Development and Characterization of Basalt Fiber-Reinforced Green Concrete Utilizing Coconut Shell Aggregates

Abstract

Lightweight aggregate concrete (LWAC) is gaining interest due to its reduced weight, high strength, and durability while being cost-effective. This research proposes a method to design an LWAC by integrating coconut shell (CS) as coarse lightweight aggregate and a high volume of wet-grinded ultrafine ground granulated blast furnace slag (UGGBS). To optimize the mix design of LWAC, a particle packing model was employed. A comparative analysis was conducted between normal-weight concrete (M40) and the optimized LWAC reinforced with basalt fibers (BF). The parameters analyzed include CO₂ emissions, density, surface crack conditions, water absorption and porosity, sorptivity, and compressive and flexural strength. The optimal design was determined using the packing density method. Also, the impact of BF was investigated at varying levels (0%, 0.15%, and 1%). The results revealed that the incorporation of UGGBS had a substantial enhancement to the mechanical properties of LWAC when BF and CS were incorporated. As a significant finding of this research, a grade 30 LWAC with demolded density of 1864 kg/m³ containing only 284 kg/m³ cement was developed. The LWAC with high-volume UGGBS and BF had the minimum CO₂ emissions at 390.9 kg/t, marking a reduction of about 31.6% compared to conventional M40-grade concrete. This research presents an introductory approach to sustainable, environmentally friendly, high-strength, and low-density concrete production by using packing density optimization, thereby contributing to both environmental conservation and structural outcomes.

1. Introduction

Emissions from the energy, industrial, and agricultural sectors have posed severe threats that impact the global environment and public health. Also, the usage of raw materials is on the rise and is expected to reach 3.4 billion tons by 2050 [1]. Furthermore, the increase in the global population increases the demand for food/agricultural products [2]. In recent years, advancements in green technologies have assisted in the further development of agricultural land, resulting in a threefold growth of the agricultural sector [3]. However, the disposal of agricultural waste products using open dumping and burning methods has significantly stressed the surrounding environment [4]. This is further aggravated by the insufficient capacity of waste disposal systems [5]. Sabiiti et al. [6] stated that underdeveloped countries use the burning of agricultural waste as a common practice.

Researchers have investigated the use of agricultural waste in the construction industry to reduce the overall carbon emissions of the agricultural and construction sectors. The increasing volume of contemporary construction and the production of reinforced concrete structures aim to decrease the expense of building materials while maintaining structural integrity. This also raises inquiries regarding the feasibility of utilizing environmentally sustainable natural resources as substitutes for conventional mineral components, either partially or entirely [7]. Nations with tropical climates produce excessive amounts of coconut shells (agricultural byproducts). Indonesia, India, and the Philippines are the top three producers of coconut shells, with a capacity of 16.8, 14.7, and 14.5 million tons [8]. Coconut shells can be used as coarse aggregates (a granular material) for concrete addressing landfill concerns, preserving natural resources, and reducing the concrete density. Moreover, multiple studies have demonstrated that incorporating coconut shell fibers into concrete enhances its strength and alters its deformation properties, resulting in a more ductile failure mode compared to conventional concrete [9]. The packing parameters of granular materials have an impact on the rheological and mechanical properties of concrete [10,11,12]. Particle packing models can be used to estimate the packing density and void ratio of concrete to improve the overall performance through optimization of mixture design [13,14,15,16]. In general, an increase in the packing densities can reduce the porosity (i.e., voids) of concrete, enhancing the strength and durability of the material [17,18,19]. The packing density is associated with the particle’s geometrical shape [20,21]. A spherical particle provides the maximum packing density and interlocking mechanism [22]. The weak strength of lightweight aggregate concrete (LWAC) is associated with the high porosity of the concrete block [23,24,25,26]. Additionally, the convexity of aggregate particles influences the packing density, with concave sections resulting in difficulty in filling the voids, specifically for aggregates with large sizes [27,28,29]. As such, fibers and filler materials are used to increase the packing density [30,31].

To further reduce the environmental impact of the manufacturing of cementitious composites, supplementary cementitious materials, such as silica fume and fly ash, were utilized. The steel industry generates granulated blast furnace slag, which is a glassy by-product with a high calcium content. The use of granulated blast furnace slag provides enhanced durability and favorable mechanical properties [30,31,32]. Furnace slag has high pozzolanic reactivity that reduces porosity, pore refinement, and inhibition of chloride ion transmission [33,34,35]. In recent studies, the wet grinding process was deemed an effective method for activating granulated blast furnace slag [36,37,38]. The wet grinding process reduces particle size and surface defects and enhances the particle’s reactivity [36,37,39,40,41]. As a result, an improved cementitious microstructure during the early stages of hydration was achieved [36,37]. As a result, the cementitious matrix is packed densely [42,43,44].

In this research, the cementitious matrix was optimized using the packing density method. Also, the impact of integrating wet-grinded granulated blast furnace slag, and coconut shells on the microstructure and mechanical properties of the cementitious composites was investigated. Additionally, the particle size of the coarse and fine particles was optimized to increase the packing density. Furthermore, the impact of various volume fractions of BF was examined to enhance the compressive and flexural strengths of the developed concrete. Finally, the mechanical and environmental impact of the developed concrete was compared to the traditional normal-weight concrete (M40). Importantly, this study addresses a gap in the existing literature by investigating low-cementitious, more economical, high-strength LWAC incorporating non-spherical waste aggregates using the packing density method, offering a novel contribution to the field of structural LWAC.

2. Packing Density Methodology

The packing density method was employed to analyze the impact of the particle size of the dry materials (coarse aggregate, fine aggregate, and binder) of concrete on the packing density. The packing density method was implemented on each dry material and a mixture of the dry materials. In general, the wet condition is preferred in determining the packing density of concrete [11,45,46,47,48]. However, for fine aggregates with particle sizes less than 100 μm, the dry condition is recommended to prevent agglomeration and porosity and eliminate the risk of underestimating the overall packing density [11,49]. As such, the wet condition was employed for all dry materials, except for fine aggregates. The packing density (τ_dry) for the dry condition and the solid concentration (φ_wet) for the wet condition were computed by Equations (1) and (2) in accordance with Li et al. [48].

$${\mathsf{\tau }}_{\mathrm{d}\mathrm{r}\mathrm{y}}={\displaystyle \frac{{\mathsf{\rho }}_{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}}{{\mathsf{\rho }}_{\mathsf{\alpha }}{\mathrm{R}}_{\mathsf{\alpha }}+{\mathsf{\rho }}_{\mathsf{\beta }}{\mathrm{R}}_{\mathsf{\beta }}+{\mathsf{\rho }}_{\mathsf{\gamma }}{\mathrm{R}}_{\mathsf{\gamma }}}}$$

(1)

$${\mathsf{\phi }}_{\mathrm{w}\mathrm{e}\mathrm{t}}={\displaystyle \frac{{\mathsf{\rho }}_{\mathrm{b}\mathrm{u}\mathrm{l}\mathrm{k}}}{{\mathsf{\rho }}_{\mathrm{w}}{\mathrm{u}}_{\mathrm{w}}+{\mathsf{\rho }}_{\mathsf{\alpha }}{\mathrm{R}}_{\mathsf{\alpha }}+{\mathsf{\rho }}_{\mathsf{\beta }}{\mathrm{R}}_{\mathsf{\beta }}+{\mathsf{\rho }}_{\mathsf{\gamma }}{\mathrm{R}}_{\mathsf{\gamma }}}}$$

(2)

$\rho$ and R represent the density and volumetric ratio of each material to the total solid material. ρ_bulk is associated with bulk density. α, β, and γ correspond to cement, fine aggregate, and fiber, respectively. ${\mathrm{u}}_{\mathrm{w}}$ denotes the volumetric water-to-solids (w/s) ratio.

