Fly ash is finely divided residue resulting from the combustion of powdered coal and transported by flue gases and collected by electrostatic precipitation. Fly ash is most used pozzolanic material all over the world.
In recent time, the importance and use of fly ash has grown so much that it has almost become a common ingredient in concrete, particularly in making high strength and high performance concrete. The use of fly ash as concrete admixture not only extends technical advantages but also contributes to the environmental pollution control. Extensive research is being carried out in most part of the world that could be accrued in the utilization of fly ash, which is basically a waste product.
The volume of fly ash produced is about 75 million tons per year, the disposal of which has become a major concern. Only about 5% of the total fly ash is utilized in India the remaining of which has to be disposed. Instead of doing so, it can be utilized in a major way. The role of it plays in the field of concrete technology. In this discussion it would be interesting to discuss on fly ash from the point of concrete making.
There are two ways that the fly ash can be used: one way is to intergrind certain percentage of fly ash with cement clinker at the factory to produce Portland pozzolona cement (PPC) and the second way is to use the fly ash as an admixture at the time of mixing the concrete at the site of work. But the main problem is that the fly ash produced in the 75 thermal power plants in India is not of the similar characteristics. The suitability of fly ash used in the making of the concrete has to be further processed. The quality of fly ash should be of the standard of IS: 3812-1981. For better utilization of fly ash it becomes important to know the hydration reactions, pozzolanic activity evaluation, effect of fly ash on fresh and hardened concrete, durability etc.
2. Scope of the study of fly ash characteristics:
The importance of fly ash cannot be exaggerated. It is no longer a cheap substitute for cement, nor an extender or an addition to the mix. Fly ash bestows important advantages upon concrete, and it is, therefore, essential to understand the role and influence of fly ash. The variability of the properties of fly ash arises from the fact that fly ash is not a specially manufactured product and cannot, therefore, be governed by strict requirements of a standard. The power station which produces fly ash varies its operation in response to the power demand. The variation in the fly ash are those in glass content, carbon content, particle shape and size distribution, as well as in the presence of magnesia and other minerals, and even in colour. It is possible to improve the size distribution of fly ash particles by classification and by grinding.
As said earlier the fly ash is an artificial pozzolan and it exhibits different characteristics with different compounds of cement. The cement might contain different compositions of the above said Tri calcium silicate, Di calcium silicate, tri calcium aluminate etc. hence it becomes necessary to study the reactions exhibited by the fly ash with each of these compounds so that the necessary concrete can be obtained. Hence the cement should match with the fly ash at the first instance. This becomes important because it might have an affect on the strength and durability of the concrete.
3. Types of fly ash as per ASTM
ASTM broadly classify fly ash into two classes.
Class F: Fly ash normally produced by burning anthracite or bituminous coal, usually has less than 5% CaO. Class F fly ash has pozzolanic properties only.
Class C: Fly ash normally produced by burning lignite or sub-bituminous coal. Some class C fly ash may have CaO content in excess of 10%. In addition to pozzolanic properties, class C fly ash also possesses cementitious properties.
4. Hydration reactions:
4.1:Hydration of Portland cement:
The setting and hardening of Portland cement occur as a result of the reaction between the compounds of cement and water. The major compounds of cement that produce reaction products and the products are shown in the equation below.
2C3S + 6H C3S2H3 + 3CH
Tri calcium silicate
2C2S + 4H C3S2H3 + CH
Di calcium silicate
C3A + 3CSH2+ 26H C3A (C’S)3H32 C3AC’SH12
Tri calcium gypsum ettringite monosulphoaluminate
Aluminate hydrate
4.2:The fly ash hydration reactions:
High calcium fly ash, which contains mainly of glass phase and some crystalline phases, has self-hardening properties. Ettringite, monosulphoaluminate hydrate, and C-S-H cause hydration of fly ash when mixed with water.
Low calcium fly ash, which has very little or no self-cementatious properties, hydrates when alkalis and Ca (OH) 2 are added. As more Ca (OH) 2is supplied, more of it is fixed by silica and alumina in fly ash. The degree of hydration of fly ash is also increased in the presence of gypsum.
