Embodied carbon saving potential of using recycled materials as cement substitute in Singapore’s buildings

Abstract

Material production and construction activities are key contributors to global carbon footprints, necessitating sustainable alternatives. This study aims to investigate the potential of integrating recycled materials as Supplementary Cementitious Materials (SCMs) in concrete production to mitigate the substantial carbon emissions of Singapore’s building and construction sector. The research focuses on Ground Granulated Blast-furnace Slag (GGBFS), waste glass powder, and calcined marine clay as alternative SCMs, aiming to reduce environmental impact and waste disposal emissions in Singapore. Employing a cradle-to-gate Life Cycle Assessment (LCA) methodology for 1 m³ of concrete with different grades, this study quantifies embodied carbon savings and assesses the feasibility of substituting these SCMs in concrete. The results reveal that substituting Ordinary Portland Cement (OPC) with GGBFS in concrete offers the most significant reduction, up to 56%, in 1 m³ of concrete. In contrast, the use of calcined marine clay and glass powder in concrete results in reductions of up to 21% and 16%, respectively. Two case studies were used to exemplify the impact of using SCM concrete at the project scale. Results indicate that up to 31% of the total embodied carbon could be saved in the building. Additionally, scenario analysis suggests that the total emissions from cementitious materials in Singapore could decrease by 20% through the incorporation of locally recycled marine clay and glass powder. This reduction could potentially reach 56% if the GGBFS supply is not constrained. To further enhance sustainability in Singapore’s construction sector, the study proposes sourcing GGBFS from neighboring countries to minimize transportation emissions and localizing the production and usage of calcined marine clay and glass powder. These measures can improve material circularity and significantly contribute to achieving carbon reduction targets.

Introduction

The building and construction industry is a significant contributor to global carbon emissions, responsible for 39% of these emissions. Within this sector, carbon emissions from the production of building materials and construction activities account for 11% of the total¹. This subset of emissions, known as embodied carbon, has garnered increasing attention due to its rising share of the total carbon emissions from buildings. According to the World Green Building Council (WGBC)¹, it is projected that by 2050, the embodied impact of new constructions could contribute up to 50% of the total carbon emissions.

The use of low-carbon building materials is identified as a key strategy in reducing embodied carbon in building industry², with production falling into two primary approaches. The first approach focuses on using alternative energy sources or innovative technologies to reduce carbon emissions during the material production stage. In cement industry, greenhouse gas (GHG) emissions due to fossil fuel combustion in kiln could contribute to 40% of the GHG emissions of cement production³. Thus, efforts have been made to adopt alternative fuel in cement production. Clark et al.⁴ stated that the cumulative energy-related GHG emissions in cement industry could be up to 21% between 2020 and 2050 by fuel switching. Georgiopoulou et al.⁵ reported a 22% reduction of carbon emission in cement kiln by substituting 30% of fossil fuel by biofuel. Zhang et al.⁶ reported a 40% embodied carbon reduction in cement production when 100% fossil fuels are replaced by construction and demolition (C&D) wood waste. Carbon Capture Utilization and Storage (CCUS) also play a crucial role in decarbonizing industries like cement and steel production. The aim of CCUS is to capture the carbon emission during material production to prevent it from leaking to the environment. Then the carbon will be utilized for chemical processing or stored in other places. In roadmap for cement low-carbon transition proposed by International Energy Agency (IEA), CCUS contributes 48% of the overall carbon reduction by 2050⁷. Baker et al.⁸ examines the carbon-reduction potential of membrane-based CCUS system, which could capture 80% of carbon emitted from cement or steel industry.

Despite the great potential of utilizing alternative fuels and CCUS technology to mitigate carbon emission, implementing these decarbonization methods may be limited to countries with large-scale production capabilities. In countries like Singapore, where a considerable amount of building materials is imported, alternative strategies must be adopted. Therefore, this paper primarily concentrates on the second approach, which involves incorporating recycled substitutes into building material production.

