Classification of Pozzolanas: Natural pozzolanas and Artificial Pozzolanas

 

Classification of Pozzolanas

Pozzolanas are classified as natural and artificial:

1. Natural pozzolanas - 

All pozzolanas are rich in silica and alumina and contain only a small quantity of alkalis. 

The following are some of the naturally occurring pozzolanas:

· Clays and shales which must be calcined to become active.

· Diatomaceous earth and opaline cherts and shales which may or may not need calcination

· Volcanic tuffs and pumicites. Fine grained ashes form better pozzolana. However, tuffs solidified volcanic ash-may be ground to desired fineness for use.

· Rhenish and Bavarian trass.

 

2. Artificial Pozzolanas -

Artificial pozzolanas is a man-made material that is a byproduct of industrial processes, such as burning organic materials like coal in furnaces to reclaim ash. Some of the examples of artificial pozzolanas are:

1. Fly ash

2. Surkhi (burnt clay pozzolana)

3. Rice husk ash

4. Ground blast-furnace slag

5. Silica fume

1. Fly ash -

Fly ash or pulverized fuel ash (PFA) is residue from the combustion of pulverized coal collected by mechanical or electrostatic separators from the flue gases or power plants.

Fly ash is a heterogeneous by-product material produced in the combustion process of coal used in power stations. It is a fine grey coloured powder having spherical glassy particles that rise with the flue gases. As fly ash contains pozzolanic materials components which reach with lime to form cementitious materials. Thus, FLY ASH is used in concrete, mines, landfills and dams.

It can be used for:

a. Brick manufacture: It results in 15% to 20% of saving in fuel of bricks. Also it improves quality of bricks.

b. For concrete mix as replacement of cement.

c. As part replacement of cement in cement concrete.

d. It reduces the amount of air entrained by a given quantity of air entraining agent.

 

Fly ash is a multifunctional material and can be used for different purposes, its utilization depends on local conditions and it can be used in different ways for different products.

The government of India has issued guidelines to use at least 25% ash in the manufacture of clay bricks, blocks, or tiles within a radius of 50 km from coal or lignite based thermal power plants.

 

Chemical Composition of Fly Ash:

·The chemical composition of fly ash depends upon the type of coal used and the methods used for combustion of coal.

Chemical Composition of fly ash of different coals

Component	Bituminous Coal	Sub-bituminous Coal		Lignite
SiO2(%)	20-30	40-60		15-45
Al2O3(%)	5-35	20-30		20-25
Fe2O3(%)	10-40	4-10	4-15
CaO(%)	1-12	5-30		15-40

Physical Properties of Fly Ash: -

1. Fineness of Fly Ash -

As per ASTM, the fineness of the fly ash is to be checked in both dry and wet sieving. The fly ash sample is sieved in 45 micron sieve and the percentage of retained on the 45 micron sieve is calculated. Further fineness is also measured by Le-Chatelier method and Blaine Specific Surface method.

2. Specific Gravity of Fly Ash -

The specific gravity of fly ash ranges from a low value of 1.90 for a sub-bituminous ash to a high value of 2.96 for an iron-rich bituminous ash.

3. Size and Shape of Fly Ash -

As the fly ash is a very fine material, the particle size ranges in between 10 to 100 micron. The shape of the fly ash is usually spherical glassy shaped.

4. Color -

The color of the fly ash depends upon the chemical and mineral constituents. Lime content in the fly ash gives tan and light colors whereas brownish color is imparted by the presence of iron content. A dark grey to black color is typically attributed to an elevated un-burned content.

Effects of fly ash on cement concrete -

• On amount of mixing water: the use of fly ash in limited amounts as a replacement for cement or as an addition to cement requires a little more water for the same slump because of fineness of the fly ash. It is generally agreed that the use of fly ash, particularly as an admixture rather than as a replacement of cement, reduces, segregation and bleeding. If the sand is coarse the addition of fly ash produces beneficial results; for fine sands, its addition may increase the water requirement for a given workability.

