_{ct}={\displaystyle \frac{2P}{\pi ld}}$$

where P is the maximum load in N, l is the height of the cylinder in mm, and d is the diameter in mm.

The dynamic modulus of elasticity ($E_d$) was determined using ultrasonic pulse velocity (UPV) measurements obtained from 150 mm cube specimens. Calculations were performed in accordance with Annex D of IS 516 (Part 5/Sec 1) [66] using the following expression,

$$E_d={\displaystyle \frac{(1+\mu )(1-2\mu )}{(1-\mu )}}\rho V^2$$

where $E_d$ is the dynamic modulus of elasticity (MPa), $\rho$ is the concrete density (kg/m³), V is the pulse velocity (m/s), and $\mu$ is the dynamic Poisson’s ratio. The static modulus of elasticity was estimated using the empirical relation $E_c=5000\sqrt{f_{ck}}$ as per IS 456 [47], and also using Neville’s correlation $E_s=0.83E_d$ [67].

The rapid chloride permeability test (RCPT) was conducted in accordance with ASTM C1202-12 [68]. Cylindrical specimens of 100 mm diameter and 50 mm thickness were prepared, and their lateral surfaces were sealed using epoxy. The specimens were vacuum conditioned at a pressure below 1 mm Hg for 3 h, followed by vacuum saturation for 1 h and immersion for $18\pm 2$ h. A constant DC potential of 60 V was applied for 6 h, with one face exposed to 0.3 N NaOH solution and the opposite face to a 3% NaCl solution. The resistance of the specimen to chloride ion penetration is quantified by the total charge passed, expressed in Coulombs (C). Current values are recorded at 30 min intervals, and the cumulative charge is determined using the trapezoidal rule, as given by:

$$Q=900\left(I_0+2I_{30}+2I_{60}+\cdots +2I_{330}+I_{360}\right)$$

where Q is the charge passed in coulombs, $I_0$ is the current (A) immediately after the voltage is applied, and $I_t$ is the current (A) at t minutes after the voltage is applied.

The rapid chloride migration test (RCMT) was conducted following the NT Build 492 [69] on cylindrical samples measuring 100 mm in diameter and 50 mm in thickness. Initially, loose particles were removed, and the specimens underwent vacuum treatment at 1–5 kPa for 3 h, followed by saturation in a Ca(OH)₂ solution for 1 h and subsequent storage in the same solution for $18\pm 2$ h. During the test, the anolyte chamber was filled with 0.3 N NaOH (12 g NaOH per liter of water), while the catholyte chamber contained a 10% NaCl solution (100 g NaCl dissolved in 900 g water). Each specimen, enclosed within a rubber sleeve and secured with a clamp, was positioned between the electrodes, connecting the cathode to the negative pole and the anode to the positive pole of the power supply. Upon completion, the specimens were axially split and sprayed with a 0.1 M AgNO₃ solution. The depth of chloride penetration was recorded at 10 mm intervals, and the chloride migration coefficient was subsequently calculated using the following expression,

$$D_{\mathrm{nssm}}={\displaystyle \frac{0.0239(273+T)L}{(U-2)t}}\left(X_d-0.0238\sqrt{{\displaystyle \frac{(273+T)L{X}_d}{U-2}}}\right)$$

where $D_{\mathrm{nssm}}$ is the non-steady-state migration coefficient in units of ${10}^{-12}{\mathrm{m}}^2/\mathrm{s}$, U is the absolute value of the applied voltage (V), T is the average value of the initial and final temperatures of the solution (°C), L is the thickness of the specimen (mm), $X_d$ is the average penetration depth (mm), and t is the test duration (h).

The electrical resistivity (ER) of concrete was determined using the Wenner four-probe resistivity technique as per the guidelines of AASTHO TP 95 [70]. Concrete specimens were cast in cylindrical molds of 150 mm diameter and 300 mm height and cured in water for 28 days and 90 days duration. After curing, the specimen surfaces were cleaned to ensure proper electrical contact, and four equally spaced probes were positioned on the concrete surface following the Wenner configuration. A low-voltage electrical current (I) was applied through the two outer probes, while the resulting potential difference (V) between the two inner probes was measured using a resistivity meter. The electrical current was primarily conducted through ions present in the pore solution of the concrete. The ER ($\rho$) was calculated using the Wenner equation,

$$\rho =2\pi a{\displaystyle \frac{V}{I}}(\mathrm{k}\Omega \mathrm{cm})$$

where a represents the equal spacing between adjacent probes. The measured resistivity values were used as an indicator of concrete quality and durability-related performance, particularly with respect to ionic transport and corrosion resistance [71].

Water absorption tests were carried out on specimens of 100 mm diameter and 50 mm height, having a volume greater than 350 cm³, in accordance with ASTM C642-06 [72]. The tests considered water absorption after immersion as well as after immersion and boiling.

Water permeability was evaluated following DIN 1048 (Part 5) [73] using cylindrical specimens of 150 mm diameter and 160 mm height. A constant water pressure of 0.5 N/mm² was applied for 72 h, after which the specimens were split, and the maximum penetration depth was measured.

Drying shrinkage was measured on prismatic specimens of dimensions $75\times 75\times 300\mathrm{mm}$ in accordance with IS 516 (Part 6) [74]. The drying shrinkage was calculated using

$$\mathrm{Drying}\mathrm{shrinkage}(\%)={\displaystyle \frac{L_i-L_f}{L_0}}\times 100$$

where $L_i$ is the initial length after curing, $L_f$ is the final length after drying, and $L_0$ is the effective gauge length.

Microstructural characterization was carried out using scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS), also known as energy-dispersive X-ray (EDX) analysis, on specimens cured for 90 days. Samples were collected from fractured surfaces after compressive strength testing, vacuum-dried, gold-coated, and mounted for SEM–EDS examination. For phase analysis, powdered samples were obtained from the mortar phase by grinding hardened concrete to pass a 75 μm sieve. Phase identification was performed using X-ray diffraction (XRD) over a diffraction angle range of $2\theta =5^{°}$–${80}^{°}$ with a step size of $0.020°$.

3. Results and Discussion

3.1. Workability

The variation in slump values of concrete mixes incorporating BA as a partial replacement of river sand is shown in Figure 6. With constant cement content and water-to-cement ratio, SP dosages of 0.5%, 1.0%, 1.2%, and 1.3% by mass of cement were used for 0%, 20%, 35%, and 50% sand replacement levels with BA, respectively. The results indicate that workability is governed by both the BA replacement level. For all mixes, slump decreased with increasing BA content, with a pronounced reduction beyond 20% replacement.

At 0% sand replacement, NC exhibited a slump value of 160 mm, indicating the reference workability of the control mix. At 20% replacement, A-BTPS retained a slump value of approximately 160 mm, showing no significant variation compared to NC, while STPTS and UTPS exhibited slump values of about 155 mm, corresponding to a reduction of approximately 3.1%. KTPS showed a comparatively lower slump value of 150 mm, reflecting a reduction of about 6.3%, whereas MTPS exhibited the lowest slump value of approximately 145 mm, representing a reduction of around 9.4% relative to NC.

At 35% sand replacement, slump values further decreased to approximately 155 mm for A-BTPS, indicating a reduction of about 3.1%, while STPTS recorded a slump value of around 150 mm, corresponding to a reduction of approximately 6.3%. KTPS and UTPS exhibited slump values of about 145 mm, representing reductions of nearly 9.4%, whereas MTPS continued to show the lowest slump value of approximately 140 mm, corresponding to a reduction of about 12.5% compared to NC.

