Strategic Resource Extraction and Recycling from Waste: A Pathway to Sustainable Resource Conservation

Abstract

This study examines calcium extraction from Bottom Ash (BA) and the use of Solid Residue (SR) as a substitute for White Lump Clay (WLC) in brick production. Experimental analyses identified calcium and silicon as the main elements in BA, with 50% of calcium carbonate recovered through leaching. SR was a viable alternative to WLC in ceramic bricks, as SEM-EDS and FTIR analyses revealed changes in composition and microstructure. This approach promotes circular economy principles by recovering resources and reducing waste. Calcium extraction from BA can produce 29,000 tons of CaCO₃ annually for industrial use, while substituting SR for WLC in brick production could replace 30% of clay, saving 1500 tons of clay and producing millions of bricks annually. Less than 50% of incinerated Municipal Solid Waste (MSW) would require landfilling. The process supports sustainable construction by conserving natural resources, reducing landfill waste, and lowering CO₂ emissions. It offers annual cost savings of 2,639,250 USD and preserves 74,812.5 tons of resources through waste and clay reduction. By demonstrating a scalable model for waste valorization, this research aligns with global goals for sustainable development, resource efficiency, and ecological balance.

1. Introduction

The incineration of non-recyclable MSW offers a sustainable solution by reducing landfill dependency and harnessing energy. As waste volumes increase, incineration plays a key role in waste-to-energy strategies [1]. BA is a significant byproduct of this process, which requires detailed examination to understand its composition, properties, and environmental impact.

MSW, comprising diverse materials from households and businesses, is increasingly challenging to manage sustainably. Traditional landfilling is seen as unsustainable due to space limitations and environmental concerns, prompting the adoption of MSW incineration. This process reduces waste volume and generates energy, producing fly ash and bottom ash. BA, found at the bottom of incinerators, is a complex mix of inorganic materials, including minerals, metals, and unburned carbon, reflecting the original waste’s variability [2,3,4].

BA’s physical and chemical properties, such as granulometry, density, and mineral content, are crucial for handling, disposal, and potential reuse. Understanding these characteristics helps address environmental concerns like heavy metal leaching, necessitating strict regulatory oversight [2,5,6,7,8,9].

Despite being a waste product, BA holds potential for industrial applications, particularly in construction. Its mineral-rich composition makes it suitable for materials like concrete and road infrastructure. Advances in technology offer ways to enhance BA’s properties for broader uses [10].

Using bottom ash obtained after the incineration of MSW as a replacement component in clay brick production is a sustainable way to recycle waste materials. Bottom ash can partially substitute traditional clay, offering benefits in both environmental sustainability and material performance. However, extracting calcium from BA will offer environmental and economic benefits [11,12]. BA is a byproduct of the incineration of a non-recyclable fraction of MSW with significant amounts of calcium, which can be repurposed rather than disposed of in landfills. Recovering calcium contributes to sustainability by reducing waste and conserving natural resources like limestone, typically mined for calcium production. Additionally, extracted calcium can be used in various industries, such as construction (cement and brick production) [13] and environmental applications (carbon capture and water treatment), supporting circular economy initiatives while reducing the need for new raw materials.

After calcium extraction, the remaining SR typically contains oxides and trace metals, making it suitable for clay brick production. This article explores a dual-strategy approach to valorizing BA, focusing on extracting calcium and using the resulting SR as a substitute for traditional clay in eco-friendly brick production. The research adopts an integrated methodology combining material characterization, experimental leaching, and performance evaluation of calcium recovery and SR utilization. Calcium is extracted from BA through a controlled leaching process, providing a sustainable alternative to limestone-derived calcium, with applications in construction materials and environmental solutions such as carbon capture and water treatment. The remaining SR, rich in oxides and trace metals, is tested as a partial replacement for clay in ceramic bricks. The study investigates the bricks’ physical, chemical, and mechanical properties to assess their suitability and performance in construction. The approach promotes resource conservation, circular economy practices, and sustainable waste management by reducing clay usage and waste sent to landfills. The objectives of this research are to optimize calcium recovery from BA, enhance the applicability of SR in construction materials, and demonstrate a scalable model for waste valorization that contributes to achieving sustainable development goals.

