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LETTERS
PUBLISHED ONLINE: 21 NOVEMBER 2016 | DOI: 10.1038/NGEO2840
Substantial global carbon uptake by cement
carbonation
Fengming Xi1,2,3, Steven J. Davis1,4, Philippe Ciais5, Douglas Crawford-Brown6, Dabo Guan7,
Claus Pade8, Tiemao Shi3, Mark Syddall6, Jie Lv9, Lanzhu Ji1, Longfei Bing1, Jiaoyue Wang1, Wei Wei10,
Keun-Hyeok Yang11, Björn Lagerblad12, Isabel Galan13, Carmen Andrade14, Ying Zhang15
and Zhu Liu16,17*
Calcination of carbonate rocks during the manufacture of
cement produced 5% of global CO2emissionsfrom all industrial
process and fossil-fuel combustion in 20131,2. Considerable
attention has been paid to quantifying these industrial process
emissions from cement production2,3, but the natural reversal
of the process—carbonation—has received little attention in
carbon cycle studies. Here, we use new and existing data
on cement materials during cement service life, demolition,
and secondary use of concrete waste to estimate regional
and global CO2uptake between 1930 and 2013 using an
analytical model describing carbonation chemistry. We find
that carbonation of cement materials over their life cycle
represents a large and growing net sink of CO2, increasing
from 0.10 GtC yr−1in 1998 to 0.25 GtC yr−1in 2013. In total,
we estimate that a cumulative amount of 4.5 GtC has been
sequestered in carbonating cement materials from 1930 to
2013, osetting 43% of the CO2emissions from production
of cement over the same period, not including emissions
associated with fossil use during cement production. We
conclude that carbonation of cement products represents a
substantial carbon sink that is not currently considered in
emissions inventories1,3,4.
A tremendous quantity of cement has been produced worldwide
for the construction of buildings and infrastructure, namely:
76.2 billion tons of cement between 1930 and 2013, and 4.0 billion
tons in 2013 alone1. When making cement, the high-temperature
calcination of carbonate minerals (for example, limestone rocks)
produces clinker (mainly calcium oxide), and CO2is released
into the atmosphere from this process. These ‘process’ CO2
emissions from cement production (as opposed to related emissions
from fossil-fuel energy that may have been used during cement
production) comprise approximately 90% of global CO2emissions
from all industrial processes and 5% of global CO2emissions
from industrial processes and burning fossil fuels combined2–4.
Cumulative cement process emissions are estimated to have released
38.2 Gt CO2from 1930 to 20132–4.
However, the calcium oxide in cement materials is not stable
over time, and cement hydration products gradually reabsorb
atmospheric CO2through a physiochemical process called
carbonation5–8. Carbonation occurs when CO2diffuses into the
pores of cement-based materials and reacts with hydrated products
in the presence of pore water8,9 (see Methods). The carbonation
starts at the surface of the concrete or mortar and progressively
moves inwards. Although carbonation reactions are known to
civil engineers due to their effects on the strength and safety of
structures5,10, the resulting large-scale CO2uptake flux has not
been quantified. In contrast to the instantaneous emissions of CO2
during manufacture of cements, carbonation is a slow process
that takes place throughout the entire life cycle of cement-based
materials5,11. The CO2uptake through carbonation of cement
materials is thus proportional to the time-integral of cement
consumption. Previous studies have applied a life cycle assessment
to estimate concrete carbon sequestration over 100–200-year
timescales5,11,12. However, these studies were limited to concrete
materials in specific regions, and did not account for CO2uptake
in other types of cement materials found in built infrastructure:
cement mortar, construction cement waste, and cement kiln
dust worldwide.
Based on new data sets compiled from field surveys in China
and a comprehensive synthesis of existing data and studies (see
Methods), we modelled the global atmospheric CO2uptake by four
different cement materials (concrete, mortar, construction cement
waste, and cement kiln dust) between 1930 and 2013 in four regions
(China, the US, Europe, and the rest of the world) and analysed
the sensitivity of our uptake estimates to 26 different variables
(see Methods).