The use of recycled substitutes in building materials offers substantial potential for reducing embodied carbon. By 2050, it is anticipated that 37% of carbon emission in cement industry could be saved by substituting clinker with SCMs⁷. Commonly used SCMs in concrete production include silica fume (SF)⁹, fly ash (FA)¹⁰ and GGBFS¹¹. Among which GGBFS is the most widely used SCM in Singapore. GGBFS is a by-product of steel-making process. Slagging agents are added to the blast furnace or basic oxygen furnace to remove the impurities of iron ore during smelting. The slag lid floats on the liquid metal to protect it from outside oxygen and remain temperature. The slag could be easily removed from the surface of the metal. The removed slag will undergo quick water quenching to form a granular product that will be further grounded to fine powders^11,12. Concrete incorporating GGBFS have generally slower hydration process, resulting in a reduced strength in early stage. But ultimate strength of concrete will be increased due to the pozzolanic behavior of slag in slag-cement blend^11,12,13,14. As GGBFS replace the cement in the concrete it tends to improve the concrete pore structure and Interfacial Transition Zone (ITZ) turns out to be denser¹⁵. As a result, inclusion of GGBFS could reduce the permeability and provide better chemical resistance thus improving the durability of concrete¹⁶.

Besides of GGBFS, Singapore has also conducted research of utilizing local waste like glass powder and marine clay into concrete production to promotes local material circularity. Du et al¹⁷. collected waste soda lime glasses from a local recycler in Singapore. Crusher and ball miller were used to break and finely grind the recycled glass into powders until the desired fineness was achieved. Du and Tan^18,19 investigated the mechanical and transport and durability properties of concrete with up to 60% finely ground glass powder replacing OPC. If the replacement is below 30%, due to the pozzolanic reaction between glass powder and cement hydration products, the 28-days compressive strength of the concrete will not be decreased by the substitution of glass powder. Additionally, the resistance to chloride ions and water penetration continuously increases with higher glass powder content. These improvements in durability properties are due to the refined microstructures, particularly at the interfacial transition zone. Hassani et al.²⁰ conducted a literature review on the use of glass powder in concrete through 78 published scientific articles. They concluded that incorporating waste glass powder has advantages in higher mechanical and durability properties as well as lower environmental impact.

Marine clay, a type of low-grade kaolinite clay commonly found in coastal areas worldwide, is typically produced during excavation activities. These clays are characterized by their high silt content and low kaolinite content, making them of limited value for industrial applications²¹. However, recent scientific research has explored their potential utility in several areas, including as a partial replacement for cement, as an additive to enhance the pozzolanic properties of cement²² or silica sand²³, as a component in low-carbon lightweight strain-hardening cement composites²⁴, and as a filler replacement in ultra-high-performance concrete²². In studies by Du et al.^21,22, marine clay, initially supplied in large lumps, underwent a process where it was dried at 50 °C for three days, manually pulverized, and then ground in a ball mill for 30 min. The resulting fine powder was subsequently calcined at specific temperatures (600 °C, 700 °C, and 800 °C) for one hour in a furnace. During this process, the heating ramp was maintained at 10 °C/min from room temperature to the designated calcination temperature. Afterward, the calcined clay was removed from the furnace and rapidly cooled by spreading it on a metal tray at room temperature. According to Du and Pang²², without pre-treatment, kaolinite in marine clay is approximate 20% by mass. Owing to the low kaolinite content, the pozzolanic reaction could be completed within the first week. The calcined clay mortar with 30% replacement ratio shows a high strength activation index of approximately 0.9. Raw marine clay samples can also be transformed into a value-added product through 2 h thermal treatment at 700 °C, enhancing their pozzolanic activity and accelerating heat flow and calorimetric responses²⁵. Calcined marine clay refines mortar pores, reducing threshold, critical and average pore sizes, but increasing total pore volume and porosity with higher replacement ratios. The compressive and flexural strengths decrease as the cement replacement ratio increases. Compared to the reference, mortar with 15% fine calcined marine clay achieves the highest strength. However, the strength significantly drops beyond 45% replacement due to dilution, porous structure, and agglomeration effects.

