Carbon Emission Assessment During the Recycling Phase of Building Meltable Materials from Construction and Demolition Waste: A Case Study in China

Abstract

The improper disposal of construction and demolition waste (CDW) exacerbates the consumption of raw materials and emissions of greenhouse gasses. In this study, due to the high recycling rate, focusing on the meltable materials of CDW, the recycling phase of CDW is divided into four stages, namely the on-site disposal stage, the transportation stage, the reprocessing stage, and the reproduction stage. Second, based on these four stages, a carbon emission accounting model (CEAM) is established to evaluate the carbon emission benefits of meltable materials during these stages. Third, the CEAM is applied to a typical old residential area to evaluate the carbon emission reduction benefits of the CDW recycling. The results indicate that (1) the full-process carbon emissions of recycled steel, recycled flat glass, and recycled aluminum per unit mass are 677.77 kg/t, 1041.54 kg/t, and 845.39 kg/t, respectively, which are far lower than their corresponding ordinary meltable building materials (OMBMs); (2) the carbon emissions during the reproduction stage represent the primary component of carbon emissions in the MW recycling phase, accounting for 88.52% to 97.45% of the total carbon emissions; and (3) the carbon emissions generated by the recycling of cullet per unit mass are very high, reaching 1768 kg/t, which is 4.3 times that of scrap steel (409.05 kg/t) and 3.6 times that of scrap aluminum (483.76 kg/t). The research findings could provide theoretical methods and experimental data for decision-makers to formulate treatment plans for meltable materials in CDW, thereby empowering urban carbon emission reduction and promoting sustainable development. Construction parties engaged in demolition tasks should enhance on-site sorting and collaborate with recycling companies to ensure its efficient recycling. Recycling companies need to focus on high-carbon-emission stages, such as the reproduction stage, and strengthen technological research to improve carbon reduction benefits.

1. Introduction

Global warming is related to human well-being. In recent years, global warming has brought about a series of ecological problems. This has posed a challenge to the sustainable development of the natural environment and human society. The massive emission of greenhouse gasses, primarily carbon dioxide, represents a significant contributor to global warming [1]. According to the data released by the United Nations Environment Programme, 40% of global energy is consumed by the construction industry, which generates 38% of greenhouse gasses and 30% of waste [2]. In China, the construction industry, accounting for 50.9% of the country’s total carbon emissions, is the primary contributor for both direct and indirect carbon emissions [3]. Therefore, the assessment of carbon emissions in the construction industry is regarded as a pivotal step to promote China’s achievement of the “carbon peaking and carbon neutrality goals”, which is outlined in the 75th Session of the United Nations General Assembly [4].

According to the bulletin released by the National Bureau of Statistics of China, the urbanization rate in China reached 66.16% in 2023, representing an increase of only 0.94% compared to the previous year [5]. With the slowdown of urbanization, China’s current focus on urban development has shifted from urban expansion to urban renewal [6]. The target of this endeavor is the vast number of existing old buildings in cities, which were typically constructed at the end of the last century or the beginning of this century and can no longer adequately meet the use demands of modern urban life. The renovation work of these buildings, which includes transformation, repair, or demolition, inevitably generates a significant amount of CDW.

Building waste, also known as construction and demolition waste (CDW), primarily refers to solid waste generated during the construction, renovation, and demolition stages of buildings [7]. It is estimated that more than 10 billion tons of CDW are generated globally each year [8]. Currently, CDW has emerged as a prominent environmental concern in numerous countries, accounting for approximately 30% of total waste generated in the US, 36% in the EU, and 50% in the UK, thereby constituting the largest contributor to overall municipal waste production [9]. As the largest producer of CDW in the world, China generates a volume of CDW that exceeds the combined totals of the United States and the 27 member states of the European Union [10]. In 2022, China generated 2.6 billion tons of CDW [11], which is more than 10 times the amount of household waste produced in the same year [12]. The large amount of landfilled CDW not only causes a series of environmental impacts, such as raw material consumption, energy consumption, and greenhouse gas emissions but also poses a threat to people’s health [13].

Cited from the Technical Standard for Construction and Demolition Waste Treatment (CJJ/T 134-2019), which was enacted by the Ministry of Housing and Urban-Rural Development of China, CDW includes engineering spoil, engineering mud, construction waste, renovation waste, and demolition waste [14]. This standard provides a scientific methodology for calculating the production of various types of waste and the production cardinality of these wastes, which are summarized in

Table 1. The production cardinality of various types of waste.

Waste	Production Cardinality	Proportion
Engineering spoil, engineering mud	Determine comprehensively based on the site topography, design data, and construction techniques	—
Construction waste	300–800 t/104 m2	2.3–9.1%
Renovation waste	0.5–1.0 t/household annually	3.6% × 10−5–12% × 10−5
Demolition waste	8000–13,000 t/104 m2	90.9–97.7%

.

It is evident that the production cardinality of demolition waste is substantially greater than that of construction waste and renovation waste. According to Table 1, during the life cycle of the same building, the proportion of demolition waste ranges from 90.9% to 97.7%. Selecting demolition waste, which constitutes the vast majority, as the subject of study has strong practical significance and is in line with the current situation in China. In summary, the research in this article only considers CDW in the demolition stage of the building life cycle.

Due to different evaluation criteria, the classification of CDW materials generally exists in variance. According to material melting properties, various CDW materials could be divided into two categories, including meltable materials and non-meltable materials. “Meltable” performance signifies smelting and casting materials during the processes of production and reproduction. Common meltable materials primarily include steel, iron, glass, and non-ferrous metals. Meltable materials include both meltable waste and meltable building materials. Non-meltable materials refer to those that cannot undergo melting and casting processes, including concrete, tiles, wood, and other similar substances. Additionally, the non-recyclable waste in CDW needs to be landfilled in a traditional manner, referred to as landfilled waste. By comparison, due to the high recycling rate, advanced recycling technologies, and substantial demands of meltable materials, these substances could be preferable [15].

The carbon emissions from meltable materials account for a significant proportion of the global total carbon emissions. Specifically, the direct carbon emissions from the steel industry account for about 7% of the global carbon emissions [16]; the carbon emissions from the aluminum industry account for more than 3% [17]; the copper industry accounts for about 0.3% [18]; and glass accounts for about 1% [19]. In addition, there are carbon emissions from other materials such as zinc, lead, rubber, and plastics.

It is of great significance to analyze their process units and quantify carbon emissions throughout the entire recycling phase. Therefore, after posing the question, a literature review was conducted to analyze the problem and determine the scope of the study and the meltable materials in CDW. Based on this, the present study aims to define the various stages and steps in the recycling phase of meltable materials and establish a systematic CEAM to assess carbon emissions. The established CEAM was applied to the selected case for the accounting and assessment of carbon emission data. After analysis and discussion, the results of the study were obtained. The method framework of the study is illustrated in Figure 1.

