1st International Conference on Grand Challenges in Construction Materials – March 17-18, 2016
Using CO2 to reduce the carbon footprint of concrete
Sean Monkman1, Mark MacDonald2 and Doug Hooton3
Abstract
Carbon dioxide emissions are recognized as a significant issue relating to cement production and the use of
concrete as a building material. It is estimated that 5% of the world’s annual CO2 emissions are attributable to
cement production while the global demand for concrete is projected to climb ever higher. The industry has
previously recognized a number of approaches to reduce the emissions intensity of the cement produced and used
but the most significant improvements in efficiency and cement substitution are likely to be already known and
under deployment. Future emissions improvements will come from innovative approaches used as part of a
portfolio strategy.
Carbon dioxide has been developed as a beneficial admixture through addition to freshly mixing concrete. The
reaction between the CO2 and the hydrating cement results in the in-situ formation of finely distributed nano-scale
calcium carbonate reaction products. The carbonation product creation can have beneficial effects on the concrete
properties. A series of industrial trials established the potential for industrial integration and the expectations for
material properties.
The addition of the optimal amount of carbon dioxide had no effect on the slump or workability of the fresh
concrete. The average strength gain of the trial concretes was observed to be 14% at 24 hours, 13% at 7 days and
10% at 28 days. The durability was determined to be acceptable. One outcome of using a tool that can unlock a
consistent strength benefit could be to adjust the mix design to include greater fractions of low-embodied CO2
components. An increased use of SCMs could potentially accompany the CO2 addition to create acceptable
concrete with a reduced carbon footprint.
1.
2.
3.
Sean Monkman is Vice President of Technology Development with CarbonCure Technologies; 60 Trider Crescent,
Dartmouth, NS, B3B 1R6; (email: [email protected]).
Mark MacDonald is the Director of Research with CarbonCure Technologies; (email:
[email protected]).
Doug Hooton is a Professor in Civil Engineering at the University of Toronto, Toronto, ON, M5S 1A4; (email:
[email protected]).
I. INTRODUCTION
Carbon dioxide emissions are recognized as a significant
aspect inherent to cement production. It is estimated that 5%
of the world’s annual industrial CO2 emissions are
attributable to cement production [1]. Still, global cement
and concrete production is projected to see long term
increases in response to rising demand [2]. The challenge is
then to reduce the intensity of carbon dioxide emissions
associated with the production and use of cement. The
cement industry announced a plan to reduce CO2 emissions
from 2 Gt in 2007 to 1.55 Gt in 2050, while projecting that,
over the same period, cement production would increase by
about 50 % [3].
The cement industry’s plan recognized a number of
approaches to reduce the carbon dioxide emissions intensity
of cement and concrete (i.e. increased thermal and electric
efficiency in cement kilns, increase use of alternative fuels
for cement production, increased SCM usage, and carbon
capture and storage) [3]. It is clear, however, that practical
limits on the impacts of these measures mean that meeting
the goal will be difficult [4]. The most significant
improvements in production efficiency and clinker
substitution with supplementary cementitious materials are
known or subject to practical constraints on their future
impact [2]. Future emissions improvements will likely be
incremental. Therefore, innovative approaches are sought
that can be a part of a portfolio strategy.
One potential method is to upcycle waste carbon dioxide
into concrete during production. The mechanism of reacting
carbon dioxide with freshly hydrating cement was
systematically studied in the 1970s at the University of
Illinois [5]. The main calcium silicate phases in cement were
shown to react with carbon dioxide, in the presence of water,
to form calcium carbonate and calcium silicate hydrate gel
as shown in Equations 1 and 2:
{1} 3CaO·SiO2 + (3-x)CO2 + yH2O → xCaO·SiO3·yH2O +
(3-x)CaCO3
{2} 2CaO·SiO2 + (2-x)CO2 + yH2O → xCaO·SiO3·yH2O +
(2-x)CaCO3
Further any calcium hydroxide present in the cement paste
will react with carbon dioxide, in the presence of water, as
shown in Equation 3:
{3} Ca(OH)2 + CO2 → CaCO3 + H2O
The reaction occurs in the aqueous state when Ca2+ ions
from the cementitious phases react with CO32- ions from the
applied gas. The carbonation reactions are exothermic. The
carbonation heats of reaction for the main calcium silicate
phases are 347 kJ/mol for C3S, 184 kJ/mol for β-C2S [5] and
74 kJ/mol for Ca(OH)2 [6]. The carbonate phases are
thermodynamically stable when formed and offer permanent
fixation of the CO2 gas.