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1Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China. 2Key Laboratory of Pollution Ecology and Environmental Engineering,
Chinese Academy of Sciences, Shenyang 110016, China. 3College of Architecture and Urban Planning, Shenyang Jianzhu University, Shenyang 110168,
China. 4University of California, Irvine, Department of Earth System Science, Irvine, California 92697, USA. 5Laboratoire des Sciences du Climat et de
l’Environnement, CEA-CNRS-UVSQ, CE Orme des 14 Merisiers, 91191 Gif sur Yvette Cedex, France. 6Cambridge Centre for Climate Change Mitigation
Research, Department of Land Economy, University of Cambridge, 19 Silver Street, Cambridge CB3 9EP, UK. 7School of International Development,
University of East Anglia, Norwich NR4 7TJ, UK. 8Danish Technological Institute, Gregersensvej, 2630 Taastrup, Denmark. 9College of Economy and
Management, Shenyang Agricultural University, Shenyang 110000, China. 10CAS Key Laboratory of Low-carbon Conversion Science and Engineering,
Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, China. 11 Department of Plant Architectural Engineering, Kyonggi
University, Gwanggyosan-ro 154-42, Yeongtong-gu, Suwon, Kyonggi-do 16227, South Korea. 12 Swedish Cement and Concrete Research Institute, CBI
Betonginstitutet, SE-100 44 Stockholm, Sweden. 13Department of Chemistry, University of Aberdeen, Aberdeen AB24 3UE, UK. 14 Eduardo Torroja Institute
CSIC, Serrano Galvache 4, 28033 Madrid, Spain. 15Shenyang Pharmaceutical University, Shenyang 110016, China. 16Resnick Sustainability Institute,
California Institute of Technology, Pasadena, California 91125, USA. 17 John F. Kennedy School of Government, Harvard University, Cambridge,
Massachusetts 02138, USA. *e-mail: zhuliu@caltech.edu
880 NATURE GEOSCIENCE | VOL 9 | DECEMBER 2016 | www.nature.com/naturegeoscience
NATURE GEOSCIENCE DOI: 10.1038/NGEO2840 LETTERS
Annual uptake (GtC per year)
−0.20
−0.10
−0.05
−0.15
0.00
0.05
a
−0.25
19501930
Year
19901970 2013
Annual uptake (GtC per year)
−0.20
−0.10
−0.05
−0.15
0.00
0.05
−0.25
19501930
Year
19901970 2013
Annual uptake (GtC per year)
−0.20
−0.10
−0.05
−0.15
0.00
0.05
−0.25
19501930
Year
19901970 2013
1.4
0.3 1.7
1.2 1.2
0.2
0.3
2.7
2.4
0.9
0.6
0.4
0.2
<0.1 0.1
<0.1
China
United States
Europe (including Russia)
ROW
Concrete
Mortar
Waste
Cement kiln dust
1980s
1990s
2000−2013
1950s
1960s
1970s
1930s
1940s
Year of production
Cement materials
Region
bc
Figure 1 | Annual carbon sequestration by cement 1930–2013. a–c, Worldwide annual uptake of atmospheric CO2by cement, disaggregated by regions
(a), by cement materials (b) and by years of which the cement produced (c). The numbers in each panel indicate the cumulative carbon sequestration
(median values from our uncertainty analysis). ROW, rest of world.
Details of our calculations are available in the Methods. In
summary, carbon sequestration from concrete was calculated
from three stages in the life cycle of this material: service life,
demolition, and secondary use of concrete waste5. In each case,
we estimated exposed surface areas5,10, thicknesses10,13 , exposure
conditions including atmospheric CO2concentrations in different
regions5,9,14,15, and exposure time5,16–18, then modelled carbon uptake
by applying Fick’s diffusion law15 and concrete carbonation rate
coefficients derived from both experimental measurements5,18,19
and an extensive review of relevant literature15,20. The effect of
different concrete strength classes, exposure conditions, additions,
and coatings were explicitly modelled5. Exposure time in service
life (t) was assumed to be the average building lifetime, ranging
from 35 to 70 years5,10–12, and carbon sequestration in demolition
and secondary use stages was modelled assuming a spheric concrete
shape for particles in waste21, with carbonation fractions affected by
waste concrete treatment methods, waste concrete particle size13,22,
and changing exposure conditions during phases of demolition
and either reuse or disposal13. The carbon sequestration from
mortar was calculated based on mortar utilization thickness23 and
annual carbonation depth using Fick’s diffusion law. The carbon
uptake from construction cement waste and cement kiln dust was
estimated using the generation rate and carbonation fraction24,25.
Model uncertainties and sensitivities to assumptions were evaluated
by a Monte Carlo analysis that varies 26 individual parameters over
100,000 iterations (see Methods).