2. Literature Review

2.1. Recycling of CDW

Ibrahim et al. [20] explored the current environmental impacts of CDW management based on relevant data and found that recycling CDW significantly enhances environmental benefits. Wang et al. [21] demonstrated that the environmental benefits of recycling CDW are far greater than those of landfilling and incineration. The results from the United States Environmental Protection Agency’s (EPA) Simplified Greenhouse Gas Emissions Calculator also indicate that the direct landfilling of CDW leads to direct emissions of greenhouse gasses such as methane and carbon dioxide [22]. Recycling CDW can achieve significant carbon emission reductions. Meng et al. [23] showed that the secondary recycling of CDW can effectively reduce carbon emissions by 31.01% during the building demolition phase. Hao et al. [24] found that the comprehensive utilization of 1 ton of CDW can reduce carbon emissions by 0.698 tons.

The “3R” theory (i.e., reuse, reduce, and recycle), proposed earlier in the last century, is a widely accepted principle for managing waste generated during the demolition phase (Figure 2) [25]. For structural elements that can be dismantled and used again, reuse represents an economic and environmentally friendly method [26]. Reduce aims to decrease the production of CDW through standardized operations and engineering technologies. For CDW that has already been generated, recycling is considered an effective method to reduce its quantity and carbon emissions [27].

The circular model of CDW management possesses a wider range of applications compared to the open 3R principles [28]. Research on the recycling of CDW has been initiated early in developed countries. A series of documents has been previously issued by the European Union. It clearly required its member states to promote the recycling of CDW in the market [29]. In Europe, CDW has been classified as “special waste” by various frameworks, recognizing its significant potential for generating secondary raw materials. This view is reflected in the EU Framework Level [30] and the Strategy for a Sustainable Environment [31]. Germany has implemented a series of laws and regulations, including the Law on Limited Disposal of Waste, the Law on Circular Economy and Waste Management, and the Law on Construction Waste, which are pivotal in the entire process of construction waste resource utilization, spanning from initial design, collection, and transportation supervision to the final treatment stage [32]. The United States has introduced a range of incentive measures, encompassing tax breaks for enterprises engaged in construction waste recycling and government programs that prioritize the procurement of recycled materials [33,34], as well as penalties imposed on entities that fail to adhere to regulations concerning the acquisition of recycled materials. Japan has clarified the responsibilities of all relevant parties, including clients, contractors, demolition companies, and recycling operators, through laws such as the “Construction Material Recycling Law” and the “Waste Management and Public Cleansing Law” [35]. All parties need to collaborate to ensure the effective recycling of CDW.

Developing countries like China can adjust and optimize their practices in areas such as regulations, technology, and public participation based on their own circumstances. The earliest policy document on CDW management in China was the Regulations on the Management of Construction Waste in Cities in 1996, which provided an initial legal framework for the management of construction waste in China [36]. However, the enforcement of this document was insufficient, and it lacked detailed operational guidelines. In recent years, the Chinese government has issued a series of documents to guide the management and recycling of CDW. The Guidelines for the Pilot Projects of Resource Utilization of Construction Waste in 2018 launched pilot projects for the resource utilization of CDW in some regions, exploring feasible management models and technical pathways [36]. However, the overall coverage rate was still low. The Guidelines on Promoting the Reduction of Construction Waste in 2020 [37] and the Opinions of the General Office of the State Council on Accelerating the Construction of a Waste Recycling System in 2024 [38] both emphasized the need to accelerate the recycling and resource utilization of CDW and to build a circular resource system. This concept coincides with the circular strategies of Western countries.

China can learn from the successful experiences of Western countries to further improve the legal and regulatory system for CDW management, clarify the responsibilities of all relevant parties, strengthen policy incentives, and promote the efficient recycling and resource utilization of CDW.

Despite achieving high recycling rates for CDW in certain developed countries, such as 90% in the EU and 76% in the US, the global recycling rate remains at approximately 30% on average [10]. In contrast, China’s recycling rate for CDW stands at only around 10%, and the utilization rate of secondary building materials is less than 5% [39]. The Chinese construction industry is viewed as a linear economic model, characterized by a “take-make-consume-dispose” cycle, and it remains heavily reliant on the consumption of raw materials [40]. Although the Chinese government has initiated efforts to advance the utilization of CDW in its “14th Five-Year Plan for the Development of Circular Economy” [41], the transition towards a circular economy in the Chinese construction industry is still in its nascent stage [42].

China’s pioneering cities, such as Shanghai and Beijing, have made a certain amount of progress in enhancing the recycling rate of CDW, yet the rate remains limited, estimated to be only 3–10% [43]. Municipal governments across China have also implemented relevant policies to enhance the recycling of CDW, including “Administrative Measures on Financial Subsidies for the Comprehensive Utilization of Construction and Demolition Waste in Guangzhou” and “Incentive Measures for the Reduction and Comprehensive Utilization of Construction and Demolition Waste in Shenzhen” [44].

Considering the relatively backward state of CDW recycling in China, scholars have conducted active explorations. Yi et al. propose a CDW management model to identify the waste reduction effects of various policies in China [32]. Hao et al. evaluated the carbon emissions of CDW in building energy retrofit projects [45]; Zhang et al. analyzed the carbon emission reduction benefits for the technical process of a typical CDW recycling project in Hangzhou [46]; Wang et al. evaluated the recycling equivalent of CDW in Shanghai and estimated the carbon emissions based on the Life Cycle Assessment (LCA) gray model [47]; Liu et al. established a quantitative model to analyze the carbon emission reduction potential of CDW recycling in Jiangsu Province [48].

However, the majority of studies focus on the overall carbon emissions of CDW within the scope of a single project [45,46] or across cities and regions [47,48]. The carbon emissions during the recycling process of various waste materials in specific projects need to be quantified specifically. This is beneficial for engineering decision-makers in formulating material recycling strategies and determining the carbon emission benefits of recycling projects. The carbon emission accounting model (CEAM) becomes particularly significant in this phase. This study takes meltable materials from CDW as an example to establish a CEAM, which aims to quantify the carbon emissions during the recycling phase and the carbon reduction benefits associated with the use of recycled building materials.

2.2. Research Boundary Based on LCA

Recently, LCA has become a prevalent tool for assessing and contrasting the environmental consequences of various waste management options [49]. Zhang et al. evaluated the carbon and economic benefits of producing recycled aggregates from CDW based on LCA [50]. Ortiz-Barrios et al. [51] and Çalik [52] conducted an analysis utilizing LCA to examine the environmental benefits of using alternative building materials within the supply chain. Currently, LCA has been widely employed for the quantification and analysis of carbon emissions across the entire life cycle of buildings.

Cited from the Handbook of Building Carbon Emission Calculation Standards (GB/T 51366-2019), which was enacted by the Ministry of Housing and Urban-Rural Development of China, the carbon emissions of buildings represent the sum of greenhouse gas emissions emitted by buildings during the production, transportation, construction, demolition, and operation stages of building materials related to them [53]. The LCA for building carbon emissions is frequently employed to quantify and calculate the carbon emissions at various stages of building products [54], such as the studies of them conducted by Cao et al. [55] and Su et al. [56]. As illustrated in Figure 3, the current research scope of building carbon emissions is mostly limited to the various stages from “cradle” to “grave” within a single life cycle. In contrast, there has been relatively little research on the recycling phase of CDW, which is generated during the demolition stage after the “grave”.

