Mitigation of carbon dioxide by accelerated sequestration in concrete debris

Renewable and Sustainable Energy Reviews 117 (2020) 109495

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Mitigation of carbon dioxide by accelerated sequestration in concrete debris Ning Zhang a, Huabo Duan a, *, Travis R. Miller b, Vivian W.Y. Tam a, c, Gang Liu d, Jian Zuo e, ** a

School of Civil and Transportation Engineering, Shenzhen University, Shenzhen, 518060, PR China Department of Chemical and Environmental Engineering, Yale University, New Haven, CT, 06511, United States c School of Computing Engineering and Mathematics, Western Sydney University, Locked Bag 1797, Penrith, NSW, 2751, Australia d SDU Life Cycle Engineering, Department of Chemical Engineering, Biotechnology, and Environmental Technology, University of Southern Denmark, Odense, 5230, Denmark e School of Architecture & Built Environment, The University of Adelaide, Adelaide, 5001, Australia b

A R T I C L E I N F O

A B S T R A C T

Keywords: Concrete debris Carbon mitigation Tradeoff Economic benefits Worldwide

Carbon capture and storage is becoming increasingly feasible. This study provides a novel quantitative analysis of the global CO2 mitigation potential through accelerated carbon sequestration in concrete debris separated from construction and demolition waste. We consider the economic and environmental tradeoff. Based on data collection from 14 large regions and countries, we created a method to estimate and project the generation of concrete debris, recycling and carbonation rates, and cost. The overall concrete debris generation was more than 3.0 (�0.6) billion tonnes (Bt) in 2017 worldwide, mainly from emerging countries such as China and India. This debris has the potential to mitigate 62.5 (�8.9) million tonnes (Mt) CO2 under optimal carbonation conditions determined by pressure, temperature, humidity, time, CO2 concentration, and debris size. Our scenario analysis reveals that the global cumulative carbonation of concrete debris could be as high as 3.0 Bt CO2 between 2018 and 2035, which equals approximately one third of the total CO2 emissions from fuel combustion of China in 2016. In our scenarios, the economic benefits of storing CO2 by concrete debris are mainly from recycled con­ crete aggregate rather than carbon sequestration, but also consider the current carbon price in major carbon markets. These findings highlight an effective and practical approach to reuse concrete debris as well as enhancing economic benefits. This approach could be helpful to better manage the fast-growing concrete debris and need for carbon mitigation as well as bridging the gap of CO2 sequestration by concrete debris between research and application.

1. Introduction 1.1. Motivation Carbon dioxide (CO2) is one of the most critical greenhouse gases (GHG) responsible for climate change [1]. In recent years, the CO2 emissions from the cement industry have received wide attention. Carbonation, the natural process of concrete infrastructure slowly uptaking some atmospheric CO2, has received less attention than the production phase [2]. The natural carbonation of concrete building materials is a noteworthy pathway of capturing atmospheric CO2 [3]. Also, CO2 curing of concrete provides an innovative use of waste CO2 from industrial processes. It relies on the chemical reactions between

CO2 and cement clinker inside the concrete specimens, which can quickly harden the concrete. This process has recently been adopted by some technology companies such as CarbonCure [4]. Cement is a critical component of concrete and mortar, which constitute modern buildings and structures; concrete is the most widely used building material in the world [5]. With the rapid urbanization of global emerging regions, a burst of construction and demolition activ­ ities are resulting in massive construction and demolition waste (CDW). Although the recycling of CDW has become a growing trend around the world, the landfill is unfortunately still the main disposal approach. Crushed construction debris -recycled concrete aggregate (RCA), can be reused as aggregate for road base, or in paving blocks [6,7]. Recycling concrete debris to sequestering CO2 in RCA is another option to reduce the environmental pollution and safety risk from CDW

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Duan), [email protected], [email protected] (J. Zuo). https://doi.org/10.1016/j.rser.2019.109495 Received 7 July 2019; Received in revised form 1 October 2019; Accepted 14 October 2019 Available online 28 October 2019 1364-0321/© 2019 Elsevier Ltd. All rights reserved.

