Concrete main binding material in concrete. Over

Concrete
is the most used construction material in the world. Cement is the main binding
material in concrete. Over the past 3 decades, the production of cement has
grown rapidly all over the world. The cement production in India is expected to
grow three-folds by 2050, as can be seen in Figure 1.1 (WBCSD-IEA 2006).
However, cement production has major environmental issues that are of concern
worldwide (Barcelo et al. 2014; Davidovits 1994). For every one tonne of
clinker manufactured, approximately one tonne of CO2 is released to
the atmosphere (Benhelal et al. 2013), which contributes almost 5-7% of global anthropogenic
carbon dioxide emissions. In the manufacturing process of cement, the main
sources of gas emissions are combustion of fuels and decomposition of CaCO3
to CaO and CO2. Supplementary cementitious materials are used to
partially replace clinker, which eventually reduces the harmful emissions.

Figure 1.1  Estimated cement production (WBCSD-IEA 2006)

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Figure 2 CO2
emission by ten largest emitters worldwide (adapted from IEA 2014)

 India is the second largest cement producer in
the world after China. India produced 280 million tonnes of cement in the year
2014, with an expected annual growth of approximately 8% to 10% annually in the
coming years. As the pressure on the industry is expected to continue to grow,
the prices of cement have been rising constantly due to increasing demand and
costs of raw materials. Being relatively new, most of the cement plants in India
utilise state-of-the-art technologies that are energy and resource efficient.
It has been projected that the production of cement in India is expected to
reach 550 MT by 2020, as shown in Figure 1.1, and thus, further improvements in
these efficiencies are required. Figure 1.2 shows the current contribution of
the Indian industry to emissions, with respect to the global emissions from the
top ten emitters. As the production of cement increases, the emissions are also
expected to increase proportionally, unless a change in the cement production
technology or cement composition takes place. Supplementary cementitious
materials such as fly ash offer the most promising means of reducing these
emissions. Pulverised fuel ash extracted from flue gases by electrostatic
precipitators or cyclone separators of a coal-fired thermal power plant is
known as fly ash and the coarse ash particles that are too heavy to fly with
the flue gases and settle in the boiler are known as bottom ash.

While
bottom ash has coarse and crystalline particles, fly ash is usually as fine as
cement and consists of glassy spherical particles as well as residues of
quartz, mullite, hematite, magnetite, char and other crystalline phases formed
during cooling. The glassy and amorphous particles of fly ash are known to be
reactive in the alkaline conditions offered by cement pastes. In India, almost
70% of the total electricity generation is from coal based thermal power plants
(CEA 2014). The coal used in India has a higher ash content of about 35-40%,
which produces more quantity of fly ash during combustion of coal in
electricity generation (Haque 2013).

The
energy consumption and CO2 emissions associated with the
manufacturing of cement can be reduced when fly ash is used as a partial
replacement of clinker. This, at the same time, allows the productive use of an
industrial by-product, which would otherwise have ended in landfills. The
utilization of fly ash as cement replacement material in concrete or as an
additive has many benefits from economical, technical and environmental points
of view. The cost of fly ash is much lower than cement, approximately Rs. 300
per tonne of fly ash compared to Rs. 6000 per tonne of cement. Inclusion of fly
ashes improves the workability of concrete by reducing segregation and bleeding
(Thomas 2007); it also improves resistance to corrosion of reinforcing steel
and chemicals by lowering the permeability of concrete (Bouzoubaâ et al. 2001;
Jiang et al. 2004; Malhotra 1990; Plowman and Cabrera 1996). Except in regions
experiencing freeze thaw cycles (Valenza and Scherer 2007), which are not
common in India, the use of fly ash has also been associated with the
improvement in the long term performance of concrete (Siddique 2004).

The
use of fly ash is known to result in three main advantages:

1)
use of low cost raw materials,

2)
conservation of natural resources

3)
the elimination of wastes (Cheerarot and Jaturapitakkul 2004). It can be said
that fly ash is not a waste and is a valuable resource material (Kumar et al.
2005).