In contrast to the linear and single-life-cycle approach employed in previous studies, this research establishes a circular approach for the multi-life cycle of buildings through the “cycle concept” and discusses the industrial processes of meltable materials across different life cycles. The research boundary starts from the “grave” stage of the old life cycle and ends at the “cradle” stage of the new life cycle. The established CEAM has improved the calculation standards.

2.3. Representative Meltable Materials

Meltable materials, primarily composed of non-inert materials such as metals and glass, possess significant recycling value. Previous research has typically focused on materials with significant weight proportions, such as concrete and non-metallic minerals, while overlooking valuable recyclable materials like steel and aluminum [57]. The study by Rosado et al. indicates that the recycling of meltable materials can generate more environmental credits compared to non-meltable materials such as concrete and bricks [58]. The studies by Ghaffar et al. [59] and Wang et al. [60] recognize the significant economic, social, and environmental benefits of recycling key materials such as steel, aluminum, and copper as “resources for secondary raw materials.”

The meltable materials in buildings include metallic materials such as steel, aluminum, and copper, as well as non-metallic materials like glass, rubber, and plastic [53]. These materials are the main meltable materials in CDW, and this study is based on three of them. There are two principles for selecting these three representative meltable materials.

On the one hand, the proportion of waste in CDW is an important factor to consider during selection, as it affects the quality in practical projects. Choosing materials with a high proportion holds practical significance. Based on the research of Liu et al., the proportion of various common materials in CDW is shown in Figure 4 [48]. Steel and glass, which are the majority of meltable materials, account for 7% and 4%, respectively. Given their relatively high proportions, these two materials are selected as a part of the representative meltable materials.

However, on the other hand, the materials that Liu et al. [48] statistically analyzed in their study mainly account for a large proportion in CDW. Meltable materials that account for a smaller proportion in CDW, such as non-ferrous metals and rubber, are not included in Figure 4. These meltable materials are also indispensable for the study, and one of them has been selected in this research. Correspondingly, the ratio of a material’s consumption in the construction industry to its annual total production has become another important factor for selection. Like steel, non-ferrous metals, which serve as important building and decorative materials, also possess high recycling value. Non-ferrous metals, serving as significant enclosing and decorative materials in buildings, should be included in research. There exist numerous types of non-ferrous metals, and it is reasonable to select one that is heavily utilized in the construction industry for inclusion in the research. In 2023, China’s total production of non-ferrous metals was 74.7 million tons, of which 55.7% was aluminum (Figure 5). The aluminum consumed by the construction industry accounted for 40% of the total aluminum production [61] and 22.28% of the total production of non-ferrous metals. Due to this significant proportion, aluminum, as a representative of non-ferrous metals, is selected as one of the representative meltable materials.

3. Carbon Emission Accounting Model

There are technical differences and challenges in the recycling of three representative meltable materials. For example, the recycling of scrap steel involves the cleaning of contaminants on the surface of the scrap. Impurities such as coatings and oils on the surface of scrap steel can affect the quality of recycled steel building materials, which in turn puts corresponding demands on the processes in the reprocessing stage [62]. The technical difficulties in aluminum scrap recycling mainly lie in the reproduction stage. Existing primary aluminum refining technologies, such as the three-layer electrolysis method and the segregation method, are not well-suited for refining scrap aluminum, even though aluminum recycling still has significant carbon reduction benefits [63]. Waste glass may undergo downcycling in some waste management schemes due to quality degradation, a phenomenon that is often overlooked in the secondary production of glass [64]. Relevant professions should focus on research in this area to avoid quality issues in recycled flat glass.

Figure 6 shows a simplified process of the recycling practices of meltable materials waste, which has five major industrial units that produce various recycled building meltable materials. The first step is on-site disposal. The CDW at the demolition site is typically a mixture of various types of waste. On-site disposal allows for the classification of this waste, enabling their rapid progression into subsequent processes. As such, when CDW containing meltable waste (MW) is obtained, site disposal can be first performed before transportation to a reprocessing plant. The second step is the reprocessing of MW. The industrial units in the reprocessing plant usually include crushing, shaking, washing, dust removal, and other processes [21]. MW with diverse shapes and specifications could be transformed into regular sizes through crushing and selection. Furthermore, the surface of MW in CDW is often contaminated by mortar and plaster, making the dust removal process essential. The third step is reproduction of recycled meltable building materials (RMBMs). The reprocessed waste could be used as raw material for reproduction. The RMBMs obtained from reproduction could be utilized in new engineering projects. The transportation steps are interspersed among these three main processes. These industrial activities accomplish the transformation of meltable materials from waste to recycled building materials. Materials are transferred from the “grave” of the old building life cycle to the “cradle” of the new life cycle, achieving the recycling of materials.

These stages are the engineering basis for the carbon emission assessment of the recycling phase and also reflect the material flow path of the meltable materials. The analysis of industrial activities that generate carbon emissions within each stage is the foundation of carbon emission assessment work. The carbon emission assessment relies on the CEAM. The CEAM, which is established based on the analysis results of each stage, provides technical support for calculating the carbon emissions corresponding to each stage.

The accounting of carbon emissions requires systematic consideration of all processes in a recycling activity. The CEAM corresponds one-to-one to these steps and analyzes the specific influencing factors for each one. It is important to emphasize that since the principles of carbon emissions generated in the two transportation processes are the same, it is reasonable to unify these two processes into a CEAM for a single transportation stage. The established carbon emission CEAM is as follows:

$$C=C_O+C_T+C_{RPC}+C_{RPD}$$

(1)

where C is the total carbon emissions generated during the recycling phase (kg) and C_O denotes the carbon emissions generated during the on-site disposal stage (kg). The carbon emissions during this stage are mainly from the on-site disposal equipment; C_T denotes the carbon emissions generated during the transportation stage (kg). The carbon emissions during this stage are mainly from transportation vehicles; C_RPC denotes the carbon emissions generated during the reprocessing stage (kg). The carbon emissions during this stage are mainly from the reprocessing machinery; C_RPD denotes the carbon emissions generated during the reproduction stage (kg). The carbon emissions during this stage are related to the quality of the recycled meltable building materials and their carbon emission factors.

Due to the differences in the working principles and influencing factors for each step, separate CEAMs have been established to quantify carbon emissions at each stage. The detailed CEAM and explanations for these four stages can be found in Section 3.2, Section 3.3, Section 3.4 and Section 3.5, respectively.

3.1. Quality of MW

It should be particularly noted that MW is derived from construction waste generated from demolition, so the accounting of the quantity of CDW is primary. The accounting model of CDW is as follows:

$$W_{CDW}=A\times {AM}_{CDW}$$

(2)

where W_CDW denotes the generation of CDW in the demolition stage (t), A is the building area (m²), and AM_CDW denotes the amount of CDW generated per unit building area (t/m²). The value of AM_CDW depends on the structural type of the building. For general buildings, the value ranges between 0.8 and 1.3, which is cited from the Handbook of Construction and Demolition Waste Treatment Technical Standards (CJJ/T 134-2019) enacted by the Ministry of Housing and Urban-Rural Development of China [14].