4.3.The effect of fly ash on the hydration of cement compounds:
The fly ash reaction with that of C2S, C3A AND C3S is more complicated. According to V.N.Malhotra (proffesor, CANMET, Natural resources, Canada) the schematic explanation of fly ash and its reaction with C3S is given below:
“In C3S pozzolan system, calcium ions dissolved from C3S run about freely in liquid and rare absorbed on the surface of pozzolan particles. C-S-H formed by the hydration of C3S precipitates as the hydrates of high Ca/Si ratio on the surface of C3S grains and as the porous hydrates of low Ca/Si ratio on the surface of pozzolan particles. Attack of the pozzolan surface in water brings about gradual dissolution of Na+ and K+, resulting in Si and Al rich amorphous layer on the surfaces. Dissolved Na+ and K+ increase the OH- concentration and accelerate the dissolution of SiO44- combine with Ca2+ to increase the thickness of the layer. Due to the osmotic pressure, the layer swells gradually and the void between the layer and pozzolan particle is formed. When the pressure in the void ruptures the film, SiO44- and AlO2- diffuse into the Ca± rich solution. Additional C-S-H and Ca-Al hydrate precipitate on the surface of outer hydrates of C3S particles and to slight extent on the ruptured film. Vacanted film, vacant spaces remains inside the film as the hydrates do not precipitate there because of high concentration of alkalis. For pozzolan with low alkalis, destruction of amorphous Si, Al rich film enables Ca2+ to move into the inside of the film and precipitate calcium silicate and calcium aluminate hydrates on the surface of pozzolan grain. There fore no space is observed between pozzolan grains and hydrates.”
It was observed that the degree of hydration of C3S alone after one day was 35%, while that of the mixture with fly ash was 45% (ACC designers manual-2003 &XRD). This result obviously shows that adding fly ash accelerates the hydration of C3S.
Many investigations have shown that fly ash retards the hydration of C3A. The degree of this retardation mainly depends on the Sulphate content of fly ash, the amount of dissolved alkalis, and the calcium absorption capacity. Ca2+ and SO42-are dissolved from high calcium fly ashes. Dissolved ca2+ is absorbed on to the Al rich surface (produced by non-stoichiometric dissolution) lowering the hydration activity. Further more, the absorption of SO42- also retards the hydration of C3A. Sulphate in fly ash retards the hydration more than the equivalent amount of added gypsum. . The schematic explanation of the mechanism of hydration in the C3A- pozzolona system: The early reactions of the fly ash-cement system was described by diamond et al. He explained the occurrence of so-called duplex films, which rapidly develop in hydrating cement compounds around exposed surfaces, such as sand grains and coarse aggregates. He also described about the formation of uniform continues layer of calcium hydroxide and a thin layer of less widely spread C-S-H gel. The gel is usually formed roughly perpendicular to the calcium hydroxide layer. The duplex is formed usually after one day when kept at room temperature.
5. Physical and Chemical requirements as per IS: 3812-1981:
Table 1: Physical requirements
Sl no
|
Characteristic
|
Requirement
Grade of fly ash
| |
I
|
II
| ||
1
|
Fineness- specific surface in m2/kg by Blaine’s permeability method, Min
|
320
|
250
|
2
|
Lime reactivity- Average compressive strength in N/mm2,Min
|
4.0
|
3.0
|
3
|
Compressive strength at 28 days N/mm2, Min
|
Not less than 80% of the strength of corresponding plain cement mortar cubes
| |
4
|
Dry shrinkage, %, Max
|
0.15
|
0.10
|
5
|
Soundness by autoclave test expansion of specimen, %, Max
|
0.8
|
0.8
|
Table 2: Chemical requirements
Sl no.
|
Characteristic by mass
|
Requirement
|
1
|
SiO2 + Al2O3 + Fe2O3, Min
|
70.0
|
2
|
SiO2, Min
|
35.0
|
3
|
MgO, Max
|
5.0
|
4
|
SO3, Max
|
2.75
|
5
|
Na2O (refer Note 1) Max
|
1.5
|
6
|
Loss on ignition, Max
|
12.0
|
Note 1: Applicable only when reactive aggregates are used in concrete and only if the purchaser specially requests for.
| ||
6. Engineering properties
Some of the engineering properties of fly ash that are of particular interest when fly ash is used as an admixture or a cement addition to PCC mixes include fineness, LOI, chemical composition, moisture content, and pozzolanic activity. Most specifying agencies refer to ASTM C618 when citing acceptance criteria for the use of fly ash in concrete.