We find that a large fraction of global cement process CO2
emissions, both cumulatively and annually in recent years, are
reabsorbed by carbonation of cement materials. Figure 1a shows
the annual carbon sequestration by cement materials between 1930
and 2013, disaggregated by world region. Based on our uncertainty
analysis, we find a mean estimated global carbon uptake by all
cement materials was 0.24 Gt C (2σ=±10.0%) in 2013. Prior to
1982, the majority of sequestration occurred in Europe and the US,
corresponding to the legacy carbon sink of cement building and
infrastructure built during the 1940s and 1950s (Fig. 1a,c). Since
1994, cement materials used in China have absorbed more CO2than
the other regions combined, due to its rapidly increasing cement
production (Fig. 1a). Mortar cement consistently sequestered the
most carbon, even though only ∼30% of cement is used in
mortar (Fig. 1b). This is because mortar is frequently applied in
thin decorative layers to the exterior of building structures, with
higher exposure surface areas to atmospheric CO2, and thus higher
50
%
68
80
90
95
Median
Net emissions
5.9
−0.50
−0.40
50
40
30
20
Annual fluxes (GtC per year)
1940 1950 1960 1970 19901930
0.10
0.30
0.60
a
Year
−0.20
0.40
2000 2013
0.00
1980
1940 1950 1960 1970 1990
1930
Year
2000 20131980
Sequestration
rate (%)
−0.10
−0.30
0.20
0.50
Process cement emissions
10.4
Uptake by
carbonating
cement
4.5
b
Figure 2 | Net cement emissions and annual sequestration rate
1930–2013. a, Between 1930 and 2013, 10.4 GtC was emitted by the
cement industrial process2(dashed black line). Over the same period,
however, carbonating cements absorbed 4.5 GtC (2.8–7.5GtC, p=0.05,
green lines), or 43% of the cumulative cement emissions. b, Existing
cement is thus a large and overlooked carbon sink, sequestering roughly
44% of cement emissions each year since 1980.
carbonation rate coefficients (see Supplementary Data)23. Despite a
relatively smaller exposure area, and therefore lower carbonation
rate coefficients, concrete is the second largest contributor to the
carbon sink because ∼70% of all produced cement is used in
concrete. Figure 1c shows the legacy effects of accumulating cement
stocks; on average, between 2000 and 2013, 25.0% of the carbon
sequestered each year was absorbed by cement materials produced
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881
LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2840
Rest of
World
Europe
United
States
China
Region
33%
25%
7%
In service
Landfill and recy.
Current life cycle
89%
6%
Demolished
5%
Concrete
Mortar
CKD
Loss
Cement materials
Sequestered
In atmosphere
Current status
43%
56%
35%
68%
27%
3%
2%
Figure 3 | Allocations of global historical cement process emissions 1930–2013. Between 1930 and 2013, 7%, 33%, 25% and 35% carbon dioxide
emissions from cement production are from United States, China, Europe, and rest of world, respectively (Region). The emissions are 68% from concrete,
27% from mortar, 2% from loss cement in construction stage and 3% from CKD generation (Cement materials). The emissions are 89% in service life
cement, 5% attributed to demolished cement, and 6% attributed to demolition cement landfill and recycling (Current life cycle). The emissions are 43%
are sequestered by cement materials and 57% are remaining in atmosphere (Current status).
more than five years earlier and 14% produced more than ten years
earlier. Demolition causes an increase in carbonation rates by
exposing large and fresh surfaces. Because the average 35-year
service lifetime of structures in China16 is shorter than the average
65–70 years in the US17 and Europe5, the turnover of cement with
respect to carbonation has been increasing over time, accelerating
the uptake of CO2(Fig. 1c).
Figure 2 shows the net annual CO2emissions related to industrial
process of cement production minus the estimated annual CO2
sequestration from carbonation of cement materials. Between 1990
and 2013, the annual carbon uptake has been increasing by 5.8%
per year on average, slightly faster than process cement emissions
over the same period (5.4% per year; Fig. 2a). Using a bookkeeping
model to estimate the carbon sink in established buildings and
infrastructure each year, we estimate that a cumulative amount
of 4.5 GtC (2.8–7.5, p=0.05) has been sequestered by cement
materials since 1930. The annual carbonation carbon sink increased
from 0.10 GtC yr−1in 1998 to 0.25 GtC yr−1in 2013, which is
consistent with previous estimations of carbon sequestration from
cement-based materials (0.1–0.2 GtC yr−1) from 1926 to 20087. In
total, we estimate that roughly 43% of the cumulative cement
process emissions of CO2produced between 1930 and 2013
have been reabsorbed by carbonating cement materials, with an
average of 44% of cement process emissions produced each year
between 1980 and 2013 offset by the annual cement carbonation
sink (Fig. 2b).
Figure 3 traces the cumulative cement process CO2emissions
between 1930 and 2013 according to regional production and use
of cement in different materials, and to the life cycle of each type
of materials. In the case of concrete, an average of 16.1% of the
initial emissions are absorbed during the service life of the material,
with an additional 1.4% being absorbed during the demolition of
cement structures and another 0.1% absorbed during the disposal
or reuse of the concrete waste. In the case of mortar cement, an
average of 97.9% of the annual initial emissions is absorbed during
the material’s service life, and the remaining 2.1% is absorbed in
the demolition stage (Fig. 3). Given expected demolition, waste
disposal, and reuse of cement materials from the large amount of
concrete structures and infrastructure built in the past half-century,
and the still-increasing cement consumption in China and other
developing countries, the carbon sink of cement materials can be
anticipated to increase in the future.