Previous studies have examined the embodied carbon saving potential of using SCMs in concrete production. Crossin²⁶ examined the carbon emission of 1 m³ of concrete adopting 70% of GGBFS substitution based on Australian database, which resulted in 48% of carbon reduction comparing to OPC concrete. Tait and Cheung²⁷ studied a cradle to gate concrete containing 30% OPC and 70% GGBFS and reported a 62% of carbon reduction based on UK context. Regarding lower GGBFS replacement ratio, Aslani et al.²⁸ assessed the environmental impact of concrete mix using by-products and waste materials, including GGBFS, FA and glass powder. They implemented cradle-to-gate LCA adopting Ecoinvent 3 database and concluded that GGBFS concrete with 50% replacement ratio could achieve the highest carbon reduction compared with other waste/by product concrete, which could reach 41% when compared to controlled OPC concrete. Hossain et al.²⁹ reported 28% of GHG reduction in 50% GGBFS concrete using data from Chinese Life Cycle Database (CLCD).

It is noted that the carbon saving potential of SCM concrete varies significantly according to the studied region. This is mainly resulted from the regional technology variance and material transportation distance³⁰. To address this issue and achieve more accurate environmental impact evaluation, it is necessary to quantify the localized embodied carbon saving potential of the SCMs based on local context. This research employed cradle-to-gate life cycle assessment (LCA) to assess the embodied carbon of various SCM concrete mixtures based on their functional performance in Singapore. Two local case studies are utilized to exemplify their embodied carbon saving potential. Scenario analysis reflecting various SCM adopting level and sustainable strategies are performed. This assessment will enable a more informed comparison and facilitate strategic decision-making regarding SCM adoption in Singapore’s construction sector.

Results

ECF of OPC and SCMs

The embodied carbon emission of waste glass powder, calcined marine clay, GGBFS and OPC are assessed based on Singapore’s local context and quantified by embodied carbon factor (ECF). The assessment includes the material production and transportation process. Life cycle inventory (LCI) data of these material are listed in Tables 1, 2, 3 to Table 4. The ECF assessment of waste glass powder and marine clay are not considering the raw material extraction process, since they are treated as local wastes; yet, the additional processing (e.g., grinding to the appropriate fineness of waste glass powder) are considered in the analysis. The LCI data of waste glass powder are retrived from the paper in preperation by H. Du. The ECF of OPC and GGBFS over past 20 years are assessed based on their import amount and the corresponding local production emissions in original country, as shown in Figs. 1, 2. No allocation and economic allocation are separately applied to GGBFS assessment. We assume the importing structure of OPC and GGBFS from the past 3 years are representative for future analysis. Thus, the average ECF of OPC and GGBFS in past three years are adopted in the following discussion and case studies. The detailed import data and the corresponding ECF of OPC and GGBFS are listed in Tables 3, 4, respectively.

Table 1 ECF and LCI data for production of 1 ton waste glass powder⁴⁴

Table 2 ECF and LCI Data for production of 1 ton calcined marine clay

Table 3 ECF of OPC in Singapore

Fig. 1: Singapore OPC import amount and ECF from 2003 to 2022³⁶.

The changing importing structure of OPC has no strong impact on its ECF.

Fig. 2: Singapore GGBFS import amount and ECF from 2003 to 2022³⁶.

The significant change of GGBFS ECF indicates the strong impact of importing structure on the embodied carbon emission of GGBFS in Singapore. The economic allocation method generates higher ECF results for GGBFS than no allocation method.

Table 4 ECF of GGBFS in Singapore

Embodied carbon emission of concrete mixtures

This research applied cradle-to-gate LCA methodology to assess the embodied carbon emission of 1 m³ of concrete with different grades (i.e., G40, G50 and G60). The impact of cement substitution using GGBFS, calcined marine clay and glass powder was also evaluated for each concrete grade. The assessment results are shown in Fig. 3. It can be observed that embodied carbon of concrete generally rises with higher grade, due to the increasing cement content. Among all SCMs, GGBFS could achieve the highest embodied carbon reduction regardless of the allocation method. The embodied carbon reduction for no allocation GGBFS concrete is 55% to 56% and for economic allocation GGBFS concrete is 53% to 54%. Marine clay concrete could achieve 20% to 21% emission reduction while glass powder concrete could generally achieve 16% of reduction. The reduction ratio of using SCMs slightly varies with concrete grades. Generally, higher grade concrete with more cement content could exhibit more significant emission reduction, since more cement content could be replaced by SCMs.

