Fly ash as sustainable material

Full text

AR.SHRUTI.H.KAPUR RASA-2011
Theme :-!Methodologies+of+sustainable+design
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++Ar. Shruti.H.Kapur
Assistant Professor, Department of Architecture,
LovelyProfessional University , Phagwara, Punjab, India
shrutihkapur@gmail.com
Sub-theme: Fly Ash: A Sustainable Material For Today’s Buildings
ABSTRACT
Last few decades have seen the emergence of various innovative as well as low cost building
materials & construction techniques that satisfy functional & specification requirements of
conventional construction. One such product is known for its proven stability for variety of
applications as admixture in cement, concrete, mortar, lime pozzolan mixture (bricks.blocks) etc.
Flyash, an industrial by-product from Thermal Power Plants with current annual generation of
approximately 108 million tones, has many advantages like cost effectiveness & environment
friendly. Its use as a building material helps increase buildings strength & stability. The aim of
the paper is to discuss properties of geopolymer concrete, how these differ from properties of
ordinary Portland cement, durability properties of fly ash based geopolymer concrete and its
advantage when used as a construction material as well.
Keywords: Flyash; clay brick; flyash brick; strength, Geopolymer, durability, residual alkalinity.+
INTRODUCTION
The Building Industry depends heavily on energy-intensive materials and resources. These
building materials contribute to large environmental costs incurred due to extraction, production,
manufacture, transportation, and installation and recycling of these materials. It is the need of
the hour to use such material that results in minimum possible environmental cost. This can be
achieved with the aid of sustainable material. An overall evaluation of the total energy and
environmental impact of the material and its creation process, as well as the long-term efficiency

AR.SHRUTI.H.KAPUR RASA-2011
of the material is needed to determine the sustainability of a material. A material can be made
more sustainable in various ways, including reducing the travel time of transport, reusing
materials, selecting materials that have a manufacturing process with a low energy requirement,
using recyclable products and so on. One of the most important requirements of the green
building is to use environmental friendly building materials such as the industrial waste products
like flyash – Greener materials.
2.0 Flyash
Fly ash, fine glass powder recovered from the gases of burning coal during the production of
electricity, are micron-sized earth elements made primarily of silica, alumina and iron. During
the combustion of coal, the products formed are classified into two categories viz. bottom ash
and flyash(Figure1). The residue that gets fused into particles is termed as bottom ash and the
part of the ash that is entrained in the combustion gas leaving the boiler is Flyash. Flyash thus
formed is collected either through mechanical collectors or electrostatic.
2.1 Physical Properties of Flyash :
Colour: Flyash derives its color mainly from Carbon and iron. High carbon content changes the
color to grey or black and high iron content gives tin colored ash (Figure 2).
Figure 2. Typical ash colors.
Fineness: Fineness of flyash is expressed in terms of specific surface area, which varies
from 2x107 to 5x107 N/mm2. Greater fineness of flyash increases the air entraining
admixture demand. The large amount of surface energy is utilized to combine with lime.

AR.SHRUTI.H.KAPUR RASA-2011
The fineness of the flyash used as a percentage by weight retained on 90 micron sieve is 9%.
Shape: Flyash is basically spherical in shape and its size ranges from about 150
Micron to less than 1mm(Figure 2).
Figure 2. Typical ash colors
2.2 Microstructure of fly ash
Fly ash is a fine, glass powder recovered from the gases of burning coal during the production of
electricity. The particle size may correspond to that of silty sand to silty clay i.e. between 5-120
microns (Figure 3).
Figure 3. Microstructure of flyash
2.3 Chemical composition of Flyash :
Fly ash can be categorized into the following two categories: –
1.Class F Fly ash: Fly ash normally produced from burning anthracite or bituminous coal falls in
this category. This class of fly ash exhibits pozzolanic property but rarely if any, self-hardening
property.
2. Class C Fly ash: Fly ash normally produced from lignite or sub- bituminous coal is the only

AR.SHRUTI.H.KAPUR RASA-2011
material included in this category. This class of fly ash has both pozzolanic and varying degree
of self cementious properties.
2.4 Engineering Features of fly ash
• Spherical shape
• Ball bearing effect
• Higher Strength
• Decreased Permeability
• Reduced Sulfate Attack.
• Reduced Efflorescence and Shrikage
• Reduced Heat of Hydration
• Reduced Alkali Silica
• Workability
• Improved Finishing
• Reduced Bleeding , Segregation and Slump Loss
2.5 Hydration of Flyash:
Hydration is the process of formation of cementitious material by the reaction of free lime (CaO)
with the pozzolans(AlO3, SiO2, Fe2O3) in the presence of water. The hydrated calcium silicate
gel or calcium aluminate gel (cementitious material) binds inert material together. The calcium
oxide (lime) material of Class C Flyash reacts with the siliceous and aluminous materials
(pozzolans) of the fly ash itself. Since the lime content of class F fly ash is relatively low,
addition of lime is necessary for hydration reaction with the pozzolonas of the fly ash
3.0 FLY ASH BASED INNOVATIVE BUILDING PRODUCTS
Some of the innovative and commonly manufactured environmental friendly building materials
utilizing Fly Ash are covered below;

