CEN/TR 17310:2019
(Main)Carbonation and CO2 uptake in concrete
Carbonation and CO2 uptake in concrete
This document provides detailed guidance on the carbonation and carbon dioxide (CO2) uptake in concrete. This guidance is complementary to that provided in EN 16757, Product Category Rules for concrete and concrete elements, Annex BB.
Typical CO2 uptake values for a range of structures exposed to various environmental conditions are presented. These values can be incorporated into EPDs for the whole life cycle for either: a functional unit, one tonne or one m³ of concrete, without necessarily having any detailed knowledge of the structure to be built.
In the rest of the document, the data will be given per m³.
Carbonatisierung und CO2-Aufnahme von Beton
Dieses Dokument gibt ausführliche Hinweise zur Karbonatisierung und Kohlenstoffdioxid(CO2)-Aufnahme von Beton. Diese Hinweise ergänzen die in EN 16757, Produktkategorieregeln für Beton und Betonelemente, Anhang BB, angegebenen Hinweise.
Es werden typische Werte für die CO2-Aufnahme einer Reihe von Tragwerken, die unterschiedlichen Umweltbedingungen ausgesetzt sind, dargestellt. Diese Werte können in die EPD für den gesamten Lebenszyklus entweder einer funktionellen Einheit, einer Tonne oder eines Kubikmeters (m3) Beton einbezogen werden, ohne detaillierte Kenntnisse über das zu errichtende Tragwerk haben zu müssen.
In allen anderen Teilen des Dokuments werden die Daten je m3 angegeben.
Carbonatation et absorption du CO2 dans le béton
Le présent document fournit des recommandations détaillées concernant la carbonatation et l’absorption du dioxyde de carbone (CO2) dans le béton. Ces lignes directrices viennent en complément de celles de l’EN 16757, Règles régissant la catégorie de produits pour le béton et les éléments en béton, Annexe BB.
Les valeurs typiques d’absorption du CO2 sont présentées pour diverses structures soumises à différentes conditions environnementales. Ces valeurs peuvent être prises en compte dans les DEP pour l’ensemble du cycle de vie pour une unité fonctionnelle, une tonne ou un m3 de béton, sans connaître nécessairement tous les détails de la structure à construire.
Dans tout le reste du présent document, les données seront indiquées par m3.
Karbonatizacija in absorpcija CO2 v beton
Ta dokument podaja podrobne smernice o karbonatizaciji in absorpciji ogljikovega dioksida (CO2) v beton. Te smernice dopolnjujejo smernice iz standarda EN 16757: Pravila za kategorije proizvodov za beton in betonske elemente, dodatek BB.
Navedene so značilne vrednosti absorpcije CO2 za nabor konstrukcij, izpostavljenih različnim okoljskim pogojem. Te vrednosti je mogoče vključiti v okoljske deklaracije za proizvode za celoten življenjski cikel za funkcionalno enoto, eno tono ali en kvadratni meter betona, ne da bi bilo nujno potrebno podrobno poznavanje konstrukcije za izgradnjo.
V preostalih delih dokumenta bodo podatki podani v kvadratnih metrih.
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
01-maj-2019
Karbonatizacija in absorpcija CO2 v beton
Carbonation and CO2 uptake in concrete
Karbonatisierung und CO2-Aufnahme von Beton
Carbonatation et absorption du CO2 dans le béton
Ta slovenski standard je istoveten z: CEN/TR 17310:2019
ICS:
91.100.30 Beton in betonski izdelki Concrete and concrete
products
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
CEN/TR 17310
TECHNICAL REPORT
RAPPORT TECHNIQUE
January 2019
TECHNISCHER BERICHT
ICS 91.100.30
English Version
Carbonation and CO uptake in concrete
Carbonatation et absorption du CO dans le béton Karbonatisierung und CO -Aufnahme von Beton
2 2
This Technical Report was approved by CEN on 30 December 2018. It has been drawn up by the Technical Committee CEN/TC
104.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATIO N
EUROPÄISCHES KOMITEE FÜR NORMUN G
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 17310:2019 E
worldwide for CEN national Members.
