CHAPTER ON FLY ASH FOR GRADUATE (CIVIL ENGINEERING) COURSE STUDENTS Subhash Chandra Sr Engineer AUD NTPC Limited Noida Office
Fly Ash – A Resource Material
1.Fly ash Introduction
1.1.General 1.2.Production and Classification 1.3.History of Fly Ash Utilization 1.4.Properties of Fly Ash 1.5.Geo-technical Properties 1.6.Chemical Properties 1.7.Quality of Fly Ash as per BIS, ASTM
2.Main avenues for use of fly ash
2.1Cement concrete and mortar 2.2Fill Material – Road and embankments construction 2.3Reclaim wasteland 2.4Mine filling 2.5Bricks, blocks and tiles Manufacturing 2.6Agricultural soil 2.7Ash Dykes 2.8Autoclaved Aerated Concrete 2.9Lightweight aggregates 2.10Cenosphere
3.Legal Issues – Ash Utilization
4.Major Successful examples
1Fly Ash Introduction 1.1General
The Fly ash word sounds its own definition i.e. material which flies along with gases and ash – a residue of burnt material. This name is used for residue of coal which is used as pulverized fuel for generation of electricity in thermal power station. In present life style electricity has become one of the most important part of day to day of human life. For generation of electricity coal is continued to be used as a major source of fuel in many countries in the World including India. In the process of electricity generation, large quantity of fly ash gets produced and becomes available as a by-product of coal-based power stations.
Fig. 1: Fly Ash mound Fig. 2: Fly Ash-microscopic view
1.2Production and Classification
In thermal power station, pulverized coal is fed into boiler furnace, after burning; the lighter and finer particles fly along with hot flue gases which are passed through Electrostatic Precipitators (ESP). In ESP these particles are arrested and then taken out either pneumatically to a storage silos in dry form or sluiced through with water and sent to ash ponds. The particles collected in ESP are called Fly Ash. It is then collected by either using electrostatic precipitators, baghouses or a combination of both.
There are four categories of Ash normally available from a coal based thermal power station. a.Fly Ash: This kind of ash is collected from different rows of Electro-Static Precipitators (ESP) in dry form. This is characterized by comparatively lower carbon content and higher fineness. Fineness of fly ash is more in subsequent field of ESP as compared to initial fields. This ash comprises about 80% of total ash produced in a station. b. Bottom Ash: This kind of ash is collected at the bottom of Boiler furnace as a result of coal burning activity. This is characterized by comparatively higher carbon content and coarse size. This comprises about 20% of total ash produced in a station. c.Pond Ash: The slurry formed after mixing ash with water is pumped to the nearby ash ponds wherein water gets drained away. The ash thus stored in ash ponds is called Pond Ash. d.Mound Ash: Fly ash conveyed in dry form and deposited dry in mounds.
1.3History of Fly Ash Utilization
Utilization of fly ash is not a new phenomenon rather it is about seven decades old. The fly ash became available in coal based thermal power station in the year 1930 in USA. In order to find out its properties, scientist started research activities and in the year 1937, R.E. Davis and his associates at university of California published research details on use of fly ash in cement concrete. This research had laid foundation for its specification, testing & usages. The research had established that fly ash possesses specific property called as pozzolanic property which was similar to volcanic ash and deserves to utilize in lime/ cement concrete works.
The pozzolanic property of a substance is defined as -A Siliceous or Siliceous and Aluminous material, which in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
The pozzolana term came from Roman city. About 2,000 years ago, Roman used volcanic ash along with lime and sand to produce mortars, which possesses superior strength characteristics & resistances to corrosive water. The best variety of this volcanic ash was obtained from the locality of pozzoli and thus the volcanic ash had acquired the name of Pozzolana. The investigations had also established that the geo-technical properties of fly ash are similar or better than many of the soils and can be utilized for structural fill & embankment construction substituting soil.
In India, to establish more uses of fly ash produced from Indian coal, research activities were continued at various research institutes of Council of Scientific and Industrial Research (CSIR) like Central Building Research Institute, Roorkee, Central Institute for Mining Fuel Research Institute, Dhanbad, Central Road Research Institute, New Delhi etc and other Institutes like National Council for Cement & Building Materials, Ballabhgarh, IIT Kanpur, IIT Chennai, NTPC (R&D)etc. Studies have revealed that fly ash can be used in manufacture clay ash bricks, fly ash lime/ cement bricks and other building products, in agriculture, road embankments construction and fill applications.
1.4Properties of Fly Ash
Fly ash is complex material having wide range of chemical, physical and mineralogical compositions. The chemical composition of fly ash depends on the type of coal burnt in boiler furnace, oil firing, oxygen flow temperature of furnace, degree of pulverization of coal, efficiency of ESP etc. Physical properties of fly ash such as fineness or particle size, reactivity with lime / cement etc depends on design and efficiency of boiler, combustion process, coal grinding techniques employed, process of ash collection and handling method and efficiency of Electrostatic Precipitator etc.
The majority of fly ash particles are glassy, solid or hollow, slightly to highly porous and spherical in shape. The specific gravity falls within the range of 2.1 to 2.6 and dry density is commonly found to be in the range of 950-1200 kg/m3 depending on the degree of compaction. The particle size of fly ash generally ranges from 1 micron to 100 micron in diameter of glassy spheres and 7 to 300 microns in diameter for angular carbon particles. In terms of soil grain size analysis, most fly ash particles fall within the silt range, with small percentages in the fine sand and clay sizes.