Fig. 3: Comparison of embodied carbon emission of 1 m³ of concrete with different grades and SCMs.

The embodied carbon emission of concrete is primarily impacted by the concrete strength and SCM usage.

Case studies

In order to exemplify the embodied carbon saving potential of using recycled materials in SCM concrete, case studies were conducted using two local projects adopting different structural systems.

Case study 1 involved a 19-storey composite building with 4-level basements. The GFA of this project is 186,166 m². This project utilized composite columns and composite slabs in superstructure and cast in-situ reinforced concrete (RC) slabs in the basement. The concrete strength for all superstructure elements and basement elements (except for basement walls) is Grade 40. The basement wall from basement 4 to basement 1 utilized G60 concrete. All steel elements in composite structural members adopt S355 steel. Due to the limited data available, only main structural elements are considered in embodied carbon calculation including columns, beams, slabs and walls. The material information for the structural elements is estimated and tabulated in Table 5. The structural foundation as well as the interior decoration materials are ignored in this case study. The rebar volume is estimated as 2% of total concrete volume. The steel amounts are provided by the designer. The ECF of rebar and steel is taken as 2.51 kgCO₂e/kg and 2.84 kgCO₂e/kg from SGBC calculator³¹. The density of rebar and steel is assumed to be 7850 kg/m³.

Table 5 Material information of case study 1 (Composite building)

Case study 2 (Fig. 4) involved a 9-story RC building with GFA of 30,972 m². This project adopted conventional RC frame structure, with Grade 60 concrete utilized in structural columns and walls, Grade 50 concrete utilized in structural beams and slabs, and Grade 40 concrete utilized in stairs. Similar to case study 1, the foundations and interior decoration materials are ignored. The material information for case study 2 is listed in Table 6.

Fig. 4: Structural layout of case study 2.

Typical structual members in case study 2, including columns, beams, slabs and walls.

Table 6 Material information of case study 2 (RC building)

Concrete adopting three kinds of recycled SCMs are applied to the case study according to its strength. The usage of rebar and steel section are assumed to be unchanged. The embodied carbon intensity (ECI) of original projects, which is quoted as kgCO₂e/m², is compared with that of the building after substitution of SCM concrete. The results for two case studies are listed in Tables 7, 8. These ECI results are further compared with Singapore benchmark as shown in Fig. 5. Singapore BCA³² set 1000 kgCO₂e/m² as the benchmark of non-residential buildings. It is observed that the original building is already below the BCA benchmark. However, the ECI value could be further reduced by adopting SCM concrete in the building. In case study 1, it is observed that the GGBFS concrete achieved the highest reduction of 25% and 24% for no allocation method and economic allocation method, respectively. Marine clay concrete achieved 9% of overall ECI reduction and Glass powder concrete achieved 7% reduction. For case study 2, GGBFS concrete achieved 31% and 30% of reduction. Marine clay concrete and glass powder concrete achieved 12% and 9% of reduction, respectively. For both case studies, adopting GGBFS concrete could result in the highest embodied carbon saving, followed by marine clay concrete and glass powder concrete. Besides, the carbon saving potential of using SCM concrete in case study 2 is generally higher than in case study 1, which may be due to the difference in structural system. Case study 1 adopted composite structure, which includes large amount of structural steel usage. As shown in Fig. 6, OPC concrete, rebar and steel contributes to 46%, 43% and 11% of overall embodied carbon emission. For RC structure in case study 2, the OPC concrete and rebar contribute to 56% and 44%. The higher concrete usage in the structure can lead to more significant emission reduction by adopting SCM concrete.

Table 7 Comparison of original building (OPC) with SCM concrete substitution in case study 1

Table 8 Comparison of original building (OPC) with SCM concrete substitution in case study 2

Fig. 5: ECI of case study 1 and case study 2.

The ECI of case study 1 and case study 2 are lower than BCA benchmark. ECI is impacted by the SCM usage.

Fig. 6: Embodied carbon contribution by material in case study 1 and case study 2.

