Updated: Nov 26, 2022
Lightweight aggregates are used to produce lightweight concrete when the weight of aggregates, is lower than 1200 kg/m3. Lightweight aggregates have many sources: natural materials such as shales, clays, pumice, diatomite, volcanic cinders, and slates or artificial materials (by-products) such as iron blast furnace slag, clay, sintered fly ash, and shale. Some lightweight aggregates occur naturally, while others are produced artificially. Lightweight aggregates are generally classified according to the production process and method. Natural lightweight aggregates are generally of volcanic basis and are therefore only found in certain parts of the world.
The properties of lightweight aggregate concrete depend on the type of lightweight aggregate in the concrete, structural lightweight aggregates can produce concretes with compressive strengths in excess of 35 MPa, and a limited number of lightweight aggregates can be used in concretes that develop cylinder strengths from 48 to more than 69 MPa
Artificial Lightweight Aggregate
The artificial aggregates are classified on the basis of raw materials used and the method of manufacture.
Aggregates classified on the basis of raw material are those aggregates that are produced by the application of heat. They are in order as expanded clay, shale, slate, diatomaceous shale, perlite, obsidian, and vermiculite.
The second category of aggregates is produced by the special cooling process through which an expansion of blast furnace slag is obtained.
Industrial cinders are categorized in the third group.
The materials which have partly fused particles arising from the combustion of coal are termed cinder, clinker, or Breeze. Nowadays the use of these materials is diminishing due to the wider use of pulverized 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, screening over flat roofs, and plastering purposes, etc.
As a result of the production of artificial aggregate from waste materials, the consumption of non-renewable natural resources will decrease and lighter aggregates can be produced compared to natural aggregates. Today, solid waste materials are preferred in the production of materials used in construction. Scientific researchers have used solid waste materials in the production of artificial aggregates. The fresh density of lightweight concrete; In addition to cement and pozzolanic components, if any, mixing ratios and water requirements; The density of the lightweight aggregate is a function of the available moisture content and the pre-saturation water. During the curing period; The light aggregate will slowly release the water it has absorbed, and as the concrete gains strength with internal curing, a partial decrease in density will be observed.
1. Fly Ash Aggregate
Artificial lightweight fly ash aggregates are produced using a sintered bead. At first, the clayey fly ash is mixed with water and pelletized, and heat is applied at 1100ºC on the grid or on the sintering wire. pellets; The fly ash is subjected to drying, grinding, sintering, and cooling processes until light aggregate is produced. At the end of the process, the pellets are distinguished by natural or mechanical means. Aggregates passing through a 2.5 mm sieve are distinguished, while other aggregates remaining on a 2.5 mm sieve are used in the production of lightweight concrete. Autoclaving, which is a hydrothermal process, is the hardening of fly ash pellets by curing them with saturated steam. While producing ash aggregate by autoclave method, 45% quartz sand, 47% fly ash, 4.5% lime, 2% additives, and 1.5% water are used. The entire mixture is pelletized and then superheated at high humidity. In order to produce lightweight aggregates, which are also used in brick production, they are heat treated at 200 ºC for 6.5 hours.
The cold bonding method is a method that describes the reaction of fly ash with calcium hydroxide at ambient temperature. In the main production process of fly ash lightweight aggregate, fly ash is mixed with Portland cement and water into pellets and cured for several days.
Fly Ash aggregate
2. Expanded or foamed slag
Expanded slag is one of the most important types of lightweight aggregates. The blast furnace slag is a by-product of the manufacture of pig iron. The expanded or foamed slag is produced by rapidly quenching blast-furnace slag.
The bulk density of expanded slag varies from 300 to 1100 kg/m3 depending upon the details of the 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.
3. 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 the 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 a sinter strand. In this case, the moistened material is carried by a traveling metal frame called a 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 a 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 surfaces. 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 the 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 lightweight aggregates.
Expanded Shale Aggregate
4. Bleated Clay
When a certain variety of glass and shale are heated to a temperature of the beginning of fusion or melting gases are formed within 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 lightweight aggregate and is used for the production of lightweight aggregate concrete.
Bloated Clay aggregate