Based on the classification of materials in CDW mentioned in the previous section, CDW consists of meltable waste (MW), non-meltable waste (NMW), and landfilled waste (LW), leading to the following formula:

$$W_{CDW}={\sum }_{i=1}^n{M}_{MW, i}+{\sum }_{i=1}^n{M}_{NMW,i}+{\sum }_{i=1}^n{M}_{LW,i}$$

(3)

where i refers to the types of materials; M_MW,i denotes the mass of the ith type of MW in CDW (t); M_NMW,i denotes the mass of the ith type of NMW in CDW (t); and M_{LW, i} denotes the mass of the ith type of LW (t).

Therefore, under the condition of clarifying the proportion of a certain type of MW in CDW, its mass could be obtained according to the following accounting model:

$$M_{MW,i}=W_{CDW}\times R_{MW,i}$$

(4)

where R_MW,i denotes the proportion of the ith type of MW in CDW, which is crucial for calculating its mass.

As mentioned above (Section 2.2), steel, glass, and aluminum have been selected as representative meltable materials. The proportions of steel and glass in CDW are 7% and 4%, respectively (Figure 4). On the contrary, other meltable materials, such as aluminum, etc., account for a small proportion of CDW and thus have not been included in relevant CDW research studies for statistical purposes. For this kind of meltable material, a separate accounting model needs to be established to calculate their proportions in CDW. Taking aluminum as an example, the material information of aluminum is substituted into Equation (4) for the following calculation:

$$M_{Al}=W_{CDW}\times R_{Al}$$

(5)

where M_Al denotes the mass of aluminum scrap in CDW (t), W_CDW denotes the generation of CDW in the demolition stage (t), and R_Al denotes the proportion of aluminum scrap in CDW (%).

In addition, according to the research from Lu et al., the density of CDW is 528 kg/m³ [65]. Meanwhile, the research conducted by Liu et al. indicates that the quality of aluminum in buildings is approximately 3–4.05 kg/m³ [66]. Therefore, the following formula could be established:

$$M_{Al}={\displaystyle \frac{W_{CDW}}{D_{CDW}}}\times O_{Al}$$

(6)

where D_CDW denotes the average density of CDW (t/m³) with a value of 0.528 [65] and O_Al denotes the content of aluminum in buildings (t/m³) with a value ranging from 0.5 to 0.6 [66].

As shown in the steps in

Table 2. The calculation process for the proportion of aluminum scrap in CDW.

Step1. Combine Equations (5) and (6) to Form a System of Simultaneous Equations.	Step2. Solve the System of Equations.	Step3. Substitute the Values.
$\left\{\begin{array}{c}{M}_{Al}=W_{CDW}\times R_{Al}\\ M_{Al}={\displaystyle \frac{W_{CDW}}{D_{CDW}}}\times O_{Al}\end{array}\right.$	$R_{Al}={\displaystyle \frac{O_{Al}}{D_{CDW}}}$	$R_{Al}={\displaystyle \frac{[0.003-0.00405]}{0.528}}$ 0.057–0.077%

, by converting the two equations above, the proportion of aluminum in construction and demolition waste can be determined with a value ranging from 0.057% to 0.077%. This calculation method is also applicable to other meltable materials. The proportions of representative meltable materials in CDW are summarized in

Table 3. The proportions of representative meltable materials in CDW.

	Steel Waste	Glass Waste	Aluminum Waste
RMW,i	7%	4%	0.057–0.077%

.

3.2. On-Site Disposal Stage

CDW is preliminarily classified and selected on the construction site. The carbon emissions at this stage are mainly generated by the energy consumption (EC) from manually operated machinery and equipment. The CEAM is as follows:

$$\left\{\begin{array}{c}{C}_O=\sum _{a=1}^n{E}_{O,a}\times F_{E,a}\\ E_{O,a}=\sum _{b=1}^n{EC}_{a,b}\times {CM}_{a,b}\end{array}\right.$$

(7)

where C_O is the total carbon emissions generated during the on-site stage (kg); E_O,a denotes the total consumption of the ath type of energy in the on-site disposal stage (kWh or kg); F_E,a denotes the carbon emission factor of the ath type of energy (kg/kWh or kg/kg), and it is determined by Appendix A (the Standard for Building Carbon Emission Calculation (GB/T 51366-2019)) of the Standard for Building Carbon Emission Calculation (GB/T 51366-2019) [53]. EC_a,b denotes the EC per machine team for the bth type of machinery with the ath type of EC (kWh/machine team or kg/machine team). CM_a,b denotes the consumption of the bth machinery machine team with the ath type of EC (machine team); a denotes the serial number of energy and b denotes the serial number of machinery. The mechanical equipment used for on-site disposal is selected according to the requirements of the engineering project, with some commonly used machines shown in

Table 4. Machinery for on-site disposal of meltable materials.

Machinery Name	Specifications	EC
		Diesel Oil (kg/Machinery Team)	Electricity (kWh/Machinery Team)
Crane	Lifting mass (5–60 t)	18.42–47.17	—
Reinforcing steel cutting machine	Diameter (40 mm)	—	32.10
Semi-automatic cutting machine	Thickness (100 mm)	—	98.00
Pipe cutting machine	Pipe diameter (150–200 mm)	—	12.90–22.50
Section steel shearing machine	Cutting width (500 mm)	—	53.20

Note: One machinery team indicates that 1 piece of machinery works for 8 h.

[53]. Some common types of energy and their corresponding carbon emission factors are shown in

Table 5. Common types of energy and their carbon emission factors.

Type of Energy	Carbon Emission FACTOR
Electricity	0.54 kg/kW·h [67]
Crude oil	72.23 t/TJ [53]
Fuel oil	75.82 t/TJ [53]
Diesel oil	72,59 t/TJ or 3.11 kg/kg [53]
Petrol	67.91 t/TJ [53]
Anthracite coal	94.44 t/TJ [53]
Bituminous coal	89.00 t/TJ [53]
Natural gas	55.54 t/TJ [53]

.

3.3. Transportation Stage

As mentioned before, the transportation stage involves two processes with regard to the first transportation process and the second transportation process. After on-site manual disposal, MW is transported from the demolition site to the reprocessing plant, which is the first transportation process. The second transportation process involves the transportation of reprocessed meltable materials from the reprocessing plant to the reproduction plant. The CEAM for the transportation stage is similar to that in LCA, as shown below.

$$C_T={\sum }_{c=1}^n{M}_{MW,c}\times T_c\times F_{T,c}$$

(8)

where C_T is the carbon emission generated during the transportation stage (kg), M_MW,c denotes the mass of MW to be transported during the cth transportation process (t), and T_c denotes the transportation distance during the cth transportation process. The transportation distance is calculated based on the actual transportation distance within the city (km). When the distance is not explicitly known, a value of 25 km can be assumed for the distance from the demolition site to the reprocessing plant within the city [68]. Like production plants, reproduction plants are typically located approximately 500 km away from urban centers [53]. Considering the distance of the reprocessing plant from the urban center, the distance between the reproduction plants and the reprocessing plants is approximately 450–500 km. F_T,c denotes the carbon emission factor per unit weight of transportation distance under the selected transportation mode during the cth transportation process [kg/(t·km)]. The values are taken from Appendix E (the Standard for Building Carbon Emission Calculation (GB/T 51366-2019)) of the Standard for Building Carbon Emission Calculation (GB/T 51366-2019) [53]; c denotes the serial number of the transportation processes.

