AAC Blocks or Autoclaved Aerated Concrete Blocks are a lightweight, load-bearing, a durable, eco-sensitive, and economical alternative for conventional bricks. AAC Blocks can be customised to satisfy dimensional requirements. When compared to concrete,wood ,brick and stone AAC block possess excellent insulation, fire-resistant, and thermal resistant properties. AAC block can enhance construction speed and reduce costs.
AAC Blocks Manufacturing
AAC block constitute 80% voids and hence it is lightweight and easy to handle. They are manufactured using easily available materials. The ingredients include sand, fly ash, lime, cement, gypsum, and aluminum powder. The manufacturing process is clean and does not emit any hazardous chemicals. Major raw materials used for AAC block manufacturing are industrial wastes like fly ash and gypsum. The manufacturing process is energy efficient eco-friendly certified green building materials.
Ingredients of AAC blocks
The main ingredients of AAC blocks are Fly ash, Sand, Cement, Lime, Gypsum, water, and aluminum powder. Aluminum powder is used as a foaming agent.
Cement used for AAC block manufacturing is OPC 43 or OPC 53 grade cement and is used as a binding material.
Fly ash is an industrial waste from power plants. The density ranges from 600kg/cm3 to 1400kg/cm3. The presence of fly ash is responsible for imparting insulation and fire-resistant properties of AAC blocks.
The lime is obtained by grinding limestone in the block manufacturing factory or outsourcing as lime powder.
Aluminum is used as a foaming agent and is responsible for the development of voids inside the blocks.
Gypsum is an industrial waste generated from fertiliser plants and is abundantly available.
AAC Blocks manufacturing process
The manufacturing process of AAC blocks is associated with a series of operations starting from, feeding of ingredients to the stacking of blocks. Every stage is associated with strict manufacturing quality guidelines for ensuring requisite strength and density. AAC block manufacturing processes are broadly classified as follows.
- Raw material preparation
- Dosing & mixing
- Moulding, rising and pre-curing
- De-moulding and Wire cutting
Raw material Preparation
The basic ingredients required for AAC block manufacturing are fly ash or sand. Fly ash is abundantly available as industrial waste from power plants. Hence fly ash is used for manufacturing AAC blocks. It is mixed with water to form fly ash slurry which is stored in containers for the next stage of operation.
Dosing and mixing
The fly ash slurry is stored in a container which is connected to a mixing drum. All other ingredients like lime powder, gypsum, and cement are stored in different storage containers and are fed into the mixing drum using screw conveyors. Automatic feeding systems are attached to each storage container to ensure correct ingredient proportions. The proportioned ingredients are simultaneously fed to a mixing drum where it is mixed thoroughly. Aluminum dosing is done through a feeding attachment and is controlled according to the requirement. The ingredients are thoroughly mixed and ready to be poured into the moulds.
Moulding, rising and Pre-curing
The casting of AAC blocks is done in moulds. The moulds are cleaned and coated with oil to avoid any sticking of the ingredients on the surfaces of the mould which in turn can hamper the surface finish of blocks. Already mixed slurry is poured into the oiled moulds.
Inside the moulds, the aluminum reacts with calcium hydroxide (lime) and water and produces a lot of unconnected hydrogen bubbles and expands the slurry. The expansion is almost twice that of the original volume. This process is called rising. These hydrogen bubble spaces are responsible for the lightweight characteristics of AAC blocks. The process of rising is followed by de-moulding and cutting the blocks.
The cutting of the blocks into the required sizes requires some strength for the cube. For that, the mould is allowed to settle and cure for a while just enough for the wire cutting process. The pre-curing time can be anywhere between 1-3 hours or more depending on weather conditions.
This can be done by indirect heating also. After the pre-curing process, the green cake is strong enough for a wire cut.
De-moulding and Wire cutting
The pre-cured moulds are carried to the cutting area. The moulds are removed and the green cake is wire cut as per the required sizes. The wire cutting process is done in two stages. The first stage is a vertical cut and the second stage is a horizontal cut. De-moulding and cutting give the dimensional perfection and finish of the AAC blocks. This process has to be done with utmost care or else rejection may happen.
Autoclaving of AAC blocks
The final process of manufacturing is autoclaving. The wire cut blocks are transported to an autoclaved chamber where is heated to the required temperature. This process gives the desired durability and structural stability to AAC blocks. The baking is done for about 8-12 hours at a temperature of around 180 degrees. The temperature and period of heating determine the grade of the materials. Curing in an autoclave reduces drying shrinkage.The autoclave curing process requires more energy and a more expensive kiln, but it can produce blocks in less time.
The blocks are stacked on pellets on completion of autoclaving process for transporting to the required destinations. All quality tests are conducted in the factory itself.