AR.SHRUTI.H.KAPUR RASA-2011
3.1 Cellular Light Weight Concrete (CLC) Blocks
Cellular Light Weight Concrete (CLC) blocks are substitute to bricks and conventional concrete
blocks in building with density varying from 800 kg/m3 to 1800 kg/m3. This Foaming Agent
based technology from Germany constitutes mainly of cement, Fly Ash, sand, water and foam
CLC has better strength to weight ratio and can thus be used for producing walling & roofing
panels.
3.2 Fly Ash Based Polymer Composites as Wood Substitute
Fly ash based composites have been developed using fly ash as filler and jute cloth as
reinforcement. After treatment, the jute cloth is passed into the matrix for lamination. These
laminates are then cured at specific temperature and pressure. In order to achieve the required
thickness, a number of laminates are used.
3.3 Portland Pozzolana Cement (Fly Ash based)
Upto 35% of suitable fly ash can directly be substituted for cement as blending material.
Addition of fly ash significantly improves the quality & durability characteristics of resulting
concrete
3.4 Ready mixed Fly Ash concrete
Ready Mix concrete is quite popular in developed countries but in India it consumes less than 5
percent of total cement consumption. Recently its application has started growing at a fast rate.
On an average 20% Fly ash in the country is being used which can easily go very high. In ready
mix concrete various ingredients and quality parameters are strictly maintained/controlled which
is not possible in the concrete produced at site and hence it can accommodate still higher
quantity of fly ash.
3.5 Fly Ash- Sand-Lime-Gypsum(Cement) Bricks /Blocks
Fly Ash can be used in the range of 40-70% along with lime, gypsum (cement), sand, stone

AR.SHRUTI.H.KAPUR RASA-2011
dust/chips etc. Minimum compressive strength (28 days) of 70 kg/cm2 can easily be achieved
and this can go upto 250 Kg/cm2 (in autoclaved type). Lower requirement of mortar in
construction, plastering over brick can be avoided. Controlled dimensions, edges, smooth and
fine finish & can be in different colours using pigments . Cost effective, energy-efficient &
environment friendly (as avoids the use of fertile clay) .
3.6 Clay-Fly Ash Bricks
In Clay Fly Ash Bricks Fly Ash content can be 20 to 60% depending on the quality of clay.
Process of manufacturing is same as for the burnt clay bricks. Fuel requirement is
considerably reduced as fly ash contains some percentage of unburnt carbon. These bricks
are known for better thermal insulation, Cost effectiveness and being environment friendly
EXPERIMENTS:
4.0 DURABILITY OF FLY ASH GEOPOLYMER CONCRETE :
Fly Ash Geopolymer Concrete has the capability to resist weathering action, chemical attack and
abrasion while maintaining its desired engineering properties. This results in a lifespan of trouble
free performance.
4.1 RESISTANCE TO SULPHATE AND CHLORIDE ATTACK:
Extensive research has revealed that after being exposed to sulphuric acid solution, fly ash based
geopolymer concrete remains structurally inert except development of some fine cracks on the
surface whereas OPC concrete shows sign of severe damage. Even the mass change of fly ash
based geopolymer concrete is pretty less compared to OPC concrete. Two graphs exported from
the work “Durability of fly ash based Geopolymer concrete against sulphuric acid attack” by X.J.
Song et al are shown below to illustrate the comparison between fly ash geopolymer concrete
and OPC in respect of mass change. When OPC is used in concrete exposed to sulphurc acid
solution surface of concrete gets damaged resulting in exposure of aggregates. But since fly ash
geopolymer concrete shows very little roughness test results have shown that compressive
strength of around 40 MPa has been found even after two months of exposure.

AR.SHRUTI.H.KAPUR RASA-2011
FIG: 4 Comparison between fly ash geopolymer and OPC w.r.t mass change in sulphate solution.
So it testifies strong bonding and thus improved resistance of fly ash geopolymer to sulphate
attack. Inspite of this good quality care should be taken to improve permeability and use of
aggregates. Use of latite aggregates must be avoided since gypsum formation and thus cracking
due to expansion occurs when it comes in contact with suphuric acid.
4.2 ALKALI- AGGREGATE REACTION:
Research by Stanton in early 1940s showed that failure of concrete due to alkali aggregate
reaction was due to expansion caused by chemical reaction between alkali in cement and silica
contained in aggregate. So if we replace cement to some extent by by mineral admixture like fly
ash alkali aggregate reaction can be controlled substantially. If we use class F fly ash ( low lime
content) in geopolymer concrete only 15-20% cement replacement will give enough
protection.While geopolymer ensures improved bonding by acting as a binder prohibiting
disastrous result by alkali aggregate reaction, fly ash acts as a spice to it by controlling alkali
silica reaction. Also it is recommended that water cement ratio should be low to control alkali
aggregate reaction since water helps alkali-silica gel to swell. Use of fly ash geopolymer
concrete utilizes low water-cement ratio maintaining desired workability and hence can make
concrete more impermeable and less vulnerable to such reaction.
4.3 EXPOSURE TO HIGH TEMPERATURE:
Fly ash based geopolymer concrete can sustain when exposed to exposed to considerably high
temperature. While OPC concrete degrades and degenerates at high temperature, it has been