At 50% replacement, a more pronounced reduction in slump was observed for all BAC. Slump values decreased to approximately 145 mm for A-BTPS, 140 mm for STPTS, 135 mm for UTPS, 130 mm for KTPS, and 125 mm for MTPS, corresponding to reductions of about 9.4%, 12.5%, 15.6%, 18.8%, and 21.9%, respectively, relative to NC. Among all the BA sources, MTPS exhibited the lowest slump at all replacement levels due to its relatively high water absorption capacity and irregular particle morphology, which increased the water demand of the concrete mix.

The reduction in workability with increasing BA content is attributed to its angular shape, rough surface texture, and porous structure, which increase specific surface area and inter-particle friction compared to river sand [17]. In addition, the higher water absorption of BA reduces the availability of free water for lubrication, further lowering flowability [15,75]. These effects become more pronounced at higher replacement levels. The results indicate that sand replacement levels up to 20% maintained workability comparable to NC and provided satisfactory consistency for practical casting applications. Although mixes containing 35% BA still exhibited acceptable slump values for placement, a comparatively lower workability was observed for certain BA sources, particularly MTPS, indicating increased water demand at higher replacement levels.

To mitigate excessive loss of workability, BA was pre-conditioning, and a PCE-based SP was incorporated [39,76]. Under these conditions, slump values ranging from approximately 125 to 160 mm were achieved. Similar trends have been reported in previous studies [77,78,79], confirming that appropriate moisture conditioning and the use of PCE-based admixtures can effectively enhance the workability of BAC [76]. Although the utilization of BA contributes positively to waste valorization and the conservation of natural resources, the increased demand for SP at higher replacement levels may increase material cost and slightly reduce the economic sustainability of the mixes. Based on the experimental results, BA replacement up to 20% provided comparatively higher workability, while replacement levels up to 35% remained acceptable depending on the desired performance requirements.

3.2. Compressive Strength

Compressive strength is a reliable performance indicator governed by the quality, proportion, and physicochemical characteristics of the constituent materials used in concrete [15,61]. Column 3 in

Table 6. Strength properties of concrete containing 0% bottom ash.

Sample Name	Age (Days)	${\mathit{f}}_{\mathit{ck}}\pm \mathbf{SD}$ (MPa)	${\mathit{f}}_{\mathit{cr}}\pm \mathbf{SD}$ (MPa)	${\mathit{f}}_{\mathit{ct}}\pm \mathbf{SD}$ (MPa)
NC
OPC + 0% BA + 100% RS	7	$29.33\pm 0.44$	$3.53\pm 0.12$	$2.69\pm 0.14$
	28	$38.52\pm 0.68$	$4.17\pm 0.25$	$3.30\pm 0.16$
	56	$38.67\pm 0.89$	$4.24\pm 0.21$	$3.51\pm 0.18$
	90	$39.56\pm 0.44$	$4.56\pm 0.21$	$3.82\pm 0.14$

Note: $f_{ck}$ = compressive strength; $f_{cr}$ = flexural strength; $f_{ct}$ = splitting tensile strength.

,

Table 7. Strength properties of concrete containing 20% bottom ash.

Sample Name	Age (Days)	${\mathit{f}}_{\mathit{ck}}\pm \mathbf{SD}$ (MPa)	${\mathit{f}}_{\mathit{cr}}\pm \mathbf{SD}$  (MPa)	${\mathit{f}}_{\mathit{ct}}\pm \mathbf{SD}$  (MPa)
BAC-AB20
OPC + 20% A-BTPS BA + 80% RS	7	$29.78\pm 0.44$	$3.84\pm 0.26$	$2.74\pm 0.16$
	28	$39.11\pm 0.44$	$4.58\pm 0.69$	$3.40\pm 0.14$
	56	$39.41\pm 1.12$	$4.60\pm 0.20$	$3.91\pm 0.22$
	90	$41.04\pm 1.12$	$4.76\pm 0.46$	$3.96\pm 0.28$
BAC-KT20
OPC + 20% KTPS BA + 80% RS	7	$29.33\pm 0.44$	$3.58\pm 0.18$	$2.78\pm 0.08$
	28	$39.41\pm 0.68$	$3.99\pm 0.39$	$3.07\pm 0.22$
	56	$40.30\pm 1.36$	$4.21\pm 0.32$	$3.68\pm 0.14$
	90	$41.33\pm 0.44$	$4.63\pm 0.29$	$4.01\pm 0.22$
BAC-MT20
OPC + 20% MTPS BA + 80% RS	7	$26.81\pm 0.68$	$3.47\pm 0.42$	$2.59\pm 0.22$
	28	$37.48\pm 0.93$	$4.23\pm 0.78$	$3.35\pm 0.08$
	56	$40.00\pm 1.94$	$4.27\pm 0.24$	$3.73\pm 0.08$
	90	$41.04\pm 1.12$	$4.56\pm 0.26$	$3.91\pm 0.22$
BAC-TS20
OPC + 20% TSTPS BA + 80% RS	7	$29.04\pm 1.36$	$3.47\pm 0.42$	$2.74\pm 0.16$
	28	$38.81\pm 0.68$	$4.30\pm 0.26$	$3.30\pm 0.59$
	56	$39.56\pm 1.60$	$4.33\pm 0.42$	$3.49\pm 0.08$
	90	$41.63\pm 0.68$	$4.47\pm 0.31$	$3.68\pm 0.14$
BAC-UT20
OPC + 20% UTPS BA + 80% RS	7	$27.56\pm 0.89$	$3.30\pm 0.18$	$2.78\pm 0.22$
	28	$38.96\pm 1.12$	$3.71\pm 0.35$	$3.16\pm 0.29$
	56	$40.89\pm 0.44$	$4.25\pm 0.47$	$3.40\pm 0.14$
	90	$41.93\pm 0.93$	$4.53\pm 0.23$	$3.58\pm 0.22$

Note: $f_{ck}$ = compressive strength; $f_{cr}$ = flexural strength; $f_{ct}$ = splitting tensile strength.

,

Table 8. Strength properties of concrete containing 35% bottom ash.

Sample Name	Age (Days)	${\mathit{f}}_{\mathit{ck}}\pm \mathbf{SD}$ (MPa)	${\mathit{f}}_{\mathit{cr}}\pm \mathbf{SD}$ (MPa)	${\mathit{f}}_{\mathit{ct}}\pm \mathbf{SD}$ (MPa)
BAC-AB35
OPC + 35% A-BTPS BA + 65% RS	7	$31.11\pm 0.44$	$3.52\pm 0.11$	$2.59\pm 0.16$
	28	$41.33\pm 1.18$	$4.74\pm 0.96$	$3.49\pm 0.16$
	56	$43.56\pm 0.44$	$4.93\pm 0.12$	$4.10\pm 0.14$
	90	$45.63\pm 3.88$	$5.09\pm 0.10$	$4.24\pm 0.28$
BAC-KT35
OPC + 35% KTPS BA + 65% RS	7	$29.47\pm 1.26$	$3.70\pm 0.43$	$2.83\pm 0.14$
	28	$39.52\pm 0.50$	$4.03\pm 0.08$	$3.07\pm 0.08$
	56	$44.30\pm 0.64$	$4.43\pm 0.06$	$3.73\pm 0.22$
	90	$46.42\pm 0.17$	$4.80\pm 0.20$	$4.24\pm 0.28$
BAC-MT35
OPC + 35% MTPS BA + 65% RS	7	$24.74\pm 1.12$	$3.31\pm 0.69$	$2.45\pm 0.08$
	28	$36.59\pm 1.12$	$3.70\pm 0.10$	$3.02\pm 0.16$
	56	$39.26\pm 0.93$	$4.30\pm 0.26$	$3.49\pm 0.22$
	90	$40.74\pm 1.56$	$4.89\pm 0.25$	$3.68\pm 0.28$
BAC-TS35
OPC + 35% TSTPS BA + 65% RS	7	$26.67\pm 2.22$	$3.33\pm 0.42$	$3.11\pm 0.16$
	28	$38.37\pm 0.68$	$4.05\pm 0.33$	$3.11\pm 0.75$
	56	$40.15\pm 2.00$	$4.99\pm 0.23$	$3.68\pm 0.14$
	90	$43.11\pm 1.33$	$4.53\pm 0.31$	$3.82\pm 0.14$
BAC-UT35
OPC + 35% UTPS BA + 65% RS	7	$25.16\pm 0.20$	$3.23\pm 0.36$	$2.26\pm 0.14$
	28	$38.66\pm 0.49$	$3.46\pm 0.09$	$3.05\pm 0.07$
	56	$41.98\pm 0.30$	$4.34\pm 0.63$	$3.54\pm 0.14$
	90	$45.22\pm 0.94$	$5.02\pm 0.20$	$3.91\pm 0.08$

