Geotechnical properties of lightly cemented fly ash

Abstract

In India, about 76% of electrical energy is generated using coal as fuel in thermal power plants.Presently in India,170 millions of tones of fly is being produced by the of thermal power plants, out of which a vast majority is fly ash having low lime content. Fly ash is a solid waste generated by thermal power plants where coal is used as fuel. As the need of power is increasing with a very fast rate for development purpose, the production of fly ash is increasing rapidly while generating electrical energy by thermal power plant. Disposal of this enormous amount of fly ash faces problem of huge land requirement, transportation, and ash pond construction and maintenance, which can be reduced by utilizing fly ash as a construction material for civil engineering structures. For increasing the use of fly ash as a construction material, it is required to enhance some properties by stabilizing raw fly ash with suitable stabilizer like lime or cement. Fly ash becomes an attractive construction material because of its self hardening characteristics for which available free lime is responsible. The variation of its properties depends upon the nature of coal, fineness of pulverization, type of furness and firing temperature. Fly ash is of two types; Class C and Class F. Class C fly ash contains high calcium content which is highly reactivity with water even in absence of lime. Class F ash contains lower percentage of lime. The main work carried out is to investigate the suitability of class F fly ash, containing CaO as low as 1.4%, modified with added lime as a construction material in different civil engineering fields. Large scale utilization of Fly ash in geotechnical constructions will reduce the problems faced by the thermal power plants for its disposal mostly because of its property closely related with the natural earth material. So assessment of the behavior fly ash at different condition is required before its use as a construction material in Civil engineering structure. For judging the suitability of any material for different geotechnical engineering works its consistency properties, compaction properties, strength parameters and settlement properties are the most important parameters to be evaluated. In this work an attempt was made to evaluate the above said geo-engineering properties of fly ash collected from NSPCL-RSP captive plant along with the fly ash treated with different proportion of lime. The overall testing program is conducted in two phase. In first phase the physical and chemical characteristics of the fly ash samples were studied by conducting Hydrometer analysis, UCS test, Permeability test and CBR test. In second phase of the test programme fly ash mixed with 1%, 2%,5% and 10% of lime. Lime added in percentage of dry weight of Fly ash. The geotechnical property of this lime stabilized fly ash sample were evaluated and compared with that of Fly ash. Based on the experimental findings the following conclusions are drawn:
 The fly ash consists of grains mostly of fine sand to silt size with uniform gradation of particles. The percentage of Fly ash passing through 75μ sieve was found to be 86.62%. Coefficient of uniformity (Cu) and coefficient of curvature (Cc) for Fly ash was found to be 5.88 & 1.55 respectively, indicating uniform gradation of samples. The specific gravity of particles is lower than that of the conventional earth materials.
 An increase in compaction energy results in closer packing of particles resulting in an increase in dry density where as the optimum moisture content decreases.
 Dry unit weight of compacted specimens is found to change from 1.142 to 1.255 kJ/m3 with change in compaction energy from 118.6kJ/m3 to 2483 kJ/m3, whereas the OMC is found to decrease from 30.2 to 24.2 %. This shows that fly ash sample responds very poorly to the compaction energy. With addition of lime maximum dry density decreases and optimum moisture content increases. Addition of lime results in filling the voids of the compacted fly ash thus increases the density.
 The failure stresses as well as initial stiffness of samples, compacted with greater compaction energies, are higher than the samples compacted with lower compaction energy. However the failure strains are found to be lower for samples compacted with higher energies. The failure strains vary from a value of 0.75 to 1.75%, indicating brittle failures in the specimen.
 A linear relationship is found to exist between the compaction energy and unconfined compressive strength.
 The UCS value is found to change from 32.764 to 47.271 kPa with change in compaction energy from 118.6kJ/m3 to 2483kJ/m3 indicating that the gain in strength is not so remarkable. It revealed from the test results that a linear relationship exists between the initial tangent modulus with unconfined compressive strength and deformation modulus.
 Increase in curing period of lime treated fly ash specimen show improvement in the UCS value. However the gain in strength with curing period is more in initial days of curing which tends to decreases with increase in curing period.
 With increase in compaction energy followed by curing period shows a significant increase in strength due to closer packing of particles. Besides, when lime is small in quantity, that’s about 1%, the strength improvement is practically negligible, even if cured for long. With increased lime content the pozzolanic reaction peaks up producing adequate amount of cementitious compounds leading to visible increase in strength. As the lime percentage increases this facilitates the pozzolanic reaction that form cementiceous gel that binds the particles. This process is catalyst by increase in curing period. Increased duration of curing, leading to prolonged pozzolanic reaction and result in increase in strength.
The unit cohesion and the angle of internal friction vary from 10.7 to 13.4kPa and 24.84 to 27.34 degree with the change in compaction energy from 118.6 kJ/m3 to 2483kJ/m3. Low value of angle of internal friction is due to lack of proper interlocking among particles as the fly ash mostly contains spherical particles with uniform gradation. There is negligible increase in cohesion component with compaction energy.
 The highest unsoaked and soaked CBR value are found to be 25.39% and 1.546% at compaction energy of 2483 kJ/m3.This indicates that CBR value of compacted ash is very susceptible to degree of saturation.
 The unsoaked CBR value is more than soaked CBR value. Even after 28 days of curing of samples with lime content of 10% the soaked CBR value do not show significant improvement over unsoaked CBR. This indicates that, relatively large amount of the lime is needed to bind all the fly ash particles together, leading to visible increase in strength.
 Permeability decreases with increase in either compactive energy or lime content. Permeability is basically a function of grain size and compactive effort. With increase in lime content, pozzolanic reaction occurs which result in blocking of the flow paths thus reducing the value of coefficient of permeability of the lime treated fly ash specimens.