AR.SHRUTI.H.KAPUR RASA-2011
found from different study that fly ash geopolymer concrete can maintain its desired compressive
strength even at 400 degree centigrade. Strength starts dropping once temperature crosses 400
centigrade and remains almost constant at higher temperatures. It is noteworthy here that
geopolymerization process continues even at high temperature and it is the strength of the binder
or bonding that prevents the concrete from disintegration. However lowest residual strength has
been marked at 600-700 degree Celsius. The below graph shows compressive strength of fly ash
geopolymer concrete ( plotted along Y-axis) at different temperatures (plotted along X-axis).
FIG 5: Compressive strength of flyash geopolymer concrete at different temperatures.
4.4 DRYING SHRINKAGE OF FLY ASH BASED GEOPOLYMERS:
Drying shrinkage is the reduction in volume that is primarily caused by loss of water during the
drying process. Factors that affect drying shrinkage of concrete also influences rate of drying
shrinkage and intensity. The aggregate plays an important role in affecting shrinkage of concrete.
Davidovits suggested that the smaller drying shrinkage strain of fly ash-based geopolymer
concrete may be explained by the block- polymerization concept. According to this concept, the
silicon and aluminum atoms in the fly ash are not entirely dissolved by the alkaline liquid. The
polymerization that takes place only on the surface of the atoms is sufficient to form the blocks
necessary to produce the geopolymer binder. Therefore, the insides of the atoms are not
destroyed and remain stable.In other words, the presence of the “micro- aggregates” increases
the restraining effect of the aggregates on drying shrinkage. Drying shrinkage is even less in fly

AR.SHRUTI.H.KAPUR RASA-2011
ash geopolymer concrete that are heat cured. In heat-cured fly ash- based geopolymer concrete,
most of the water released during the chemical reaction evaporates during the curing process.
Because the remaining water contained in the micro-pores of the hardened concrete is small, the
induced drying shrinkage is also very low.
4.5 RESISTANCE AGAINST FREEZING AND THAWING:
Non air entrained concrete with low water to binder ratio is durable against freezing and thawing.
So it implies that if water-binder ratio is low all water can mix up with binder and other
components resulting in low permeability of concrete. It can so hinder the saturation of paste
during freezing. If saturation of paste does not take place concrete won’t crack even in absence
of any air entraining admixture. Fly ash plays the important role by lowering water-cement ratio
and reducing concrete permeability. Experiments conducted by Frantisek Skvara et al has stated
that mass od fly ash geopolymer concrete slightly changes after 150 freezing and defrosting
cycles and also there was no deformation and cracking. However concrete with upto 50 % fly
ash and 10% silicafume with geopolymer binder has proved most efficient. This hints at the
disadvantage of having too much fly ash particles in the mixture. It means that excess or
unreacted fly ash will not bind well with geopolymer matrix and will weaken the bonding only.
CONCLUSION :
Use of Fly Ash is increasing day by day as there is an imperative need to produce more building
materials for various elements of construction. So the role of alternative and innovative options is
coming into sharp focus, considering the short supply, increasing cost and energy and environment
considerations for traditional and conventional materials. The possibility of using innovative building
materials and technologies, more so covering waste material like Flyash have been considered as a
felt need. Series of institutional support for land, finance, regulatory, media, marketing support,
testing support and awareness creation would be needed and some of the existing initiatives would
have to be substantially strengthened, more importantly, entrepreneurship for the production of
appropriate flyash based walling, roofing and flooring materials including Portland Pozzolana
Cement and other cements to ad1ieve better strength, energy saving, conservation of natural
resources besides cost efficiency, would have to be increasingly supported and developed.

AR.SHRUTI.H.KAPUR RASA-2011
REFERENCES:
• Census of India 2001.
• MOEF Notification dated September 14, 1999 regarding Utilisation of Fly Ash and
Amendments to this dated August 27, 2003.
• “Building Materials in India: 50 Years” A commemorative volume published by
BMTPC.
• Hwai-Chung Wu, Peijiang Sun. New building materials from fly ash-based lightweight
inorganic polymer, International Journal of Construction and Building Materials, No. 1,
21(2007) 211–7.
• Nevellie AM. Properties of Concrete, Pearson Education (Singapore) Pte Ltd., Indian
• Branch, 482 FIE Patparganj, Delhi 110 09.
• J, D. (1994). High Alkali cement for 21st century Concrete. Concrete Technology past
present and future ACI special publication SP 144. Farmington Hills,Michigan,