Contents Page
European foreword . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Carbonation, the uptake of carbon dioxide . 5
4.1 Compounds, chemistry and notation . 5
4.2 Carbonation . 6
4.2.1 Carbonation reactions . 6
4.2.2 Process of carbonation . 7
4.2.3 Degree of carbonation . 8
4.2.4 Effect of carbonation on cement paste structure. 10
4.2.5 Carbonation rate . 11
4.2.6 Carbonation rate controlling factors . 11
4.2.7 Carbonation rate of concrete with blended cements or with additions. 15
4.3 CO binding capacity in concrete, Degree of carbonation . 16
4.3.1 General . 16
4.3.2 Theoretical binding capacity of Portland cement . 17
4.3.3 Normal binding capacity of Portland cement . 17
4.3.4 Normal binding capacity of blended cements. 18
4.4 Carbonation in different environments . 19
4.4.1 General . 19
4.4.2 Dry indoor concrete . 19
4.4.3 Concrete exposed to rain. . 20
4.4.4 Concrete sheltered from rain . 20
4.4.5 Wet or submerged concrete . 20
4.4.6 Buried concrete . 21
5 Practical experiences of CO uptake in concrete life stages . 21
5.1 CO uptake during product stage (module A) . 21
5.2 CO uptake during use stage (module B) . 22
5.3 CO uptake during end of life stage . 29
5.3.1 CO uptake during end of life stage – demolition, crushing and waste handling
(module C1-C3) . 29
5.3.2 CO uptake during end of life stage – landfill (module C4) . 32
5.4 CO uptake beyond the system boundary (module D) . 32
6 Figures for "direct estimation” of CO uptake in whole structures during use stage . 33
6.1 General . 33
6.1.1 General . 33
6.1.2 CO uptake for a portal frame bridge . 34
6.1.3 CO uptake for a residential building . 35
6.2 Average CO uptake for construction types, strength classes and exposure . 36
7 Additional information . 37
7.1 CO uptake in the long term, beyond the service life of the structure . 37
7.2 CO uptake of crushed concrete in new applications . 38
8 Society perspective – Carbonation and CO uptake in mortar . 38
9 National calculation models and methods . 39
9.1 General . 39
9.2 Calculation of Carbonation of concrete in use phase (Swiss approach) . 39
9.2.1 General . 39
9.2.2 Water/CaO . 39
9.2.3 CO concentration, relative humidity and CO buffer capacity . 39
2 2
9.2.4 A simple approach of assessing the CO uptake of concrete components . 40
9.2.5 Ratio of CO uptake/CO emission as a function of thickness of concrete element . 43
2 2
Bibliography . 45
European foreword
This document (CEN/TR 17310:2019) has been prepared by Technical Committee CEN/TC 104
“Concrete and related products”, the secretariat of which is held by SN.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
1 Scope
This document provides detailed guidance on the carbonation and carbon dioxide (CO ) uptake in
concrete. This guidance is complementary to that provided in EN 16757, Product Category Rules for
concrete and concrete elements, Annex BB.
Typical CO uptake values for a range of structures exposed to various environmental conditions are
presented. These values can be incorporated into EPDs for the whole life cycle for either: a functional
unit, one tonne or one m of concrete, without necessarily having any detailed knowledge of the
structure to be built.
In the rest of the document, the data will be given per m .
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 Carbonation, the uptake of carbon dioxide
4.1 Compounds, chemistry and notation
4.1.1 Carbon dioxide: Chemically expressed as CO and present in the atmosphere as a gas. When
CO is dissolved in water, H O, it may form carbonic acid, H CO , where this may release carbonate,
2 2 2 3
2− −
CO , and bicarbonate, HCO ions.
3 3
4.1.2 Calcium hydroxide: Chemically expressed as Ca(OH) and often called Portlandite. It is a
product of the hydration of Portland cement and is always present in concrete. For simplicity, cement
chemists often denote calcium hydroxide as CH. Calcium hydroxide is not very soluble in water but is
+ −
does dissolve to the ions Ca and 2OH . The presence of calcium hydroxide in concrete is largely
responsible for maintaining its alkaline environment, which is at a pH around 12,5. Around 25 % of
hardened hydrated cement is Ca(OH) .
4.1.3 Calcium oxide: Chemically expressed as CaO. Portland cement clinker contains 61 % to 67 %
CaO by oxide analysis, and where typically the assumed value is 65 %. Nearly all the calcium oxide in
Portland cement is not present as calcium oxide but as part of more complicated compounds such as
di-calcium silicates, tri-calcium silicates, tri-calcium aluminate and tetra-calcium alumina ferrite.
Fortunately using the oxide analysis figure of 65 % CaO is sufficient for the calculation of potential
carbonation without going into the more complex chemistry.
4.1.4 Calcium silicate hydrates, and other hydration products: When Portland cement reacts with
water, that is when it hydrates, it forms calcium hydroxide and a larger proportion of complex
hydration products where the bulk of these are made up of calcium and silica. The hydration products,
or gel as described by concrete technologists, are called calcium-silica-hydrates, often simplified to CSH.
