Center for
By-Products
Utilization
DRAFT REPORT
CARBON DIOXIDE SEQUESTRATION IN
CEMENTITIOUS PRODUCTS
By Tarun R. Naik, Rakesh Kumar, and Rudolph N. Kraus
Report No. CBU-2009-02
January 2009
REP-640
A Report Submitted to the Electric Power Research Institute, Palo Alto, California, January 2009
Department of Civil Engineering and Mechanics
College of Engineering and Applied Science
THE UNIVERSITY OF WISCONSIN-MILWAUKEE
CARBON DIOXIDE SEQUESTRATION IN
CEMENTITIOUS PRODUCTS
Progress Report
by
Tarun R. Naik, Rakesh Kumar, and Rudolph N. Kraus
UWM Center for By-Products Utilization
Department of Civil Engineering and Mechanics
University of Wisconsin-Milwaukee
Submitted to the Electric Power Research Institute
January 2009
UWM Center for By-Products Utilization
Department of Civil Engineering and Mechanics
College of Engineering and Applied Science
The University of Wisconsin - Milwaukee
P.O. Box 784
Milwaukee WI 53201
Ph: (414) 229-6696
Fax: (414) 229-6958
Table of Contents
Item
Page
Task 1: Literature Review of Carbon Dioxide Sequestration
Technologies............................................................................................. 1
1.1 Literature Review .................................................................................................... 1
1.2 Sequestration of CO2 in Cementitious Materials .... ………………………………… 8
1.3 Carbon Dioxide Sequestration, Concrete and other CementBased Products ..................................................................................................... 10
1.4 Claims by Industries for the Sequestration of CO2 in CementBased Products .................................................................................................... 14
1.5 Theoretical Basis of Carbonation of Concrete …………………… ......................... 20
1.6 Mechanism of Carbonation ................................................................................... 23
1.7 Modes of Carbonation ........................................................................................... 23
1.8 Carbonation Related Microstructural Change ....................................................... 24
1.9 Effect of Pozzolans on Carbonation of Concrete ................................................... 26
1.10 Rate of Carbonation ............................................................................................. 28
1.11 Measurement Methods of Carbonation Profiles/Depth in Concrete .................... 31
1.12 Limitations of Phenolphthalein Test Method ......................................................... 32
1.13 Other Methods for the Measurements of Carbonation Profile in Concrete ............ 32
1.13.1 Thermal Gravimetric Analysis (TGA) ............................................................ 33
1.13.2 Gammadensimetry Method .......................................................................... 34
Task 2: Theoretical Quantification of CO2 Sequestration Potential in
Cementitious Materials ......................................................................................... 35
Summary ................................................................................................................ 38
Acknowledgements ................................................................................................ 38
References ............................................................................................................... 39
ii
List of Tables
Item
Page
Table 1 Stable Phases in Portland Cement Paste at Different pH................................................. 22
Table 2 Carbonation Rate Constants (CEM 1) for Various Concrete Cylinders
Strength and Exposure Conditions .............................................................................................. 29
Table 3 Temperature Ranges of Hydrate Decomposition during TGA Measurements .............. 33
List of Figures
Item
Page
Figure 1 Depth of Carbonation by Phenolphthalein and Analytical Methods ............................. 18
iii
Task 1: Literature Review of Carbon Dioxide Sequestration
Technologies
1.1 Literature Review
The April 17, 2006 issue of Fortune magazine states: ―We recognize the accumulation of
greenhouse gases in the Earth‘s atmosphere poses risks that may prove to be significant for
society and ecosystems. We believe that these risks justify actions now. But the actions must
consider the costs and uncertainties that remain.‖1
It has been reported that ―CO2 capture and
sequestration remain a discussion issue with those people who are concerned about climate
change and the impact that manmade (anthropogenic) emissions may have on increasing the rate
of climate change. Some people involved in the debate believe that climate change can be
controlled by reducing CO2 emissions from burning of all fossil fuels. As power plants are large
sources of CO2 emissions, they have been a focal point of discussion in some areas.‖2 The
process commercially available for CO2 removal from pulverized coal-fired plants is known as
amine scrubbing, which is an old technology and not particularly efficient. Therefore, without
improving the technology, CO2 capture will add significantly to the operating costs of the
existing fleet of fossil-fuel-based plants.
The Joint Sciences Academies with representation from Brazil, Canada, China, France,
Germany, India, Italy, Japan, Russia, United Kingdom, and the U.S. issued a joint statement in
May 2007 regarding climate change. The group acknowledges uncertainty and that the climate
system will be slow to change. The joint statement said ―There will always be uncertainty in
understanding a system as complex as the world‘s climate. However, there is now strong
1
evidence that significant global warming is occurring.
The evidence comes from direct
measurements of rising surface air temperatures and subsurface ocean temperatures and from
phenomena such as increases in average global sea levels, retreating glaciers, and changes to
many physical and biological systems. It is likely that most of the warming in recent decades
can be attributed to human activities. This warming has already led to changes in the Earth‘s
climate.
Major parts of the climate system respond slowly to changes in greenhouse gas
concentrations. Even if greenhouse gas emissions were stabilized instantly at today‘s levels, the
climate would still continue to change as it adapts to the increased emission of recent decades.
Further changes in climate are therefore unavoidable. Nations must prepare for them.‖3
The Electric Power Research Institute (EPRI) has announced a program to improve CO2
capture technology and thereby reduce costs. EPRI‘s work was planned to begin in late 2006 or
early 2007. ―The carbon capture project is basically a two-step process. A slip stream of boiler
flue gas will be dramatically cooled using chilled ammonia, and then through a chemical reaction
process, carbon dioxide gas will be separated from the flue gas. Once the carbon dioxide gas is
isolated, it is considered captured. After successful separation, the carbon dioxide probably will
be re-introduced to the flue gas and released through the chimney. Final discussions about what
will be done with the separated CO2 are still in progress.‖2
Douglas4 in the Fall 2007 EPRI Journal that is titled ―Pathways to Sustainable Power In a
Carbon Constrained World‖ identifies seven advanced technology options that if implemented
aggressively could reduce CO2 emissions together, by about 45% from 2007 levels in 2030. The
portfolio of technologies include: end-use energy efficiency, renewable energy, advance light
2
water nuclear reactors and life extension for existing reactors, advanced coal power plants, CO 2
capture and storage, plug in hybrid electric vehicles, and distributed energy sources.
―The capture of CO2 is not the biggest challenge involved in addressing the issue of
greenhouse gas reduction from steam or electricity producing power plants. Certainly one could
simply replace the entire fleet of subcritical coal-fired power plants in the world with
supercritical units and reduce the emissions of CO2 by 20 to 25 percent. This would have a
massive cost implication to the industry and society at large. Assuming that USA, as a country,
chooses to limit emissions of CO2 by capturing it from the existing fleet of plants, then how to
transport and where to hold massive amounts of CO2 effectively in perpetuity is a far bigger
challenge. The challenge is how does the USA move and place in permanent repository huge
volumes of CO2? As an example, in 2005 We Energies plants in southeastern Wisconsin emitted
some 23.5 million tons of CO2 (some 23 million tons from coal plants and 600,000 tons from
gas-fired combustion turbines operating in both simple and combined cycle mode). The largest
CO2 sequestration projects in the world today will handle one million tons per year. This
technological challenge needs to be addressed. Additionally, society will need to address the
impacts of moving these large volumes of CO2 around the country and into appropriate geologic
formations below ground. Most probably, in 2012 We Energies will generate 31 million tons of
CO2. If 90 percent of this volume were to be captured and shipped to storage sites, it would
require about one million truck loads of liquefied CO2 to be shipped to storage. This is not a
likely alternative. Such infrastructure development to meet these sequestration needs will have
significant regulatory hurdles and likely to face public opposition to CO2 transportation as well.