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List of abbreviations BAU Bt CCS CDW CO2 cRCA C–S–H EPHC EU GDP

GHG LCCA MSW Mt OECD RCA RH SI USD USEPA w/c

Business-as-usual Billion tonnes Carbon capture and storage Construction and demolition waste Carbon dioxide Carbonated recycled concrete aggregate Calcium silicate hydrates Australia Environment Protection and Heritage Council European Union Gross Domestic Product

landfilling [8]. This potential has attracted the attention of researchers and engineers who have performed pilot studies [9–13]. The carbonated recycled concrete aggregate (cRCA) has improved properties compared to standard RCA for use as an aggregate in new concrete, including lower water adsorption, increased density, and strength [14]. Some of the existing research on concrete recycling and carbon capture also takes into account other scenario - passively absorbs CO2 at atmospheric concentrations apply on concrete debris to make it a tool for carbon capture, in addition to, or instead of, curing and reusing them into construction [3,15,16]. The potential emissions from existing power plants are considered represent 85 billion tonnes (Bt) CO2 only in China, if they continue to run at present workload for their remaining years, even if smaller units will retire early [17], which suggest a great potential of carbon capture and storage (CCS). Yang et al. [15] estimated that the CO2 absorption of concrete during a building’s service lifecycle in South Korea (including recycling stage) is about 15.5%–17% of the CO2 emissions from cement production. This is also a considerable ab­ sorption. However, compared with another method of reusing concrete debris to absorb CO2 is relatively limited. It has been reported that the total CO2 emissions in 2030 cannot exceed 42 Bt if a low-carbon path is followed. This threshold represents about 80% of 2016’s CO2 emissions, which totaled 52 Bt [18]. Such imperatives are a key driver for innovations in concrete technology [19]. Achieving the target requires aggressive strategies, which should include intentional, accelerated concrete carbonation if it is proven to be feasible and cost-effective at scale.

react with CO2 as shown in Eq. (5) and Eq. (6) [3].

CaðOHÞ2 þ CO2 →CaCO3 þ H2 O

(2)

C

S

H þ CO2 →CaCO3 þ SiO2 ⋅αH2 O

3CaO ⋅ 2SiO2 ⋅3H2 O þ 3CO2 →3CaCO3 þ 2SiO2 ⋅3H2 O

(5)

ð3CaO ⋅ SiO2 Þ þ 3CO2 þ αH2 O→3CaCO3 þ SiO2 ⋅αH2 O

(6)

1.3. Review of cRCA experiments Several researchers have explored this potential through experi­ ments and detailed engineering models. Given the variety of concrete mixes, carbonation results vary but generally show considerable potential. After a holistic review of papers related to cRCA experiments, Zhan et al. [22] presented an innovative method for the production of con­ crete blocks, which cannot only reuse the concrete debris for new con­ struction materials but also capture CO2 from flue gas permanently. Kou et al. [23] conducted an experimental study on the performance of concrete prepared by modifying recycled mortar aggregate with CO2 curing method. Engelsen et al. [24] carried out experiments related to the carbonation of demolished concrete debris in Northern Europe. Monkman and Shao [25] investigated the carbonation behavior of concretes in different ages, toughness, and pH change, etc. Yang et al. [15] estimated the CO2 sequestration of concrete during the service stage of the building and after demolition. Thiery et al. [11] designed an experiment to study the carbonation mechanism of a pile of RCA and the effect of the cementitious phase characteristics attached to the original aggregate on the CO2 sequestration rate. Lagerblad [26] used data from literature combined with experiments to calculate the carbonation rate in some simplifications. Across these existing studies, seven critical parameters of carbon­ ation were revealed: water/cement (w/c) ratio of the concrete, particle size, relative humidity (RH), temperature, CO2 partial pressure, CO2 concentration, and exposure time. The past research has shown that the w/c ratio of the concrete plays a major role in the carbonation rate; however, w/c ratio is unmeasurable from concrete debris. Overall, existing studies have made some attempts to estimate the generation of concrete debris, but only from a city or regional level. There are also a few studies to focus on the CO2 sequestration potential by natural carbonation of cement and measured the properties of cRCA