When the calcium silicates carbonate, the formed CaCO3
is understood to be co-formed with calcium silicate hydrate
(C-S-H) gel [7]. C-S-H gel formation has been observed to
form even in the model cases of reacting β-C2S and C3S with
100% CO2. It was found that the amount of calcium silicate
that reacted exceeded the amount that would be attributable
to the formation of the carbonate products alone [5].
The reaction of carbon dioxide with a mature concrete
microstructure is conventionally acknowledged to be a
durability issue due to such effects as reduced pore solution
pH, and carbonation induced corrosion. In contrast, a
carbonation reaction integrated into concrete production
reacts CO2 with freshly hydrating cement, rather than the
hydration phases present in mature concrete, and does not
have the same effects. Rather, by virtue of adding gaseous
CO2 to freshly mixing concrete the carbonate reaction
products are anticipated to form in situ, are of nano-scale
and homogenously distributed.
Preliminary investigations into using carbon dioxide as a
feedstock in ready mixed concrete production had sought to
maximize the carbon dioxide absorption [8]. A limited
reaction time and effects on workability were identified as
challenges to overcome. Subsequent lab work using
isothermal calorimetry identified the potential performance
benefit of using an optimized low dose of carbon dioxide to
promote the development of finely distributed carbonate
reaction products. It was concluded that a small dose of
carbon dioxide could feasibly be used to improve the
performance of ready-mixed concrete. If the carbon dioxide
reaction can lead to a tangible compressive strength benefit
then it is intended that the concept would support mix
optimizations to achieve concrete mixes with a reduced
carbon footprint. An industrial experimental program was
launched to investigate the potential to directly use carbon
dioxide to reduce the carbon emissions intensity of ready
mixed concrete.
II. EXPERIMENTAL
Experimental work was conducted whereby carbon
dioxide was delivered to industrially-produced ready mixed
concrete during batching and mixing. In some cases the CO2
was added to a central mixer, while in other cases it was
added to a truck mixer. A supply of liquid CO2 was
connected to a gas control system and injector. The liquid
was metered for injection into the mixer or truck whereupon
it converted into a mixture of CO2 gas and solid carbon
dioxide “snow”. The liquid is not stable at atmospheric
temperature and pressure and converts immediately upon
delivery from the injection hardware. The carbon dioxide
was delivered into the fresh concrete at a specified flow rate
2
Figure 1: Influence on CO2 treatment on concrete air content
Figure 2: Direct impact on CO2 treatments of concrete air content
and over a fixed injection interval, whereupon it reacted with
the hydrating cement during the earliest stages of hydration.
The concrete was then subjected to assessment and testing.
The experiments were intended to consist of a first
attempt optimization wherein up to three doses of carbon
dioxide were investigated in an abbreviated search for an
optimum dose. No intentional adjustments to admixtures or
mix designs were included in the experiments though typical
quality control procedures were followed to ensure that all
concrete (both reference batches and CO2-treated batches)
were produced with an acceptable consistency.
The experimental program visited 13 locations and
investigated 24 different mix designs. In addition to different
cements at each location, the testing included concretes that
included blast furnace slag, Class C fly ash and Class F fly
ash.
The goal was to translate the potential performance
benefit observed in the lab into a strength benefit in
industrially produced concrete and to establish the potential
to reformulate the concrete mix design to contain less
embodied CO2. The process would involve a small amount
of directly absorbed carbon dioxide but should an increased
replacement of cement with lower embodied CO2
alternatives (fly ash, slag, limestone) be viable then it would
represent a larger improvement to the concrete CO2
footprint.
Concrete was assessed in the fresh and hardened states.
Air content and slump was measured at the time of casting.
Isothermal conduction calorimetry was used to characterize
the early hydration. Compressive strength was measured at
1, 3, 7 and 28 days. Durability testing include linear
shrinkage (OPS LS-435: similar to ASTM C157 with 28
days drying at 50% RH after 7 days of moist curing), the
rapid chloride permeability test (ASTM C1202), bulk
resistivity, hardened air properties, freeze/thaw deicing salt
scaling mass loss (OPS LS-412: a modification of ASTM
C672) and freeze/thaw durability (ASTM C666). (OPS
signifies Ontario Provincial Standards, as used by the
highway agency in Ontario, Canada).
III. RESULTS
The results, as presented, show the outcomes of the CO2
treated batches wherein results are simplified to only
concern batches with an optimal dose, as determined by the
best strength results amongst any given first pass
optimization series.