Note: $f_{ck}$ = compressive strength; $f_{cr}$ = flexural strength; $f_{ct}$ = splitting tensile strength.

and

Table 9. Strength properties of concrete containing 50% bottom ash.

Sample Name	Age (Days)	${\mathit{f}}_{\mathit{ck}}\pm \mathbf{SD}$ (MPa)	${\mathit{f}}_{\mathit{cr}}\pm \mathbf{SD}$ (MPa)	${\mathit{f}}_{\mathit{ct}}\pm \mathbf{SD}$ (MPa)
BAC-AB50
OPC + 50% A-BTPS BA + 50% RS	7	$24.74\pm 2.19$	$3.37\pm 0.24$	$2.26\pm 0.14$
	28	$34.81\pm 1.36$	$3.90\pm 0.10$	$2.69\pm 0.14$
	56	$36.15\pm 0.51$	$4.19\pm 0.22$	$2.92\pm 0.16$
	90	$38.37\pm 0.51$	$4.36\pm 0.41$	$3.11\pm 0.28$
BAC-KT50
OPC + 50% KTPS BA + 50% RS	7	$21.93\pm 1.80$	$3.24\pm 0.32$	$2.17\pm 0.22$
	28	$34.22\pm 0.44$	$3.70\pm 0.30$	$2.74\pm 0.16$
	56	$36.44\pm 0.89$	$4.27\pm 0.61$	$3.11\pm 0.14$
	90	$38.96\pm 1.12$	$4.40\pm 0.20$	$3.25\pm 0.14$
BAC-MT50
OPC + 50% MTPS BA + 50% RS	7	$21.19\pm 1.12$	$2.91\pm 0.34$	$2.07\pm 0.08$
	28	$31.11\pm 2.22$	$3.71\pm 0.35$	$2.59\pm 0.22$
	56	$32.74\pm 1.43$	$4.19\pm 0.22$	$2.83\pm 0.14$
	90	$33.93\pm 0.93$	$4.23\pm 0.49$	$3.25\pm 0.14$
BAC-TS50
OPC + 50% TSTPS BA + 50% RS	7	$23.85\pm 1.43$	$3.20\pm 0.20$	$2.31\pm 0.22$
	28	$34.52\pm 0.93$	$4.03\pm 0.32$	$2.97\pm 0.14$
	56	$36.44\pm 0.89$	$4.27\pm 0.61$	$3.40\pm 0.14$
	90	$37.19\pm 0.68$	$4.36\pm 0.34$	$3.49\pm 0.29$
BAC-UT50
OPC + 50% UTPS BA + 50% RS	7	$21.78\pm 2.04$	$3.18\pm 0.24$	$2.31\pm 0.22$
	28	$30.07\pm 1.12$	$3.71\pm 0.24$	$2.78\pm 0.22$
	56	$31.41\pm 0.51$	$4.12\pm 0.24$	$3.25\pm 0.14$
	90	$33.33\pm 1.33$	$4.33\pm 0.42$	$3.63\pm 0.08$

Note: $f_{ck}$ = compressive strength; $f_{cr}$ = flexural strength; $f_{ct}$ = splitting tensile strength.

represents the compressive strength of concrete incorporating 0% to 50% sand replacement with different BA sources, while Figure 7 illustrates the comparative strength variation of BAC at 20%, 35%, and 50% replacement levels with respect to NC. The reported results represent the average of three cube specimens tested for each mix at different ages under water curing conditions.

At an early age (7 days), NC attained approximately 77% of its target strength, whereas concrete containing BA exhibited lower strength at all replacement levels. At 20% replacement, the reduction in early-age compressive strength (26.81–29.78 MPa) was marginal compared to higher replacement levels, indicating that limited incorporation of BA does not severely impair early strength development. However, at 35% and 50% replacement, a pronounced negative variation was observed, attributable to the relatively weaker and more porous nature of BA particles compared to river sand [10,31]. At this age, effective stress transfer across the interfacial transition zone (ITZ) is limited, and the pozzolanic reaction of BA remains largely inactive [26].

The compressive strength scenario changes significantly with curing age. At 28 days, concrete with 20% BA incorporation achieved compressive strengths in the range of 37.48–39.41 MPa, confirming that this replacement level does not adversely affect the target strength requirement. At 35% replacement, mixes incorporating A-BTPS, KTPS, TSTPS, and UTPS BA also achieved the target 28-day compressive strength, whereas MTPS-based concrete exhibited comparatively lower strength. SEM analysis of BAC-MT35 revealed the presence of calcium hydroxide (CH) and ettringite, indicating incomplete hydration and microstructural instability. Furthermore, the lower Si peak observed in the EDS analysis suggests reduced participation in pozzolanic reaction development, thereby contributing to the lower strength gain. In contrast, A-BTPS and KTPS concretes exhibited positive strength variation compared to NC at this stage, which may be attributed to the higher SiO₂ content and higher FM value of A-BTPS. In the case of KTPS, the spherical particle morphology observed in the SEM images (Figure 4S(ii)), together with the higher Si peak identified from EDS analysis (Figure 4E(ii)), likely contributed to the improved strength performance. However, at 50% replacement, the compressive strength values remained consistently lower than those of NC and failed to satisfy the target strength criteria.

With increasing curing duration, hydration in NC progresses gradually, whereas in BAC, pozzolanic reactions become more dominant due to the availability of reactive silica, leading to the formation of secondary C–S–H gel that contributes to pore refinement and enhanced strength development [10,18]. This mechanism is clearly reflected at 56 days, where concrete containing 20% and 35% sand replacement exhibited superior strength gain compared to NC. However, at 50% replacement, the strength development remained comparatively lower, primarily due to gap gradation and the excessive presence of finer particles, which adversely affected particle packing density and resulted in a weaker microstructure.

At 90 days, BAC demonstrated commendable long-term strength performance, particularly at 20% and 35% replacement levels. While NC exhibited only a 2.69% increase in strength relative to its 28-day value, BAC at 35% replacement showed substantial gains of 10.39%, 17.46%, 11.34%, 12.36%, and 16.99% for A-BTPS, KTPS, MTPS, TSTPS, and UTPS BA, respectively. The 20% replacement mixes also displayed consistent strength enhancement, confirming their long-term structural viability. At 50% replacement, only A-BTPS and KTPS BAC achieved their target strength at 90 days, while the remaining mixes failed to do so. Moreover, none of the 50% replacement mixes surpassed the compressive strength of NC, indicating that excessive sand substitution adversely affects strength performance.