For a typical composition of hardened hydrated cement it is assumed 50 % is CSH, around 25 % is
calcium hydroxide, 10% calcium monosulfoaluminate-AFm, 10 % ettringite-AFt leaving 5 % undefined.
4.1.5 Calcium carbonate: Chemically expressed as CaCO , normally present in concrete as calcite.
4.2 Carbonation
4.2.1 Carbonation reactions
The reaction between carbon dioxide and calcium hydroxide, to form calcium carbonate, CaCO , and
water, H O, is called carbonation. The reaction can be expressed as a formula:
CO + Ca OH → CaCO + H O
(1)
( )
2 3 2
This formula makes it appear that the reaction is simply carbon dioxide as a gas reacting with calcium
hydroxide as a solid, but in reality the kinetics of the reaction are more complicated in that moisture
2− −
must be present for the intermediate stages, where carbonate, CO , bicarbonate, HCO , calcium
3 3
+ −
Ca and hydroxide 2OH ions interact.
In addition to carbonation of calcium hydroxide directly, the calcium-silicate-hydrates also carbonate.
This is a complex reaction where CSH is made up of short silica chains bound together by calcium ions,
2+ −
Ca , and hydroxide, OH , ions with the water more or less firmly bound. The makeup of CSH can be
characterized by its calcium to silica ratio, expressed as either CaO/SiO or Ca/Si, where the
2+
un-carbonated value is around 1,85. As carbonation lowers Ca content of the pore solution this is
2+
compensated by the release of Ca from the CSH [6], and its Ca/Si ratio drops to around 0,85. As the
Ca/Si ratio falls below 1, and the pH is reduces to around 10, the CSH transforms into a silica gel but
where there is always some calcium in the silica gel.
Carbonation is the neutralization of the natural alkalinity of hydrated cement in concrete, reducing the
pH from around 12,5 to below 9 and possibly as low as 8. The pH of freshly exposed surfaces of concrete
can be assessed by the application of an indicator. Phenolphthalein is particularly useful as it remains
clear when sprayed onto concrete where the pH is below 9, but is purple for higher pH, as shown in
Figure 1. The concrete shown in Figure 1 is a high quality concrete and indicates that the pH is above 9
except for the very outer surface of the concrete.
Figure 1 — Phenolphthalein indicator as used on a freshly exposed surface of good quality
concrete
4.2.2 Process of carbonation
Carbonation starts at the concrete surface as this is where it is first exposed to carbon dioxide gas in the
atmosphere. Carbonation also occurs where concrete is in contact with water containing soluble carbon
compounds such as carbonic acid, bicarbonate and carbonate ions. Carbonation under water is slower
than in air because ion diffusion through water is slower than CO gas through concrete pores. There
are two simultaneous processes:
— the transport and dissolution of any CO gas and soluble carbon compounds into the concrete from
the surface;
— the reaction of the soluble carbon compounds with the hydrated cement to produce calcium
carbonate. The precipitated calcium carbonate is normally calcite but may be vaterite or aragonite.
The demarcation between carbonated concrete and uncarbonated concrete is called the carbonation
front, and the depth of the carbonation is generally assumed to be the distance from the surface of the
concrete to the purple zone revealed by the application of phenolphthalein. Figure 2 is a diagrammatic
representation of carbonation through concrete.
a) uncarbonated concrete showing an air void, pore solution and hydrated cement
b) the progression of CO through air void and pore solution to carbonate concrete
Key
1 air void in pore 4 carbonation front
2 pore solution 5 carbonation depth
3 hydrated phases
NOTE The red line is the limit of the phenolphthalein colour change.
Figure 2 — Progression of carbonation through concrete
4.2.3 Degree of carbonation
It is important to note that the phenolphthalein colour change corresponds to areas of low pH, where
some of the calcium hydroxide and CSH has carbonated but not that they have totally carbonated. The
degree of carbonation is generally defined as the percentage of reactive CaO present in hydrated cement
in concrete that is converted to CaCO , or the CO bound in concrete related to the CO emitted during
3 2 2
calcination in the carbonated zone of the concrete (not taking into account CO emitted by fuel),both
giving the same results.
This figure has to be distinguished of the percentage of CO bound in the total functional unit related to
the CO emitted by carbonation, this percentage taking into account the not carbonated zone.
In practice the degree of carbonation can only be determined by sophisticated techniques such as
thermogravimetric analysis (TG).
There is no precise relationship between carbonation depth as determined by phenolphthalein and the
degree of carbonation expressed as the amount of CO as percentage of calcination emissions
(Figure 3).