In short, there are many more questions than answers with respect to this issue at the current
3
time.‖2 Patulski2 implies that CO2 transport pipelines would need to be constructed to permanent
sequestration sites. If CO2 can be mineralized at the emission source and be converted into
useful products, such processes could sequester and possibly reduce the quantity transported and
placed at storage sites, such as deep geologic formations. Because of the overwhelming volume
of CO2 to be sequestered, it is likely that several approaches will have to be developed to
contribute towards the future goals of a society.
Since the first step in some current ideas about CO2 sequestration is carbon dioxide
separation, industry has been very focused on technology options for capturing CO2.5 ―At
present, CO2 is routinely separated at some large industrial plants such as natural gas processing
and ammonia production facilities, although these plants remove CO2 to meet process demands
and not for storage. There are three main approaches to CO2 capture, for industrial and power
plant applications. Post-combustion systems separate CO2 from the flue gases produced by
combustion of a primary fuel (coal, natural gas, oil, or biomass). Oxy-fuel combustion uses
oxygen instead of air for combustion, producing a flue gas that is mainly H2O and CO2 that is
readily captured. This is an option still under development. Pre-combustion systems process the
primary fuel in a reactor to produce separate streams of CO2 for storage and H2 which is used as
a fuel. The lowest CO2 capture costs (averaging about $12/ton of CO2 captured or $15/ton of
CO2 avoided) were found for industrial processes such as hydrogen production plants that
produce concentrated CO2 streams as part of the current production process; such industrial
processes may represent some of the earliest opportunities for CO2 capture and storage.‖6
4
EPRI has selected a power plant in Wisconsin for the first large-scale demonstration of
separating and capturing CO2 gas from the combustion emission gases of a coal-fueled electric
power plant. A five megawatt pilot plant will be built in order to demonstrate a carbon dioxide
(CO2) capture process, capturing CO2 from a portion of boiler flue gas at the We Energies power
plant in Pleasant Prairie, Wisconsin. This capture process was developed by the engineering
firm ALSTOM. It uses chilled ammonia to capture the CO2. This specific process has not yet
been demonstrated in the US. The process reduces the required energy for capture and isolation
of the CO2 in a highly concentrated, highly pressurized form. In previous laboratory testing, the
process was found to be 90 percent efficient at removing CO2 at a much lower cost than other
methods. The pilot project was commissioned in mid-2007 at the We Energies Pleasant Prairie
Power Plant. It will be operated for approximately one year. An engineering/environmental
performance and cost analysis will be conducted by EPRI during the operation.
The USA has huge reserves of coal to help meet energy needs of the future. The US
Department of Energy believes carbon dioxide sequestration techniques need to be developed
over the next 10 to 15 years that meet these criteria:
Be effective and cost-competitive;
Provide stable, long-term storage; and,
Be environmentally benign7.
Accurate models and CO2 monitoring and recordkeeping systems will be needed to
document actual CO2 emissions and reductions from processes designed to capture and sequester
CO2 emissions prior to release to the atmosphere. ―Many believe an eventual carbon cap-and-
5
trade system in USA is likely. Under such a system, companies would need accurate greenhouse
gas accounting systems and provisions to document ownership of emission reduction credits.
Already, markets such as the Chicago Climate Exchange allow trading of greenhouse gas
reduction credits. If greenhouse gas reduction credits can be designated for the use of fly ash as
supplementary cementitious materials, coal-based utilities will need to establish agreements with
fly ash processors, commodities brokers, and Portland cement manufacturers on the ownership of
those credits.‖8
Interestingly, CO2 gas is now being used as a refrigerant in place of hydro-fluorocarbons
that had about 1,400 times more greenhouse warming potential than the same quantity of CO2.
CO2 has become the refrigerant of choice for the future and the amount released is miniscule
compared to that released by fossil-fuel combustion and cement manufacturing.
The high
coefficient of performance of CO2 ensures that less power will be used in the refrigeration
process, further reducing the CO2 impacts of refrigeration systems. Vallort9 points out one
challenge for engineers and the society, ―The engineering community and government
regulations continue to strive to reduce both global warming potential gases and ozone depleting
gases, but we must also realize that the health of the global community needs refrigeration to
maintain the cold chain to ensure the freshness of food and vaccines.‖
Coddington and Reynolds10 state that another important and high volume use for CO2 is
in enhanced oil recovery processes. ―Carbon dioxide‘s chemical properties make it uniquely
suited to recover hydrocarbon resources. The supercritical carbon dioxide molecule enters the
reservoir as a super-critical fluid and literally scrubs large volumes of otherwise unrecoverable
6
oil from the injection zone. In 2005, the Permian Basin in West Texas produced its billionth
barrel of oil from injected carbon dioxide. The amount of CO2 needed to lift this oil is
significant.
Department of Energy has estimated the enhanced oil recovery demand for
anthropogenic CO2 at 10,500 trillion cubic feet, or 151 gigatons of anthropogenic emissions of
carbon dioxide. Enhanced coal bed methane production is another commercial opportunity for
the gas which exists today. New technologies suggest that the molecule could be used to recover
oil from shale. In Texas alone, there are 10,000 permitted carbon dioxide injection wells, 8,000
of which inject carbon dioxide exclusively. Those wells safely inject over 1 billion cubic feet of
CO2 daily.‖
Another emerging high end use of carbon dioxide is as a green solvent i.e. supercritical
carbon dioxide.11 Supercritical carbon dioxide is a term for CO2 that has characteristics of both a
liquid and a gas. By virtue of its unique features such as higher diffusion rate, Supercritical CO 2
can penetrate a solid sample faster than liquid solvents. It can rapidly transport dissolved solutes
from the sample matrix because of its low viscosity. Supercritical CO2 is gaining popularity due
to its increasing uses such as an extracting solvent12,
13
to produce micro- and nano-scale
particles, to produce supramics(low-cost substitutes for rigid thermoplastic and fired ceramics,
made using supercritical carbon dioxide as a chemical reagent), in foaming of polymers, to
enhance recovery in mature oilfields, in heat pumps, and other similar applications
7
1.2 Sequestration of CO2 in Cementitious Materials
Concrete and other cement-based materials absorb carbon dioxide through a natural
process known as carbonation reaction that results in carbon dioxide sequestration in these
materials.
About 19% of the carbon dioxide produced during manufacture of cement is
reabsorbed by the concrete over its lifecycle (i.e., its service life and secondary life following
crushing and reuse14). The natural process of carbonation in conventional concrete is very slow
– on average about 1 mm/year.15 The rates of carbonation of concrete and other cement-based
materials mainly depend on the type of cement, quality of concrete, environmental conditions,
and permeability of concrete.16 Benefits of carbonation generally include increased concrete
strength and increased impermeability as compared to the same concrete prior to the carbonation.
Processes for promoting carbonation in the production of higher quality precast-concrete
products were proposed in the early 1900s.
The disadvantage of carbonation is possibly
accelerated corrosion of steel in reinforced concrete and the resulting possible effect on the life
of a structure. However, carbonation of concrete and other cement-based materials provide an
alternate means for the sequestration of carbon dioxide.