The results establish 20% sand replacement as a safe and efficient level, offering adequate early-age strength and significant long-term performance, while 35% replacement provides improved later-age strength gain. Replacement beyond this level leads to compromised mechanical performance due to microstructural and gradation-related deficiencies. These findings are consistent with those reported in previous studies [10,26,43,80].

3.3. Flexural Strength

The significance of flexural strength becomes evident from the fact that concrete inherently possesses low tensile capacity [67]. Although concrete exhibits high compressive strength, a concrete with superior compressive strength does not also exhibit adequate flexural strength [67]. IS 456 [47] provides an empirical relationship between flexural strength and characteristic compressive strength, expressed as per the following equation,

$$f_{cr}=0.7\sqrt{f_{ck}}$$

Flexural strength is governed by the elastic behavior of concrete and the characteristics of the cement paste [61]. The formation of hydration products, quality of the ITZ, and aggregate orientation significantly influence the flexural performance of concrete [10]. Column 4 of Table 6 presents the flexural strength of NC. The flexural strength results of BAC with 20%, 35%, and 50% BA as partial replacement of river sand are reported in Table 7, Table 8, and Table 9, respectively. The test results represent the average values obtained from testing three beam specimens for different mixes at different curing ages. The variation in flexural strength at 7, 28, 56, and 90 days with respect to NC for different BA sources is illustrated in Figure 8.

At 7 days, NC achieved approximately 85% of its 28-day flexural strength. Concrete incorporating BA exhibited lower flexural strength at all replacement levels, with the reduction being marginal at 20% replacement and more pronounced at 35% and 50% replacement levels. The reduction was maximum at 50% replacement, indicating a detrimental effect of excessive BA content at early ages. At this stage, the absence of pozzolanic reaction results in the lack of secondary C–S–H gel formation, leading to a relatively weak and porous cement matrix [10]. Similar observations were reported by [29], who noted that increasing BA content as sand replacement adversely affects early-age flexural strength.

At 28 days, concrete with 20% BA achieved flexural strength values comparable to NC, demonstrating that limited sand substitution does not adversely affect flexural performance. At 35% replacement, an improvement in flexural strength was observed, although the strength remained slightly lower than that of NC, except in concrete prepared with A-BTPS BA. This behavior may be attributed to the relatively higher specific gravity, lower water absorption, and the presence of a substantial amount of SiO₂ in A-BTPS, which contributed to comparable or marginally higher strength development. In contrast, at 50% replacement, all BAC mixes exhibited lower flexural strength compared to NC, as evident from Figure 8. Kurama and Kaya [81] also reported that BAC can achieve flexural strength comparable to NC at 28 days, and the findings of the present study are in good agreement with earlier research [11].

The trend of flexural strength development closely followed that of compressive strength. At a curing age of 56 days, concrete with 20% and 35% replacement exhibited higher strength gain compared to NC, indicating the beneficial role of delayed pozzolanic reactions. However, at 50% replacement, flexural strength remained lower than that of NC at 56 days, except for concretes incorporating A-BTPS and KTPS BA, which showed relatively better performance.

With further increase in curing age, BAC developed a denser and more stable cementitious matrix due to the formation of secondary C–S–H and C–A–S–H gels through pozzolanic reactions [10,82]. These reactions reduced early-age deficiencies such as weak bonding and porous microstructure, resulting in improved flexural performance and reduced CH content. At 90 days, NC exhibited a flexural strength increase of approximately 9.48% relative to its 28-day strength, whereas BAC at 35% replacement showed a significantly higher strength gain in the range of 19–32% [15,18,81,83]. In contrast, concrete with 50% sand replacement with BA failed to achieve flexural strength comparable to NC, even at later ages, indicating that excessive sand replacement adversely affects flexural performance [75,84].

The present study found that 20% BA incorporation is structurally safe for flexural performance, while 35% replacement provides enhanced long-term flexural strength. Replacement beyond this level resulted in a reduction in flexural strength.

3.4. Splitting Tensile Strength

The splitting tensile strength of concrete is an important hardened-state property, as it indirectly reflects the tensile resistance of concrete and plays a significant role in evaluating cracking behavior, durability, and overall structural performance. This property is strongly influenced by the quality of the ITZ and the characteristics of the cement paste matrix [10,29]. Column 5 of Table 6 presents the splitting tensile strength of NC, while Table 7, Table 8 and Table 9 present the corresponding results for BAC mixes incorporating 20%, 35%, and 50% bottom ash as partial replacement of river sand at curing ages of 7, 28, 56, and 90 days. The reported values represent the average of three specimens tested for each replacement level and curing age. The variation in splitting tensile strength of BAC with respect to NC is illustrated in Figure 9.

At early curing ages, BAC exhibited lower splitting tensile strength than NC at all replacement levels. The reduction was marginal at 20% replacement but became more pronounced at 35% and 50% replacement levels, primarily due to the weaker paste matrix and limited bonding at the ITZ with limited resistance to tensile cracking [11].

With increasing curing age, the pozzolanic reaction of BA contributed to gradual improvement in the paste matrix, leading to a noticeable increase in splitting tensile strength, consistent with observations reported by Ghafoori [39]. At 28 days, the splitting tensile strength of mixes BAC-AB35, BAC-KT35, BAC-MT35, BAC-TS35, and BAC-UT35 exceeded that of NC at the 35% replacement level, while the 20% replacement mixes achieved strength values comparable to or marginally higher than NC. A similar positive trend in strength gain was observed at 56 and 90 days, highlighting the beneficial role of delayed pozzolanic reactions in enhancing tensile performance [10,84].

Despite later-age gains, splitting tensile strength showed a smaller improvement than compressive strength, underscoring the governing role of aggregate gradation, FM, and ITZ quality. At 50% replacement, excess fines and gap grading likely increased voids and water films around CA, weakening stress transfer across the ITZ; consequently, tensile strength remained lower than that of NC despite ongoing pozzolanic reactions. With appropriate chemical admixture dosage and curing, the development of splitting tensile strength followed the trend of compressive strength [85].

The ratio and the relationship between splitting tensile strength to compressive strength, as presented in

Table 10. Ratio of splitting tensile strength to compressive strength expressed as a percentage.

Age	0% BA	20% BA					35% BA					50% BA
	NC	BAC-AB20	BAC-KT20	BAC-MT20	BAC-TS20	BAC-UT20	BAC-AB35	BAC-KT35	BAC-MT35	BAC-TS35	BAC-UT35	BAC-AB50	BAC-KT50	BAC-MT50	BAC-TS50	BAC-UT50
7 days	9.16	9.19	9.48	9.67	9.42	10.10	8.34	9.60	9.91	9.90	9.00	9.15	9.89	9.79	9.69	10.61
28 days	8.57	8.68	7.78	8.93	8.50	8.11	8.44	7.76	8.25	8.11	7.90	7.72	7.99	8.34	8.61	9.25
56 days	9.09	9.93	9.13	9.31	8.82	8.30	9.42	8.41	8.89	9.16	8.42	8.09	8.54	8.64	9.32	10.36
90 days	9.66	9.65	9.70	9.54	8.84	8.55	9.30	9.14	8.74	8.86	8.65	8.11	8.35	9.59	9.38	10.89

Note: NC = normal concrete; BAC = bottom ash concrete.

