GREEN CONCRETE STRUCTURES BY REPLACING CEMENT WITH POZZOLANIC MATERIALS TO REDUCE GREENHOUSE GAS EMISSIONS FOR SUSTAINABLE ENVIRONMENT

Abstract

The concrete industry is known to leave an enormous environmental footprint on Planet Earth. First, there are the sheer volumes of material needed to produce the billions of tons of concrete worldwide each year. Then there are the CO 2 emissions caused during the production of Portland cement. Together with the energy requirements, water consumption and generation of construction and demolition waste, these factors contribute to the general appearance that concrete is not particularly environmentally friendly or compatible with the demands of sustainable development. One important issue is the use of environmental-friendly concrete ("green") concrete to enable worldwide infrastructure-growth without increase in CO 2-emission. Another, probably even more important issue, is the use of more environmental friendly structural designs incorporating more environmental-friendly. Therefore, it is necessary to look for sustainable solutions for future concrete construction. The solution of this problem is use the green concrete which eliminates the negative impact of the cement industry, minimizing environmental impact, therefore, we should try to reduce the quantity of concrete used in buildings, to replace as much Portland cement as possible by supplementary cementitious materials, especially those that are by-products of industrial processes, such as fly ash, rice husk ash, palm oil fuel ash, slag, metakaolin and silica fume, and use that concrete wisely.

Full text

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GREEN CONCRETE STRUCTURES BY REPLACING
CEMENT WITH POZZOLANIC MATERIALS TO
REDUCE GREENHOUSE GAS EMISSIONS FOR
SUSTAINABLE ENVIRONMENT
Nurdeen M. Altwair and Shahid Kabir
School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong
Tebal, P. Pinang, Malaysia
ceshahidkabir@eng.usm.my
Abstract
The concrete industry is known to leave an enormous environmental footprint on Planet
Earth. First, there are the sheer volumes of material needed to produce the billions of
tons of concrete worldwide each year. Then there are the CO2 emissions caused during
the production of Portland cement. Together with the energy requirements, water
consumption and generation of construction and demolition waste, these factors
contribute to the general appearance that concrete is not particularly environmentally
friendly or compatible with the demands of sustainable development. One important
issue is the use of environmental-friendly concrete ("green") concrete to enable world-
wide infrastructure-growth without increase in CO2- emission. Another, probably even
more important issue, is the use of more environmental friendly structural designs
incorporating more environmental-friendly. Therefore, it is necessary to look for
sustainable solutions for future concrete construction. The solution of this problem is
use the green concrete which eliminates the negative impact of the cement industry,
minimizing environmental impact, therefore, we should try to reduce the quantity of
concrete used in buildings, to replace as much Portland cement as possible by
supplementary cementitious materials, especially those that are by-products of
industrial processes, such as fly ash, rice husk ash, palm oil fuel ash, slag, metakaolin
and silica fume, and use that concrete wisely.
Keywords: Sustainable development; Cement; CO2; Green concrete; Supplemental
cementitious materials
Introduction
Sustainable development and sustainability have become increasingly popular over the
last few decades, although they are amorphous concepts, and many governments,
corporations and institutions are adopting them as policy [1-3]. The sustainability
concept has also been applied to characterize a type of development, known as
sustainable development [4]. According to the World Commission on Environment and
Development, sustainability means sustainable development, which meets the needs of
the present without compromising the ability of future generations to meet their own
needs [5]. A growing global population is straining the finite resources available on the
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AMERICAN SOCIETY OF CIVIL ENGINEERS
6th International Engineering and Construction
Conference (IECC’6), Cairo, Egypt, June 28-30, 2010