6.1 Fineness: Fineness is the primary physical characteristic of fly ash that relates to pozzolanic activity. As the fineness increases, the pozzolanic activity can be expected to increase. Current specifications include a requirement for the maximum allowable percentage retained on a 0.045 mm (No. 325) sieve when wet sieved. ASTM C618 specifies a maximum of 34 percent retained on a 0.045 mm (No. 325) sieve. Fineness can also be assessed by methods that estimate specific surface area, such as the Blaine air permeability test commonly used for Portland cement.
6.2 Pozzolanic Activity (Chemical Composition and Mineralogy):Pozzolanic activity refers to the ability of the silica and alumina components of fly ash to react with available calcium and/or magnesium from the hydration products of Portland cement. ASTM C618 requires that the pozzolanic activity index with Portland cement, as determined in accordance with ASTM C311 be a minimum of 75 percent of the average 28-day compressive strength of control mixes made with Portland cement.
6.3 Loss on Ignition: Many state transportation departments specify a maximum LOI value that does not exceed 3 or 4 percent, even though the ASTM criteria is a maximum LOI content of 6 percent. This is because carbon contents (reflected by LOI) higher than 3 to 4 percent have an adverse effect on air entrainment.
Fly ashes must have a low enough LOI (usually less than 3.0 percent) to satisfy ready-mix concrete producers, who are concerned about product quality and the control of air-entraining admixtures. Furthermore, consistent LOI values are almost as important as low LOI values to ready-mix producers, who are most concerned with consistent and predictable quality.
6.4 Moisture Content: ASTM C618 specifies a maximum allowable moisture content of 3.0 percent.
Some of the properties of fly ash-concrete mixes that are of particular interest include mix workability, time of setting, bleeding, pumpability, strength development, heat of hydration, permeability, resistance to freeze-thaw, sulfate resistance, and alkali-silica reactivity.
6.5 Workability: At a given water-cement ratio, the spherical shape of most fly ash particles permits greater workability than with conventional concrete mixes. When fly ash is used, the absolute volume of cement plus fly ash usually exceeds that of cement in conventional concrete mixes. The increased ratio of solids volume to water volume produces a paste with improved plasticity and more cohesiveness.
6.6 Time of Setting: When replacing up to 25 percent of the Portland cement in concrete, all Class F fly ashes and most Class C fly ashes increase the time of setting. However, some Class C fly ashes may have little effect on, or possibly even decrease, the time of setting. Delays in setting time will probably be more pronounced, compared with conventional concrete mixes, during the cooler or colder months.
6.7 Bleeding: Bleeding is usually reduced because of the greater volume of fines and lower required water content for a given degree of workability.
6.8 Pumpability: Pumpability is increased by the same characteristics affecting workability, specifically, the lubricating effect of the spherical fly ash particles and the increased ratio of solids to liquid that makes the concrete less prone to segregation.
6.9 Strength Development: Previous studies of fly ash concrete mixes have generally confirmed that most mixes that contain Class F fly ash that replaces Portland cement at a 1:1 (equal weight) ratio gain compressive strength, as well as tensile strength, more slowly than conventional concrete mixes for up to as long as 60 to 90 days. Beyond 60 to 90 days, Class F fly ash concrete mixes will ultimately exceed the strength of conventional PCC mixes. For mixes with replacement ratios from 1.1 to 1.5:1 by weight of Class F fly ash to the Portland cement that is being replaced, 28-day strength development is approximately equal to that of conventional concrete.