Although the Intergovernmental Panel on Climate Change
(IPCC) Guidelines for National Greenhouse Gas Inventories
provides methods for quantifying CO2emissions during the
cement production process, they do not consider carbon absorbed
by carbonation of cement materials. Furthermore, the rate of
sequestration by carbonating cement is increasing rapidly (by an
average of 5.8% per year during the period 1990–2013) as the
stock of cement buildings and infrastructure increases, ages and
gets demolished and disposed. The overall size of the cement
sink between 1930 and 2013 is significant for the global carbon
cycle. We estimate that the global carbon uptake by carbonating
cement materials in 2013 was approximately 2.5% of the global CO2
emissions from all industrial processes and fossil-fuel combustion in
the same year2, which is equivalent to 22.7% of the average net global
forest sink from 1990 to 200726. The cement carbon sink of China
alone in 2013 was about 0.14 GtC yr−1, which accounts for 54% to
74% of the average net annual carbon sink in terrestrial ecosystems
during the 1980s and 1990s27.
It is well known that the weathering of carbonate and silicate
materials removes CO2from the atmosphere on geologic timescales
(104years)28. However, the potential for removal by the weathering
of cement materials has only recently been recognized29. Our
results indicate that such enhanced weathering is already occurring
on a large scale; existing cement stocks worldwide sequester
approximately 1 billion tons of atmospheric CO2each year. Future
emissions inventories and carbon budgets may be improved by
including this cement sink. Moreover, efforts to mitigate CO2
emissions should prioritize the reduction of fossil-fuel emissions
over cement process emissions, given that produced cement entails
creation of concomitant carbon sink. Indeed, if carbon capture
and storage technology were applied to cement process emissions,
the produced cements might represent a source of negative CO2
emissions30. Finally, policymakers might productively investigate
ways to increase the completeness and rate of carbonation of cement
waste (for example, as a part of an enhanced weathering scheme)31
to further reduce the climate impacts of cement emissions.
882
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NATURE GEOSCIENCE | VOL 9 | DECEMBER 2016 | www.nature.com/naturegeoscience
NATURE GEOSCIENCE DOI: 10.1038/NGEO2840 LETTERS
Methods
Methods, including statements of data availability and any
associated accession codes and references, are available in the
online version of this paper.
Received 17 July 2016; accepted 10 October 2016;
published online 21 November 2016
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Acknowledgements
Supported by the NSFC (41473076, 41501605, 51578344, 31100346, 41629501,
71533005), Fund of Youth Innovation Promotion Association, Chinese Academy of
Sciences, Fund of Fellowships for Young International Distinguished Scientists in
Institute of Applied Ecology, Chinese Academy of Sciences, National Key R&D Program
of China (2016YFA0602604), the UK Economic and Social Research Council funded
project ‘Dynamics of Green Growth in European and Chinese Cities’ (ES/L016028/1),
and the UK Natural Environment Research Council funded project ‘Integrated
assessment of the emission-health-socioeconomics nexus and air pollution mitigation
solutions and interventions in Beijing’ (NE/N00714X/1).
Author contributions
F.X. and Z.L. designed the paper. F.X. conceived the research. F.X., C.P., T.S., J.W., K.H.Y.,
L.B., I.G., C.A. and P.C. provided the data from different countries and regions. T.S., F.X.,
J.W. and Y.Z. provided the survey statistics and experimental measurements data. S.J.D.,
F.X., Z.L., P.C., M.S. and D.C.B. performed the analysis. C.P., J.L., Z.L., L.J., P.C., K.H.Y.,
L.B., I.G. and Y.Z. provided the reference data. F.X., L.B., D.C.B., Z.L. M.S. and T.S.
performed uncertainty analysis. S.J.D. and L.B. drew the figures. All authors contributed
to writing the paper.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints.
Correspondence and requests for materials should be addressed to Z.L.
Competing financial interests
The authors declare no competing financial interests.
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883
LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2840
Methods
Cement material carbonation. Civil engineers use the term ‘carbonation’ to
describe a complicated physicochemical reaction between CO2and hydrated
cement products in the presence of pore water, which ultimately sequesters carbon
in cement material15,32. In solution of pore water, CO2reacts with Ca(OH)2, and in
turn reacts with calcium silicate, dicalcium silicate, tricalcium silicate, tricalcium
aluminate and other hydrated products. The carbonation reactions start at the
surface of the cement or concrete and moves inwards over time10,33. The main
chemical reactions of carbonation are as follows:
Ca(OH)2+CO2
H2O
−→CaCO3+H2O
(3CaO ·2SiO2·3H2O)+3CO2
H2O
−→3CaCO3·2SiO2·3H2O
(2CaO ·SiO2)+2CO2+xH2OH2O
−→2CaCO3+SiO2·xH2O
(3CaO ·SiO2)+3CO2+xH2OH2O
−→3CaCO3+SiO2·xH2O
(3CaO ·Al2O3·6H2O)+3CO2
H2O
−→2Al(OH)3+3CaCO3+3H2O
Process model of cement carbonation. A life cycle assessment (LCA) method is
used to estimate carbon uptake by cement materials over time (see Supplementary
Information). Total carbon uptake of cement (Cu) is calculated as
Cu=XCon +XMor+XWaste +XCKD (1)
PCon: carbon uptake by concrete cement.