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planet. Sustainability seeks to balance the economic, social, and environmental impacts,
recognizing that population growth will continue [6]. Sustainable development brings
this evaluation to the design and construction industries, which have significant
potential to reduce the negative impact of human activities on the environment.
Sustainability initiatives can be traced back to the environmental movement of the
1960s and the energy crisis of the 1970s [7]. The promotion of sustainable development
has put pressure on the adoption of proper methods to protect the environment across all
industries, including construction. The building process requires high expenditures of
energy and causes a wide range of quantifiable environmental effects, including gas
emissions, water use and solid and liquid wastes. According to a recent USA
Department of Energy (DoE) report, annual energy demands will increase from a
current capacity of 363 million kilowatts to 750 million kilowatts by 2020. The world’s
energy consumption today is estimated at 22 billion kWh/yr; this means 53 billion kWh
by 2020. Such ever-increasing demands could place a significant strain on the current
energy infrastructure and potentially damage world environmental health by CO, CO2,
SO2, NOx effluent gas emissions and global warming. Achieving solutions to
environmental problems that we face today requires long-term potential actions for
sustainable development [8, 9]. The exponential and unsustainable forecast of CO2
emissions during the 21st century (Fig. 1) is based on an estimated population increase
from 6 to 9 billion; a corresponding growth in industrial development and urbanization
would result in three-fourths of the earth’s inhabitants living in urban communities, and
assuming little or no change in today’s wasteful consumption pattern of natural
resources [10].
One of the most pressing concerns for this industry is global warming, which is an
increase in the average temperature of the Earth's atmosphere and oceans as a result of
the buildup of greenhouse gases in our atmosphere. Global warming, sometimes called
climate change, is causing an increase in the earth’s near-surface temperature due to
changes in the atmospheric composition. Many scientists believe recently observed
global warming is partially caused by greenhouse gas emissions from energy
production, transportation, industry and agriculture [9]. The greenhouse effect is one
result of the differing properties of heat radiation when it is generated at different
temperatures [11]. During the past century, global surface temperatures have increased
at a rate near 0.6 ºC/century and the average temperature of the Atlantic, Pacific and
Indian oceans, which cover 72% of the earth's surface, have risen by 0.06 ºC since
1995. Global temperatures in 2001 were 0.52 ºC above the long-term 1880–2000
average (the 1880–2000 annually-averaged combined land and ocean temperature is
13.9 ºC). Also, according to the USA Department of Energy, world emissions of carbon
are expected to increase by 54% above 1990 levels by 2015, making the earth likely to
warm 1.7–4.9 ºC over the period 1990–2100, as shown in Fig. 2.
Figure (1): Historical and future
atmospheric CO2 concentrations [10].
Figure (2): Global mean temperature
changes over the period of 1990–2100 and
1990–2030 [11].

271
Such observations among others demonstrate that interests will likely increase
regarding energy related environment concerns and that energy is one of the main
factors that must be considered in discussions on sustainable development [11, 12]. This
paper reviews the impacts on the environment, by one of the largest industries in the
world, the concrete industry. The role of the cement industry in global CO2 emissions,
and alternative ways to reduce the use of large quantities of cement in mortar and
concrete are especially discussed.
Environmental impact of concrete
The concrete industry is no exception in its role in the negative impact on the
environment. Concrete is the most popular construction material and more than 12
billion tons/year of concrete is consumed annually worldwide. This equals more than ½
cu. yd. of concrete produced per person worldwide. Concrete is a very versatile building
material, but it is not very green. A sustainable concrete structure is one that is
constructed such that the total societal impact during its entire life cycle is minimal [13-
15] Designing with sustainability in mind includes accounting for the short-term and
long-term consequences of the structure. In particular, building in a sustainable manner
means to focus attention on the effects on human health, energy conservation, and
physical, environmental & technological resources for new and existing buildings. It is
also important to take into account the impact of construction technologies and methods
when creating sustainable structures [16].
Cement is considered one of the most important building materials around the world. It
is mainly used for the production of concrete. Concrete is a mixture of inert mineral
aggregates, such as sand, gravel, crushed stones, and cement. Cement consumption and
production is closely related to construction activity, and therefore to general economic
activity. Cement is one of the most produced materials around the world. Due to the
importance of cement as a construction material, and the geographic abundance of the
main raw material, limestone, cement is produced in virtually all countries. The
widespread production is also due to the relatively low price and high density of cement
[17].
Figure (3): BAU cement consumption by region [19].
In general, a mortar consists of about 45% by volume of cement and about 10 to 15%
cement; as for concrete, a typical cubic yard (0.7643 m3) contains about 10% cement by
weight. The infrastructure needs of developing countries have led to huge increases in
demand for Portland cement (Table 1) [9,18]. According to the BAU scenario, cement
consumption will grow at high rates on world levels in the 2000–2030 period. On a
global level, the 1600 Mt of cement consumption in 2000 will increase almost two-fold
to 2880 Mt by 2030, implying an annual 2% growth rate. Figure 3 represents the
regional consumption of cement in 10-year intervals, where 1997 is given in the figure