The optimization of the concrete composition was structured in three phases: Phase 1 is focused on selecting the particle size and mixing ratio of the coarse and fine aggregates; Phase 2 is focused on the development of high-efficiency cementitious paste while utilizing furnace slag; and Phase 3 is focused on selecting the mixing ratio of aggregates (coarse and fine) to the cementitious paste to reduce porosity. Figure 1 illustrates the flowchart for the optimization methodology adopted in this research.

To achieve high packing density in LWAC, several optimization steps were undertaken. Different sizes of coconut shell aggregates were selected and combined in specific proportions to maximize the packing density, involving a range of particle sizes to effectively fill the voids between larger particles. The proportions of these various aggregate sizes were carefully calculated, requiring iterative testing and adjustments to find the optimal combination. Additionally, ultra-ground granulated blast furnace slag (UGGBS) was used as a supplementary cementitious material, with its fine particles filling micro-voids in the concrete matrix, thereby contributing to a higher packing density. Specific mixing procedures were followed to ensure uniform distribution of the aggregates and SCMs, avoiding segregation and ensuring consistent packing throughout the mix. Enhanced compaction techniques were also employed to further reduce void content and improve overall packing density.

In comparison to traditional concrete mix design methods, the packing density approach involves more complex aggregate selection and proportioning, utilizing multiple aggregate sizes to optimize void-filling capacity. Traditional methods typically use a limited range of aggregate sizes, focusing on achieving a workable mix with sufficient strength and durability rather than explicitly targeting void reduction. The packing density method prioritizes minimizing voids through precise aggregate proportioning and compaction, leading to denser and potentially more durable concrete. While both methods may use supplementary cementitious materials, the packing density approach specifically selects SCMs like UGGBS for their ability to enhance packing density by filling micro-voids. Additionally, traditional methods generally rely on basic compaction practices to achieve adequate density, whereas the packing density method emphasizes optimized compaction techniques to ensure the highest possible packing density, reducing porosity and enhancing mechanical properties.

2.1. Phase 1: Optimization of Aggregates

The coconut shells were washed and purified to eliminate the coconut fibers. The coconut shells were then dried in an oven at 100 °C for 24 h. Two coarse aggregate sizes of coconut shells were sieved with particle sizes of 9.50 to 6.30 mm and 6.30 to 4.75 mm. For fine aggregate, the particles were categorized into three groups (F, P, and FA) based on the particle size. Group F had particles with a size less than 75 μm, Group P had particles with a size between 75 μm and 150 μm, and Group FA had particles with a size in the range of 150 μm to 4.75 mm. The specific gravity of the fine and coarse aggregates was computed in accordance with ASTM C128 [50]. In phase 1, three optimization stages were used. Stages 1 and 2 optimized the gradation of the fine and coarse aggregates. Stage 3 optimized the volumetric ratios of the aggregates (i.e., coarse to fine). Figure 2 represents an illustration of the packing density needed to achieve high-strength concrete via densifying the matrix. Furthermore, using a mix of different-sized particles can create a densely packed structure. Larger particles interlock to reduce empty spaces and increase strength, while smaller particles fill in the gaps between them. This combination enhances the overall packing efficiency and structural integrity of the material. By using this packing density method with non-spherical aggregates like coconut shell aggregates, bond strength can be significantly improved.

2.2. Phase 2: Optimization of Cement Paste

In this phase, the cement-to-furnace slag ratio was optimized using two stages. In the initial stage, the wet grinding procedure was analyzed to enhance the compressive strength of the slag paste. As such, the binder-to-grinding media ratio by mass and the ratio of slag to total grinding capacity by volume were optimized. In the second stage, the cement replacement ratio was optimized to achieve high compressive strength.

2.3. Phase 3: Optimization of Aggregate to Cement Paste

Phase 3 optimizes the ratio of aggregate to cementitious paste porosity and enhances compressive strength. Additionally, the optimization goal is to reduce the reliance on cementitious paste to reduce emissions while maintaining its compressive strength. As such, the research team analyzed the fresh and hardened properties (i.e., compressive and flexural strength), water absorption and porosity, and sorptivity.

3. Materials and Experimental Setup

3.1. Materials

Type 1 Portland cement was supplied by Tasek Corporation Berhad-Cement Industry of Malaysia and used in this research. The cement had a specific gravity of 3.14 and a specific surface area of 3510 cm²/gram. The granulated blast furnace slag, a non-metallic substance, consisted of aluminosilicates, calcium silicates, and other bases. The slag has amorphous and crystalline components, with glass attributing to 85–90% of the content [51].

Table 1. Physical properties and chemical composition of grounded granulated blast slag.

Specific Gravity	Specific Surface	CaO (%)	SiO2 (%)	Al2O3 (%)	MgO (%)	Fe2O3 (%)
2.89	410–450 m2/kg	40.54	36.17	11.49	8.75	1.13

shows the composition of the used slag.

Figure 3 illustrates that coconut shells were obtained as waste materials from a local manufacturer. The coconut shells originated from the co-processing of various coconut species. The thickness of the obtained coconut shells ranged from 2 mm to 8 mm. The shells were crushed and exhibited a flaky and irregular morphology. The dosage of BF was selected based on initial feasibility studies and practical considerations regarding its impact on the properties of LWAC. Specifically chosen variables are 0%, 0.15%, and 1% to investigate a range that includes a control (0%), a lower dosage typically considered for enhancing specific properties (0.15%), and a higher dosage that could potentially maximize benefits without adverse effects (1%).

Table 2. Physical and mechanical properties of coconut shells.

Loose Bulk Density (kg/m3)	Compacted Bulk Density (kg/m3)	Specific Weight (kg/m3)	Water Absorption (%)
			15 min	1 h	24 h
583	657	1032	12.8	19	27.3

summarizes the physical and mechanical properties of the coconut shells that were used as coarse aggregates.

Table 3. Physical and mechanical properties of basalt fibers.

Length (mm)	Diameter (μm)	Specific Weight (kg/m3)	Tensile Strength (N/mm2)	Elastic Modulus (N/mm2)	Ultimate Elongation (%)
12	15	2600	2000	93,000	3

summarizes the physical and mechanical properties of the basalt fibers used. Fine aggregates, which are mining sand with a maximum nominal size of 4.75 mm and a specific gravity of 2.6, were used as described in Section 2.1.

