Puzzolanas
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1. Introduction
The term puzzolana is derived from Puzzouli, a town in Italy on the Bay of Naples near Mount Vesuvious. The sand (volcanic dust) around this town when mixed with hydrated lime was found to possess hydraulic properties. Puzzolana may be defined as a siliceous material which whilst itself possessing no cementitious properties, either processed or unprocessed and infinely divided form, reacts in the presence of water with lime at normal temperatures to form compounds of low solubility having cementitious properties. Puzzolanas may be natural or artificial, fly ash being the best known in the latter category. Before the advent of cement these were used with lime to make concrete. Currently its principal use is to replace a proportion in cement when making concrete. The advantages gained are economy, improvement in workability of concrete mix with reduction of bleeding and segregation. Other advantages are greater imperviousness, to freezing and thawing and to attack by sulphates and natural waters.In addition the disruptive effects of alkali-aggregate reaction and heat of hydration are reduced.It is generally held that the addition of natural puzzolanas reduce the leaching of soluble compounds from concrete and contributes to the impermeability of the concrete at the later ages.
The main justification for using puzzolanas is the possibility of reducing costs. If they are toreduce costs, they must be obtained locally and it is for this reason that they have not so farbeen much in use.
2. Classification
Puzzolanas are classified as natural and artificial.
- Natural Puzzolanas All puzzolanas are rich in silica and alumina and contain only a small quantity of alkalis. Following are some of the naturally occurring puzzolanas :
- Clays and shales which must be calcined to become active.
- Diatomaceous earth and opaline cherts and shales which may or may not need calcination (most active).
- Volcanic tuffs and pumicites. Fine grained ashes form better puzzolana. However, tuffs—solidified volcanic ash—may be ground to desired fineness for use.
- Rhenish and Bavarian trass.
Artificial Puzzolanas Some of the examples of artificial puzzolanas are :
1.Fly ash
2.Ground blast-furnace slag
3.Silica fume
4.Surkhi
5.Rice husk ash
3. Activity Puzzolana
When mixed with ordinary Portland cement the silica of the puzzolana combines with the free lime released during the hydration of cement. This action is called puzzolanic action. The puzzolanic activity is due to the presence of finely divided glassy silica and lime which produces calcium silicate hydrate similar to as produced during hydration of Portland cement.The silica in the puzzolana reacts with the lime produced during hydration of Portland cement and contributes to development of strength. Slowly and gradually additional calcium silicate hydrate is formed which is a binder and fills up the space, gives impermeability, durability and ever increasing strength.
Hydration of Portland cement may be expressed as
Lime produced combines with silica of puzzolana
Silicas of amorphous form react with lime readily than those of crystalline form and this constitutes the difference between active puzzolanas and materials of similar chemical composition which exhibit little puzzolanic activity. Since puzzolanic action can proceed only in the presence of water, enough moisture has to be made available for a long time to complete puzzolanic action.
It is commonly thought that lime-silica reaction is the main or the only one that takes place,but recent information indicates that alumina and iron if present also take part in the chemicalr eaction.
The optimum amount of puzzolana, as replacement for cement, may normally range between10-30% and may be as low as 4-6% for natural pouzzolanas. It may be somewhat higher for some fly ashes.
4. Effect of Natural Puzzolanas
On Heat of Hydration : The heat of hydration of a puzzolana is same as that of low heat cement.
On Strength of Concrete : When puzzolanas are used the addition of an air entraining agent may enable a reduction in the amount of water than if the air entraining agent was added to concrete containing cement only. This may lead to an increase in strength and consequentlyless cement may be permitted for the same strength. At early ages the replacement of cement by a puzzolana usually results in a decrease in the compressive strength, but the difference becomes less and may disappear at ages of 3 months or more˛
On Shrinkage and Moisture Movement : It is similar to Portland cement.
5. Application
Puzzolana finds its chief application where the reduction in the heat of hydration is of great importance and the slower rate of gain in strength is not of much conscience, i.e., where mass concreting is to be done. Also, the improvement in workability obtained by using puzzolana causes considerable advantage in the lean harsh mixes normally used in the construction of mass causes concreting. The examples are dams, retaining walls, wharf walls, breakwaters, harbour works and massive foundations. Lime-puzzolana mixtures are used for masonry mortars, plasters and for foundation concrete. Their types and physical requirements are given in Appendix 1.
6. Fly Ash
Fly ash or pulverized fuel ash (PFA) is the residue from the combustion of pulverized coal collected by mechanical or electrostatic separators from the flue gases or power plants. It constitutes about 75 per cent of the total ash produced. The properties and composition of fly ash vary widely, not only between different plants but from hour to hour in the same plant. Its composition depends on type of fuel burnt and on the variation of load on the boiler. Fly ash obtained from cyclone separators is comparatively coarse and contains a large proportion of un burnt fuel, whereas that obtained from electrostatic precipitators is relatively fine having a specific surface of about 3500 cm2/g and may be as high as 5000 cm2/g. Normally it is rather finer than Portland cement. Fly ash consists generally of spherical particles, some of which maybe like glass and hollow and of irregularly shaped particles of un burnt fuel or carbon. It may vary in colour from light grey to dark grey or even brown.