Key
X 0,5 old samples
k (mm/year )
Y degree of carbonation (%) outdoor sheltered – 0,6
outdoor non sheltered – 0,6 outdoor sheltered – 0,45
indoor – 0,6 indoor – 0,45
outdoor sheltered – 0,45
Figure 3 — Relationship between carbonation rate and the degree of carbonation for interior,
sheltered and non-sheltered concrete, after [13]
NOTE Due to the low content in clinker of CEM III the carbonation degree is higher than 100 % because the
slag carbonates. It happens also with other mineral additions. The degree of carbonation is only with respect to
emissions due to the clinker.
It is known from a reviews of laboratory testing, such as those by Parrot, 1987, that the depth of
carbonation is a maximum where the relative humidity is around 60 %. At lower RH the lack of
moisture inhibits the ability of CO to dissolve and react, whereas at higher RH there is so much
moisture that it prevents the permeation of CO gas through the concrete pore system. For
Northern-Europe external sheltered exposure approximates to RH 60 %, and so it is not surprising that
the measurements shown in Figure 3 confirm the highest depth and degree of carbonation. It is
expected that drier internal conditions or wetter un-sheltered exposure will give lower depths of
carbonation.
Tables 1 and 2 show semi quantitative x-ray diffraction of a cement paste taken from indoor concrete
(Table 1) and outdoor concrete (Table 2) [4]. The measurements are taken at various depths as
tabulated, where the results are semi quantitative because the materials are identified from spot
counting sample areas without reconciling with the actual proportions in the concrete. The depth of
carbonation was determined using phenolphthalein indicator on a split concrete cylinder, where for
indoor concrete the depth of carbonation is at 35 mm, and for outdoor concrete around 20 mm. In the
tables carbonated is indicated as ‘Carb.’, at the carbonation front ‘Carb. Front’ and where it is assumed
the concrete is at a depth largely uncarbonated identified as 'intact'.
Table 1 — Semi quantitative x-ray diffraction of a cement paste taken from indoor concrete
(10 to (15 to (40 to
(0 to 5) mm (5 to 10) mm
15) mm 40) mm 50) mm
Material identified
Carb. Carb.
Carb. Carb. front Intact
Alite 11 0 0 0 0
Alite+Belite 513 250 92 98 72
Portlandite, CH 0 291 1 037 1 794 2 115
Calcite 827 529 761 456 121
Quartz 3 707 4 125 4 309 2 234 1 335
Feldspar 1 010 1 310 1 289 865 828
Table 2 — Semi quantitative x-ray diffraction of a cement paste taken from outdoor concrete
(0 to 7) mm (7 to 16) mm (16 to 25) mm (25 to 50) mm
Material identified
Carb. Carb. Carb. front Intact
Alite 0 0 0 0
Alite+Belite 59 66 121 123
Portlandite, CH 0 0 912 2 443
Calcite 2 382 2 919 1 179 0
Quartz 3 201 2 318 2 871 2 138
Feldspar 1 059 869 2 084 736
The materials identified are alite, alite+belite, portlandite, calcite, quartz and feldspar. Alite and
alite+belite are compounds found in unhydrated Portland cement, where there will always be some of
these materials present in concrete however old, although the amount will reduce with time if there is
water present for continued hydration. Portlandite and calcite are as described before, where a
comparative reduction in portlandite and an increase in caltite means the concrete is carbonatining.
Quartz and feldspar are minerals from the aggregate.
From Table 1 it is evident that the carbonated zone as indicated by phenolphthalein contains significant
amounts of both portlandite, and unhydrated cement grains as shown by the presence of alite and
alite+belite. In outdoor concrete some unhydrated cement is present close to the surface but the
portlandite is carbonated to the carbonation front.
The measurements indicate that the degree of carbonation in the drier indoor concrete is significantly
lower than the wetter outdoor concrete.
From wider research it is evident that outdoor concrete, generally with a lower depth of carbonation
than drier indoor concrete, the degree of carbonation is higher at around 75 % to 90 % [2], [4], [7], [15].
4.2.4 Effect of carbonation on cement paste structure
The transformation of portlandite to calcite represent a volume change of 11 %, but the theoretical
volume change from transformation of CSH to calcite is less certain and will depend on the water
content. It is known that the volume changes do not affect the mechanical stability of the carbonated
zone, it is stable and hard. Cracks induced by carbonation are rare although there may be microcracks.
Normally the calcite precipitated just fills the available space in the capillary system and thus densifies
the cement paste locally.
In general the effect of carbonation on porosity will depend on the locations and mode of calcite
precipitation as well as the type of cement. As diffusion and transport in cement paste is mainly through
the capillary pores then precipitation in these pores will diminish flow.