3.2. Experimental Procedures

Water absorption and porosity tests were performed according to the ASTM C642-06 standard [52]. Sorptivity tests were conducted in accordance with the ASTM C1585-04 standard [53]. 100-mm cubic specimens were prepared and were subjected to compression at a loading rate of 5 kN/sec in accordance with ASTM C39-14a [54]. The compressive strength was examined for specimens cured in water for 1 day, 7 days, and 28 days. Beam specimens (100 × 100 × 500 mm) were prepared and subjected to a four-point flexural loading test at a rate of 0.0675 kN/sec after 28 days of water curing in accordance with ASTM C78 [55]. For the flexural and compressive strength values given, three specimens were used for each batch and curing condition. The hardened density was computed for all specimens after demolding. The saturated surface dry (SSD) was computed for all specimens after curing for 1 day, 7 days, and 28 days. To make the results comparable, all procedures, including curing and testing, were applied identically to both M40-grade concrete and LWAC specimens.

4. Results and Discussions

4.1. Composition Optimization

In this section, the results of the packing density for the potential proportions to select the optimal composition are described.

4.1.1. Coarse Aggregates

Figure 4a,b illustrates the packing density for the loose and compacted conditions of the coarse aggregates (i.e., coconut shells). The packing density was computed by varying 5% between the two sizes of the coconut shells used. The results indicated that the maximum packing density is achieved with a ratio of 15–20% of coconut shells with a size ranging from 9.5 mm to 6.3 mm of the total coarse aggregates. In general, increasing the ratio of large coarse aggregates increases the packing densities up to a critical value. The flaky concave structure of coconut shells alters the void structure and may lead to a decrease in packing density [56]. To further optimize the ratio of sizes of coarse aggregates, the packing densities were computed with an incremental ratio of 1% from 15% to 20%. Figure 4c demonstrates the packing densities (loose and compact) for the ratio of coconut shells with a size between 6.3 mm and 9.5 mm of the total coarse aggregates ranging from 15% to 20% (i.e., 16%/84%–17%/83%–18%/82%–19%/81%). The results indicate that 18% of the coarse aggregates should have a size between 6.3 mm and 9.5 mm.

4.1.2. Fine Aggregates

Figure 5 illustrates the packing density for fine aggregates that were computed for various mixing ratios of the fine aggregate’s sizes (i.e., Group F, Group P, and Group FA). A water-to-solid volumetric ratio (i.e., V_w/V_s) of 0.35 was maintained for all the mixtures and computations. The results indicate that increasing the volume ratio of fine particles in Group F (<75 μm) and Group P (75 μm < size < 150 μm), increased the packing density. The primary reason for this enhancement is that powders and fines can occupy voids that would otherwise be filled by water or air. The optimum proportions of the fine aggregates were determined to be Group F at 60 vol.%, Group P at 15 vol.%, and Group FA at 25 vol.%.

4.1.3. Coarse to Fine Aggregate

The irregular shape of coconut shells has an impact on the microstructure and interfacial transition zone (ITZ) when compared to traditional granite aggregates [57,58]. The irregular shape reduces the interlocking strength (i.e., contact area and bond strength) and increases porosity [59,60]. The variations in packing density are attributed to particle shape 22, particle size distribution [61], interlocking/compaction effects [29,47], and porosity [62]. The shape irregularity reduces the packing density [63]. Figure 6 illustrates the packing density for the composition of fine to coarse aggregates. In general, as the percentage of fine aggregates is reduced, the packing density increases. The trend is then reversed beyond 25% of the aggregates composed of fine aggregates. As such, the optimal fine-to-coarse aggregate ratio is 1/3.

4.1.4. Cementitious Paste

The wet-grinding process employs innovative technology to refine the fineness of solid particles by conducting the process in a liquid environment [36,64,65]. To maintain a water-to-powder GGBS ratio of 0.3, polycarboxylate-based superplasticizers were utilized as dispersants at 1% of total solid mass. The impact of the binder-to-grinding media ratio by mass and the ratio of the material amount to the total grinding capacity by volume on the efficiency of the reactivity of UGGBS slurry is investigated. Three distinct UGGBS slurry mixes were examined based on varying binder-to-total volume (V_GGBS/V_T) and grinding media-to-total volume ratio (V_GM/V_T) within the rotating chamber. Steel grinding media, each with a mass of 400 g, a volume of 47.67 cm³, and a rotating chamber volume of 4785 cm³, were employed. The blending of GGBS powder, water, and superplasticizer underwent milling at a rotation frequency of 50 Hz for a duration of 1.5 h. The wet milling process and the equipment used are displayed in Figure 7. The density of the processed UGGBS was measured at 2000 kg/m³. 7-day normalized compressive strength (NCS) test results for paste mixtures with identical UGGBS slurry mass replacement and water-to-binder ratios are detailed in

Table 4. Effect of grinding media volume on the compressive strength of cement paste.

	VGGBS/VT	VGM/VT	Cement (kg)	UGGBS Slurry (kg)	Water (kg)	7 Days NCS
1	0.052	0.04	1103	400	451	126
2	0.052	0.05	1103	400	451	110
3	0.079	0.05	1103	400	451	100

.

In the process of producing UGGBS slurry through wet grinding, a comprehensive assessment was conducted to evaluate the impact of different conditions on the resulting properties. Three distinct scenarios were examined, characterized by varying ratios of GGBS volume to the total volume of the rotating chamber, namely 0.052, 0.052, and 0.079. Additionally, the volume of grinding media in relation to the volume of the rotating chamber was also assessed with ratios of 0.04, 0.05, and 0.04 for the separate conditions. A total of three composition scenarios were examined: two for a 0.052 VGGBS/VT ratio (at 0.04 and 0.05 VGM/VT ratio) and one for a 0.079 VGGBS/VT ratio (at 0.05 VGM/VT ratio). The primary performance indicator considered in this evaluation was the NCS of the conditions compared to a reference condition of 100 (where the lowest strength is represented by 100). The determined NCS values were 126, 110, and 100 for the three conditions for cement-included hardened pastes. It can be concluded from Table 5 that the conditions with V_GGBS/V_T and V_GM/V_T at 0.052 and 0.04, respectively, were determined optimal and selected to produce UGGBS slurry. In comparative analysis, an increase in VGGBS/VT was associated with a reduction in the compressive strength of the resulting slurry. This phenomenon can be attributed to the likelihood of a higher average particle size resulting from the decreased free-fall distance of the grinding media. Figure 8 illustrates the impact of the ball milling ratio on grinding efficiency.

Consequently, the grinding process may have been less efficient in reducing particle size and enhancing the reactivity of the GGBS particles. Similarly, an increase in the VGM/VT ratio was also correlated with a decrease in strength. This can be explained by the larger volume of grinding media causing heightened interaction between the grinding balls, thereby diminishing the speed and impact of the free balls on the grinding process. Consequently, the overall effectiveness of the grinding action was compromised, leading to a lower compressive strength of the slurry-mixed cement paste [67,68].

4.1.5. Binder

The substitution of wet-grinded UGGBS slurry for cement represents a promising initiative aimed at reducing the cement content in concrete production. This substitution is executed in 10% intervals by volume of cement. The UGGBS slurry consists of finely divided particles with pozzolanic properties, allowing them to actively participate in the hydration reaction [36,69,70]. Incorporating UGGBS slurry into the concrete mixture enhances the reactivity of these fine particles, leading to the development of additional hydration products and consequently improving the overall strength and performance of the concrete matrix [36,71]. This substitution not only decreases the cement content but also harnesses the supplementary cementitious properties of GGBS, contributing to improved workability, enhanced durability, and a reduction in the heat of hydration in the concrete [72,73].

