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UWM Center for By-Products Utilization
GLOBAL WARMING AND
CEMENT-BASED MATERIALS
By Tarun R. Naik and Rakesh Kumar
ii
This first edition is being published at the Second International Conference on
Sustainable Constructions Materials and Technologies, Ancona, Italy, June 2010.
Copyright 2010 by Tarun R Naik, UWM Center for By-Products Utilization,
Milwaukee, Wisconsin USA
Cover Photo: Fontana di Piazza Roma, Ancona, Marche, Italy; courtesy of Irene de Venecia.
iii
GLOBAL WARMING
AND CEMENT-BASED MATERIALS
First Edition
Professor Tarun R. Naik
Research Professor and Academic Program Director
UWM Center for By-Products Utilization
University of Wisconsin-Milwaukee
USA
Dr. Rakesh Kumar
Scientist, Rigid Pavements Division
Central Road Research Institute (CSIR)
Mathura Road, New Delhi
INDIA
iv
Preface
The principal aim of this book is to provide an overview of the CO2 emission coming from the
construction industry and the capability of cement-based materials such as concrete and controlled low
strength material (CLSM) for the sequestration of carbon dioxide gas to help reduce global warming. The
information and data contained in this book have been selected from a wide variety of sources. An
economical, efficient, and viable technology for the sequestration of carbon dioxide in typical concrete,
no-fines concrete, CLSM, and other similar cement-based materials, with and without using fly ash are
described in this book. The capability of earning carbon credit by sequestration of CO2 in cement-based
materials is also demonstrated. The authors believe that this book should be useful to cement
manufacturers, thermal power plants owners, practicing engineers, academicians, environmental
regulatory authorities, and the construction industry.
The authors are grateful for editorial review and revisions made by Dr. Margaret Lansing and
formatting and style improvements made by Dr. John Zachar.
v
Contents
Chapter 1. CARBON DIOXIDE EMISSION AND
CONCRETE CONSTRUCTION ...………………………………………….. 1
1.1 Carbon Dioxide and Global Warming ...………………………. ……….. 1
1.2 Carbon Dioxide Emission from Cement Industry...……………………... 4
1.3 Carbon Dioxide Sequestration ...………………………………………... 6
1.4 Carbon Credits...…………………………………………………………. 8
Chapter 2. CARBON DIOXIDE (CO2) SEQUESTRATION IN
CEMENT-BASED MATERIALS...………………………………………….. 9
2.1 CO2 and Cement-Based Materials...……………………………………. 9
2.2 Carbon Dioxide Sequestration, Concrete, and
other Cement-Based Products...…………………………………………... 11
2.3 Other Reports by Industries for the Sequestration
of CO2 in Cement-Based Products...……………………………………… 15
Chapter 3. CARBONATION OF CEMENT-BASED MATERIALS...………………... 21
3.1 Theoretical Basis of Carbonation of Concrete and
other Cement-based Materials ...………………………………………. 21
3.2 Mechanism of Carbonation ...………………………………………….. 24
3.3 Modes of Carbonation...………………………………………………... 25
3.4 Carbonation Related Microstructural Change...………………………... 26
3.5 Effect of Pozzolans on Carbonation of Concrete ...……………………. 27
3.6 Rate of Carbonation...…………………………………………………... 29
vi
3.7 Measurement Methods of Carbonation Profile/Depth in Concrete...…... 32
3.8 Limitations of the Phenolphthalein Test Method ...……………………. 32
3.9 Other Methods for the Measurement of the Carbonation
Profile in Concrete………………………………………………………………33
3.9.1 Thermal Gravimetric Analysis (TGA) ...……………………… 33
3.9.2 Gammadensimetry Method ...………………………………….. 34
3.9.3 X-ray Diffraction Analysis (XRDA) Method……………………35
Chapter 4. CO2 SEQUESTRATION POTENTIAL IN
CEMENT-BASED MATERIALS ...…………………………………………. 36
4.1 Theoretical Quantification of CO2 Sequestration
Potential in Cement-Based Materials…………………………………...36
4.2 Experimental Investigation on CO2 Sequestration Potential in Concrete
and CLSM……………………………………………………………….38
4.2.1 Portland Cement ...……………………………………………… 39
4.2.2 Fly Ash ...……………………………………………………….. 40
4.2.3 Fine Aggregate (Sand) ...……………………………………….. 41
4.2.4 Coarse Aggregates ...…………………………………………… 42
4.2.5 Chemical Admixtures...………………………………………... 44
4.3 Mixture Details……………………………………………..………….. 44
4.3.1 Trial Concrete Mixtures ...………………………………………45
4.3.2 Final Concrete Mixtures ...……………………………………... 47
4.4 No-Fines Concrete Mixtures ...…………………………………………. 49
4.4.1 Specimens Preparation and Curing ...…………………………………... 50
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4.5 Carbonated Depth of Concrete...………………………………………... 53
4.5.1 Carbonated Depth of Concrete Mixtures made with MRWRA.... 54
4.5.2 Carbonated Depth of Concrete without MRWRA ...………….... 56
4.5.3 Carbonated Depth of No-Fines Concrete ………………………. 58
4.6 CO2 Sequestration in Concrete through Carbonation…………………. 59
4.7 Controlled Low Strength Material (CLSM) Mixture………………………….. 65
4.7.1 Carbonated Depth of CLSM...……… ……………………….... 67
4-8 Quantification of Carbon Dioxide Sequestered
in Portland Cement Used in Concrete and CLSM ...…………………… 70
Chapter 5. OBSERVATIONS ...……………………… ………………………………….. 72
Chapter 6. ACKNOWLEDGEMENTS…………………………………………………....76
REFERENCES…………….....……………………………………………………………….. 77
1
Chapter 1
CARBON DIOXIDE EMISSION AND CONCRETE
CONSTRUCTION
1.1 Carbon Dioxide and Global Warming
Carbon dioxide (CO2) exists in gaseous form in the Earth‟s atmosphere at a standard temperature (0 0C
i.e. 273.15 K) and pressure (0.986 atm. i.e. 100 kPa) [Lingen, 1986]. It is a trace gas currently (2008)
being 0.038% (380 ppm – parts per million) of the Earth‟s atmosphere. However, it can also exist in
different phases such as solid, liquid, and supercritical fluid with change in pressure and temperature (Fig.
1) [http://en.wikipedia.org/wiki/Carbon_dioxide].
Fig. 1. Pressure – Temperature Phase Diagram of Carbon Dioxide
[http://en.wikipedia.org/wiki/Carbon_dioxide]
2
The gaseous phase of carbon dioxide has a very critical effect on the Earth‟s ecosystems because it is a
greenhouse gas (GHG). 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” [Schwartz, 2006]. US-EPA declared in April 2009 that
CO2 is a danger to human health [Financial Times, April 18/19, 2009]. 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” [Patulski, 2006].
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 [http://www.nationalacademies.org/includes/G8Statement_Innovation_07_May.pdf]. 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 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” [http://www.nationalacademies.org/includes/
G8Statement_Innovation_07_May.pdf].
3
It is now generally accepted that the global warming is caused by an increase in the concentration of
greenhouse gases (GHGs) in the Earth‟s atmosphere from human activities. Carbon dioxide gas is the
principal greenhouse gas. The major human activities which contribute to the emission of the CO2 gas in
the Earth‟s atmosphere include combustion of fossil fuels and deforestation. A natural emitter of large
amounts of CO2 gas is volcanoes, hot springs, and geysers. However, the emissions of carbon dioxide gas
by human activities are about 130 times greater than the quantity emitted by volcanoes; and, it is about 27
billion (metric) tonnes/year (about 30 billion (short) tons) [USGS 2009]. The increasing concentration of
carbon dioxide gas in the Earth‟s atmosphere has raised concerns about global warming, climate change,
and their subsequent effects on its inhabitants. The global average atmospheric carbon dioxide
concentration has risen from 280 ppm (by volume) at the beginning of the industrial revolution (about
150 years ago) to 387 ppm today (2008). The annual growth of the global average CO2 concentration in
the Earth‟s atmosphere since the beginning of continuous monitoring in 1959 is given in Fig. 2.
Fig. 2. CO2 Concentrations Measured at Mauna Loa Observatory [NOAA, 2009]
The maximum growth rate of atmospheric carbon dioxide was 1.9 ppm/year during 2000 – 2006
[Canadell et al., 2007]. Carbon dioxide is a toxic gas. Its effects on the human body increase with an
increase in its concentration. Some of these effects as reported by Davidson [2003] are:
4
1% increase can cause drowsiness with prolonged exposure;
2% increase is mildly narcotic and causes increased blood pressure and pulse rate; and,
at about 5% increase it causes stimulation of the respiratory organs, dizziness, confusion, and
difficulty in breathing.
Scientists, engineers, researchers, environmentalist, geologists, and others, along with carbon dioxide
contributing industries, are making tireless efforts to develop efficient and viable technologies in their
respective areas that could help in reducing carbon dioxide concentration in the atmosphere.
1.2 Carbon Dioxide Emission from Cement Industry
From 1850 to 2006 about 330 x 109 metric tons of CO2 gas have been accumulated in the Earth‟s
atmosphere from burning of fossil fuel and emissions from cement industry [Canadell et al., 2007].
Fossil fuel and cement emissions increased from 7.0 PgC/y (1PgC = 1 petagram = 109 metric tons of C) in
2000 to 8.4 PgC/y in 2006, which is 35% above the emissions in 1990. The average growth rate of fossil
fuel and cement emissions increased from 1.3% per year for 1990-1999 to 3.3% per year for 2000-2006.
The annual atmospheric CO2 concentrations measured at Mauna Loa Observatory by NOAA [2009] is
presented in Fig. 2.
Cement is a backbone of the construction industry. From an environmental prospective, the concrete
construction industry is a very large consumer of natural resources such as stone, sand, and drinking
water; and, at the same time, it is also one of the biggest generators of large amounts of waste. Each of
the primary ingredients of concrete, i.e., cement, aggregate, and water has some adverse environmental
impacts [Mehta 2001; Mehta 2002]. The production of cement, the main ingredient of concrete, is a
highly energy-intensive process and releases green-house gases (GHGs). The cement industry contributes
5
approximately 6% of the total anthropogenic CO2 emission [Hendriks et al. 2004; Naik 2008]. Since
global warming has emerged as the most serious issue of recent time, and at the same time sustainability
is becoming an important economical and political issue, there is an urgent need for the carbon dioxide
contributing industries to develop technologies that could help in reducing carbon dioxide concentration
in the atmosphere. The major environmental issue associated with the concrete construction industry is
the CO2 emissions from the production of portland cement.
Naik [2008] has recommended increasing use of blends of portland cement as a way to reduce CO2
emission from the concrete construction industry. In such blended cements, increasing quantities of by-
products materials such as fly ash and ground granulated blast furnace slag are used to replace the
portland cement in concrete. However, the potential to reduce CO2 emission by using blended cement
varies from country to country depending on the availability of blending materials on the basis of coal
combustion, pig iron production, and production of cement. Worrell et al. [1995] estimated the potential
for carbon emission reduction in 24 countries in the OECD, Eastern Europe, and Latin-America and
reported a potential for CO2 emission reduction between 0% and 29%. He further reported an average
emission reduction of 22% for all countries accounted in their study. It was negligible for those countries
already producing a large share of blended cement or countries without iron production or coal fired
thermal power plants. A significant potential for blended cement exists in countries without much
production of blended cement (such as in the USA) and having coal fired thermal power plants and iron
industries. Hendriks et al. [2004] estimated that the global potential for CO2 reduction through blended
cement is at least 5% of the total carbon dioxide emissions from cement making, but it may be up to 20%.
Therefore, innovators and researchers working in the fields related to cement-based materials are
exploring possibilities to develop economical, practical, and environmentally friendly technologies for
CO2 reduction and sequestration in cement-based materials for lowering the carbon dioxide gas
concentration already present in the Earth‟s atmosphere.
