Autoclaved Aerated Concrete Production

Sand processing and storage

Aerated concrete is made from the raw materials silica sand, lime, cement, gypsum (anhydrite), aluminium powder/paste, and water.

The precisely measured raw sand and the required quantity of water are fed into the wet ball mill where the raw sand is processed into sand slurry. The consistency of the sand slurry is defined exactly in terms of sand slurry density and grinding fineness. The throughput of the mill is adapted to the production capacity of the AAC factory in respect of the required maximum density of the aerated concrete. The milling process itself is carried out by means of grinding bodies (Cylpebs or steel balls).

After the grinding process, the sand slurry is pumped into sand slurry tanks for storage. Integrated stirrers serve to maintain the optimum consistency.

Dosing, mixing, casting

All additional raw materials - lime, cement, gypsum (anhydrite) and aluminium powder/paste - are stored according to safety regulations, partly pre-mixed, weighed and fed into the Masa mixer together with sand slurry and water. The mixer itself is equipped with all required dosing and weighing devices and a temperature measuring device. The mixer is installed above the casting device.

When the mixing process has been completed, the casting device feeds the mixture into a casting mould. If required, the mixer as well as the casting pipe can be rinsed automatically with water. The rinsing water is either collected in a rinsing water tank or used for the preparation of the return slurry.

The complete mixing tower with its different levels is characterized by its compact design.

The complete mixing process is controlled and supervised via the Masa mixing plant control system .

Fermentation

A transfer platform transports the filled casting mould to the closed and heated pre-hardening area, also called the fermentation area. Due to the alkaline reaction of the aluminium in the slurry, many small bubbles develop in the compound. At the same time, the cake begins to harden. After a waiting time of approx. 2.5 – 4 hours, the preset "green strength" (cutting strength) is reached. The waiting time depends on the temperature in the fermentation area, but also on the quality of the raw materials used and the chosen recipe.

Transport to the cutting line and removal of the mould

When the cutting strength has been reached and/or the preset waiting time has elapsed, the fresh ("green") aerated concrete cake is transported to the Masa cutting line while the casting mould is still closed.

In the mould turning plant, the mould is removed from the aerated concrete cake. This procedure comprises several steps. First, the mould is grabbed by the turning plant, then tilted by 90°. Next the mould, with the side plate acting as carrier plate, is placed on the cutting car. Finally, the mould is automatically taken off. The advantage of this method is that the aerated concrete cake remains on the side plate and can thus travel through the individual stations of the cutting line.

Reassembly and oiling of the casting mould

The empty casting mould is transported to the return track where a side plate is inserted so that the casting mould is complete again. It is then tilted back by 90° so that the opening is to the top. The complete mould is placed on the track leading to the mixing plant. The necessary oiling of the mould before it is re-filled can be carried out manually or automatically with the the latest generation of the Masa oil distribution system .

Cutting line

The cutting line comprises several stations that are installed above a trench to collect the cutting waste.

On precisely placed rails, the cutting car moves the side plate with the fresh aerated concrete cake through the pre-cutting plant / tongue and groove profiling station , the longitudinal cutting plant , the cross cutting plant/recessed grip mill and the blowing-off station .

As the aerated concrete block stands upright, it is possible to use short cutting wires. These enable highly precise geometrical cuts that comply with current European standards.

All waste material generated in the cutting process is collected in a basin underneath the cutting line, recycled , and returned to the production process.

The optimum coordination of the individual components results in a smooth and efficient production process.

Transport plant and tilting table

In the transport plant, the side plate with the cut aerated concrete cake is placed on the tilting table. The cake is immediately turned by 90° and placed on a hardening grid.

In this process, the side plate separates from the aerated concrete cake. The side plate is returned to the mould turning plant, and on the way to this, the so-called bottom cut is removed. This bottom cut, too, is recycled and returned to the production process.

The aerated concrete cake that is now on the hardening grid is transferred to a hardening car. When three aerated concrete cakes have been stacked on top of each other, the hardening car is transferred to the autoclave platform.

Transport platform and waiting area in front of the autoclaves

The transport platform arranges the fresh aerated concrete cakes on the hardening cars into a train (according to the autoclave filling pattern). This takes place in the closed and heated waiting area. The transport platform both loads and unloads the autoclaves.

Heating the waiting area prevents the fresh aerated concrete cakes from cooling and has a positive effect on energy consumption during the hardening process.

Autoclave/steam boiler

The aerated concrete is hardened under steam pressure in special, sealed pressure vessels, the autoclaves, thereby achieving its final strength.

The steam pressure is approx. 12 bar, the temperature approx. 192° C.

The hardening procedure is defined precisely. The hardening time depends on the density and open-cell character of the aerated concrete cake. Usually, the hardening time is between 10 and 12 hours.

The various autoclaving steps can be determined individually in the Masa autoclave control system . The complete hardening process runs automatically and is recorded continuously.

The used steam for the hardening process in the autoclaves is generated in a steam boiler plant. Usually, high-capacity steam boilers that are operated by gas or oil are used to generate the steam.

Re-tilting table, hardening grid transport, and hardening car return

When the hardening process is completed, the aerated concrete blocks are ready for use.

From the transport platform, the hardened aerated concrete cake is transported to the re-tilting table by a second transport plant. There, it is tilted by 90° and moved on to the separating machine.

The transport plant transfers the empty hardening grid to a roller track. This transports the brushed-off and slightly oiled grids back to the tilting table.

The empty hardening cars are also transferred by the transport plant and returned to the tilting table.

Separating process

Depending on the raw material composition, aerated concrete blocks tend to stick to each other. To reduce the proportion of waste the blocks should be separated smoothly.

As only supplier worldwide, Masa can offer two completely different methods for the separation of blocks in an aerated concrete block making plant. Depending on the product qualities, Masa recommend to use a separating machine for "green" or already hardened "white" products. The optimum separating method is chosen when the customer-specific requirements have been analyzed and discussed with the customer.

"Green" separation "White" separation

Block transfer device

The Masa block transfer device takes the cut blocks from the transport conveyor and places them on wooden pallets that are provided by a pallet magazine.

In case where the blocks have been cut to different sizes in the longitudinal cutting plant, the Masa block transfer device is able to sort the blocks so that each type can be packaged separately.

Moreover, the block transfer device can perform quality control.

Packaging

A high-quality aerated concrete block requires smooth handling.

Therefore, the finished product has to be packaged so that it is protected from dirt and moisture before it is removed from the factory. Depending on the plant concept, the packaging process can be carried out automatically or semi-automatically.

The filled transport pallets have to be bundled, preferably with plastic straps, for safe transport and storage.

Other individual packaging options, eg. with stretch film or shrink-wrapping plants, can be integrated into the packaging line as per customer requirements.

Reinforced AAC products

To make sure that large-size aerated concrete products such as wall, ceiling and roof elements comply with the static requirements of a building, the products have to be reinforced. Beside sections for the preparation of the steel reinforcement cages and the reinforcement frames, the manufacturing concept VARIO PANEL comprises a loading and unloading unit to place and remove the reinforcement frames. The hardening process inside the autoclaves can optimally be adapted with the Masa autoclave control system. The ready-to-use products are transported to the packaging area and packaged individually.