Furthermore, the incorporation of UGGBS slurry as a partial replacement for cement aligns with sustainable construction practices by effectively repurposing an industrial waste material and reducing the environmental impact associated with cement production. Striking a reasonable balance involves carefully adjusting the UGGBS slurry content to ensure cost-effectiveness and sustainability while maintaining the desired engineering properties of the concrete. In contrast to regular GGBS, UGGBS demonstrates a noteworthy capacity to expedite the hydration process, especially during its initial stages [74]. This observation suggests that UGGBS slurry exhibits greater reactivity in the early phases of the reaction. Moreover, after undergoing the wet grinding treatment, the UGGBS exhibits characteristics that even enable self-hydration [75]. UGGBS, with its finer particle size compared to GGBS, leads to a pronounced filling effect. Moreover, smaller particles exhibit increased reactivity, leading to the production of a greater quantity of hydration products and consequently contributing to increased strength. This phenomenon can be attributed to the reduced particle size and the homogeneous distribution achieved in GGBS particles through the implementation of the wet grinding technique. This specific procedure serves to enhance the interfacial contact area among particles, thereby resulting in the improvement of mechanical characteristics [75]. Figure 9 illustrates that the NCS, which considers control paste with no UGGBS slurry replacement (100/0) represents 100, tends to decrease slightly as the volume of UGGBS slurry replacement increases. For the ratios of 95/5, 85/15, and 75/25, the NCS remains relatively close to the 100/0 mix ratio. However, at the ratio of 65/35, there is a slightly decreasing trend in the NCS. Noticeably, at the ratio of 55/45, a more pronounced reduction is observed, with the NCS dropping to 88.73. This suggests that a higher volume of UGGBS slurry replacement (45%) can lead to a moderate decrease in compressive strength compared to the 100/0 mix. Considering these findings, it is important to carefully assess the trade-off between the potential benefits and the slight reduction in compressive strength when considering a high volume of UGGBS slurry replacement in concrete mixtures.

4.1.6. Optimum Parameters and Casting Procedure

The casting phase represents a pivotal stage in concrete production, exerting a substantial influence on the quality and ultimate performance of the product. Commencing the process, the UGGBS slurry underwent wet grinding for a duration of 1.5 h. The critical step of continuous stirring is to prevent the segregation of GGBS particles to the bottom and the initiation of the self-hydration process, ensuring a uniform distribution of UGGBS slurry throughout the mixture. Given the high water absorption capacity of CS aggregates, standing at 27% for the first day, an additional pre-treatment step becomes imperative. This involves immersing the aggregates in water for a duration of one day to mitigate their capacity for excessive water absorption. Subsequently, the aggregates were subjected to a drying process until reaching a saturated surface-dry condition in the last 30 min of the mixing process. This drying interval is important in preventing water evaporation from the voids within the CS aggregates.

To maintain a consistent water/binder ratio during the casting process, coarse CS aggregates and fine aggregates from mining sand are pre-mixed for a duration of 5 min. This preliminary mixing stage effectively addresses potential water demand and flowability concerns arising from the increased water absorption of the fine content. Following the pre-mixing of aggregates, OPC is introduced and dry-mixed for an additional 2 min, ensuring the complete integration of cement particles within the mixture. Subsequently, half of the UGGBS slurry and water are gradually incorporated into the mix, with each component undergoing mixing for 2 min. This gradual addition and careful mixing contribute to achieving a homogeneous mixture where UGGBS and water are evenly dispersed throughout the concrete blend.

Finally, the controlled addition of BF to the mixture is performed gradually to ensure a well-distributed integration. This procedural measure holds significant importance in elevating the mechanical attributes of the concrete while simultaneously mitigating the potential issues of fiber clustering or agglomeration. By meticulously implementing this optimized casting process, the inherent challenges linked to GGBS segregation from UGGBS slurry, self-hydration, water absorption by CS aggregates, and overall flowability are effectively addressed. The outcome is a concrete mix characterized by improved homogeneity and enhanced performance metrics, rendering it well-suited for diverse applications. Detailed specifications of the mixtures’ design are elucidated in Table 5.

Table 5. Optimal compositions based on packing density method (kg).

Mix	Cement	GGBS Slurry	Fine Aggregate			Crushed Stone	Coconut Shells		Basalt Fibers	Free Water	Super Plasticizer
			FA	P	F		Large	Small
M40	487	-	894.7	18.2	-	805	-	-	-	198	-
LWAC-Control	284	307	234	58.5	97.5	-	82	374	-	43.5	8
LWAC-0.15%	284	307	234	58.5	97.5	-	82	374	4	43.5	8
LWAC-1%	284	307	234	58.5	97.5	-	82	374	23	43.5	8

Table 5. Optimal compositions based on packing density method (kg).

Mix	Cement	GGBS Slurry	Fine Aggregate			Crushed Stone	Coconut Shells		Basalt Fibers	Free Water	Super Plasticizer
			FA	P	F		Large	Small
M40	487	-	894.7	18.2	-	805	-	-	-	198	-
LWAC-Control	284	307	234	58.5	97.5	-	82	374	-	43.5	8
LWAC-0.15%	284	307	234	58.5	97.5	-	82	374	4	43.5	8
LWAC-1%	284	307	234	58.5	97.5	-	82	374	23	43.5	8

4.2. Fresh and Hardened Properties

4.2.1. Fresh Density and Particle Packing

Figure 10 shows the results of the fresh density and packing density values of the studied mixtures. As depicted in the figure, M40 grade concrete has the highest fresh density at 2390.5 kg/m³, highlighting the significant impact of crushed stone aggregate on density, especially when compared to mixtures using only CS aggregate replacement. However, its packing density, standing at 0.794, suggests a potential inefficiency in the arrangement of solid particles. In contrast, the LWAC-Control mix, characterized as UGGBS slurry-replaced non-fibrous concrete, exhibits a lower fresh density of 1792.4 kg/m³ with a 25% decrease but compensates with a relatively higher packing density of 0.887. The incorporation of BF follows a consistent trend, resulting in increased fresh density across all mixes. LWAC-0.15% demonstrates a fresh density of 1821.6 kg/m³, while the LWAC-1% mix achieves a fresh density of 1855.1 kg/m³, with a 24% and 22% reduction compared to M40-grade concrete, respectively.

The addition of BF further enhanced the packing efficiency for fibrous mixes. The mix with 0.15% BF resulted in a packing density of 0.901, and the 1% BF-added mix achieved an even higher packing density of 0.915. These findings indicate that the inclusion of BF fosters a more effective arrangement of solid particles within the mixture, leading to increased packing density. The impact of fiber type on raising packing density is visually depicted in Figure 11. The incorporation of BF as flexible fibers consistently raised both fresh and packing densities in all concrete mixtures. This improvement in density values signifies the valuable role of BF in enhancing the overall density and packing efficiency of concrete mixes, potentially leading to improved mechanical and durability characteristics.