6
1.3 Carbon Dioxide Sequestration
Carbon dioxide (CO2) sequestration is a geo-engineering technique for the long-term storage of CO2, or
other forms of carbon, for the mitigation of accumulation of greenhouse gases in the Earth‟s atmosphere.
In this technique CO2 is captured from the flue gases through chemical, biological, or physical processes.
The capture of CO2 is not the biggest challenge involved in addressing the issue of greenhouse gas
reduction from human activities including from industries, such as thermal power plants and cement
manufacturing. Since the first step in CO2 sequestration is carbon dioxide separation, industry has been
very focused on technology options for economically separating CO2 [Elwell and Grant 2006]. “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 separated and 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” [IPCC 2005]. This would have a massive cost implication for the industry and
society at large. If USA, as a country, chooses to limit emissions of CO2 by capturing it from the existing
fleet of thermal power 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? This technological challenge needs to be addressed.
Additionally, society will need to address the impacts of moving these large volumes of CO2 around the
7
country and into appropriate geologic formations below ground. Such infrastructure development to meet
these sequestration needs will have significant regulatory hurdles and are 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 time [Patulski 2006]. Patulski also implied that CO2 transport pipelines would need to
be constructed in permanent sequestration sites. 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 benign [Paul 2006].
In the chemical process of carbon dioxide sequestration, CO2 can be removed from the atmosphere and
stored in stable carbonate mineral forms. Such a process is termed as CO2 or carbon sequestration by
mineralization. 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.
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-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 material, coal-based utilities will need to establish
8
agreements with fly ash processors, commodities brokers, and portland cement manufacturers on the
ownership of those credits” [Farland et al. 2006].
1.4 Carbon Credits
Carbon credit is treated as a universal and global trading currency. Cutting of one ton of CO2 or carbon
from the emission by an industry or process yields one carbon credit. One can purchase carbon credit for
compensating CO2 emissions coming out from ones industry or other activities related to CO2 emissions.
Therefore, the purchased amount of carbon credit offsets the carbon emission by the same amount. Today
(2009), one carbon credit is equivalent to over 25 USD. Cement-based construction materials have
enormous potential for earning of carbon credits by sequestrating carbon dioxide in them by
mineralization.
9
Chapter 2
CARBON DIOXIDE (CO2) SEQUESTRATION
IN CEMENT-BASED MATERIALS
2.1 CO2 and Cement-Based Materials
Cement-based materials including concrete absorb carbon dioxide (CO2) through a process known as
carbonation reaction with alkalis in the cement-based materials that results in carbon dioxide
sequestration in these materials. About 19% of the carbon dioxide produced during the manufacture of
cement is reabsorbed by the concrete over its lifecycle (i.e., its service life and secondary life following
crushing and reuse) [http://www.sustainableconcrete.org.uk/main.asp?page=85]. The normal process of
carbonation in conventional concrete is very slow, about one mm/year [Vasburd et al. 1997]. The rate 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 [Fattuhi 1986]. Benefits of carbonation
generally include increased concrete strength and increased impermeability 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.
10
United States Patents that utilize carbonic acid, H2CO3, for strengthening cementitious materials date back
to 1870 [Rowland 1870]. Rowland [1870] was issued a patent for the improvement in the manufacture
of artificial stone in 1870: clean washed sands were combined with cementitious materials and steam
cured in a carbon rich environment to yield a strong, hard, durable, and inexpensive artificial stone.
Rowland [1872] was also issued a 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. Heinzerling [1897] was issued a patent in 1897 for
the production of artificial stone with carbonic acid gas under pressure.
Ball [1978] was issued a patent in 1978 for portland-cement products, with or without added gypsum:
carbon dioxide gas was homogeneously reacted with the cement slurry during the cement and water
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. Malinowski [1982] 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 was cast and subjected to a vacuum treatment to have it
de-watered and compacted. CO2 gas was then supplied to the concrete mass where it diffused into the
capillaries formed for rapid hardening. Jones [1996] was issued a patent in 1996 for concrete treated with
high-pressure CO2. Jones introduced the use of dense-phase or supercritical CO2 conversion of calcium
hydroxide in the concrete to calcium carbonate and water yielding closely packed and aligned crystals in
the cured concrete products.
There are numerous [Murray 1978; Moorehead and Davis 1982; Murray 1984; Alpar et al. 1991; Cowan
et al. 1994; Jones 1997; Jones 1997a; Baglin 1999; Knopf and Dooley 2002; Deppen 1984; Oshio 1990;
Suzuki 1994] other patents that have been issued in which carbon dioxide and carbonic acid are utilized in
various forms and pressures for the production of concrete and concrete products. The common primary
advantages of using carbon dioxide and carbonic acid in various forms for concrete and concrete products
11
are increased strength, increased density, and increased impermeability from the resulting carbonation
products [Murray 1978; Moorehead and Davis 1982; Murray 1984; Alpar et al. 1991; Cowan et al. 1994;
Jones 1997; Jones 1997a; Baglin 1999; Knopf and Dooley 2002; Deppen 1984; Oshio 1990; Suzuki
1994]. 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.
2.2 Carbon Dioxide Sequestration, Concrete, and other Cement-Based Products
Several recent studies [Naik et al. 2009; Shah 2005; Naik et al. 2007; Shao 2008; Ramme 2008;
Monkman et al. 2006; Shao et al. 2006; Shao and Monkman 2006] deal with carbon dioxide sequestration
potential in concrete and other cement-based products. If cement-based materials could be utilized to
mineralize carbon dioxide to a stable calcium carbonate form during their production, and thereafter, then
this method of carbon dioxide sequestration would have 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 coming from these industries.
Carbon dioxide mineralization in the hydrates (i.e., alkalis) of cement in cement-based material occurs
either in the natural process of carbonation or by some specifically designed engineered process. The
natural process of carbonation of concrete is quite slow. Normally, good quality concrete of normal
strength (21, 28, and 35 MPa (3000, 4000, and 5000 psi)) carbonates at a rate of one mm/year [Vasburd et
al. 1997]. The Portland Cement Association (PCA) [Gajda 2001] indicates that virtually all structures
constructed with portland cement concrete have the potential to absorb atmospheric CO2 through
carbonation. A comprehensive study was undertaken involving 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 tonnes (300,000 tons) of
12
atmospheric CO2 during the first year of construction. The concrete continues 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, respectively. An overall average carbonation rate was calculated to be 2.1
mm/yr0.5
. When fully hydrated, 100 tonnes (110 tons) of the ordinary portland cement produces 31.1
tonnes (34.4 tons) of calcium hydroxide. Accounting for the average unhydrated cement content of about
seven % in typical concrete reduces the calcium hydroxide yield to 29.0 tonnes (32.0 tons). When fully
carbonated, this quantity of calcium hydroxide can absorb up to 17.3 tons (19.1 tons) of CO2. Portland
cement consumption in U.S. is about 100 million tonnes (110 million tons) per year, leading to potentially
17.3 million tonnes (19.1 million tons) of sequestration of CO2 per year in concrete; or, a market value of
about 350 million dollars (at about $20 per tonne of CO2).
The most widely adopted engineered way for the mineralization of carbon dioxide in cement-based
materials is through their early age carbonation curing. The early age carbonation is more efficient
because the pore structure is still not very dense. This covert cement hydrates to stable calcium carbonate
and silica gel; hence, it provides an efficient means for carbon dioxide sequestration in cement-based
materials. Numerous studies [Vasburd 1997; Shah 2005; Naik et al. 2007; Shao 2008; Ramme 2008;
Monkman et al. 2006; Shao et al. 2006; Shao and Monkman 2006] have shown many advantages of this
early age carbonation curing for concrete and other cement-based materials. Earlier age carbonation also
helps accelerate strength gain. Therefore, carbonation shortens the time required for the production and
the use of the concrete element, resulting in enhanced productivity.
Naik et al.[2009] based on their study on carbon dioxide sequestration in cementitious products reported
“ASTM Class C fly ash is very effective in sequestration of carbon dioxide in cementitious materials,
such as concrete and controlled low strength materials (CLSM)”. They further observed a “three folds
increase in the rate of carbonation of concrete with ASTM Class C fly ash in comparison to the concrete
13
without fly ash.” Furthermore, they found a much higher carbonation rate for CLSM than concrete and
concluded that such concretes and CLSM have immense potential to sequester CO2.
Shah [2005] studied carbonation in non-air entrained and no-fines concrete. He reported 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 other alkalis) 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 and other alkalis becomes carbonated,
the pH value of the pore solution is reduced from about 12.5 to 8.3. The rate of carbonation is the highest
when the relative humidity of the surrounding environment is in the range of 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. [2007] also investigated the effect of different curing environments on carbon dioxide
sequestration in concrete containing Class C fly ash. In this study they used Class C fly ash at 0%, 18%,
and 35 % of total cementitious materials and three different curing environments (i.e., moist-curing
(100% RH) and 0.15% of CO2, 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 upon 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
a carbon dioxide chamber (at 50% R. H. and 5% of CO2 concentration) showed mechanical properties at
par with specimens cured in a moist curing room at 100% R. H.
14
Ramme [2008] 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. [2006] studied the potential of calcium silicate concrete for sequestration of CO2 through the
early age (two hours) carbonation curing. The curing chamber was maintained under 0.5 MPa pressure
and at an ambient temperature (23 ºC) for the duration of two hours with a 100% concentration of CO2.
They used Type 10 (ordinary) and Type 30 (high-early strength) portland cements as a binder in
concretes. The CO2 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 the portland
cement could be sequestered in two hours. This study shows that concrete and other cement-based
materials have the potential for carbon dioxide sequestration through early age carbonation curing.
However, the consumption of carbon dioxide is dependent on the quantity of the cement in the concrete
and the 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 Wu [2008] examined the effects of different parameters, such as water-to-cementitious materials
ratio, curing time, carbon dioxide pressure during curing, and temperature, on carbon dioxide
consumption and strength of the concrete product. They reported that accelerated reactions between CO2
and hydrated cement minerals happen mainly during the first 15 minutes regardless of carbon dioxide
pressure and pre-conditioning environment. Further, they found that an increase in carbon dioxide
pressure increases the CO2 consumption but does not show significant effect on the increase in the
strength of the concrete. They also reported optimum water-to-cementitious materials ratios of 0.36 to
0.43 for the reaction between CO2 and hydrated cement minerals. They showed that preconditioning of
concrete in the environment of relative humidity of 55 ± 10% at 22 ± 3 ºC increases CO2 consumption in
15
the concrete compared with the specimens pre-conditioned in the moist environment with relative
humidity greater than 95% at 22 ± 3 ºC. The reason behind this may be the loss of water from the
specimens in a dry environment which might have resulted in an easier transport of CO2 inside the
concrete. They also reported a gain in strength by the specimens kept in a moist environment with relative
humidity greater than 95% at 22 ± 3 ºC after CO2 curing.
2.3 Other Reports by Industries for the Sequestration of CO2 in Cement-Based Products
Carbon dioxide sequestration has been a hot topic for study and discussion recently (2008). Many
companies claim to have found constructive use for carbon dioxide sequestration in cement and cement-
based materials. California-based Calera [http://www.inhabitat.com/2008/08/11/using-Co2-to-make-
concrete] claimed to have found technology for using carbon dioxide-rich flue gas to make cement.
Calera utilizes seawater and carbon dioxide-rich flue gas for making carbonates. By bubbling the flue gas
through the seawater, Calera plans to create cement, which can be used in the production of concrete
among other things. The company employs spray dryers that use the heat of the flue gas to dry the
seawater-slurry (seawater is reach in minerals of calcium and magnesium) that results from mixing the
flue gas and seawater. With this process, Calera says, “it can capture close to 90% of the Carbon dioxide
emissions emitted by power plants and other industrial giants.” Calera further states that it can remove a
half ton of carbon dioxide emission from the environment for every ton of cement it produces
[http://www.technologyreview.com/Energy/21117/page2/ (1 of 5)].
