In this article we will discuss about the natural and artificial types of light weight aggregates.
1. Natural Aggregate:
They are not of uniform quality, and not found in many countries. As such they are not used universally for making light weight concrete. Out of all the natural light weight aggregate, only pumice is used widely.
This is further classified as:
i. Pumice:
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It is a rock of volcanic origin which occurs in many parts of the world. They are light in colour or nearly white. They are light in weight enough yet sufficient strong to be used as light weight aggregate. The light weight of these rocks is due to escaping of gas from the molten lava when erupted from beneath the earth’s crest. It is of fairly even texture of inter connected cells. Chemically it is inert and has about 75% silica content.
Pumice is one of the oldest kinds of light weight aggregates which even have been used in Roman structures. It is mined, washed and used. To get the stronger aggregate, pumice may be heated to the beginning of fusion. The bulk density of pumice varies from 600 to 900 kg/m3. Good variety pumice produces concrete with a density of 700 to 1400 kg/m3. The insulating property of this concrete is very good, but it has high absorption and high shrinkage.
ii. Diatomite:
This is hydrated amorphous silica derived from the remains of microscopic aquatic plants called diatoms. The deposits of these aquatic plants are formed under deep ocean bed. Later when the ocean bed is raised and became continent, the diatomaceous earth became available on land. In short diatomite is a semi consolidated sedimentary deposits formed in cold water environment. The average weight of pure diatomite is about 450 kg/m3, whereas the weight of impure diatomite is more than 450 kg/m3.
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iii. Scoria:
It is a hollow (vesicular) glassy rock of volcanic origin. Usually it is dark in colour and contains larger and irregularly shaped cells unconnected with each other.
iv. Volcanic Cinders:
These are also of volcanic origin loose products resembling artificial cinder.
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v. Saw Dust:
Saw dust has been used as a light weight aggregate in flooring and in the manufacture of precast products on a very small scale. A lot of difficulties have been experienced in the use of saw dust. The shrinkage and moisture movement is also very high of saw dust.
vi. Rice Husk:
It has been used as light weight aggregate on a very limited scale.
2. Artificial Aggregates:
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The artificial aggregates are classified on the basis of raw materials used and the method of manufacture.
i. Aggregates classified on the basis of raw material are those aggregates which are produced by the application of heat. They are in order as expand clay, shale, slate, diatomaceous shale, perlite, obsidian and vermiculite.
ii. The second category of aggregates are produced by the special cooling process through which an expansion of blast furnance slag is obtained.
iii. Industrial cinders are categorised in the third group.
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The materials which have partly fused particles arising from the combustion of coal are termed as cinder, clinker or Breeze. Nowadays the use of these materials is diminishing due to the wider use of pulverised coal than lump coal. Cinder aggregates undergo high dry shrinkage and moisture movement. The swelling or unsoundness of cinder or clinker often is due to the presence of excessive un-brunt coal particles.
Sometimes the un-burnt particles may be as much as 15 to 25%. These particles expand on wetting and contract on drying, which is responsible for the unsoundness of concrete made with cinder or clinker etc. Cinder aggregate has been used for producing building blocks for partition walls, for screeding over flat roofs and for plastering purposes etc.
This light weight aggregate is further classified as:
i. Clinker Aggregate:
It is called cinder in U.S.A. It is made from well burnt residue of industrial high temperature furnaces, fused into lumps. The clinker should be free from harmful varieties of un-burnt coal, which is a main source of unsoundness in the concrete made from this aggregate. B.S. 3799-1990 has laid down limits of loss on ignition and of soluble sulphate content in clinker aggregate to be used in plain concrete for general purposes and in-situ interior concrete not normally exposed to damp conditions.
Iron or Pyrites in the clinker may result in staining of concrete surfaces. It should be removed. Unsoundness due to hard burnt lime can be avoided by allowing the clinker to stand wet for a period of several weeks. Due to wetness, the lime becomes slaked and there will be no expansion in the concrete.
When cinders are used as coarse as well as fine aggregate, the density of the concrete will be of the order of 1100 to 1400 kg/m3. But in order to improve the workability of the mix, often natural sand is used as fine aggregate. In this case the density of the resulting concrete will vary from 1750 to 1850 kg/m3.
ii. Breeze:
It is the name given to a material similar to clinker, but more lightly burnt and less well burnt. There is no clear cut demarcation between breeze and clinker.
iii. Expanded Clay, Shale, and Slate:
These are obtained by heating suitable raw materials in a rotary kiln to a temperature of 1000°C to 1200°C known as incipient or beginning to fusion temperature. At this temperature, the gases produced are entrapped in a viscous plastic mass, causing expansion of the material. This porous structure is retained on cooling. Thus the apparent specific gravity of the expanded material is lower than before burning. Generally the raw material is reduced to the desired size before heading, but crushing may also be applied after expansion.
Expansion of material may also be achieved by the use of sinter strand. In this case the moistened material is carried by travelling metal frame called grate having under burners such that heating gradually penetrates the full depth of the material bed. The viscosity of the material at this stage is such that the gases are entrapped into it and cause expansion of the material. The cooled mass either is crushed or material is broken into small pieces before heating as in the case of rotary kiln.
The broken or pelletized material produces particles with a smooth shell of about 0.05 to 0.1 mm thick coating over the cellular interior. These particles are nearly spherical and have glazy surface. Due to this glazy surface they have lower water absorption than uncoated particles whose absorption ranges from 12 to 30%. Coated particles are easier to handle and mix. These coated particles produce concrete of higher workability, but their cost is more than the uncoated aggregate.