Carbonation refers to the process of CO2 diffusing through pores of the concrete and reacting with the alkaline materials [20]. Calcium attached to the pore surface from cement paste Ca(OH)2 and calcium silicate hydrates (C–S–H) in hydrated cement react with CO2. The carbonation process results in the creation of limestone (CaCO3) mole­ cules within the concrete pores, increasing the concrete’s density and reducing further water adsorption. Unlike the calcination process during cement production which requires significant energy, the carbonation process releases energy during the reaction; it is a stable, thermody­ namically favorable process. The main chemical reactions of carbon­ ation are as Eqs. (1)–(4) [16]. (1)

ð2CaO ⋅ SiO2 Þ þ 2CO2 þ αH2 O→2CaCO3 þ SiO2 ⋅αH2 O

In different circumstances, the likelihood of producing carbonate from chemical reaction products in cement paste varies. Researchers have shown that the degree of carbonation is between 30% and 90% in RCA under atmospheric, the lower values is due to the dry indoor climate [16]. In order to ensure a higher CO2 capture rate, the conditions of the external environment should be optimized. The simplified carbonated cycle of concrete debris is shown in Fig. 1. As the traditional disposal method, landfilling and dumping will create negative environmental impacts, such as land subsidence [21]. Mean­ while, compared with the RCA recycling process, the production process of cRCA will produce both economic and environmental benefits.

1.2. Carbonation chemistry

H2 O þ CO2 →H2 CO3

Greenhouse gases Life cycle cost analysis Municipal solid waste Million tonnes Organization for Economic Co-operation and Development Recycled concrete aggregate Relative humidity Supporting information USA dollar United States Environmental Protection Agency Water/cement ratio

(3) (4)

If unhydrated cement of concrete is contained in hardened cement, the mineral components dicalcium silicate and tricalcium silicate will 2

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Fig. 1. Framework of the carbonated cycle of concrete debris.

according to experimental study. However, there is relatively little work to take economic benefits of CO2 sequestration potential into consider­ ation together with the environmental benefits from industrial appli­ cation perspective. Improving the potential of CCS through the concrete debris could help to address the current research limitations and provide an accessible approach for a comprehensive consideration of CDW recycling. Therefore, this study aims to estimate both the optimal CO2 mitigation potential and the economic benefits by using concrete debris.

developing regions. All regions in the study except the United States, Japan, Australia, Canada, and Europe are developing regions. It is generally difficult to collect data from developing regions as available data sources are limited. Therefore, the reliability and timeliness of data cannot be ensured. To address this problem, this paper uses a tiered approach, inspired by IPCC guidelines (in IPCC guidelines Tiers change form simplified to specific) [28]. Fig. 2 shows the detailed data gath­ ering methods in different regions (see detailed data source in Table S9 in SI). The tiers are as follows:

2. Objective and scope

Tier 4) Official report/academic paper on CDW

2.1. Objectives

If per capita generation of CDW is given in the reports or papers, Eq. (7) is used to simply estimate the total. Gci refers to the generation of CDW per capita in year i, Pi is population, and Gi is the CDW generation in the year i in all equations.

The main aim of this work is to estimate the potential for accelerated CO2 sequestration by cRCA on a regional basis. The historic cement debris generation in major countries across the world is estimated firstly and then the CO2 sequestration of concrete debris using these techniques is projected under several scenarios. We conclude with an analysis of the economic and environmental benefits of CO2 sequestration with con­ crete debris.

Gi ¼ Gci � Pi

(7)

Tier 3) Municipal solid waste (MSW) generation data

2.2. Scope

The MSW and CDW are both parts of general waste; general esti­ mates exist on their proportion in general waste: 24% (R1 ) and 36% (R2 ), respectively [29]. In reality, this ratio may vary throughout the world and over time, but data availability is limited, the ratio variation is not taken into consideration. CDW can be backed out in Eq. (8) or Eq. (9), depending on if MSW generation data is total GMSWi or per capita GcMSWi .