Carbon Sense Solutions [http://www.inhabitat.com/2008/07/30/new-co2-sucking-precast-concrete] of
Canada claimed to have developed a faster way to store more carbon dioxide in concrete through CO2-
accelerated curing of precast concrete elements. Their method allows storage of up to 60 tons of carbon
dioxides in 1000 tons of precast concrete (containing about 150 tons of portland cement). The company
16
has further claimed that the technology has the potential to sequester 20% of all cement industry carbon
dioxide emission. The company also claimed to have developed a process by which CO2 emissions from
the cement industry can be captured and converted into bicarbonate-ions, which are used to generate
limestone to be used in cement manufacturing [http://www.newswire.ca/fr/releases/archieve/
February2007/19c9873.html (1 of 2)]. 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 produce portland
cement.
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 concrete [Naik et al. 2007; Naik 2006; Ramme et al. 2005;
McKelvy et al.; Naik 2002]. Opportunities exist 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 fines [Ramme 2008; Ramme et al. 2005]. In the U.S.,
over 2.7 million tonnes, (3 million tons) of cement-kiln dust were removed from the cement
manufacturing process with only 573,000 tonnes (634,000 tons) being beneficially re-used [Elwell and
Grant 2006]. Approximately, one ton of CO2 and other greenhouse gases are emitted to the atmosphere
for each ton of portland cement produced [Naik 2006]. The portland cement industry has established a
voluntary goal of a 10% reduction in CO2 intensity from 1990 levels by 2020 [Carter 2006].
O‟Connor [www.netl.doe.gov/publications/proceedings/01/carbon_seq/6c2.pdf] 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‟Connor said there have been more than 600 autoclave tests
undertaken concerning such 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
17
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. These materials might be used in a diverse range of products (e. g., ceiling tiles).
However, the vast majority of such carbonate products would likely be used to reclaim the silicate
minerals from a mine that supplies the minerals to react with the carbon dioxide. O‟Connor 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). 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 such a process. This would require a large open-pit mine for
the carbonation process for each power 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 for the carbonation process was
used. This would allow the use of less expensive, narrow diameter pipes rather than large diameter high
pressure tank reactors. It was estimated that the mineral carbonation step, in the CO2 sequestration
process, would add about eight cents per kilowatt hour to the consumer‟s electricity bills. Carbon dioxide
capture and transportation would further add to this cost. Such 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 a total cost of about $10 per ton.
Malhotra [2006] and others [Naik 2002; 2008] have persuasively pointed out that the replacement of
cement by pozzolans also effectively decreases the net emissions from cement manufacturing. Malhotra
[1999] elegantly concluded that “the combined use of superplasticizers and supplementary cementing
materials (along with application-specific high-quality aggregates) 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
18
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 portland cement is one such industry” [Malhotra 1999]. Power generation using fossil
fuels for combustion could be another targeted industry. The PCA [Gajda 2001] 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 have absorbed approximately 69 million tonnes
(76million tons) of atmospheric CO2.
The commonly employed phenolphthalein color staining RILEM test was confirmed by the PCA [Gajda
2001] to accurately describe the depth and degree of carbonation. Fig. 2-1 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 from the top/exterior surface.
Fig. 2-1. Depth of carbonation by phenolphthalein and analytical methods
[Shao and Monkman 2006]
19
Slices were then immediately ground to a fineness of 45 microns (passing 200 mesh). Powders from the
ground specimens were then subjected to thermo-gravimetric analysis to determine the relative
concentrations of carbonates.
In the report by the International Panel on Climate Change [IPCC 2005] 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 by-product, 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 Lawrence Berkeley National Laboratory
[Martin et al. 1999] 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 occurred between 1970 and 1997. The report further asserts 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. The use of blended cement may reduce about 25% CO 2 emissions due to reduction
20
in cement quantity and the CO2 emissions from fossil fuel combustion during cement production. 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.
21
Chapter 3
CARBONATION OF CEMENT-BASED MATERIALS
3.1 Theoretical Basis of Carbonation of Concrete and Other Cement-based Materials
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 aluminates hydrates,
and calcium sulfoaluminate hydrates (mainly ettringite), as well as other small amounts of alkalis, in
cement-based materials react with carbonic acid (CO2 + H2O = H2CO3). CO2 is available from air, and
water is available in the cement-based material as well as in the air. If all available hydrates in the
cement-based material are carbonated, the pH of concrete is reduced from around 12.5 to below 9 through
the absorption of carbon dioxide [Lagerblad 2005]. In theory, the carbonation process is very simple, but
in reality it is a complex set of chemical reactions. CO2 in the gaseous form cannot react directly with the
hydrates in the cement paste. Therefore, for carbonation, the CO2 gas has to first dissolve in the water to
form carbonate ions of carbonic acid, which in turn react with the calcium ions (Ca2+
) and other hydrates
of the pore water. Therefore, for a cement-based material, carbonation is a chemical reaction in which
atmospheric carbon dioxide penetrates the material and reacts with the alkaline calcium hydroxide and
other cement hydrates 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.
22
A very simple model showing some of the characteristics of the carbonate system is provided by
equilibrating natural water with a gas phase (e.g. atmosphere) containing CO2 at a constant partial
pressure. A partial pressure of CO2 (pCO2=10-3.5
atm.) combined with the Henry‟s law gives
2
0
1COHT pKC
; and
`2
12
0
13 COHCOH pK
H
KpKHCO
2
212
0
22
3 COHCOH pKH
KKpKCO
where α is the ionisation fraction. In other words the equilibrium concentration of all the soluble
component can be calculated with the help of Henry‟s law, the acidity constants and the proton condition
or the charge balance if, in addition to temperature, one variable such as pCO2 , [HCO3] or [H+] is given
[Fava 2009].
Inside the cement-based materials, the pH is quite high (about 12.5+). 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 carbonate ions form due to higher pH
leading to the precipitates of calcium carbonates crystals [Lagerblad 2005]. Carbonation starts from the
surface and moves inwards. The process can be expressed by the following chemical equations:
1. CO2 (gas) + 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
23
This reaction lowers the concentration of Ca2+
in the pore solution, which in turn leads 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 or carbonate ions are consumed. This results in lowering of the pH which may
destabilize other cement hydration products. Once the concentration of calcium ions drops, C-S-H starts
dissolving. Monosulphate (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 10.6[Lagerblad 2005].
Therefore, Ettringite is usually absent in the matrix of the carbonated cement-based material. 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
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 the silica gel.
Lagerblad [2005] has made an attempt to summarize the stability of different hydration products of
cement with respect to carbonation. He divided the carbonation process into five stages with respect to
the lowering of pH as shown in Table 2-1. The calcium carbonate (CaCO3) that formed by the 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 vaterite [Slegers and Rouxhet 1976; Sauman 1971].
24
Table 3-1. Stable Phases in Portland Cement Paste at Different pH [from Lagerblad 2005]
Non-Carbonated Concrete Carbonated Concrete
Stage 1 Stage 2 Stage 3 Stage 4 Stage 5
Calcium
Hydroxide,
Ca(OH)2
- - - -
Calcium
Silicate
Hydrates, CSH
Ca/Si > 1.5
CSH
Ca(OH)3
1.5 < Ca/Si > 0.5
Ca(OH)3
Ca(OH)3
SiO2 with some CaO, Ca/Si
< 0.5
AFm,
Monosulphate
AFm Al(OH)3 Al (OH)3 Al(OH)3
Aft,
Ettringite
Aft Aft Fe(OH)3 Fe(OH)3
pH > 12.5 pH < 12.5 pH < 11.6 pH <10.5 pH < 10
3.2 Mechanism of Carbonation
The mechanism behind carbonation in cement-based materials is inward diffusion of carbon dioxide gas,
as well as subsequent carbonate ions as they are formed. This process starts with the exposed surface of
the cement-based material that is exposed to an environment containing CO2, for example, air. This
carbonation process lowers the amount of Ca2+
ions in the pore solution, which, in turn, triggers
dissolution of CH and Ca2+
and diffusion from the interior of the concrete to the carbonation front. At
this front, the concentration of both components will be at a low point due to low solubility of calcium
carbonates [Lagerblad 2005]. The speed of diffusion of both Ca2+
and carbonate ions govern the
mechanism of carbonation. Besides the concentration gradient of ions, the process of diffusion in
concrete is controlled by its pore system and pore saturation (i.e., how full the liquid is 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 greater depth, but carbonation does not occur due to a lack of
25
available 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.
3.3 Modes of Carbonation
Carbonation front is dependent on the relative concentration and speed of Ca2+
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 hydrated phases, as long as the
Ca2+
concentration is high [Chi et al. 2002]. CH is the most soluble phase in the hydrates of the 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 CH, and the process leaves a shell of calcium carbonate
around CH. This shell slows down the carbonation process for CH. However, since the product is
porous, it only delays the carbonation process. If Ca2+
moves faster than carbonate ions, then CH will
dissolve, and the calcium carbonate will precipitate in the capillary pore system. In such a situation,
volume change will result in densification and decrease in the porosity of the concrete. Such situations
may happen more readily in ordinary portland cement (OPC) paste [Lagerblad 2005]. After the calcium
hydroxide (CH) is consumed, the carbonation will start consuming C-S-H. The C-S-H dissolves in a
different manner 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 vaterite [Lagerblad 2005; Slegers and
Rouxhet 1976]. The reaction is linked to the value of pH and also depends 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 Ludwig [1997] have reported a coarser microstructure for concrete
made with slag-cement due to carbonation. Similar improvement in microstructure can also be expected
with OPC, especially with fly ash (because fly ash is a similar pozzolanic material as the slag used by
Stark and Ludwig [1997]).
26
Chen et al.[2005] 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 lower than 1.5.
3.4 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, their distribution,
and pore connectivity). Carbonation of cement-based materials involves diffusion of calcium and
carbonate ions. In addition to the concentration and gradients of these ions, the process of diffusion in
concrete is controlled by the pore system and pore saturation. The diffusion coefficients and water
permeability are also affected by the reduction of porosity and supply of water during the carbonation
process [Shah 2005; Thiery et al. 2005]. Therefore, the carbonation of concrete is affected by the pore
system of the cement-based materials. Silva et al. [2002] studied the effects of carbonation on the
microstructure of concrete by using mercury intrusion porosimetry and scanning electron microscopy
(SEM). They measured the open porosity. Open pores are the pores that 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 in accordance with RILEM [RILEM Commission 25 PEM 1980] procedure.
There are also closed isolated pores that are not accessible to the fluid from the surface of the concrete (by
using the RILEM procedure to obtain the porosity accessible to water). Based on the mercury
porosimetry results, Silva et al. [2002] 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
improved for carbonated concrete verses noncarbonated concrete. Measurement of porosity accessible to
water showed that carbonated concrete could become more compacted, with a reduction of 5% to 12% of
27
the open porosity, compared with the noncarbonated concrete. Using SEM, they also observed
microstructure to be more uniform for the carbonated concrete.
Villain and Thiery [2005] studied the impact of carbonation on the 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 components such as the calcium hydroxide (CH) or C-
S-H. Transformation of CH to calcite and metastable vaterite provides a volume reduction of 11% and
14%, respectively. These volume changes decrease the porosity in the carbonated zone. The increase in
volume due to calcite and vaterite normally fills empty space in the capillary system and hence densifies
the cement matrix, leading to improved durability.
Björn and Peter [2001] investigated microstructural changes caused by the carbonation of cement mortars
and found an 8% increase in the specific surface area (i. e., deceasing in the pore size and/or increasing
the number of pores) in the case of well-carbonated mortar compared to the noncarbonated mortar. They
also reported about two times increase in the volume of small pores for the carbonated specimens.
