Study Of Properties Of Concrete Using Processed Flyash Engineering Essay

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In light of increased awareness towards environment, pollution and conservation it is high time for Construction industry to incorporate ways and means to reduce the pollution potential of various construction activities. One major area where construction industry is making steady progress is the use of various industrial wastes and by products generated in concrete and concrete based applications. One major industrial waste/by-product that is abundantly used now a days is flyash.

This paper tries to present the test results obtained at the end of experimental studies on concrete manufactured using Processed Flyash. Processed Flyash is a type of sub-bituminous fly ash that is self cementing and pozzolanic in nature. As per ASTM standards Processed flyash is categorized as Class C flyash because of its high calcium content. In particular the fineness and carbon content properties of the flyash have great influence on the air content and water demand of concrete. The air content and water demand of concrete greatly affect the durability and strength of concrete.

The experiments undertaken in this study project utilized the Processed flyash obtained from DIRK flyash processing plant. The original flyash was acquired from local power plant which was subjected to processing. Trial mixes for M-20 grade of concrete were prepared by progressively replacing cement with Processed flyash in stages with maximum replacement upto 35%. Regular workability tests as well as compressive strength tests were performed on cubes cast of normal concrete as well as concrete manufactured with Processed flyash with varying percentages.

The paper presents the results of above experimental tests performed and discusses possible applications of Processed flyash.

Key Words: Environment, Pollution, Conservation, Concrete, Processed Flyash.

INTRODUCTION:

Flyash, a by-product of coal fired power station, is now accepted on a worldwide basis as a concreting material but this recognition has taken a long time due to conservative attitude of construction industry vis-à-vis flyash as well as variability in the quality of flyash. The properties of flyash vary depending upon the coal that is burnt and the furnace conditions. This paper focuses on study of properties of flyash resulting from coal combustion and not with flyash from waste incineration or from heavy fuel oil. In general coal fired power stations normally use bituminous coal but can also use lignite, anthracite or a mixture of coal and other waste materials. Previously use of coal based flyash was restricted in mass concreting works like dams, where a high degree of variability would not jeopardize structural integrity.

FLYASH AND ITS TYPES:

Flyash is a by-product of pulverized coal blown into a fire furnace at a power generating plant. Coal ground to the consistency of flour ignites when blown into the furnace and a certain amount of non-burnable material residue remains as either slag or air borne particles known as flyash. These air borne particles are removed by mechanical collectors, electrostatic precipitators or wet scrubbers. Flyash looks very similar to cement in appearance. However when magnified flyash will appear as spherical particle, similar to ball bearings, whereas cement appears angular, more like crushed rock coarse aggregates. Spherical shape of flyash gives it a ball-bearing effect in the concrete mix. One of the important aspects to be considered in choice of flyash is the Loss of Ignition (LOI). LOI is the measure of unburnt carbon. In modern day processes use of pulverized coal and improved efficiencies of combustion mechanisms ensure LOI within permissible limits in most of the flyash.

The chemical composition of flyash depends on the source of coal and the operating conditions of the furnace2. As per ASTM practice flyash is categorized as Type F or Type C. Type F flyash, also called as low calcium flyash, is the combustion product of anthracite or bituminous coal and has a particle size in range of 1μ to 150μ. In this type of flyash the amount of lime present is less than 10%. Type C flyash also termed as high calcium flyash, is the combustion product of lignite or sub-bituminous coal. Class C flyash has a particle size in range of 1μ to 15μ. In this type of flyash the lime content is more than 20%. The class C flyash is more reactive than class F flyash. Both categories of flyash exhibit wide range of properties. Concrete manufactured using type C flyash has higher early strength and ensures early initiation of pozzolanic action3. Type F flyash provides sulphate resistance equivalent to or at times superior to Type V Cement. It effectively moderates the heat gain during concrete curing operation.

Country

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O + K2O

SO3

LOI

France

48.45

25.89

8.07

5.95

2.36

4.58

1

3.72

Germany

41.13

24.39

13.93

5.06

1.85

-

0.77

9.65

UK

46.16

26.99

10.44

3.06

1.96

4.18

1.59

5

India

55.34

23.72

9.39

3.22

2.13

-

0.94

5.96

Japan

57.96

25.86

4.31

3.98

1.58

3.64

0.34

0.73

USA

44.11

20.81

17.49

4.75

1.12

2.7

1.19

7.83

Russia

55.08

25.97

7.83

5.08

1.81

-

1.63

-

The general constituents of flyash are glass (60 to 85 percent) and crystalline components (10 to 30 percent) and unburnt carbon (upto 5%). Silica and alumina are chiefly contained in glass, formulated into solid or hollow spherical particles with some iron oxide, lime, alkalies and magnesia. In coal with high sulpher content, SO3 is present either as anhydrite or as gypsum or even associated with glass. Carbon is present in the form of cellular particles larger than 45. More than 5 percent carbon in a flyash meant for use as a mineral admixture in concrete is considered undesirable because the cellular particles of carbon tend to increase both the water requirement for a given consistency and the admixture requirement for air entrainment.

