Pozzolanic materials are siliceous and aluminous materials, possessing little or no cementitious value by themselves, but in finely divided form and in the presence of moisture react chemically with calcium hydroxide liberated on hydration of cement at ordinary temperature to form compounds, possessing cementitious properties.
The Silicious or aluminious compounds in a finely divided form react with calcium hydroxide to form highly stable, cementitious substances of complex composition involving calcium, silica and water. Generally amorphous silicate reacts much more rapidly than the crystalline form. The calcium hydroxide a water soluble material is converted into insoluble cementitious material by the reaction of pozzolanic materials.
Initially the pozzolanic reaction is slow and thus the production of heat of hydration and development of strength is also slow. The reaction involves the consumption of Ca(OH)2 and thus there is no production of Ca(OH)2. The reduction of Ca(OH)2 increases the durability of cement paste by making the cement paste dense and impervious.
Pozzolanic materials may be divided into two groups:
1. Natural Pozzolanic Materials:
Under this group following materials can be grouped:
1. Calcined diatomaceous earths
2. Volcanic ash, tuffs and pumicites
3. Opaline cherts
4. Clay and shales
Natural pozzolans need further grinding and calcining to activate them. Nowadays they have lost their popularity due to the availability of more active artificial pozzolans.
2. Artificial Pozzolanas:
Under this group following substances are grouped:
1. Silica fume
2. Fly ash
3. Blast furnace slag
1. Silica Fume:
It is an artificial pozzolanic material. It is produced by the reduction of high quality quartz with coal in an electric arc furnance in the manufacture of silicon or ferro silicon alloy. In the process silica fume rises as oxidised vapour. It cools, condenses and collected in cloth bags. The silica fume so collected is further processed to remove impurities and to control particle size.
The condensed silica fume is essentially silicon dioxide (SiO2) more than 90% in non-crystalline form. Its shape is spherical and extremely fine in size less than 1 micron. Its average diameter is about 0.1 micron. It is about 100 times smaller than average cement particles. The specific surface area of silica fume is about 200000 cm2/gram as against 2300 to 3000 cm2/gram that of cement.
The use of silica fume along with super plasticizers has been found very effective in the high performance of concrete. About twenty five years ago perhaps no one would have imagined that the production of concrete of 1200 kg/cm2 compressive strength could be achieved, which is possible now with the use of silica fume with super Plasticizers.
As a matter of fact silica fume by itself does not contribute to the strength dramatically. It contributes to the strength property as a very fine pozzolanic material and creating a dense packing pore filler of cement paste. Really speaking the high strength of the high performance concrete containing silica fume is attributed to a larger extent to the reduction in water content which is possible in the presence of high dose of super plasticizers and dense packing of cement paste.
Some Information about Silica Fume:
Silica fume is also called micro silica. It is a very important new material.
Some details about this material are as follows:
(a) Initially it was produced as an ultra-fine un-densified powder.
(b) It contained at least 85% SiO2
(c) The mean size of diameter is between 0.1 to 0.2 micron.
(d) Its minimum specific surface area is 150000 cm2/gram.
(e) The shape of its particles is spherical.
It is available in the following forms:
(a) Un-densified form with bulk density of 200 to 300 kg/m3
(b) Densified form with bulk density of 500 to 600 kg/m3.
(c) Micro pellitised form with bulk density of 600 to 800 kg/m3.
(d) Slurry form with density 1400 kg/m3.
(e) Slurry of silica fume is the easiest and most practical to use in the concrete mix. The slurry is produced by mixing un-densified silica fume powder and water in equal proportion by weight usually 50:50.
(f) Its surface area is 15 to 20 x 104 cm2/gram.
(g) The pH value of standard slurry is 4.7, specific gravity 1.3 to 1.4, dry content of silica fume 48 to 52%.
Silica fume or micro silica is much more reactive than fly ash or any other natural pozzolana. The reactivity of a pozzolana can be quantified by measuring the amount of calcium hydroxide in the cement paste at different times. Most of the researchers agree that the C-S-H formed by the reaction between the silica fume and Ca(OH)2 appears dense and amorphous. The 15% silica fume has been found to reduce the quantity of Ca(OH)2 of samples of cement from 24% to 12% in 90 days and from 25% to 11% in 180 days.
Influence of Silica Fume on Fresh Concrete:
The demand of water in concrete increases in proportion to the amount of silica fume added to it. The increases in water demand of concrete having silica fume will be about 1% for every 1% cement substituted. It has been observed that 20 mm maximum sized aggregate concrete having 10% silica fume will need an increased water content of about 20 litres/m3. Thus steps should be taken to avoid this increase in water by adjusting the grading and using some plasticizers.
The addition of silica fume to concrete has the following effects:
i. It lowers the slump of the mix, but increases its cohesiveness to a great extent.
ii. It makes the fresh concrete sticky in nature and hard to handle.
iii. It reduces the bleeding of the concrete. Hence the concrete can be handled and transported without segregation.
iv. It introduces plastic shrinkage cracking in the concrete.
v. Silica fume produces more heat of hydration at the initial stages, but total generation of heat will be less than that of concrete without silica fume.
Influence on Hardened Concrete:
Silica fume has the following effects on the hardened concrete.
(a) With the use of silica fume, concrete of 60 to 80 MPa strength can be obtained easily.
(b) The modulus of elasticity of silica fume concrete is less than that of ordinary concrete without silica fume.
(c) It increases the durability of concrete, but its frost resistance has been found less.
(d) It has been found effective in reducing the alkali-aggregate reaction, but some researchers are of the view that addition of silica fume even in small quantities actually increases the expansion in concrete.