Table 1: Geo-technical properties of Fly Ash
Specific gravity g/cc1.90-2.55 PlasticityNon Plastic Proctor compaction – Maximum dry density (g/cc)0.90-1.60 Optimum moisture content (%)38.0-18.0 Angle of internal friction ( o)300-400 Cohesion (kg/cm2 )Negligible Compression index0.05-0.4 Permeability (cm/sec)105-103 Particle size distribution Clay size fraction (%) Silt size fraction (%) Sand size fraction (%) Gravel size fraction (%) 1-10 8-85 7-90 0-10 Coefficient of uniformity3.1-10.7
The major constituents of most of the fly ashes are Silica (SiO2), alumina ((Al2O3), ferric oxide (Fe2O3) and calcium oxide (CaO). The other minor constituent of the fly ash are MgO, Na2O, K2O, SO3, MnO, TiO2 and unburnt carbon. There is wide range of variation in the principal constituents – Silica (25-60%), Alumina (10-30%) and ferric oxide (5-25%). When the sum of these three principal constituents is 70% or more and reactive calcium oxide is less than 10% -technically the fly ash is considered as Siliceous fly ash and also called as Class F fly ash. Such type of fly ash is produced by burning of anthracite or bituminous coal. Siliceous fly ash characteristically contains a large part of silicate glass of high silica content and crystalline phases of low reactivity mullite, magnetite and quartz. The active constituents of this type of fly ash is siliceous or alumino-silicate glass. If the sum of these three constituents is equal or less than 70% and reactive calcium oxide is not less than 10%, fly ash will be considered as Calcareous fly ash and also called as Class C fly ash. This type of fly ash is commonly produced by burning of lignite or sub-bituminous coal and possesses self setting property similar to cement.
Table 2: Chemical composition
ComponentBituminous Sub bituminous Lignite
SiO2 (%)20-6040-6015-45 Al2O3 (%)5-3520-3020-25 Fe2O3 (%)10-404-104-15 CaO (%) 1-125-3015-40 LOI (%) 0-150-30-5
1.7Quality of Fly Ash as per BIS, ASTM
To utilize fly ash as a Pozzolana in cement concrete and cement mortar, Bureau of Indian Standards (BIS) has formulated IS: 3812 Part-1 2003. In this code quality requirement for siliceous fly ash and calcareous fly ash with respect its chemical and physical composition has been specified. These requirements are given as below.
Table 3: Chemical Requirements of fly ash as a Pozzolana in cement concrete and cement mortar as per Bureau of Indian Standards (BIS)
S. No.CharacteristicsRequirements Siliceous fly ashCalcareous fly ash 1Silicon dioxide (SiO2) + Aluminium oxide (Al2O3) + Iron oxide (Fe2O3), in percent my mass, minimum7050 2Silicon dioxide in percent my mass, minimum3525 3Reactive silica in percent my mass, minimum (Optional test)2020 4Magnesium Oxide (MgO), in percent my mass, max.5.05.0 5Total sulphur as sulphur trioxide (SO3), in percent my mass, max.3.03.0 6Available alkalis as sodium oxide (Na2O), in percent my mass, max.1.51.5 7Total chlorides in percent my mass, max0.050.05 8Loss on Ignition, in percent my mass, max.5.05.0
Table 4: Physical Requirements of fly ash as a Pozzolana in cement concrete and cement mortar as per Bureau of Indian Standards (BIS)
S. No.CharacteristicsRequirements Siliceous fly ash and Calcareous fly ash 1Fineness – Specific surface in m2/kg by Blaine’s permeability method, Min.320
2Particles retained on 45 micron IS sieve (wet sieving) in percent, Max. (Optional Test)34 3Lime reactivity – Average compressive strength in N/mm2, Min4.5
4Compressive strength at 28 days in N/mm2, Min.Not less than 80 percent of the strength of corresponding plain cement mortar cubes 5Soundness by autoclave test – Expansion of specimen in percent, Max.0.8
ASTM International for fly ash: ASTM International C-618-03 specifies the chemical composition and physical requirements for fly ash to be used as mineral admixture in concrete. The standards requirements are given as below.
Table 5: Chemical Requirements of fly ash as a Pozzolana in cement concrete and cement mortar as per ASTM International
S. No.CharacteristicsRequirements Class F (Siliceous fly ash)Class C (Calcareous fly ash) 1Silicon dioxide (SiO2) + Aluminium oxide (Al2O3) + Iron oxide (Fe2O3), in percent my mass, minimum7050 2Sulfur trioxide (SO3), max. Percent5.05.0 3Moisture content, max., percent3.03.0 4Loss on ignition, max., percent6.06.0
Table 6: Physical Requirements of fly ash as a Pozzolana in cement concrete and cement mortar as per ASTM International
S. No.CharacteristicsRequirements for class F & class C fly ash 1Fineness – amount retained when wet-sieved on 45 micron (No. 325) sieve, Max., percent.34
2Strength activity index oWith portland cement, at 7 days, min., percent of control oWith portland cement, at 28 days, min., percent of control75c
75c 3Water requirement, max., percent of control105
4Soundness Autoclave expansion or contraction, max., percent0.8 5Uniformity requirements: The density and fineness of individual samples shall not vary from the average established by ten preceding tests, or by all preceding tests if the number is less than ten, by more than Density (max.) variation from average percent. Percent retained on 45 micron (no. 325), max variation, percentage points from average
2Main avenues for use of fly ash
2.1Cement concrete and mortar 2.2Fill Material – Road and embankments construction 2.3Reclaim wasteland 2.4Mine filling 2.5Bricks, blocks and tiles Manufacturing 2.6Agricultural soil 2.7Ash Dykes 2.8Autoclaved Aerated Concrete (AAC) 2.9Lightweight aggregates 2.10Cenosphere
2.1Using fly ash in cement concrete and mortar
Portland Pozzolana Cement fly ash based is manufactured either by intimately intergrinding Portland cement clinker and fly ash with addition of gypsum or calcium sulphate or by intimately and uniformly blending with OPC and fine fly ash. The pozzolanic property of fly ash makes it suitable as raw material for manufacture of PPC. IS:1489 Part-1-1991(Reaffirmed 2005) is BIS specification for this cement. As per the BIS specification, fly ash constituent shall not be less than 15 percent and not more than 35 percent by mass of Portland Pozzolana Cement.