PROCESSED FLYASH:

The fineness of flyash is one of the important properties which governs the properties like lime reactivity and contribution in compressive strength. In case of a thermal power station, flyash gets trapped in between number of rows of electrostatic precipitators. About two-third of total flyash is generally trapped in the first row of electrostatic precipitator whereas the remainder gets trapped in subsequent rows. The coarsest particles are trapped in the first row and finer particles are trapped in later rows. It is this flyash which meets the fineness requirements as prescribed by IS 3812:2003. however the quantity of this type of flyash is less and its properties and fineness has to be checked before utilization in concrete. To ensure substantial quantity of flyash the coarser particles need to be ground, but due to the grinding action the flyash particles loose their perfect spherical shape thereby the advantage of improved workability is lost. The best possible option in the scenario would be subjecting the flyash to mechanical air clarifying process which segregates the flyash on the basis of size. This is known as processing of flyash where by no alteration in chemical properties and physical appearance takes place only the coarser particles are segregated from the finer particles.

The particle size of flyash has significant role in strength contribution as well as pozzolanic action. The particle size of raw flyash range from 1μ to 100μ3. Particles under 10μ, regardless of type of flyash are the ones that contribute to the 7 and 28 day strengths. Particle size of 45μ and more are considered as inert and do not participate in any pozzolanic reaction. A typical flyash should satisfy provisions of IS 3812: 2003 with respect to following parameters:

Table 1: Test Parameters for flyash as prescribed by IS 3812: 2003 (Part 1)4

Sr. No.

Parameter

IS Code provision

1.

Fineness

Specific Surface Area = 320m2/kg

Max. limit of 45μ particle = 34% by wet sieve analysis.

2.

Lime Reactivity

Minimum compressive strength after 10 days for fly-ash lime mortar cube should be 4.5N/mm2

3.

Compressive Strength

28 day compressive strength of cement-flyash mortar cubes should be 80% of the corresponding plain cement mortar cubes.

4.

SiO2 Content

Minimum SiO2 in percent by mass should be 35%.

5.

LOI

Max. permissible limit is 5%.

CONCRETE MIX TEST WITH PROCESSED FLYASH:

The study project focused on testing basic properties of concrete - workability and compressive strength- using Processed flyash by replacing cement from the concrete mix with 0% replacement to 35% replacement. The test procedure parameters are as follow:

Source of flyash: Dirk Flyash Processing Plant, Nashik - Dirk Pozzo 60.

Specific Gravity of Flyash: 2.3

Grade of concrete: M20.

Specified minimum strength: 20 MPa.

Target strength: 27 MPa.

Water cement ratio adopted 0.52.

Batching process: Weigh Batching.

Mixing Process: Mechanized using pan mixers.

Workability measured immediately after mixing process was over as well as after 30 minutes, 60, 90 and 120 minutes.

Workability measurement test: Slump Cone Test.

Compressive strength measured at interval of 1, 3, 7 and 28 days.

Table No. 2: Properties of Processed Flyash used in the study:

Sr. No.

Test Parameter

IS 3812:2003 (Part 1) Recommendation

Test Result

1.

Fineness Specific Surface by Blaine's Permeability Method (Min.)

320 m2/kg

378 m2/kg

2.

Loss of Ignition (Max.)

5 %

0.90 %

3.

Residue on Sieve (Wet Sieve analysis) 45µ particle (Max.)

34 %

16.33 %

4.

Chemical Analysis Test

i.

SiO­2 + Al2O3 + Fe2O3 (Min.)

70 % by mass

92.84 %

ii.

MgO (Max.)

5 % by mass

2.14 %

iii.

Na2O (Max.)

1.5 % by mass

0.56 %

Sr. No.

Test Parameter

IS 3812:2003 (Part 1) Recommendation

Test Result

iv.

SO3 (Max.)

3 % by mass

0.88 %

v.

Total Chlorides (Max.)

0.05 % by mass

0.029 %

Table No. 3: Details of the Concrete Mix.

Material

Source

0% Replacement

20% Replacement

25% Replacement

30% Replacement

35% Replacement

Cement

OPC G53

340 kg

264 kg

255 kg

238 kg

221 kg

Flyash

Dirk Pozzo - 60

0

66 kg

85 kg

102 kg

119 kg

River Sand

Malegaon

772 kg

772 kg

772 kg

761 kg

750 kg

Crushed Rock Fines

Dindori Sinner

365 kg

360 kg

357 kg

362 kg

355 kg

10 mm CA

Dindori Sinner

242 kg

237 kg

233 kg

240 kg

248 kg

20 mm CA

Dindori Sinner

692 kg

686 kg

683 kg

697 kg

705 kg

Water

176 kg

168 kg

165 kg

162 kg

160 kg

Admixture

Sikament 610 UT

1.5 %

1.45 %

1.45 %

1.40 %

1.35 %

The test results are tabulated below:

Table No. 4: Test Results on Workability and Compressive Strength.