, and Figure 10 showed reasonable agreement with values reported in the literature [11]. Singh and Siddique [10] reported this ratio to be approximately 8–9% for moderate-strength concrete and about 7% for high-strength concrete. In the present study, the NC ratio increased with curing age, from approximately 8.9% at 7 days to about 9% at 90 days. A similar increasing trend was observed for BAC, although the absolute values differed slightly from those reported in previous work [84,86]. Furthermore, the experimentally obtained splitting tensile strength values showed good agreement with the CEB–FIP empirical relationship [87].

$$f_{ct}=0.3{\left(f_{cyl}\right)}^{2/3},\mathrm{where}{f}_{cyl}=0.8f_{cu},$$

$$f_{ct,author}=0.31{\left(f_{ck}\right)}^{0.66},R^2\approx 0.93,$$

3.5. Static and Dynamic Modulus of Elasticity

The static modulus (E_s) estimated from the dynamic modulus using Neville’s correlation and code-based modulus (E_c) are shown in Figure 11. The dynamic modulus of elasticity (E_d) was calculated from the average of three UPV tests, as shown in Figure 12, to evaluate the stiffness characteristics of concretes incorporating BA as a partial fine aggregate replacement at 28 and 90 days. In general, all concretes exhibited an increase in elastic modulus with curing age, indicating continued hydration and progressive microstructural densification [15,18]. The mathematically determined E_s and experimentally determined E_d values demonstrated a clear dependence on both BA source and replacement level. At 20–35% replacement, most BAC exhibited comparable or enhanced stiffness relative to the NC, attributable to improved particle packing, filler effects, and the formation of secondary C–S–H gel. Among all sources, TSTPS and A-BTPS BA exhibited superior elastic performance, particularly at 35% replacement. The TSTPS mix attained the highest E_d values of 43.78 GPa and 45.98 GPa at 28 and 90 days, respectively, along with corresponding E_s values of 36.34 GPa and 38.17 GPa, indicating improved matrix densification and stronger aggregate–paste interaction. Similarly, A-BTPS and UTPS mixes also demonstrated significant enhancement in elastic stiffness at 35% replacement. In contrast, MTPS BA showed relatively lower modulus values compared to other sources, which may be associated with its comparatively porous and weaker particle characteristics.

At 50% replacement, both E_s and E_d decreased for all BA sources, primarily due to the higher porosity, lower stiffness, and weaker interfacial bonding associated with excessive BA incorporation, resulting in reduced load-transfer efficiency. Nevertheless, the elastic moduli at 50% replacement remained comparable to or slightly higher than the NC for certain sources. As expected, E_d values were consistently higher than E_s because dynamic measurements are less influenced by microcracking and loading duration effects. However, both parameters followed similar trends with replacement level and curing age, indicating strong agreement between static and dynamic stiffness behavior.

The code-based modulus E_c also followed trends similar to experimentally measured E_s, showing good agreement up to 35% replacement. A strong linear relationship was observed among E_d, E_s, and E_c, with E_s being approximately 84% of E_d, demonstrating the suitability of dynamic modulus measurements for estimating static elastic properties of BAC. Overall, the results indicate that BA replacement levels up to 20–35% can effectively maintain or enhance the elastic stiffness of concrete, whereas excessive replacement adversely affects elastic behavior irrespective of BA source.

3.6. Rapid Chloride Permeability Test

This test provides a rapid assessment of chloride ion penetrability in concrete [68]. Chloride ingress disrupts the passive film around reinforcing steel, initiating pitting corrosion and reducing structural service life [88]. Consequently, the RCPT value is a widely used durability indicator for evaluating resistance to chloride penetration.

The reported RCPT values represent the average obtained from three test specimens. As shown in Figure 13, BAC exhibited higher resistance to chloride ingress than NC. While NC showed moderate chloride penetrability, BAC mixes generally fell within the low to very low penetrability range, consistent with earlier findings [76]. For all mixes (A-BTPS, KTPS, MTPS, TSTPS, and UTPS), the charge passed at 90 days was lower than that at 28 days, reflecting continued hydration, secondary pozzolanic reactions, and progressive pore refinement.

Increasing BA content resulted in a considerable reduction in RCPT values relative to NC, with the most pronounced improvement observed at 35% replacement. At 28 days, KTPS and TSTPS concretes exhibited the lowest charge passed values of approximately 1062.48 C and 1102.25 C, corresponding to reductions of about 51.2% and 49.4%, respectively, compared to NC (2179.29 C). Similarly, A-BTPS concrete exhibited an RCPT value of approximately 1250.52 C at 35% replacement, representing a reduction of about 42.6%. In contrast, MTPS BAC mixes exhibited comparatively higher RCPT values ranging from 1504.19 C to 1752.87 C, corresponding to reductions of approximately 19.6–31.0% relative to NC. At 90 days, further reductions in RCPT values were observed for all mixes. KTPS concrete exhibited the lowest charge passed value of approximately 947.28 C at 35% replacement, corresponding to a reduction of about 51.6% compared to NC (1956.85 C), whereas TSTPS and A-BTPS concretes exhibited reductions of approximately 48.1% and 42.3%, respectively. The lower RCPT values observed in KTPS and TSTPS concretes indicate the formation of a denser and less permeable matrix, whereas the comparatively higher charge passed values of MTPS BAC mixes suggest a relatively more connected pore structure.

The reduction in chloride ion penetrability with BA incorporation may be attributed to the filler effect and the formation of additional secondary C–S–H gel, which contributed to pore refinement and reduced capillary connectivity [11]. The results further demonstrate that the chloride resistance performance of BAC is strongly influenced by the physical and chemical characteristics of the BA source, particularly at higher replacement levels and longer curing durations.

3.7. Rapid Chloride Migration Test

RCMT is a widely used method for evaluating concrete’s resistance to chloride-ion penetration and the associated risk of corrosion of reinforcement [69,89].

Table 11. Average non-steady-state migration coefficient (×10⁻¹² m²/s) at different sand replacement levels at 28 and 90 days.

Age	0%	20%					35%					50%
	NC	BAC-AB20	BAC-KT20	BAC-MT20	BAC-TS20	BAC-UT20	BAC-AB35	BAC-KT35	BAC-MT35	BAC-TS35	BAC-UT35	BAC-AB50	BAC-KT50	BAC-MT50	BAC-TS50	BAC-UT50
28 days	6.75	5.72	7.18	8.95	7.14	6.83	4.87	7.86	9.82	8.25	6.86	6.92	8.81	11.20	8.76	8.54
90 days	6.50	5.65	6.95	8.70	6.95	6.45	4.60	7.15	9.44	7.93	6.38	6.25	8.32	10.95	8.56	8.10

Note: Chloride ion migration classification based on non-steady-state migration coefficient: <2 × 10⁻¹² m²/s = Very good; 2–8 × 10⁻¹² m²/s = Good; 8–16 × 10⁻¹² m²/s = Normal; >16 × 10⁻¹² m²/s = Poor.

presents the average non-steady-state chloride migration coefficients of the concrete mixes, determined from tests conducted on three specimens for each mix. Compared with NC, the BAC mixes exhibited lower migration coefficients, indicating improved resistance to chloride ingress.

The improvement was most pronounced at BA replacement levels of 20–35%, which is consistent with SEM observations showing a denser cement matrix and more uniform C–S–H gel formation. Reduced migration coefficients at these levels are attributed to refined pore structure, lower pore connectivity, and secondary C–S–H formation resulting from pozzolanic reactions [11].