272
as the base year. The chart shows that most growth takes place in the developing
regions [19]. Generally, the sustainability of the cement and concrete industries is
imperative to the well-being of our planet and to human development. However, the
production of Portland cement, an essential constituent of concrete, leads to the release
of a significant amount of CO2 and other greenhouse gases (GHGs).
Carbon dioxide emission from cement production
The environmental issues associated with CO2 will play a leading role in the sustainable
development of the cement and concrete industry during this century. One of the
biggest threats to the sustainability of the cement industry is the dwindling amount of
limestone in some geographical regions. The production of Portland cement leads to the
release of a significant amount of CO2 and other greenhouse gas (GHGs) [20]. Cement
production is a highly energy-intensive production process. Energy consumption by the
cement industry is estimated at about 3% of the global primary energy consumption, or
almost 5% of the total global industrial energy consumption [18,21]. Cement is
manufactured from a combination of naturally occurring minerals - calcium (60% by
weight) mainly from limestone or calcium carbonate, silicon (20%), aluminum (10%),
iron (10%) and small amounts of other ingredients and heated in a large kiln to over
1500° C (2700° F) to convert the raw materials into clinker. The theoretical energy
consumption for producing cement can be calculated based on the enthalpy of
formation of 1 kg of Portland cement clinker, which is about 1.76 MJ [22]. This
calculation refers to reactants and products at 25±C and 0.101 MPa. In addition to the
theoretical minimum heat requirements, energy is required to evaporate water and to
compensate for the heat loss. Heat is lost from the plant by radiation or convection and,
with clinker, emitted kiln dust and exit gases leaving the process. Hence, in practice,
energy consumption is higher. The kiln is the major energy user in the cement-making
process. Energy use in the kiln basically depends on the moisture content of the raw
meal. Most electricity is consumed in the grinding of the raw materials and finished
cement. Power consumption for a rotary kiln is comparatively small, and generally
around 17 and 23 kWh/t of clinker (including the cooler and preheater fans) [23].
Additional power is consumed for conveyor belts and packing of cement. Total power
use for auxiliaries is estimated at roughly 10 kWh/t of clinker [24].
The cement industry contributes to carbon dioxide emissions in two ways. Both the
manufacturing process and resulting by-products of the chemical reaction that produces
ordinary Portland cement (OPC) contribute to total carbon dioxide emissions. An
indirect and significantly smaller source of CO2 is from consumption of electricity
assuming that the electricity is generated from fossil fuels. Roughly half of the emitted
CO2 originates from the fuel and half originates from the conversion of the raw material
[25]. Ordinary Portland cement results from the calcination of limestone and silica in
the following reaction [9].
Limestone + silica (1500 °C) = Portland cement + carbon dioxide
5CaCO3+2SiO2→(3CaO,SiO2)(2CaO,SiO2)+5CO2 (2.1)
This reaction produces roughly 597kg of CO2 gas for every ton of cement produced
[26]. The production of 1 ton of cement produces 0.5 tons of chemical CO2, in a
reaction that takes place at 1450 °C. An additional 0.4 ton of CO2 is given off as a result
of the burning of carbon fuel to provide this heat [27]. To put it simply, 1 ton of cement
produced equals 1 ton of CO2 released [19]. Without altering the chemistry of cement
the reaction component of this CO2 cannot change. The other 40% of CO2 is produced
by burning of fossil fuels to make OPC clinker [28].