2. Materials and Methods

2.1. Study Object and Sample Collection

The bottom ash was produced by incinerating non-recyclable municipal solid waste at the Kaunas Cogeneration Power Plant (CPP). After the combustion process, large metal parts were removed, and the remaining BA was taken to a regional landfill for sieving, where ferrous and non-ferrous metals were recovered. Samples of BA were collected weekly over a month, with each one-kilogram sample taken from various locations for accuracy. The samples were then homogenized and aged six months to complete the oxidation process.

Next, calcium was tried to extract from the BA. First, the BA was dried at 105 °C for 24 h and ground into a fine powder to improve leaching efficiency. For the leaching process, 10 g of BA was mixed with 100 mL of 1 M HCl and stirred for an hour at 70 °C to dissolve the calcium. After cooling, the mixture was filtered to collect the dissolved calcium ions, while the solid residue was set aside for the following recycling stage. Calcium ions were then recovered from the filtrate by adding 1M Na₂CO₃, precipitating CaCO₃. During this addition, the pH of the solution was monitored, and once it reached between 8 and 9, calcium carbonate began to precipitate. This reaction was stirred for 30 min, and then the calcium carbonate was filtered, washed with distilled water, and dried at 105 °C for 24 h. Initial and products obtained during the leaching process are presented in Figure 1. The dried calcium carbonate was weighed to determine the extraction yield.

Finally, X-ray diffraction (XRD) and SEM-EDS analysis were used to confirm the purity and structure of the extracted calcium carbonate. The solid residue remaining on the filter was used as a replacement component in the clay bricks production.

2.2. Preparation of the Test Samples

The remaining WLC and SR, after the extraction of calcium ions from BA, were used to form clay bricks. The clay bricks samples were formed by replacing clay with SR 10–30%, dried and fired at 1000 and 1100 °C. Based on previous research results [14], the authors used other fire temperatures and replacement components to produce ceramic bricks using bottom ash. Their scientific results provide positive prerequisites for a more in-depth and detailed study of such output. Upon cooling, samples were stored in a muffle oven down to room temperature to prevent moisture absorption. Physical and mechanical properties, including density, shrinkage, compressive strength, and water absorption of ready clay bricks, were assessed to evaluate the impact of solid residue replacement and firing temperature and other testing needed for construction materials. Fired clay bricks were characterised according to [15] for density determination, [16] for water sorption, and [17] for compressive strength. Additional calculations were conducted to ascertain the economic and environmental feasibility of utilising solid residue as a replacement component in clay brick production.

2.3. Test Instrument

Scanning electron microscopy with an energy dispersive spectroscopy detector (SEM-EDS) was employed to examine the morphology and elemental composition of BA, WLC, SR, CaCO₃ and clay bricks. The specimens underwent sieving and subsequent drying at 40 °C. Before SEM analysis, the analysed samples (excluding the powder) were resized to approximately 5 mm³ and coated with gold. SEM analysis was conducted on a microscope (Zeiss, Oberkochen, Germany) at an accelerating voltage of 20 kV. The detector “Bruker AXSX Flash 6/10” reflected all chemical elements in the analysed samples with a precision of 1% weight [18].

XRD analysis was carried out for raw materials (BA, SR, and CaCO₃) and fired ceramic bricks using a diffractometer “Bruker D8 Advance” with a scanning step of 0.02° in the interval 2Θ from 5 to 70° (Bruker, Karlsruhe, Germany).

Differential thermal analysis of solid residue in comparison with initial BA and WLC was done using Simultaneous Thermogravimetry and Differential Scanning Calorimetry (TG-DSC)—high-pressure ability (Linseis Messgeraete GmbH, Selb, Germany).