3.5 Effect of Pozzolans on Carbonation of Concrete
It has become a common practice to add fly ash, silica fume, blast furnace slag, or other pozzolanic
materials during the production of concrete to derive technical and environmental benefits over concrete
without pozzolanic materials. In comparison to hydrated ordinary portland cement, hydrated cement
containing pozzolan has less calcium hydroxide (CH) and more C-S-H because calcium hydroxide is
consumed in the pozzolanic activity to produce C-S-H. Furthermore, C-S-H in such cement-pozzolan
paste also contains more Aluminum and Magnesium due to the use of the pozzolan. Therefore, the
28
carbonation process and the structure of the carbonated paste are different in cement with pozzolan
compared to that of ordinary portland cement paste. The amount of calcium ions to be carbonated is less
in cement with pozzolan and, thus, the carbonate ions can penetrate to a greater depth, leading to an
increased carbonation rate, and, therefore, an increased CO2 sequestration, with the amount of cement
replaced with fly ash [Shah 2005; Lagerblad 2005; Nagataki 1986; Paillere et al.1986]. Such increased
carbonation depends upon the type and amount of pozzolans.
The results of carbonation of cement-based materials can be either beneficial or harmful depending on the
time, rate, and extent to which the carbonation occurs, the environmental exposure, and whether or not
steel reinforcement and other embedded items of steel are present in the material. It is known that
carbonation can provide higher strength and increased hardness to mortar, plaster, concrete, and other
cement-based products. However, carbonation also results in an increased possibility of 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. At a lowered value of pH, the steel‟s passive oxide film may be destroyed, thus
accelerating the corrosion. Exposure to CO2 and the subsequent hardening process can also result in
carbonation shrinkage and affect the finished surface, e. g., for slabs, by leaving a soft dusting of
carbonated products (powdered calcite) thus making the surface less wear-resistant. The reaction of
hydrated portland cement exposed to 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 . By
increasing the temperature and pressure, it is possible to increase the rate at which carbonation occurs in
the concrete [Fauth and Soong 2001]. Carbonation can also occur from exposure to the groundwater
where CO2 may have dissolved in the water and combined to form carbonic acid H2CO3 [Chi et al. 2002;
ACI 201.2R13 2003].
29
3.6 Rate of Carbonation
Carbonation of cement-based materials starts from the surface exposed to the air and moves inwards. The
rate of carbonation is mainly influenced by the permeability and alkali content of the concrete besides
ambient atmospheric conditions (i.e., amount of carbon dioxide available, 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 of time relationship developed on the
basis of Fick‟s law for the diffusion of CO2 in the cement-based material as given below. This
relationship is frequently used for the determination of carbonation depth in cementitious materials
[Lagerblad 2005].
Xc = K(t0.5
)
where Xc, K, and t are depth of carbonation (in mm), constant of carbonation rate, and age in years of
concrete at the time of evaluation, respectively. For most situations the above equation is accepted as a
good approximation. However, for high-strength concrete, or concrete under exposed outdoor conditions,
this equation may be accepted as an approximation. The carbonation rate constant (K) is generally higher
for the indoor concrete than that exposed to the outdoor due to the fact that the carbonation rate is
dependent upon the amount of CO2 in the air as well as relative humidity [Pade and Guimaraes 2007].
Lagerblad[2005], based on literature review, suggested various carbonation rate constants for concrete of
different strength and exposure conditions, Table 3-2. Table 3-2 shows the variation of the carbonation
rate constant, K, from 0.5 to 15. It reflects the effect of strength of concrete and exposure condition on
the carbonation of the concrete. The process of carbonation and the rate of carbonation penetration in a
concrete product depend on porosity and pore structure of the concrete, availability of Ca(OH)2 and other
30
alkalis, moisture content of the concrete product, relative humidity and CO2 concentration of the
surrounding environment, and use of mineral admixtures in concrete [Lea 1971].
Table 3-2. Carbonation Rate Constants, K for Various (Ordinary Portland Cement)
Concrete Cylinder Strength and Exposure Conditions, per Lagerblad [2005]
Exposure
condition
Compressive strength
< 15 MPa 15–20 MPa 25–35 MPa > 35 MPa
K (mm/(year)0.5
)
Wet/submerged 2 1.0 0.75 0.5
Buried 3 1.5 1.0 0.75
Exposed
(Outdoor)
5 2.5 1.5 1.0
Sheltered 10 6.0 4.0 2.5
Indoor 15 9.0 6.0 3.5
Atis [2004] also reported lower depths of carbonation at higher strength levels. He also showed higher
depth of carbonation of concrete with higher porosity. Neville [1995] has stated that “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 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
conditions [BRE Digest 1995; Kumar 1997; Kumar and Bhattacharjee 2003 and 2004]4. Sulapha et al.
31
[2003] found that a lower water-to-binder ratio and a long-term curing in water resulted in a slower rate of
carbonation, apparently due to improved microstructure of the concrete.
Several studies [Atis 2004; Neville 1995] have reported that the highest rate of carbonation occurs for the
relative humidity of the surrounding environment between 50 % and 70 %. Concrete with high internal
moisture shows a 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 pores [Sulapha et al. 2003] necessary to form carbonic acid from the CO2 gas.
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. Verbeck [1958] concluded that carbonation
produces little shrinkage at relative humidity (R.H.) of 25 % and proceeds slowly at R. H. of 100 %. 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) affects the carbonation of the concrete. An
increased carbonation rate results in a loss of moisture thus causing shrinkage [Verbeck 58].
Sagüés et al. [1997] 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), 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 the depth of carbonation as the compressive
strength of concrete increased. Collepardi et al. [Collepardi 2004] 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 %.
32
Concrete containing fly ash, if not cured sufficiently, may have a higher degree of carbonation. Class F
fly ash, meeting ASTM C 618 requirements, used in concrete can show the same trend of carbonation as
concrete made without such fly ashes [Malhotra and Ramezanianpour 1994].
3.7 Measurement Methods of Carbonation Profile/Depth in Concrete
To evaluate the carbonation profile/depth in concrete, use of various experimental methods are reported
[Villain and Platret 2006; Chang and Chen 2006; Villain et al. 2007]. 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 indicates the depth of
carbonation. It gives a carbonation depth (i.e., color change) corresponding to a pH value near to 9
[Villain et al. 2007].
The carbonated area remains grey in color whereas the noncarbonated area turns
fuchsia in color.
3.8 Limitations of the Phenolphthalein Test Method
This RILEM test cannot detect the existence of a partially carbonated zone of concrete where pH is higher
than 9 or in areas difficult to spray and detect [Chang and Chen 2006; Fukushima et al. 1998; RILEM
Committee CPC-18 1998; http://findarticles.com/p/articles/mi_qa5379/is_200301/ai_n21325892].
Furthermore, this method cannot distinguish loss of concrete alkalinity resulting from a specific cause
(such as carbonation or other causes such as exposure to acids). Chang and Chen91
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%”.
33
3.9 Other Methods for the Measurement of the Carbonation Profile in Concrete [Villain
and Platret 2006; Chang and Chen 2006; Villain et al. 2007]
In order to improve the understanding of the carbonation process and to measure quantitative carbonation
profile, the following methods are sometimes used for the determination of the carbonation depth in
concrete:
Thermal Gravimetric Analysis (TGA);
Gammadensimetric Method;
X-Ray Diffraction Analysis (XRDA); or,
Infrared spectrometry
These methods are used either alone or together. Among these methods, the TGA and
Gammadensimetric methods are frequently used by some researchers [Villain and Platret 2006; Chang
and Chen 2006; Villain et al. 2007].
3.9.1 Thermal Gravimetric Analysis (TGA)
TGA method determines the portlandite and the calcium carbonates resulting from carbonation of
concrete [Villain and Platret 2006; Parrott and Killoh 1989]. 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 emissions
[Villain and Platret 2006; Villain et al. 2007]. Table 3-3 shows the temperature ranges of the cement-
hydrate decomposition during TGA measurements, as used by Villain and Platret [2006].
34
Table 3-3. Temperature Ranges of Hydrate Decomposition during TGA Measurements
[Villain and Platret 2006]
Field Temperature range Decomposition of hydrates or carbonated products
1 25 to 430 ºC
Free and adsorbed H2O, H2O from C-S-H, AFt, AFm,
gypsum, and CO2 adsorbed in C-S-H
2 430 to 520 ºC H2O from portlandite Ca(OH)2
3 520 to 620 ºC
OH- from structure of hydrates, structure H2O or 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
3.9.2 Gammadensimetry Method
This is a non-destructive test method. It is commonly used to measure density variations due to variations
of water content during a drying or water soaking process and, also, due to segregation of aggregates
[Chang and Chen 2006]. Villain and Platret [2006] and Villain et al.[2007] have demonstrated the use of
this method in determination of density variations related to CO2 penetration in concrete during the
carbonation process.
Villain and Platret [2006] used two experimental methods, TGA and Gammadensimetry, to determine the
carbonation profile that was related to the amount of chemically-fixed carbon dioxide at various depths in
35
concrete. Gammadensimetry was used to monitor the progress of carbonation 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. This method can
be used to validate the mathematical models for carbonation depth.
3.9.3 X-ray Diffraction Analysis (XRDA) Method
Chang and Chen [2006] 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 with different values of pH
(i.e., fully carbonated, partially carbonated, and noncarbonated) in the carbonated concrete. The fully
carbonated zone was identified with pH of less than 9.0 with a degree of carbonation greater than 50%.
The degree of carbonation in partially carbonated zone was between 0 - 50% (9.0 < pH < 11.5).
Noncarbonated zone was marked by the zone where a sign of the carbonation was not detected. They
reported that the depth of carbonation, determined by TGA, XRDA, and FTIR, in the carbonated zone,
where the phenolphthalein remained colorless (indicating presence of carbonation), was found to be twice
that shown by 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 remained
colorless, is generally 9.0 at which the degree of carbonation is 50%. They further concluded that when
the pH of the 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.
36
Chapter 4
CO2 SEQUESTRATION POTENTIAL IN
CEMENT-BASED MATERIALS
4.1 Theoretical Quantification of CO2 Sequestration Potential in Cement-Based
Materials [Naik et al. 2009; Ramme 2008]
Theoretically the maximum carbon dioxide uptake by portland cement concrete can be estimated on the
basis of the chemical compositions of the cement (and binder) by using the Steinour [Ramme 2008;
Steinour 1959] formula, as given below:
CO2 (wt%) = 0.785 (CaO – 0.7SO2) + 1.09 MgO + 1.42 Na2O + 0.935 K2O
Monkman and Shao [2010] conducted research on the carbonation behavior of six types of 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 five bars for two hours. The CO2 uptake for all materials was
significantly less than the theoretical maximum as predicted by the Steinour [1959] formula. Their
conclusions suggested that the carbonation reaction may be limited to about 25% of its potential due to
the lack of water necessary for the carbonation process. The primary product of the carbonation was
calcite (CaCO3). In comparing the cements, it was noted that an increase in the fineness of the cement
resulted in an increase of carbonation.
37
Estimated potential for sequestration of carbon dioxide by mineralization in cement-based materials can
be calculated by knowing the cement content and CaO content of other cementitious material. If it is
assumed that 100% of the calcium oxide found in calcium hydroxide, Aft, and AFm, as well as 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 [Chi et al. 2002]. Therefore, the 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 CaO
CO
M
M 2 (kg/m
3)
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 oxide
CaO
CO
M
M 2= 44/56 = 0.786
Example of the Carbon Dioxide Sequestration Potential in Cement-Based Materials
If a concrete contains 350 kg of portland cement per cubic meter of the concrete mixture and the cement
contains 64% of Cao, then the potential for carbon dioxide uptake is:
= 0.75 x 350 x 0.64 x 0.786 kg/m3
= 132 kg/m3
Therefore, percentage carbon dioxide sequestration potential of this mixture of cement
38
in the concrete = 350
132x 100 = 38 %.
Jahren [2003] also reports “that a fully carbonated concrete could bind about 0.3 kg of CO2 per kg of
cement and (also) get a strength increase of 30%”.
The method of direct mass gain has also been used by researchers [Shao et al. 2006 and 2008] to estimate
the CO2 uptake in carbonation of concrete and other cement-based materials. This method involves a
mass comparison before and after carbonation. A dry cement binder is used as a reference, as given
below:
Mass gain (%) = binderdry
lostCObefCOatf
mass
watermassmass
,
2,2,
)(
)()(
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 inside core of the sample are equally
carbonated.