3.4. Reprocessing Stage

Fragmentation and selection processes and dust removal processes have been regarded as industrial steps for handling meltable materials in the reprocessing plant. These processes rely on the mechanical equipment and in turn generate carbon emissions from its EC. The established accounting model is as follows:

$$C_{RPC}={\sum }_{d=1}^n{M}_{MW,RPC}\times E_{RPC,d}\times F_{RPC,d}$$

(9)

where C_RPC denotes the carbon emissions generated during the reprocessing stage (kg), M_MW,RPC denotes the mass of the meltable materials that need to be reprocessed during this stage (t), E_RPC,d denotes the EC of the equipment for processing unit weight of meltable materials during the dth reprocessing process (kWh/t), F_RPC,d denotes the carbon emission factor of the energy consumed during the dth reprocessing process (kg/kWh), and d denotes the serial number of the reprocessing processes. EC and carbon emission factors of selection and dust removal machines are shown in

Table 6. Equipment EC and carbon emission factors of reprocessing machines.

Reprocessing Machines	EC (kWh/t)		Carbon Emission Factor (kg/kWh)
Selection machine	Electricity	12.62 [46,69]	0.54 [67]
Dust removal machine	Electricity	28.46 [46,69]	0.54 [67]

[46,69].

LW is also generated during this process (Figure 7). The proportion of each meltable material’s trash is different. As shown in the following formula, it could be reasonable to express this indicator in terms of material recovery rate:

$$M_{MWRPCD,i}=M_{MW,i}\times Q_i$$

(10)

where M_MWRPCD,i is the mass of the reprocessed ith type of MW (t); M_MW,i denotes the mass of the ith type of MW prior to reprocessing (t); and Q_i denotes the recovery rate of the ith type of MW during the reprocessing stage (%). Based on the studies conducted by Liu et al. [66], Luo et al. [70], and Eheliyagoda et al. [71], the recycling rates of three representative MW types are presented in

Table 7. Recovery rates of representative meltable materials waste.

Representative Meltable Materials Waste	Steel	Aluminum	Glass
Recovery rate (%)	75% [70,72]	60–76% [66,71,73]	50% [70]

; i denotes the serial number of MW.

3.5. Reproduction Stage

A carbon emission assessment for meltable materials waste during the reproduction stage is the most critical. MW could be converted into building materials so that it can be applied in new projects after a series of industrial activities in reproduction plants. The impact of this stage primarily concerns the carbon emissions during the building materials production stage in LCA. It is meaningful to correctly establish the CEAM. Consistent with the carbon emission CEAM in LCA [53], the carbon emissions in this stage are positively correlated with the quality of the RMBM and their carbon emission factors. The CEAM is shown below.

$$C_{RPD}={\sum }_{j=1}^n{M}_{R.j}\times F_{R,j}$$

(11)

where C_RPD is the carbon emissions generated during the reproduction stage (kg), M_R,j denotes the mass of the jth type of RMBM produced in the reproduction plant (t), and F_R,j denotes the carbon emission factor of the jth type of RMBM (kg/t). For the same type of meltable material, j represents the serial number of the RMBM, while i denotes the serial number of the reprocessed MW. Their corresponding relationship is illustrated in

Table 8. The corresponding relationship between j and i.

Meltable Materials Category	Steel	Aluminum	Glass
j RMBM	Steel and iron building materials such as steel reinforcement bars	Aluminum products	Plate glass and flat glass
i reprocessed MW	Steel and iron scrap	Aluminum scrap	Cullet

.

3.5.1. Mass of RMBM

The reprocessed meltable material wastes are transported to the reproduction plant for regenerative activities. As shown in Figure 8, these reprocessed wastes cannot replace all mineral materials, only a part of them. Taking glass materials as an example, in the reproduction process of recycled flat glass, cullet can only replace 20–30% of the mineral raw materials [74]. On the other hand, there will be some material loss in the reproduction processes that cannot be incorporated into the quality of the recycled materials. For example, one ton of scrap steel cannot produce the same weight as steel building materials. Considering the impact factors of these two aspects, an accounting model is established as follows:

$$M_{R,j}={\displaystyle \frac{M_{MWRPCD,i}}{P_i}}\times Y_j$$

(12)

where M_R,j is the mass of the jth type of recycled RMBM (t), M_MWRPCD,i denotes the mass of the reprocessed ith type of MW (t), and P_i denotes the proportion of the ith type of reprocessed MW in mineral raw materials (%). The proportions of three representative meltable materials are shown in

Table 9. Reprocessed materials proportion in mineral raw materials.

Meltable Materials Waste	Steel	Aluminum	Glass
Reprocessed waste proportion (%)	100% [75]	100% [76]	20–30% [74]

. In the reproduction of steel and aluminum, mineral raw materials can be 100% replaced by reprocessed wastes [74,75]. In contrast, only 20–30% of reprocessed cullet can be used in the reproduction of flat glass [74]. Y_j denotes the material output ratio of RMBMs in the reproduction processes, which is the ratio of the quality of RMBMs to the quality of raw materials consumed in reproduction processes (%).

The reproduction processes of different materials are different, resulting in variations in their material output ratio. A single accounting model cannot consider the overall influencing factors in each reproduction process. Therefore, based on the characteristics of these materials, two targeted accounting models have been established to calculate Yi, as shown in Equations (13) and (14):

$$Y_j={\displaystyle \frac{1}{M_{MWRPD,i}}}\times 100\%$$

(13)

$$Y_j=1-P_{L,j}$$

(14)

where M_MWRPD,i is the total mass of the type i reprocessed MW from the reprocessed stage and mineral materials consumed in the reproduction of 1 t of type j RMBM during the reproduction stage (t); P_L,j denotes the casting loss rate of the jth type of RMBM during the reproduction processes (%).

These two accounting models are designed to accommodate the different and varied reprocessing techniques for various smeltable materials. Taking three representative smeltable materials as examples, steel and glass are calculated according to Equation (13), because their M_MWRP,i are commonly found in their industrial activities [71,74,75]. Aluminum is calculated according to Equation (14), as the loss of raw materials is often expressed in terms of casting loss rates [77].

The mass of scrap steel consumed in the reproduction of 1 t of recycled steel building materials is approximately 1.1 t [75]. The mass of mineral materials, including cullet, consumed in the reproduction of one ton of flat glass ranges from 1.1 to 1.2 tons, which is cited from the Handbook of Flat Glass (GB 11614-2022) enacted by the State Administration for Market Regulation and Standardization Administration of China [78]. In addition, the casting loss rate of recycled aluminum building materials is 7% [77]. The calculated material output ratios are shown in

Table 10. Material output ratio of representative meltable materials.