Porosimetry analysis was carried out on the fired clay bricks. The physical N₂ adsorption method estimated the specific surface area, total pore volume, and average pore diameter using the BET method—a Quantachrome Autosorb iQ analyser (Quantachrome, Boynton Beach, FL, USA) measured between p/p 0 and 0.99.

Fourier Transform Infrared Spectroscopy (FTIR) analysis of clay brick samples fired at 1000 and 1100 °C was performed. The Bruker Alfa FTIR (Bruker, Karlsruhe, Germany) spectrophotometer was used for sample analysis within the 4000 to 300 cm⁻¹ region.

Clay bodies’ drying and firing shrinkage were calculated according to Equations (1) and (2).

$${\mathrm{S}}_{\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{i}\mathrm{n}\mathrm{g}}=\left[\left({\mathrm{S}}_0-{\mathrm{S}}_1\right)/{\mathrm{S}}_0\right]·100\%$$

(1)

$${\mathrm{S}}_{\mathrm{f}\mathrm{i}\mathrm{r}\mathrm{i}\mathrm{n}\mathrm{g}}=\left[\left({\mathrm{S}}_0-{\mathrm{S}}_2\right)/{\mathrm{S}}_0\right]·100\%$$

(2)

where S₀ is the distance between two reference marks in a wet sample, mm; S₁ is between two reference marks in a dried sample at 105 °C, mm; S₂—is the distance between two reference marks in a fired specimen at 900 and 1000 °C, mm.

2.4. Application of the Economic and Environmental Benefit Model

2.4.1. Economic Benefit Model

To write the model, we use the next output data. W(t)—the amount of incinerated waste at time t in t/y. BA(t) = 0.23⋅W(t)—the amount of BA generated (23%). C_ext(t) = 0.5⋅BA(t)—the extracted CaCO₃ from the BA (50% of BA). SR(t) = C_ext(t)—the amount of SR available for brick production (50%).

Total economic benefits could be modelled by:

$${\mathrm{B}}_{\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}}={\displaystyle \underset{0}{\overset{\mathrm{T}}{\int }}\left({\mathrm{P}}_{\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3}·{\mathrm{C}}_{\mathrm{e}\mathrm{x}\mathrm{t}}\left(\mathrm{t}\right)+{\mathrm{P}}_{\mathrm{S}\mathrm{R}}·\mathrm{S}\mathrm{R}\left(\mathrm{t}\right)-{\mathrm{C}}_{\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{l}}·\mathrm{B}{\mathrm{A}}_{\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{l}}\left(\mathrm{t}\right)\right)\mathrm{d}\mathrm{t}}$$

(3)

where

${\mathrm{P}}_{\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3}$—price per ton of CaCO₃.

P_SR—the value of SR as a clay substitute (per ton).

C_landfill—cost for landfill disposal (per ton).

BA_landfill(t) = BA(t) − C_ext(t)—the BA remaining after extraction is sent to the landfill.

2.4.2. Environmental Impact Model

This can be defined by calculating the reduction in environmental burden over time. Assume that

E_CO2(t)—CO₂ emissions avoided per ton of extracted CaCO₃ and SR used in brick production.

Then, the cumulative environmental benefit can be calculated using the formula:

$${\mathrm{B}}_{\mathrm{e}\mathrm{n}\mathrm{v}}={\displaystyle \underset{0}{\overset{\mathrm{T}}{\int }}\left({\mathrm{E}}_{\mathrm{C}{\mathrm{O}}_2}·\left({\mathrm{C}}_{\mathrm{e}\mathrm{x}\mathrm{t}}\left(\mathrm{t}\right)+\mathrm{S}\mathrm{R}\left(\mathrm{t}\right)\right)\right)\mathrm{d}\mathrm{t}}$$

(4)

In both cases, the integrals sum up the benefits over a period T (e.g., 1 year), accounting for the economic and environmental impacts of continuous waste treatment and resource recovery.