For all mixes, the migration coefficient decreased from 28 to 90 days, reflecting continued hydration and pore refinement. According to the adopted classification criteria, BAC falls within the range of 2–$8\times {10}^{-12}{\mathrm{m}}^2/\mathrm{s}$, corresponding to good durability performance. In contrast, concretes with 50% BA replacement showed relatively higher migration coefficients. Overall, the RCMT results confirm that partial replacement of fine aggregate with BA up to 35% provides an optimal balance between durability and mechanical performance through microstructural refinement.

3.8. Electrical Resistivity Test

The ER of concrete was measured using the Wenner four-probe method. ER, calculated from the measured current, voltage, and probe spacing, is widely used as a non-destructive indicator of concrete quality, corrosion risk, and resistance to chloride ingress [71].

Figure 14 presents the ER results of concretes incorporating different levels of BA at 28 and 90 days. The reported ER values represent the average obtained from three cylindrical specimens. For each specimen, measurements were recorded at four circumferential positions corresponding to 0°, 90°, 180°, and 270°. The measurement procedure was subsequently repeated, resulting in two sets of readings at each position. Consequently, a total of 24 readings were obtained for each mix at different curing ages, and the average values were reported in accordance with AASHTO TP 95 [70]. At 0% replacement, the mix exhibited similar resistivity values of approximately 22–25 kΩ-cm at both 28 and 90 days, indicating comparable pore connectivity in the NC. With increasing BA content, the ER increased systematically for all mixes, with the effect becoming more pronounced at higher replacement levels and longer curing ages. At 28 days, mixes containing 35% and 50% BA exhibited higher resistivity than NC. TSTPS and A-BTPS concretes showed the greatest enhancement, with resistivity increasing from 22.3 kΩ-cm for NC to 44.5–47.2 kΩ-cm and 42.8–46.5 kΩ-cm, respectively, corresponding to increases of approximately 92–112%. KTPS and UTPS mixes reached resistivity values of approximately 34.7–38.6 kΩ-cm, representing increases of about 56–73%, whereas MTPS mixes exhibited comparatively lower increases, with resistivity values limited to 27.6–30.1 kΩ-cm (approximately 24–35% increase). The increase in resistivity reflects pore refinement resulting from the filler effect and delayed pozzolanic reactions of BA.

At 90 days, further increases in resistivity were observed for all mixes, confirming continued microstructural densification with curing. The A-BTPS and TSTPS mixes exhibited the highest resistivity values at 50% replacement, reaching approximately 48.1 kΩ-cm and 49.2 kΩ-cm, respectively, compared to 24.5 kΩ-cm for NC, corresponding to increases of approximately 96% and 101%. KTPS and UTPS concretes exhibited resistivity values ranging from approximately 36.2 kΩ-cm to 40.2 kΩ-cm, whereas MTPS mixes showed comparatively lower resistivity values of 29.2–32.6 kΩ-cm. The higher ER values indicate reduced ionic transport and enhanced resistance against the ingress of aggressive agents.

3.9. Water Absorption

The average water absorption behavior of concrete mixes with different sand replacement levels under immersion and immersion–boiling conditions at 28 and 90 days is presented in Figure 15. The NC exhibited the lowest water absorption at both ages, ranging from 2.50–2.76% under immersion and 2.64–3.02% under immersion–boiling conditions, indicating comparatively lower water absorption characteristics.

Incorporation of BA generally increased water absorption with increasing replacement level. At 20% replacement, only marginal increases were observed, whereas 35% and 50% replacement levels resulted in noticeably higher absorption, particularly for MTPS BA. Under 28-day immersion and immersion–boiling conditions, the MTPS 50% mix exhibited the highest absorption values of 4.08% and 4.24%, respectively, corresponding to increases of nearly 48% and 40% compared to NC. Even at 90 days, the MTPS 50% mix maintained relatively high absorption values of 4.00% and 4.15%, respectively. SEM images shown in Figure 4C(iii) revealed porous, hollow, and irregular shell-like structures of MTPS BA, which increased the absorption capacity of the concrete.

For all mixes, water absorption decreased at 90 days compared to 28 days, with reductions generally ranging from 3 to 12% because of continued hydration and matrix densification. Among the investigated mixes, BA replacement up to 20% showed comparatively acceptable water absorption behavior, whereas higher replacement levels reduced resistance to water penetration.

The water absorption characteristics were strongly influenced by the intrinsic properties of the BA particles. In contrast, RCPT, RCMT, and ER were more strongly governed by the pore structure and densification of the cementitious matrix [67]. The filler effect and limited pozzolanic activity of BA contributed to pore refinement and reduced pore connectivity at later ages, resulting in improved transport-related durability properties despite the relatively higher water absorption observed in some mixes. Singh and Siddique [26] reported that water absorption of BAC increased with increasing BA content at 28 days, while mixes with 30% BA replacement exhibited absorption values comparable to NC. They also observed that prolonged curing reduced water absorption because of continued hydration and microstructural densification.

3.10. Water Permeability

The water permeability test evaluates the resistance of concrete to water ingress, where a lower penetration depth indicates improved durability. The average penetration depths of three specimens for NC and BAC at 28 and 90 days are summarized in

Table 12. Average water penetration depth (mm) at different sand replacement levels at 28 and 90 days.

Age	0%	20%					35%					50%
	NC	BAC-AB20	BAC-KT20	BAC-MT20	BAC-TS20	BAC-UT20	BAC-AB35	BAC-KT35	BAC-MT35	BAC-TS35	BAC-UT35	BAC-AB50	BAC-KT50	BAC-MT50	BAC-TS50	BAC-UT50
28 days	9	10	10	11	9	11	10	10	13	10	12	10	12	15	12	14
90 days	8	8	9	10	8	10	9	10	12	9	11	10	11	14	11	12

Note: NC = normal concrete; BAC = bottom ash concrete.

.

At 28 days, NC exhibited a penetration depth of 9 mm. BAC mixes with 20% sand replacement showed comparable penetration depths of 9–11 mm, corresponding to a 0–22% increase relative to NC. Among these, BAC-TS20 matched the NC, whereas BAC-MT20 and BAC-UT20 exhibited the highest increase. At 90 days, penetration depths decreased for all mixes; NC recorded 8 mm, while BAC-20 mixes ranged from 8 to 10 mm, with BAC-AB20 and BAC-TS20 showing values comparable to NC.

At 35% replacement, penetration depths increased to 10–13 mm at 28 days (11–44% higher than NC) and reduced to 9–12 mm at 90 days (13–50% higher). BAC-MT35 consistently showed the highest penetration, whereas BAC-AB35 and BAC-KT35 exhibited relatively lower values. The highest penetration depths were observed at 50% replacement, ranging from 10–15 mm at 28 days and 10–14 mm at 90 days, corresponding to increases of 11–67% and 25–75%, respectively, compared to NC.

Water penetration depth generally increased with increasing BA replacement level, particularly at 35% and 50% replacement, due to the porous nature and higher absorption capacity of BA particles. However, for all mixes, the penetration depth decreased from 28 to 90 days of curing, indicating continued hydration and gradual pore refinement within the concrete matrix [11,76]. The present study shows BAC mixes incorporating up to 20% BA exhibited water penetration depths comparable to NC at both curing ages, suggesting that this replacement level maintained adequate resistance to water ingress.

3.11. Drying Shrinkage

Saturated cement paste undergoes drying shrinkage when exposed to ambient temperature and humidity due to the loss of physically bound water. The average drying shrinkage values of NC and BAC at 28 and 90 days are summarized in

Table 13. Average drying shrinkage test results (%) at different sand replacement levels at 28 and 90 days.