United States Patents that utilize carbonic acid, H2CO3, for strengthening cementitious
materials date back to 187017. The Columbia Encyclopedia18 defines carbonic acid as ―a weak
dibasic acid formed when carbon dioxide dissolves in water; it exists only in solution. Carbonic
acid forms carbonate and bicarbonate salts by reaction with bases. It contributes to the sharp
taste of carbonated beverages.‖ Rowland17 was issued a patent for the improvement in the
manufacture of artificial stone in 1870 where clean washed sands were combined with
cementitious materials and steam cured in a carbon rich environment to yield a strong, hard,
8
durable, and inexpensive artificial stone.17 Rowland17 was also issued second patent in 1872 for
the improvement and hardening of artificial stone walls, floors, pavements, roofs, and
foundations produced with artificial stone, and for hardening other cementitious products with
carbonic acid gas.19 Heinzerling20 was issued a patent in 1897 for the production of artificial
stone with carbonic acid gas under pressure.
Ball21 was issued a patent in 1978 for portland-cement products, with or without added
gypsum, where carbon dioxide gas was homogeneously reacted with the cement slurry during the
water and cement mixing. The use of carbon dioxide with ground cement was shown to control
setting and also resulted in hydraulic cement mixtures, which were more stable following
hydration. Malinowski22 was issued a patent in 1982 for his method of casting different types of
concrete products without the need for using a curing chamber or an autoclave. The concrete is
cast and subjected to a vacuum treatment to have it de-watered and compacted. CO2 gas is then
supplied to the concrete mass where it diffuses into the capillaries formed for rapid hardening.
Jones23 was issued a patent in 1996 for cement treated with high-pressure CO2.
Jones23
introduced the use of dense-phase or supercritical CO2 conversion of calcium hydroxide in the
cement to calcium carbonate and water yielding closely packed and aligned crystals in the cured
concrete products. Taylor et al.24 explains the concept of a supercritical fluid. They stated ―let
us examine the situation of a quantity of liquid in a closed container, subjected to slow, uniform
heating. As the container is heated, the density of the liquid decreases because of normal thermal
expansion. Simultaneously, the density of the vapor increases as more molecules leave the liquid
and enter the gas phase. If the heating continues, a temperature will be reached called the critical
temperature, where the density of the liquid is so reduced, and that of the vapor is so increased
9
that the density of the two phases become equal. When this occurs, then since the density and
temperature inside the container is everywhere equal, and the pressure is everywhere equal, we
have reached a supercritical fluid.‖ The critical conditions of temperature and pressure for CO 2
exist at 31°C and 1072 psi.
There are numerous25-37 other patents that have been issued where carbon dioxide and
carbonic acid are utilized in various forms and pressures for the production of cement, concrete,
and concrete products. The common primary advantages of using carbon dioxide and carbonic
acid in various forms for concrete and concrete products are increased strength, increased
density, and increased impermeability from the resulting carbonation products25-37. Today, the
mineralization of CO2 in concrete products can help to fulfill a new purpose in contributing to
the long-term sequestration of increased CO2 levels in the air that can result from
industrialization of a society.
1.3 Carbon Dioxide Sequestration, Concrete, and other CementBased Products
There is a very limited number of studies38-44 dealing with carbon dioxide sequestration
potential in concrete and other cement-based products. If concrete and other cement-based
materials can be utilized to mineralize carbon dioxide to stable calcium carbonate during their
production, then this method of carbon dioxide sequestration will be of both environmental and
economical benefits. Furthermore, this technology of carbon dioxide sequestration would help
cement, thermal power plants, concrete, and other similar industries to reduce carbon dioxide
emission. Carbon dioxide mineralization in the hydrates of cement in cement-based material
10
occurs either in natural way of carbonation or by some specific design in an engineered way.
The natural process of carbonation of concrete is very slow. Normally, good quality concrete of
normal strength, widely used in the construction of buildings, carbonates at a rate of one
mm/year.15 The Portland Cement Association (PCA)45 indicates that virtually all structures
constructed with portland cement concrete have the potential to absorb atmospheric CO2 through
carbonation. A comprehensive study was undertaken that involved the use of extensive data
collected from more than 1000 concrete samples of absorbed CO2 in concrete. These data were
collected from locations across the USA. Calculations indicated that all the concrete produced
during a single year of typical concrete construction in the USA will absorb approximately
274,000 metric tons (300,000 short tons) of atmospheric CO2 during the first year of
construction. The concrete goes on to absorb CO2 throughout its life. PCA states that the
durability of plain concrete is not impaired by carbonation, and it may even be improved.
Carbonation rates of 8.5, 6.7, and 4.9 mm/yr0.5 were achieved for 21, 28, and 35 MPa (3000,
4000, and 5000 psi) concrete. An overall average carbonation rate was calculated to be 2.15
mm/yr0.5. When fully hydrated, 100 metric tons (110 short tons) of the average portland cement
produces 31.1 metric tons (34.3 short tons) of calcium hydroxide. Accounting for the average
unhydrated cement content of 6.8% in typical concrete, reduces the calcium hydroxide yield to
29.0 metric tons (32.0 short tons). When fully carbonated, this quantity of calcium hydroxide
can absorb up to 17.3 metric tons (19.1 short tons) of CO2. Portland cement consumption in U.S.
is about 100 million metric tons (110 million short tons) per year, leading to potentially 17.3
million metric tons (19.1 million short tons) of sequestration of CO2 per year in concrete; or, a
market value of about 350 million dollars (at about $20 per metric ton).
11
The most widely adopted engineered way for the mineralization of carbon dioxide in
cementitious material is their early age carbonation curing. The early age carbonation curing
coverts cement hydrates to stable calcium carbonate and silica gel; hence, it provides a means to
carbon dioxide sequestration in cement-based materials. Numerous studies15, 38-44 have shown
many advantages of this early age carbonation curing for concrete and other cement-based
materials. Earlier age carbonation accelerates strength gain. Therefore, carbonation shortens the
time required for the production, resulting in enhanced productivity.
Shah38 performed work at UWM-CBU on carbonation in non-air entrained and no-fines
concrete and observed that ―Carbonation occurs in the pores near the surface of concrete and
progresses towards the center of the concrete element, and is dependent upon the pore structure
of the concrete, relative humidity and CO2 concentration in the environment, availability of
Ca(OH)2 and water, and replacement of cement with mineral additives‖. He also stated that
―Other hydrates also react with dissolved CO2 such as hydrated silica, alumina, and ferric oxide.
When all Ca(OH)2 becomes carbonated, the pH value of the pore solution is reduced from 12.5
to 8.3. The rate of carbonation is the highest when the relative humidity of the surrounding
environment is 50% to 70%. During the carbonation of calcium hydroxide, one mole of water is
being released with every mole of CO2 being consumed. Due to the higher molar weight of CO2
than water, concrete gains weight. Carbonation also causes shrinkage in concrete. On the other
hand, pretreatment of concrete by CO2 reduces drying shrinkage.‖
Naik et al.39 investigated the effect of different curing environments on carbon dioxide
sequestration in concrete containing Class C fly ash. In the study they used Class C fly ash at
12
0%, 18%, and 35 % of total cementitious materials and three different curing environments (i.e.,
moist-curing (100% RH) and 0.15% of CO2, room with 50% RH and 0.15% of CO2, and CO2
chamber with 50% RH and 5% of CO2 concentration) to investigate the carbon dioxide
sequestration potential and subsequent effects on mechanical properties of concrete. Based on
their finding they reported that the rate of carbonation was the highest in the carbon dioxide
chamber. They further reported that the concrete specimens kept in carbon dioxide chamber (at
50% R. H.) showed mechanical properties at par with specimens cured in moist curing room at
100% R. H.