3. Results

3.1. Results Study of Initial and Post-Leachate Products

3.1.1. SEM-EDS Analysis

Before the leaching process, SEM-EDS analysis of the initial BA was performed. Similar studies were carried out with the solid residue that remained after the extraction of calcium ions and the directly extracted calcium carbonate. The results obtained are shown in Figure 2.

The results show 13 chemical elements in both materials, the initial BA and the solid residue (Figure 2a,b). The content of 10 chemical elements in the materials exceeds 1%. The predominant elements are calcium, silicon, iron, aluminium, and sodium; oxygen and carbon are also present. All these predominant elements can form oxides and enter into chemical reactions with other components in the subsequent processing of the solid residue.

Figure 2c shows the mapping and EDS analysis results of the calcium carbonate obtained from BA’s leaching process. As expected, the calcium content is relatively high at 21.72%, and other chemical elements that form pure calcium carbonate are also present. The results of the X-ray structural analysis confirm this. After calculations, it was found that the yield of calcium ions was 68.3%. A magnetic stirrer was used to leach calcium ions. After leaching, 7% of metals were found on the magnet. The dominant metal is iron, and its content is relatively high, at 43%.

3.1.2. XRD Analysis Result

The initial bottom ash XRD analysis revealed a mix of minerals, including quartz, calcite, hematite, akermanite, wollastonite, and anorthoclase. This diverse composition reflects the complex nature of the incinerated waste, with quartz being the dominant silica phase, calcite providing a source of calcium, and the other minerals representing silicates and oxides formed during high-temperature combustion (Figure 3).

After the leaching process, the solid residue showed a change in composition. While quartz, hematite, akermanite, and anorthoclase remained, calcite and wollastonite were no longer present. This indicates that these calcium-rich minerals dissolved during leaching, meaning the acid extraction successfully removed the calcium content from the bottom ash. The remaining minerals, more resistant to the acid, stayed in the residue.

Finally, the XRD analysis of the extracted calcium carbonate revealed that it primarily consists of magnesian calcite. This form of calcite contains calcium and magnesium, suggesting that a small amount of magnesium was co-precipitated along with the calcium during recovery. The formation of magnesian calcite confirms that the leaching process effectively extracted calcium from the bottom ash, which was then successfully recovered as calcium carbonate.

To sum up, the XRD analysis clearly shows the transformation of the minerals: calcium from calcite and wollastonite were extracted during leaching, and the final product was magnesian calcite, indicating successful calcium recovery.

3.2. Results Study of Raw Materials

3.2.1. SEM-EDS Analysis of WLC

The SEM-EDS analysis of WLC revealed that it primarily consists of 67.39% SiO₂, with additional components including CaO (0.37%), Al₂O₃ (27.74%), and Fe₂O₃ (1.09%) and show on the Figure 4.

Titanium dioxide (TiO₂) accounted for 2.31%, contributing to the clay’s whitish tint. Other components included sodium (0.53%), potassium (2.9%), and magnesium oxide (0.56%). This study’s SiO₂ content of WLC was comparable with earlier findings, measuring 66.72%.

A comparative elemental analysis between the solid residue formed after calcium ion leaching and the original clay material revealed that the solid residue retained no heavy metals. This indicates its potential as a replacement material for clay in certain applications. SEM images of the solid residue showed a heterogeneous structure with varied shapes and sizes, while WLC particles displayed a more uniform morphology, as supported by previous research.

3.2.2. XRD Analysis Result of WLC

The XRD analysis of the clay sample confirmed the presence of several minerals commonly found in WLC, as presented in Figure 5.

The predominant minerals included quartz, a typical silica phase, and microcline, a potassium feldspar. Titanium oxide was also detected, contributing to the clay’s chemical properties.

Furthermore, clay-specific minerals such as kaolinite and illite were identified, indicating the sample’s fine-grained structure. Wollastonite, a calcium silicate mineral, suggests that the clay may have undergone metamorphism or high-temperature processes, influencing its mineralogical composition.