A. Fresh Properties
The air contents of the CO2 treated batches were
compared to their appropriate reference batches [Fig. 1]. It is
shown that the average impact of CO2 was to reduce the
fresh air content by 0.4% while the median impact was a
0.2% reduction. A reduction of this magnitude is not
significant given the variability of the test in the field. The
results supported the conclusion that the air content was not
negatively impacted by the application of the carbon
dioxide.
This is reinforced through review of a subset of the
overall trial data wherein, for certain concrete mixes, the air
content was measured both before and after carbon dioxide
injection [Fig 2]. This direct evaluation removes batch-tobatch variation since the same batch of concrete is measured
in both the untreated and treated state. It was shown that the
maximum increase associated with the CO2 was 1.0%, the
greatest decrease was 0.8% while the average and median
impacts were no change.
The slumps of the CO2 treated batches were compared to
their appropriate reference batches [Fig. 3]. The data reveals
that the average impact was to reduce the slump by 0.6
inches with the median being a 0.5 inch decrease. A further
comparison, [Fig. 4], of concrete slumps before/after CO2
wherein the measurements were made on the same batch
shows that in 55% of cases a decrease was observed.
However, within this subset, the average impact was a
reduction by 0.4 inches and the median was a reduction by
0.2 inches.
The overall conclusion from the fresh air and slump
testing indicated that the addition of carbon dioxide to the
concrete mix had a negligible effect on the fresh properties.
3
Figure 3: Influence on CO2 treatments on concrete slump
B. Early Hydration
The early hydration of the concrete was assessed by
isothermal calorimetry. It was observed that typically an
optimal dose of CO2 accelerated and increased the early
hydration. The heat of hydration energy curves would be
shifted to earlier times while the total heat of hydration at 24
hours would increase [Fig 5]. In the presented case the CO2
shifted the onset of the main hydration heat evolution about
two hours and increased the total energy release at 24 hours
by 14%. In the example, the shape of the calorimetry curve
has also changed. The first part of the hydration peak, as
attributable to silicate hydration, is greater in the CO2 treated
case than in the reference case. The accelerating action of
the carbon dioxide is observed to relate to silicate activity.
Across the test program a variety of calorimetric changes
were observed. In some cases hydration acceleration was
observed without an increase in overall hydration. In other
cases no acceleration was observed but overall hydration
increased. It is justifiable to say that the specific outcome is
sensitive to the dose of carbon dioxide and to cement and
system chemistry.
C. Compressive strength
The experimental work focused on achieving a
compressive strength benefit. Strengths were assessed at 1,
3, 7 and 28 days [Fig. 6]. The data is presented in terms of a
box and whisker plot wherein the box represents 90% of all
results with the average plotted as a line crossing the box.
The plot is scaled to focus on the bulk of the data with
outliers at the three earliest ages (189%, 153% and 143%
respectively) being placed outside the plotted area.
The average 1 day strength benefit of the CO2 treated
batches was a 14% improvement with 90% of the results
falling in the range of 98 to 125% of the control strength. At
3 days the average benefit was 16% with 90% of the results
in the range of 105% and 132% of the control strength. At 7
days the average benefit was 13% with 90% of the results in
the range of 102% and 124% of the control strength. At 28
Figure 4: Direct impact of CO2 treatments of concrete slump
days the average benefit was 10% with 90% of the results in
the range of 102% and 119% of the control strength.
The neutral test results (e.g. strengths between 95 and
105% of the control strength) may have either been an
indication of an unfavorable cement compatibility or been an
outcome of the limited test program wherein the search for
an optimum dose was restricted to three or fewer doses of
CO2. More research and optimization would be required to
conclude which of the two explanations, if either, is valid or
the most significant. However, the promise shown by the
average and median compressive strength test results
discovered during the limited investigation merits optimism
for unlocking strength benefits in the neutrally performing
mixes.
D. Durability
The durability of the concrete produced using added CO2
was assessed as part of one of the trials [Table 1].