Ramme41 studied CO2 sequestration through mineralization by a process that utilized a
foaming agent and CO2 gas in the manufacturing of controlled low-strength materials (CLSM).
The carbonated product was then crushed to make aggregates suitable for a variety of
construction uses. The results found were encouraging for CO2 sequestration potential in CLSM
and subsequent aggregates production.
Shao et al.43 studied the potential of calcium silicate concrete for sequestration of CO2
through the early age (two hours) carbonation curing in a chamber under 0.5 MPa pressure and at
ambient temperature (23 ºC) for a duration of two hour with a 100% concentration of CO 2. They
used Type 10 and Type 30 portland cements as a binder in concretes. The CO 2 uptake was
quantified by direct mass gain and by infrared-based carbon analyzer. Based on the results, they
reported that by adopting their approach 9 to 16% CO2 by mass of portland cement could be
sequestered in two hours. The study show that concrete and other cement-based materials have
the potential for carbon dioxide sequestration through early age carbonation curing. However,
13
the consumption of carbon dioxide is dependent on quantity of the cement and concentration of
carbon dioxide in the curing environment. The specimens used were press-formed concrete
prepared by pressing them under a constant pressure of 8 MPa.
Shi and Wu46 examined the effects of different parameters such as water-to-cement ratio,
curing time, carbon dioxide pressure during curing, and temperature on carbon dioxide
consumption and strength of concrete products. They reported that accelerated reactions between
CO2 and cement minerals happen mainly during the first 15 minutes regardless of carbon dioxide
pressure and pre-conditioning environment. Further, they found that increase in carbon dioxide
pressure increases the CO2 consumption but does not show significant effect on strength of the
concrete. They also reported optimum water-to-cement ratios of 0.36 to 0.43 for the reaction
between CO2 and cement minerals. They have further shown that preconditioning of concrete in
the environment of relative humidity of 55 ± 10% at 22 ± 3 ºC increases CO 2 consumption
compared with the specimens pre-conditioned in moist environment with relative humidity
greater than 95% at 22 ± 3 ºC. The reason behind it may be the loss of water from the specimens
in dry environment which might have result in an easier transport of CO2 inside the concrete
specimens. They also reported gain in strength by the specimens kept in moist environment with
relative humidity greater than 95% at 22 ± 3 ºC after CO2 curing.
1.4 Claims by Industries for the Sequestration of CO2 in CementBased Products
Carbon dioxide sequestration has been a hot topic for study and discussion. Many
companies claim to have found constructive use for carbon dioxide sequestration in cement and
14
cement-based materials. California-based Calera47 claimed to have found technology for using
carbon dioxide rich flue gas to make cement. Calera47 utilizes ocean water and carbon dioxide
rich flue gas for making carbonates. By bubbling the flue gas through seawater, Calera plans to
creates cement, which can be used in production of concrete among other things. The company
employs spray dryers that use the heat of the flue gas to dry the slurry (ocean water reach in
mineral calcium and magnesium) that results from mixing (flue gas). With this process, Calera47
says, ―it can capture close to 90% of the Carbon dioxide emissions emitted by power plants and
other industrial giants.‖ Calera47 further states that it can remove a half ton of carbon dioxide
emission from the environment for every ton of cement it produces48.
Carbon Sense Solutions49 of Canada claimed to have developed a faster way to store
more carbon dioxide in concrete through CO2-accelerated concrete curing of precast concrete
elements. The method allows storage of up to 60 tons of carbon dioxides in 1000 tons of precast
concrete. The company has further claimed that the technology has the potential to sequester
20% of all cement industry carbon dioxide emission. Further, the company claimed to have
developed a process by which CO2 emissions from cement factories can be captured and
converted into bicarbonate-ions, which are used to generate limestone to be used in cement
manufacturing50.
The company plans to use flue gas and the water leftover after mining
operations commonly known as mine slime rich in magnesium and calcium to create cements.
Processes are under development for sequestration of CO2 in magnesium oxides (such as
those present in dolomitic limestone) by using calcium-rich ASTM C 618 Class C fly ash, and
accelerating the carbonation surface area by using permeable concrete39, 51-54. Opportunities exist
15
to develop carbon sequestration processes with high-surface area, calcium-rich, secondary
materials such as cement-kiln dust, blast furnace slag, Class C fly ash, lime-kiln dust, and
crushed recycled concrete fines41, 52. In the U.S, over 3,000,000 tons (2,730,000 metric tons) of
cement-kiln dust were removed from the manufacturing process with only 634,000 tons (573,000
metric tons) being beneficially re-used7. Approximately, one ton of CO2 and other greenhouse
gases are emitted to the atmosphere for each ton of portland cement produced51. The portland
cement industry has established a voluntary goal of a 10% reduction in CO2 intensity from 1990
levels by 202055.
O‘Connor56 described his vision of CO2 mineralization and stated ―This would require
capturing the carbon dioxide and mixing it into a slurry of ground up minerals. The minerals
react with the carbon dioxide and when the water is removed, a solid carbonate product is
produced.‖
O‘Connor56 said there have been more than 600 autoclave tests undertaken
concerning mineral sequestration. A filter press was used for solid/liquid separation and the
solids were dried. A value-added benefit from the mineral carbonation process could also be the
development of materials from the recovered carbon dioxide and slurry minerals.
The
carbonation reaction products of this process consist of magnesite, free silica, and residual
silicates. Potential uses for the magnesite/silica product include soil amendments, replacing
materials such as lime (CaO), limestone, and/or dolomite. Those materials might be used in a
diverse range of products, e. g., ceiling tiles. However, the vast majority of the carbonate
product would likely be used to reclaim the silicate minerals from a mine that supplied the
minerals to react with the carbon dioxide. O‘Connor56 also said a 1.3 gigawatts coal-fired power
plant produces about 24,000 tons of carbon dioxide per day (or, over 8,500,000 tons per year).
16
So, it would take a huge quantity (up to 70,000 tons per day; or, about 25 million tons per year)
of minerals to supply the process. This would require a large open pit mine for the carbonation
process for each plant. A process evaluation indicated that cost could be as much as $2 billion
for one mineral carbonation plant designed for the 1.3 gigawatts coal-fired power plant.
However, this cost could be greatly reduced if a continuous flow reactor was used. This would
allow the use of less expensive, narrow diameter pipes rather than large diameter high pressure
tank reactors. At the time, it was estimated that the mineral carbonation step in the CO2
sequestration process would add about eight cents per kilowatt hour to consumer‘s electricity
bills. Carbon dioxide capture and transportation would further add to this cost. Current mineral
carbonation systems would cost about $53 per ton of carbon dioxide sequestered, plus another
$25 per ton in energy used. The goal is to develop systems that would be effective for about $10
per ton.
Malhotra57, 58
effectively points out that the replacement of cement by pozzolans also
effectively decreases the net emissions from cement manufacturing.