4.2 Experimental Investigation on CO2 Sequestration Potential in Concrete and CLSM
[Naik et al. 2009; Shah 2005; Ramme 2008; Ramme 2005]
In order to investigate the CO2 sequestration potential in concrete and CLSM, the following materials
were used for the manufacturing of concrete and CLSM.
39
4.2.1 Portland Cement
ASTM Type I portland cement meeting the requirements of ASTM Standard Specifications for Portland
Cement (C 150) was used throughout this investigation. Table 4-1 and Table 4-2 present the chemical
composition and physical properties of the cement along with the requirements of ASTM C 150,
respectively.
Table 4-1. Chemical Composition of Portland Cement
Item Test results
(% by mass)
Standard requirement of ASTM C
150 for Type I cement
Silicon dioxide, SiO2 20.2 …
Aluminum oxide, Al2O3 4.5 …
Ferric oxide, Fe2O3 2.6 …
Calcium oxide, CaO 64.2 …
Magnesium oxide, MgO 2.5 6.0 maximum
Sulfur trioxide, SO3 2.4 3.0 maximum, when C3A 8%
3.5 maximum, when C3A > 8%
Loss on ignition 1.4 3.0 maximum
Insoluble residue 0.4 0.75 maximum
Free lime 1.5 …
Tricalcium silicate, C3S 67 …
Tricalcium aluminate, C3A 8 …
Equivalent alkalis, Na2O + 0.658K2O 0.53 …
40
Table 4-2. Physical Properties of Portland Cement
ASTM Item Test
results
Standard requirement of
ASTM C 150 for Type I
cement
C 185 Air content of mortar (volume %) 6.0 12 maximum
C 204 Fineness (specific surface) by Blaine air-
permeability apparatus (m2/kg)
364 280 minimum
C 151 Autoclave expansion (%) 0.07 0.80 maximum
C 109 Compressive strength of cement mortars
(psi):
1 day 2080 …
3 days 3590 1740 minimum
7 days 4400 2760 minimum
28 days 5620 …
C 191 Initial time of setting by Vicat needle
(minutes)
105 Between 45 to 375
C 188 Density (g/cm3) 3.15 …
Note: 145 psi =1 MPa
4.2.2 Fly Ash
ASTM Class C fly ash with the chemical composition and physical properties as shown in Table 4-3 and
Table 4-4, respectively, along with the requirements of ASTM C 618, “Specification for Coal Fly Ash
and Raw or Calcined Natural Pozzolan for Use in Concrete” was used in this study.
41
Table 4-3. Chemical Composition of Fly Ash
Item Fly ash
(% by mass)
Requirement of ASTM C 618
for Class C fly ash
Silicon dioxide, SiO2 36.2 …
Aluminum oxide, Al2O3 19.0 …
Ferric oxide, Fe2O3 5.6 …
SiO2 + Al2O3 + Fe2O3 60.8 50 minimum
Calcium oxide, CaO 23.4 …
Magnesium oxide, MgO 3.7 …
Sulfur trioxide, SO3 2.1 5.0 maximum
Sodium oxide, Na2O 1.0 …
Potassium oxide, K2O 1.0 …
Table 4-4. Physical Properties of Fly Ash
Item Fly ash
Requirement of ASTM C 618
for Class C fly ash
Strength activity index
(% of Control)
7 days
28 days
98
99
75 minimum, at either 7
or 28 days
Water requirement (% of Control) 91 105 maximum
Autoclave expansion (%) 0.05 ± 0.80
Density (g/cm3) 2.53 …
42
4.2.3 Fine Aggregate
Natural sand was used as fine aggregate in this investigation. The water absorption, specific gravity, and
bulk density of fine aggregate are given in Table 4-5. The grading (particle-size distribution) of fine
aggregate and the grading requirements of ASTM C 33, “Standard Specification for Concrete
Aggregates” is presented in Table 4-6. The sand met the requirements of ASTM C 33.
Table 4-5. Absorption, Specific Gravity, and Bulk Density of Fine Aggregate
Absorption (%) Specific gravity on
oven-dry basis
Specific gravity on
SSD* basis
Bulk density
(kg/m3)
Bulk density
(lb/ft3)
1.3 2.62 2.66 1800 112
* Saturated surface-dry
Table 4-6. Grading of Fine Aggregate
Amounts finer than each sieve (mass %)
Fineness
modulus
9.5 mm
3/8 in.
4.75 mm
No. 4
2.36 mm
No. 8
1.18 mm
No. 16
600 µm
No. 30
300 µm
No. 50
150 µm
No. 100
Sand 2.7 100 99 87 71 50 18 4
ASTM C 33 2.3~3.1 100 95-100 80-100 50-85 25-60 5-30 0-10
4.2.4 Coarse Aggregate
Crushed stones of maximum size ¾ inches (19 mm) were used as coarse aggregate for this project. The
physical properties and gradation of coarse aggregate (crushed stones) are shown in Table 4-7 and Table
43
4-8, respectively, along with the requirements of ASTM C 33, “Standard Specification for Concrete
Aggregates.” The crushed stones met the requirements of ASTM C 33.
Table 4-7. Physical Properties of Coarse Aggregates
ASTM Test Coarse Aggregates,
(Crushed Stones)
Requirements of
ASTM C33
C117
Materials Finer Than 75µ by
Washing, %
0.5 1.0
C128
Bulk Specific Gravity
Bulk Specific Gravity at
SSD
Apparent Specific Gravity
Absorption
2.65
2.66
2.68
0.40
None
C29
Unit Weight and Voids in
Aggregates:
Unit Weight, lb/ft3
Void Content, %
96.8
41.5
None
Table 4-8. Gradation of Coarse Aggregates
Percent Passing
Sieve Size → 1-in., 25.4
mm
3/4-in.,
19.05 mm
1/2-in,
12.7 mm
3/8-in. 9.5
mm
No. 4, 4.75
mm
No. 8,
2.36 mm
Coarse
Aggregates
(Crushed
Stones)
100.0 94.9 55.2 30.7 2.8 1.7
ASTM C 33
Requirements 100 90 - 100 - 20 - 55 0 - 10 0 – 5
44
4.2.5 Chemical Admixtures
Chemical admixtures, a mid-range water reducing admixture (MRWRA) and a high-range water reducing
admixture (HRWRA), were used to get the required slump/workability for the concrete. The specific
gravity and recommended dosage rates of these water-reducing admixtures are shown in Table 4-9.
Table 4-9. Specific Gravity and Dosage recommended of Water-Reducing Admixtures
Admixture Brand name
Specific
gravity
Manufacture‟s recommended
dosage rate
Mid-Range Water-
reducing admixture
Sikament 686
1.1
195-780 ml/100 kg (3-12 fl. oz./100 lb)
of cementitious materials
High-Range Water-
reducing admixture
Sika ViscoCrete 2100
1.1
130-390 ml/100 kg (2-6 fl. oz./100 lb) of
cementitious materials
4.3 Concrete Mixture Details [Naik et al. 2009]
Naik et al. [2009] used a control concrete mixture for a compressive strength of 4000 psi at the age of 28
days, with a slump between 3 to 4 inches at a water-to-cementitious material ratio of about 0.50 with and
without water reducing agent. Ten trial concrete mixtures, seven conventional concrete mixtures that
included three concrete mixtures with MRWRA and four without MRWRA, one CLSM mixture, and nine
no fines concrete mixtures were used. The details for these mixtures are given in the following sections.
45
4.3.1 Trial Concrete Mixtures
Trial concrete mixtures were prepared to arrive at a mixture proportions having a slump between 3 to 4
inches, with or without water reducing admixtures. First trial mixture “T-1” was without any water
reducing agent with the water-to-cementitious materials ratio of 0.58. The second trial mixture “T-2” had
the same water-to-cementitious materials ratio (i.e., 0.58), but more fine aggregate and less coarse
aggregates were used compared with Mixture T-1. Following these initial mixtures, two trial concrete
mixtures series (i.e., Series 1 and Series 2) were developed. Series 2 trial mixtures had comparatively
more sand and less coarse aggregates than the trial mixtures of Series 1. Both trial series of concrete
mixtures were manufactured by using two different dosages (i.e., one with about half of the maximum
and another near maximum of the manufacturer‟s prescribed dosages) of MRWRA and HRWRA.
Table 4-10. Mixture Proportions Details of Trial Concrete Mixtures Series 1[Naik et al.
2009]
Mixture Designation T - 1 T - 7 T - 8 T - 9 T - 10
Curing Environment Curing Room with 100% RH
Cement, lbs/yd3 500 500 500 500 500
Fly Ash, lbs/yd3 - - - - -
% Cement
Replacement - - - - -
Sand, SSD, lbs/yd3 1503 1499 1500 1500 1500
3/4" Aggregates,
SSD, lbs/yd3
1755 1755 1755 1755 1755
Water, lbs/yd3 290 250 250 250 250
MRWRA, fl. oz. -- -- 55 -- 19.2
HRWRA, fl. oz. -- 28.3 -- 19.2 --
Water to
Cementitious
Materials Ratio,
W/Cm
0.58 0.50 0.50 0.50 0.50
Slump, inches ¾ 3 – 3/4 3 – 1/8 1 – 7/8 5/8
Wet-density, lbs/cu.
ft. 151.2 149.6 150.6 150.4 151.0
46
The trial Mixtures T-3, T-4, T-8, and T-10 contained different dosages of mid-range water reducing
admixture (MRWRA), while trial Mixtures T-5, T-6, T-7, and T-9 were made with different dosage of
high-range water reducing admixture (HRWRA). Table 4-10 and Table 4-11 show the details of these
trial concrete mixtures Series 1 and Series 2, respectively.
Table 4-11. Mixture Proportions Details of Trial Concrete Mixtures Series 2 [Naik et al.
2009]
Mixture
Designation T - 2 T - 3 T - 4 T - 5 T - 6
Curing Environment Curing Room with 100% RH
Cement, lbs/yd3 498 500 500 504 500
Fly Ash, lbs/yd3 - - - - -
% Cement
Replacement - - - - -
Sand, SSD, lbs/yd3 1607 1593 1593 1610 1600
3/4" Aggregates,
SSD, lbs/yd3
1647 1638 1641 1659 1648
Water, lbs/yd3 289 290 250 252 250
MRWRA, fl. oz. -- 18.3 55.0 -- --
HRWRA, fl. oz. -- -- -- 19.2 28.3
Water to
Cementitious
Materials Ratio,
W/Cm
0.58 0.58 0.50 0.50 0.50
Slump, inches 2-1/8 4 – 1/4 4 – 1/4 3 – 1/4 4 – 1/4
Wet-density,
lbs/cu. ft. 149.8 149.6 150.6 150.8 148.0
47
In spite of having a higher water-to-cementitious materials ratio of 0.58, the slump of concrete Mixture-1
was low, only ¾ inches. At the same W/Cm ratio a second trial Mixture T-2 was carried out by
increasing the sand content of the mixture and reducing coarse aggregates (Table 4-11), but the slump
was only slightly improved to about 2 inches. Therefore, MRWRA and HRWRA were used for obtaining
the desired slump at W/Cm of 0.50. Since the slump loss was very quick in the case of concrete mixtures
with HRWRA, Naik et al. [2009] decided to consider concrete mixtures with MRWRA only.
4.3.2 Final Concrete Mixtures
Based on the performance of trial concrete mixtures, Naik et al. [2009] selected Mixture T-4 for further
investigation. The cement content of the control mixture, M-1, was replaced by ASTM Class C fly ash at
two different replacement levels (30% and 50% by mass) and the resulting concrete was designated as
Mixtures M-2 and M-3, respectively. The replacement ratio of cement to fly ash was 1:1.25 by mass.
These concrete mixtures contained MRWRA. Additionally, four more concrete mixtures at different
levels of cement replacement by fly ash were also manufactured without using any water reducing
admixture. This was done to avoid effect of the plasticizer on the smoothness of the surface of the
concrete test specimen (if any), which may affect the mixture‟s CO2 sequestration potential. The mixture
designations are shown in Table 4-12.
The details of the mixture proportions and fresh properties of the final series of concrete mixtures are
presented in Table 4-13.