Portland Pozzolana cement produces less heat of hydration and offer greater resistance to the attack of aggressive waters than Ordinary Portland Cement. It is particularly useful in marine and hydraulic construction and other mass construction structures. This cement is equivalent to Ordinary Portland cement on the basis of the 28 days and above compressive strength. Use of fly ash in cement manufacturing helps to conserve raw material such lime stone, clay etc and also conserve coal required for clinkerization for manufacture of cement clinker. During this process generation of CO2 is also reduced, therefore, eco-friendly.
It may be noted that, Bureau of Indian Standard also permits use of Fly ash up to 5% as a Performance Improver in the manufacturing of 33 grade, 43grade and 53grade of Ordinary Portland Cement.
2.1.1How fly ash woks with cement?
Fly ash being a pozzolanic material is found to be useful for part replacement of Ordinary Cement in cement concrete and Mortar. In the concrete, cement chemically reacts with water and produces strength bearing gels called as C-H-S gels that binds other component together and provide strength to concrete and creates strength bearing materials. The reaction process is called ‘hydration’ of cement. In this process some amount of lime [Ca (OH)2] is also liberated. Fly ash available in concrete reacts with this lime and forms additional similar strength bearing gel which provides strength to concrete. As we know Ordinary Portland Cement (OPC) is a product of four principal mineralogical phases. These phases are Tricalcium Silicate – C3S(3CaO.SiO2), Dicalcium Silicate – C2S(2CaO.SiO2), Tricalcium Aluminate – C3A(3CaO.Al2O3) and Tetracalcium alumino-ferrite – C4AF(4CaO.Al2O3Fe2O3). The setting and hardening of the OPC takes place as a result of reaction between these principal compound and water [H2O(H)] as shown under:
2C3S + 6H = C3S2H3 + 3CH (tricalcium silicate) water (C-S-H gel)(Calcium hydroxide) 2C2S +4H = C3S2H3 + CH (dicalcium silicate) (water) (C-S-H gel)(Calcium hydroxide)
The hydration products from C3S and C2S are similar but quantity of calcium hydroxide (lime) released is higher in C3S as compared to C2S. The reaction of C3A with water takes place in presence of sulphate ions supplied by dissolution of gypsum present in OPC. This reaction is very fast and shown under:
C3A + 3(CSH2) + 26H =C3A(CS)3H22 (tricalcium Aluminate (gypsum) (water) (ettringite)
C3A + CSH2 + 10H =C3ACSH2
(monosulphoaluminate hydrate) (S – SiO2, C – CaO, A – Al2O3, F – Fe2O3)
Tetracalcium alumino-ferrite forms hydration product similar to those of similar of C3A, with iron substituting partially for alumina in the crystal structures of ettringite and mono sulpho-aluminate hydrate.
Above reaction indicate that during the hydration process of cement, lime is released out and remains as surplus in the hydrated cement. This leached out surplus lime renders deleterious effects to concrete such as make the concrete porous, give chance to the development of micro-cracks, weakening the bond with aggregates and thus affects the durability of concrete.
If fly ash is available in the mix, this surplus lime becomes the source for pozzolanic reaction with fly ash and forms additional C-S-H gel having similar binding properties in the concrete as those product by hydration of cement paste. The reaction of fly ash with surplus lime continues as long as lime is present in the pores of liquid cement. The process is illustrated as below.
2.1.2Advantage of use of Fly ash with concrete
Fly ash concrete3 was as early as used in the U.S. for the Hoover Dam, where engineers found that it allowed for less total cement. It is now used across the world. Consisting mostly of silica, alumina and iron, fly ash is a pozzolana substance containing aluminous and silicious material that forms cement in the presence of water. When mixed with lime and water it forms a compound similar to portland cement. The spherical shape of the particles reduces internal friction thereby increasing the concrete’s consistency and workability, permitting longer pumping distances. Improved workability means less water is needed, resulting in less segregation of the mixture.
