Fly Ash by harish1991

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									                                 A STUDY ON



J.P.R.V.VEMKATESH                             K.N.A.G.K.MANIKANTA
III/IV B.TECH (CIVIL)                         III/IV B.TECH (CIVIL)        



       One of the challenges of civil engineering community in near future will be to realize projects in
harmony with the concept of suitable development of innovative construction materials that suit the end
requirements of the structures. This involves the use of high performance materials produced at
reasonable cost with lowest possible environmental impact. The most abundantly available
supplementary cementing material is flyash, a by-product of thermal power stations. It has a
tremendous potential to be utilized and is produced in huge quantities due to rapid industrialization
.The crux of the problem is in its disposal as it requires vast areas of valuable land. Further its
hazardous effects to the environment and mankind. This paper presents some of the issues that have
long been problematic to the flyash producing industries as its utilization is far less than production.
The suitability and superiority of flyash in various applications is also briefly discussed. Hence the
knowledge and awareness of evolution of use of flyash will go a long way in building up better
constructions in the future.

             Flyash is a finely divided residue resulting from the combustion of pulverised
coal and transported by the flue gases of boilers fired by pulverised coal as defined by IS:
3812-1981. Realisation dawned on the tremendous environmental problems being caused
by large scale dumping of flyash on scarce land resource. keeping in view the health
hazard, India launched the ‘Flyash Mission’, a Technology Project in Mission Mode in 1994
towards promoting safe disposal and utilization of flyash in the country. To cater this
problem, flyash is used in different civil engineering constructions such as building materials,
construction of ash dykes, embankments, laying of cement concrete roads etc.

             With the trend of development of various Building materials and application of
flyash in the construction industries, it is expected that 50% of the total production of flyash
could be properly utilized, provided the major constraints are removed as regards high value
added suitable technology is yet to be developed and needs the research. Rough estimation
of existing utilisation of flyash is around 10 % (1998) of the total generated quantity as
against 3 - 5 % (1994). In absolute terms it has increased about three folds. However,
countries like France, Belgium, Germany, and Netherlands have utilisation levels of 90 % or
higher. This paper highlights the methods to develop proper utilisation of the flyash for the
construction industries.

Physical Properties of Flyash:

       Flyash particles can be spherical and rounded, sub-rounded, irregular and angular. Fineness is
probably a single important physical characteristic which influences the activity of flyash more than
any other physical factor. The surface area is found to range between 3627 and 6091 cm2/gm showing
India’s flyashes to be quite fine. Carbon content in flyash influences the colour, fineness and
temperature reactivity of flyash.
Chemical Composition:
       Indian flyash is rich in SiO2, Al2O3, Fe2O3 and unburnt carbon. A typical chemical
composition of Indian flyash is given in Table 1.

Table 1: Chemical Composition of Indian Flyash:

                       Constituent                           Percentage
                           SiO2                                 35 – 62
                          Al2O3                                 19 – 28
                          Fe2O3                                  4 – 20
                           CaO                                  0.6 – 3.0
                           MgO                                  0.2 – 3.9
                           SO3                               Trace – 2.5
                           LOI                               0.8 – 15.81


Flyash Bricks:

       With the fast urban development, the demand for bricks has been increasing, allowing
the brick industry to exploit the top soil which is a social crime. In order to meet this demand, the
flyash based bricks can be an alternative material to the conventional bricks with the improved
engineering properties. In India about 25% of usage of flyash bricks envisaged to consume around 30
- 40 million tones of flyash every year.

The various suggested proportions of flyash with sand, clay and lime/cement are given as:
Lime based bricks:70 % flyash, 5 - 10 % lime, 20 - 25 % sand.
Cement based bricks: 70 % flyash, 10% cement, and 20% sand.
Clay bricks: 30 % flyash, 50 % clay, 20 % sand.
Typical properties of flyash (flyash + sand + lime + gypsum) bricks and burnt clay bricks are given in

Table 2 Properties of fly ash

                     Property                   Burnt clay brick            Flyash brick
     % Water absorption                                21                        15
     Density (gm/cc)                                  1.44                      1.80
     Compressive strength (kg/cm2)                   19 -42                     58-78

Advantages of flyash based bricks over the conventional bricks.
The main advantages of flyash based bricks over the conventional bricks are follows.

   Uniform and standard product size resulting in 10 % less consumption of bricks per unit
   Cement consumption is less in cement mortar.
   Compressive strength is more than conventional red bricks (>100 kg/cm2).
   Less load on foundation due to light weight.
   Due to less water absorption and no weathering effects, surfaces can be left exposed without
    plastering and direct application of paint is also possible.