Class C fly ashes often exhibit a higher rate of reaction at early ages than Class F fly ashes. Some Class C fly ashes are as effective as Portland cement in developing 28-day strength. Both Class F and Class C fly ashes are beneficial in the production of high-strength concrete. However, the American Concrete Institute (ACI) recommends that Class F fly ash replace from 15 to 25 percent of the Portland cement and Class C fly ash replace from 20 to 35 percent.
6.10 Heat of Hydration: The initial impetus for using fly ash in concrete stemmed from the fact that the more slowly reacting fly ash generates less heat per unit of time than the hydration of the faster reacting Portland cement. Thus, the temperature rise in large masses of concrete (such as dams) can be significantly reduced if fly ash is substituted for cement, since more of the heat can be dissipated as it develops. Not only is the risk of thermal cracking reduced, but greater ultimate strength is attained in concrete with fly ash because of the pozzolanic reaction. Class F fly ashes are generally more effective than Class C fly ashes in reducing the heat of hydration.
6.11 Permeability: Fly ash reacting with available lime and alkalies generates additional cementitious compounds that act to block bleed channels, filling pore space and reducing the permeability of the hardened concrete. The pozzolanic reaction consumes calcium hydroxide (Ca(OH)2), which is leachable, replacing it with insoluble calcium silicate hydrates (CSH). The increased volume of fines and reduced water content also play a role.
6.12 Resistance to Freeze-Thaw: As with all concretes, the resistance of fly ash concrete to damage from freezing and thawing depends on the adequacy of the air void system, as well as other factors, such as strength development, climate, and the use of deicer salts. Special attention must be given to attaining the proper amount of entrained air and air void distribution. Once fly ash concrete has developed adequate strength, no significant differences in concrete durability have usually been observed. There should be no more tendency for fly ash concrete to scale in freezing and thawing exposures than conventional concrete, provided the fly ash concrete has achieved its design strength and has the proper air void system.
6.13 Sulfate Resistance: Class F fly ash will generally improve the sulfate resistance of any concrete mixture in which it is included. Some Class C fly ashes may improve sulfate resistance, while others may actually reduce sulfate resistance and accelerate deterioration. Class C fly ashes should be individually tested before use in a sulfate environment. The relative resistance of fly ash to sulfate deterioration is reportedly a function of the ratio of calcium oxide to iron oxide.
6.14 Alkali-Silica Reactivity: Class F fly ash has been effective in inhibiting or reducing expansive reactions resulting from the alkali-silica reaction. In theory, the reaction between the very small particles of amorphous silica glass in the fly ash and the alkalis in the Portland cement, as well as the fly ash, ties up the alkalis in a nonexpansive calcium-alkali-silica gel, preventing them from reacting with silica in aggregates, which can result in expansive reactions. However, because some fly ashes (including some Class C fly ashes) may have appreciable amounts of soluble alkalis, it is necessary to test materials to be used in the field to ensure that expansion due to alkali-silica reactivity will be reduced to safe levels.
Fly ash, especially Class F fly ash, is effective in three ways in substantially reducing alkali-silica expansion:
1) It produces a denser, less permeable concrete;
2) When used as a cement replacement it reduces total alkali content by reducing the Portland cement; and
3) Alkalis react with fly ash instead of reactive silica aggregates.
Class F fly ashes are probably more effective than Class C fly ashes because of their higher silica content, which can react with alkalis. Users of Class C fly ash are cautioned to carefully evaluate the long-term volume stability of concrete mixes in the laboratory prior to field use, with ASTM C441 as a suggested method of test
.
7. Fly ash in the production of high performance concrete:
HPC is usually produced using high quality materials. These constituents drastically increase the initial cost of HPC, thus hindering its more widespread usage. But the research done by university of Minho shows that Using low quality as received materials like fly ash and locally available crushed aggregates low cost HPC can be produced. The effect of the amount of fly ash was evaluated using 0%, 20%, 40% and 60 % of cement, replacing fly ash. Workability, mechanical and durability properties aspects were also studied. The results obtained indicated that it is possible to produce HPC with up to 60Mpa by replacing 40% of cement by fly ash. Hence it is possible to conclude that the use of fly ash is beneficial, in terms of durability, workability, etc., when there is a compromise for the late development of the strength. For more information in the development of HPC using fly ash can be obtained from University of Minho, Department of Civil Engineering, Portugal.