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Reducing carbon dioxide emission through green concrete
One approach to help achieve higher infrastructure sustainability is the development
and use of new materials, deliberately designed with sustainability as a primary goal, in
terms of improved social well being, increasing economic prosperity, and reduced
environmental impact. This can be accomplished through many methods, such as the
replacement of dwindling raw materials with suitable waste products, the development
of improved materials to replace less sustainable materials, or the use of new materials
to extend infrastructure service life.
Reductions will be achieved not only as a result of modifications to existing cement
production methods, and the solution to this environmental problem is not to replace
concrete with other materials, but to reduce the environmental impact of cement. Again,
even a small reduction of the environmental impact per ton of concrete will result in
large environmental benefits because of the vast amount of concrete produced today. A
reduction in cement use is desirable in terms of energy and this can be achieved by
using other cementitious materials or admixtures [29]. These cementitious materials
will need to show comparable or better properties and costs compared with the existing
ordinary Portland cement. The cement industry, realizing the need to reduce carbon
emissions, began an initiative to bring down the industry’s contribution to GHG. There
have been a number of articles written about reducing the CO2 emissions. There are
many steps to remove problems that affect sustainability, as well as to reach green
concrete, including the use of supplemental cementitious materials (SCMs) to reduce
cement consumption, through the use of lower amounts of cement and reasonable
amounts of supplementary cementitious material (SCM). The proportion of 'pure'
cement in a cement based mixture can be reduced by replacing some of it with other
pozzolanic material, which has the ability to act as a cement like binder. Industrial
wastes including fly ash, slag (a by-product of the coal power industry), silica fume and
rice husk ash all have the combined benefit of being pozzolana that would otherwise be
destined for landfill. While every ton of pozzolana effectively saves a ton of cement
there are often engineering constraints limiting the percentage of cement that can be
replaced. In the past, these limits have typically been in the range of 10-15%, but more
recently, structures containing high volumes of pozzolanic materials can be seen. In
addition to using pozzolanic materials as supplementary materials to reduce
environmental impacts resulting from the use of cement, there are various other reasons,
particularly for reducing construction costs, since lower amounts of cement required for
making concrete and mortar will lead to a reduction in the costs of construction.
Incorporating industrial by-products/pozzolanic materials is becoming an active area of
research because of their improved properties such as workability, long-term strength
and durability. The common blending agents used are fly ash (PFA), rice husk ash
(RHA), palm oil fuel ash (POFA), slag, silica fume (SF), calcined clay etc. Improved
properties such as rheology and cohesiveness, lower heat of hydration, lower
permeability and higher resistance to chemical attack are reported in the literature [30].
Aside from industrial wastes, ashes from agricultural sources, such as rice husk, palm
oil husk, peanut shell or fiber shell, etc. have been used for making cement substitutes
[31]. Furthermore, the benefits from using pozzolanic materials include reductions in
energy consumption, greenhouse gas releases, and other pollutant emissions from initial
mining of limestone, calcination, and grinding.

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Types of supplemental cementitious materials
Pulverized-Fuel Ash (PFA) or Fly Ash
PFA is a by-product of burning pulverized (finely ground) coal to generate electric
power. The shales and clays (contents of silica, alumina and iron oxide) and other
contents in coal, melt while in suspension, and then with rapid cooling they are carried
out by the flue gases and form into fine spherical particles. Fly ash particles are generally
spherical in shape and range in size from 0.5 µm to 100 µm. They consist mostly of
silicon dioxide (SiO2) [32]. The physical and chemical properties of fly ash can vary
considerably from power plant to power plant, primarily because of the differences in the
sources of coal. Two major classes of fly ash are specified in ASTM C 618 on the basis
of their chemical composition resulting from the type of coal burned; these are
designated Class F and Class C. Class F is fly ash normally produced from burning
anthracite or bituminous coal and Class C is normally produced from the burning of sub-
bituminous coal and lignite [33]. Class F is low free lime and contains CaO less than
10%. Whereas Class C is high free lime and contains CaO more than 10%. The sum of
three significant oxides namely SiO2, Al2O3 and Fe2O3 for class F has a minimum value
of 70% and 50% for class C fly ash. Class C fly ash usually has cementitious properties
in addition to pozzolanic properties due to free lime, whereas Class F is rarely
cementitious when mixed with water alone. In the Class F of PFA, the glass is siliceous
or alumino-silicate composition, while Class C of PFA is calcium aluminate
composition. The high lime PFA is more reactive because it contains most calcium in the
form of reactive crystalline compounds such as C3A, C2S and C4A3S [32]. PFA
normally results in lower early strength but improved workability [34]. Fly ash is an
important pozzolan, which has a number of advantages compared with regular Portland
cement. First, the heat of hydration is lower, which makes fly ash a popular cement
substitute for mass structures. Previous studies have found that the use of fly ash as an
additive material for concrete gives positive results in terms of mechanical and chemical
properties..
Silica Fume (SF)
SF is a by-product of the manufacture of silicon and ferrosilicon alloys from high purity
quartz and coal in a submerged-arc electric furnace. It is a powder with particles having
diameters 100 times smaller than those of anhydrous Portland cement particles. The SiO2
content in range 85-98% mean particle size in range 0.1-0.2μm, spherical particle shape
with a number of primary agglomerates, and amorphous particles. The amorphous silica
is highly reactive, and the smallest of the particles speeds up the reaction with CH,
which the compound of cement hydrates. The very small particles of SF be able to go
through the void between the particles of cement hence improve packing. The specific
surface of SF, determine by nitrogen absorption, is 20,000m2/kg. It also has a very low
bulk density (200 to 300 kg/m3) [35]. The most important influences in the use of silica
fume as an admixture in cement based materials are increases in tensile strength,
compressive strength, compressive modulus, flexural modulus and the tensile ductility,
but decreases in the compressive ductility. In addition, it enhances the freeze-thaw
durability, the vibration damping capacity, the abrasion resistance, the bond strength
with steel rebar, the chemical attack resistance and the corrosion resistance of
reinforcing steel. Furthermore, it decreases the alkali-silica reactivity, the drying
shrinkage, permeability, creep rate, coefficient of thermal expansion and dielectric
constant. Moreover, it increases the specific heat and decreases the thermal conductivity,
though the thermal conductivity is increased if silica fume is used with silane, another