There is evidence that concrete made with Portland slag cement produces a coarser pore system when
carbonated, then that of a pure Portland cement concrete. As the slag cement paste contains less
calcium hydroxide and more CSH it will affect the carbonation front, and generally increase the rate of
carbonation.
Although the carbonation may appear to be quite simple in practice the process is complex in that the
concrete quality, cement type and carbonation will affect the structure, and changes in the structure will
affect the rate and degree of carbonation.
4.2.5 Carbonation rate
The rate at which concrete carbonates reduces with time. This is because carbonation is a diffusive
process, that is as carbonation progresses the carbon dioxide has further to travel to carbonate the
interior of the concrete In addition the precipitation of calcite may reduce the penetrability of the
concrete. For all practical purposes this can be described by the following expression:
d = kt (2)
c
where
d = depth of carbonation;
c
k = rate;
t = time.
The rate of carbonation, "k", includes the effects of transport and reaction and enable the depth of
carbonation to be calculated at any time. To calculate the amount of CO combined within the concrete
other models are required to cover transport and reaction separately.
The rate of carbonation is influenced by many factors. Apart from the amount of CO in the atmosphere
it also depends on; the size, geometry and interconnectivity of pores, as well as t the type of cement or
binder, the relative humidity and other factors.
4.2.6 Carbonation rate controlling factors
4.2.6.1 Humidity and temperature
A very dry concrete does not carbonate as water is needed for ions to form and subsequently react with
carbon hydroxide or CSH and form calcite. In normally dry concrete, say RH around 50 % to 85 %, the
rate of carbonation depends largely on gas diffusion where the driving mechanism in stagnant air is the
difference in partial pressure between the surface and the interior of the concrete. The CO gas then
dissolves in the pore water and consumed at the carbonation front. In wetter concrete, say RH > 85 %,
the rate of carbonation is controlled by ion diffusion in the pore solution, and this is slower than the
permeation of CO as a gas. Figure 4 shows the influence of relative humidity, water/cement (w/c)
ratio, and temperature upon depth of carbonation at sixteen years, after [5].
Key
X relative humidity in %
Y
average depth of carbonation d , in mm
x
Figure 4 — Influence of relative humidity, w/c ratio, and temperature upon depth of carbonation
at sixteen years, after [5]
From Figure 4 it is evident that there is a peak rate of hydration between 55 % to 80 % RH.
The effect of temperature on carbonation is unclear, probably due to the inter-relationship with relative
humidity, and is generally considered negligible for practical purposes [16].
4.2.6.2 Binder content
For concretes containing natural or normal weight aggregates CO diffusion will be through the cement
paste and not through aggregate. In itself the amount of cement does not influence the rate of
carbonation provided the w/c ratio is constant. This is due to fact that the flow is measured as material
passing through a unit area, where increasing the cement content just increase the total area but where
the rate of flow and hence carbonation will be the same. In order to estimate the volume of carbonated
paste the amount of cement in the concrete must be known.
The effect of different cement, CEM I, content on carbonation depth can be estimated with the help of a
diagram in [34], showing the effect on carbonation depth as a function of time for different amounts of
CaO available for carbonation. This shows that for normal ranges of cement content, say ±40 kg/m for
w/c ratio of 0,50, the variation in the depth of carbonation and k-value can be estimated to within
±10 %. This is small compared to the variation depending of strength, w/c ratio and exposure.
4.2.6.3 Concrete strength
In general terms concrete with a lower w/c ratio, or a lower water/binder ratio will have a higher
strength and enhanced durability. In addition the longer a concrete is allowed to cure in moist
conditions the higher the degree of hydration and the greater the strength. The higher degree of
hydration leads to a lower penetrability to fluids, and hence a lower rate of carbonation for all
environments. It is for these reasons that strength is a rough indicator for the resistance to carbonation,
as shown in Figure 5.
a) w/c = 0,6 – 300 kg/m building
b) w/c = 0,45 – 400 kg/m building
Key
X compression strength (Mpa) sheltered
Y 0,5 indoors
k (mm/year )
non-sheltered
Figure 5 — Relationship between strength and carbonation rate, for sheltered, non-sheltered
and interior exposure, after [13]
4.2.6.4 Concrete structure/texture and cracks
Carbonation is a reaction that moves from the exposed surface of the concrete inwards. For this reason
the area of the exposed surface is important and the exposed surface to volume ratio. Although CO will
travel though concrete mainly through the cement paste there may be easier paths such as the
interfacial zone between paste and aggregate, as well as cracks in concrete. The interfacial zone is
generally more permeable than cement paste tends to increase the average carbonation rate. Any
cracks in the concrete surface will be preferential routes for carbon dioxide to penetrate and hence
increase the rate of carbonation around the crack.