In this study, 14 global regions are selected for being in the top ten for population and/or Gross Domestic Product (GDP) in 2017 [27]: China, India, EU, USA, Indonesia, Brazil, Pakistan, Nigeria, Bangladesh, Russian, Mexico, Australia, Canada, Japan. These 14 regions represent about 68% global population and 82% GDP (see Table S2 in SI). In addition, the GDP growth rate of these regions also ranked top. The historical timeframe is 2000–2017, and the projection timeframe is 2018–2035. 3. Methods and data 3.1. Estimate and projection of CDW

Gi ¼

GMSWi � R2 R1

(8)

Gi ¼

GcMSWi � R2 � Pi R1

(9)

The latest data on CDW production of several regions were gathered through official websites of national governments and international organizations including the Europe, United States Environmental Pro­ tection Agency (USEPA), Organization for Economic Co-operation and

Since concrete debris is a fraction of CDW, ideally CDW generation data would be used to estimate the generation of concrete debris. However, the lack of official statistics is a common problem in many 3

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Fig. 2. The tiers of data sources for estimate and projection of CDW.

Development (OECD), Australia Environment Protection and Heritage Council (EPHC) etc., and from previously published studies and esti­ mation where data were not available through government departments (see Table 2). In spite of some definitions of MSW including CDW, using the pro­ portion of CDW in MSW to estimate the generation is much smaller than real value. Therefore, based on an estimation of TEKES (Finnish Funding Agency), as shown in Eqs. (8) and (9), R2 =R1 is used to estimate the MSW generation [29], the second type of data is calculated with this ratio. The main MSW generation data of OECD countries come from the OECD database [30].

where Gx and Px refers to the CDW generation and population in data known year x (n＜3), Gi and Pi means the CDW generation and popu­ lation in data unknown year i. 3.2. Generation of concrete debris In order to estimate annual concrete debris generation GCDi in a given year, its proportion in CDW (R3 ) is estimated. The composition of CDW in different regions are provided in a number of research papers and official reports (see Table 2). The concrete debris generation can be calculated according to the CDW generation and the materials compo­ nent proportion (see Fig. 3 and Eq. (11)).

Tier 2) Linear Interpolation

GCDi ¼ Gi � R3

The final two tiers are reserved for regions lacking sufficient CDW or MSW data. But when there are three or more years of data, linear interpolation is expected to be more accurate.

(11)

3.3. Estimation of CO2 sequestration Considering that the experimental materials and procedures in the literature are similar, six parameters described in section 1.3 are taken into account, which is used to provide a theoretical optimal CO2 sequestration rate to calculate mitigation (CO2 sequestration per kg concrete debris). After a systematic collation of the literature data [23, 24,26,51], Fig. 4 shows the comprehensive results of the CO2 seques­ tration experiments. A, B, C, and D refer the samples with different particle size and other parameters. During the first few hours of expo­ sure in a Model 30–50 L 150-kPa pressure chamber, the experimental

Tier 1) Weighted average per capita CDW generation When there are fewer than three years of data, the population of the present year multiplies the average per capita of the known data (Eq. (10)). Pn Gx Gi ¼ Px¼1 � Pi (10) n x¼1 Px 4

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of CO2 sequestered increases monotonically with exposure time and CO2 concentration. Since particle size and CO2 concentrations can be controlled in industrial settings, the particle size of 1–8 mm and the highest CO2 concentration considered (50%) will be assumed. However, the industry efficiency should be considered, and therefore 5-h is assumed to balance sequestration with throughput. The rate of sample D after 5 h (120 g CO2/kg concrete debris) is adopted. By using the pa­ rameters in this section, the global estimated CO2 sequestration mass is estimated in the next section. The sequestration of CO2 in RCAGCO2 i is a function of the recycling rate ðR4 Þ and the sequestration rate (s), shown in Eq. (12) (recycling rate can be seen in Fig. 3). We assume that concrete debris in landfills does not appreciably sequester CO2. Eq. (12) presents a simple calculation of CO2 sequestration potential. (12)

GCO2 i ¼ GCDi � R4 � s Fig. 3. Regional concrete composition and recycling rate of CDW. Data sources: (1) US [31,32], EU [33,34], China [35], Japan [36], India [37], Brazil [38], Canada [39,40], Russia [41], Australia [42,43], Mexico [44,45], Indonesia [46], Nigeria [47,48], Pakistan [49,50]. The data source of concrete proportion in China derives from the field investigation and in Indonesia and Bangladesh refer to the India. The data source of recycling rate in Russia refers to the EU.

