Autoclaved Aerated Concrete (AAC) is a lightweight, concrete-like material with many small, closed internal voids. AAC typically weighs one-sixth to one-third as much as conventional concrete and is about one-sixth to one-third as strong. It is suitable for bearing walls and shear walls of low- to medium-rise structures. Its thermal conductivity is one-sixth or less that of conventional concrete, making it energy efficient. Its fire rating is slightly longer than that of conventional concrete of the same thickness, making it useful in applications where fire resistance is important. Because of its internal voids, AAC has a low sound transmission, making it useful acoustically.
AAC Blocks and Panels are becoming popular building material due to its lightweight and high strength. AAC is produced from the common materials lime, sand, cement and water, and a small amount of rising agent. AAC is economically and environmentally superior to the more traditional structural building materials such as concrete, wood, brick and stone. AAC offers incredible opportunities to increase building quality and at the same time reduce costs at the construction site.
The autoclaved aerated concrete production process differs slightly between individual production plants but the principles are similar. The AAC block manufacturing process involves the following steps:
Raw Material Preparation: Key ingredient for manufacturing Autoclaved Aerated Concrete (AAC) blocks is silica rich material like fly ash, pond ash or sand. Most of the companies in India use fly ash to manufacture AAC blocks. Fly ash is 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 quantities consistent with the recipe. Alternately sand can also be used to manufacture AAC blocks. A 'wet' ball mill finely grinds sand with water converting it into the sand slurry. Sand slurry is mixed with other ingredients just like fly ash slurry.
Dosing & Mixing: A dosing and mixing unit is used to form the correct mix to produce Autoclaved Aerated Concrete (AAC) blocks. Fly ash/sand slurry is pumped into a separate container. Once the desired weight is poured in, pumping is stopped. Similarly lime powder, cement and gypsum are poured into individual containers using screw conveyors. Once the required amount of each ingredient is filled into their individual containers control system releases all ingredients into mixing drum. Mixing drum is like a giant bowl with a stirrer rotating inside to ensure proper mixing of ingredients. Steam might also be fed to the unit to maintain the temperature in the range of 40-42oC. A smaller bowl type structure used for feeding Aluminium powder is also attached as a part of the mixing unit. Once 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 mixture as per set quantities into moulds for foaming. Dosing and mixing process is carried out continuously because if there is a long gap between charging and discharging of ingredients, the residual mixture might start hardening and choke up the entire unit.
Casting, Rising and Precuring: Before casting, moulds are coated with a thin layer of oil. This is done in order to ensure that green-cake does not stick to moulds. While slurry is mixed and poured into greased moulds, Aluminium reacts with Calcium Hydroxide and water to form Hydrogen. Millions of tiny Hydrogen bubbles are released due to this reaction. This leads to the formation of tiny unconnected cells causing the slurry mix to expand. Such expansion may be twice its original volume. This process is called rising. These cells are the reason behind the lightweight and insulating properties of AAC blocks. Once the rising process is over, green-cake is allowed to settle and cure for some time. This ensures the cutting strength required for wire cutting.
Usually rising and the pre-curing process takes around 1-4 hours. Rising is dependent on raw material mix and weather conditions. As the reaction is affected by weather conditions, it is recommended to maintain a constant temperature in the pre-curing area. Due to this, pre-curing is also referred to as 'heating room pre-curing'. This can be achieved by deploying radiating pipes for indirect heating. It should be ensured that green-cake is not subjected to vibrations during pre-curing or else it might develop cracks.
At end of the pre-curing process, green-cake is hard enough to be wire cut as per requirements. Pre-curing is not a complicated process, but it should be monitored constantly. Pre-form defects (cracking, sinking, etc) mainly occur during the process.
Demoulding and Cutting: In earlier process slurry is allowed to rise and gain strength. Once the green cake has achieved cutting strength, it is ready to be demoulded and cut as per requirements. Demoulding and cutting are very critical processes in AAC blocks manufacturing. These two processes play a major role in defining the amount of rejection as well as dimensional accuracy of the final product.
While all previous processes like raw material preparation, dosing & mixing and casting are pretty much same across all technologies, demoulding and cutting process vary vastly depending on technology provider.
Once a mould is out of the pre-curing room, it is lifted by a crane or rolled on tracks for demoulding operation. Primarily cutting process may be classified as flat-cake and tilt-cake based on how green cake is demoulded and sent to cutting line. Most commonly used technology is tilt-cake technology. Usually, 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.
While in most cases green cake is sent to autoclave queue after cutting operation, in certain cases it is sent to a tilting table. Tilting table is used to remove the top and bottom layer of crust in the green stage. This crust may be recycled by sending it to recycling tank. In absence of a tilting table, the crust may have to be separated in white stage and may cause damage to the final product due to the use of tools like chisels.
Autoclaving: After cutting, the blocks are transported to a large autoclave (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 a pressure of about 174 psi (12 Bars) and a temperature of about 360oF (180oC), depending on the grade of material produced.
After the autoclaving process, the material is ready for immediate use on the construction site. Depending on its density, up to 80% of the volume of an AAC block is air. AAC's low density also accounts for its low structural compression strength. It can carry loads of up to 8 MPa (1,160 PSI), approximately 50% of the compressive strength of regular concrete.