4.2.2. Saturated Surface Dry Density and Surface Cracking

The study investigated fresh density, 1-day demold density, and densities at 7 and 28 days under SSD conditions for the concrete samples. The acquired data offers insights into the evolution of concrete density over time and the influence of incorporating BF. The results of density for the mixtures are shown in

Table 6. Density of concrete mixes in different ages.

Mix	Fresh Density	Demolding Day	7-Day SSD	28-Day SSD
M40	2390	2394	2431	2452
LWAC-Control	1792	1800	1826	1880
LWAC-0.15%	1822	1830	1858	1906
LWAC-1%	1855	1864	1883	1922

. It can be observed that all mixtures subjected to packing density considerations yielded lightweight concrete densities. This aligns with existing literature, which typically categorizes lightweight concrete as having densities lower than 2000 kg/m³ according to EN 206-1 [77]. When examining the 1-day demolded density, it becomes evident that all mixtures show a slight increase compared to their fresh densities.

For the 7- and 28-day periods under SSD conditions, an increase was noted, reflecting an upward trend in density for all mixtures. In general, the results suggest that incorporating BF enhances the density properties of concrete, contributing to improved packing and densification. After 28 days of water curing, minor cracks were observed on the surface of the CS aggregate-replaced concrete samples, as depicted in Figure 12. These cracks may be attributed to autogenous shrinkage, which is the uniform reduction of internal moisture due to cement hydration. Autogenous shrinkage is especially prevalent in high-strength concrete, where the water-to-cement ratio is typically lower than 0.42 [78]. However, these cracks were less pronounced with increasing fiber volumes, indicating the bridging effect of BF-causing reinforcement to reduce the internal stress of the matrix through the microstructure and their ability to prevent decomposition, thereby potentially preserving structural properties [79]. Moreover, the presence of tensile stress in the fiber results in the formation of a variety of bond stress distributions near the cracks. This, in turn, hinders the development of the crack tip due to the limits and barriers imposed by the fiber. Therefore, the inclusion of fibers in concrete can effectively mitigate autogenous shrinkage and deformation [80].

4.2.3. Water Absorption and Porosity

Table 7. Water absorption, and volume of permeable voids of mixtures.

Mix	Water Absorption (%)	Volume of Permeable Voids (%)
M40	3.87	9.19
LWAC-Control	14.96	27.13
LWAC-0.15%	14.92	26.88
LWAC-1%	13.95	25.56

presents the detailed results of water absorption and the volume of permeable voids for the mixtures. Previous studies [79,80,81] have shown that BF is effective in reducing the water absorption of concrete mixtures. This phenomenon can be attributed to the enhanced compactness of the concrete; the addition of BF blocked the development of cracks and plugged the pores, causing a decrease in water absorption and permeability [81,82,83]. Water absorption of CS aggregate concrete samples showed a similar increase in the range of 260–286%, compared to granite aggregate concrete. The mechanisms behind the significant increase in water absorption in LWAC with CS aggregates compared to granite aggregates can be attributed to the porous structure of the CS aggregates, which inherently absorb more water [84]. Additionally, the systematic increase in the use of coconut shell aggregates of varying sizes, according to the packing density method in concrete, further enhances water absorption. Despite achieving higher packing density values, the intrinsic porosity of the CS leads to increased water uptake, explaining the substantial rise in water absorption compared to less porous granite aggregates. Gunasekaran et al. [85] conducted a study on the water absorption of coconut shell aggregate concrete under full water curing conditions. In early ages, the absorption of water by the specimens does not appear to have a significant impact. This phenomenon may be attributed to the water that is absorbed by the CS during the soaking process and subsequently stored, acting as a reservoir. However, at later ages, there were notable disparities in the curing process, with a reduction of 16% at 56 days and 24% at 90 days, respectively. A similar trend is evident when examining the volume of permeable voids. The M40 grade concrete exhibited the lowest value at 9.19%, signifying a diminished volume of voids capable of permitting fluid passage. In contrast, the LWAC-Control and the mixtures with BF show higher volumes of permeable voids, indicative of the increased porosity despite having higher packing density values. The results indicate that M40 concrete displays comparable lower water absorption and lower permeable voids compared to LWAC mixtures. The introduction of CS aggregates into lightweight mixes leads to increased water absorption and porosity. Notably, the incorporation of BF in the mixes generally has a marginal impact on these properties.

4.2.4. Sorptivity

The sorptivity test results from 1 to 24 h reveal distinctive differences in absorption rates among various concrete mixes for earlier and later ages. Figure 13 illustrates the water sorptivity test results for all mixes. Specifically at 1 min for LWAC-Control concrete, a 1351% rise was observed compared to M40 concrete, which has a significantly high sorptivity value, highlighting its inferior resistance to immediate water penetration. The LWAC-Control mix exhibits remarkably high sorptivity at these early time intervals, indicating its susceptibility to rapid water ingress. The 0.15% and 1% BF-added mixes demonstrate intermediate sorptivity values with 465% and 358% rises compared to M40 concrete, respectively, suggesting that the inclusion of BF contributes to reducing early water absorption rates. This trend revealed itself by having 275%, 197%, and 66% increases for LWAC-Control, LWAC-0.15%, and LWAC-1% mixtures compared to M40 concrete, respectively, in 24-h duration. At the early stage, the LWAC-Control mix shows a high sorptivity of approximately 6 mm, while the M40 mix remains low at about 0.5 mm, illustrating the control mix’s rapid initial water uptake. The introduction of 0.15% BF reduces this early sorptivity to 2.5 mm, and 1% BF further lowers it to around 2 mm, indicating a reduction of over 58% and 67%, respectively, compared to the control. Over the longer term, the LWAC-Control’s sorptivity reaches around 12.5 mm, while the LWAC-0.15% and LWAC-1% mixes exhibit reductions to approximately 8 mm and 7 mm, reflecting 36% and 44% lower sorptivity, respectively. The M40 mix remains consistently low, at about 4 mm, but the LWAC-1% mix’s final sorptivity value demonstrates a significant improvement, bringing its performance closer to that of M40 concrete. The analysis also reveals that while increasing BF content from 0.15% to 1% continues to reduce sorptivity, the rate of improvement decreases, suggesting diminishing returns at higher BF levels.

As evident in the figure, the sorptivity values of M40 concrete remain consistently low. Notably, the mix incorporating 1% BF displays a consistently lower sorptivity trend compared to the control mix. Over time, sorptivity values of the LWAC-1% mix increase at a slower rate, approaching those observed for M40 concrete. For the increased rate of BF mixes, both the LWAC-0.15% and LWAC-1% formulations demonstrate significant reductions in sorptivity compared to the LWAC-Control mix at both testing intervals, which may be attributed to the fact that the inclusion of BF imparts effective, sustained protection against water penetration owing to the filler effect by blocking the capillary pathways [86,87]. Furthermore, the contrast in sorptivity between early and late-age results is more pronounced in the LWAC-Control mix than in the BF-added mixes. This implies the critical role of BF reinforcement in sustaining consistently lower sorptivity values over time. Overall, the findings underscore the significant role of BF in reducing sorptivity, with particular emphasis on the 1% BF-added mixture, which achieves sorptivity values akin to those of M40-grade concrete. These findings contribute to the understanding of how BF incorporation can effectively control sorptivity, offering valuable insights for enhancing the durability and performance of concrete structures.