3.2.3. Differential Thermal Analysis Results

The DTA curve for bottom ash, solid residue, and WLC illustrates the thermal reactions as the temperature rises (Figure 6).

The DTA curve for WLC exhibited three distinct endothermic peaks at 95 °C, 505 °C, and 570 °C. These correspond to the evaporation of moisture, the decomposition of organic materials, and the dehydration of clay minerals with the release of chemically bound water [19]. The TGA data indicated a 1.45% weight loss at 120 °C due to moisture evaporation, followed by a 1.38% weight loss between 120 °C and 405 °C, likely due to the combustion of organic materials. A 3.12% weight loss occurred between 410 °C and 592 °C, culminating in an overall mass loss of 7.84%.

The thermal behaviour of BA and SR showed similarities, as BA serves as the initial material for calcium extraction. BA exhibited two endothermic peaks at 86–100 °C and 781 °C, while SR showed an endothermic peak at 570 °C. BA’s TGA data indicated a 1.55% weight loss at 100 °C and a further 3.33% loss between 110 °C and 620 °C. SR showed smaller weight losses, including a 0.83% loss at 780 °C, with total mass losses of 9.39% and 4.71% for BA and SR, respectively.

The significant peaks in the 300–500 °C range for both BA and SR correspond to the incineration of organic matter and the release of crystallization water. Carbonate decomposition and CO₂ release occurred between 720 °C and 860 °C for BA and SR [20].

3.3. Results Study of Fired Clay Bricks Samples

3.3.1. SEM-EDS Analysis Result of Clay Bricks

Figure 7 presents the SEM-EDS analysis of fired samples containing SR as a replacement for WLC, which revealed significant changes in the chemical composition of the resulting materials.

The concentration of elements such as silicon (Si), aluminium (Al), potassium (K), and titanium (Ti) consistently decreased across all firing temperatures. This reduction can be attributed to the lower concentrations of these elements in SR than WLC.

In contrast, the levels of calcium (Ca), iron (Fe), magnesium (Mg), sodium (Na), and sulfur (S) increased as the percentage of SR used in place of WLC rose. This suggests that SR introduces higher concentrations of these elements into the fired materials, likely due to their more abundant presence in SR. Notably, sulfur in the fired samples was observed after SR was introduced, which could affect the final material’s properties, such as its colour, strength, and durability.

3.3.2. FTIR Analysis of Fired Clay Bricks

Figure 8 illustrates the infrared spectra of WLC compositions containing varying percentages of SR, ranging from 10% to 30%, and fired at 1000 °C and 1100 °C. The FTIR spectra showed characteristic bands corresponding to the minerals present in the WLC compositions. This included kaolinite, illite, montmorillonite, and others. Specific peaks were observed. A peak at 1150 cm⁻¹ was attributed to vibrations associated with the sulphate tetrahedron. Peaks at 1033 cm⁻¹ and 1037 cm⁻¹ correspond to Si-O bond vibrations. Peaks between 798 cm⁻¹ and 774 cm⁻¹ were identified as stretching vibrations of Si-O bonds in quartz. Other peaks near 690 cm⁻¹ and 680 cm⁻¹ correspond to Si-O, Si-O-Al (Mg) vibrations, while bands at 560 cm⁻¹ and 562 cm⁻¹ indicate Si-O-Mg bond elongation. Peaks near 450 cm⁻¹, 453 cm⁻¹, and 594 cm⁻¹ suggest the presence of Si-O-Al bonds.

These peaks reveal the composition of WLC, with mineralogical characteristics such as Si-O bonds indicative of silica and alumino-silicates.

At higher temperatures, clay minerals such as kaolinite, illite, and montmorillonite (typically observed between 1200 cm⁻¹ and 800 cm⁻¹) undergo chemical and structural changes, including dehydration, phase transformations, and the formation of new mineral phases like mullite and quartz.