(i)Reduced Heat of Hydration
In concrete mix, when water and cement come in contact, a chemical reaction initiates that produces binding material and consolidates the concrete mass. The process is exothermic and heat is released which increases the temperature of the mass. The large temperature rise of concrete mass exerts temperature stresses and can lead to formation of micro cracks. When fly ash is used as part of cementitious material, quantum of heat liberated is low and staggers through pozzolanic reactions and thus reduces micro-cracking and improves soundness of concrete mass. (ii)Improved Workability of Concrete Fly ash particles are generally spherical in shape; this gives a ball bearing effect to the mixing of aggregates & cement. This way part mixing is achieved and leads for reduced water requirements for a given slump. The spherical shape helps to reduce friction between aggregates and between concrete and pump line and thus increases workability and improve pumpability of concrete. Fly ash use in concrete increases volume of fines and decreases water content and thus reduces bleeding of concrete. (iii)Reduced Permeability & increased corrosion protection Water is essential constituent of concrete preparation. When concrete is hardened, part of the entrapped water in the concrete mass is consumed by cement mineralogy for hydration. Some part of entrapped water evaporates, thus leaving porous channel to the extent of volume occupied by the water. Some part of this porous volume is filled by the hydrated products of the cement paste. The remaining part of the voids consists capillary voids and give way for ingress of water. Similarly, the liberated lime by hydration of cement is water-soluble and is leached out from hardened concrete mass, leaving capillary voids for the ingress of water. Higher the water cement ratio, higher will be the porosity and thus higher will be the permeability. The permeability makes the ingress of moisture and air easy and is the cause for corrosion of reinforcement. Higher permeability facilitate ingress of chloride ions into concrete and is the main cause for initiation of chloride induced corrosion. Additional cementitious material results from reaction between liberated surplus lime and fly ash, blocks these capillary voids and also reduces the risk of leaching of surplus free lime and thereby reduces permeability of concrete. (iv)Improved Resistance for Carbonation Phenomena Carbonation phenomenon in concrete occurs when lime of the hydrated Portland Cement react with carbondioxide from atmospheres in the presence of moisture and form calcium carbonate. To a small extent, calcium carbonate is also formed when calcium silicate and aluminates of the hydrated Portland cement react with carbon dioxide from atmosphere. Carbonation process in concrete results in two deleterious effects (i) shrinkage (ii) Corrosion. Concrete immediately adjacent to steel reinforcement may reduce its resistance to corrosion. The rate of carbonation depends on permeability of concrete, quantity of surplus lime and environmental conditions such as moisture and temperature. When fly ash is available in concrete; it consumes surplus lime by way of pozzolanic reaction, reduces permeability and as a result improves resistance of concrete against carbonation phenomenon.
(v)Increased Sulphate Resistance
Sulphate attacks in concrete occur due to reaction between sulphate from external origins or from atmosphere with surplus lime leads to formation of etrringite, which causes expansion and results in volume destabilization of the concrete. Increase in sulphate resistance of fly ash concrete is due to continuous reaction between fly ash and leached out lime, which continue to form additional C-S-H gel. This C-S-H gel fills in capillary pores in the cement paste, reducing permeability and ingress of sulphate ions.
(vi)Reduced alkali- aggregate reaction Certain types of aggregates react with available alkalis and cause expansion and damage to concrete. These aggregates are termed as reactive aggregates. It has been established that use of adequate quantity of fly ash in concrete reduces the amount of alkali aggregate reaction and reduces/ eliminates harmful expansion of concrete. The reaction between the siliceous glass in fly ash and the alkali hydroxide of Portland cement paste consumes alkalis thereby reduces their availability for expansive reaction with reactive silica aggregates. (vii)Greater long – term strength
The pozzolanic reaction between fly ash and lime liberated from hydration cement continue for longer period & generates additional cementitious gel which provide greater strength to concrete mass.
In a nutshell, it can be summarized that permeability and surplus lime liberated during the hydration of Portland cement are the root causes for deleterious effect on the concrete. Impermeability is the foremost defensive mechanism for making concrete more durable and is best achieved by using fly ash as above. Use of fly ash in concrete saves the cement requirement for the same strength thus saving of raw materials such as limestone, coal etc required for manufacture of cement and thus activity is eco-friendly.
The Indian Standards IS: 456-2000-entitled Plain and Reinforced cement concrete code of practice permits use of fly ash (conforming to IS:3812 Part-1) up to 35% as part replacement of OPC in the concrete. 2.2Use of Fly ash as a fill material i.e. for road embankment construction and similar projects One of the most significant characteristics of fly ash in its use as a fill material is its strength. Well-compacted fly ash has strength comparable to or greater than soils normally used in earth fill operations. In addition, fly ash possesses self-hardening properties which can result in the development of shear strengths. Significant increases in shear strength can be realized in relatively short periods of time and it can be very useful in the design of embankments6.
Construction of road embankments using fly ash, involves encapsulation of fly ash in earthen core or with RCC facing panels. Since there is no seepage of rain water into the fly ash core, leaching of heavy metals is also prevented. When fly ash is used in concrete, it chemically reacts with cement and reduces any leaching effect. Even when it is used in stabilization work, a similar chemical reaction takes place which binds fly ash particles. Moisture content of pond ash to be nearer to OMC after having been taken out from pond and stored for one or two days.
Use of fly ash in the construction of road embankment of the second Nizamuddin Bridge in New Delhi, has amply demonstrated its suitability as a material for road/embankment construction. The site proximity to the river Yamuna posed a challenge to use the conventional material; moreover, the embankment of 7 to 8 metre height was to be constructed for a stretch of about two kilometers in a flood zone.
The construction work has been completed in a record time. The on-site quality control was maintained with simple tests and procedures. These were similar to the corresponding requirements in case of earthworks. No operational or execution problems have been faced on technical aspects, rather the working with fly ash has been found to be easier and better. Further, use of fly ash results in an enormous amount of cost saving.