Malhotra elegantly
concludes that ―the combined use of superplasticizers and supplementary cementing materials
can lead to economical high-performance concrete with enhanced durability. It is hoped that the
concrete industry would show leadership and resolve, and make contributions to the sustainable
development of the industry in the 21st century by adopting new technologies to reduce the
emission of the greenhouse gases, and thus contribute towards meeting the goals and objective
set at the 1997 Kyoto Protocol. If the above leadership and bold initiatives are not forthcoming,
it is certain that the bureaucrats will impose unpleasant regulations and taxes on the industries
contributing significant amounts of greenhouse gases to the atmosphere. The manufacturing of
17
portland cement is one such industry.‖58 Power generation using fossil fuels for combustion
would be another targeted industry. The PCA45 has used proportions of 86.4% portland cement
and 13.6% fly ash in the CO2 absorption calculations. In 2001, all the concrete placed in the
USA from 1950 to 2000 was calculated to absorb approximately 69.2 million tons (76.3 million
short tons) of atmospheric CO2.
The commonly employed phenolphthalein color staining test was also confirmed to
accurately describe the depth and degree of carbonation by the PCA.45 Figure 1 shows the
percent of carbonated material measured by standard gravimetric analysis versus the carbonation
indicated by the phenolphthalein color staining test. The concrete specimens were sliced into a
series of eight consecutive 5 mm (0.2 in) thick increments parallel discs to the top exterior
surface. Slices were then immediately ground to a fineness of 45 microns (200 mesh). Powders
from the ground specimens were then subject to thermo gravimetric analysis to determine the
relative concentrations of carbonates.
Figure 1 Depth of Carbonation by Phenolphthalein and Analytical Methods.45
18
In the report of the International Panel on Climate Change6 (IPCC) on carbon dioxide
capture and storage, Chapter 7 was dedicated to the topic of mineral carbonation and industrial
uses. ―In the case of mineral carbonation, captured CO2 is reacted with metal-oxide bearing
minerals thus forming the corresponding carbonates and a solid byproduct, silica for example.
Natural silicate minerals can be used in artificial processes that mimic natural weathering
phenomena, but also alkaline industrial wastes can be considered. The products of mineral
carbonation are naturally occurring stable solids that would provide storage capacity on a
geological time scale. Moreover, magnesium and calcium silicate deposits are sufficient to fix
the CO2 that could be produced from the combustion of all fossil fuel resources.‖ The IPCC
report describes in-situ carbonation with geologic storage and ex-situ storage that involve the
mining, grinding, and activation necessary to accommodate mineral carbonation. The report
recognizes that ―On a smaller scale, industrial wastes and mine tailings provide sources of
alkalinity that are readily available and reactive. Even though their total amounts are too small
to substantially reduce CO2 emissions, they could help introduce the technology.‖ The report
also acknowledges that mineral carbonation today is an immature technology.
A study prepared by the Energy Analysis Department of the Ernest Orlando Lawrence
Berkeley National Laboratory59 discussed energy efficiency measures employed in the
manufacture of portland cements. It shows that a 30% reduction of primary physical energy
intensity and a 25% reduction in CO2 emissions for cement production have occurred between
1970 and 1997. The report goes on to identify that the production of blended cement in the
USA, which is already common in many other parts of the world, could result in an additional
reduction of 18% of energy use and a 16% reduction in CO2 emissions from cement production.
19
The combined effect from reduced CO2 emissions from fossil fuel combustion and offsetting the
CO2 from the resulting reduction in cement usage combines for a total 25% reduction in CO2
emissions with the use of blended cement.
This report demonstrates that blended cement
production could be a key strategy to a cost-effective energy efficiency improvement and CO2
emission reductions in the cement industry.
1.5 Theoretical Basis of Carbonation of Concrete
Carbonation is a chemical reaction in which solid products of cement hydrates, primarily
calcium hydroxide (Ca(OH)2), and to a lesser extent calcium silicate hydrates (CSH), calcium
aluminate hydrates, and calcium sulfoaluminate hydrates (mainly ettringite), in cement-based
materials react with carbonic acid (CO2 + H2O = H2CO3). In concrete technology, carbonation
may be defined as a chemical process in which the pH of concrete is reduced from around 12.5
to below 9 through the absorption of carbon dioxide60. In theory, carbonation process is very
simple but in reality it is a complex set of chemical reactions. CO2 in gaseous form can not react
directly with the hydrates of the cement paste. Therefore, for carbonation, the CO2 gas has to
first dissolve in water to form carbonate ions which in turn react with the calcium ions (Ca2+) of
the pore water. Therefore, as a simplified way, carbonation is a chemical reaction in which
atmospheric carbon dioxide penetrates the concrete and reacts with the alkaline calcium
hydroxide and other cement hydrates of concrete to form carbonates, thereby liberating water
and/or metal oxide depending upon the hydration product involved. The type of carbonate ions
depends on the pH. When carbon dioxide comes into contact with water at neutrality (pH about
7.5), it forms bicarbonates. Inside cement-based materials, the pH is very high (about 12.5+).
20
Therefore, as a result the bicarbonate dissociates and forms carbonate ions. Hence, in the
carbonated layer of a cement-based material, bicarbonate forms; but, closer to the noncarbonated
cement paste carbonates ions form due to higher pH leading to the precipitates of calcium
carbonates crystals.61 Concrete carbonation starts from surface and moves inwards.
Carbonation process can be expressed by the following chemical equations:
1. CO2 (g) + H2O = HCO3 – (bicarbonate ion) + H+
2. HCO3 – = CO3 2- (carbonate ion) + H+
The carbonate ions react with Ca2+ in the pore solution to form calcium carbonate crystals.
3. Ca2+ + CO3 2- = CaCO3
This reaction lowers the concentration of Ca2+ in the pore solution, which in turn lead to
dissolution and reduction of the primarily calcium hydroxide.
4. Ca(OH)2 = Ca2+ + 2OH -
Thus, calcium hydroxide (CH) dissolves and calcium carbonate precipitates. This reaction
will continue until all of the CH is consumed. This results in lowering of the pH which
destabilizes other cement hydration products. Once the concentration of calcium ions drops,
then C-S-H start dissolving.
Monosulphate (the ettringite reacts with C3A to form the
monosulphate phase; i.e., 3CaO4Al2O3SO312H2O) decomposes at a pH of around 11.6.
Ettringite is carbonated easily and is not stable at slightly lowered pH.
The ettringite
decomposes at a pH of around 10.661. Therefore, Ettringite is usually absent in carbonated
residue. The reaction with silicates and aluminates are as given below:
5. 3CaO.2SiO2.3H2O + 3CO2 = 3CaCO3 + 2SiO2 + 3H2O
6. 4CaO.Al2O3.13H2O + 3CO2 = 4CaCO3 + 2Al(OH)3 + 10H2O
21
At a pH of less than 9.2 none of the original calcium containing phases remains. Most of the
calcium from the C-S-H will be bound to calcium carbonate but some Ca will always remain in
silica gel.
Lagerblad61 has made an attempt to summarize stability of different hydration
products of cement with respect to carbonation. He divided the carbonation process in five
stages with respect to the lowering of pH as shown in Table 1.
Table 1: Stable Phases in Portland Cement Paste at Different pH (from Lagerblad 61)
Non-Carbonated Concrete
Stage 1
Calcium
Hydroxide,
Ca(OH)2
Calcium
Silicate
Hydrates, CSH
Ca/Si > 1.5
AFm
Monosulphate
Aft
Ettringite
pH > 12.5
Carbonated Concrete
Stage 2
Stage 3
Stage 4
Stage 5
-
-
-
-
CSH
1.5 < Ca/Si > 0.5
Ca(OH)3
Ca(OH)3
Ca(OH)3
CaO, Ca/Si < 0.5
AFm
Al(OH)3
Al (OH)3
Al(OH)3
AFt
Aft
Fe(OH)3
Fe(OH)3
pH < 12.5
pH < 11.6
pH <10.5
pH < 10
SiO2 with some
The calcium carbonate (CaCO3) formed by reaction of lime (CaO) is calcite. However,
the C-S-H reacts to form amorphous silica gels and calcium carbonates of different types: calcite,
aragonite, or vaterite62, 63.