PMor: carbon uptake by mortar cement.
PWaste: carbon uptake by construction cement waste.
PCKD: carbon uptake by cement kiln dust (CKD).
Carbon uptake by concrete cement. The concrete life cycle divided into three phases:
service life (for example, in buildings), demolition and secondary use (including
both disposal in a landfill and recycling)5. In this case, we calculate CO2uptake as
XCon =Ctl
l+Ctd
d+Cts
s(2)
Ctl
l: carbon uptake during the service life.
Ctd
d: carbon uptake during the demolition.
Cts
s: carbon uptake during the secondary use stage.
Service life. Concrete categories. We further break down cement utilization for
different categories of concrete because the details of structure category are
important for assessing strength class, cement content, exposure condition,
exposed surface area, and service life5,10,34–36 .
Concrete strength classes. The strength classes of concretes are estimated based on
the survey statistics and previous studies in the US37,38, Europe and the rest of
world39 and Nordic countries5.
Concrete cement content. The cement content for concrete (Ci) is the mass of
cement used in one cubic metre of concrete (kgm−3)34,37,39–43 .
Exposure conditions, CO2concentrations, and additives. We estimate carbon
uptake under five different categories of exposure conditions: exposed, sheltered,
indoors, wet and buried5. Specifically, relative humidity, ambient CO2
concentration14,44 and additives have been shown to affect carbonation rate
coefficients9. The range of applicable conditions are estimated based on the
previously referenced, region-specific studies and survey statistics5,9,10,14 .
Coating and coverings. Application of surface coating and coverings such as paints
can reduce the rates of cement carbonation by 10–30%36,45. Based on previous
studies46–50, we assess carbonation using carbonation correction coefficients meant
to reflect the potential effects of coatings, including decreases in carbonation rates
of up to 50% over the life cycle of concretes51,52.
Concrete carbonation rates. Based on our estimates of concrete category, cement
content, exposure conditions, additives and coatings, we use relevant concrete
carbonation rate coefficients from various region-specific references5,10,19. We
further calculated concrete carbonation rate coefficients by considering the impacts
of compressive strength class and exposure conditions (βcsec)12 , cement additives
(βad)36 , CO2concentration (βCO2)9,14, and coating and cover (βCC )47,53, according
to ref. 5.
kli=βcsec ×βad ×βCO2×βcc (3)
Service life duration. The concrete service life (tl), the duration of the demolition
stage (td), and the duration of the secondary use stage (ts) are provided based on
the previous, region-specific references5,16–18,54.
Carbonation depth. The applicable carbonation rate coefficients and exposure
times are used to calculate the carbonation depth (di) of concrete in each strength
class and set of exposure conditions using Fick’s diffusion law (equation (4))5,
where kliis the carbonation rate coefficient of concrete in strength class iand tlis
the time of service life in years:
di=kli×√tl(4)
Exposed surface area. The exposed surface area (Ai) of concrete in the US, China,
Europe, and other countries based on average thickness of concrete structures are
listed in the literature5,10,21.
Volume of carbonated concrete in service life. The carbonated concrete volume Vi
is calculated as
Vi=di×Ai(5)
where Aiis exposed surface area and direpresents the product of carbonation rate
coefficient and carbonation depth for each concrete strength class i.
The carbonated cement in service life (Wli) can then be calculated as
Wli=
n
X
i=1
Vi×Ci(6)
where Ciis the cement content of concrete in different strength classes (kg
cement m−3)34,37,39–43. Next, we calculate the cumulative carbon uptake of
carbonated concrete in service life (Ctl
l)
Ctl
l=Wl×Cclinker ×fCaO ×γ×Mr(7)
where Cclinker is clinker to cement ratio ranged from 75% to 97% according to IPCC
guidelines of 1997 and 2006, fCaO is the average CaO content of clinker in cement
(65%, ranging from 60% to 67%)55,γis the proportion of CaO within fully
carbonated cement that converts to CaCO3(0.80, ranging from 0.50 to 1.00;
refs. 5,10–12,56,57), and Mris the ratio of C element to CaO (a constant equal to
the molar fraction in (CO2/CaO)×(C/CO2); 0.214)5.