48
Table 4-12. Mixture Designations of Final Concrete Mixtures [Naik et al. 2009]
Type of Concrete Mixture Designations % Cement Replacement
With MRWRA
M – 1 0
M – 2 30
M – 3 50
Without MRWRA
(plasticizer)
M – 4 50
M – 5 40
M – 6 30
M – 7 0
Table 4-13. Mixture Proportions and Fresh Properties of Final Series of Concrete Mixtures
Mixture Designation M - 1 M - 2 M - 3 M - 4 M - 5 M - 6 M - 7
Curing Environment 65 ± 25% RH and 20 ± 2 °C Temp
Cement, lbs/yd3 505 351 250 250 300 351 508
Fly Ash, lbs/yd3 - 185 315 315 252 185 -
% Cement Replacement - 30 50 50 40 30 -
Sand, SSD, lbs/yd3 1600 1580 1563 1563 1570 1580 1620
3/4" Aggregates, SSD,
lbs/yd3
1665 1650 1650 1650 1650 1650 1650
Water, lbs/yd3 253 268 283 283 252 268 261
MRWRA, fl. oz. 58 58 58 0 0 0 0
Water to Cementitious
Materials Ratio, W/Cm 0.50 0.50 0.50 0.50 0.50 0.50 0.51
Slump, inches 3 – 1/2 5 – 1/4 8 - 1/4 5 4-1/4 1-7/8 3/4
Air Content, % 3.2 2.1 0.5 0.9 0.9 2.1 2.2
Concrete Temp. (°F) 75 75 73 70 77 75 77
Ambient Air Temp. (°F) 70 68 76 79 74 68 72
Wet-density, lbs/cu. ft. 150.6 150.2 150.2 150.6 150.6 151.2 151.2
49
4.4 No-Fines Concrete Mixtures
A total of nine no-fines concrete mixtures were used in the investigation carried out by Naik et al. [2009].
These concrete mixtures contained 40% and 80% less fine aggregate (i.e., sand) by mass in comparison
with their corresponding normal/regular concrete mixtures used in this investigation, except for Mixture
N-9, which had 100% of sand taken out from the mixture. Further, these concrete mixtures were made at
0%, 30%, 40%, and 50% replacement of cement by fly ash at cement to fly ash ratio of 1:1.25 by mass.
The mixture designations are shown in Table 4-14.
Table 4-14. Mixture Designations for No-Fines Concrete [Naik et al. 2009]
Mixture Designations % Cement Replacement
% Sand Taken Out
N – 1 0 40
N – 2 0 80
N – 3 30 40
N – 4 30 80
N – 5 40 40
N – 6 40 80
N – 7 50 40
N – 8 50 80
N – 9 0 100
The details of the mixture proportions and fresh properties of these no-fines concrete mixtures are shown
in Table 4-15.
50
Table 4-15. Mixture Proportions and Fresh Properties of No-fines Concrete
Mixture Designation N- 1 N - 2 N - 3 N - 4 N-5 N-6 N-7 N-8 N-9
Curing Environment 65 ± 25% RH and 20 ± 2 °C Temp
Cement, lbs/yd3 476 400 350 330 295 260 250 240 387
Fly Ash, lbs/yd3 0 0 185 170 255 225 310 300 0
% Cement
Replacement 0 0 30 30 40 40 50 50 0
Sand, SSD, lbs/yd3 905 255 955 300 970 285 950 305 0
% Sand Replaced
compared to regular
mixture
40 80 40 80 40 80 40 80 100
3/4" Aggregates,
SSD, lbs/yd3
2180 2335 2290 2738 2315 2630 2285 2804 2480
Water, lbs/yd3 238 200 267 250 272 243 279 268 194
Water to
Cementitious
Materials Ratio,
W/Cm
0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.5 0.50
Concrete Temp. (°F) 75 75 77 76 79 71 77 75 75
Ambient Air Temp.
(°F) 78 79 80 79 73 78 79 79 79
Wet-density, lbs/cu.
ft. 141.6 118.7 149.9 139.7 152 135.1 151 145.6 113
4.4.1 Specimens Preparation and Curing
Concrete mixtures were prepared following standard ASTM Test Methods in an electrically driven,
revolving drum, tilting mixer. Cylindrical specimens of 100 mm diameter and 200 mm length were
prepared for the evaluation of CO2 sequestration along with compressive strength and splitting tensile
51
strength at 7, 28, 56, and 91 days. Test specimens of concrete were prepared in accordance ASTM C 192
in reusable plastic molds. The surface of the plastic molds for the specimens to be tested for splitting
tensile strength of concrete and consequently for carbonation depth was not oiled to minimize any
possible effect due to smooth surface on carbonation potential.
In the case of no fine concrete, test specimens were prepared in metallic molds. The procedure for mixing
remained the same as the regular concrete. However, the procedure to prepare and cast no-fines concrete
specimens for various tests was followed in accordance with ASTM C 1435, “Practice for Molding
Roller-Compacted Concrete in Cylinder Molds Using a Vibrating Hammer.” Figures 4-1 and 4-2 show a
typical no-fines concrete and the compaction process for preparing test specimens, respectively.
Fig. 4-1. A typical no-fines concrete in fresh state
52
Fig. 4-2. Compaction of no-fines concrete for test specimen preparation
The test specimens were removed from their molds within 24 ± 4 hours after casting. The specimens
were kept in an environment of 65 ± 25% RH and 20 ± 2 °C Temperature for curing until the date of test.
This method of curing was adopted to accelerate the carbonation by providing a favorable environment
for “natural” carbonation to the specimens without increasing carbon dioxide concentration in its
surrounding. In the case of trial mixture, the concrete cylinders, after demolding, were kept in a curing
room of RH not less that 95% and temperature of 20 ± 2°C (70 ± 3.5°F) until they were tested. The tests
performed on fresh concrete are as shown in Table 4-16.
Table 4-16. Test Methods for Fresh Concrete Properties
Property Test Method
Slump ASTM C 143
Density ASTM C 138
Air content by the pressure method ASTM C 231
Concrete temperature ASTM C 1064
53
4.5 Carbonated Depth of Concrete
CO2 sequestration potential of the concrete was measured by determining the carbonated depth of the
specimens at specific test ages. To evaluate the carbonation depth in concrete specimens, the simplest
and most well known method, the RILEM phenolphthalein test [RILEM Committee CPC-18] was used.
The method involved spraying of the phenolphthalein solution on the freshly broken concrete specimen
and measuring the depth of concrete up to the depth at which color has changed to fuchsia. The
carbonation depths were typically measured at four to five locations on the split-cylinder obtained from
splitting tensile strength testing at each age. Figure 4-3 shows a split portion of a cylinder showing some
carbonation depth indicated by grey portion in the concrete and non-carbonated portion is indicted by
fuchsia color. The outside layer of the cylinder has carbonated, and the grey color of the concrete did not
change. Figure 4-4 shows the carbonation depth measurement using a ruler.
Fig. 4-3. Spilt concrete cylinder showing some depth of carbonation in the outer layer
54
Fig. 4-4. A view of the measurement of carbonation depth (as measured to be 5 mm) in the
freshly-split concrete cylinder
4.5.1 Carbonated Depth of Concrete Mixtures made with MRWRA
Figure 4-5 presents average carbonation depth obtained from three specimens of concrete mixtures made
with MRWRA at different test ages i.e. 7, 28, 56, and 91 days.
55
Fig. 4-5. Carbonation depth of concrete mixtures with MRWRA
The carbonation test results presented in Fig. 4-5 by Naik et al. [2009] indicate that the depth of
carbonation of concrete increased with increase in the fly ash content in the concrete (i.e., increase in the
replacement level of cement by fly ash). The rate of carbonation in case of Mixture M-3 is almost three
times higher than concrete containing cement only, Mixture M1. This finding confirms results reported
earlier [Shao and Monkman 2006; Shi and Wu 2008; Sim 1994; Schutter and Audenaert 2004;
Khunthongkeaw et al. 2006]. Concrete containing 50% replacement of cement by fly ash (Mixture M-3)
showed highest carbonation depth at all the test ages. It can be noticed that Mixture M-3 showed a
carbonation depth of 3.5 mm at 28 days while Mixture M-2 (concrete with 30% fly ash) and Mixture M-1
(0% fly ash) were yet to have any measurable depth of carbonation. This is because when fly ash is used,
the effect of reduction of CH by pozzolanic reaction of fly ash and reduced amount of CH generated due
to reduced amount of cement used lead to a higher carbonation rate that dominates over the pore
refinement effect (see Section 3.5) due to the pozzolanic reaction allowing more efficient ingress of CO2
in the test specimen. Moreover, at the same W/Cm ratio, fly ash usually slows the hydration reactions
and increases the porosity of the concrete (especially at an early age). It is apparent from the data that it is
M-1, 0% Fly ash
M-2, 30% Fly ash
M-3, 50% Fly ash
0
1
2
3
4
5
6
7 28 56 91
Carb
on
ati
on
Dep
th (
mm
)
Age Days
56
possible to have a higher rate of carbonation and, therefore, higher CO2 sequestration potential, in
concrete made with replacing 50% cement with ASTM Class C fly ash. It is also apparent that in the case
of concrete containing 30% replacement of cement with fly ash (Mixture M-2), the rates of carbonation
were 30 to 50% higher than the portland cement only concrete (Mixture M-1). In the case of Mixture M-
3 it was possible to have a much higher carbonation rate, and, therefore, higher carbon dioxide
sequestration potential (about twice compared with Mixture M-1). Therefore, CO2 sequestration in such
high fly ash content concretes; it is possible to achieve a rate more than twice compared with that of
portland cement concretes only. These results show that there is an enormous potential for CO2
sequestration in concrete containing 50% cement replaced by ASTM Class C fly ash.
4.5.2 Carbonated Depth of Concrete without MRWRA
The carbonation test results of concrete mixtures without MRWRA are presented in Figure 4-6. They
indicate that the depth of carbonation of the concrete without MRWRA increased with an increase in the
fly ash content in the concrete (similar to concrete made with MRWRA).
Fig. 4-6. Carbonation depth of concrete mixtures without MRWRA
M-4, 50% Fly ash
M-5, 40% Fly ash
M-6, 30% Fly ash
M-7, 0% Fly ash
0
0.5
1
1.5
2
2.5
3
3.5
4
7 28 56 91
Carb
on
ati
on
Dep
th (
mm
)
Age (days)
57
Concrete containing 50% replacement of cement by fly ash (Mixture M-4) and containing 40% fly ash
(Mixture M-5) showed the highest carbonation depth at all test ages compared to Mixtures M-6 and M-7
(concrete containing 30% and 0% fly ash, respectively). It can be seen that Mixture M-5 (40% fly ash
concrete) even shows a higher carbonation depth at the 91-day age compared to Mixture M-4 (50% fly
ash concrete) at the same age. Mixtures M-4 and M-5 have the same carbonation depth at all ages, except
at the age of 91 days Mixture M-4 with higher fly ash (50 %) had slightly lower carbonation depth than
Mixture M-5 (with 40 % fly ash). The higher carbonation rate for concretes containing fly ash is similar
to that discussed in Section 4.5.1 for concrete mixtures made with fly ash and MRWRA. It is also
apparent that a higher percentage replacement of cement by ASTM Class C fly ash makes concrete
carbonate at an early age (i.e., 28-day due to increase in porosity at early age). Further, a comparative
evaluation of Figure 4-5 and Figure 4-6 reveals that the absence of MRWRA in concrete starts the
concrete to carbonate at an earlier age than concrete with MRWRA, possibly because of the improved
microstructure of concrete mixtures with MRWRA, due to the use of a chemical admixture, leading to
slower ingress of CO2 in such concretes.
It is also apparent from Fig. 4.6 that it is possible to have a higher rate of carbonation and, therefore,
higher CO2 sequestration potential in concrete containing 40 to 50% replacement of cement with ASTM
Class C fly ash. Furthermore, in the case of concrete containing 30% replacement of cement with fly ash,
the rate of carbonation was just one-half that of Mixture M-4 (50% fly ash) or Mixture M-5 (40% fly ash).