22
1.6 Mechanism of Carbonation
The mechanism behind carbonation is inward diffusion of carbon dioxide gas and
carbonate ions, starting with the concrete surface(s) exposed to air (or, any environment
containing CO2). Carbonation process lowers the amount of
Ca2+ ions in the pore solution,
which in turns trigger dissolution of CH and Ca2+ and diffusion from the interior of the concrete
to the carbonation front, where the concentration of both components will be at low point due to
low solubility of calcium carbonates61. The speed of diffusion of both Ca2+ and carbonate ions
govern the mechanism of carbonation. Beside the concentration gradients of ions, the process of
diffusion in concrete is controlled by its pore system and pore saturation (i.e., how full is the
liquid in the connective pore system). In fully saturated concrete, only carbonate ions can move
and carbonation is slow. In dry concrete, carbon dioxide can penetrate to a deeper depth but
carbonation does not occur due to a lack of water. Therefore, pore saturation plays a vital role in
the mechanism of carbonation of concrete. Hence, porous concrete with favorable relative
humidity should be very good for accelerated carbonation reaction.
1.7 Modes of Carbonation
Carbonation front is dependent on the relative concentration and speed of Ca and
carbonates ions. If the concentration of carbonate ion is high then calcium carbonate may
precipitate on the surface of calcium hydroxide (CH). It may also precipitate in the pore
solution, or on other phases, if the Ca2+ concentration is high61. CH is the most soluble phase in
hydrates of cement. It is first to dissolve and forms carbonate. If the carbonate ions move faster
than Ca2+ then calcium carbonate (CC) precipitates on the surface of calcium hydroxide making
23
a shell of calcium carbonate around it. This shell slows down the carbonation process for CH.
However, since the product is porous, it will only delay the carbonation process. If Ca2+ moves
faster than carbonate ions then CH will dissolve and calcium carbonate will precipitate in
capillary pore system. In such a situation, volume change will result in densification and
decrease in porosity. Such situations may happen in pure ordinary portland cement (OPC)
paste61. After the calcium hydroxide (CH) is consumed the carbonation will start consuming CS-H. The C-S-H dissolves in different mode than CH. The calcium silicate hydrate (C-S-H)
reacts to form amorphous silica gels and calcium carbonates of different types (i.e., calcite,
aragonite or vaterite61, 64). The reaction is linked to pH and depended on Ca/Si ratio. In this
case, calcium carbonate will precipitate close to the C-S-H and to a larger extent affect the gel
porosity rather than the capillary porosity.
Stark and Ludwig65 have reported a coarser
microstructure for concrete made with slag-cement due to carbonation.
Chen et al.66 reported a decrease of Ca/Si ratio of C-S-H from a value about 1.5 to 0.11 at
pH of 9.54. A pH of 9.54 is in the upper range of the phenolphthalein indicator suggesting that
the Ca/Si ratio of C-S-H in carbonated paste may even be even lower than 1.5.
1.8 Carbonation Related Microstructural Change
Almost all the transport phenomena occurring through cement-based materials, including
concrete is governed by their microstructural properties (i.e., porosity, pore sizes, types of pores
and its distribution, and pore connectivity). Carbonation of cement-based materials involves
diffusion of calcium and carbonate ions. Beside the concentration and gradients of these ions,
the process of diffusion in concrete is controlled by the pore system and pore saturation. The
24
diffusion coefficients and water permeability are also affected by the reduction of porosity and
supply of water during the carbonation process38, 67. Therefore, the carbonation of concrete is
affected by the pore system of the cement-based materials. Silva et al.68 studied the effects of
carbonation on the microstructure of concrete by using mercury intrusion porosimetry and
scanning electron microscopy (SEM). They also measured the open porosity (Open pores are the
pores which are accessible to fluid. They are interconnected. The most common method to
determine open porosity is the amount of water absorbed by a dried concrete specimen. Besides
open and interconnected pores there are also closed isolated pores which are not accessible to the
fluid from surface) of the concrete specimens by using the RILEM69 procedure to obtain the
porosity accessible to water. Based on the mercury porosimetry results, they reported a lower
total porosity for carbonated concrete in comparison with the controlled noncarbonated concrete.
Furthermore, the pore system features such as the surface area of pores, threshold diameter,
average pore size, and similar features were reported to be significantly different for carbonated
and noncarbonated concrete.
Measurement of porosity accessible to water showed that
carbonated concrete could become more compacted, with a reduction of 5 to 12% of the open
porosity, in comparison with the noncarbonated concrete. Using SEM, they also observed
microstructure to be more uniform for carbonated concrete.
Villain and Thiery70 studied the impact of carbonation on microstructure and transport
properties of concrete and reported that carbonation significantly affected the transport properties
by modifying and densifying the microstructure of the concrete. They further reported reduction
in both the total porosity and pore size distribution due to precipitation of products such as
calcium carbonates and silica gels, which have a bigger molar volume than the initial
25
components such as the calcium hydroxide (CH) or C-S-H. Transformation of CH to calcite and
metastable vaterite gives a volume reduction of 11% and 14%, respectively. These volume
changes decrease the porosity in the carbonated layer. The increased volume of calcite and
vaterite normally fills empty space in the capillary system and hence densifies the cement paste.
Björn and Peter71 investigated the microstructural changes caused by the carbonation of
cement mortar and found 8% increase in specific surface area (i. e., deceasing the pore size
and/or increasing the number of pores) in case of well carbonated sample than the noncarbonated
sample. They also reported about two times increase in the volume of small pores for the
carbonated sample.
1.9 Effect of Pozzolans on Carbonation of Concrete
It has become a common practice to add fly ash, silica fume, blast furnace slag, and other
pozzolanic materials during concrete-making to derive technical and environmental benefits over
concrete without pozzolanic materials. In comparison of ordinary portland cement, cement
containing pozzolan has less calcium hydroxide (CH) and more C-S-H because calcium
hydroxide will be consumed in the pozzolanic activity to produce C-S-H. Further, C-S-H in such
cement paste also contains more Aluminum and Magnesium. Therefore, the carbonation process
and the structure of the carbonated paste will be different than in cement with pozzolan verses
that of the ordinary portland cement paste. The amount of calcium ions to be carbonated is less
in cement with pozzolan and, thus, the carbonates ion can penetrate to a greater depth and
26
carbonation rate increases with the amount of fly ash38,
61, 72, 73
. Such increased carbonation
effect depends upon the type and amount of pozzolans.
The results of carbonation of concrete or other cement-based materials can be either
beneficial or harmful depending on the time, rate, and extent to which the carbonation occurs and
the environmental exposure, as well as if steel reinforcement, and other embedded items of steel,
is present or not. It is known that carbonation can provide higher strength and increased
hardness to mortar, plaster, concrete, and other cement-based products. However, carbonation
can also result in deterioration due to the decrease in pH of the cement paste leading to corrosion
of reinforcing steel, if steel is present in the carbonated zone. Carbonation in concrete is the
reaction of the cement and water hydration products dissolved in pore water with the carbon
dioxide, usually from the ambient air, which reduces the pH of the concrete pore solution from
about 12.6 to 9. At this value of pH, the steel‘s passive oxide film may be destroyed, thus
accelerating its corrosion. Exposure to CO2 during the hardening process can also result in
carbonation shrinkage and affect the finished surface (e. g., of slabs) by leaving a soft dusting of
carbonated products (powdered calcite) making the surface less wear-resistant. The reaction of
hydrated portland cement in the air is generally a slow process and dependent on the relative
humidity of the environment, temperature, permeability of the concrete, and concentration of
CO2 available. By increasing the temperature and pressure, it is possible to increase the rate at
which carbonation occurs in concrete74.