3.3.3. The Characteristics of Clay Bricks in Terms of Physical and Mechanical Properties

The density of the fired clay brick samples varied depending on the firing temperature and the amount of SR used as a replacement for WLC. As the SR content increased, the density of the samples decreased, with values ranging from 1.62 to 2 g/cm³ (Figure 9). This decrease is a natural consequence of incorporating SR, which has a lower density than WLC.

The SR content and the firing temperature directly influenced the compressive strength of the fired clay bricks. As SR increased, the samples’ density and compressive strength decreased. However, bricks with SR proportions up to 30% and fired at 1000 °C and 1100 °C exhibited the highest compressive strength, ranging from 21.6 to 24.4 MPa. These values surpass the minimum recommended compressive strength of 14.5 MPa for construction applications.

Regarding water absorption and open porosity, both properties increased with increasing SR content but showed a decreasing trend as the firing temperature rose. This trend suggests that higher temperatures may help reduce the porosity and water absorption of the fired samples, improving their overall quality for specific applications.

Lastly, the clay bricks’ firing shrinkage was measured, ranging from 4.96% to 6.94%, depending on the SR content and firing temperature. At 1000 °C, the shrinkage was 6.05%, and at 1100 °C, it slightly increased to 6.94%. The variation in shrinkage is influenced by the amount of SR incorporated, with a more significant amount of SR leading to more shrinkage during firing.

3.3.4. Nitrogen Adsorption Analysis of Clay Bricks

The pore size distribution of fired clay bricks was analyzed using nitrogen adsorption and Quenched Solid Density Functional Theory (QSDFT) [21,22]. The QSDFT method accounts for pore wall heterogeneity and provides reliable characterization of mesopores (ranging from 2 to 35 nm). This method considers the roughness and variability of pore surfaces, a key factor in accurately determining pore size in clay materials. In comparison, methods like the BJH and Mercury Intrusion Porosimetry (MIP) assume cylindrical pores, which may not fully capture the heterogeneity in clay materials [23,24].

The analysis revealed that brick samples fired at 1000 °C and 1100 °C exhibited Type II isotherms according to IUPAC classification, indicating they are nonporous or macroporous (Figure 10a). Additionally, all samples displayed H3 hysteresis loops, typical of slit-shaped pores. The desorption curve’s slope was correlated with tensile strength effects.

The DFT analysis of pore diameter and surface area showed that all fired clay brick samples possessed a combination of micro-, meso-, and macropores. With increasing firing temperature and higher SR content, the surface area decreased significantly, from 0.6 m²/nm/g at 1000 °C to 0.18 m²/nm/g at 1100 °C (Figure 11).

Although macropores were present in all samples, their contribution to the overall pore structure was less than 1%. The micro- and mesopores were predominant, which is typical for clay materials.

3.3.5. Environmental and Economic Aspects of Calcium Extraction from Bottom Ash and Clay Bricks Production from Solid Residue

The extraction of calcium from BA and using SR as a replacement component for clay in brick production offer promising insights into resource conservation and waste reduction. Approximately 50% of the calcium carbonate in BA was successfully extracted, with the remaining 50% forming a solid residue that was recycled to manufacture ceramic bricks.

Traditionally, producing 1000 clay bricks requires 5 tons of clay. The study observed significant benefits by replacing 30% of the clay with SR. The non-recyclable, burnable fraction of waste incinerated at the CPP amounts to 255,000 tonnes annually, generating 58,650 tonnes of bottom ash. From this, about 29,325 tonnes of calcium carbonate can be extracted yearly, leaving an equivalent amount of solid residue. Up to 1500 tonnes of SR can replace 30% of the clay by producing one million clay bricks. The remaining 20% of SR can be utilized in other construction industries or, if necessary, sent to landfill.

Using the mathematical model presented in Section 2.4, Equations (3) and (4), the economic and environmental benefits of extracting CaCO₃ from BA and clay brick production using SR as a replacement component can be calculated.