The embodied carbon contribution from material differs in composite building in case study 1 and RC building in case study 2. After SCM substitution, the contribution from concrete generally decrease.

Discussion

The ECF of waste glass powder, calcined marine clay and GGBFS are assessed under Singapore’s local context. Glass powder has the lowest ECF, followed by GGBFS and calcined marine clay. Glass powder and calcined marine clay have the advantage of local production, which avoids marine transportation emission. The production of glass powder involves physical treatment of crushing and grinding, which only requires electricity input. Meanwhile, the production of calcined marine clay includes heating process by fuel combustion, which results in higher emission. As shown in Fig. 1, the ECF of OPC in Singapore is relatively stable over past 20 years, regardless of the change in importing structure. This could be explained by the high contribution ratio of OPC production towards its total embodied emission, which accounts for up to 90%. On the contrast, the ECF of GGBFS could be significantly influenced by its importing structure as more than 55% are contributed by transportation emission. As shown in Fig. 2, the ECF of GGBFS drops significantly from 2019, this is because increasing amount of GGBFS were imported from Singapore’s neighboring country like Malaysia and Indonesia. Around 50% of GGBFS are imported from Malaysia and Indonesia since 2019, while is share is only less than 10% before 2019. The ECF of GGBFS under economic allocation is generally 25% higher than no allocation in the past 20 years. However, economic allocation has disadvantage of unstable due to the potential fluctuation of the market price³³. Transportation emission contributes to the majority of the embodied carbon emission of GGBFS under both allocation methods. This indicates that Singapore should source GGBFS from neighboring countries to avoid the emission from long-distance international transportation.

When the three materials are adopted as SCM in concrete, replacement ratio is identified as a critical parameter of embodied carbon reduction. GGBFS concrete achieves the highest embodied carbon reduction of 56%. This is mainly due to the high replacement ratio of GGBFS with up to 70%. Despite glass power has the lowest ECF among three SCMs, its 20% replacement ratio results in only 16% of embodied carbon reduction. Similar to calcined marine clay, its 30% replacement ratio results in 21% carbon reduction.

Compressive strength/embodied carbon ratio is calculated for 1 m³ of concrete to determine the most efficient concrete mixture. Higher compressive strength/embodied carbon ratio indicates a more efficient concrete mixture in terms of concrete performance and environmental impact. As shown in Figs. 7, 8, the compressive strength/embodied carbon ratio for concrete mixtures ranges from 0.09 to 0.25 MPa/kgCO₂e. G60 concrete with 70% GGBFS substitution is identified as the most efficient concrete. The efficiency of concrete mixture generally increases with concrete strength, which indicates that higher grade of concrete has better efficiency in environmental impact. Nevertheless, concrete mixtures adopting high GGBFS substitution are more efficient than marine clay concrete, glass powder concrete and OPC concrete under both allocation methods. Thus, high strength concrete with high GGBFS replacement ratio is recommended to improve the environmental efficiency, provided that the requirement of other concrete mechanical properties is fulfilled. The use of high GGBFS replacement ratio is also suggested by Onn et al.³⁴ to improve the compressive strength/embodied carbon ratio of concrete.

Fig. 7: Compressive strength/embodied carbon ratio for concrete mixtures.

The compressive strength /embodied carbon ratio measures the embodied carbon efficiency of concrete mixture. The ratio is impacted by the concrete grade and SCM usage.

Fig. 8: Scenario analysis of total embodied carbon emission of cementitious materials in Singapore in 2022.

The scenario analysis tests the impact of sustainable strategies on the total embodied carbon emission of cementitious materials in Singapore.