The variables Cclinker,fCaO and Mrin the following equations (13), (21), (26),
(29), (32) and (33) are the same as in (7).
Annual carbon uptake by concrete in service. Finally, we combine the results of the
above calculations to calculate the annual carbon uptake in year tl(1Ctl
l) as the
cumulative carbon uptake in year tlminus the cumulative carbon uptake in year
tl−1:
1Ctl
l=XCtl
l−XC(tl−1)
l(8)
Demolition stage. During demolition, concrete structures are crushed into smaller
pieces so that contained steel reinforcing can be recycled and the concrete can be
more easily transported. The fate of demolition waste in different regions is taken
from different sources in the literature. In China, sources suggest that more than
97% of concrete waste is landfilled, with less than 3% recycled58. In contrast,
roughly 60% of concrete is recycled in the US, with the remaining 40% sent to
landfills17,22. Recycling rates are even higher in Europe, with data showing that
61.1% recycled and only 38.9% was sent to landfill5,21. Other studies indicate that
recycling rates in rest of world are quite low: about 25%18,59.
Size and surface area of waste concrete pieces. The surface area of concrete
pieces after demolition is difficult to estimate. We use available data to estimate a
range and particle size distribution of different types of demolished concrete in
each region59,60.
Exposure time. We estimate the average exposure time (td) of concrete during the
demolition stage is about 0.4 years in the whole world5,11–13. Almost all these
demolished and crushed concrete pieces are exposed to open air; only very small
proportions are stockpiled under shelter12,59.
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Carbonation of demolished concrete. We estimate the proportion (Fdi) of concrete
that will be carbonated during the demolition stage by assuming the shape of
concrete particles and pieces is spherical21. The carbonation fraction is calculated
according to particle size distributions and carbonation depths using Fick’s
diffusion law:
D0i=2ddi=2kdi×√td(9)
Fdi=
100% −Zb
a
π
6(D−D0i)3/Zb
a
π
6D3×100% (a≥D0i)
100% −Zb
D0
π
6(D−D0i)3/Zb
a
π
6D3×100% (a<D0i<b)
100% (b<D0i)
(10)
where Fdiis the fraction of demolished concrete in strength class ithat is
carbonated, D0iis the maximum diameter of particles that undergo full
carbonation in strength class i, ddithe carbonation depth of particles in strength
class i,kdiis the carbonation coefficient of concrete in strength class iin open air
exposure conditions, tdis the average time for the demolition stage, Dis the
diameter of demolished and crushed particles, aand bare the minimum and
maximum diameter of crushed concrete particles in a given size distribution. All
the particles less than D0iwill finish carbonation in tdyears or less, such that Fdiwill
be 100%. For particle sizes larger than D0i,Fdican be calculated by integration
(equation (10)).
Using the fraction of concrete that will undergo carbonation for equation (10),
we next calculate the mass of concrete cement carbonated during the demolition
stage (Wd) as
Wdi=(Wci−Wli)×Fdi(11)
Wd=
n
X
i=1
Wdi(12)
where Wdiis the concrete cement carbonated during demolition for each concrete
strength class i,Wciis the cement consumed for each strength class iof concrete,
Wliis the concrete cement carbonated during service life for each strength class i
(Wliin equation (6)), Fdiis the fraction of carbonated cement in concrete strength
class iin the demolition stage, and Wdis the total mass of concrete cement
carbonated in the demolition stage.
Total carbon uptake during demolition stage. Finally, we estimate total carbon
uptake during the demolition stage (Ctd
d) based on cement carbonated in
demolition stage and carbonation fraction of differently treated concretes:
Ctd
d=Wd×Cclinker ×fCaO ×γ×Mr(13)
Ctd
dis carbon uptake of concrete cement during demolition and γis the proportion
of CaO within fully carbonated concrete that converts to CaCO3. The other
parameters are the same as in equation (7).
Secondary use stage. After demolition, concrete materials continue to absorb
carbon dioxide during the secondary use stage. In sum, more than 91% of crushed
concrete particles worldwide are buried, either in landfills or as part of their
recycled use, such as for road base or backfill aggregates13,18,22,58,59.