4.2.6.5 Partial pressure of CO
The carbonation rate will be higher in urban areas and indoor environments where the partial pressure
of CO is higher than average.
For accelerated carbonation testing CO concentrations of 3 % to 5 % have been used but as the higher
concentrations do not permit a reduction in the time it takes to carry out the test, the European
Standard level is set at 3 %.
4.2.6.6 Carbonation rate model
As set out in 4.2.5 the k-value is useful in characterizing the carbonation characteristics of concrete, and
values are included in EN 16757:2017, Annex BB for Portland cement, where Table 3 is a summary of
the values given.
Table 3 — Value of k for the calculation of carbonation depth for ranges of strength and
exposure conditions
Compressive cylinder strength
MPa
Concrete exposure conditions
< 15 15 to 20 25 to 35 > 35
In ground, below water table 0,2 0,2 0,2
In ground 1,1 0,8 0,5
Exposed to rain 5,5 2,7 1,6 1,1
Sheltered from rain 11 6,6 4,4 2,7
Indoors 16,5 9,9 6,6 3,8
Figure 6 shows the relationship between k-values determined from field measurements of carbonation
depth [24] and k-values estimated using Table 3. From this Figure it evident that although the scatter is
large the slope of the best fit relationship is close to unity, indicating that for a range of structures
covering various strength classes and exposure conditions then the use of EN 16757:2017, Annex BB
method for estimating carbonation depth is fairly realistic.
Key
X k according to Table 3
Y k calculated from measured carb. depth
Figure 6
Figure 6 shows the relationship between k-values determined from field measurements of carbonation
depth [24] and k-values estimated using Table 3.
4.2.7 Carbonation rate of concrete with blended cements or with additions
Portland cement (CEM I) has traditionally been the most common type of cement used in Europe up to
the turn of the century, but today the most common cements are blended cements such as CEM II, or
combinations of CEM I or CEM II with additions such a ground granulated blastfurnace slag (ggbs), fly
ash, natural and calcined pozzolanas and silica fume. The most common type of cements, or
combinations of cements and additions, varies between countries due to local tradition and availability.
Across Europe the most common cement type used for building and general applications is CEM II,
where CEM II/A contains at least 80 % Portland cement clinker and CEM II/B contains at least 65 %
Portland cement clinker.
NOTE The ‘CEM’ notation refers to European common cements conforming to EN 197-1, Cement — Part 1:
Composition, Specification and conformity criteria for common cements. In this standard, the proportions are
expressed as percentage by mass of the main and minor additional constituents.
Additions such as ggbs, fly ash, pozzolanas and silica fume all have a pozzolanic reaction when
combined with water and Portland cement. This is important because the pozzolanic reaction reduces
the amount of calcium hydroxide in the concrete, and increases CSH. In particular ggbs changes the
composition of the hydrates in that it lowers the Ca/Si ratio of the CSH and there is less calcium
hydroxide available to maintain the pH. This is also true for the other additions but the effect may not
be so significant as the proportions of non-ggbs addition tend to be less than ggbs.
In practice, the presence of additions as part of the cement reduce the amount of calcium hydroxide,
and as there is less of this to carbonate then the rate of carbonation as measured by phenolphthalein is
faster than that measured in Portland cement concretes. In addition the pozzolanic reaction is slower
than the Portland cement reaction, which means that there are less hydration products available at
early ages, and so the early age permeability/diffusivity will be higher. This will be particularly true if
the concrete is not cured sufficiently.
Portland-slag cement (CEM II/A-S) contains up to 20 % slag and CEM II/B-S contains up to 35 % slag.
Blast furnace cement (CEM III) contains between 36 % and 95 % slag. Ggbs does have a pozzolanic
reaction but is a latent hydraulic binder, i.e. it will react with water by itself but for practical
applications it is used with Portland cement.
Portland fly-ash cements (CEM II/-V or W) and Portland Pozzolana cement (CEM II/ P or Q) contain up
to 35 % fly ash or pozzolana, but up to 55 % may be incorporated into Pozzolanic cements (CEM IV).
Portland-limestone cement (CEM II/-L or LL) can contain up to 35 % limestone. Although limestone
fines are not inherently reactive the ultrafine particles are effective nucleation points that enhance
hydration. Due to the reduction of clinker in the cement the capacity to react with CO , the buffering
capacity, is reduced and hence increase the carbonation rate.
Portland-silica fume cement, CEM II/A-D, may contain up to 10 % silica fume. Silica fume is an efficient
pozzolana that due to inherit fineness and reactivity reacts at rates comparable to Portland cement.
Table 4 is a summary of indicative comparative rates of carbonation for commonly used additions and
proportions in cement, for equal strength concrete, after [1] the rate of CEM I being 1,00.