Representative Meltable Materials Waste	Steel	Aluminum	Glass
Equation	(13) [75]	(14) [71,77]	(13) [74]
Material output ratio (%)	90.9%	93%	83.3–90.9%

.

3.5.2. Carbon Emission Factor of RMBM

The carbon emission factor is an important indicator for carbon emissions in material reproduction. Taking the three selected representative meltable materials as examples, the carbon emission factors for recycled steel and recycled aluminum are referenced from the studies by Shangguan et al. [75], Li et al. [76], and Peng et al. [79], as shown in

Table 11. Carbon emission factors of representative RMBMs.

Recycled Representative Meltable Materials	Recycled Steel	Recycled Aluminum	Recycled Glass
Carbon emission factor (kg/t)	500–700 [75]	680–850 [76,79]	990–1040

.

It should be noted that the carbon emission factors for some meltable materials, represented by recycled flat glass, are still uncertain and lack definitive research. Therefore, based on the material information obtained through the literature review, it is necessary to establish a reasonable accounting model to calculate their carbon emission factors during the reproduction stage.

As shown in Figure 9, in the material production stage, the carbon emissions of flat glass mainly consist of two parts, energy consumption (EC) and carbonate decomposition (CD) in raw materials. Among them, carbon emissions from EC account for 80% of the total carbon emissions in this stage, while those from the decomposition of carbonate materials account for 20% [80]. The carbon emission factor (kg/t) refers to the amount of carbon dioxide released per ton of building material produced during the production and reproduction stage. Therefore, the carbon emission factors of flat glass also include the two aspects mentioned above. Thus, Equations (15) and (16) could be established as follows:

$$F_{Flat glass}=F_{EC,Flat glass}+F_{CD,Flat glass}$$

(15)

$$F_{Recycled flat glass}=F_{EC,Recycled flat glass}+F_{CD,Recycled flat glass}$$

(16)

where F _{Flat glass} is the carbon emission factor of flat glass during the production stage (kg/t), taken as 1.13 [53]; F_{EC, Flat glass} denotes the carbon emission factor related to the EC of flat glass during the production stage (kg/t), which accounts for 80% [80]. The calculated value is 0.90; F_{CD, Flat glass} denotes the carbon emission factor related to the CD of flat glass during the production stage (kg/t), which accounts for 20% [80]. The calculated value is 0.23; F _{Recycled flat glass} is the carbon emission factor of recycled flat glass during the reproduction stage (kg/t); F_{EC, recycled flat glass} denotes the carbon emission factor related to the EC of recycled flat glass during the reproduction stage (kg/t); and F_{CD, Recycled flat glass} denotes the carbon emission factor related to the CD of recycled flat glass during the reproduction stage (kg/t).

Incorporating cullet as a part of the raw materials in the reproduction stage of flat glass could directly reduce the consumption of carbonate materials [81], thereby decreasing carbon emissions generated from the decomposition of carbonates. Based on this, Equation (17) can be established.

$$F_{CD,Recycled flat glass}=F_{CD,Flat glass}\times \left(1-P_{Cullet}\right)$$

(17)

where P_Cullet denotes the proportion of reprocessed cullet in mineral raw materials (%), and it has a value range of 20% to 30% [74].

In addition, because cullet has been previously processed, it can effectively reduce EC during the reproduction process. Relevant research indicates that within the range of 20–30% of raw materials, every 10% increase in cullet in the raw materials can lead to a reduction of 2.5% in EC [81]. On the one hand, this is because the addition of cullet reduces the energy consumption of mineral raw material processing. On the other hand, cullet can act as a catalyst for some mineral raw materials, thereby affecting energy consumption [82]. This study measures this effect using an energy-saving index, the value of which is ${\displaystyle \frac{2.5\%}{10\%}}$.

$$F_{EC,Recycled flat glass}=F_{EC,Flat glass}\times \left(1-P_{Cullet}\times I_{Cullet}\right)$$

(18)

where I_Cullet denotes the energy-saving index of cullet, with a value of ${\displaystyle \frac{2.5\%}{10\%}}$ [81].

Through the accounting model, the carbon emission factor of recycled flat glass during the reproduction stage can be derived. When the proportion of cullet in the raw materials is between 20% and 30%, the carbon emission factor ranges from 900 to 1040 kg/t, as shown in

Table 12. Calculation process of carbon emission factor for recycled flat glass.

P Cullet	FCD, Recycled flat glass	FEC, Recycled flat glass	F Recycled flat glass
20% (min)	0.23 × (1–20%)	0.90 × [1–20% × (2.5%/10%)]	1040 kg/t
30% (max)	0.23 × (1–30%)	0.90 × [1–30% × (2.5%/10%)]	900 kg/t

.

3.6. Model Validation

Model validation is a crucial step in assessing whether a model can reliably link accounting data with predictive variables [83]. Through validation, it can be ensured that the model has sufficient accuracy and reliability in practical applications, thereby providing strong support for decision-making [84]. During the model validation process, potential issues in the model’s design and implementation, such as data error values, can be identified. By discovering and resolving these issues, the model’s generalization ability and applicability can be enhanced.

This study randomly selected three case buildings and simulated data for the recycling of three types of castable waste. The carbon emission accounting engine of the Jiangsu Province Building Materials and Construction Carbon Emission Accounting and Monitoring Technology Public Service Platform (Accounting Platform) was used for data accounting [85]. The data from the Accounting Platform were statistically compared with the accounting results of CEAMs to analyze data errors and judge the accuracy of the model. The carbon emissions generated by recycling one ton of meltable waste calculated by the two methods are shown in Figure 10. The error between the two sets of data is within 4%, with the errors for steel and aluminum not exceeding 3%. This indicates that the accuracy of CEAMs can be guaranteed. The reason for the error may be that the specific transportation distance in the transportation stage of the case is different from the default value in CEAMs.

3.7. Sensitivity Analysis

Sensitivity analysis is an important tool for identifying key input parameters and their impacts on model outputs, which helps enhance the reliability and applicability of the CEAM. The Sobol method can provide both the individual sensitivity indices of each parameter and their interaction indices [86]. It decomposes the system output variance into the sum of variances determined by each input variable. This allows for a quantitative analysis of the impact of input variables on the system output. This study employs the Sobol global sensitivity analysis method to assess the sensitivity of multiple input parameters and their interactions on the output of the CEAM. By calculating the first-order sensitivity indices (S_i) and the total sensitivity indices (S_Ti), the study quantifies the impacts of individual parameters and their interactions on carbon emissions.