Carbonation depth in secondary use stages. The carbonation rate coefficients of
waste concrete in the secondary use stage will be slow and decreasing due to the
layer of carbonated cement (ddi) that was during the demolition stage45 and the fact
that most of the concrete is buried and not exposed to the air5. The total
carbonation depth in the demolition stage and secondary use stage (dti) can be
estimated by carbonated depth in the demolition stage (ddi) plus the new
carbonation depth (dsi) during the secondary use stage. There is a time lag 1tifor
the same carbonation depth (ddi) from air exposure condition to buried condition
as follows:
ddi=Kdi×√tdi=Ksi×√tsi(14)
kdi×√tdi=ksi×√tsi+1ti(15)
1ti=tdi× k2
di
k2
si−1!(16)
The total carbonation depth in the demolition and secondary use stages dtican
then be calculated by
dti=ddi+dsi=ksi×√tsi+tdi+1ti(17)
ddi: carbonation depth at the end of the demolition stage.
kdi: carbonation rate coefficient in the demolition stage (exposed to air).
tdi: carbonation time for existing carbonated depth ddiduring the demolition stage.
ksi: carbonation rate coefficient of concrete particle in strength class iin the
secondary use stage (buried condition).
tsi: carbonation time for ddiif waste concrete in the secondary use stage (buried
condition).
1ti: time lag for the same carbonation depth (ddi) from buried condition to air
exposure condition.
dti: total carbonation depth in the demolition stage and secondary use stage.
Fraction carbonized. The carbonation fraction of cement in concrete rubble (Fsi)
during the secondary use stage is calculated as:
D1i=2dti=2Ksi×√tsi+tdi+1ti(18)
Fsi=
100% −Zb
a
π
6(D−Dti)3/Zb
a
π
6D3×100% −Fdi(a≥D1i)
100% −Zb
D1
π
6(D−Dti)3/Zb
a
π
6D3×100% −Fdi(a<D1i<b)
100% −Fdi(b<D1i)
(19)
where D1iis the maximum diameter of particles that undergo full carbonation in
strength class iin the demolition and secondary use stages, Dis the diameter of
demolished and crushed particles, Fdiis the fraction of carbonated waste concrete
particle in strength class iin the demolition stage, aand bare the minimum and
maximum diameter of crushed concrete particles in a given size distribution. All
the particles less than D1iwill finish carbonation in tsi+tdi+1tiyears, so there Fsi
is 100% - Fdi. The value of Fsifor particle size larger than D1ican be calculated by
integration.
Cumulative and annual carbon uptake during the secondary use stage. The
cumulative and annual carbon uptake in the secondary use stage can be calculated
by the following:
Wsi=Wci−Wli−Wdi(20)
Cts
s= n
X
i=1
Wsi×Fsi!×Cclinker ×fCaO ×γ×Mr(21)
1Cts
s=Cts
s−C(ts−1)
s(22)
where Wsiis the weight of cement used for strength class iin the secondary use
stage, Wciis the weight of cement used for strength class iin the building
construction stage, Wliis the carbonated concrete cement in strength class iin the
service stage, Wdiis carbonated concrete cement for concrete strength class iin the
demolition stage, Fsiis the fraction of carbonated strength class iconcrete cement
in the treatment and secondary use stage, γis proportion of CaO within fully
carbonated cement that converts CaO to CaCO3,Cts
sis the cumulative carbon
uptake in year ts, and C(ts−1)
sis the cumulative total carbon uptake in year ts−1,
1Cts
sis annual carbon uptake in year tsin the secondary use stage. The other
parameters are the same as in equation (7).
Carbon uptake by mortar cement. Cement utilization for mortars. Cement mortar is
used for rendering and plastering (that is, decorating), masonry (brick-laying),
maintenance and repairing of concrete structures, and various other
applications23,61,62. Most mortar is used for rendering, plastering and decorating61.
The typical thickness of cement mortar utilization. Rendering and plastering
mortar is usually applied in a thickness of 10–30 mm and decorating (finishing)
mortar is typically much thinner, only 1–5 mm23,61. When used as tile adhesive or
grout, mortar is typically applied in thicknesses of 15–30mm and 3–30 mm,
respectively23. For self-levelling under layers, thicknesses vary from 5 to 30 mm,
and the thickness of mortar for screeds is 30–80 mm61,62. Most of these cement
mortar thicknesses are about 20 mm23,33,45. The thickness of mortar for masonry is
about 10 mm, except for a small proportion in 2–3 mm for very even blocks23.
Mortar used for maintaining and repairing (that is, patching concrete structures
and building surfaces) is applied similarly to rendering and adhesive uses, with a
mean thickness of 25 mm.
Carbonation rate coefficients of cement mortar. Cement mortars have been shown
to undergo carbonation at a faster rate than concrete63,64. The carbonation rate
coefficients of cement mortar are between 6.1mm yr−1/2and 36.8 mm yr−1/2in
outdoor and indoor exposure conditions, respectively (in temperate climate
conditions and according to our field survey and experiment data using the 1%
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alcohol phenolphthalein solution). Carbonation depth will increase if the cement
contains more additives48. In this study, we use an average carbonation rate for
mortar of 19.6 mmyr−1/2, but evaluate uptake assuming the full range
6.1 mm yr−1/2and 36.8 mm yr−1/2.