I Economic benefits calculation

Given data:

• Annual incinerated waste—255,000 tons/year
• BA generated—58,650 tons/year
• Calcium carbonate (CaCO₃) extracted from BA—29,325 tons/year
• Solid residue used as clay replacement in brick production—29,325 tons/year
• Clay needed for 1,000,000 bricks—1500 tons
• BA to the landfill—27,825 tons/year

Economic parameters (for illustrative purposes, adjust with actual market prices if available):

• Market price of calcium carbonate (CaCO₃)—80 USD/ton
• Cost of clay—30 USD/ton
• Processing cost for calcium carbonate extraction—20 USD/ton

Economic benefit calculation

I-1. Savings from extracted CaCO₃

I-1.1 Amount saved by using extracted CaCO₃:

CaCO_3(savings) = (80−20) × 29,325 = 1,759,500 USD/year

I-2. Savings from using SR as clay replacement:

I-2.1 Since 29,325 tons of SR can replace clay in bricks, calculate how many bricks this could produce:

Bricks with SR = 29,3251,500 × 1,000,000 = 19,550,000 bricks/year

I-2.2 Clay replacement savings:

Clay replacement (savings) = 30 × 29,325 = 879,750 USD/year

I-3. Total economic benefit

Economic benefit _total = CaCO_3(savings) + Clay_{repl. (savings)} = 1,759,500 + 879,750 = 2,639,250 USD/year

II Environmental benefits calculation

Using the provided figures, we can calculate the environmental impact of reduced landfill usage, clay conservation, and potential CO₂ savings from CaCO₃ production.

Environmental impact calculation

II-1. Landfill diversion:

II-1.1 The total BA generated is 58,650 tons, with 27,825 tons still going to landfills.

II-1.2 Landfill diversion (avoided):

Landfill diversion = 58,650 − 27,825 = 30,825 tons/year

II-2. Resource conservation (WLC):

II-2.1 Amount of clay conserved due to solid residue replacement

Clay conservation = 29,325 tons/year

II-3. CO₂ emissions reduction from CaCO₃:

II-3.1 Assume a reduction of 0.5 tons of CO₂ per ton of calcium carbonate extracted (adjustable if you have specific CO₂ savings data).

II-3.2 CO₂ reduction:

CO₂ (reduction) = 0.5 × 29,325 = 14,662.5 tons CO₂/year

II-4. Total environmental benefit

II-4.1 The sum of avoided landfill, clay conservation, and CO₂ reduction can represent the total benefit:

Environmental benefit _total = Landfill diversion + Clay conservation + CO_2reduct. = 30,825 + 29,325 + 14,662.5 = 74,812.5 tons of resource conservation and CO₂ reduction

Economic benefits using such a strategy consist of 2,639,250 USD/year and environmental benefit—of 74,812.5 tons (including landfill diversion, clay conservation, and CO₂ reduction). This approach not only helps to conserve natural resources but also maximizes the use of waste by-products. After implementing this process, no more than 20% of the solid residue from the waste generated by MSW incineration would need to be sent to landfills, making this a highly sustainable solution.

4. Discussion

The leaching process efficiently isolates calcium from bottom ash, yielding 21.72% calcium in the recovered calcium carbonate. This confirms the effectiveness of acid leaching in targeting calcium-rich minerals like calcite and wollastonite while leaving behind an SR viable for use in secondary applications such as construction materials and corresponds to the results of other authors [25,26,27]. Optimizing the leaching process could further enhance calcium and iron recovery, increasing economic viability and supporting sustainable resource management.

The residue left after calcium extraction is free of heavy metals [28,29], making it an environmentally safe substitute for natural clay in brick production. SR can replace up to 30% of clay in fired clay bricks, with its heterogeneous structure enhancing mechanical bonding and its thermal stability supporting sustainable construction practices. In contrast to the uniform morphology of WLC, SR’s diverse structure provides opportunities to tailor material properties for specific applications [30,31]. The high silica, alumina, and titanium oxide content in WLC contributes to its strength, durability, heat resistance, and lighter colouration, making it suitable for ceramics and construction.