Figure 11 illustrates the sensitivity analysis results of key input parameters on the overall carbon emissions in the CEAM. By calculating the first-order sensitivity indices and the total sensitivity indices of each parameter, the independent and combined effects of different input parameters on the output of the CEAM were evaluated. The results indicate that the amount of CDW generated (W_CDW) and energy consumption (E_O,a, E_RPC,d) have the most significant impact on carbon emissions, with higher sensitivity indices, suggesting that changes in these parameters can greatly affect the output of the CEAM. Additionally, the proportion of recyclable waste (R_MW,i) and transportation distance (T_c) also exhibit a certain level of sensitivity. Although the sensitivity of carbon emission factors (F_E,a, F_R,j) is relatively lower, their interaction effects should not be overlooked.

Figure 12 presents the interaction sensitivity among input parameters in the CEAM using a heatmap. In the heatmap, redder colors indicate higher interaction sensitivity. A strong interaction is observed between the amount of CDW generated (W_CDW) and the proportion of recyclable waste (R_MW,i), suggesting that these two parameters significantly influence each other in carbon emission accounting. In addition, the interaction between energy consumption (E_O,a, E_RPC,d) and these parameters is also significant.

4. Carbon Emission Assessment

4.1. Case Selection

Selecting a reasonable case to conduct systematic simulation experiments could effectively verify the accuracy of the CEAM in engineering projects. Correspondingly, the typicality and representativeness of the selected case are indispensable.

In China, the total carbon emissions across the whole life cycle of residential buildings account for approximately 79.3% of those of urban buildings [3], and relevant research shows that this figure will continue to rise until 2037 [87]. At present, a large number of old residential areas exist in various regions of Chinese cities. Correspondingly, urban building demolition projects also cover many old residential areas.

Old residential areas in China were mostly constructed in the 1970s and 1980s, as well as the early 2000s [88]. The buildings are primarily multi-story (with six floors or fewer) and high-rise (with more than seven floors). The housing units include one-bedroom apartments, two-bedroom apartments, and three-bedroom apartments, with residents typically being families of three or four members [89]. The road and utility systems in these residential areas are also relatively aged.

After research and field investigation, this experiment selected an old residential area in Qixia District of Nanjing City as the established research case. As illustrated in Figure 13, there are 25 residential buildings with different heights located in this residential area, encompassing both multi-story and high-rise structures. The housing units include the types mentioned above (Figure 14), and the residents are mostly middle-aged and elderly people. Based on these conditions, it was selected as the target for simulated demolition.

4.2. Result Analysis

The residential area covers 149,856 m², and the amount of CDW is calculated using Equation (2). Since the range of CDW generated per m² is between 0.8 t and 1.3 t [14], the mass of CDW generated from the demolition of this residential area lies between 119.88 kt and 194.81 kt. Construction waste data typically exhibit a skewed distribution. In such cases, the mean can be inflated by a few high values, whereas the median better reflects the typical level of the data [90]. Hyndman et al. also pointed out that the robustness of the median against outliers makes it superior to the mean in forecasting and describing data [91].

Based on the aforementioned reasons, the median value of this range is appropriate, which is 157.35 kt. Among them, the masses of scrap steel, cullet, and scrap aluminum are 11.01 kt, 6.29 kt, and 0.11 kt, respectively (Figure 15). Based on the accounting, three representative RMBMs obtained from reproduction are 7.51 kt, 10.96 kt, and 0.07 kt, respectively (Figure 16). Meanwhile, during the entire recycling phase, the total carbon emissions of these three representative materials are 5.08 kt, 11.42 kt, and 0.06 t, respectively.

The data presented in Figure 16 are determined by the quality and proportion of materials. Due to its minimal proportion in CDW, the data related to aluminum materials are barely visible in the figure. Thus, discussion of the carbon emissions of various materials based on their total mass lacks objectivity. It is imperative to conduct data analysis and discussion based on the unit mass of materials.

The carbon emission assessment based on the unit mass of materials could be discussed from two aspects, namely MW and RMBM. Due to the influence of the different industrial processes, there is a certain difference in the quality of waste and recycled materials for the same kind of materials (Figure 16). Accordingly, the carbon emissions per unit mass of materials also vary. Therefore, it is reasonable to conduct a classified discussion.

Table 13. Carbon emissions at various stages of recycling 1 t MW (kg/t).

	On-Site	Transportation	Reprocessing	Reproduction	Total
Steel scrap	0.19	21.73	31.09	409.05	462.07
Cullet	0.18	14.96	31.10	1768.13	1814.37
Aluminum scrap	0.17	19.82	31.11	483.76	534.87

provides a good overview of the carbon emission data for MW per unit mass during the recycling and reproduction stages. Among them, the carbon emissions throughout the entire process for steel scrap and aluminum scrap per unit mass are similar, while the carbon emissions for cullet are approximately three to four times higher than those of the other two materials (Figure 17). The main reason for this phenomenon is the significant amount of carbon emissions generated by glass during the reproduction stage (Figure 18). It is worth mentioning that in the transportation stage, the carbon emissions of cullet are actually the lowest. This is because the recovery rate of cullet is very low, at only 50% [70], which leads to a reduction in carbon emissions generated during the second transportation process of cullet. The carbon emissions of the three representative materials that were generated during the on-site disposal and reprocessing stages are similar.

The carbon emission data of RMBM per unit mass during the recycling and reproduction stages are shown in

Table 14. Carbon emissions at various stages of reproducing 1 t RMBM (kg/t).

	On-Site	Transportation	Recycling	Reproduction	Total
Recycled steel	0.28	31.88	45.61	599.99	677.77
Recycled flat glass	0.10	8.59	17.85	1014.99	1041.54
Recycled aluminum	0.27	31.33	49.18	764.62	845.39

. Like the carbon emissions from waste recycling, the carbon emissions of recycled steel and aluminum per unit mass are not significantly different, and they both maintain a stable trend overall (Figure 19 and Figure 20). The carbon emissions of these two types of recycled materials have increased throughout the entire process, which is attributed to the reduction in the quality of recycled materials compared to waste. However, since the proportion of cullet in the raw materials is only 20–30%, the quality of the recycled flat glass is much greater than that of cullet, which leads to a reduction in the carbon emission quality per unit mass of recycled flat glass compared to cullet. During the on-site disposal, transportation, and reprocessing stage, the carbon emissions per unit mass of recycled flat glass are already much lower than those of recycled steel and aluminum. Comparatively, although the carbon emissions of recycled flat glass during the reproduction stage have decreased, it is still significantly higher (Figure 20).

Compared with ordinary meltable building materials (OMBMs), the overall carbon emissions of RMBMs per unit weight are lower during the reprocessing and reproduction phases. For every ton of building material, the carbon emissions of recycled steel, flat glass, and aluminum are reduced by 66.9%, 7.8%, and 95.8%, respectively, compared to ordinary carbon steel, flat glass, and electrolytic aluminum (Figure 21). Among them, the difference in carbon emissions between recycled aluminum and electrolytic aluminum is almost 20 times, which reflects the strong value of energy conservation and emission reduction in recycled aluminum. However, in contrast, the difference in carbon emissions between recycled flat glass and ordinary flat glass per unit mass is relatively small, with only about 88.5 kg, indicating a need for further research in this area.