Carbonation can also occur from exposure to
groundwater where CO2 had dissolved in water and combined to form carbonic acid H2CO3.60, 75
27
1.10 Rate of Carbonation
Carbonation of cement-based materials starts from the surface and moves inwards. The
rate of carbonation is mainly influenced by the permeability and calcium content of the concrete
besides ambient atmospheric conditions (i.e., amount of carbon dioxide, relative humidity, and
temperature). The carbonation process is controlled by the law of diffusion. At the carbonation
front, carbon dioxide reacts with alkalis of the pore solution to form various types of carbonate
phases. The depth of carbonation is generally calculated by using a square root time relationship
as given below:
Xc = K x t0.5
where Xc, K, and t are depth of carbonation, constant of carbonation rate, and age of
concrete at the time of evaluation, respectively. For most cases the above equation is accepted as
a good approximation. However, for high-strength concrete, or concrete under exposed outdoor
conditions, this equation may not be suitable. The carbonation rate constant (K) is generally
faster for indoor concrete than that exposed to outdoor due to the fact that carbonation rate is
dependent upon the amount of CO2 in the air and relative humidity76. Lagerblad61, based on
literature review, suggested various carbonation rate constants for concrete of different strength
and exposure conditions, Table 2.
28
Table 2: Carbonation Rate Constants, K (for ordinary portland cement) for Various
Concrete Cylinder Strength and Exposure Conditions, per Lagerblad61
Compressive strength
Exposure
condition
< 15 MPa
15–20 MPa
25–35 MPa
> 35 MPa
(mm/(year)0.5)
(mm/(year)0.5)
(mm/(year)0.5)
(mm/(year)0.5)
Wet/submerged
2
0.1
0.75
0.5
Buried
3
1.5
1.0
0.75
Exposed
5
2.5
1.5
1.0
Sheltered
10
6.0
4.0
2.5
Indoor
15
9.0
6.0
3.5
Table 2 shows the variation of carbonation rate constant, K, from 0.5 to 15. It reflects the
effect of quality/strength of concrete and exposure condition on the carbonation of concrete.
The process of carbonation and the rate of carbonation penetration in a concrete product
depend on porosity and pore structure of concrete, availability of Ca(OH)2, moisture content of
the concrete product, relative humidity and CO2 concentration of the surrounding environment,
and use of mineral admixtures in concrete77. Atis78 reported lower depths of carbonation at
higher strength levels. He also reported higher depth of carbonation of concrete with higher
porosity.
Neville79 has mentioned ―the fundamental factor controlling carbonation is the diffusivity
of the hardened cement paste, which is a function of the pore system of the hardened cement
29
paste during the period when the diffusion of CO2 takes place.‖ Pore structure has a direct effect
on the permeability of concrete. The permeability of concrete to air and water mainly depends
on the type and amount of cementitious materials, the degree of hydration, the water to
cementitious materials ratio, type, size, and grading of aggregates, the degree of compaction, and
curing conditions80-83. Sulapha et al.84 found that a lower water to binder ratio and a long-term
curing in water resulted in a slower rate of carbonation.
Several studies78,
79
have reported that the highest rate of carbonation occurs for the
relative humidity of the surrounding environment between 50 % to 70 %. Concrete with high
internal moisture shows a much lower rate of carbonation because the diffusion of CO2 becomes
difficult when pores are saturated with water. Carbonation rate also reduces at a lower internal
moisture level due to insufficient water in the pores85 necessary to form carbonic acid. Besides
relative humidity, the CO2 concentration in the surrounding environment of the concrete is also a
very important factor that affects the rate of carbonation. Verbeck86 concluded that carbonation
proceeds slowly and produces little shrinkage at relative humidity of 100 % and 25 %; and, he
found maximum carbonation shrinkage at 50 % relative humidity. He also concluded that
besides the relative humidity, the dimension of the specimen (carbonation in large dense concrete
members will be limited to surface layers and hence shrinkage may be insignificant) and the CO2
concentration (with increase in CO2 concentration carbonation rate increases.
Increased
carbonation rate results in quick loss of moisture causing shrinkage.) in the curing environment
also have significant influences on the carbonation and carbonation shrinkage86.
30
Sagüés et al.87 found that for concrete mixtures made by 20 % cement replacement with
fly ash and having 444 kg/m3 of cementitious materials (cement plus fly ash, in this case), the
depth of carbonation increased as the water to cementitious materials ratio increased from 0.37 to
0.50. They also found that at a given water to cementitious materials ratio, the depth of
carbonation increased as the cement replacement by fly ash increased from 20 to 50 %. They
reported a decrease in depth of carbonation as the compressive strength of concrete increased.
Collepardi et al.88 concluded that at a given water to cementitious materials ratio, the rate of
carbonation increased when the cement replacement rate with fly ash increased beyond 15 %.
Concrete containing fly ash, if not cured sufficiently, may have a higher degree of
carbonation. Fly ash, meeting ASTM C 618 requirements, used in concrete can show the same
trend of carbonation as concrete made without fly ash89.
1.11 Measurement Methods of Carbonation Profile/Depth in Concrete
To evaluate the carbonation profile/depth in concrete, use of various experimental
methods are reported64, 90, 91. The simplest and most well known method to determine the depth
of carbonation in concrete in laboratory, as well as at a site, is a pH indicator, such as the RILEM
phenolphthalein test. The method involves phenolphthalein spraying on the freshly cut or split
concrete specimen and observation of color change that indicate the depth of carbonation. It
gives a carbonation depth corresponding to a pH value near to 991.
31
1.12 Limitations of Phenolphthalein Test Method
This test cannot detect the existence of partially carbonated zone of concrete where pH is
higher than 9 or difficult to detect90, 92-94. Furthermore, this method cannot distinguish loss of
concrete alkanality caused by carbonation or by other causes such as exposure to acids. Chang
and Chen93 reported that ―at a pH value of 9.0 of pore solution indicated by phenolphthalein test
the degree of carbonation is 50% while at a pH of 7.5 the degree of carbonation is 100%‖.
1.13 Other Methods for the Measurements of Carbonation Profile in
Concrete64, 90, 91
In order to improve the understanding of the carbonation process and to measure quantitative
carbonation profile over time, the following methods are also being used for the determination of
carbonation depth in concrete:
Thermal Gravimetric Analysis (TGA)
Gammadensimetric Method
X-Ray Diffraction Analysis (XRDA)
Infrared spectrometry
These methods are used either alone or together with other methods. Among these methods the
TGA and Gammadensimetric methods are frequently used by the researchers64, 90, 91.
32
1.13.1
Thermal Gravimetric Analysis (TGA)
TGA method quantifies the portlandite and the calcium carbonates resulting from
carbonation of concrete64, 95. TGA involves continuous measurement of the mass of a sample
subjected to a variation in temperature. Each chemical component is characterized by its own
temperature range of decomposition and a specific mass loss involving gaseous emissions64, 91.
Table 3 shows the temperature ranges of cement-hydrate decomposition during TGA
measurements as used by Villain and Platret64.