275
admixture. Silica fume addition also increases the air void content, decreases the density,
enhances the dispersion of microfibers, and decreases the workability. In addition, the
defects associated with the interface between silica fume and cement contribute to the
defect dynamics during elastic deformation. The use of silane treated silica fume in place
of untreated silica fume increases the consistency, tensile strength and compressive
strength, but decreases the compressive ductility [36].
Ground granulated blast furnace slag
Ground granulated blast furnace slag (GGBS or GGBFS) is obtained by quenching
molten iron slag (a by-product of iron and steel making) from a blast furnace in water or
steam, to produce a glassy, granular product that is then dried and ground into a fine
powder. ASTM C 989-82 and AASHTO M 302 were developed to cover ground
granulated blast furnace slag for use in concrete and mortar. The most important
influences in the use of slag with concrete and cement paste are, effects of slags on
properties of fresh concrete. Use of slag or slag cements usually improves workability
and decreases the water demand due to the increase in paste volume caused by the lower
relative density of slag. An increase of slag content from 35 to 65% by mass can extend
the setting time by as much as 60 minutes. This delay can be beneficial, particularly in
large pours and in hot weather conditions. The rate and quantity of bleeding in concrete
containing slag or slag cements is usually less than that in concrete containing no slag
because of the relatively higher fineness of slag. The higher fineness of slag also
increases the air-entraining agent required, compared to conventional concrete. In
general, the strength development of concrete incorporating slags is slow at 1-5 days
compared with that of the control concrete. Between 7 and 28 days, the strength
approaches that of the control concrete; beyond this period, the strength of the slag
concrete exceeds the strength of control concrete [37]. The significant reduction in
permeability is achieved as the replacement level of the slag increases from 40 to 65% of
total cementitious material by mass. Because of the reduction in permeability, concrete
containing granulated slag may require less depth of cover than conventional concrete
requires to protect the reinforcing steel. Malhotra [38] reported results of freeze-thaw
tests on concrete incorporating 25-65% slag, where test results indicated that regardless
of the water-to-cement (+ slag) ratio, air-entrained slag concrete specimens performed
excellently in freeze-thaw tests, with relative durability factors greater than 91% .
Rice Husk Ash (RHA)
Globally, approximately 600 million tons of rice paddy is produced each year. On
average, 20% of the rice paddy is husk, giving an annual total production of 120 million
tons. Rice husk is an external covering of rice, which is generated (about 90% by mass)
during de-husking of paddy rice [39]. The RHA is rich in silica content, obtained by
burning rice husk to remove volatile organic carbon such as cellulose and lignin. It is
estimated that, one ton of rice yields 200kg of husk and about 40kg of ash [40]. The
silica present in the ash can be amorphous or crystalline and its reactivity depends
primarily on burning conditions. The burning method and the fineness of the particles
are two major factors that primarily affect the reactivity of RHA [40]. According to
Mehta [34], the amorphous silica powders with high surface area are more reactive than
the crystalline form of silica. The fineness of ash will significantly affect the reactivity of
RHA in lime, mortar or concrete mix. These ashes should be complying with the
standard for PFA such as ASTM C 618-84 (1994). The ash has to achieve a maximum of
34% retained on 45μm sieve, when wet sieve analysis is done as per specifications of