Advantages of using fly ash for road construction
a)Fly ash is a lightweight material, as compared to commonly used fill material (local soils), therefore, causes lesser settlements. b)Easy to handle and compact because the material is light and there are no large lumps to be broken down. Can be compacted using either vibratory or static rollers. c)High permeability ensures free and efficient drainage. After rainfall, water gets drained out freely ensuring better workability than soil. d)Work on fly ash fills/ embankments can be restarted within a few hours after rainfall, while in case of soil it requires much longer period. e)Conserves good earth, which is precious topsoil, thereby protecting the environment. f)Higher value of California Bearing Ratio as compared to soil provides for a more efficient design of road pavement. g)Pozzolanic hardening property imparts additional strength to the road pavements/ embankments and decreases the post construction horizontal pressure on retaining walls. h)Amenable to stabilization with lime and cement. i)Can replace a part of cement and sand in concrete pavements thus making them more economical than roads constructed using conventional materials.
2.3Use of Ash in waste land Reclamation
It is a very difficult task to convert a wasteland to greenery. Tata Energy and Resources Institute (TERI) had identified certain strains of naturally occurring mycorrhizal fungi that provide nutritional support and high level of stress tolerance to the plants. These were applied to plants on fly ash dumps in Korba Super Thermal Power Station with additional doses of organic and Mycorrhizal fertilizers. The fungi form a reciprocating relationship with the living roots by providing nutrition to plants from the substrate and receive carbon in turn from them. The mycelial network of the Mycorrhizal fungi, accumulate heavy metal from fly ash and retain them in their living cells. In a short period of time, the grey, toxic fly ash laden waste land was converted to that of green vegetation. The technology was successfully replicated in Badarpur and Vijayawada Thermal Power Stations
Waste5/degraded land/low lying areas near in the vicinity of different TPPs viz. Farakka STPP, Bhusawal TPS, Chandrapur STPP, Harduaganj/ Obra/ Anpara TPPs of UPRVUNL, NLC and in the State of Jharkhand have been reclaimed and made suitable for crop farming /social forestry /medicinal plants through bulk use of fly ash. Thus Fly Ash has been established to act as excellent soil conditioner/ modifier.
2.4Use of fly ash in Mine filling
Fly ash in back filling of opencast mines and stowing of underground mines can be used. In open cast mines after extraction of coal and completion of mining operation, mines are back-filled with over burden materials and remaining area can be backfilled with ash.
In case of under ground mines the conventional material for back filling is river sand. NTPC has got studies conducted (including fields trials) through Central Mining Research Institute, Dhanbad for use of bottom ash from Ramagundam for stowing operation in the under ground mines of Singreni. The study indicates that it is technically feasible to utilize bottom ash in stowing operations. More than 1,00,000 tonnes of bottom ash have already been stowed in these underground mines.
2.5Use of Fly Ash in Bricks and other building product Manufacturing
Fly ash can be used for making a variety of building materials such as Bricks, Blocks, Cellular concrete, Tiles etc. Brick is a basic need for the development works. Several studies show that fly ash brick is a far superior building material than burnt clay brick. The use of fly ash brick provides a stronger, more durable construction that is better protected from efflorescence and salinity with meaningful savings in construction costs. Fly Ash Brick Making Machines used for manufacturing bricks of different sizes from fly ash.
The process of manufacturing lime fly ash bricks is based on the reaction of lime with silica of fly ash to form calcium silicate hydrates(C-S-H) which binds the ingredients to form a brick. The quality of bricks8 obtained is highly dependant on the quality of fly ash. The manufacturing of Lime-Fly ash bricks are generally designed such that they stand at par with burnt clay bricks used conventionally. Fly ash bricks have good compressive strength, low water absorption, high density and low shrinkage value as compared to burnt clay bricks. 3.5.1Fly ash bricks can be divided into the following types
Clay Fly ash Bricks: Manufacturing process of clay fly ash bricks by manual or extrusion process involves mixing of fly ash (20 % – 80 %) with clay of moderate plasticity. The green bricks are dried under ambient atmospheric conditions or in shed to equilibrium moisture level of below 3 %. Dried bricks are fired in traditional brick kilns at 1000 30 C with a soaking period of 5 – 7 hours at maturing temperature. Fly ash Sand Lime Bricks: In presence of moisture, fly ash reacts with lime at ordinary temperature and forms a compound possessing cementitious properties. After reactions between lime and fly ash, calcium silicate hydrates are produced which are responsible for the high strength of the compound. This process involves homogeneous mixing of raw materials (generally fly ash, sand and lime), moulding of bricks and then curing of the green bricks. 3.5.2Advanced work: Cold Bonded Lightweight Fly ash Bricks, Blocks and Tiles: The material can be produced in a variety of building blocks, bricks and tiles, depending on local markets and regulations. The mixed raw material is cast in moulds, after which the moulds are processed in a microwave oven for transportation to the building site. Flux Bonded Fly ash Bricks Blocks and Tiles: The process is similar to the one in the conventional tile industry: fly ash is mixed with less than 10 % plastic clay and a few additives and tiles, bricks or blocks are pressed. These shapes are fired in the range of 900C to 1000C to make the final product. More than 85% of fly ash is used in the process. 3.5.2Advantages of fly ash bricks over clay bricks:
1)Growing cold crushing strength 2)No efflorescence 3)Compact construction & Consistent quality 4)Better thermal insulation properties 5)Availability through out the year. 6)Better shape & finish 7)Rationalization of Mortar with optional need of Plaster.