Carbon content in fly ash is important consideration for use with cement; it should be as low as possible. The fineness of fly ash should be as high as possible. The silica contained in fly ash should be present in finely divided state since it combines slowly over a very long period with the lime liberated during the hydration of the cement. Curing at a temperature of 38°C has been found to greatly accelerate its contribution to the strength of concrete. Curing at high pressure and temperature in autoclave promotes the reaction between the lime liberated during hydration of cement and the silica in the fly ash. However, this reaction should tend to prevent the release of free lime to reduce efflorescence.
Fly ash is supplied in two grades; grade I and grade II. There general use is incorporating it in cement mortar and concrete and in lime pozzolana mixture. However, only grade I is recommended for manufacture of Portland pozzolana cement.
Spesification
Fly ash consists of spherical glassy particles ranging from 1 to 150 m, most of which passes through a 45 m sieve. More than 40 per cent of the particles, which are under 10 microns contribute to early age strength (7 and 28 day). Particles of sizes 10 to 45 microns reacts slowly and are responsible for gain in strength from 28 days to one year. Physical and chemical requirements of fly ash are given in Tables 1 and 2.
Tabel 1. Physical Requirements
Tabel 2. Chemical Requirements
Effect of Fly Ash on Cement Concrete
On A 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 Stregth In Compression : Since the puzzolanic action is very slow, an addition of fly ashup to 30 per cent may result in lower strength at 7 and 28 days, but may be about equal at3 months and may further increase at ages greater than 3 months provided curing is continued.
On Modulus Of Elasticity : It is lower at early ages and higher at later ages.
On Curing Condition : 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 pulverised fly-ash-concrete may be three times as permeable as ordinary concrete butafter 6 months it may be less than one quarter permeable.
On Resistance Chemical Attack : Fly ash slightly improves the resistance of concrete to sulphate attack.
On Hit Hydration : Fly ash reduces the heat of hydration in concrete. A substitution of30 per cent fly ash may result in a reduction of 50-60% heat of hydration.
On Air Entertainment : The presence of fly ash reduces the amount of air entraining agent.
Setting Time : A 30 per cent substitution of fly ash may result in an increase of initial settingtime up to 2 hours.
7. Calcined Clay Puzzolana (Surkhi)
It is one of the artificial puzzolana obtained by burning clay soils at specified predeterminedtemperatures. In doing so the water molecules are driven off and a quasi-amorphous material,reactive with lime, is obtained. However, in practice, calcined clay puzzolana is manufacturedby grinding the brick bats in the grinding mills until an impalpable powder is obtained. This puzzolana is called surkhi in India, semen merah in Indonesia and homra in Egypt.
The best surkhi is obtained by burning clay in field or in a kiln and is classed as grade I whereas that obtained by grinding brick bats is classed as grade II. Soils containing little amount of clay are placed alternately with fuel layers in a pit and are fired. The residue obtained from firing is friable and needs no pulverization. In the kiln method of surkhi production, 50-l00 mm clay lumps along with coal fuel are placed in shaft kiln (Pic. 1). Coal is fired and the clay is calcined at 600 to 1000°C depending upon the type of clay. The temperature is regulated by the air blower and feed input.
Surkhi is extensively used in making mortar and concrete as an adulterant for economy. But its chief function is to impart strength and hydraulic properties to mortar. When mixed with cement to react with lime liberated during the setting and hardening of cement it makes dense,compact and impermeable concrete.
Specification
The clay or clay products used in the manufacture of surkhi must not contain a high percentage of silica. A good surkhi should be clean, cherry red in colour, and free from any foreign matter. Calcined clay puzzolana should conform generally to the chemical requirements on an oven dry basis (at 105°C) as given in Table 3
Tabel 3. Chemical Requirements
The physical requirement are same as that given in Table 9.1 except that for soundness which is not applicable.
Pic.1 Vertical Shaft Kiln
8. Ground Blast Furnace Slag
Blast furnace slag is a by product obtained while smelting iron ore in blast furnace. By melting the iron ore at 1400-1600°C pig iron is produced and the floating impurities, containing main lylime, silica and alumina from the blast furnace slag. By slow cooling of the slag crystalline material is produced, which is used as aggregate and has no cementing properties. Glassy pallets (> 4 mm) produced on rapid cooling form excellent light weight aggregate and granules(> 4 mm) on grinding possess hydraulic properties. This granulated ground blast furnace slag(GBFS) is used for the production of blast-furnace cement. The specific surface is 3000 to 3500cm2/g. The specifications of GBFS are given in Table 4.
The ground blast furnace slag exhibits hydraulic action in the presence of calcium hydroxide liberated by Portland cement when hydrated. The ground slag is blended with Portland cement to produce Portland blast furnace slag cement, the proportion of the former not exceeding65 per cent. The early strength of the cement so produced might be less but the ultimate strength is comparable. Because of low heat of hydration the ground blast furnace slag cement finds its application in mass concreting. The other advantages of addition of blast furnace slag to cement are improved workability, resistance to chemical attack and the protection provided to reinforcement that makes it suitable for reinforced concrete and prestressed concrete.
