How to plaster aerated concrete?

When building walls.

Aerated concrete and gas silicate are modern building materials that resemble foam concrete in their structure, but differ in the structure of air bubbles inside the material.

Due to the hollow structure, aerated concrete and gas silicate absorb moisture well. Therefore, walls made of aerated concrete (gas silicate) cannot be left without exterior finishes.

Between themselves, aerated concrete and gas silicate differ only in that gas silicate is made from lime and quartz sand, and aerated concrete is based on cement (50-60% by weight). The basic physical characteristics of both materials are very similar.

In this article, let's look at the basic requirements for plaster used to finish aerated concrete walls.

What is the difference between aerated concrete and foam concrete?

In foam concrete, air bubbles are closed and isolated from each other, due to which its ability to withstand wetting is significantly increased.

Aerated concrete, on the contrary, has an open structure of air bubbles, due to which moisture quickly penetrates into the material.

It is because of this that aerated concrete has lower rates of resistance to heat transfer and frost resistance and needs high-quality exterior and interior finishing, which allows minimizing the disadvantages of open porosity of the material.

Let's see how to properly plaster aerated concrete walls and what plasters should be used for this.

When is the best time to plaster aerated concrete walls?

Many craftsmen start plastering immediately after the walls are erected. In the case of aerated concrete, this can lead to many different problems.

When building walls from aerated concrete, it is best to produce plaster for the next season.

If it is necessary to complete the finish immediately after construction, then you should carefully consider the choice of plaster, which should have high plasticity and freely let water vapor out.

A feature of the thermal physics of aerated concrete is rapid wetting, long drying and the inadmissibility of oversaturation with water vapor.

It is recommended that when erecting buildings from aerated concrete, first do the interior decoration, and only then the exterior. For example, interior decoration should be done in the fall, and exterior plastering in the spring of next year.

Rare exceptions can be houses built on the sea coast, when it is necessary at all costs to protect the walls of aerated concrete from damp winds in the first place.

Another option for finishing can be the simultaneous execution of finishing from the outside and from the inside. But this option is the least preferred.

There is no need to hurry. After construction, aerated concrete should dry well, and the process is not fast. As a result of finishing heavy cement plaster on non-dried aerated concrete walls during the cold period, when it becomes warm inside the building, water vapor begins to move to the outer edge of the wall.

But because of the heavy vapor-tight plaster, they have nowhere to go and water vapor falls out in the form of condensate, the freezing of which can lead to the complete separation of the plaster from the wall.

A particularly unfavorable time for plastering walls made of aerated concrete blocks is the period from November to March.

Is it necessary to insulate the walls of aerated concrete from the outside?

When making walls from aerated concrete blocks, their thickness is calculated based on local climatic conditions. As a rule, a properly selected wall thickness does not require its external insulation. Moreover, in some cases this can lead to the destruction of the building.

For wall insulation, many advise using expanded polystyrene, believing that this will improve the thermal properties of the building. However, material impervious to water vapor can cause condensation to form at the boundary of the insulation and masonry from water vapor released from aerated concrete blocks. Freezing of condensate can lead to the appearance of cracks in the walls and further to their destruction.

That is, one should not take the thickness of the walls so that they still need to be insulated. Aerated concrete has high thermal performance and does not require the construction of thick walls to save heat inside the building.

So, for country houses in the middle lane, an aerated concrete wall thickness of 300 mm will be quite sufficient. For the construction of a bath, a wall thickness of 200 mm is enough, and in warmer regions - 100 mm.

In the case of building insulation with the help of an external laying of a layer of expanded polystyrene, its thickness should be chosen so as to prevent condensation at the boundary of the masonry and insulation. The thickness of the insulation layer made of expanded polystyrene or polyurethane should be taken at least 80 mm, and the thermal resistance of the insulation layer should be at least 50% of the total thermal resistance of the wall.

How to plaster aerated concrete?

It would seem that the easiest way is to prepare an ordinary cement-sand mortar and plaster the walls. However, there may be problems that have already been mentioned above.

As a result of the different physical properties of the cement plaster and the wall of gas silicate blocks, cracking and peeling of the plaster can occur.

Plaster for aerated concrete should pass water vapor well, not get wet, have good adhesion to the surface of aerated concrete blocks and high frost resistance.

Lime-cement plaster for aerated concrete

All the necessary properties are possessed by light thin-layer plasters, specially created for finishing walls made of aerated concrete.

An example of such a plaster mixture is Baumit HandPutz sand-lime plaster for hand finishing with a grain size of 1 mm, available in 25 kg bags.

The main physical properties of lime-sand plaster for aerated concrete are given in the table:

Before plastering aerated concrete walls with this plaster, it is recommended to apply a spray of Baumit Vorspritzer, which is part of the plaster, performed as a thin glue line, on the cleaned surface of aerated concrete.

Acrylic plaster for aerated concrete

For a more decorative finish, acrylic plasters are used. It is a very durable material that can be used wherever extra strength is required.

However, in terms of vapor permeability, acrylic plasters are inferior to lime-sand plasters, and as a result of finishing with such plaster, water vapor can accumulate in the thickness of the wall.

To prevent this, you should use reinforced waterproofing of the wall from the inside, as well as an improved ventilation system.

In addition, acrylic plasters are combustible and therefore used with restrictions.

An example of acrylic plaster for aerated concrete is Bolix and Bolix Complex plaster.

Silicate plaster for aerated concrete

There are also silicate plasters for aerated concrete. An example of such a plaster is the ready-to-use paste-like mixture Baumit SilikatTop based on potassium liquid glass.

Silicate plasters have excellent vapor permeability and low water absorption, which is very important when finishing walls made of aerated concrete blocks.

The disadvantages of such plasters include an extremely meager choice of colors and loss of appearance due to dust settling on the surface of the plaster.

Silicone plaster for aerated concrete

Another type of plaster mixtures used in the decoration of walls made of aerated concrete blocks is silicone plaster based on organosilicon polymers.

Silicone plaster is resistant to weathering, it practically does not get wet (hydrophobic), has high vapor permeability, is easy to apply and does not lose its attractive appearance over time.

The only disadvantage of such plaster is the higher price. Such plaster can no longer be attributed to the "economy class".

Plastering walls with lime-sand plaster

When using a mixture of Baumit HandPutz, the sequence of actions will be as follows.

A bag of the mixture (25 kg) should be poured into a container with 6-7 liters of clean water and thoroughly mixed with a low-speed mixer. Mixing time is 3-5 minutes.

After that, apply a Baumit Vorspritzer spray on the wall, which promotes better adhesion of the plaster to the surface and reduces the moisture absorption of aerated concrete.

A layer of plaster is applied with a trowel and drawn out with a rule. After setting, it is rubbed with a grater.

Within 2 days, the applied plaster should be protected from rapid drying.

The hardening time of the layer is 10 days per 10 mm thickness. At this time, the plaster should be protected from mechanical damage.

All work must be carried out at an air temperature not lower than zero degrees.

For better protection against getting wet, a thin layer of water repellent can be applied over the plaster. If prolonged rains are not uncommon in your area of ​​​​residence, then such a protective layer will significantly shed the life of the plaster.
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