4.2.5. Compressive Strength

The 1-, 7-, and 28-day compressive strengths of tested specimens are shown in Figure 14. Notably, in the context of 1-day compressive strength, a remarkable observation developed in the UGGBS slurry-replaced mixtures. The 1-day compressive strength of the LWAC-Control, LWAC-0.15%, and LWAC-1% mixes are 49%, 49%, and 61% of M40 concrete. Insignificant early compressive strength difference between LWAC-Control and LWAC-0.15% mixtures due to the low fiber content of BFs which is lower compared to that of LWAC-1%. The results also align with Wu et al. [88] that low fiber content has negligible impact on the early compressive strength of LWACs. All packing density optimized LWAC mixtures demonstrated substantial strength development even after a one-day curing period, a characteristic that is distinctive in comparison to the high cement content LWAC behavior as presented in the existing literature [89,90]. The enhanced packing density of both UGGBS slurry with cement as paste and coarse-fine aggregate combination is likely effective in this early-age strength development. These findings align with existing literature, where UGGBS-based concretes accelerated early-age strength attributed to the activation efficiency [36,91].

The 7-day compressive strength shows a similar pattern compared to 1-day results, where the UGGBS slurry-replaced LWAC-Control, LWAC-0.15%, and LWAC-1% mixtures had 85%, 86%, and 94% of the compressive strength of M40 concrete, respectively. The results imply that these mixtures have the potential to attain strength levels comparable to traditional concrete within a relatively short curing period. The relatively higher compressive strength of LWAC mixes compared to the existing literature can also be attributed to the influence of fine content in decreasing total porosity, improving compactness, and enhancing the microstructural development of concrete [92]. Hence, the enhancement in the strength of LWACs is not solely attributable to hydration reactions but also the grain size distribution associated with the formation of particle interlocking within the matrix [93]. It is crucial to emphasize that the incorporation of BF also contributes to this increased strength development, likely attributable to their reinforcing effects and the potential enhancement of packing density [94,95,96]. These outcomes reveal that BF incorporation can develop both the early- and long-term strength of concrete.

The data emphasizes the remarkable early-age strength attributes of UGGBS slurry-replaced mixtures [36], owing to the synergistic influences of UGGBS and CS aggregates, which potentially improved packing density both in mortar [61] and aggregate [46] phases. The incorporation of BF further increases both early-age and long-term strengths, positioning these mixtures as promising contenders for diverse concrete applications. These results represent the importance of optimizing packing density, aligning with insights from the existing literature regarding early-age strength development in concrete.

The compressive strength results demonstrate that the LWAC-1% mix consistently outperforms the LWAC-Control and LWAC-0.15% mixes across all curing times. At 1 day, the LWAC-1% mix achieves a compressive strength of 15.2 MPa, notably higher than the 12.2 MPa and 12.3 MPa of the LWAC-Control and LWAC-0.15% mixes, respectively. By the 7th day, this mix further distinguishes itself, reaching 36.7 MPa, which is approximately 10% higher than the LWAC-Control. At 28 days, the LWAC-1% mix attains a compressive strength of 40.4 MPa, showing a significant 17.4% improvement over the LWAC-Control and marking the highest strength among the LWAC mixes. This performance underscores the critical role of higher BF content in enhancing both early and long-term compressive strength, making it particularly advantageous for applications requiring rapid strength development and durability.

Results of the investigated mixtures revealed that the 28-day compressive strengths of M40 concrete exhibit a higher development compared to other concretes, characterized by its conventional composition with higher cement content, which exhibits a steady and consistent development in strength, resulting in a final compressive strength of 52 MPa. In contrast, the UGGBS slurry-replaced LWAC-Control mixture, despite demonstrating comparable early-age strengths, displays a relatively lower 28-day strength of 34.4 MPa. This difference can be attributed to the prolonged pozzolanic reaction of GGBS, which tends to result in slower strength gain beyond the initial 7 days [97,98]. However, with the introduction of BF, particularly evident in the 1% BF mixture, a noticeable enhancement in 28-day strength is observed (40.4 MPa). The addition of BF reinforces the tension of capillary pores induced by water evaporation, thereby mitigating the formation of microcracks. Consequently, the long-term compressive strength of BF-reinforced concrete demonstrates an increase compared to non-fibrous concrete [99]. Moreover, as can be seen from the above results, the influence of packing density remains significant in shaping the 28-day strengths of these mixtures. It is important to state that a negligible increase in strength was observed between the 7th and 28th days for the LWAC-Control, LWAC-0.15%, and LWAC-1% concrete mixtures with 4%, 6%, and 10% rises, respectively.

4.2.6. Flexural Strength

The 28-day flexural strength of mixtures is presented in Figure 15. The flexural strength results at the 28-day illustrate the influence of involving BF in concrete. M40 concrete resulted in a flexural strength of 7.1 MPa; additionally, the LWAC-Control mix demonstrated a marginally lower flexural strength of 7.0 MPa, where the flexural strength of the LWAC-Control mix is notably comparable to that of the M40. Adding 0.15% and 1% of BF raised the flexural strength by 33.80% and 59%, respectively. Previous studies also revealed that the inclusion of BFs enhanced the flexural strength of cementitious LWAC composites. In a study conducted by Zeng et al. [100], the flexural strength of BF-reinforced concrete specimens increased compared to non-fibrous control mixtures in 0.5%, 1.0%, and 1.5% volume fractions. The flexural strength results at 28 days reveal a clear distinction in performance among the concrete mixtures when BF is incorporated. The M40 concrete and LWAC-Control mix show similar flexural strengths of 7.1 MPa and 7.0 MPa, respectively, indicating that the absence of BF maintains a comparable baseline strength. However, the addition of BF markedly alters this dynamic. The LWAC-0.15% mix reaches a flexural strength of 9.4 MPa, which is 2.4 MPa higher than the LWAC-Control, reflecting a 34% enhancement in flexural capacity. The LWAC-1% mix further improves this value to 11.1 MPa, resulting in a 59% increase over LWAC control. Notably, the incremental improvement in flexural strength from 0.15% to 1% BF (an additional 1.7 MPa) highlights the effectiveness of increasing BF content in significantly boosting the concrete’s flexural performance. These results demonstrate that even a small increase in BF content can lead to substantial gains in the material’s ability to resist bending stresses, with the LWAC-1% mix showing a 4.1 MPa higher strength than the LWAC-Control, underscoring the material’s enhanced structural capacity and potential for broader application in load-bearing scenarios. As the stress increases, the plain concrete initially approaches its maximum flexural strength, resulting in a brittle fracture. When fiber is introduced, the fibers dispersed in a random orientation decrease the stress intensity factor at the crack tip. A higher stress is necessary to initiate fracture propagation and result in material failure for the same matrix material. The high-elastic-modulus fiber enhances the crack resistance during the entire stress process, hence improving the flexural strength of the concrete [101].