Age	NC	BAC-20					BAC-35					BAC-50
	0%	AB	KT	MT	TS	UT	AB	KT	MT	TS	UT	AB	KT	MT	TS	UT
28 days	0.006	0.007	0.007	0.008	0.006	0.008	0.008	0.006	0.009	0.007	0.009	0.010	0.008	0.011	0.009	0.010
90 days	0.005	0.006	0.006	0.007	0.005	0.007	0.007	0.005	0.008	0.006	0.008	0.008	0.007	0.009	0.007	0.009

Note: NC = normal concrete; BAC = bottom ash concrete.

. All mixes exhibited low shrinkage values (<0.011%), indicating satisfactory dimensional stability.

At 28 days, NC recorded a shrinkage of 0.006%. BAC mixes with 20% sand replacement showed values between 0.006% and 0.008%, corresponding to a 0–33% increase. At 90 days, NC shrinkage reduced to 0.005%, while BAC-20 mixes ranged from 0.005% to 0.007% (0–40% increase). BAC-AB20 and BAC-KT20 exhibited shrinkage comparable to NC at both ages.

For 35% replacement, shrinkage increased to 0.006–0.009% at 28 days (0–50% increase) and 0.005–0.008% at 90 days (0–60% increase). BAC-MT35 and BAC-UT35 showed relatively higher values, whereas BAC-KT35 remained close to NC.

The highest shrinkage was observed at 50% replacement, with values of 0.008–0.011% at 28 days (33–83% increase) and 0.007–0.009% at 90 days (40–80% increase). BAC-MT50 consistently recorded the maximum shrinkage, while BAC-KT50 and BAC-TS50 showed comparatively lower increases.

Although drying shrinkage increased with higher BA content, values decreased with age for all mixes due to continued hydration [90,91]. The moderate increase in shrinkage is attributed to the higher porosity and water absorption capacity of BA particles. Overall, BAC mixes remain within acceptable shrinkage limits.

3.12. Scanning Electron Microscopy

The formation of hydration products in the cement matrix is governed by hydration reactions and chemical interactions among the constituent phases [61]. The topographic images obtained from SEM, along with EDS analysis, provide direct evidence of hydration products and their spatial distribution within the hardened matrix [10,11].

As shown in Figure 16a, the microstructure of NC is characterized by the presence of dense fibrous calcium silicate hydrate (C–S–H) gel along with well-defined hexagonal-prism morphology of CH crystals, which are typical hydration products of OPC [11]. The SEM image also reveals crumbled hexagonal plate-like crystals of monosulfate hydrate (C₄A$\overline{\mathrm{S}}$H₁₈) (Figure 17a), formed through the reaction of tricalcium aluminate (C₃A) with sulfate ions during cement hydration, making the concrete vulnerable to sulfate attack [61]. In addition, the presence of micro-voids within the matrix indicates a relatively porous microstructure, which may adversely affect the mechanical and durability properties of NC by facilitating moisture and ion transport.

Figure 16b–f presents the SEM micrographs of BAC mixes containing 35% BA after 90 days of curing. Compared with NC, the BAC mixes exhibit a comparatively denser and more homogeneous microstructure due to the synergistic effect of cement hydration and secondary pozzolanic reactions. The incorporation of silica-rich BA promotes the consumption of CH through pozzolanic activity, resulting in the formation of additional C–S–H gel. This secondary hydration product effectively fills capillary pores and refines the pore structure, thereby enhancing matrix compactness. The corresponding EDS spectra (Figure 18b–f) further support this observation by indicating increased silica content in BAC mixes, confirming the active participation of BA in secondary reactions.

The SEM image shown in Figure 17b provides direct evidence of the pozzolanic reactivity of BA particles. The spherical ash particles embedded within the hydrated cement matrix appear to react with surrounding CH crystals, leading to the progressive formation of secondary C–S–H gel at the particle–paste interface. This reaction contributes to improved interfacial bonding and microstructural densification at later curing ages. The observed microstructural modifications are consistent with the findings of Cheriaf et al. [37], who reported that pozzolanic reactions in ash-based cementitious systems become increasingly significant after approximately 28 days of curing and contribute substantially to long-term strength development.

Among the BAC mixes, BAC-AB35 (Figure 16b) exhibits the most refined and compact microstructure, characterized by a uniformly distributed C–S–H gel with minimal visible voids. The dense morphology indicates efficient pozzolanic activity, supported by the high SiO₂ content (70.42%) identified through chemical analysis, along with improved particle packing. These characteristics correlate well with the superior compressive strength observed for this mix. In contrast, the BAC-KT35 mix (Figure 16c) shows localized voids and partially reacted particles within the hydrated matrix, which can be attributed to its relatively lower SiO₂ content (58.36%). This suggests incomplete matrix densification and comparatively lower pozzolanic reactivity.

The BAC-MT35 mix (Figure 16d) contains relatively large CH crystals together with needle-shaped ettringite (C₆A${\overline{\mathrm{S}}}_3$H₃₂). The persistence of substantial CH content suggests limited CH consumption through pozzolanic reactions, resulting in comparatively lower secondary C–S–H gel formation. Moreover, excessive ettringite formation may contribute to localized microcracking and reduced matrix integrity, thereby explaining the comparatively lower mechanical performance of this mix. The comparatively lower performance of MTPS BA was mainly attributed to its particle morphology rather than its chemical composition, LOI, or grading characteristics. Although the BA exhibited a high SiO₂ content (70.19%) and acceptable LOI and particle gradation, SEM analysis (Figure 4C(iii)) revealed predominantly popcorn-like, porous, and hollow shell-like particles. These irregular structures increased water absorption, weakened interfacial bonding, and reduced stress-transfer capacity within the concrete matrix. In addition, the comparatively higher Al content with the relatively high Al/Si ratio observed in the EDS analysis (Figure 4E(iii)) may have promoted the formation of aluminate-rich hydration products such as ettringite and increased CH presence due to limited silica availability for pozzolanic reaction and secondary C–S–H formation. Consequently, these factors resulted in a less dense and less durable microstructure, thereby reducing the mechanical performance of the MTPS BAC mixes.

Figure 16e and Figure 16f show relatively uniform and compact C–S–H gel structures with tiny micro-voids in BAC-TS35 and BAC-UT35 mixes, respectively, indicating effective secondary hydration and matrix refinement. In particular, BAC-UT35 demonstrates clear evidence of BA particles reacting with CH, accompanied by extensive secondary C–S–H gel formation and reduced porosity. The dense microstructure can be attributed to the relatively high SiO₂ contents of TSTPS BA (63.89%) and UTPS BA (66.75%), which promoted pozzolanic reactivity. In addition, the well-graded particle distribution of TSTPS BA, falling within Zone III, enhanced particle packing and improved the ITZ characteristics.

3.13. Energy Dispersive Spectroscopy

EDS systems are equipped with software for quantitative elemental analysis based on the ZAF correction method, which accounts for atomic number (Z), absorption (A), and fluorescence (F) effects. Quantitative measurements are generally conducted on polished sections, with elemental peaks represented along the x-axis as X-ray energy and the y-axis as intensity (counts/s), where higher intensities indicate greater elemental abundance

The EDS results, shown in Figure 18, illustrate the elemental composition of NC and BAC after 90 days of curing and provide further insight into the hydration and pozzolanic mechanisms observed in the SEM micrographs.