GGBFS is an excellent replacement material for cement. However, according to the IStructE’s report³⁵, the production of clinker is currently twelve times higher than GGBFS, and this situation will be remained beyond 2030. It is reported that around 90% of the steel slag have already been processed to GGBFS, which indicates that the production rate of GGBFS could be hardly improved in the future. As a GGBFS importer, Singapore also suffers from the carbon emission caused by GGBFS transportation, which dilutes the environmental benefits of using GGBFS as SCM. Besides, due to the shortage of GGBFS globally, increasing the use of GGBFS will be ineffective to tackle global emission. Based on the cement import data in 2022³⁶, the annual demand for cement in Singapore is 4,348,849 tons. The annual import amount of GGBFS stands at 1,378,000 tons, capable of achieving 24.06% of the cement replacement assuming all GGBFS are used in concrete industry. However, it is crucial to note that global GGBFS production has already reached its limit due to steel production constraints, and the produced GGBFS is fully utilized worldwide. Therefore, increasing GGBFS imports would not effectively address the global emission issue. Singapore’s focus should instead shift towards enhancing local material circularity.

Marine clay presents a significant opportunity, with an annual excavation potential of 12,750,000 tons. If fully recycled, this quantity could entirely meet the local cement demand. However, the replacement potential of calcined marine clay is constrained by the functional requirements and max replacement ratio of 30%. The recycling rate of waste glass was only 14% in 2022³⁷, indicating that if waste glass could be fully recycled into glass powder, it could replace 1.27% of the cement used. Efforts must be directed towards maximizing the cement replacement ratio of calcined marine clay and glass powder to leverage their potential in reducing carbon emissions. The availability of three recycled materials is outlined in Table 9³⁷.

Table 9 Availability analysis of GGBFS, calcined marine clay and glass powder

Given the current availability of materials, this study delineates four scenarios concerning the future adoption of recycled Supplementary Cementitious Materials (SCMs) in Singapore’s concrete production (Table 10). The baseline scenario (S1) adheres to existing practices, maintaining the current import levels and sources of Ground Granulated Blast-Furnace Slag (GGBFS) without integrating calcined marine clay and waste glass powder. Scenario two (S2) incorporates the locally sourced calcined marine clay and waste glass powder, while keeping GGBFS importation constant. Scenario three (S3) proposes a shift in GGBFS import strategy by sourcing exclusively from Malaysia, Singapore’s closest significant trading partner. The fourth scenario (S4) assumes no constrain of GGBFS supply and sourcing only from Malaysia. This scenario focuses solely on GGBFS as the SCM due to the lack of studies on mixed use with marine clay or glass powder.

Table 10 Scenario analysis of Singapore SCMs utilization

The impacts of these scenarios are analyzed and depicted in Fig. 8. Findings suggest that S2 can lead to a 20% reduction in embodied carbon emissions from concrete production. This reduction could increase to 23% if the source of GGBFS is shifted to Malaysia (S3). However, to achieve more substantial reductions, Singapore would need to augment its GGBFS imports (S4), potentially decreasing embodied carbon emissions by up to 56%.

Methods

LCA methodology

In this research. LCA methodology is adopted to analyze the -environmental impact of recycled building materials. A four-phase methodology is specified by ISO14040³⁸. The first phase is to define the goal and scope of LCA. The system boundary and intended use of the LCA study is to be identified. The second phase is life cycle inventory (LCI) analysis, the quantification of energy and material input and output of the system is determined. The following phase is life cycle impact assessment (LCIA) phase. The environmental impacts of the system are calculated from the LCI result. The final interpretation phase is to summarize and discuss the LCA results as a basis of conclusion and recommendation.

System boundary

The system boundary of an LCA study defines the life stages that should be included in the environmental impact evaluation. In this research, cradle-to-gate methodology is adopted to quantify the environmental impact of one cubic meter of concrete product using recycled SCM materials. The cradle-to-gate system boundary includes raw material extraction, material transportation and manufacturing process. The cradle-to-gate system boundary was also implemented to calculate the environmental impact of recycled materials in other research^34,39,40, since production stage covers most of the embodied carbon emission of building materials⁴¹. The system boundary and the included processes are illustrated in Fig. 9.

Fig. 9: Illustration of cradle-to-gate system boundary and included processes.

The studied cradle-to-gate system boundary involves A1-A3 stages of concrete production in Singapore. The extraction of waste marine clay and waste glass are excluded scince they are treated as wastes.

Functional unit

The functional unit serves as a reference to which the inputs and outputs are related. The functional unit could be various according to the studied object. In this research, the functional unit of concrete product is defined as one cubic meter of concrete with equivalent 28-day compressive strength. The environmental impact is measured in kgCO₂e/m³. For the building projects in case study, the environmental impact is normalized by Cross Floor Area (GFA), which is quoted as kgCO₂e/m^2 42.