The density of expanded shale and clay aggregate made by the sinter strand method varies from 650 to 900kg/m3 and when made in rotary kiln the density varies from 300 to 650 kg/m3. Usually the density of concrete made from expanded shale and clay aggregate varies from 1400 to 1800 kg/m3. The strength of concrete made from expanded clay or shale aggregate is higher than concrete made with other light weight aggregates.
iv. Perlite:
It is a glassy volcanic rock found in America, Italy and Ulster etc. When it is heated rapidly to a temperature of 900 to 1000°C, it expands due to the evolution of steam and forms a cellular material. The bulk density of this material is as low as 30 to 240 kg/m3. The strength of concrete made with perlite aggregate is very low. Its shrinkage is very high as its modulus of elasticity is very low. Mainly it is used for insulation purposes. This concrete dries very soon and can be finished rapidly. This is the main advantage of the concrete made with perlite aggregate.
v. Vermiculite:
Raw vermiculite is a micaceous material, whose structure is laminar (made of layers) similar to mica. It is found in Africa and America. When heated with certain amount (percentage) of water to a temperature of 650 to 1000°C, it expands by delamination in the same way as that of slate or shale. This type of expansion is known as Exfoliation. The vermiculite expands many times of its original volume. Due to exfoliation of its thin layers it expands even upto 30 times of its original volume. The bulk density of exfoliated vermiculite varies from 60 to 130 kg/m3. The strength of concrete made with vermiculite as aggregate is very low.
The density of this concrete also is very low, but its shrinkage is very high. As the insulating properties of this concrete are very good, it is mainly used for insulating purposes. It is also used for in-situ roof and floor screeds or for the manufacture of blocks, slabs, tiles, which are used for sound and heat insulation. The products of vermiculite concrete can be cut, sawn, screwed or nailed. The prefabricated panels of this concrete can be used for floor, wall and partition sound deadener. The hollow blocks of this concrete can be used for encasing steam or hot water pipe lines.
This concrete is non-inflammable. Thus it can be used as a heat resistant material. The vermiculite mixed with gypsum is completely incombustible and it also possesses sound absorption and thermal insulation characteristics. Thus, it is used as heat resistant material.
vi. Expanded or Foamed Slag:
Expanded slag is one of the most important types of light weight aggregates. The blast furnance slag is a by-product of the manufacture of pig iron. The expanded or foamed slag is produced by rapidly quenching blast furnance slag.
The quenching of slag can be done by the following two methods:
(a) Water jet process.
(b) Machine process.
(a) Water Jet Process:
In this process a limited amount of water in the form of spray is made to come in contact with the molten slag as it is discharged from the furnance. By spaying the water, steam is generated. The slag hardens in a porous form. This steam is absorbed by the molten mass producing a porous, honeycombed material. This material resembles the pumice.
(b) Machine Process:
In this process, the molten slag is rapidly agitated with a controlled amount of water, the steam is entrapped in the molten mass of slag. There is some gas formation also due to the chemical reactions of some of the slag constituents with water vapour. Such a product is also called expanded or foamed slag.
In case the cooling is done with a large excess of water, granulated slag is formed, which is used in the manufacture of blast furnance slag cement.
The strength and texture of foamed slag depends upon the chemical composition and the method of production. In general the structure is similar to that of natural pumice.
The expanded slag must be:
1. Free from excess of sulphate
2. Free from volatile impurities such as coal.
3. Free from contamination of heavy impurities.
The bulk density of expanded slag varies from 300 to 1100 kg/m3 depending upon the details of cooling process and to some extent on the particle size and grading. Concrete made with expanded slag has a density of 950 to 1750 kg/m3.
In India expanded or foamed slag is manufactured in many steel mills and industries have come up near the steel mills to manufacture readymade building blocks and partition wall panels. Such prefabricated products are lighter in weight. Hence their cost of cartage or transportation is cheaper. Expanded slag can also be used for the manufacture of R.C.C. lintels and other structural members. By controlling the density, foamed slag can be used for load bearing walls and also for the production of structural light weight concrete.
vii. Swelled or Bleated Clay:
When certain variety of glass and shale are heated to a temperature of beginning of fusion or melting gases are formed with in the mass at this temperature. Due to the formation of gases the heated materials swell or, expand many times more than their original volumes and cellular structure is formed. The expansion of materials is known as bloating. The cellular structure so formed is retained on cooling. The product so formed is known as light weight aggregate and is used for the production of light weight aggregate concrete.
In western countries bloated clay products have been produced by different techniques and are available in the marked by different trade names as Haydite, Rocklite, Leca, Aglite, Gravelite, etc. In India Central Building Research Institute of India has developed a technique for the manufacture of bloated clay for structural use. The experimental building constructed at CBRI using bloated clay as structural light weight aggregate has given good performance.
viii. Sintered Fly Ash:
Fly ash is the finely divided residue from thermal power plants and railway locomotives, obtained from burning of powdered coal. These particles are glassy spherical. If these small particles of fly ash are heated further, they combine forming small porous nodules or pellets, which have considerable strength.
A limited quantity of water is added to the fly ash and their pellets are made. After making pellets, they are burnt or heated to a temperature of 1000°C to 1200°C. This sintering process is similar to that adopted in the manufacture of port-land cement. The fly ash may contain certain un-burnt coal. The quantity of un-burnt coal may vary from 2 to 15% or more depending upon the efficiency of burning. While sintering the fly ash, the aim should be to make use of the fuel present in the fly ash and no extra fuel should be used.
This process improves the quality of sintered fly ash. Sintered fly ash is one of the most important types of structural light weight aggregate. In U.K. it is sold by the trade name ‘Lytag’. It has low drying shrinkage and high strength/density ratio. As the light weight aggregates are factory made under strict control conditions, they are less variable than many natural aggregates. The physical properties of light weight aggregate are shown in Table 22.2.