Table 1. Concrete durability test results
Condition
Property
Control
Linear shrinkage at 28 days
Hardened Air Content (%)
CO2 Treated
-0.033 %
-0.034%
4.9
4.3
-1
Hardened Air - Specific surface (mm )
38.19
38.49
Hardened Air – Spacing factor (mm)
0.119
0.130
Bulk Resistivity at 28 days (kΩ -cm)
10.0
9.9
Bulk Resistivity at 56 days (kΩ -cm)
12.9
13.3
RCPT at 28 days – Charge passed
(coulombs)
RCPT at 56 days – Charge passed
(coulombs)
Freeze/thaw deicing salt scaling mass
loss (kg/m2)
Freeze/thaw durability factor
8.2
7.0
6.3
6.7
0.40
0.24
43.2%
44.5%
1.66%
0.84%
Freeze/thaw durability mass loss
4
Figure 5: Conduction calorimetry of CO2-treated concrete
Figure 6: Relative compressive strength of CO 2 treated concretes
(boxes represent 90% of all results)
Both the CO2–treated and the reference batches were
found to have linear shrinkage lower than the CSA A23.1
limit for low-shrinkage concrete of 0.04% after 28-days
drying at 50% RH. The CO2 did not change the linear
shrinkage.
The total hardened air for both the reference and the
CO2–treated batch was lower than desired but the specific
surface values and the spacing factors were acceptable in
both cases. The specific surface was not changed by the
carbon dioxide. The spacing factor was increased in the
carbon dioxide treated sample but this aspect is consistent
with the observation of reduced hardened air content.
The bulk resistivity results indicated that both batches
were on the cusp between moderate and low risk of chloride
penetration at 28 days and low risk at 56 days. The carbon
dioxide did not change the bulk resistivity.
The RCPT results suggested that the chloride ion
penetrability would be low for both conditions samples at 28
and 56 days.
It was observed that the batch treated with CO2 exhibited
lower freeze/thaw deicing salt scaling mass loss than did the
reference batch. The scaling mass loss was reduced by 40%
though, it can be noted, neither of the samples approached
the scaling limit of 0.80 kg/m2.
The ASTM C 666 durability factor for both concretes
was lower than intended, possibly due to the low hardened
air content. The CO2 did not impact the durability factor, but
did reduce the mass loss by about 50%.
may be creating nuclei to promote earlier and denser
hydration. An example of a carbonate reaction product was
observed though activation of a cement sample with carbon
dioxide that was then freeze dried after 1 minute hydration
[Fig. 7]. The morphology and composition (via a Helios
NanoLab 650 Focused Ion Beam SEM equipped with an
EDS detector and an EDAX TEAM Pegasus system) was
consistent with amorphous calcium carbonate.
The chemical action of calcium ions combining with
carbonate ions would therein direct calcium away from the
co-formed C-S-H gel and the ratio of Ca to Si in the gel
would decrease. Perhaps this modification of the earliest
formed gel reduces the density or thickness of gel and
contributes to the reduced dormant period observed [Fig 5].
A replicable strength benefit is at the heart of using the
CO2 utilization concept to achieve a potential carbon
emissions reduction. The direct utilization of CO2 is limited;
the optimal doses explored during the experimental work
ranged between 0.1 and 0.5% by weight of cement. A
strength benefit may be economically attractive (a producer
may price the mix according to a higher specified strength)
but a carbon dioxide emissions reduction can be realized by
targeting the original, unimproved, strength. As an example,
a 10% strength benefit could be the basis to make an
untreated and treated mix that both achieve 100% of the
original strength. A simple approach would be to reduce the
cement loading in the treated mix. A rough contemplation of
cement loading and strength would consider a generic 30
MPa concrete. If the compressive strength of a mix that
would otherwise reach 27.3 MPa was improved 10%
through the action of a carbon dioxide addition then the
strength boosted mix would attain 30 MPa. The cement
loading for a 30 MPa concrete would be about 326 kg/m3
whereas for the 27.3 MPa (CO2 treated) mix it would be
about 305 kg/m3 [9]. A cement reduction of roughly 7%
could potentially be realized through an optimized mix
design that uses carbon dioxide for a strength boost.
A direct CO2 emissions reduction would be achieved by
reducing the cement loading. If Portland cement clinker
IV. DISCUSSION
The injection of carbon dioxide into concrete while
mixing can be associated with no impact on the fresh
properties, an increase in the energy of hydration observed
through isothermal calorimetry, a neutral to positive effect
on compressive strength, and no negative effect on the
durability properties.
The mechanism of a strength benefit is the subject of
further investigation. The early action of the carbon dioxide
5
Figure 7: SEM micrograph of carbonate reaction product
produced through reaction of cement with CO2
typically has embodied CO2 on the order of 866 kg
CO2e/tonne of clinker [4] then the utilization of 0.61 kg of
carbon dioxide (0.2% of the 305 kg/m3 reduced cement
loading) could unlock a reduction in the specific CO2
emissions of the concrete by 18.2 kg/m3.