3.4. Scenarios analysis To project the potential CO2 mitigation potentials in the future, scenarios have been developed that vary the recycling rates (see Table 1) and generation of concrete debris for 2018–2035. We assume that the future generation of concrete debris is aligned with economic development and therefore correlates with future GDP estimates; it does not change with the Scenarios [52]. The change in recycling rates is divided according to the national development level (Table 1). Scenario 1 is business-as-usual (BAU) with 2017 as a baseline.

environment-controlled temperature of 22 � 2 � C and an RH of 55 � 5%, this is an optimized environment based on existing literature [23]. The tests samples’ growth rate of carbonation achieved the highest as illus­ trated in Fig. 4a. However, after approximately 5–10 h of exposure, the CO2 sequestration rate slows down. This unexpected behavior seems to be due to a thin carbonation layer formed that sealed off the surface of the sample [51]. Concerning the impact of w/c ratios on the CO2 sequestration, in Fig. 4b, one can draw a parallel between columns which both demonstrate that the CO2 sequestration capacity of the studied systems is higher at CO2 ¼ 50% than at CO2 ¼ 10% and CO2 ¼ 0.04% (natural concentration). The bars also show an optimum condition for w/c ratios of 0.45 which could be ascribed to the fact that at low w/c ratios, there is growth in the number of carbon hydration products (CH and C–S–H) and reduced competition for the accessibility to CO2 in denser matrix [68]. Considering the results in Fig. 4, the mass

Table 1 Recycling rate in scenarios setting. Scenario

Underdeveloped

S1 (Business-asusual) S2 (Moderate) S3 (High)

Unchanged Unchanged 30%

Emerging*

Developed

Targeted year 2035

30% 60%

99% 99%

2035 2035

Note: *, Emerging Regions are defined by different groups (see Table S7), in the study included BRIC countries (India, China, Brazil, Russian) and Mexico.

Fig. 4. Quantification CO2 sequestration subject different situations. 5

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national statistics, international statistics and research papers such as USEPA and OECD. Based on the method model established previously, a large amount of intermediate data has been calculated (the CDW gen­ eration, concrete debris generation and their recycling amount in 14 regions in 2000–2035) to determine the tradeoff between environmental and economic benefits in these regions. As shown in Fig. 2, there was one deficiency in data obtaining, the missing data were supplemented by proper estimation or assumption, but with consideration of uncertainty. Detailed data sources are shown in Table 2.

Table 2 Detailed data sources. Symbol

Source

Applicability

Remarks

Camortized

Field investigation

Eq. (13)

Cdiesel

[70]

Eq. (13)

Celectricity

[69]

Eq. (13)

Clabor

Eq. (13)

Gci

Field investigation and research paper [71] Detailed sources are shown in SI

GcMSWi

[44]

Eq. (9)

Gi

Detailed sources are shown in SI

Eq. (11)

GMSWi

Detailed sources are shown in SI

Eq. (8)

ICO2

[72–75]

Eq. (13)

IcRCA

Eq. (13)

Pi

Field investigation and official report [76] [77]

R1

[29]

Eqs. (7), (9) and (10) Eqs. (8) and (9)

See Table S7 in SI See Table S7 in SI See Table S7 in SI See Table S7 in SI See Table S9 in SI See Table S9 in SI See Table S9 in SI See Table S9 in SI See Table S5 in SI See Table S6 in SI See Table S2 in SI –

R2

Eq. (7)

[29]

Eqs. (8) and (9)