Construction of Ash Dykes:

The constructional methods for an ash dyke can be grouped into three broad categories namely
Upstream method, Downstream method and Centerline method. Fig.1 shows the typical
configurations of embankments constructed using the different methods. The construction procedure
of an ash dyke includes surface treatment of lagoon ash, spreading and compaction and benching.

                                           Up Stream Method
                                         Downstream Method

                                         Center line Method

                            Fig. 1. Typical Ash Dyke Raising Configurations

       An important aspect of design of ash dykes is the internal drainage system. The seepage
discharge from internal surfaces must be controlled with filters that permit water to escape freely and
also to hold particles in place and the piezometric surface on the downstream of the dyke. The
internal drainage system consists of construction of rock toe, 0.5 m thick sand blanket and sand
chimney. After completion of the final section including earth cover the turfing is developed from sod
on the downstream slope.

Flyash as Fill Material :

       Large scale use of flyash as a fill material can be applied to replace another material and is
therefore in direct competition with that material, flyash itself is used by the power generating
company producing the flyash to improve the economics of the overall disposal of surplus flyash and
at some additional cost, flyash disposal is combined with the rehabilitation and reclamation of land
areas desecrated by other operations.
       Fills can be constructed as structural fills where the flyash is placed in thin lifts and
compacted. Structural flyash fills are relatively incompressible and are suitable for the support of
buildings and other structures. Non-structural flyash fill can be used for the development of parks,
parking lots, playgrounds and other similar lightly loaded facilities. One of the most significant
characteristics of flyash in its use as a fill material is its strength. Well compacted flyash has strength
comparable to or greater than soils normally used in earth fill operations. In addition, lignite flyash
possesses self-hardening properties, which can result in the development of shear strengths. The
addition of illite or cement can induce hardening in bituminous flyash, which may not self-harden
alone. Significant increases in shear strength can be realized in relatively short periods of time and it
can be very useful in the design of embankments.

Sintered light weight aggregates:

       The sintered light weight aggregates are one of the important constituents of cement. It
produced by pyro processing, mining fly ash with water, sintering at 1000 - 13000 C. The product is
finely graded into different size fractions for use as part of secondary concrete products such as pre-
cast lightweight concrete panels and lightweight blocks.

Portland Pozzolana cement:

       Pozzolana is the clay matter either natural or synthetic, which when ground with lime or
clinker and mixed with water, produce cementitious compounds. Portland pozzolana cement should
not contain more than 25 % of flyash. It has much lower heat of hydration and is also fairly sulphate
resistant. It has all the physical properties of ordinary portland cement (OPC) but has lower shrinkage
and can be used for all construction works for which OPC is used.

Flyash as Partial Replacement of Cement:

       The flyash, a pozzolana, that is, a substance although not cementitious itself, has different
constituents. This combines with the lime to form a material having cementing properties. Use of
flyash in concrete started in the United States in the early 1930’s. An addition of flyash gives
economic and ecological benefits in concrete improves its workability which inturn reduces
segregation, bleeding, heat evolutions and permeability, inhabits alkali aggregate reaction, and
enhances sulphate resistance. Flyash concretes show considerable lower rate of development of
compressive strength at early ages. At 28 days and at later ages with 15 and 20 % replacement of
cement by flyash exhibits comparable cube compressive strength, split-tensile strength and flexural

High Volume Flyash Concrete:
       The challenge for the civil engineering community in the near future will be to realize projects
in harmony with the concept of sustainable development, and this involves the use of high
performance materials produced at reasonable cost with the lowest possible environmental impact.
The most available supplementary cementing material worldwide is flyash, a by-product of thermal
power stations. For considerable increase in the utilization of flyash that is otherwise being wasted
and to have a significant impact on the production of cement, it is necessary to advocate the use of
concrete that will incorporate large amounts of flyash as a replacement for cement. In 1985,
CANMAT developed a concrete incorporating large volumes of flyash that has all the attributes of
high performance concrete, that is, one that has excellent mechanical properties, low permeability,
superior durability, and that is environmentally friendly.

Cellular Light Weight Concrete:

       Cellular light weight concrete can be manufactured by a process involving the mixing of
flyash, cement, coarse sand, fine sand and a foaming agent in a mixer to form thin slurry. This slurry
is then poured in moulds and allowed to set. The blocks are then removed from the moulds and are
cured by spraying water on the stacks. The bulk density of this product varies from 0.4 - 1.8 gm/cc.
These blocks are specially used in high rise construction to reduce the dead weight of the structure.