Sr. No.

Test Parameter

Interval

0% Replacement

20% Replacement

25% Replacement

30% Replacement

35% Replacement

1.

Workability

Initial

170 mm

180 mm

180 mm

180 mm

180 mm

After 30 min.

160 mm

160 mm

170 mm

170 mm

170 mm

After 60 min.

150 mm

150 mm

160 mm

150 mm

150 mm

After 90 min.

130 mm

140 mm

150 mm

140 mm

140 mm

After 120 min

130 mm

130 mm

130 mm

130 mm

130mm

2.

Compressive strength

1 day

9.11 N/mm2

8.45 N/mm2

8.35 N/mm2

8.22 N/mm2

8.14 N/mm2

3 days

19.31 N/mm2

14.51 N/mm2

12.64 N/mm2

12.22 N/mm2

12.06 N/mm2

7 days

27.59 N/mm2

24.64 N/mm2

22.08 N/mm2

23.92 N/mm2

24.34 N/mm2

28 days

28.39 N/mm2

32.89 N/mm2

32.6 N/mm2

31.68 N/mm2

31.56 N/mm2

Fig No. 1: Graph showing variation in compressive strength with respect to varying % of flyash.

5.0 TEST RESULT DISCUSSION:

From the above observations and the graphs plotted we observe that, as more flyash is added to the concrete mix, a decrease in the rate of strength gain is observed. Early strength gain within 3 - 7 days generally decreases as more flyash is added in the concrete mix. Flyash affects the early strength gain probably due to free lime that is still reacting during the curing process. As concrete is adequately cured the 28 day compressive strength is more than the target strength projected. Further it can be stated that concrete mix with 20% - 25% flyash content gives optimal compressive strength. It can also be observed that all the mixes achieved 70% - 80% of the its strength in first seven days of curing.

6.0 ADVANTAGES OF USE OF PROCESSED FLYASH:

Pozzolanic Reaction: Due to hydration of cement the alkalinity of mix reaches upto 12.55. The glassy structure of flyash gets dissolved due to high alkalinity and individual flyash particles are free to react. The availability of free silica and alumina in calcium hydroxide gel leads to pozzolanic reaction which results in formation of stable crystalline compounds thereby increasing the density of the binder matrix.

Contribution to chemical stability: Pure calcium hydroxide is an unstable compound easily soluble in water. If this Ca(OH)2 is exposed to acidic elements there will be leaching action causing degradation of concrete and increased permeability through the binder matrix. However due to addition of flyash there will be pozzolanic reaction wherein the unstable Ca(OH)2 is stabilized due to formation of stable crystalline structure with silica and alumina.

Improvement in workability: Due to lower specific weight of flyash the total volume of binding matrix increases substantially which improves workability leading to ease in handling and compaction. Due to perfect spherical shape of individual flyash particle they act as ball bearings and effect adequate lubrication in the binder matrix.

Influence on heat of hydration: Replacement of cement by flyash not only reduces the total amount of heat of hydration developed and released, but also delays upto certain extent the heat release reducing the peak temperatures that may cause thermal shrinkage.

Contribution to durability: Due to use of flyash the water cement ratio is significantly lower. The binder matrix is denser and less pervious to water and other environmental agents.

Ecofriendly: Use of flyash as replacement of cement to some limit can help in reduction of Green House Gas emissions.

Versatile Applications: Flyash has versatile applications apart from concrete applications such as use in Cellular light weight concrete blocks, flyash based composites reinforced with jute can act as wood substitute, in brick manufacture, in road construction.

7.0 CONCLUSION:

For a variety of reasons concrete industry is not sustainable. It consumes large amount of virgin materials, manufacture of Portland cement - main binder material in concrete - emits substantial amount of green house gases and lastly most concrete structures fail in durability due to adverse effect of changing environment.

Use of flyash can help in overcoming these limitations and address the sustainability issues also.

Processed flyash can be used as a replacement for Portland cement to produce high performance concrete.

Use of Processed flyash benefits one and all by improving the durability of the structure, yielding better workability of fresh concrete and most importantly aiding in making our planet earth a better place for the next generation to live in.

In conclusion use of Processed flyash offers a holistic solution to the problem of meeting the increasing demand for concrete in future in a sustainable manner and at a reduced or no additional cost and at the same time reducing the environmental impact if two industries that are vital to economic development viz. cement industry and coal fired thermal power plant.

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