–

R3

[31,36,37,42,78–80]

Eq. (11)

R4

[32,34–36,38,40,41,43,45,46, 48,50] [23,24,26,51,68]

Eq. (12)

See Table S1 in SI See Table S4 in SI See Fig. 4

s

Eq. (12)

4. Results 4.1. Generation of concrete debris Fig. 5 shows the estimated concrete debris generation from 2000 to 2017. The average annual generation is 2.13 Bt, in which the largest proportion was contributed by China. The total generation peaked in 2016 at about 3.13 Bt, China generated about 2.26 Bt, followed by the United States at about 0.34 Bt (see Table S3 in SI). The United States and China have not only the largest total generation but also the largest per capita generation at 1.65 tonnes/person and 1.02 tonnes/person, respectively. In addition, China and the United States are the largest economies around the world with well-developed construction in­ dustries [54]. In other Asian regions, such as Indonesia, Bangladesh, and Pakistan, there are large populations, however, less CDW and concrete debris are generated because of lower-level productivity and development [55]. In the developed regions such as Canada, Australia, and Japan, the gen­ eration is steady with small fluctuation. However, in major developing regions, the generation shows a general growing trend. Taking China as an example of a fast-growing developing country, the growth rate of its concrete debris generation is increasing yearly. While the growth po­ tential in other developing regions is much smaller, China still repre­ sents the development tendency. Even if steel-concrete structure buildings were only built in recent decades in China, most commercial or residential buildings were demolished within the designed life span due to the urban renewal, resulting in short-lived buildings with an average age of 23-years in China [56]. Analogously, the increasing urban pop­ ulation, booming economy (particularly construction sector), and the rise in community living standards will occur in India and other fast-growing regions following China [57]. As an economy matures, rapid urbanization and population growth are unavoidable, the amount of concrete debris will probably increase drastically with economic growth in the coming years. There is a time lag between when concrete is used in construction, and when it becomes demolition waste, urban­ ization poses a positive impact on growing generation of CDW. Partic­ ularly in developing regions, where a gap in economic activities and living standards between rural and urban areas is wide and people are willing to migrate to urban areas [58]. More demand for urban con­ struction will be accompanied by more CDW. Therefore, much potential of CO2 mitigation by construction concrete debris in the future.

In Scenario 2, the recycling rate of emerging regions and developed regions increases linearly to 30% and 99% by 2035, respectively. For instance, considering the present recycling rate in Russia is over 30%, 60% would be Russia’s development target in Scenario 2, which aligns well with their recycling target. Regarding Japan as the target of developed regions, setting China’s target for emerging regions, the recycling rate in undeveloped regions will remain at 2017 levels. While the developed regions in Scenario 3 achieve the same goal (99%) by 2035, emerging regions and undeveloped regions increases linearly to 60% and 30%. Regarding China’s target for undeveloped regions, where the 60% recycling rate refers to the baseline of the minimum recycling rate in developed regions, setting it as the target of emerging regions. 3.5. Evaluation of economic benefit In this study, a life cycle cost analysis (LCCA) is developed to eval­ uate the economic benefit from CO2 capturing industry, the year 2017 is set as an example to measure the comprehensive benefit. Because the study only considers the carbon capturing from recycled concrete debris, that means only the boundary of this study involves the period after buildings’ service life. For the purpose of comparison, the study esti­ mates the economic gain from CO2 sequestration using the price of the four world’s largest carbon markets: Europe, China, United States, and New Zealand [53] and makes an evaluation analysis comparing the cost of recycling (crushing). The profit (NP) means the total income of cRCA (IcRCA ) and CO2 sequestration (ICO2 ) deduct from the cost of electricity for recycling (Celectricity ), diesel (Cdiesel ), amortized equipment (Camortized ) and labor (Clabor ) (Eq. (13)). The typical price of cRCA is about 60% of the price of natural aggregates (see detailed data source in Table 2). P ¼ IcRCA þ ICO2