Table 4 — Indicative comparative rates of carbonation for commonly used additions and
proportions in cement, for equal strength concrete, after [1]
Proportion of addition or constituent in cement
%
Cement constituent or
< 10 10 to 20 20 to 30 30 to 40 40 to 60 60 to 80
addition
Limestone 1,05 1,10
Silica fume 1,05 1,10
Fly-ash 1,05 1,10
Ggbs 1,05 1,10 1,15 1,20 1,25 1,30
4.3 CO binding capacity in concrete, Degree of carbonation
4.3.1 General
The carbonation chemistry and kinetics as summarized 4.2.1 and 4.2.2 shows that the reactants CO
and CaO need to be dissolved in the aqueous phase in order to form CaCO . The phenolphthalein test
only indicates areas where the pH is above 9,0 and does not indicates the percentage of reactive CaO
present in hydrated cement in concrete that is converted to CaCO , as stated in 4.2.3.
The degree of carbonation is defined as:
CO bound under practical carbonation conditions
(3)
CO emitted during calcination
The theoretical CO -binding capacity in concrete can be calculated but studies show that the CO bound
2 2
under practical carbonation conditions is always lower.
4.3.2 Theoretical binding capacity of Portland cement
The net stoichiometric, that is net quantitative, formula for carbonation may be expressed as the
reactive calcium oxide of Portland cement combining with carbon dioxide to form calcium carbonate, as
expressed in Formula (3). Underneath this formula are the molecular weights of each compound, where
the element weights are C = 12, O = 16 and Ca = 40.
CO +=CaO CaCO
Molecular weight (4)
12+ 2×=16 44 40+ 16 56 40+ 12+ 3×=16 100
( ) ( )
In terms of mass it takes 56 g of calcium oxide to react with 44 g of carbon dioxide from the atmosphere
to precipitate 100 g of calcium carbonate. Portland cement contains around 95 % Portland cement
clinker, where 65 % of that clinker is calcium oxide available to react with carbon dioxide. So 1,00 kg of
Portland cement represents = 1,00 × 0,95 × 0,65 kg = 0,62 kg of CaO. From Formula [4] 0,62 kg of CaO
will react with 0,62 × (44/56) = 0,49 kg CO .
In summary, 1 kg of Portland cement in concrete will potentially react with up to 0,49 kg of CO .
4.3.3 Normal binding capacity of Portland cement
There is no fixed or normal CO uptake for all concrete in all environments. The most favourable
conditions for carbonation is where the concrete is outdoors. Under these conditions the CO -binding
capacity can be estimated [24] in terms of the main hydrate phases, as described in 4.1. From [24] the
main hydrate phases and their individual degrees of carbonation are summarized in Table 5.
Table 5 — Ideal cement hydrate phases and degree of carbonation
Cement hydrate phase Amount of carbonation and chemical changes
100 % carbonation, complete dissolution of calcium
Calcium hydroxide, CH
hydroxide
Calcium-silicate hydrate, CSH 50 % carbonation
Calcium monsulfoaluminate, AFm 75 % carbonation
Ettringite, AFt 50 % carbonation
Table 6 shows the calculation of the binding capacity of Portland cement in terms of the total CaO, as a
summation of the four main cement phases: CSH, CH, AFm and Aft.
=
Table 6 — Binding capacity of Portland cement in terms of total CaO
Hydrate phases
Calculations
CSH CH AFm AFt All
a) Hydrate phase content, % (4.1.4) 50 25 10 10 95
b) CaO molar ratio 0,42 0,76 0,36 0,27 -
CaO in cement hydrate phase, %
a
c) 21 19 3,6 2,7
{= a) × b)}
Assumed degree of carbonation in hydrated
d) 0,5 1 0,75 0,5 -
cement, %, (Table 5)
CaO available for carbonation in cement
hydrate, %
e) 11 19 2,7 1,3 33
{= c) × d)}
CaO available relative to the total CaO, %
23 41 5,8 2,9 72
a
{= [e)/46 )] × 100}
a
Value of last column in line c).
From Table 6, the available CaO content for normal carbonation is estimated to be around 72 % of the
total CaO. This value for sheltered outdoor concrete is used as an average value for carbonated concrete
in general, where measured experimental values of the reactive CaO vary between 50 % and 98 %. An
indicative value of 75 % of the CaO is often used, as in this report and in EN 16757:2017, Annex BB.
From 4.3.2, 1 the potential binding capacity of 1 kg of Portland cement is 0,49 kg of CO , but as
calculated in Table 6 only an average of around 75 % of the potential value is realized. In summary, for
1 kg Portland cement in concrete with react with 0,75 times the 0,49 kg of CO , as determined in 4.3.2.