• On strength in compression: since the pozzolanic action is very slow, an addition of fly ash up to 30% may result in lower strength at 7 and 28 days, but may be about equal at 3 months and may further increase ages greater than 3 months provided curing is continuing.

• On modulus of Elasticity: It is lower at early ages and higher at later ages

• On curing conditions: it is similar to Portland cement concrete.

• On shrinkage of concrete: Coarser fly ashes and those having a high carbon content are more liable to increase drying shrinkage than the finer fly ashes and those having a low carbon content.

• On permeability: The permeability of concrete reduces on addition of fly ash to cement. 28 days pulverized fly-ash-concrete may be three times as permeable as ordinary concrete but after 6 months it may be less than one quarter permeable.

• On resistance to chemical attack: fly as slightly improves the resistance of concrete to sulphate attack.

• On heat of hydration: fly ash reduces the heat of hydration in concrete. A substitution of 30% fly ash may result in a reduction of 50-60% heat of hydration.

• On air Entrainment: the presence of fly ash reduces the amount of air entraining agent .

• Setting time: A 30% substitution of fly ash may result in an increase of initial setting time up to 2 hours.

 

2. Surkhi (burnt clay pozzolana) -

It is one of the artificial pozzolana obtained by burning clay soil at specified pre-determined temperatures. In doing so the water molecules are driven off and a quasi-amorphous material(अर्ध-अनाकार), reactive with lime, is obtained. However, in practice, calcined clay pozzolana is manufactured by grinding the brick bats in the grinding mills until and impalpable powder is obtained. This pozzolana is called surkhi in India, semen merah in Indonesia and homra in Egypt.

Before modern Portland cement was introduced in India, lime surkhi was used in the construction of old structures. Even after modern cement came to India, surkhi started being used as an admixture. Using surkhi as an admixture helped in overcoming the shortcomings of cement concrete.

Surkhi, was the commonest pozzolanic materials used in India. Surkhi is an artificial pozzolana made by powdering bricks or burnt clay balls and then powdered.

Characteristics of Surkhi -

Surkhi is a complex material whose quality differs depending on the type of clay.

Characteristics of surkhi depend on the following factors-

·       Composition of soil (it should have low silica content)

·       Degree of burning

·       Fineness of the powder during grinding

 

Good Surkhi -

Following characteristics determine good surkhi-

·       Color- cherry red

·       It should be clean

·       It should be free from impurities

Mechanism of Manufacturing -

In the factory; clay is burnt the same as bricks but at a high temperature of about 600° C to 1000° C in a furnace. Then, over burnt product gains a reddish color.

It is then transferred to a mill and ground into powder such that powder is fine enough to pass IS No. 9 sieve and the residue should not be more than 10% by weight. Thus obtained product is surkhi.

Uses -

a. It can be used as sand in mortar known as Surkhi mortar. (Surkhi mortar is the mixture of lime, surkhi, and water. It is economically accessible too.)

b. It can be used for waterproofing treatment.

c. It can be used as plaster. (External wall plastering is not preferred with surkhi mortar. But external pointing work can be carried out.)

d. It can be used as an admixture in cement mortar or cement concrete.

Advantages -

a. Resistive to alkalis and salt solutions.

b. Shows pozzolanic properties.

Disadvantages -

a. Disproportionate amount of silica-containing clay in surkhi can cause less cohesion, cracking, shrinking, and warping.

b. It cannot resist the long exposure to humidity, weathering, etc.

 

3. Rice husk ash -

Rice milling industry generates a lot of rice husk during milling of paddy which comes from the fields. This rice husk is mostly used as a fuel in the boilers for processing of paddy.

Rice husk is also used as a fuel for power generation. Rice husk ash (RHA) is about 25% by weight of rice husk when burnt in boilers.

Rice husk contains around 75% of organic fickle matter. The other 25% of the weight of this husk is converted into ash during the firing process. This ash is known as rice husk ash (RHA). It is also known as a rice hull ash.

Rice husk ash is obtained by burning rice husk in a controlled manner without causing environmental pollution.

When properly burnt it has high SiO₂ content and can be used as a concrete admixture. Rice husk ash exhibits high pozzolanic characteristics and contributes to high strength and high impermeability of concrete.