Celectricity

Cdiesel

Camortized

Clabor

4.2. Comparison of global CO2 sequestration potential About 3.67 Bt of CDW and 2.13 Bt of concrete debris were generated on average annually in selected regions from 2000 to 2017. There is no doubt that the huge potential of CO2 mitigation is concealed in the mass of concrete debris. However, the present recycling rate in most regions is very low, which in developing regions like Pakistan and China around 8% and 5% respectively [35,50], but in some developed regions such as Japan and Canada that rates are high at 99% and 75% respectively [36, 40] (see Table S4). The current challenges in recycling CDW in devel­ oping regions, including a lack of equipment (e.g., crushing machine), lower rates of CDW reuse efficiency, and low public awareness of waste recycling, may have negative effects on recycling rate. The main driver for recycling is the revenue for RCA.

(13)

3.6. Detailed data sources In this study, 18 years of data from 14 regions were comprehensively collected. All of the basic data in this paper were mainly obtained from 6

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Fig. 5. Concrete debris generation in the major regions and countries around the world.

Fig. 6 shows the CO2 mitigation potential in different scenarios. Aside from BAU, the annual CO2 sequestration potential projected in future Scenarios is much greater than what could have been sequestered in the past with accelerated sequestration due to higher recycling rates. In Scenarios 2 and 3, there is strong growth of CO2 uptake over time. The increase is attributable to emerging regions due to the large base of concrete debris production combined with the increase of recycling rate. China is expected to have high concrete debris generation coupled with low recycling rates, while in EU countries there is declining generation but high recycling rates. Therefore, the baseline results of this analysis indicate the great potential of carbon mitigation ability, with the annual uptake rate rising 395 Mt between 2017 and 2035 in Scenario 3. Fig. 6 shows that within two decades (2018–2035), Scenario 2 (3034 Mt) and Scenario 3 (4526 Mt) could achieve a cumulative absorption of 1476 and 2968 Mt of CO2, respectively, compared to BAU (1558 Mt). Compared with the forecasts of the International Energy Agency [59], if all of the existing CCS projects were adopted widely across the world, the maximum sequestration rate would be less than 70 Mt CO2 each year in 2025. Davis et al. [60] estimated committed CO2 emissions annual from transport infrastructure in operation worldwide to be 115 Bt from 2010 to 2060, which accounts one-tenth of BAU’s case. If full undertaken, the accelerated CO2 sequestration of concrete debris could be the largest CCS project ever.

be generated during demolition. Concrete debris is transferred to dump and landfill in most developing regions which will lead to much envi­ ronment pollution, such as land subsidence and clearing of ground vegetation [61]. Recycling of the concrete debris will not only reduce environmental pollution but also generate economic benefits. In Fig. 7a, the cost of crushing grows from 1.7 billion USD in the present recycling situation to 7.4 billion USD as the global average recycling rate grows in our Scenarios to 70% (which aligns well with the EU recycling target in the CDW 2008/98/EC). The economic value of cRCA drives the profit. To make a profit in this scheme, it requires that the recycling cost per tonne of concrete debris (cost of diesel, electricity, amortized equipment and labor) is lower than the value of the cRCA and the value of captured carbon. Due to the low price of CO2 in global carbon markets, the economic value of CO2 sequestration is still less than the recycling cost. With the growth of the recycling rate, the profit of recycling concrete debris would increase from 1.1 USD/t to 1.9 USD/t (see Tables S5–S7 in SI). Fig. 7b shows the profit of the CO2 capturing industry in all the selected regions; there is positive growth except in Japan which has a high recycling rate (99%) at present. The reason why labor cost is higher in developing regions is that the advanced recycling technology and equipment in developed regions reduces the labor cost per tonne. The positive profit in China because there is high concrete debris generation and a low present recycling rate of 5% [32,35]. Diesel and electricity price are major drivers of the recycling cost. Historically, drivers of a high oil price including political affairs in crude-oil producing countries, fast-increasing demand for crude oil, and demand for above-ground oil inventories [62]. While unlike crude oil where there is an international futures market, the electricity price varies greatly between countries. Based on statistics, in standard regu­ lated monopoly markets, electricity prices are typically higher for commercial and residential consumers than industrial customers [63]. Generally, the electricity price relates to the degree of development; in developed regions, it will often be higher than in less developed regions but possibly less reliable and more environmentally intensive [64,65]. Overall, due to the advanced concrete debris recycling technology and machinery in developed regions with lower demand for labor, the average total cost (labor, amortized equipment, diesel, and electricity) is fairly even across countries. The economic benefits mainly come from cRCA, rather than CO2 sequestration. Nonetheless, with the low recy­ cling rate of concrete debris, there is a huge economic potential in emerging and undeveloped regions on cRCA and CO2 markets, it still needs a strong promotion from the government.