That is 0,37 kg of CO per kg of hydrated Portland cement in concrete.
4.3.4 Normal binding capacity of blended cements
4.3.4.1 Conservative approach
The CO uptake in the fly ash and slag reaction products is not as well understood as it is for Portland
cement clinker. A conservative approach is just to assume that it is only the Portland cement clinker
proportion of blended cements, that is CEM II, CEM III, CEM IV and CEM V, that react with CO .
For example a CEM II/A cement with 80 % Portland cement clinker, compared to a normal CEM I with
95 % Portland cement clinker, has a maximum theoretical CO uptake equal to:
0,49 × (80/95) = 0,41 kg CO /kg cement, and the normal binding capacity of 75 % of this.
That is 0,75 × 0,41 = 0,31 kg CO /kg of CEM II/A cement.
4.3.4.2 Binding capacity of slag cement, according to [24]
Using the conservative approach as outlined in 4.3.4.1, and for example a CEM II/B-S Portland–Slag
cement comprised 65 % Portland cement clinker and 30 % granulated blast furnace slag, the
CO -binding capacity is reduced from 0,37 kg CO /kg cement, as shown in 4.3.3, to
2 2
0,37 × (65/95) = 0,25 kg CO /kg Portland-slag cement by simple dilution, assuming none of the CaO in
the from the slag carbonates.
Granulated blast-furnace slag is a latent hydraulic binder, where by oxide analysis is comprised around
40 % CaO, 35 % SiO , 12 % Al O , 9 % MgO and 4 % other oxides. There is very little data concerning
2 2 3
carbonation in concrete. However, should 70 % of the CaO from slag carbonate then this would equate
to and extra 0,70 % of 40 % CaO for (30/95) = 0,09 kg CO /kg of granulated blast-furnace slag in
cement. This would be 0,34 kg of CO /per kg of Portland-slag cement.
Whether the binding capacity should be 0,25 kg or 0,34 kg CO /kg Portland-slag cement will depend on
the hydration products of the slag and to what extent they will carbonate under the normal exposure to
CO .
4.3.4.3 Binding capacity of fly ash cement according to [24] and [28]
In [24] it is assumed that the pozzolanic reaction, where 80 % of the glassy phase reacts, consumes
0,22 kg of calcium hydroxide/kg of fly ash to form CSH where the Ca/Si = 1, and is not available for
carbonation.
Hence for a CEM II/A-V cement where 20 % clinker is replaced with fly ash, the CO binding is reduced
from 0,37 kg CO /kg Portland fly ash cement to 0,37 × (80/95) = 0,31 kg CO /kg Portland fly ash
2 2
cement simply by dilution. Due to the pozzolanic effect removing Ca(OH) and forming CSH of low Ca/Si
then a further reduction of 0,01 kg is estimated, so the total binding capacity is around 0,30 kg CO /kg
Portland fly ash cement.
In [28] it is reported that the CO uptake of CEM I and CEM II/B-V (30 % fly ash) mortar of the same
water-to-binder ratio (0,55) is of similar magnitude. The testing conditions were however rather
special. The CO content was 1,5 % and the start of the testing was already after 14 days of sealed
curing. The clinker of the CEM II cement was found to be well hydrated, (high water-to-clinker ratio)
while not hydrated cement was frequently found in the CEM I cement. Not hydrated cement does not
carbonate.
The curing and time to the start of testing was probably too short to give realistic uptake, which also is
indicated by the fast speed of carbonation in the CEM II mortar, more than 50 % faster than with CEM I,
which is a very high value.
4.4 Carbonation in different environments
4.4.1 General
The rate and degree of carbonation depend on the exposure environment. Although most concrete is
exposed to the atmosphere, and the atmosphere contains CO , there are other exposures, such as
concrete permanently submerged concrete, where the mode of carbonation may be significantly
different.
4.4.2 Dry indoor concrete
In a dry concrete the rate of carbonation is controlled by gas diffusion and the driving mechanism in
stagnant air is the difference in partial pressure between the surface and the interior of the concrete.
This gives a faster carbonation, where carbonation is understood as lowering of pH value but not
necessarily a higher degree of carbonation because there may be a lack of moisture for the reaction. To
get an improved understanding of both rate and degree of carbonation, concrete from older buildings
has been sampled and analysed, see [21]. In general the rate of carbonation is high but in many cases it
is slowed down by surface coverings, such as paint. The major difference between dry indoor and wet
outdoor concrete is the degree of carbonation. The carbonation of indoor concrete is generally deeper
b
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