Figure 16 illustrates the crack propagation in the mixtures following the flexural strength test. Notably, all mixtures, except the one containing 1% BF, exhibited a single crack on the surface of the specimen. In contrast, the presence of a double crack in the mixture with 1% BF signifies a bridging effect of the BF, which enhances the bending crack resistance and ductility of LWAC [102,103,104]. This effect results in the transfer of load energy through the BF, providing a positive impact on the structural concrete. Furthermore, the substantial increase in flexural strength observed in the BF-added mixtures indicates the potential for further enhancing the mechanical properties of lightweight structural concrete. These findings indicate that incorporating BF, in conjunction with sustainable mix designs, can be a promising strategy for constructing eco-friendly yet robust structures with improved flexural strength.

4.2.7. Relationship between Compressive and Flexural Strength

Analyzing the 28-day values reveals a clear correlation between compressive and flexural strengths, with a correlation coefficient of 0.967: as the compressive strength increases, so does the flexural strength. This trend is particularly evident in the LWAC-0.15% and LWAC-1% mixes, where the inclusion of basalt fibers significantly enhances the flexural strength compared to LWAC-Control. For instance, the LWAC-0.15% mix shows a modest increase in compressive strength (35.5 MPa) over LWAC-Control (34.4 MPa), but a notable improvement in flexural strength (9.5 MPa versus 7 MPa). Similarly, the LWAC-1% mix achieves a substantial increase in both compressive strength (40.4 MPa) and flexural strength (11.3 MPa).

Several factors contribute to this observed correlation between compressive and flexural strengths. Firstly, M40, a normal-weight concrete, inherently possesses higher compressive and flexural strengths due to its dense matrix and the use of coarse aggregates. In contrast, LWAC mixes utilize lightweight aggregates, such as coconut shells, which generally result in lower densities and strengths. Secondly, the inclusion of basalt fibers in LWAC-0.15% and LWAC-1% mixes provides significant benefits in terms of tensile and flexural performance. Fibers bridge cracks and distribute stresses more evenly, enhancing the composite material’s ability to resist flexural loads. This is evident in the dramatic increase in flexural strength observed with higher fiber content (11.3 MPa in LWAC-1%).

Additionally, the optimized packing density in LWAC-Control contributes to better compaction and reduced porosity, leading to improved compressive strength. However, without fibers, the flexural strength remains limited due to the brittle nature of the lightweight aggregate matrix. The combination of optimized packing density and basalt fiber reinforcement in LWAC-0.15% and LWAC-1% mixes creates a synergistic effect, enhancing both compressive and flexural strengths. The fibers improve ductility and crack resistance, while the dense packing ensures a robust load-bearing matrix. Thus, the correlation between compressive and flexural strengths in these mix designs is influenced by the intrinsic properties of the materials used, the optimization of packing density, and the inclusion of basalt fibers, collectively contributing to the mechanical performance of the concrete mixes.

4.2.8. Impact of Packing Density

The analysis provides valuable insights into the correlation between packing density and various critical properties of concrete. Packing density stands out as a fundamental parameter for evaluating concrete quality, particularly concerning its long-term structural integrity. The correlation values indicate a strong positive correlation between packing density and compressive strength, as well as flexural strength, with correlation coefficients of 0.991 and 0.939, respectively. This suggests that an increase in packing density correlates with a corresponding increase in both compressive and flexural strengths, highlighting the crucial role of effective particle packing in enhancing the material’s mechanical performance. Conversely, packing density exhibits a strong negative correlation with water absorption, porosity, and sorptivity, with correlation coefficients of −0.885, −0.931, and −0.977, respectively. These negative correlations imply that as packing density increases, the water absorption, porosity, and sorptivity of the concrete decrease. This suggests that a denser packing of concrete particles results in reduced permeability and increased resistance to moisture ingress, which are essential attributes for the durability and extended service life of concrete structures.

Figure 17 presents a high-magnification view of a packing density-optimized LWAC structure. The close packing of aggregates significantly enhances the mechanical properties of concrete by optimizing load distribution within the material. By tightly arranging the aggregates, the spaces are minimized, reducing voids and enhancing interlocking between particles. This results in improved load-bearing capacity and structural integrity, making the concrete more resistant to external forces. Additionally, the reduced porosity resulting from close packing diminishes water absorption capabilities, thereby enhancing the concrete’s durability against moisture ingress and chemical attacks. Furthermore, the decreased porosity also leads to lower sorptivity, meaning the concrete absorbs water at a slower rate, contributing to its longevity and resistance to weathering effects. In summary, close packing of aggregates in concrete not only improves its mechanical strength under load but also enhances its resistance to water absorption, porosity, and sorptivity, thereby prolonging its service life and overall performance.

4.2.9. Sustainability

Assessing green building materials considers economic benefits and carbon emissions as important factors. Despite the LWAC mixed with high-volume UGGBS demonstrating approximate compressive strength with a 1% BF addition compared to M40 at 28 days, it is essential to account for manufacturing expenses and carbon footprint.

Table 8. Carbon footprint of green LWAC with high volume UGGBS slurry.

Component	CO2 Emission (kg/t)	28 Days Compressive Strength (MPa)	Emission per Compressive Strength (kg/MPa)
Cement	990 [105]	-	-
UGGBS Slurry	298.87 [36]	-	-
Sand	26 [106]	-	-
Filler	0.974 [107]	-	-
Basalt Fibers	0.85 [108]	-	-
Coarse Aggregate	81 [106]	-	-
Lightweight Aggregate	-	-	-
Superplasticizer	1.48 [109]	-	-
Water	0 [110]	-	-
M40	571.1	52	10.98
LWAC-Control	391.4	34.4	11.36
LWAC-0.15%	392.4	35.5	11.05
LWAC-1%	399.9	40.4	9.89

provides a comprehensive assessment of individual and cumulative CO₂ emissions for various components and mixtures. The quantities of materials used in each mix were determined based on the mix design proportions provided in Table 5. These quantities include cement, GGBS slurry, fine aggregate, crushed stone, coconut shells, basalt fibers, water, and superplasticizer. The CO₂ emission factors for each material were sourced from credible and standardized databases and literature, ensuring accuracy and relevance. The carbon footprint for each mix was calculated by multiplying the quantity of each material by its respective emission factor and then summing these values to obtain the total CO₂ emissions per cubic meter of concrete.