Figure 18a corresponds to NC, where dominant calcium (Ca) peaks and comparatively very low silica (Si) intensity indicate the predominance of primary cement hydration products C-S-H, along with CH. The relatively high Ca/Si ratio suggests limited secondary hydration and lower formation of additional C-S-H gel. This observation agrees well with the SEM image of NC, which shows CH crystals (Figure 16a), monosulfate hydrate formations (Figure 17a), and visible micro-voids, indicating a comparatively porous and less refined microstructure.

In contrast, the BAC mixes (Figure 18b–f) exhibit comparatively higher Si peak intensities, except for BAC-MT35, confirming the active participation of silica-rich BA in pozzolanic reactions with Ca(OH)₂ to form secondary C–S–H gel [5,10]. The simultaneous reduction in relative Ca intensity in BAC mixes indicates the progressive consumption of CH during secondary hydration. Furthermore, the presence of Al, Fe, and Mg peaks confirms the incorporation of mineral constituents originating from BA particles. These observations are consistent with the SEM micrographs shown in Figure 16b,c,e,f, which reveal denser C–S–H gel formation, reduced pore volume, and improved matrix compactness at later curing ages.

Among the BAC mixes, BAC-AB35 (Figure 18b) exhibits a comparatively higher Si peak, which is consistent with the chemical analysis results, along with balanced Ca intensity, indicating enhanced hydration products and effective pozzolanic behavior. This observation correlates well with the SEM image (Figure 16b), which reveals a highly compact and homogeneous cement matrix. Similarly, BAC-KT35 and BAC-UT35 show enhanced Si peaks accompanied by dense C–S–H structures in the SEM images, suggesting continued hydration and progressive pore refinement. The relatively higher Ca intensity observed in BAC-KT35 (Figure 18c) is also supported by the chemical analysis, which showed that KTPS BA contained comparatively higher CaO content than the other BA samples.

The comparatively lower Si content observed in BAC-MT35 correlates with slower hydration development and reduced strength gain. This is further supported by the SEM image (Figure 16d), which shows the persistence of residual CH crystals and needle-shaped ettringite. In addition, the EDS analysis of MTPS BA (Figure 4E(iii)) exhibits a relatively high Al peak, which has promoted ettringite formation [61]. The comparatively stronger Ca peak observed in the EDS spectrum of BAC-MT35 further indicates limited CH consumption during hydration.

In BAC-TS35 and BAC-UT35, enhanced Si peaks together with reduced relative Ca intensity indicate continued interaction between BA particles and CH at later curing ages. These findings correspond closely with the SEM observations (Figure 16e,f), where BA particles appear well integrated within the hydrated matrix and surrounded by dense reaction products. The resulting microstructural refinement improves the ITZ.

Overall, the combined SEM and EDS analyses demonstrate that the incorporation of BA significantly modifies the microstructure and chemical composition of concrete. The development of silica-rich hydration products and the reduction in CH content contribute to matrix densification, reduced capillary porosity, and enhanced long-term mechanical and durability performance.

3.14. X-Ray Diffraction Analysis

XRD analysis was used to identify the crystalline phases and broad amorphous humps associated with poorly crystalline hydration products present in the cement paste matrix. Powdered samples were analyzed to determine the mineralogical composition based on characteristic diffraction peaks. XRD analysis of NC and BAC after 90 days of curing is shown in Figure 19.

Both NC and BAC exhibit diffraction peaks corresponding to quartz (Q), calcium hydroxide (CH), calcium silicate (CS), and ettringite (E), together with broad diffuse humps associated with poorly crystalline C–S–H and calcium aluminate silicate hydrate (C–A–S–H) phases. Ettringite was detected in NC and BAC-35MT, but was absent in BAC mixes incorporating A-BTPS, KTPS, TSTPS, and UTPS BA. Quartz exhibited the highest peak intensity, indicating its significant contribution to matrix hardness and compressive strength [10,18]. The prominent quartz peaks observed near $2\theta \approx 26°$ confirm the presence of crystalline silica [18].

Compared with NC, the BAC mixes generally exhibited reduced CH peak intensity together with enhanced silica phases and broader diffuse humps associated with C–S–H, indicating ongoing pozzolanic interactions. The reduction in CH peaks suggests the consumption of Ca(OH)₂ released during cement hydration, while the increased diffuse nature of the C–S–H region indicates the development of additional poorly crystalline hydration products. This behavior confirms the reaction between reactive silica present in BA and CH, leading to the formation of additional C–S–H gel [37]. The development of supplementary C–S–H and C–A–S–H phases contributed to matrix densification and pore refinement, which is consistent with the SEM observations of a compact microstructure and the EDS spectra showing increased silica content in BAC mixes.

Among the BAC specimens, BAC incorporating A-BTPS and KTPS BA exhibited comparatively stronger hydration product peaks, suggesting higher pozzolanic reactivity and improved binder matrix development. The presence of moderate ettringite peaks in BAC-35MT further indicates continued hydration and filler effects without evidence of excessive expansive behavior. Overall, the XRD results demonstrate that BA incorporation enhanced secondary hydration reactions, reduced portlandite content, and promoted the development of a denser and mechanically stronger cementitious matrix, thereby contributing to improved strength and durability performance.

4. Conclusions

This study demonstrates that BA can partially replace natural river sand in concrete, making it a sustainable secondary construction material. It supports the reuse of industrial waste while reducing dependence on natural aggregates. However, the properties of BA may vary depending on its source, which can influence concrete performance and necessitate detailed physical, chemical, and microstructural investigations, along with stringent quality control measures.

• The findings of this study are limited to the BA sources and concrete mixes incorporating 0%, 20%, 35%, and 50% BA as partial replacement of river sand, produced with an OPC-based binder system at a constant water-to-cement ratio of 0.45 and cured under water curing conditions.
• The suitability of BA as a fine aggregate was governed by its gradation, morphology, water absorption, and chemical composition. The poor morphology and high water absorption of MTPS BA adversely affected concrete performance, whereas the favorable physical and chemical properties of A-BTPS, TSTPS, and UTPS BA, along with the morphology of KTPS BA, improved its suitability. Partial incorporation of BA enhanced particle packing and strength, while the pozzolanic reaction associated with the high SiO₂ content promoted secondary C–S–H gel formation.
• The most favorable performance for the materials and sources investigated in this study was achieved at 35% BA replacement, showing improved 90-day compressive, flexural, and splitting tensile strengths across all mixes. The splitting tensile-to-compressive strength ratio remained within the typical range of 7–9%.
• Durability evaluation at the 35% replacement level showed improved performance of BAC, particularly at 90 days. Water permeability tests indicated low penetration depths, while RCPT and ER results confirmed enhanced resistance to chloride ion ingress (up to 16.14%). The calculated non-steady-state migration coefficients were within the range of good-quality concrete. However, drying shrinkage values were slightly higher than those of NC.
• Microstructural analysis revealed the development of dense, fibrous C–S–H gel with reduced CH content, confirming the pozzolanic reactivity of BA. EDS characterization indicated higher silica levels in BAC mixes, which promoted continued secondary reactions and enhanced mechanical strength. The BAC-MT35 mix exhibited lower strength, consistent with its relatively lower silica content, as corroborated by SEM and EDS observations.

The results demonstrate that BA can effectively replace river sand in concrete at an optimum replacement level up to 35%, providing satisfactory mechanical and durability performance. However, performance depends on BA properties and replacement ratio, as excessive replacement may increase cement or admixture demand, reducing sustainability benefits. Since the findings are specific to the materials and conditions investigated in this study, further research on long-term durability, field applications, and standardized quality control is recommended.