Compared to Mixture M-7, concrete with cement only, all concrete mixtures with fly ash (Mixtures M-4,
M-5, and M-6) have higher potential for CO2 sequestration. In the cases of Mixtures M-4 and M-5, it is
possible to have even a much higher rate of carbon dioxide sequestration potential (i.e., about three times
compared to Mixture M-7 without fly ash). To ascertain a more precise, quantitative cause of higher
carbonation amount in fly ash concrete, a detailed chemical analysis might be required to determine
whether only the lime content of fly ash or some other critical factors are responsible for a faster rate of
carbonation. In general CO2 sequestration in fly ash concrete (such as Mixtures M-4 and M-5), it is
58
possible to have a much higher rate of sequestration compared with concrete without fly ash. These
results show that there is an increased potential for CO2 sequestration in concrete containing 40 to 50 %
(or, possibly even higher amount) of cement replaced by ASTM Class C fly ash.
4.5.3 Carbonated Depth of No-Fines Concrete
Figure 4-7 presents the carbonation depths of no-fines concrete mixtures as reported by Naik et al. [2009].
Results presented in the figure indicate that no-fines concretes having fly ash (Mixtures N-2 thorough N-
8) showed carbonation started from the 28-day age. Mixture N-8 (50 % fly ash and 80 % reduced sand
from the mixture) had the maximum depth of carbonation at all ages. Carbonation rate increased with an
increase in the replacement levels of cement with fly ash. Furthermore, concrete mixtures containing
80% reduced sand (Mixtures N-2, N-4, N-6, and N-8) showed higher carbonation depth than concrete
containing 40% reduced sand (Mixture N-1, N-3, N-5, and N-7). This indicates that the reduction in sand
content and increase in fly ash content in no-fines concretes increases the carbonation rate of the no-fines
concrete. Therefore, such no-fine concrete provides a better opportunity to sequester a higher amount of
CO2 compared to other types of concrete.
59
Fig. 4-7. Carbonation depth of no-fines concrete mixtures
4.6 CO2 Sequestration in Concrete through Carbonation [Shah 2005]
Shah [2005] used two series of concrete i.e. non-air entrained concrete and no-fines concrete. Each
series of concrete included three mixtures with the same mixture proportions. Class C fly ash was used
as a cement replacement material. Series 1 mixtures were produced without fly ash; Series 2 and 3
mixtures were produced with the cement replacement levels of 15 and 30 %, respectively. Two series of
no-fines concrete mixtures – Series 4 and 5 -- were produced with 0 and 16 % cement replacement with
fly ash, respectively. One mixture of each series was cured in an environment where 100 % relative
humidity was maintained and CO2 concentration was 0.15 ± 0.02 %. The second mixture of each series
was cured under 50 ± 5% relative humidity environment with 0.15 ± 0.02 % CO2 concentration. The
third mixture of each series was cured in the CO2 chamber, where 5 ± 1.25 %. CO2 concentration and 50
0
1
2
3
4
5
6
7
8
9
10
11
7 28 56 91
Carb
on
ati
on
De
pth
(m
m)
Age (days)
N-1, 0% FA, 40% sand
N-2. 0% FA, 80% sand
N-3, 30% FA, 40% sand
N-4, 30% FA, 80% sand
N-5, 40% FA, 40% sand
N-6, 40% FA , 80% sand
N-7, 50% FA, 40% Sand
N-8, 50% FA, 80% Sand
N-9, 0% FA, 0% sand
60
± 5 % relative humidity were maintained. He reported the carbonation depths for non-air entrained
concrete series as shown in Fig. 4.8.
F1 F2 F3 F4 F5 F6 F7 F8 F9
0
2
4
6
8
10
12
14
16
18
20
22
24
26
37
2891
0 0
0 0
0 0
3
6
3.5
7.5
12
22
0 0
0 0
0
1
5.5
6.5
3
7
13.5
22.5
0 0 0
0
0
2
6
7.5
4
7.5
16
24
Mixture Designation
Dep
th o
f C
arb
on
ati
on
, m
m
Test Age, days
F1: 0 % Cement Replacement, Curing - 100 % RH & 0.15 % CO2. F2: 0 % Cement Replacement, Curing - 50 % RH & 0.15 % CO2. F3: 0 % Cement Replacement, Curing - 50 % RH & 5 % CO2. F4: 15 % Cement Replacement, Curing - 100 % RH & 0.15 % CO2. F5: 15 % Cement Replacement, Curing - 50 % RH & 0.15 % CO2. F6: 15 % Cement Replacement, Curing - 50 % RH & 5 % CO2. F7: 30 % Cement Replacement, Curing - 100 % RH & 0.15 % CO2. F8: 30 % Cement Replacement, Curing - 50 % RH & 0.15 % CO2. F9: 30 % Cement Replacement, Curing - 50 % RH & 5 % CO2.
Fig. 4.8. Depth of carbonation of non-air entrained concrete
61
He concluded that concrete cured in high-CO2 concentration of 5 % at 50 % relative humidity showed
much higher carbonation than the concrete cured in 100 % relative humidity with 0.15 % CO2
concentration or 50 % relative humidity with 0.15 % CO2 concentration. Shah [2005] further observed
that concrete with fly ash showed a higher depth of carbonation than the concrete without fly ash. He
also found an increased carbonation rate with an increase in cement replacement levels from 0 to 30 %.
Table 4.17: Mixture Proportions of Series 4 Mixtures (0% Cement Replacement)
Mixture Designation F10 F11 F12
Curing Environment
100 % RH &
0.15 % CO2
Concentration
50 % RH &
0.15 % CO2
Concentration
50 % RH &
5 % CO2
Concentration
Cement, lbs/yd3 193 190 194
Fly Ash, lbs/yd3 0 0 0
% Cement Replacement 0 0 0
3/4" Aggregates, SSD, lbs/yd3 2700 2665 2715
Water, lbs/yd3 51 50 51
Water to Cementitious Material Ratio,
W/Cm 0.26 0.26 0.26
Air Temperature, °F 69 69 69
Concrete Temperature, °F 69 69 69
Concrete Density, lb/ft3 109.0 107.6 109.7
Shah [2005] used two series of mixtures of no-fines concrete, Series 4 and 5, with the cement
replacement levels of 0 and 16 %, respectively. Tables 4.17 and 4.18 present the mixture proportions of
62
series 4 and series 5 no-fines concrete, respectively. The procedure for mixing remained the same as
ASTM C192, “Practice for Making and Curing Concrete Test Specimens in the Laboratory”. The
procedure to prepare no-fines concrete specimens was the same as discussed in Naik et al. 2009.
Specimens were de-molded approximately 24 hours after the production of test specimens. Immediately
after the de-molding of specimens, the specimens were put in the appropriate curing environments.
Table 4.18: Mixture Proportions of Series 5 Mixtures (16% Cement Replacement)
Mixture Designation F13 F14 F15
Curing Environment
100 % RH &
0.15 % CO2
Concentration
50 % RH &
0.15 % CO2
Concentration
50 % RH &
5 % CO2
Concentration
Cement, lbs/yd3 163 162 162
Fly Ash, lbs/yd3 39 39 39
% Cement Replacement 16 16 16
3/4" Aggregates, SSD, lbs/yd3 2710 2690 2685
Water, lbs/yd3 57 57 57
Water to Cementitious Material Ratio,
W/Cm 0.28 0.28 0.28
Air Temperature, °F 68 68 69
Concrete Temperature, °F 69 69 69
Concrete Density, lb/ft3 110.0 109.2 109.0
For non-air entrained concrete specimens, the degree of carbonation was expressed as the depth of
carbonation. The depth of carbonation was measured in a similar way as discussed in Naik et al. [2009].
63
But in the case of Shah [2005] no-fines concrete it was not possible to measure the depth, because of its
highly porous structure. Therefore, Shah [2005] developed a test method to analyze the degree of
carbonation of no-fines concrete by visual analysis. This method is based on the appearance of the
fractured surface of the no-fines concrete beam specimen after spraying phenolphthalein solution on it,
and based on the coloration of the surface of the paste around the aggregates and fractured contact areas
of aggregates, the degree of carbonation of no-fines concrete was ranked in an order of 0 to 4, “0”
indicating very low degree of carbonation and “4” indicating very high degree of carbonation. Table
4.19 defines five ranks (0 to 4) for visual analysis of degree of carbonation.
4.19: Ranking for Degree of Carbonation of No-Fines Concrete
Rank Degree of
Carbonation
Appearance of Fractured Surface of Beam 24 Hours after
Spraying Phenolphthalein Solution.
0 Very Low The entire cross-section shows pink coloration.
1 Low
Surface of the paste around aggregates shows pink coloration
partially; entire fractured contact area of aggregates show pink
coloration.
2 Medium
Surface of the paste around aggregates does not show pink
coloration; entire fractured contact area of aggregates show pink
coloration.
3 High
Surface of the paste around aggregates does not show pink
coloration; fractured contact area of aggregates show pink
coloration partially.
4 Very High
Surface of the paste around aggregates does not show pink
coloration; fractured contact are of aggregates does not show
pink coloration or shows a very few pink spots.
64
As described in Table 4.19, ranking for degree of carbonation of no-fines concrete was based on the
visual appearance of the fractured surface of the beam 24 hours after spraying phenolphthalein solution
on it, which led to coloration (or lack thereof) of the surface of the paste around aggregates and coloration
(or lack thereof) of the fractured contact area of aggregates. Rank “0” indicated “very low” degree of
carbonation, Rank “1” indicated “low” degree of carbonation, Rank “2” indicated “medium” degree of
carbonation, Rank “3” indicated “high” degree of carbonation, and Rank “4” indicated very high degree
of carbonation. The degree of carbonation of No-fines concrete beam specimens used to test flexural
strength of concrete were also used to study the degree of carbonation by visual analysis. 24 hours after
spraying phenolphthalein on the fractured beam surface, the appearance of the fractured surface was
observed for coloration; and the degree of carbonation was ranked in an order of 0 to 4. The degree of
carbonation of no-fines concrete is shown in Fig. 4.9.
65
For no-fines concrete the carbonation rate was highest in a curing environment of 50% RH and 5 % CO2
irrespective of cement replacement level. He finally concluded that the rate of carbonation of no-fines
concrete was highly affected by the relative humidity and CO2 concentration of the curing
environment.
4.7 Controlled Low Strength Material (CLSM) Mixture
Naik et al. [2009] used one CLSM mixture in their study to evaluate the carbonation potential of such
materials. More CLSM mixtures studied are given elsewhere by Ramme 2008; Ramme et al. 2005.
F10 F11 F12 F13 F14 F15
0
1
2
3
4
7 28
Mixture Designation
Deg
ree o
f C
arb
on
ati
on
, ra
nk
Test Age, days
F10: 0 % Cement Replacement, Curing - 100 % RH & 0.15 % CO2. F11: 0 % Cement Replacement, Curing - 50 % RH & 0.15 % CO2. F12: 0 % Cement Replacement, Curing - 50 % RH & 5 % CO2. F13: 16 % Cement Replacement, Curing - 100 % RH & 0.15 % CO2. F14: 16 % Cement Replacement, Curing - 50 % RH & 0.15 % CO2. F15: 16 % Cement Replacement, Curing - 50 % RH & 5 % CO2.
Fig. 4.9. Degree of carbonation in no-fines concrete
66
Details of one of the CLSM mixtures are presented in Table 4-20. Fresh CLSM properties for air content,
flow, unit weight, etc. were determined in accordance with ASTM standard methods.