 Despite studies showing the benefits of Indian
fly ashes (Chatterjee 2011) and Indian standards allowing up to 35% replacement
of cement by fly ash, the majority of fly ash available in India still goes to
landfills. This is mainly because of the unpredictability in the development of
mechanical properties of fly ash due to large variation in their compositions
of fly ashes around the world and the lack of awareness about the potential
advantages of mixing fly ash in concrete. However, some authors have questioned
the benefits of Indian fly ashes, which are known to be less reactive than fly
ashes elsewhere (Kar 2003; Mullick 2005).

1.1Fly ash

Fly
ash is a by-product of the combustion of pulverized coal in thermal power
plants. It is removed by the dust collection system as a fine particulate
residue from the combustion gases before they are discharged into the
atmosphere. Fly ash particles are typically spherical, ranging in diameter from
less than 1 micron to 150 micron, the majority being less than 45 micron

1.1.2 Classification of
fly ash

ASTM-
C 618-93 categories natural pozzolans and fly ashes into the following three
major types:

Class
N Fly ash: The raw or calcined natural pozzolans such as some diatomaceous
earth,volcanics ashes and pumice come in this category of pozzolans.

Class
F Fly ash: Fly ash normally produced from burning bituminous coal falls in this
category. This class of fly ash exhibits pozzolanic property but rarely if any,
self-hardening property. The proportion of CaO in class F Flyash is generally
less than 10%.

Class
C Fly ash: Fly ash normally produced from lignite or sub-bituminous coal in the
only material included in this category. This class of fly ash has both
pozzolanic and varying degree of self-cementitious properties, due to the
higher amounts of CaO(higher than 10%).

1.1.3 Effect of Fly Ash
Composition on Concrete

 It is stated earlier that there is a little
correlation between the oxide analysis of a mineral admixtures and its
performance in concrete. However, it is important to know the possible effects
on behaviour of concrete due to the chemical composition of fly ash.

Effect of SiO­2,
Al2O3 and Fe2O3

 ASTM C 350-1954, specified a minimum limit
ofSi02 content as 40%. This takes in to account the chemical
reaction of the liquid phase of the cement which possesses additional calcium
silicate hydrate and thus contributes to the development of compressive
strength. Late, in 1960, the sum of SiO2, Al2O3
and Fe2O3 by weight was made to be a minimum of 70%. In
the latest ASTM C 618 specifications the minimum limitation for class F and for
class C is fixed at a minimum weight of 50% for the above compounds. The
intention to specify this limit is to ensure that sufficient glass constituents
are present However, many researchers found that there was little effect of
these three compounds on the performance of concrete.

Effect of MgO Content

 It is well known that in portland cement
pastes the hydration of crystallaneous MgO under autoclave conditions leads to
expansion and cracking. According to the above phenomenon maximum limit on the
MgO content was specified in the portland cement In fact it is not valid in
case of fly ash because of the MgO in fly ash will be either in noncrystalline
form (glass) or in the form of non-expansive muilite phase. However, some
countrips have specified the maximum limit of 4 to 5% to prevent expansion.

Effect of SO3
Content

The
limit on the SO3 content is imposed on basis of its expansive nature
causing deterioration of concrete due to formation of ettringite. It was stated
that the SO3 content resulted in high early strength. A recent study
indicates that the 20-40% replacement offly ash/admixture mixes were having the
optimum SO3 content of 2.76%. It was also observed based on a study
that the combination ofSO3 contents of both cement and fly ash
produces the same expansion as that of control mix. The study was also extended
to sulphate resisting cements and was found that the optimum value decreases
with age. Many authorities specify the maximum limit of SO3 as 3-5%,
Haque et al., (1987).