Flyash in Road Projects:

       Researchers have already been proved about flyash’s suitability in the road construction. A
major portion of poor quality of flyash can be used in construction of the embankment of the road.
Soil lime flyash stabilization can be done by different technique. Flyash when added to lime gives
cementitive properties and can be used to stabilize weak soils. The thickness of lime flyash soil layer
for use as sub-base or base course is designed in accordance with IRC: 37 -1984 with a minimum
thickness of 150 mm. For determination of the thickness of the stabilized layer, the California
Bearing Ratio (CBR) method has to be adopted. The percentage of addition of flyash and lime in the
soil can be determined by the test of the specimens. For the construction of the concrete block
pavement, a portion of cement can be replaced by flyash. The concrete mix incorporating upto 30 %
of flyash in place of ordinary portland cement (OPC) have been utilised and observed the satisfactory
result. However, resistance to surface abrasion is lower than the OPC concrete which can be
improved by addition of some admixtures. In this way, a huge volume of flyash can be used in the
construction of highway systems with economy and the disposal as well as ecological imbalance can
be minimised upto certain level. The use of flyash in road projects is shown in Table 3.

       The construction of first dam in India using Roller Compacted Concrete (RCC) technology
with high doses of flyash has been started near Nasik under Ghatghar Pumped Storage Scheme of
Irrigation Department, Government of Maharashtra. Approximately 60 –70 % cement is being
replaced by flyash. Three dams would be constructed under this project using RCC technology with
high doses of fly ash, which includes a major dam of height about 90 m.

                               Table 3 Use of Flyash in Road Projects

   Sl. No. Type of use                Description                               Performance
                                      Taxi track, Pune Air field                Satisfactory
            Cement flyash concrete
      1                               Taxi track, Palam Air port 1970-71        Satisfactory
            (wearing course)
                                      Near Govt. College Faridabad, Haryana Very satisfactory
                                      State transport workshop, Haryana,        Satisfactory
            Lime flyash WBM           1972                                      Satisfactory
            (base- course)            Talcher Gopal Prasad Kaniha road,
                                      Orissa, 1976 -77
            Lime flyash concrete      DTC bus stop on NH-2 1977-78              --
      3     precast blocks for foot   Approach road to NH-2, near CRRI,         Satisfactory
            paths (wearing course)    1977-78
Environmental Impacts:

      The successful usage of flyash in concrete mix well helps in solving the wide problem of flyash
disposal by thermal power plants. The other positive environmental impacts of using flyash are given

     Indian coal has high flyash content resulting in high suspended particulate matter and flyash
      disposal problems restricting its use. Large scale utilization of flyash can help in proper use of
      Indian coal.
     The reduction in demand for OPC production will help in conserving lime stone for future use,
      otherwise import of limestone /OPC to India will be required in future.
     Flyash contains very small quantities of heavy metals like arsenic, chromium, selinium, titanium
      & vanadium that can leach to ground water when disposed on land. When used in concrete mix
      leaching to ground water can be prevented due to hydration reaction as the heavy metals form
      immovable complexes in the mix.
     Due to the usage of flyash in concrete , the production of OPC is minimized .Hence equivalent
      quantum of co2 release to environment will also be avoided.

       The future poses challenges to the scientists, technologists, engineers towards sound
management of flyash disposal and deposition technologies. The problem is not due to lack to
technical competence but more of adoption, implementation and better management of improved and
appropriate technologies.

The guideline for all thermal power stations as regards disposal techniques/ strategies should ensure
minimum adverse impact on the flora and fauna of a particular place. The attempt should be to
consciously reduce environmental damage. To ensure more effective management of flyash in India,
the country needs-
(i) Guidelines, facilitation and implementation mechanism.
(ii) Community participation/ raising consciousness levels of population
(iii) Fiscal & Policy support for fly ash users/product manufacturers.
(iv) Greater financial allocation for safe disposal & utilization.
(v) Enhanced facilitation of indigenous R&D expertise for field applications.
Flyash industry to be declared as "priority Industry" for all applicable concessions / support.

                This is referred by Head of the department and other Assistant professors of civil
engineering department of D.M.S.S.V.H.College Of Engineering MTM.
    1. Metha, P.K.(1985) Influence of Flyash Characteristics on the Strength of Portland Flyash
         Mixtures, Cement and Concrete Research, Vol.15, 1985, pp.669-674.
    2. Prabhu, C., Perumal, P., and Jeyaprakash, S.K. (2004) Effect of using Flyash in Chemical
         Admixtures in Plain and Reinforced Concrete Members, Proceedings of the National
         Conference on Recent Advances in Civil Engineering, PP.395 - 402.
    3. Behera, J.P., Sarangi, B., Nayak, B.D., and Ray, H.S.(2000) Investigations on the
         Development of Blended Cement using Activated Flyash, The Indian Concrete Journal,
         Vol.74, pp.260 - 263.
    4.   BIS: 3812 - 1981, Influence of Flyash for use as Pozzolana and Admixture, BIS Standards,
         New Delhi.

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