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Aluminium Safety in AAC Plants

Aluminium is required for the production of autoclaved aerated concrete.

A chemical reaction takes place after adding aluminium to the raw slurry in the process. This causes the gas bubbles in the aerated concrete slurry.

As the aluminium powder is highly flammable, it needs to be added to the aerated concrete slurry through a special ‘aluminium dosing system’.

Unfortunately, most of the automatic dosing systems were closed down in India due to multiple issues which will be discussed later.

All are now using manual dosing only even in corporate companies.

Aluminium is widely used as a foaming or rising or reaction agent in AAC production worldwide.

When aluminium is added (usually at about 0.3% to 0.7% by dry weight of cement) to the mixing ingredients, it reacts with hydroxide of calcium or alkali which liberates hydrogen gas (3H2) and forms bubbles.

The speed at which the air bubbles form is critical to the success of the final aerated concrete product.

2 Al + 3Ca(OH)2 + 6 H2O ------> 3 Cao.Al2O3.6H2O + 3H2

There have been problems regarding the dosing of aluminium as long as AAC has been manufactured.

Fires and dust explosions are therefore no exceptions.

Because of these problems, especially in Indian corporates, increasing pressure from local authorities exists to improve safety.

Aluminium is available in various forms.
 
In Indian AAC industry the most used form is powder and next is the paste.

Either one of these has its specific advantages and disadvantages regarding the production of autoclaved aerated concrete and will be described below.

Aluminium powder

Aluminium powder is often divided into three classifications:

atomized
flake
granules

In case of an atomized particle, its length, width and thickness are all of roughly an equivalent order.

But in flake particles, the length or width maybe several hundred times its thickness.

Aluminium powder within the AAC industry is usually made up of foil scrap and exists of microscopic flake-shaped aluminium particles.

Aluminium powder with grain size less than 100μm and particularly with fractions less than 50μm can easily form highly flammable aero suspensions (dust clouds) during pouring, on vibration, under shock, and so on.

This can cause dust explosions within the presence of an ignition source.
In order to extend safety, it's advisable to use coarser aluminium powders with larger grain sizes (from 0.1 to 0.5 mm).

However, the production of AAC requires aluminium powders that contain fractions finer than 100 or 50μm.

This is important so as to get required mechanical properties of the aerated concrete.

The finest aluminium powder for AAC production that is used by Europe plants even has an average grain size of 20-45μm.

A major advantage of using powders is that its reactivity is preserved when kept dry.

This takes out much uncertainty from the casting process since the required aluminium can be accurately dosed.

Aluminium pellets

In the case of very small grain sizes, it is advisable to use micro-encapsulation (phlegmatization) techniques whereby a non-contaminating (organosilicon) binder is used to create bigger aluminium pellets from the tiny grain.

This phlegmatized powder is a smaller amount susceptible to explode or erupt.
The microencapsulated powder is safer in handling and use than powders, but care remains necessary thanks to the flammability of its organic solvent.

Although safety in handling and use is increased, a major disadvantage of pellets, aside from increased cost, is that they negatively affect the standard of the top product significantly.

AAC blocks don't show a homogeneous pore structure as is obtained by using powder.

This is due to the fact that microencapsulated aluminium cannot be dispersed so finely through the slurry and does negatively affect homogeneity in the mechanical properties (eg. compressive strength) of the blocks produced.

For the reasons mentioned above, this technique has been tested in the past but has never become widely used.

Aluminium paste

Another option is to use aluminium paste.

Aluminium paste is made from its powder and generally consists of microscopic flake shaped particles finely dispersed in an organic-solvent or aqueous-based carrier.
Just like aluminium pellets, paste is safer in handling and use than powder.

Also here care is still necessary, especially when paste based on a (flammable) organic-solvent is used.

Unlike powders and pellets, it is not possible to dose aluminium paste in batches without mixing the entire drum with water first.

This is due to the fact that paste is very sticky which makes it impossible to dose without watering it down.

Another problem with aluminium paste is that the standard of the aluminium brings uncertainty in the process.

Aluminium in paste already reacts with its solvent at the moment it is mixed.
From this moment until the moment it is mixed in the AAC production process, the aluminium reactivity will be lost slightly.

This makes that the amount of aluminium paste needed becomes uncertain for every casting and drum and may result in a high amount of aluminium waste and thus high cost.

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