LCI for OPC and SCMs

The foreground emission data of calcined marine clay and waste glasses are evaluated by considering these two materials as waste (no allocation), which ignores the potential emissions from raw material extraction process such as soil excavation and waste glass collection. The LCI phase of marine clay and waste glasses measures the transportation emission and the production emission related to energy input. Marine clay and waste glass are transported from sites to processing plants by heavy trucks. The average local transportation distance is assumed to be 20 km. The input-out data of glass powder manufacturing process are collected from local lab equipment, given the energy efficiency of crushing and milling process is similar between lab and industrial equipment. However, the production of calcined marine clay involves heating process, the energy efficiency of laboratory heating furnace is much lower than the industrial heating furnace. Thus, the emission data of producing calcined clay is adopted from Ecoinvent 3 database⁴³. The inventory and ECF of glass powder production is listed in Table 1⁴⁴. It should be noted that the inventory data of glass powder is collected from laboratory-scale equipment, which leads to a more conservation result in terms of carbon emission when compared with industrial-scale evaluations. The environmental impacts of glass powder and calcined marine clay are calculated from LCI results and electric grid emission data of 0.4168 kgCO₂e/kWh in Singapore⁴⁵.

The embodied carbon of GGBFS could be accessed based on several allocation methods. The approach of allocation remains debates and different allocation methods could significantly change the LCA outcome⁴⁶. Firstly, the concrete industry could treat GGBFS as a waste product. This will result in no allocation of GGBFS from steel making process and only emission from GGBFS grinding is accounted³³. However, since GGBFS has been widely used as SCM in concrete production, increasing number of studies are treating GGBFS as a co-product of steel. This means that GGBFS should be awarded certain percentage of environmental burden during steel making process⁴⁷. According to ISO 14044³⁸, the partitioning of the environmental impact should be based on the mass of co-product or the economic value of the co-product. The mass allocation method indicates the equivalent importance of by-products and main products⁴⁸. In terms of GGBFS, the result of mass allocation will result in higher emission of GGBFS than OCP thus induce the concrete industry to prefer using clinker than GGBFS³³. Besides, economic allocation have advantage to lowering the emission of co-products when compared to mass allocation³³. De Brito et al.⁴⁹. reviewed 33 LCA studies of industrial co-products including FA and GGBFS. They pointed out that 20% of the studies involved allocation scenario and all of them used or recommended economic allocation. Thus, no allocation and economical allocation are used to access the ECF of GGBFS in this study. The expression of economic allocation coefficient C_e is shown in Eq. (1).

$${C}_{e}=\frac{{({\$}\,\cdot\,{m})}_{{co}{-}{product}}}{{{({\$}\,\cdot\,{m})}}_{{main}{-}{product}}+{{({\$}\,\cdot\,{m})}}_{{co}{-}{product}}}$$

(1)

Where ${\left(\$\cdot m\right)}_{{co}-{product}}$ and ${\left(\$\cdot m\right)}_{{main}-{product}}$ represent the multiplication of mass value and mass produced in the system. Equation (1) indicates that the environmental impact of co-product is related to the percentage of benefit by selling the main-product and co-product⁴⁸. In GGBFS production system, around 400 kg of GGBFS are produced when producing 1 ton of steel⁵⁰. The value of steel and GGBFS are extracted from UN Comtrade database³⁶.

Singapore’s annual demand of OPC and GGBFS has reached 4.34 million tons and 1.38 million tons in 2022³⁶, respectively. The demands are mostly relied on importation. To evaluate the embodied carbon emission of these imported building materials, an alternative LCA framework based on the material import amount is adopted in this research to calculate their ECF. This framework is also adopted in previous studies to calculate the embodied carbon emission of import cement and aggregate in Singapore^30,51. The impact of regional technological variation and logistical distance are reflected in this framework. The calculation formulas are listed below.