Effect of CaO Content

The
CaO content in the fly ash mainly depends upon the type of coal used in
combustion. The CaO content of fly ashes from bituminous coal or anthracite
coal. Their CaO content rarely exceed 7% whereas from sub-bituminous or lignite
coal these values may be more than 10% and even as high as 30%. The study on
CaO content and its reaction indicates that a CaO content of about 20% contains
siliceous glass which is not so reactive, where as CaO content of more than 20%
contains calcium aluminate glass which is more reactive in concrete. It is
infered that the high CaO content in fly ash modifies its mineralogical
characteristics and reactivity. The high CaO content causes the fly ash to be
not only more reactive but also cementitious.

Effect of Loss on
Ignition (LOI) The low temperature burning (1000°C
and below) causes the presence of some volatile residue which is termed as loss
on ignition. The carbon content is the more important component of loss on
ignition and it affects the water requirement for mortar and concrete. In
general, lower the carbon 30 percentage, the better will be the fly ash. The
carbon content of fly ash which has high porosity and a very large specific
surface causes the absorption of water and organic admixtures such as water
reducing agents, air entraining agents, set-retarders etc. Different countries
have given different ranges of limitations on loss on ignition. ASTM C 618-89
has allowed only maximum of 6% in case of class C and 12% in case of class F
fly ash. The ranges on limitations of loss on ignition are about 5-12%.

Effect of Alkalies (Na2O,
K2O)

 It was stated that the presence of alkalies
(Na2O, K2O) causes the efflorescence and alkali-aggregate
reaction. It was stated that alkalies in fly ash may also cause expansion. The
main variable affecting alkali aggregate reaction in concrete is the amount of
alkalies in the cement. The effectiveness of fly ash in reducing alkali
aggregate reaction is dependent on the available alkali content of fly ash and
cement.

1.1.4 Pozzolanic
Reaction Mechanism

Cement
Reaction: C3S + H —– C-S-H + Ca(OH)2

Pozzolanic
Reaction: Ca(OH)2 + Pozzolana——- C-S-H GEL

1.1.5 Advantages of Fly
Ash

·        
It is highly economical.

·        
Use of Fly Ash is
environmentally friendly as the waste materials from industries are effectively
being used to create quality building materials.

·        
Fly Ash has very small
particles which makes the concrete highly dense and reduces the permeability of
concrete. It can add greater strength to the building.

·        
The concrete mixture generates
a very low heat of hydration which prevents thermal cracking.

·        
Fly Ash concrete is resistant
to acid and sulphate attacks.

·        
The shrinkage of fly ash
concrete is very less.

 

 

BOTTOM ASH

Coal
combustion by-products (CCPs) have been around since man understood that
burning coal generates electricity. The concept of sustainable development only
reawaken our consciousness to the huge amount of CCPs around us and the need
for proper reutilization than the current method of disposal which has severe
consequences both to man and the environment. Coal bottom ash (CBA) is formed
in coal furnaces. It is made from agglomerated ash particles that are too large
to be carried in the flue gases and fail through open grates to an ash hopper
at the bottom of the furnace. Bottom ash is mainly comprised of fused coarser
ash particles. These particles are quite porous and look like volcanic
lava. Bottom ash forms up to 25% of the total ash while the fly ash forms
the remaining 75%. One of the most common uses for bottom ash is as structural
fill. This thesis presents the result of utilization of waste from thermal
power plants to improve some engineering properties of concrete. Effect of coal
bottom ash on the properties of cement mortar such as workability, chemical
characteristics and pozzolanic activity are presented. Coal bottom ash (CBA)
were utilized in partial replacement for fine aggregates and cement. The
workability using bottom ash replacement showed that bottom ash has much higher
water absorption ratio as compared to the natural sand since the workability
decreased with the increasingly of bottom ash replacement.

1.2.1 Advantages of
BAC:

·        
More sustainable

·        
Reduces environmental pollution

·        
Later age performance is better

·        
Shrinkage value of concrete
decreases

·        
Reasonable and ecological
benefits

1.2.2 Disadvantages of
BAC:

·        
Extended setting  time

·        
Slow development of strength

·        
Low early age strength

·        
Workability reduces

·        
Bleeding increases 

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