276
ASTM C 618-84 (1994). Meanwhile the content of CaO is less than 10% and it is
classified into class F pozzolan.
Palm oil fuel ash (POFA)
Palm oil fuel ash (POFA) is produced as a result of the burning of palm oil shell and
husk (in equal volume) as fuel in palm oil mill boiler to produce steam for electricity
generation and palm oil extraction process. POFA is hazardous materials and is simply
disposed without any commercial returns. The resulting ash, palm oil fuel ash (POFA),
is 5% by weight of the original solid materials. Both physical properties and chemical
analysis indicated that POFA is a pozzolanic material. This pozzolanic material is
grouped in between Class C and Class F as specified in ASTMC618-92a. Various
researchers reported that POFA has pozzolanic properties and highly reactive and can
be used as a unique cement replacement for building construction materials if the POFA
is ground to reduce the particle size (GPOFA), the median particle size is reduced to 10
μm. Where the studies investigated use of palm oil fuel ash as a pozzolanic material to
improve the properties of concrete in terms of compressive strength, drying shrinkage,
water permeability, alkali-silica reaction, carbonation resistance, resistance to chloride
penetration and sulfate resistance, it was noted by these studies, the results were
satisfactory, as well as palm oil fuel ash consider a environmental friend material when
incorporating it as supplementary material [41-45].
Metakaolin (MK)
Metakaolin is refined kaolin clay that is fired (calcined) under carefully controlled
conditions to create an amorphous aluminosilicate that is reactive in concrete.
Metakaolin reacts with the calcium hydroxide (lime) byproducts produced during
cement hydration. Metakaolin is a valuable admixture for concrete/cement applications.
Replacing Portland cement with 8% - 20% (by weight) metakaolin produces a concrete
mix which exhibits favorable engineering properties, including: the filler effect, the
acceleration of OPC hydration, and the pozzolanic reaction. The filler effect is
immediate, while the effect of pozzolanic reaction occurs between 7 to 14 days.
Calcium hydroxide accounts for up to 25% of the hydrated Portland cement, and
calcium hydroxide does not contribute to the concrete’s strength or durability.
Metakaolin combines with the calcium hydroxide to produce additional cementing
compounds, the material responsible for holding concrete together. Less calcium
hydroxide and more cementing compounds results in stronger concrete that is very fine
and highly reactive, giving fresh concrete a creamy, non-sticky texture that makes
finishing easier.
Conclusion
In many situations concrete is superior to other materials such as wood and steel. But
cement production is very energy intensive; cement is among the most energy-intensive
materials used in the construction industry and a major contributor to CO2 in the
atmosphere. Whereas the idea of using recycled materials in concrete production was
widely unknown only a few years ago, concrete producers now know that they need to
change. One important issue is the use of environmentally-friendly or “green” concrete
to enable world-wide infrastructure-growth without increase in CO2-emission. Another,
probably even more important issue, is the use of more environmental friendly
structural designs incorporating more environmental-friendly maintenance/repair
strategies, which require less use of resources, and reduce energy and CO2-emissions at

277
all phases during the entire service life of a concrete structure. Green concrete
eliminates the negative impacts of the cement industry, minimizing environmental
impacts; therefore, we should try to reduce the quantity of concrete used in buildings, to
replace as much Portland cement as possible by supplementary cementitious materials,
especially those that are by-products of industrial processes, such as fly ash, rice husk
ash, palm oil fuel ash, slag, metakaolin and silica fume, and use that concrete wisely.
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