Fly ash Bricks v/s Normal Clay Bricks – Comparison
NORMAL CLAY BRICKFLY ASH BRICK Varying colour as per soil Uniform pleasing colour like cement Uneven shape as hand made Uniform in shape and smooth in finish Lightly bonded Dense composition Plastering required No plastering required Heavier in weight Lighter in weight Compressive strength is around 35 kg/cm2Compressive strength is around 100 kg/cm2 More porous Less porous Thermal conductivity 1.25 – 1.35 w/m2 C Thermal conductivity 0.90-1.05 w/m2 C Water absorption 20-25%Water absorption 6-12%
Subhash Chandra Sr Engineer AUD NTPC Limited Noida Office
Normal Clay Brick Fly Ash Brick
Day by day the uses of concrete blocks are gaining importance due to its own importance and advantages, except for its cost. The cost of concrete blocks can be cut drastically with the use of fly ash for its manufacturing. Manufacturing of fly ash blocks requires following ingredients Fly Ash: The fly ash should conform to IS requirement; dry fly ash with lime reactivity more than 60 kg/cm2. Cement: 43 grade Ordinary Portland Cement conforming to IS Sand: sand required for the mix should be free from dirt, impurities and fall in zone II. Coarse Aggregate: Crushed aggregate of maximum nominal size 10 mm.
Manufacturing: The fly ash concrete blocks can be manufactured on machine. Blocks are partially compacted and it requires to attend early strength so that transportation is easy. The fly ash is mix dry with the cement, before adding the raw materials to the mixer. The properly mixed concrete is placed in the block making machine. The blocks extracted from the machine is air dried and then sent for curing. Fly ash concrete requires more period for curing than ordinary concrete.
2.6Use of Ash in Agriculture Fly Ash consists of practically all the elements present in soil except organic carbon and nitrogen. Thus it was found that this material could be used as an additive material in agriculture applications. In view of the above, some agencies/ individuals/ institutes at dispersed locations conducted some preliminary studies on the effect and feasibility of fly ash as an input material in agricultural7 applications. It was generally observed that both sandy and clayey soils tend to become loamy in texture. Optimum bulk density in turn improves the soil porosity, the workability of the soil, the root penetration and the moisture retention capacity of the soil. The application of Fly Ash has been found to increase the available water content of loamy sand soil by 120% and of a sandy soil by 67%. RRL Bhopal reported that application of Fly Ash increase the porosity of Black Cotton Soil and decreases the porosity of sandy soils and thereby saves irrigation water around by 26% and 30% respectively. This improvement in water holding capacity is beneficial to the plants especially under rainfed agriculture. Further, in India most of the Fly Ash produced is alkaline in nature. Hence an application of these to agricultural soil increases the soil PH; it simultaneously adds essential plant nutrients to the soil. Some fly ash is acidic which may be used for reclamation of alkaline soils. Fly ash application helps in reducing surface encrustation, which is a problem in red soils. Studies in this context have revealed the followings. It improves permeability status of soil Improves fertility status of soil (soil health) / crop yield Improves soil texture Reduces bulk density of soil Improves water holding capacity / porosity Optimizes ph value Improves soil aeration Reduces crust formation Provides micro nutrients like Fe, Zn, Cu, Mo, B, Mn, etc. Provides macro nutrients like K, P, Ca, Mg, S etc.
Keeping the above important findings in view, pond ash at a dose of 30-50 tonne per hectare on one time basis along with recommended dose of fertilizers / manures is recommended for its use agriculture/ forestry sector/wasteland management or cultivation of different cereals / pluses/ oil seeds / vegetables etc., the repeat application of which can be made after 4-5 years as it would have significant residual effect on the yield of succeeding crops over a period of 4-5 years. 2.7Ash Dyke Raising
A number of ash disposal facilities across the country have been designed. Ash dyke9 maintaining is one of them. It can be raised both by dry disposal and wet disposal system. In dry disposal, the fly ash is transported by truck, chute or conveyor at the site and disposed of by constructing a dry embankment (dyke). 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, 0.5m 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. 2.8Autoclaved Aerated Concrete Autoclaved Aerated Concrete (AAC) is a precast structural product made with all-natural raw materials. In 1914, the Swedes discovered a mixture of cement, lime, water and sand that expands by adding aluminum powder. The material was further developed to what we know today as autoclaved aerated concrete (also called autoclaved cellular concrete). It is an economical, sustainable, solid block that provides thermal and acoustic insulation as well as fire and termite resistance. AAC is available in a variety of forms, ranging from wall and roof panels to blocks and lintels. To manufacture AAC, Portland cement is mixed with lime, silica sand, fly ash, water and aluminum powder or paste and poured into a mold. The reaction between aluminum and concrete causes microscopic hydrogen bubbles to form, expanding the concrete to about five times its original volume. After evaporation of the hydrogen, the now highly closed-cell, aerated concrete is cut to size and formed by steam-curing in a pressurized chamber (an autoclave). The result is a non-organic, non-toxic, airtight material that can be used for wall, floor, and roof panels, blocks, and lintels which according to the manufacturers, generate no pollutants or hazardous waste during the manufacturing process AAC may be beneficial in climates where outdoor temperature fluctuates over a 24-hour period from above to below the indoor temperature conditioned air set point. Advantages of AAC: 1.Light weight construction hence lighter foundation 2.Faster construction 3.Higher thermal and sound insulation 4.Higher resistively to fire 5.Lesser size of structural member i.e. lesser section of beams and columns 2.9Manufacturing lightweight aggregates from fly ash These are in great demand where stone mining is banned. However it is not true replacement of stone aggregate but good alternative where local requirement are of lower range. Lightweight aggregates have been manufactured by sintering fly ash and crushing the product into suitable sizes. These aggregates possess unique characteristics