The silica fume should be added to concrete mix in the 50 : 50 slurry as it is easy to store and to mix. The use of slurry has been found to produce significantly higher compressive and tensile strength than in other forms. However to avoid gelling and sedimentation of slurry it is to be kept agitated for few hours in a day.
Curing of a silica fume concrete is very important. If the rate of evaporation from the surface, is faster than the rate of migration of water from the interior to the surface, plastic shrinkage will take place. In the absence of bleeding and slow movement of water from the interior to the surface, early curing is essential.
2. Fly Ash:
It is a finely divided residue from the combustion of powdered coal. It is a waste product from coal fired power stations and Railway locomotive etc. It is the most common artificial pozzolana material. The fly ash particles are spherical and of the same fineness as that of cement. Thus silica is always readily available for reaction.
The pozzolanic activity of fly ash is good but it is essential that it has constant carbon content and constant fineness. The use of fly ash in concrete as an admixture not only extends the technical advantage to the properties of concrete, but it also contributes to the environmental pollution control.
Effect of Fly ash on Fresh Concrete:
The use of right quantity of fly ash is found in the reduction of water content required for the production of desired slump. With the reduction of water content in concrete, bleeding and drying shrinkage is also reduced. As fly ash is not highly reactive, the heat of hydration can also be reduced by the replacement of a part of cement with fly ash. 30% replacement of cement by fly ash has been found to reduce the rise of temperature by about 6°C as shown in Fig.6.12.
Effect of Fly Ash on Hardened Concrete:
Fly ash is an industrial waste, but the use of good quality fly ash in concrete has shown following effects on the concrete properties.
1. Fly ash being a pozzolanic material, its reaction takes place slowly. The initial strength of fly ash concrete is less than that of concrete without fly ash sufficiently but the strength at the later age is much greater than that of concrete without fly ash.
2. Fly ash also develops dense texture of concrete, resulting in the decrease of permeability of concrete.
3. As pozzolanic reaction can take place only in the presence of water, thus fly ash concrete needs long curing period for the development of strength. Thus it should be cured for longer period.
4. The use of good quality fly ash reduces the heat of hydration.
5. It also increases the durability of concrete.
High Volume Fly Ash Concrete (HVFA):
At present India generates about 100 x 106 tonnes of fly ash and out of which about 5% is utilised in making blended or pozzolanic cement, in which 10 to 30% addition of fly ash of the cement content is permissible. Thus the disposal of fly ash has become a serious problem. One of the practical approaches to reduce the disposal problem of fly ash is to popularise the use of high volume fly ash concrete.
The high volume fly ash concrete is a concrete in which 50 to 60% fly ash is incorporated. First it was developed for use for the mass concrete structures as dams where the development of low heat of hydration is the main consideration. Later on the use of this concrete showed excellent durability and mechanical properties required for structural and pavement constructions. It is also found useful for light weight concrete, shot creating and roller compacted concrete.
Properties of High Volume Fly Ash Concrete:
Experiments carried out at Canada centre for Mineral and Energy Technology with more fluid concrete having 18.0 to 20 cm slump have been observed as follows:
1. Bleeding and Setting Time:
Due to the low water content in high volume fly ash concrete, bleeding also is very low. The setting time is little longer than that of conventional concrete. Its use in cold weather concreting should be done with great care. The form work cannot be removed early as in the case of ordinary cement concrete.
2. Heat of Hydration:
Due to low contents of cement, its heat of hydration is low. Experiments have shown that the heat of hydration of high volume fly ash is 15 to 25% less than ordinary concrete without fly ash.
3. Curing Period:
The high volume fly ash concrete has to be cured effectively for longer period than ordinary as well as normal fly ash concrete to obtain the continued pozzolanic reaction for the development of strength. It should be properly protected from pre-matured drying by covering the surface properly.
4. Durability of (HVFA) Concrete:
Investigations carried out in Canada & U.S.A. has shown that high volume fly ash concrete has excellent durability.
Its mechanical properties also have been found excellent.
Use of High Volume Fly Ash:
Laboratory and field experiments have shown that concrete containing 55 to 60% high volume fly ash has excellent structural and durability characteristics. Hence after 1985, high volume fly ash has been used in the construction of many high rise buildings, industrial structures, concrete roads and Roller Compacted Concrete dams etc. There is bright future for this material due to sound economy and usefulness to utilize the large waste and harmful material.
Slag is a waste product of the manufacture of pig iron. It is a nonmetallic product consisting essentially of silicates and aluminates of calcium and other bases. The molten slag is rapidly chilled by quenching it in water to form a glassy and granulated material like sand.
3. Performance of Blast Furnace Slag in Concrete:
i. Fresh Concrete:
The replacement of cement with ground granulated blast furnance slag has been found to reduce the amount of water in the fresh concrete to obtain the same slump. The reduction in water content will be more with the increase of slag content and fineness of the slag. This is due to the different configurations and shape of slag and cements particles. Further the water added for mixing is not immediately lost due to the slight lower surface hydration of slag than cement. The reduction of water with the amount of slag and its fineness is shown in Fig. 6.13.
Reduction in bleeding has not been found significant with slag having fineness of 4000 cm2/gram. However, significant beneficial effect has been observed with fineness of 6000 cm2/gram and above of the slag.Fig. 6.14 shows the relation between fineness of Blast furnace slag and unit water content.
ii. Hardened Concrete:
Experiments carried out on the use of slag have shown that the use of slag in concrete enhances the intrinsic properties of fresh as well as of hardened concrete.
Following advantage has been noted:
1. Reduction in the evaluation of heat of hydration.
2. Refinement of pore structures.
3. Increased resistance to the chemical attack.
4. Reduction in the permeability of concrete.