4.3. Discussion

The analysis of carbon emissions from the recycling of CDW reveals significant disparities in the environmental impact of different materials. The findings highlight the importance of considering the specific characteristics associated with each material when assessing the carbon emission reduction benefits of CDW recycling. For instance, the lower emissions of recycled steel and aluminum compared to their ordinary counterparts indicate that recycling these materials can be highly effective in reducing carbon emissions. However, the relatively small reduction observed for recycled flat glass compared to ordinary glass suggests that further improvements in recycling technology and process optimization are necessary to enhance its environmental benefits.

Moreover, this study underscores the need for a nuanced understanding of the entire recycling phase, from on-site disposal to the reproduction of RMBM. Each stage contributes differently to the overall carbon emissions, with the reproduction stage often being the most significant. This highlights the potential for targeted improvements in specific stages of the recycling phase to yield substantial environmental benefits.

The carbon emission accounting method provided by the CEAM is integrated throughout the entire assessment process, comprehensively quantifying the carbon emissions of the entire recycling phase across the four stages. Compared with other accounting models, the CEAM has better versatility and a higher degree of fit with specific engineering projects. Other accounting models mostly assess carbon emissions from two aspects, the macro level of cities and regions or the life cycle of a specific material’s recycling. The former often relies on the macro data of cities to conduct simulation analysis and the evaluation of urban carbon emissions from aspects such as population and urban planning. The accounting model used by Maria et al. in their study is an example of this [92]. On the other hand, researchers in materials science often focus solely on a single material within their specific field, such as Xiao et al.’s research on recycled concrete [68]. While these studies provide specific assessments of carbon emissions from material recycling, the versatility of the accounting models is relatively low. In comparison, the CEAM in this study offers more of a conceptual framework for carbon emissions assessment. For meltable materials in CDW, as long as the relevant data are determined, this CEAM can be used to quantify and assess carbon emissions.

The stakeholders in CDW recycling include the government, construction companies, recycling enterprises, and others. The government serves as the supervisor in the management of CDW [93]. By increasing the participation rate of construction companies and waste recycling enterprises, the government can achieve the recyclability and circular utilization of CDW, as well as maximize overall social benefits. The government should encourage the use of recycled building materials with high carbon emission reduction benefits, such as recycled aluminum and steel. It can also strengthen guidance for construction companies through policy or market mechanisms. The government can enhance incentive and penalty measures to encourage public participation in supervision and to promote the recycling and management of CDW by enterprises.

At present, construction companies are reluctant to cooperate with recyclers because the economic value of CDW recycling in China is currently too low [94]. If incentives and penalties are imposed on construction companies, they can be encouraged to opt for the resource utilization of CDW [95]. Meanwhile, the recycling industry should commit to optimizing the processes for recycling meltable waste, especially for materials with relatively high carbon emissions, such as flat glass. Research on their reproduction stage will effectively reduce carbon emissions in the recycling phase. On the other hand, strengthening cooperation with construction companies and policymakers can also create a virtuous cycle of collaborative governance.

5. Conclusions

The recycling of CDW can effectively improve the urban environment, significantly reduce urban carbon emissions, and promote sustainable development. Scientifically classifying CDW and analyzing their various processes in the whole phase is highly beneficial for enhancing the efficiency of material reuse. Taking meltable materials as an example, this study divides their recycling phases into four stages, including the on-site disposal stage, the transportation stage, the reprocessing stage, and the reproduction stage. Based on this, a scientific analysis and simulation of the entire process of recycling meltable materials has been conducted. This analysis method is also applicable to the evaluation of other common materials in CDW, demonstrating a certain degree of versatility. It should be noted that the reprocessing and reproduction stages are the focus of this study, and the industrial process units and influencing factors in these two stages are the core challenges.

Based on the aforementioned stages, the CEAM presented in this paper is established, with steel, glass, and aluminum serving as representative meltable materials for the study. Based on this CEAM, this study conducted a simulated demolition experiment using actual existing urban residential area projects as examples. The results indicate the following:

(1) During the production stage of building materials, RMBM exhibits significant carbon emission reduction benefits compared to OMBM. The full-process carbon emissions of unit mass recycled steel, recycled flat glass, and recycled aluminum are 677.77 kg/t, 1041.54 kg/t, and 845.39 kg/t, respectively, which are far lower than ordinary carbon steel, flat glass, and electrolytic aluminum. The carbon emission reduction differences in them have reached 66.9%, 7.8%, and 95.8% in the respective scenarios.

The accounting results have met the requirements for the energy-saving and carbon-reducing retrofitting of building materials, as stipulated in China’s “Action Plan for Energy Conservation and Carbon Reduction (2024–2025)” [96]. The experimental data help to fulfill the carbon reduction tasks for steel, glass, and non-ferrous metal materials outlined in this document. The carbon emission reduction in recycled aluminum compared to ordinary electrolytic aluminum is most pronounced. The use of RMBM should be advocated for and stipulated in government documents.

(2) The carbon emissions during the reproduction stage represent the primary component of carbon emissions in the MW recycling phase, accounting for 88.52% to 97.45% of the total carbon emissions. The optimization of carbon emission reduction in this stage is the subsequent focus of efforts to decrease carbon emissions in the recycling phase, which may be facilitated through policies and financial subsidies.

(3) The carbon emissions of scrap steel and aluminum, which are representative of non-ferrous metals, are relatively stable, with carbon emissions during the whole phase of material wastes per unit mass being 462.07 kg/t and 534.87 kg/t, respectively. In contrast, the corresponding value for glass reaches 1814.37 kg/t. The reason lies in the differences in carbon emissions during the reproduction stage, where the carbon emissions per unit mass of cullet reach 1768.13 kg/t, which is 4.3 times that of steel (409.05 kg/t) and 3.6 times that of aluminum (483.76 kg/t). Increasing the recovery rate of cullet and optimizing the carbon emission reduction processes during the reproduction stage of recycled flat glass are the primary focuses of subsequent work. This conclusion is in line with the requirements for production processes, as specified in the document “Implementation Guide for Energy Conservation and Carbon Reduction Retrofitting and Upgrading in the Flat Glass Industry”, issued by the National Development and Reform Commission and other relevant departments of China [97].

However, correspondingly, this study also has three main limitations. The first limitation is the inadequate consideration of material products. In this study, the meltable materials were roughly classified into “material wastes” and “building materials” without further detailed categorization. Taking aluminum materials as an example, there are many different types of material products, such as aluminum window frames, aluminum tubes, aluminum decorative components, and so on. The second limitation is that only the demolition wastes of old buildings generated from demolition activities are quantified and thoroughly studied in this research. If other types of waste, such as decoration waste and renovation waste, are taken into consideration, then the CEAM will change accordingly. The third one is that this study did not compare the outcomes with other disposals, such as investing in production in other industries or traditional landfill practices. If such comparisons can be carried out, then it could provide decision-makers with a clearer understanding of the carbon-saving potential generated from recycling the meltable materials wastes of CDW. Despite the limitations, the research findings can still provide theoretical methods and experimental data for decision-makers to formulate treatment plans for meltable material wastes in CDW.