When burned in ordinary way rice husks produce a crystalline silica ash. However, if burned under suitable conditions, a highly reactive black non-crystalline silica residue having pozzolanic properties is produced. Temperature and duration of combustion are most importance for good quality rice husk ash. The right temperature is 700 ^oC for 2-3 hours. Thus, a controlled combustion of rice husk in electricity generation plants produces amorphous or non-crystalline silica with about 85-90 percent cellular particles.

India produces about 122 million ton of paddy every year. Each ton of paddy producers about 40 kg of RHA. There is a good potential to make use of RHA as a valuable pozzolanic material to give almost the same properties as that of micro silica.

 

Advantages of Rice Husk Ash -

·       Rice husk ash provides good compressive strength to the concrete.

·       It is a by-product(उद्योत्पदा); hence, it helps in cutting down the environmental pollution.

·       The high silica content makes it a good supplementary cementitious material or pozzolanic admixture.

·       The density of concrete containing rice husk ash is similar to the normal weight concrete; hence, it can also be used for the general-purpose application too.

·       The impervious microstructure of rice husk ash concrete provides better resistance to the sulphate attack, chloride ingress(प्रवेश), carbonation etc.

·       Rice hull concrete has good shrinkage property and increases the durability of concrete.

Disadvantages of Rice Husk Ash -

By the use of rise husk ash, concrete progressively becomes unworkable. Hence water-reducing admixtures should be used to obtain workable concrete for the ease of placement and compaction of concrete.

 

Uses of Rice Husk Ash -

The main uses of rice husk ash are as follows:

·       Rice hull ash is used to make high-performance concrete.

·       Rice hull ash is used as an insulator.

·       Rice husk ash has been used to produce refractory bricks.

Ø Ordinary Portland cement is expensive for undeveloped countries and the need for low cost building material is high. Due to the fact that cement is the most expensive component of concrete, replacing part of cement with rice husk ash would decline the cost of concrete greatly.

·       It is demonstrated(साबित) that, replacing 50% cement with rice husk ash would reduce cost of concrete by 25%. An Indonesian company has used mix proportion of 10% cement, 50% aggregate and 40% RHA plus water to produce blocks. The average strength of the blocks was 12MPa.

 

Chemical Properties:

SiO2: 78–86

Al2O3: 1–2.0

Fe2O3: 16–1.85

CaO: 55–4.81

MgO: 35–4.5

SO3: 24–1.18

Na2O: 1–1.14

K2O: 54–3.68

Loss in ignition: 4–8.55

Physical Properties:

The physical properties of rice hull ash vary depending on the temperature of burning and grinding of the rice husk.

Colour: Grey

Specific Gravity: 05-2.53

Specific Surface Area: 40-100 m2/g

Bulk Density: 200-300 kg/m3

• Rice husk ash when mixed with lime, gives a black cement. It can also be mixed with Portland cement and 28 day strength upto 55 MPa can be obtained. Rice husk ash cements containing not more than 20% of lime are acid-resisting. To improve its reactive properties the rice husk ash should be ground in ball mills for about one hour.
• Concrete produced with rice husk ash display low permeability and no bleeding at all. The major drawback of rice husk ash is that it is very strong absorbent of sodium, potassium and other ions which are good conductors of electricity. It can replace cement in mortars by 30% .
• The rice husk mixed with 20-50% hydrated lime is ground in a ball mill to produce ASHMOH, a hydrated binder suitable for masonry, foundation and general concerting. When rice husk is mixed with cement instead of lime, the hydraulic binder is termed as ASHMENT.
• Rice husk ash can also be used with lime sludge obtained from sugar refineries. The dried lime sludge is mixed with an equal amount of crushed rice husk. It is then mixed with water and tennis ball size cakes are prepared and sun-dried. The cakes so prepared are fired to produce powder which can be used as a hydraulic binder. Rice husk ash when mixed with soil (20 per cent), instead of lime sludge, produces excellent binding properties. This binder when used as 30 per cent in mixture with Portland cement gives the properties of Portland pozzolana cement.