that make them suitable for high strength and high performance concrete. Concrete produced using these aggregates is around 22% lighter and at the same time 20% stronger than normal weight aggregate concrete. Drying shrinkage is around 33% less than that of normal weight concrete. Moreover, the aggregates possess high durability characteristics required in high performance structures. The importance of the new aggregates lies mostly in the fact that superior qualities are achieved without having to increase the cement content. Thus it is possible to reduce the amount of cement by as much as 20% without affecting the required strength. The use of lightweight aggregate in concrete has many advantages. These include: (a) Reduction of dead load that may result in reduced footings sizes and lighter and smaller upper structure. This may result in reduction in cement quantity and possible reduction in reinforcement. (b) Lighter and smaller pre-cast elements needing smaller and less expensive handling and transporting equipment. (c) Reductions in the sizes of columns and slab and beam dimensions that result in larger space availability. (d) High thermal insulation. (e) Enhanced fire resistance. 2.10Cenosphere – Fly Ash by-product The process of burning coal in thermal power plants produces fly ash containing ceramic particles made largely of alumina and silica. The ceramic particles in fly ash have three types of structures. The first type of particles are solid and are called precipitator. The second type of particles are hollow and are called cenospheres. The third type of particles are called plerospheres, which are hollow particles of large diameter filled with smaller size precipitator and cenospheres. Thus Cenosphere is a by-product of fly ash. A cenosphere is a lightweight, inert, hollow sphere filled with inert air or gas. The color of cenospheres varies from gray to almost white and their density is about 0.4-0.8 g/cm, which gives them a great buoyancy. These are hard and rigid, light, waterproof, innoxious, and insulative. This makes them highly useful in a variety of products, notably fillers. 3Legal Issues – Ash Utilization United Nations Conference on the Human Environment held at Stockholm in June, 1972, in which India participated, to take appropriate steps for the protection and improvement of human environment. In view of this, parliament passed THE ENVIRONMENT (PROTECTION) ACT, 1986 on 23rd May, 1986 for protection and improvement of environment and the prevention of hazards to human beings, other living creatures, plants and property. Consequently Ministry of Environment and Forest, Govt of India has issued Gazette Notification regarding use of Fly Ash, which is as under:
1st Notification – 14.09.1999
Use of fly ash, bottom ash or pond ash in the manufacture of bricks and other construction activities.- (1) No person shall within a radius of fifty kilometers from coal or lignite based thermal power plants, manufacture clay bricks or tiles or blocks for use in construction activities without mixing at least 25 per cent of ash (fly ash, bottom ash or pond ash) with soil on weight to weight basis. ‘ (2) There shall be a authority for ensuring the use of specified quantity. In case of non-compliance, the said authority may cancel the license of brick kiln or may cancel mining lease (3) In case of non-availability of ash from thermal power plant in sufficient quantities as certified by the said power plant, the stipulation under para (1) shall be suitably modified (waived/ relaxed). (4) Each coal or lignite based thermal power plant shall constitute a dispute settlement committee. (5) Annual implementation report providing information about the compliance of provisions in this notification shall be submitted by the 30th day of April every year to the Central Pollution Control Board, concerned State Pollution Control Board/Committee and the concerned Regional Office of the Ministry of Environment and Forests by the coal or lignite based thermal power plants. (6) Power Plants to issue ash free of cost for ten years. (7) Use of fly ash, 100% utilization of Ash by all thermal power stations in – 15 years for existing stations – 9 years for new stations Specifications for use of ash-based products.- (1) Manufacture of ash-based products such as cement, concrete blocks, bricks, panels or any other material or the use of ash in construction activity such as in road laying, embankments or use as landfill to reclaim low lying areas including back filling in abandoned mines or pitheads or for any other use shall be carried out in accordance with specifications and guidelines laid down by the Bureau of Indian Standards, Indian Bureau of Mines, Indian Road Congress, Central Building Research institute, Roorkee, Central Road Research Institute, New Delhi, Building Materials and Technology Promotion Council, New Delhi, Central Public Works Department, State Public Works Departments and other Central and State Government agencies. (2) The Central Public Works Department, Public Works Departments in the State/Union Territory Governments, Development Authorities, Housing Boards, National Highway Authority of India and other construction agencies including those in the private sector shall also prescribe the use of ash and ash-based products in their respective schedules of specifications and construction applications, including appropriate standards and codes of practice, within a period of four months from the publication of this notification. (3) All local authorities shall specify in their respective building bye-laws and regulations the use of ash and ash-based products and construction techniques in building materials, roads, embankments or for any other use within a period of four months from the date of publication of this notification.
2nd Notification – 27.08.2003 (Amendment)
1.Radius for Use of ash for Brick manufacturing increased from 50 km to 100 km 2.Buildings within a radius of 50 Km to use ash based Bricks / Blocks /Tiles in phased manner i.e. 50% by 31st August 2004 100% by 31st August 2005 3.Buildings within a radius of 50 to 100 Km 25% by 31st August 2004 50% by 31st August 2005 75% by 31st August 2006 100% by 31st August 2007 4.It is responsibility of the construction agency either undertaking the construction or approving the design or both to ensure the implementation of the above provision 5.In case of non-compliance, consent order & mining lease of brick kilns to be cancelled. 6.Power plants to maintain month-wise record of ash made available to each brick kiln. 7.Implementing authority shall be Regional Officer of the State PCB or the Pollution Control Committee.