Tabel 4. Speficitations Of GBFS
9. Silica Fume
Silica fume also called micro silica, is a light to dark grey cementitious material composed of atleast 85 per cent ultra fine, amorphous non-crystalline (glassy) spherical silicon dioxide (SiO2)particles. It is produced as a by-product during the manufacture of silicon metal or ferrosiliconalloys by reduction of high purity quartz in a submerged-arc electric furnace heated to 2000°C with coal coke and wood chips as fuel. The individual particles are extremely fine, approximately1/50th the size of an average Portland cement particle (0.1 to 0.3 µm). The efficiency of silica fume depends upon its mineralogy and particle size distribution. The extremely fine particle size, large surface area and high content of highly reactive amorphous silicon dioxide give silica fume the super pozzolanic properties.
The effect of silica fume can be explained through two mechanisms—the pozzolanic reaction and the micro filler effect. Like other pozzolanas, silica fume does not have any binding property, but it reacts with Ca(OH2) liberated on hydration of cement. When water is added to cement, hydration occurs forming two primary products. The first product is calcium-silicate-hydrate (C-S-H) gel, that is cementitious and binds the aggregate together in concrete and the other product is calcium hydroxide Ca(OH2) which comprises up to 25 per cent of volume of hydration products. Silica fume reacts with calcium hydroxide to produce more aggregate binding C-S-H gel, simultaneously reducing calcium hydroxide. The net result is an increase in strength and durability. The second mechanism is through the micro filler effect. The extreme fineness of silica fume allows it to fill or pack the microscopic voids between cement particles and especially in the voids at the surface of the aggregate particles where the cement particles cannot fully cover the surface of the aggregate and fill all the available space. This so called interface zone influences the properties of the concrete. The effect is credited with greatly reduced permeability and improved paste to aggregate bond, and ultimate strength of concrete.
Advantages : There are various advantages in using silica fume such as reduction in bleeding and segregation of fresh concrete, and improvements in the strength and durability characteristics of hardened concrete. The combination of high reactivity and extreme fineness results in the possibility of producing more dense concrete with a very low porosity, the pores being small and discontinuous, and therefore, with a high strength and a low permeability.
Physical and Chemical Properties : The specific gravity of silica fume is 2.20. The silica fume is available as—produced in untensified from with bulk density of 200-300 kg/m3; densified from with bulk density of 500-600 kg/m3; in the form of micro pellets with bulk density of600-800 kg/m3; or in a slurry form with the desired concentration (generally with density of1400 kg/m3). The other basic characteristics are: specific surface area of 1500-2000 cm2/g,standard grade slurry pH value of 4.7, specific gravity of 1.3-1.4 and dry content of micro silica of 48-52 per cent.
Workability : With the addition of silica fume the slump loss with time is directly proportional to the increase of silica fume content. This is simply due to the introduction of large surface are ain the concrete mix by its addition. Although the slump decreases but the mix is highly cohesive. However, with the adjustment in the aggregate grading and by the use of super plasticizer the demand for additional water can be minimized.
Segregeation and Bleeding : Silica fume reduces bleeding significantly. This effect is caused mainly by the high surface area of the silica fume to be wetted, thereby reducing the free water left in the mixture for bleeding. Moreover, the silica fume reduces bleeding by physically blocking the pores in the fresh concrete. In the absence of bleeding and due to slow movement of water from interior to the surface, it is essential to finish the concrete as soon as possible after it has been placed and compacted to protect the surface from drying out. It therefore, requires early curing; membrane curing is most effective.
Setting Time : The addition of silica fume in small amounts to ordinary concrete mixtures(250-300 kg/m3) has no significant effect on setting times. However, set retarding occurs with the increased silica fume content. It can also be used in combination with fly ash or blast-furnace-slag to develop strength at early ages.
Applications : The attributes of silica fume have found their use in shotcrete applications,pumped concrete, mining and chemical industries. The high strength concrete made with silica fume provides high abrasion/erosion resistance. Silica fume influences the rheological properties of the fresh concrete, the strength, porosity and durability of hardened mass. Silica fume concrete with low water content is highly resistant to penetration by chloride ions.
10. Risk Husk Ash
The combustion of agricultural residues volatises the organic matters and a silica-rich ash is produced. Of all the agricultural wastes, rice husk yields the largest quantity of ash with about93 per cent silica which gives it puzzolanic properties.
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 puzzolanic properties is produced. Temperature and duration of combustion are of utmost importance for good quality rice husk ash. The right temperature is 700°C 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 per cent cellular particles. These particles are highly micro porous and possess a very high specific surface (5 × 105 to 10 × 105 cm2/g).
Rice husk ash when mixed with lime, gives a black cement. It can also be mixed with Portland cement and 28 day strength up to 55 MPa can be obtained. Rice husk ash cementscontaining not more than 20 per cent 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 per cent.
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 puzzolana cement.
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