8. Utilization of fly ash:
Keeping in view this versatility of fly ash, several agencies - (govt. private, public sector, NGOs) are involved in fly ash utilization and safe disposal efforts in India. These include Ministry of Environment & forests, Ministry of Urban Development, Department of Science & Technology, National Thermal Power Corporation, CSIR Laboratories, Engineering Institutes, IITs, State Electricity Boards, etc.
Areas of Fly Ash Utilization
· Fly Ash Bricks
· Embankment and Fills
· Road Pavement
· Portland Pozzolona Cement
· Cement Concrete and Mortar
· Light Weight Aggregates
· Back filling of Open Cast Mine
· Stowing of Under Ground Mines
· Agriculture
· Manufacture of Alum, Paint, Ceramic, Asbestos Cement Products.
9. Cases where fly ash has been used:
Table: Fly ash utilization in Mass concrete or Dam construction
Sl. No
|
Structures
|
State
|
Cement replaced (%)
|
Source of fly ash
|
1.
|
Gurgoan tunnel
|
Haryana
|
15
|
Delhi
|
2.
|
Jawar sagar Dam
|
Rajasthan
|
20
|
Delhi
|
3.
|
Kakki Dam
|
Kerala
|
20
|
Neyveli
|
4.
|
Navora barrage
|
U.P.
|
15
|
Harduaganj
|
5.
|
Rihad Dam
|
U.P.
|
15
|
Bokaro
|
6.
|
Sone Barrage
|
Bihar
|
15
|
Bokaro
|
7.
|
Umium Project
|
Assam
|
Not available
|
Durgapur
|
8.
|
Chandil dam
|
Bihar
|
25
|
Talcher
|
Table: Fly ash Experimental Housing Schemes By National Buildings Organisation
Location
|
Application
|
Hindustan Prefab Ltd. Staff Quarters, New Delhi
|
15% replacement of fly ash in precast RCC components.
|
Government double storied staff quarters R.K.Puram at New Delhi
|
20% fly ash in cement and mortar
|
4 storied Government quarters at dhaulakuan New Delhi
|
20% fly ash in cement and mortar
|
Double storied building at the S.E.R.C.Chennai.
|
Precast reinforced channel units
|
Dwelling units at Pankha road, New Delhi
|
Foundation, Concrete mortar flooring plaster some RCC items
|
Flats at Besant Nagar, Chennai
|
Cement mixture
|
10. Benefits of Using Fly Ash
Concrete in its hardened state — with fly ash — shows improved performance:
· Greater strength. Fly ash increases in strength over time, continuing to combine with free lime.
· Decreased permeability. Increased density and long-term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability.
· Increased durability. The lower permeability of concrete with fly ash also helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulfate, mild acid, and soft (lime hungry) water.
· Reduced alkali silica reactivity. Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates, thereby preventing destructive expansion.
· Reduced heat of hydration. The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal cracking when fly ash is used to replace a percentage of Portland Cement.
· Reduced efflorescence. Fly ash chemically binds free lime and salts that can create efflorescence. The lower permeability of concrete with fly ash can help to hold efflorescence-producing compounds inside the concrete.
The ball-bearing effect of fly ash in concrete creates a lubricating action when concrete is in its plastic state. This means enhanced concrete workability:
· Increased workability. Concrete is easier to place with less effort, responding better to vibration to fill forms more completely.
· Increased ease of pumping. Pumping requires less energy; longer pumping distances are possible.
· Improved finishing. Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.
· Reduced bleeding. Fewer bleed channels decreases porosity and chemical attack. Bleed streaking is reduced for architectural finishes. Improved paste to aggregate contact results in enhanced bond strengths.
· Reduced segregation. Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets.
· Reduced slump loss. More dependable concrete allows for greater working time, especially in hot weather.