$${ECF}=\sum {W}_{i}\times \left({EC}{F}_{{LP},i}+{EC}{F}_{T,i}\right)$$

(2)

$${EC}{F}_{{LP},i}=\left\{\begin{array}{cc}{EC}{F}_{{GGBFS},i}\, & {if\; no\; allocation}\\ {C}_{e,i}\times {EC}{F}_{{steel},i}\times \frac{{m}_{{steel}}}{{m}_{{GGBFS}}} & {if\; economic\; allocation}\end{array}\right.$$

(3)

Equation (2) describes the ECF calculation method of imported material, where W_i is the weightage of the ith trading partner, which is calculated based on the Singapore annual import amount extracted from UN Comtrade³⁶. ECF_LP represents for the ECF of local production in origin country. ECF_T represents for the ECF of transportation between Singapore and its trading partners. The transportation process considers both marine transportation and in-land transportation. The distance of marine transportation is calculated by Sea-Distances⁵², considering one-way empty run using bulk carrier. The in-land transportation distance is assumed to be 100 km. The ECF_LP of GGBFS depends on the allocation method as shown in Eq. (3). If no allocation is chosen, the local production emission of GGBFS is retrieved from literatures and local authorities’ reports as listed in Table 4. In the case of economic allocation, ECF_LP equals to the production of C_e,i, ECF_steel,i and the ratio of produced steel mass and GGBFS mass in the system. Where C_e,i is calculated from Eq. (1) based on the trading value of steel and GGBFS for each trading partner³⁶. ECF_steel is extracted from the report by World Steel Association⁵³.

This framework is used to calculate the ECF of OPC and GGBFS in Singapore according to their import amounts in past 20 years, and the calculation results are shown in Fig. 1 and Fig. 2, respectively.

LCI for concrete mixtures

Inventory data for one cubic meter of concrete are prepared based on the 28-day compressive strength of concrete and listed in Table 11. Three concrete grades, namely G40, G50 and G60 concrete are studied in this research. The original OPC concrete mixture design are referred to local concrete factory reports. Concrete admixtures are ignored in this study. For each concrete grade, glass powder, marine clay and GGBFS are used as SCM to achieve the same 28-day compressive strength as OPC concrete. In this research, the highest SCM replacement ratio without jeopardizing the concrete 28-day compressive strength is adopted to address their inherent nature of cement replacement capacity. The replacement ratio of GGBFS is selected to be 70%, which is the maximum replacement ratio defined in local standard⁵⁴. Previous studies also verified that the concrete mixture design with this GGBFS replacement ratio could achieve the targeted 28-day compressive strength^55,56. The replacement ratio of calcined marine clay and glass powder has not been regulated to the local standard yet. Thus, the maximum replacement ratio without compromising the required strength is adopted from the previous studies. According to Du et al.⁵⁷, the concrete with 30% calcined clay replacement rate shows identical 28-day compressive strength with OPC concrete, while the strength of concrete with 45% calcined clay could not match to OPC concrete. Huang et al.⁵⁸ also reported that the concrete with 30% of calcined clay replacement could match OPC concrete in terms of 28-day compressive strength. Other studies reported a minimal increase of the 28-day compressive strength^59,60,61. Therefore, based on the previous studies, the replacement ratio of calcined marine clay is selected to be 30% to achieve the targeted 28-day strength in this study. Regarding the replacement ratio of glass powder, Hassani el al.²⁰ analyzed 78 publications and concluded that samples with 5%, 10% and 15% replacement ratio had similar behavior, with about 60% samples reported same or higher compressive strength. The authors also pointed out that other factors like glass powder size, curing age and the use of additives can also affect the concrete strength properties. Jain et al.⁶² suggested that using a combination of 20% glass powder and 40% of granite powder might increase the compressive strength, while higher percentage may lead to strength decline. Thus, a rational maximum replacement ratio of 20% is selected for glass powder.

Table 11 Inventory data for one cubic meter of concrete with different grades

The embodied carbon emission of listed concrete mix proportions are assessed based on the LCI results combined with the ECF of concrete compositions. The ECF of aggregates are retrieved from Singapore Building Carbon Calculator³¹ and listed in Table 12. The aggregates are assumed to be imported from Malaysia.

Table 12 ECF of aggregates