4.3. Environmental and economic tradeoff analyses After buildings’ service life, CDW including the concrete debris will

Fig. 6. CO2 sequestration potential in the three Scenarios (S1–S3). 7

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Fig. 7. Comparison between the potential environmental and economic value (profit) of CO2 sequestration by concrete debris in 2017.

5. Discussion

showed that the cumulative mitigation achieved 2968 Mt of CO2 from 2018 to 2035. It is worth noting that 3.1 Bt concrete debris were generated in 2017 which could have resulted in 62.5 Mt CO2 sequestered with profit of 3.3 billion USD. Even the higher recycling rate followed by increased recycling cost. Nonetheless, the captured CO2 has the global potential to permanently and safely capture CO2 while producing con­ crete with improved physical properties and lesser curing time [67]. Meanwhile, it helps reduce the negative impact of concrete debris landfilling to land occupation and risk of collapse. Concrete debris can indeed make a significant contribution to a country’s economy while reducing the consumption of natural resources and meeting the needs of materials for different projects.

This study focused on the global concrete debris-related CO2 sequestration which supplemented previous studies of cRCA that were conducted at the experimental level. Results show that concrete debris present significant potential for CO2 mitigation. Importantly, it provides an accessible pathway for stakeholders. Both governments and com­ panies could share advantages from such CCS approaches. Compared with Xi et al. [3], their study reveals that a cumulative amount of 4.5 Bt CO2 has been sequestered by natural cement materials carbonation from 1930 to 2013. We estimate that over 3.0 Bt CO2 will be sequestered in 2018–2035 which is almost as much natural CO2 sequestration as we have seen in the past 80 years. Furthermore, based on the findings of Tam et al. [66], the CO2 sequestration helps to improve on the negative properties of RCA, thus providing a superior calcium carbonate chemical reaction. It can both reduce porosity and water absorbency and improve the quality of RCA. No matter from the macro or micro aspects, the sequestering of CO2 from concrete debris can be taken. In future research, a more comprehensive environmental life cycle assessment should weigh the increased emissions associated with crushing concrete debris to create RCA. In the environment section, the CO2 emission derived from diesel and electricity used to crush the concrete should be included in the future study. Similarly, other aspects of economic analysis could be taken into consideration, e.g. the costs of plant building, concrete debris transportation, and CO2 storage.

Declaration of competing interest None. Acknowledgments This work was supported by the Natural Science Foundation of Guangdong Province (2017A030313438). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.rser.2019.109495.

6. Conclusion This study developed a concise calculation model to estimate the concrete debris generation and measure the CO2 mitigation potential by recycling concrete debris globally. In this model, the tradeoff between environmental and economic benefits has been taken into consideration. The result estimated that concrete debris generation exceeded 3.0 Bt in 2017 across the world. The trading of raw limestone can be reduced and the economic benefits will be achieved if managed properly. This waste material flow can also be used effectively to reduce the use of natural resources by the ever-growing construction industry and minimize the amount of concrete debris sent to the landfill. After the estimation of concrete debris generation, the optimal capturing condition was explored. If the governments or recycle companies take proper measures to manage the fast-growing concrete debris, CO2 sequestration could rise from a missed opportunity -approximately 74.9 Mt in 2000 to 457.7 Mt in 2035. The Scenarios demonstrated the great potential of CO2 miti­ gation if the concrete debris recycling rate grows. The optimum Scenario

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