Firing experiments revealed significant effects of SR on the physical and mechanical properties of clay bricks. As SR content increases, the density and compressive strength of bricks decrease. However, bricks containing 30% SR and fired at 1000 °C and 1100 °C still meet construction standards, achieving compressive strengths of 21.6–24.4 MPa, which exceed the minimum threshold of 14.5 MPa for building materials. The firing temperature also plays a critical role, as higher temperatures reduce porosity and improve water resistance, although they may lower surface area due to material densification. The pore structure of fired bricks includes a combination of micro-, meso-, and macropores, influencing the material’s mechanical and thermal performance [21,22,23,24]. Optimizing firing conditions and SR content is crucial for balancing porosity and strength to meet industry standards [32,33].

The economic benefits of integrating calcium carbonate extraction and SR utilization are substantial [34,35]. Annually, 255,000 tons of waste incinerated at the CPP generates 58,650 tons of BA, from which 29,325 tons of calcium carbonate can be extracted. The remaining residue is sufficient to replace clay in producing 19.55 million bricks annually. By replacing natural clay with SR, 29,325 tons of clay are conserved annually, resulting in 879,750 USD in savings. The market value of the calcium carbonate extracted, after processing costs, adds an additional 1,759,500 USD in savings, bringing the total annual economic benefit to 2,639,250 USD.

This strategy’s environmental benefits [36,37] are equally significant. The approach diverts 30,825 tons of BA from landfills annually, contributing to waste reduction efforts. Additionally, 29,325 tons of clay are conserved yearly by replacing natural clay with SR. The process also helps reduce CO₂ emissions by 14,662.5 tons annually, aligning with global climate goals and supporting a circular economy. SR in brick production also promotes resource efficiency and minimizes environmental impact, with only 20% of the solid residue requiring landfill disposal.

The FTIR and XRD analyses confirm the potential of WLC and SR in ceramics and construction. Key minerals like quartz, kaolinite, and illite enhance thermal stability and mechanical strength, while titanium oxide contributes to lighter colouration and durability. During firing, these clay minerals undergo significant transformations, forming new phases such as mullite and quartz, which improve material properties. The FTIR analysis also reveals shifts in peak positions and intensities with increasing SR content and firing temperature, indicating changes in the molecular and structural composition of the material [38,39].

Further research is recommended to refine SR’s use in ceramic and construction applications. Studies should focus on improving the consistency and performance of SR-based bricks, exploring their compatibility with various materials, and expanding their applications beyond construction. Optimizing the scalability of calcium carbonate extraction and SR utilization could broaden their adoption in regions with high MSW incineration rates, maximizing economic and environmental benefits.

This sustainable approach integrates resource conservation, waste reduction, and economic efficiency, showcasing its potential as a model for industries aiming to adopt circular economy principles and reduce their environmental footprint.

5. Conclusions

This study successfully demonstrates the extraction of calcium from bottom ash and the substitution of solid residue for clay in fired brick production. The comprehensive analyses, including SEM-EDS, XRD, DTA, nitrogen adsorption, and FTIR, confirm the material transformations during leaching and firing, ensuring the effective recovery of calcium carbonate and the preservation of critical brick properties. Substituting up to 30% of clay with SR achieves adequate brick compressive strength while promoting resource sustainability and environmental benefits. The findings reveal substantial economic and environmental advantages. Annually, this approach could generate an estimated 2,639,250 USD in savings and conserve over 74,000 tons of resources, including reduced landfill use, clay conservation, and lower CO₂ emissions. These results highlight the feasibility of integrating SR into brick production, aligning with circular economy principles and sustainable construction goals.

Future research could optimise the extraction and substitution processes to enhance efficiency and scalability. Further studies might also explore the application of SR in other construction materials, broadening the scope of waste valorisation and fostering innovative pathways for sustainable resource management.