Table 3: Temperature Ranges of Hydrate Decomposition during TGA Measurements64
Field
Temperature range
Decomposition of hydrates or carbonated products
Free and adsorbed H2O, H2O from C-S-H, AFt,
1
25 to 430 ºC
AFm, gypsum, and CO2 adsorbed in C-S-H
2
430 to 520 ºC
H2O from portlandite Ca(OH)2
OH- from structure of hydrates, structure H2O or
3
520 to 620 ºC
CO2 from vaterite, and C-S-H carbonation
4
650 to 720 ºC
CO2 from calcite of carbonation
5
720 to 900 ºC
CO2 from calcite of aggregates
6
900 to 1150 ºC
Other structural H2O
Note: For heating rate of 10 ºC/minute
Villain and Platret64 used two experimental methods, TGA and Gammadensimetry, to
determine carbonation profile that is related to the amount of chemically-fixed carbon dioxide at
various depths in concrete. Gammadensimetry was used to monitor the progress of carbonation
33
during the entire experimental period. They claimed that Gammadensimetry is very useful in the
monitoring progress of carbonation in the same sample subjected to either natural or accelerated
carbonation.
Therefore, this method can be used to validate the mathematical models for
carbonation depth.
1.13.2
Gammadensimetry Method
It is a non-destructive method. It is commonly used to measure density variations due to
variations of water content during drying or water soaking process and, also, due to segregation
of aggregates90. Villain and Platret64 and Villain et al.91 have demonstrated the use of this
method in determination of density variations related to CO2 penetration in concrete during the
carbonation process.
Chang and Chen90 determined the depth of carbonation in concrete by using TGA, X-ray
Diffraction Analysis (XRDA), and Fourier transformation infrared spectroscopy (FTIR) tests
along with phenolphthalein indicator tests. They identified three zones of carbonation different
values of pH (i.e., fully carbonated, partially carbonated, and noncarbonated) in carbonated
concrete. The fully carbonated zone is identified with pH of less than 9.0 with a degree of
carbonation greater than 50%. The degree of carbonation in partially carbonated zone lies
between 0 - 50% carbonation (9.0 < pH < 11.5).
Noncarbonated zone is marked by the zone
where a sign of carbonation was not detected. They reported that the depth of carbonation,
determined by TGA, XRDA, and FTIR, in carbonated zone, where the phenolphthalein remained
colorless (indicating absence of carbonation), was found to be twice that shown by
34
phenolphthalein indicator. They further reported that the pH of the pore solution in concrete
changes with the degree of carbonation. The pH value where phenolphthalein shows color
change is generally 9.0 at which the degree of carbonation is 50%. They further concluded that
when the pH of pore solution was 7.5, the degree of carbonation was 100%.
They also
concluded that TGA, XRDA, and FTIR give similar results of carbonation depth.
Task 2: Theoretical Quantification of CO2 Sequestration Potential in
Cementitious Materials
Theoretically the maximum carbon dioxide uptake by portland cement concrete can be
estimated on the basis of the chemical compositions of the cement binder by using the Steinour96
formula, as given below:
CO2 (wt%) = 0.785 (CaO – 0.7SO2) + 1.09 MgO + 1.42 Na2O + 0.935 K2O
Monkman and Shao97 conducted research on the carbonation behavior of six cementitious
materials including CSA Type 10 (ordinary) cement, CSA Type 30 (high-early strength) cement,
fly ash, ground granulated blast furnace slag, electric arc furnace dust, and hydrated lime that
were subjected to 100% CO2 at a constant pressure of 5 bars for 2 hours. The CO2 uptake for all
materials was significantly less than the theoretical maximum as predicted by the Steinour96
formula. Their conclusions suggest that the carbonation reaction may be limited to about 25% of
its potential due to the lack of water. The primary product of carbonation was calcite (CaCO3).
35
In comparing the cements, it was noted that an increase in the fineness of the cement resulted in
an increase of carbonation.
Estimated potential for sequestration of carbon dioxide by mineralization in cementbased materials can be calculated by knowing the cement content and CaO content of the
cementitious material.
If it is assumed that 100% of the calcium oxide found in calcium
hydroxide, Aft, and AFm and 50% of the CaO found in C-S-H have been transformed into
calcium carbonate in a carbonated concrete, as indicated by the phenolphthalein indicator, then
about 75% of the CaO of the portland cement clinker is consumed by the carbonation 61.
Therefore, amount of carbon dioxide consumed in carbonation may be calculated as given
below:
Amount of carbon dioxide consumed = 0.75 x C x CaO x
M CO 2
(kg/m3)
M CaO
where,
0.75 is amount of Cao carbonated,
C is amount of portland cement in concrete per m3,
CaO = amount of CaO in cement (weight %)
MCO2 = Molecular weight of carbon dioxide
MCaO = Molecular weight of calcium dioxide
Example of the Carbon Dioxide Sequestration Potential in Cement-based Materials
If a concrete contains 350 kg of portland cement per cubic meter of mixture and the cement
contains 64% of Cao, then the potential for carbon dioxide uptake is:
36
= 0.75 x 350 x 0.64 x 0.786 kg/m3
= 132 kg/m3
Therefore, percentage carbon dioxide sequestration potential of cement in the concrete =
132
x
350
100 = 38 %.
Jahren98 also reports ―that a fully carbonated concrete could bind about 0.3 kg of CO2 per
kg of cement and get a strength increase of 30%‖.
The method of direct mass gain has also been used by researchers40, 43 to estimate the
CO2 uptake in carbonation of concrete and other cement-based materials. This method involves
a mass comparison before and after carbonation; and, a dry cement binder is used as a reference,
as given below:
Mass gain (%) =
(mass) atf ,CO 2 (mass) bef ,CO 2
waterlost
(mass) dry ,binder
where, (mass)aft,CO2 is the mass measured after carbonation (net mass gain); (mass)bef,CO2 is the
mass measured before carbonation; (mass)dry binder is the mass of dry cement used; and, water lost
is the mass of water expelled from the sample during carbonation. In this method of calculation
of carbon dioxide sequestration potential, it is assumed that the surface and the core of the
samples are equally carbonated.
37
Summary
A state-of-the-art literature review on various technologies of carbon dioxide
sequestration has been presented. The potential for sequestration of carbon dioxide in cementbased materials has also been presented. The possibilities for carbon dioxide sequestration in
cement-based materials using CO2 mineralization have been presented.
Different claims
regarding extent of CO2 sequestration in cementitious product has also been discussed. Based
upon technical conclusions presented in this report, it is apparent that carbon dioxide
sequestration in cement-based materials, including concrete, by adopting engineered way of
carbonation (i.e., early age accelerated carbonation curing) can be used as an economically
viable new approach for safe and long-term direct sequestration of carbon dioxide.
Acknowledgements
The report was prepared using the work done by Bruce W. Ramme as reported in his Ph.
D. thesis (Reference # 41).
The UWM Center for By-Products Utilization was established in 1988 with a generous
grant from the Dairyland Power Cooperative, La Crosse, WI; Madison Gas and Electric
Company, Madison, WI; National Minerals Corporation, St. Paul, MN; Northern States Power
Company, Eau Claire, WI; We Energies, Milwaukee, WI; Wisconsin Power and Light Company,
Madison, WI; and, Wisconsin Public Service Corporation, Green Bay, WI. Their financial
support and additional grant and support from Manitowoc Public Utilities, Manitowoc, WI, are
gratefully acknowledged.
38
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