Autoclaved Aerated Concrete is a Lightweight, Weight-bearing, Great insulating, Long-lasting building product, which is produced in a wide range of sizes and strengths AAC Blocks (Autoclaved aerated concrete blocks) are also known as autoclaved lightweight concrete (ALC)
AAC is the eco – friendly and certified green building materials. AAC is used by the Swedish architect in mid of 1920. It has become one of the most used building materials in Europe and Asia and is rapidly growing in many other countries around the world.
With the development of manufacturing process of Bricks using AAC Plants in India it is now easier to manufacture AAC Blocks and Bricks. Basically, AAC is a mixture of cement, fly ash, sand, water, and aluminium powder. AAC is using no aggregate larger than sand. Here, Aluminium powder reacts with calcium hydroxide and water to form H2. The hydrogen gas foams and doubles the volume of the raw mix after creating gas bubbles. After Steaming and hardening process, and reacts with calcium hydroxide to form calcium silicate hydrate, which gives AAC its high strength and lightweight properties.
Following is the process followed in manufacturing AAC Blocks in with the help of AAC Plants
Raw Material used –
1. Cement – Cement works as a binder, it hardens and binds all material together.
2. Fly Ash - Fly ash is a waste industrial product, a by-product of burning pulverised coal. It provides thermal insulation, fire resistance and sound absorption
3. Sand – sand is an aggregate material, sand with silica content more than 80 % is generally used in AAC bricks.
4. Limestone - Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3) and it can be directly purchased as limestone powder.
5.Aluminium powder - Aluminium is an expansion agent, air bubble introduced due to a reaction between calcium hydroxide, aluminium and water and hydrogen gas is released.
6. Gypsum - Gypsum is easily available in the market and is used in powder form.
A) 53-grade Ordinary Portland Cement (OPC) from a trusted manufacturer is required for manufacturing AAC Blocks.
B) Fly ash and sand are mixed with water to form fly ash slurry. The slurry thus formed is mixed with other ingredients like lime powder, cement, gypsum and aluminium powder in proportionate quantity to form blocks.
C) Lime powder required for AAC production is acquired by crushing limestone to fine powder at a factory or by directly purchasing it in powder.
After raw material preparation, the next step of AAC blocks manufacturing process is dosing and mixing. Process of dosing and mixing means the quality of final products. Maintaining the ratio as Fly ash or Sand: Lime: Cement: Gypsum = 69:20:8:3
Aluminium is used to a proportion of 0.08 % and water used in a ratio of 0.60 to 0.65 Dosing and mixing is a procedure to finely mix all the components and Dosing and mixing unit is used to form the correct mix.
Fly ash is pushed into a container. After the preferred bulk is poured in, pushing is stopped. Similarly lime powder, cement and gypsum are poured into individual containers. The required amount of each element is filled into their individual containers; the control system releases all elements into mixing drum. A smaller bowl type structure used for Aluminium powder is also attached as a part of a mixing part.
After the mixture has been churned for a set time, it is ready to be poured into moulds using the dosing unit. Dosing unit releases this blend as per set measures into moulds. Dosing and mixing process is carried out continuously because if there is a break between charging and discharging of elements, the left mixture might start hardening and choke up the entire unit.
After Dosing and Mixing, Casting is a manufacturing process in which a liquid material is usually poured into moulds which contains a hollow cavity of the desired shape and then allowed to harden the material. The solidified part is also known as a casting, which is ejected or broken out of the mould to complete the process.
After the mixture is poured into greased moulds, Aluminium reacts with Calcium Hydroxide and water to form Hydrogen. Loads of tiny Hydrogen bubbles are released due to this reaction. This leads to the formation of tiny distinct cells causing the slurry mix to enlarge. Such an increase may be twice its original size. This process is called rising. These cells are the reason behind the lightweight and insulating properties of AAC blocks
After Casting the ready material is allowed to settle, rising and the pre-curing process takes around 1-4 hours, it is recommended to put up with a persistent temperature in the pre-curing area. Due to this, pre-curing is also referred to as heating room pre-curing. It must be protected from vibrations during pre-curing or else it might develop cracks.
De moulding and cutting are very critical processes in AAC blocks manufacturing. These two processes play a most important role in outlining the amount of rejection as well as dimensional precision of the final bricks.
After a mould is out of the pre-curing room, it is lifted by a crane or rolled on tracks for de-moulding operation. Mostly cutting process may be classified as flat-cake and tilt-cake based on how green cake is de- moulded and sent to cutting line, generally used technology is tilt-cake technology.
Tilt-cake technology uses two cutting machines. The horizontal cutting machine is used in the first stage, while the vertical cutting machine is used in the second stage.
After Cutting, the blocks are conveyed to a large autoclave machine (a large pressure vessel, normally a steel tube), where the curing process is completed. Autoclaving is required to achieve the desired structural properties and dimensional stability.
During this process quartz sand reacts with calcium hydroxide to form calcium silicate hydrate, which gives AAC its high strength and other unique properties. The process takes about 8 to 12 hours under temperature of about 360oF (180oC), depending on the grade of material produced.
Advanced level of strength of these blocks gives higher stability to the structure of the building. AAC is manufactured from non-biodegradable materials.
Technology development in Autoclaved Aerated Concrete Plant in India helped brick manufacturers.
AAC block weighs up almost around 80% less as compared to the conventional red brick ultimately resulting in an excessive reduction of deadweight. Further, the reduced deadweight results into a reduction of the use of cement and steel which helps greatly in cost savings.
AAC Blocks have an attractive appearance and is readily adaptable to any style of architecture. Almost any design can be achieved with AAC.