If a direct cement reduction is not advisable then an
increased cement substitution by SCMs or inert fillers may
be pursued. The potential for the CO2 to impact early
strength development [Fig. 5 and Fig. 6] would integrate
promisingly with a reformulated binder (e.g. increased fly
ash or slag) that would be otherwise anticipated to exhibit
slower hydration.
While pore solution testing was not performed within
this study there are no concerns that the CO2 injection
process has produced “carbonated concrete”. The concrete
resulting from the CO2 injection process involves a reaction
between carbon dioxide and the freshly hydrating calcium
silicates and is not involving a reaction with calcium
hydroxide and a mature cement paste. The treated concrete
is not carbonated in the conventional sense and would not be
subject to increased concern regarding ferrous reinforcement
corrosion. The uniformly-dispersed initial nanocarbonates
that form have a chemical impact only at the earliest stages
of hydration and do not impact the later development of
calcium hydroxide and pore solution alkalinity.
It is likely that the absorption efficiency of the carbon
dioxide into the concrete is on the order of 50 to 80%. The
injection of liquid CO2 into the truck was effectively a
delivery of a two phase mixture (approximately 50/50) of
solid carbon dioxide “snow” and gas. The acceleration for
the CO2-treated batch [Fig. 5] was associated with the
reaction of roughly 0.025% CO2 by weight of cement, or,
according to molar weights, 0.057% CaCO3. While this
amount is small, the proportions and observable effect are
comparable to other admixtures such as the retarding effect
of 0.0125% sodium gluconate [10], the accelerating effect of
0.05% finely divided silica [11], or the rheological effect of
0.025%
polysaccharide-based
viscosity
modifying
admixtures [12].
Some similarity between this approach and the action of
ex-situ additions of nano-CaCO3 is recognized. Nano calcite
additions have been observed to achieve accelerated
hydration and strength improvements [13]. However, the
raw material cost can be significant and successful
integration of nano-CaCO3 additions into conventional
concrete would have to overcome the obstacle of achieving
effective dispersion [14]. The in-situ production of nanoscale calcium carbonate reaction products via CO2 injection
addresses both the cost and the dispersion challenges.
The potential performance benefit of beneficially using
carbon dioxide, combined with lack of impact on the
durability, offers an interesting prospect for using CO2 as an
admixture. A general economic impact assessment can be
estimated. Given assumptions for a generic material cost of
$385 (US) per tonne of industrial carbon dioxide (midpoint
in a range of $300 to $400 per ton), a generic dose of 0.2%
carbon dioxide by weight of cement, and generic cement
loading of 325 kg/m3, then the raw cost of the CO2 would be
about 25 cents per m3 of concrete or $2.01 per truckload (8
m3).
V. CONCLUSIONS
A series of industrial trials saw carbon dioxide injected into
concrete mixes seeking to demonstrate a performance
benefit. The addition of waste CO2 into the concrete
mixtures had no affect on either the fresh air or the slump.
Isothermal calorimetry suggested that the CO2 injection
could accelerate early hydration reactions and indicated that
the carbon dioxide reacted with the silicate phases.
A compressive strength benefit was typically observed at
all test ages (1, 3, 7 and 28 days). The average benefit was
14% at 1 day and 10% at 28 days. A reproducible strength
benefit could be the basis for a reformulated mix design
wherein a cement reduction is pursued and the normal
baseline strength is achieved. A generic consideration
suggests that a 10% strength benefit would be a tool to
reduce the cement content, and the associated carbon dioxide
emissions, by 7%.
The durability testing showed that the CO2-injection
process had a neutral to positive effect on concrete
durability. Chloride penetration resistance, drying shrinkage,
freeze-thaw, and de-icer salt scaling resistance performance
of the CO2-treated concrete was assured through testing.
The action of the small amounts of CO2 have some
similarity to ex-situ nanocalcite additions to concrete. The
6
approach would be more attractive both economically and in
terms of implementation.
VI. ACKNOWLEDGEMENTS
The authors thank the industrial partners for providing the
materials, time and staff for the experimental trials. The
durability data collection was conducted by Phil Zacarias
and Stephen Parkes of CBM (Canada Building Materials).
Further assistance provided by University of Toronto
students Gita Charmchi and Soley Einarsdottir was greatly
appreciated. The work that produced the SEM
photomicrograph was conducted by Greg Dipple of the
Department of Earth, Ocean and Atmospheric Sciences at
the University of British Columbia. Research funding to
support the research was received from Sustainable
Development Technology Canada (SDTC) and the National
Research Council’s Industrial Research Assistance Program
(IRAP).
7
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