Table 4-20. Mixture Designation and Proportions of CLSM
Mixture Designation S - 1
Curing Environment 65 ± 25% RH and 20 ± 2 °C Temp
Cement, lbs/yd3 50
Type and Source of Fly Ash ASTM Class C, Pleasant Prairie
Fly Ash, lbs/yd3 702
Sand, SSD, lbs/yd3 2641
Water, lbs/yd3 390
CLSM Temp. (°F) 74
Ambient Air Temp. (°F) 78
Slump/Spread , inches 9-3/4
Air Content, % 0.7
Wet-density, lbs/cu. Ft. 141.4
For CLSM mixture, fresh CLSM properties air content, flow, unit weight, etc. were determined as per the
standard methods mentioned in Table 4-21. Test specimens of CLSM were prepared in accordance with
ASTM D 4832. The surface of the plastic molds for the specimens to be tested for splitting tensile
67
strength and consequently for carbonation depth was not oiled to minimize any possible effect due to
smooth surface on carbonation potential.
Table 4-21. Test Methods for Fresh CLSM Properties
Property Test Method
Flow ASTM D 6103
Unit Weight ASTM D 6023
Air content by the pressure method ASTM D 6023
Cylindrical specimens of 100 mm diameter and 200 mm length were prepared for the evaluation of CO2
sequestration along with compressive strength and splitting tensile strength at 7, 28, 56, and 91 days. Test
specimens of concrete were prepared in accordance ASTM C 192. The surface of the plastic molds for
the specimens to be tested for splitting tensile strength of concrete and consequently for carbonation depth
was not oiled to minimize any possible effect due to smooth surface on carbonation potential. The test
specimens were removed from their molds within 24 ± 4 hours after casting. The demolded test
specimens were cured in a chamber maintained at a temperature of 20 ± 2°C (70 ± 3.5°F) and a relative
humidity of 65 ± 25 until the date of test.
4.7.1 Carbonated Depth of CLSM
The average depth of carbonation of CLSM obtained on three test specimens at each test age is given in
Fig. 4-10. Figure 4-11 through figures 4-14 show the depth of carbonation in various CLSM specimens at
different test ages. The CLSM cylinder carbonated, and, therefore, sequestered CO2 efficiently. Up to
68
70% of the cylindrical test specimens carbonated in 91 days. This is primarily due to the more porous
nature of the CLSM matrix.
Fig. 4-10. Carbonation depth of CLSM mixtures
Based on these carbonation test results, it is clear that CLSM has a very high potential for
efficiently sequestrating carbon dioxide.
0
5
10
15
20
25
30
35
40
7 28 56 91
Carb
on
ati
on
De
pth
(m
m)
Age (days)
S-1
69
Fig. 4-11. Carbonation depth of CLSM at Fig. 4-12. Carbonation depth of CLSM at
28-day 56-day
Fig. 4-13. Another view of CLSM specimen Fig. 4-14. Carbonation depth of CLSM at
showing carbonation depth at 56 days 91-days
70
4.8 Quantification of Carbon Dioxide Sequestered in Concrete, CLSM, and Other
Cement-based Materials
According to PCA [Gajda 2001] fully carbonated 100 tonnes (110 tons) of average portland cement
produces 31 tonnes (34 tons) of Ca(OH)2. Accounting for the average un-hydrated cement content in a
typical concrete, which is about 7 %, the Ca(OH)2 yield reduces to 29 tonnes (32 tons). Therefore, when
fully carbonated, this quantity of Ca(OH)2 can absorb 17.3 tonnes (19 tons) of CO2 (at the rate of about 56
%). This indicates that in a fully carbonated concrete, cement can absorb carbon dioxide by about 17% of
its mass. This information has been used in subsequent sections for the quantification of carbon dioxide
sequestered in concrete, CLSM, and other cement-based materials.
For simplified understanding and calculations, it is assumed that the depth of concrete and CLSM up to
which carbonation was detected, by the RILEM phenolphthalein test, is considered to be fully carbonated.
Carbonated depth for concrete Mixtures M-1, M-3, M-4, M-5, N-8, and CLSM is given in Table 4-23.
The carbonated depths at the 91-day age were used in the case of concrete Mixtures M-1, M-3, M-4, and
M-5, and CLSM. For Mixture N-8, no-fines concrete, 56-day carbonation result was used because 91-day
result was not available. The calculations are given detailed in Table 4-22. It can be seen from Table 4-
22 that the maximum sequestration of CO2 is possible in CLSM, followed by the no-fines concrete
containing 80% less sand and 50% cement replaced with Class C fly ash. For the concrete Mixture M-3
with MRWRA, containing 50% cement replaced also has high potential for CO2 sequestration. Based on
the results it could be concluded that maximum potential for carbon credit earned per ton of cement is
with the use of CLSM followed by the no-fines concrete mixture, and concrete mixture.
71
Table 4-22: Carbon Dioxide Sequestered per Ton of Cement Used
Mixture Carbonation
depth (mm)
Volume of
cylinder
carbonated (%)
CO2 sequestered by
the cement used in
one cu. yd. mixture
(lbs)
CO2 sequestered per
ton of cement (lbs)
M – 1, 0% fly ash 2.0 8 6.7 27
M – 3, 50% fly ash 5.5 21 8.8 71
M – 4, 50% fly ash 3.0 12 4.9 40.
M – 5, 40% fly ash 3.75 14 7.4 49
N-8* 50% fly ash 7.0 26 10.6 88
CLSM S-1 35 91 7.7 309
* 56-day result is used, 80% sand taken out, no-fines concrete
72
Chapter 5
OBSERVATIONS
The trends obtained on CO2 sequestration in cement-based materials have shown one of the effective and
economical ways for the reduction of CO2 from construction industry and subsequently a viable option
for carbon dioxide sequestration to help reduce global warming. Based on the information presented in
different chapters on the sequestration of carbon dioxide in concrete, no-fines concrete, CLSM,
and other similar cement-based materials using fly ash the following important observations may
be drawn.
Gaseous phase of the carbon dioxide has very critical effect on the Earth‟s ecosystems. CO2 gas
is the main GHG responsible for global warming and climate change. The maximum growth rate
of atmospheric carbon dioxide was 1.9 ppm/year during 2000 – 2006. It is a toxic gas, and its
effects on the human body increase with an increase in the concentration in air.
The average growth rate of carbon dioxide emissions from fossil fuel and cement production has
increased from 1.3% per year for 1990-1999 to 3.3% per year for 2000-2006. Cement industry
contributes approximately 6% of the total anthropogenic CO2 emission to the Earth‟s atmosphere.
Therefore, the major environmental issue associated with the construction industry is reduction of
CO2 emissions from the production of portland cement. Of course, the use of blended cement is a
way to reduce CO2 emission from the construction industry; however, it has its own limitations.
Therefore, exploring possibilities to develop economical, practical, and environmentally friendly
technologies for CO2 sequestration in cement-based materials is the need of the hour for lowering
the concentration of carbon dioxide gas already present in the Earth‟s atmosphere.
73
About 19% of the carbon dioxide produced during manufacture of cement is reabsorbed by the
concrete over its lifecycle through the natural process of carbonation. The normal process of
carbonation in conventional concrete is very slow, about one mm/year and mainly depends on the
type of cement, quality of concrete, environmental conditions, SCM material used, and
permeability of concrete etc. Carbonation of cement-based materials generally results in
increased concrete strength and increased impermeability compared to the same concrete prior to
the carbonation. This technology is used in the production of higher quality precast-concrete
products and was proposed in the early 1900s. Faster carbonation of concrete and other cement-
based materials through mineralization could be used as an alternate means for the sequestration
of carbon dioxide. Generally, 100 tonnes (110 tons) of the hydrated ordinary portland cement
can absorb up to 17.3 tonnes (19.1 tons) of CO2 leading to earning of globally tradable carbon
credits.
The most widely adopted engineered way for the mineralization of carbon dioxide in cement-
based materials is through their early age carbonation curing. The early age carbonation is more
efficient because the pore structure is still not very dense that provides an efficient means for
carbon dioxide sequestration in these materials besides several technical benefits. Claims to
have a faster way to store more carbon dioxide in concrete through CO2-accelerated curing of
precast concrete elements have also been made. However, it requires special arrangement for
pure carbon dioxide, its pressure, pre-conditioning, and equipment etc.
Opportunities have been demonstrated 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 fines.
There is also possibility to sequester carbon dioxide gas directly from atmosphere by using
cement-based materials by providing conditions favorable to carbonation.
74
Cement-based materials such as regular concrete, no-fines concrete, and controlled low strength
materials (CLSM) have great potential for the sequestration of carbon dioxide from atmosphere
directly.
ASTM Class C fly ash is very effective in direct CO2 sequestration capability of cement-based
materials.
The depth of carbonation or CO2 sequestration potential in the concrete mixtures increases with
an increase in ASTM Class C fly ash content.
Rate of carbonation of concrete can be increased up to three times compared with concrete
without fly ash by replacing 50% cement with ASTM Class C fly ash. Therefore, such concrete
has three times faster CO2 sequestration potential.
A higher percentage replacement of cement by ASTM Class C fly ash causes concrete to
carbonate or sequester CO2 at an earlier age.
Absence of water reducing admixture in concrete starts CO2 from an earlier age than concrete
with MRWRA.
CO2 sequestration in concrete mixtures with fly ash is possible at much higher rate compared
with portland cement concrete only.
CO2 sequestration potential in no-fine concrete also increases with an increase in the replacement
levels of cement with ASTM Class C fly ash.
No-fine concrete mixtures with 80% less sand showed a higher carbonation depth than no-fines
concrete with 40% less sand.
A reduction in sand content but an increase in fly ash content results in a higher carbonation rate
for the no-fines concrete.
Increase in CO2 concentration in curing environment of concrete at 50 ± 5 % relative humidity
increases the carbon dioxide sequestration potential in the cement-based materials.
CLSM has much faster rate of carbon dioxide sequestration potential i.e. 3-5 times, compared to
concrete.
75
The maximum potential for carbon credit earning per ton of cement used in conjunction with
ASTM Class C fly ash is in CLSM followed by no-fines concrete, and regular concrete without
MRWRA, respectively.
Cement used in CLSM can sequester carbon dioxide about eleven times more than the same used
in normal concrete without fly ash.
76
Chapter 6
ACKNOWLEDGEMENTS
The UWM Center for By-Products Utilization was established in 1988 with a generous grant from the
Dairyland Power Cooperative, La Crosse, Wisc.; Madison Gas and Electric Company, Madison, Wisc.;
National Minerals Corporation, St. Paul, Minn.; Northern States Power Company, Eau Claire, Wisc.; We
Energies, Milwaukee, Wisc.; Wisconsin Power and Light Company, Madison, Wisc.; and, Wisconsin
Public Service Corporation, Green Bay, Wisc. Their financial support and additional grant and support
from Manitowoc Public Utilities, Manitowoc, Wisc., are gratefully acknowledged.
77
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Authors:
Tarun R. Naik, Ph. D., P. E., is currently a Research Professor and Academic Program Director
of the UWM Center for By-Products Utilization, University of Wisconsin – Milwaukee, USA.
He received his Bachelor of Engineering degree in Civil Engineering from the Gujarat
University, India. He received his M.S. and Ph.D. degrees in Civil Engineering from the
University of Wisconsin - Madison. He is a registered Professional Engineer in Wisconsin. His
contribution in teaching and research has been well recognized nationally and internationally.
He has taught many civil engineering and mechanics courses as a part of his teaching
responsibilities at UWM since 1975. From the UWM College of Engineering and Applied
Science he received an award for Outstanding Service in 1990; an award for Outstanding
Teaching in 1997; and, an award for Outstanding Research in 2000.
Rakesh Kumar, Ph. D., was a Post-doctoral Research Associate at UWM Center for By-
products Utilization, Milwaukee, Wisconsin, USA; and, is currently a Scientist at Central Road
Research Institute (CRRI) New Delhi, India. He received his Bachelor of Engineering degree
with distinction in Civil Engineering from the Bihar University, India and his Ph.D. degree in
Civil Engineering from the Indian Institute of Technology Delhi (IITD). His research interest
includes SCC, HPC, sustainable concrete, concrete for CO2 sequestration, microstructure and
durability of concrete, NDT methods, and repair and rehabilitation of roads, bridges, and
buildings. He is a national merit scholar recipient and has published more than 45 peer-reviewed
papers. He is well recognized nationally for his SCC work.