• Example Calculation for LWAC-Control Mix:
• Cement: 284 kg × 990 kg CO₂/1000 kg = 281.16 kg/t
• GGBS Slurry: 307 kg × 298.87 kg CO₂/1000 kg = 91.80 kg/t
• Fine Aggregate: (234 + 58.5 + 97.5) kg × 26 kg CO₂/1000 kg = 9.75 kg/t
• Coconut Shells: (82 + 374) kg × 19 kg CO₂/1000 kg = 8.64 kg/t
• Water: 43.5 kg × 0.1 kg CO₂/1000 kg = 0.004 kg/t
• Superplasticizer: 8 kg × 1.48 kg CO₂/1000 kg = 0.011 kg/t
• Total CO₂ Emission: 281.16 + 91.80 + 9.75 + 8.64 + 0.004 + 0.011 = 391.36 kg/t

Statistical analysis reveals significant variations across the mixes. Notably, LWAC-Control showcases the lowest CO₂ emissions at 391.4 kg/t, representing a reduction of approximately 31.6% compared to M40’s emission rate of 571.1 kg/t. The LWAC-0.15% mix had emissions of 392.4 kg/t, slightly higher than LWAC-Control but still significantly lower than M40. The LWAC-1% mix, despite incorporating more basalt fibers, resulted in emissions of 399.9 kg/t, highlighting the effectiveness of using lightweight aggregates and UGGBS slurry in reducing the overall carbon footprint. Additionally, when evaluating emissions per compressive strength (kg/MPa), LWAC mixes demonstrated lower values than M40, indicating a more efficient use of materials in terms of CO₂ emissions per unit strength. Despite its lower CO₂ emissions, LWAC-Control achieves a compressive strength of 34.4 MPa, suggesting competitive structural performance. However, LWAC-1% emerges as the strongest mix, attaining a compressive strength of 40.4 MPa, surpassing LWAC-Control by approximately 17.4%. Nevertheless, this superior strength comes at the cost of increased CO₂ emissions due to BF inclusion, with LWAC-1% emitting 399.9 kg/t of CO₂. In terms of environmental efficiency, LWAC-Control continues to excel, demonstrating the lowest emission per unit of compressive strength at 11.36 kg/MPa, implying a reduction of 3.62% and 1.3% compared to LWAC-0.15% and LWAC-1%, respectively. This statistical analysis underscores LWAC-Control’s prominence as a sustainable concrete mix, offering competitive performance across environmental and structural metrics.

Table 9. Comparison of the current study’s mix design and compressive strength results with previous literature.

Samples	Cement Dosage (kg)	Coconut Shells (kg)	Hardened Density (kg/m3)	CO2 Emission (kg/m3)	Compressive Strength (MPa)	Emission per Compressive Strength	Reference
LWAC-1%	284	456	1864	399.9	40.4	9.89–2.81	-
C-100	450	450	1905	466.05	29.81	15.63–3.12	[59]
M11	510	331.5	1970	524.374	26.7	19.64–3.51	[111]
CCS100	400	616.9	1689	413.2	18.8	21.98–4.56	[112]
M10	408	331.5	N/A	423.4	23	18.41–3.69	[113]

provides a comparison between the current study and previous research regarding the components used in their mix designs and the compressive strength obtained.

Earlier investigations employing CS as coarse aggregates exhibited lower strength levels and required higher cement dosages. The use of high volumes of CS with improved mechanical characteristics emphasizes the importance of the packing density method for the matrix. Furthermore, the wet grinding technique for GGBS enhanced hydration properties by reducing particle size, facilitating the formation of a reliable binder between coarse and fine aggregate particles. The provided table sheds light on various concrete production samples, delineating their composition, properties, and environmental impact. Notably, the LWAC-1% sample stands out with distinctive attributes. LWAC-1% exhibits a remarkable reduction in CO₂ emissions. This reduction is attributed to the high-volume cement reduction facilitated by packing density optimization techniques. As a result, LWAC-1% boasts the lowest CO₂ emissions per cubic meter among the samples, at 399.9 kg/m³, signifying a notable decrease of approximately 14.6% compared to its counterparts. Additionally, LWAC-1% showcases a commendable compressive strength of 40.4 MPa, outperforming all other samples listed in the table.

LWAC-1% stands out with a reduction of approximately 14.4% in emissions compared to M11, which exhibits the highest emissions. Specifically, for producing 1 MPa of concrete strength, LWAC-1% emits approximately 9.89 kg of CO₂. In contrast, M11, which has the highest emissions, produces approximately 19.64 kg of CO₂ to achieve the same strength level. This stark difference underscores LWAC-1%s’ efficient resource utilization and balanced approach, making it a compelling option for environmentally conscious projects seeking sustainable concrete solutions.

The lightweight coarse aggregate’s specific gravity in a dry state, ranging between 1/3 and 2/3 of that of normal-weight aggregate, meets the criteria for achieving a 28-day compressive strength of at least 17 MPa [114], high-strength lightweight aggregate concrete (HSLWAC) that exceeds 40 MPa [115], and a unit weight of 1120–1920 kg/m³ [114]. Therefore, all LWAC mixes produced for this study are classified as structural lightweight aggregate concrete (SLWAC); in particular, the LWAC-1% mix can be defined as HSLWAC [115]. This implies that, through careful selection of supplementary cementitious materials and systematic mix design optimization, it is feasible to strike a balance between eco-friendliness and structural reliability. This study not only offers a novel approach to address challenges faced by the cement industry but also serves as inspiration for low-cement-content, high-strength LWAC with a greener profile.

5. Conclusions

This study focused on the optimization of lightweight aggregate concrete (LWAC) by incorporating coconut shells as a lightweight aggregate (LWA) and basalt fibers (BF) for reinforcement. The packing density method was employed to enhance the mix design. The significant findings of this research are summarized as follows:

• The inclusion of fines and powder content has proven to be an effective method for increasing packing density, which in turn enhances the void-filling capacity. At a volume proportion of 60/15/25, there was an 18% increase compared to a single-size mixture with 100/0/0 ratios.
• Wet grinding of GGBS effectively replaces cement with reduced particle size and enhanced hydration while decreasing slurry density. Optimal UGGBS slurry production was achieved with VGGBS/VGC and VGM/VGC ratios of 0.052 and 0.04, respectively, based on compressive strength results.
• The 1-day compressive strengths of LWAC-Control, LWAC-0.15%, and LWAC-1% are 49%, 49%, and 61% of M40 concrete, respectively, showing significant early strength even with low cement. The 28-day strength of the 1% BF mixture reaches 40.4 MPa, indicating enhanced long-term strength and classifying it as HSLWAC.
• The correlation values show a strong positive relationship between packing density and both compressive (0.991) and flexural strength (0.939), indicating improved mechanical performance. Conversely, packing density negatively correlates with water absorption (−0.885), porosity (−0.931), and sorptivity (−0.977), suggesting reduced permeability and better moisture resistance.
• Regarding the environmental and economic benefits of using LWAC with high-volume UGGBS and BF, the LWAC-Control mix demonstrates the lowest CO2 emissions at 390.9 kg/t, reducing emissions by approximately 31.6% compared to conventional M40 concrete. This reduction in carbon footprint highlights the effectiveness of these materials in promoting sustainability while achieving comparable or enhanced mechanical performance.

6. Future Recommendations

To advance the understanding of water absorption, sorptivity, and durability in lightweight concrete mixes using UGGBS, basalt fibers, and coconut shell aggregates, future research should focus on integrating rapid sorptivity measurement techniques. Leveraging methods that correlate initial sorptivity with surface-wetting characteristics, as highlighted in recent studies, could provide efficient insights into long-term performance [116]. Additionally, exploring the impact of dynamic wetting and spreading behaviors of water on porous substrates, particularly in relation to basalt fibers’ influence, could help address the increased water absorption and porosity associated with coconut shell aggregates [117]. Incorporating these approaches, current research on drop-spreading dynamics and contact angles will enhance the development of more durable and sustainable lightweight aggregate concrete mixes.