3rdNotification – 03.09.2009 (Amendment)
1.Mandate of compulsory use of fly ash in clay bricks/ tile/ block within 100 km of TPS-deleted 2.Free issue of fly ash and pond ash to Cement, Ready Mixed Concrete, Asbestos Products etc.-deleted 3.Existing TPS / expansion units in operation to achieve ash utilization levels (from date of Notification), as mentioned below: At least 50%- 1 year, 60% -2 years, 75% – 3 years,90% -4 years, 100%-5 years 4.New TPS / expansion units commissioned after this notification At least 50% – 1 year, 70% – 2 years, 90%-3 years, 100%- 4 years
5.TPS would be free to sell fly ash to User Agencies subject to: Pond ash -to be given free on -as is where is basis- to manufacturers of Bricks/ Blocks and tiles, Farmers, Central / State Road Construction Agencies, PWD and to agencies engaged in Mine filling. At least 20% of dry ESP fly ash -free of charge to units manufacturing fly ash/ clay-fly ash bricks, blocks & tiles on a priority basis over other users. No fly ash to be made available to defaulting users. 6.The amount from sale (including by subsidiary / sister concern) to be kept in separate account head and to be utilized only for development of infrastructure / facilities and promotion / facilitation activities for use of fly ash, until 100% ash utilization is achieved. 7.The amount can be utilized for other development programs as long as 100% ash utilization level is maintained. 8.TPS to constitute dispute settlement committee which shall include GM of plant, representatives of relevant construction and fly ash brick manufacturing association. 9.Fly ash means all type of ash such as ESP ash, dry fly ash, bottom ash, pond ash and mound ash. 10.Mandatory to use at least 25% of fly ash within a distance of 50km (by road) under the guidance of DGMS
4Major Successful examples World Over The ash generated from Volcanoes was used extensively in the construction of ancient Roman structures i.e. The Aqueduct of Segovia, Spain, Dome of the Pantheon, Insula in Ostia Antica, The Baths of Caracalla, etc. Colosseum (in the year 100 A.D.) is a classic example of durability achieved by using volcanic ash. Volcanic Ash acts just like our Fly Ash. In the United States of America more than six million tonnes and in Europe more than nine million tonnes are used annually in cement and concrete. So it is hard to think about concrete construction without considering the use of fly ash. Some of the most prestigious projects of recent times have relied on fly ash concrete, including dams, power stations, offshore platforms, the Channel tunnel, highways, airports, commercial and residential buildings, bridges, pipelines and silos. No wonder that fly ash is used in all sectors of the concrete industry, covering ready-mixed, precast and on-site applications. Fine examples of application of minerals from coal can be seen throughout world: Water Tower Place in Chicago, the Eisenhower Expressway in Chicago, Picasso Tower in Madrid, the Commerzbank Tower in Frankfurt, Puylaurent dam in France, the Channel Fig. 5: Burj Dubai Tunnel between France and United Kingdom, the Underground railway tunnel in Vienna, in the East Bridge in Copenhagen, and the Eindhoven Airport landing strip in the Netherlands. High performance concrete is used in the construction of the world’s tallest building (Burj Dubai) in Dubai, United Arab Emirates: 990 fine aggregate, 810 coarse aggregate, 350 Portland cement, 120 Class F fly ash (probably exported from India), 30 Microsilica, 135 water and superplasticizer (kg/m3). India Fly Ash has been used successfully in many prestigious projects in India. Some of them are: Okhla Fly over Bridge, New Delhi, Hanumaan setu, the fly over near Yamuna Bazar, Delhi, Sarita Vihar flyover in Delhi, Noida-Greater Noida Expressway, plant roads at Budge-Budge thermal power station, NH-6 four laning work near Kolkata, one km long rural road near Raichur in Karnataka etc. Use of Roller Compacted concrete technology using high doses of fly ash for construction of Saddle Dam and upper Dam of Ghatghar pumped storage Scheme near Nashik, have been implemented. 5.Conclusion:
Use of fly ash in various applications imparts several technical and environmental benefits and thus it is eco-friendly. Large quantity of fly ash is being produced every year in power generation process and huge area of agriculture land is being used for its storage / disposal. Since electricity is requirement of every individual, it is the responsibility of every user to make sincere efforts for its gainful utilization and protection of environment. References: – 1.IS 3812-2003, Pulverized Fuel Ash specification, part 1for use as pozzolana in cement, cement mortar and concrete. 2.ASTM international C: 618-03 Standard specification for coal ash and Raw or Calcined Natural Pozzolana for use in concrete. 3.V M Malhotra and A A Ramezaniapour March 1994, Fly Ash in Concrete. 4.Seminar document, May 1996 Maharshtra India Chapter of ACI, Use of fly ash in concrete. 5.Using fly ash Extracting value from waste Published by INEP. 6.Vimal Kumar and Mukesh Mathur Fly ash in roads and embankment, National seminar and Business Meet in Use of Fly Ash in Roads and Embankment, Allahabad, June 2005, pp.3-15. 7.Chand, S.K. and Rao, Bhuganga D.D. Fly ash disposal or utilization?, International Conference on Fly Ash and Utilization, 1998, Central Board of Irrigation and Power, pp. I-5, 35-40. 8.Bhanumathidas, N. and Kalidas, N. Fly ash for bricks, cement and concrete: The Indian perspective, Paper presented at the plenary session of the CANMET/ACI Conference on Fly ash, Silica fume, Slag and Natural Pozzolanas in Concrete, Chennai, July 2001. 9.Extract from paper ‘Technology: Fly ash Disposal and Utilization: The Indian Scenario’ by Rajiv Sinha, Department of Civil Engineering, IIT Kanpur Subhash Chandra Sr Engineer AUD NTPC Limited Noida Office CHAPTER ON FLY ASH FOR GRADUATE (CIVIL ENGINEERING) COURSE STUDENTS