Carbon uptake by mortar cements. We calculate annual carbon uptake based on
the proportion of annual carbonation depth65, and estimate carbon uptake as the
sum of uptake by rendering and plastering mortar (Crpt), uptake of masonry mortar
(Crmt), and uptake of maintaining and repairing mortar (Crmat):
XMor=Crpt +Crmt +Crmat (23)
Annual carbonation of cement in mortar used for rendering, plastering, and
decorating is calculated by
drp =Km×√t(24)
frpt=(drpt −drp(t−1))/dTrp ×100% (25)
Crpt =Wm×rrp ×frpt×Cclinker ×fCaO ×γ1×Mr(26)
where drp is the carbonation depth of rendering mortar, Kmis the carbonation rate
coefficient of cement mortar, drptand drp(t−1)are the carbonation depths in years
tand (t−1), respectively, dTrp is the utilization thickness of rendering mortar, frptis
the annual carbonation percentage of cement mortar for rendering, Crptis the
annual carbon uptake of carbonated mortar cement for rendering. γ1is the
proportion of CaO within fully carbonated mortar cement that converts to CaCO3,
Wmis the cement for mortar and rrp is the percentage of cement for rendering in
mortar cement.
Carbon uptake of repairing and maintaining cement mortar. Annual carbon uptake
of cement mortar for repairing and maintaining is calculated by
drm =Km×√t(27)
frmt=(drmt−drm(t−1))/dTrm ×100% (28)
Crmt =Wm×rrr ×frmt×Cclinker ×fCaO ×γ1×Mr(29)
where drm is the carbonation depth of repairing and maintaining mortar, drmtand
drm(t−1)are the carbonation depths in years tand (t−1), respectively. dTrm is the
utilization thickness of repairing and maintaining mortar. fr mtis the annual
carbonation percentage of cement for repairing and maintaining mortar. Crmtis the
annual carbon uptake of carbonated mortar cement for repairing and maintaining.
rrr is the percentage of cement for repairing and maintaining in mortar cement. γ1
is the proportion of CaO within fully carbonated mortar cement that converts to
CaCO3.
The carbon uptake by masonry cement mortar can is calculated as
Crmat =Cmbt +Cmot +Cmnt (30)
where Cmbtis carbon uptake by masonry mortar of walls with both sides rendered,
Cmotis carbon uptake by masonry mortar of walls with one side rendered, and Cmnt
is carbon uptake by masonry mortar of walls with no rendering. The carbon uptake
calculation method of Cmbt,Cmot, and Cmntis similar as that of rendering and
plastering mortar by considering wall thickness and demolition effects.
Carbon uptake by cement in construction wastes. We estimate carbon uptake of
construction waste24,54 by
X
waste =Cwastecon +Cwastemor (31)
Cwastecon = n
X
i=1
Wci×fcon ×rcont!×Cclinker ×fCaO ×γ×Mr(32)
Cwastemor = n
X
i=1
Wmi×fmor ×rmor!×Cclinker ×fCaO ×γ1×Mr(33)
where Cwastecon and Cwastemor are carbon uptake by construction waste concrete and
construction waste mortar, respectively. Wciis cement used for concrete in strength
class i,fcon is loss rate of cement for concrete in the construction stage24,40,rcontis
annual carbonation fraction of construction waste concrete, γis proportion of CaO
within fully carbonated concrete that converts CaO to CaCO3,Wmiis cement used
for mortar in strength class i,fmor is loss rate of cement for mortar54,66,rmor is annual
carbonation fraction of construction waste mortar. γ1is the proportion of CaO
within fully carbonated mortar cement that converts to CaCO3.
Carbon uptake by cement kiln dust. We estimate carbon uptake by CKD in
different regions67–69 of the world based on the cement production, CKD
generation rate, and proportion of CKD treatment in landfill (Supplementary
Data 4) as follows:
X
CKD = n
X
i=1
Wi×Cclinker ×rCKD ×rlandfill!×f2CaO ×γ2×Mr(34)
where Wiis the cement production in region i,rCKD is the CKD generation rate
based on clinker68,rlandfill is proportion of CKD treatment in landfill, f2CaO is CaO
proportion in CKD70, and γ2is the fraction of CaO within fully carbonated CKD
that has been converted to CaCO3.
Uncertainty analysis. We use a Monte Carlo method as recommended by the 2006
IPCC guidelines for National Greenhouse Gas Inventories to evaluate uncertainty
of CO2removal due to cement material carbonation71. We identify 26 causes of
uncertainties associated with carbon sequestration estimates, which we vary across
wide ranges to estimate the implications for carbon uptake (see Supplementary
Information). The mean value carbon uptake from global cement materials is
0.25 Gt C (2σstandard deviation of 10.03%) in 2013.
Data availability. The authors declare that the data supporting the findings of this
study are available within the Article and its Supplementary Information files.
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