Foam block plastering — main types of mixtures and application technology

Foam blocks are a common building material because of their low cost, affordability, and ability to withstand heat. But a good plaster coating is necessary to give them a polished appearance and shield them from the elements. This procedure strengthens the structure and increases the lifespan of the foam blocks in addition to improving their appearance.

Selecting the proper plaster mixture is essential because it influences the walls’ finish and durability. Every type, from conventional cement-based mixtures to contemporary, flexible alternatives, has advantages and uses specific to it. Gaining an understanding of these variations will enable you to complete your project with optimal outcomes.

Plastering foam blocks requires careful planning and execution. Understanding the proper procedures will guarantee a seamless, long-lasting finish whether you’re doing it yourself or hiring a professional. You will learn about the different kinds of plaster mixtures that are available and how to apply them to foam block walls using this guide.

Main Types of Plaster Mixtures	Application Technology
Cement-Sand Plaster	Apply with a trowel in layers, smooth each layer before the next one, ideal for exterior walls.
Gypsum Plaster	Use a steel float for application, work quickly as it sets fast, best for interior walls.
Lime Plaster	Spread evenly with a hawk and trowel, allow each layer to dry completely, provides a breathable finish.
Acrylic Plaster	Apply with a roller or sprayer, smooth out with a trowel, known for flexibility and durability.

What is a foam block

Concrete was utilized in the construction of numerous structures following the invention of Portland cement two centuries ago. They found artificial stone to be attractive and kept attempting to alter it, giving it different characteristics. Because of its structure and method of production, the new type of concrete that emerged at the start of the last century was dubbed foam concrete.

Cement (the primary binder M ≥ 400), sand (filler), foam concentrate (made from bone glue, protein, or pine rosin), and water are the main ingredients of foam concrete.

Two different kinds of technologies are used to create foam concrete:

• foam whipped in a foam generator is fed into a cement mortar, mixed to evenly distribute air bubbles in the volume;
• abundant foam is whipped in a mixer, then sand and cement are added to it.

The resultant mass is either poured into individual molds to form completed blocks (cast foam concrete) or poured into one large block that is sawed into commercial blocks (sawed foam concrete) after it has hardened.

The following characteristics set apart finished blocks made by casting and sawing:

• cast — often have chips (obtained during stripping), a bumpy uneven surface contaminated with lubricating oil substances, low adhesion;
• sawed — have a rough surface, more precise geometry, good adhesion.

Foam blocks have closed pores and a distinct chemical makeup from aerated concrete. Air pores give products heat-insulating qualities by lowering their weight and density. Additionally, the use of polystyrene foam—such as foam plastic and penoplex—as insulation is made possible by minimally reduced vapor permeability.

Why do you need foam block plastering

The blocks’ hydrophobic layer is harmed in transit and during laying. Two significant drawbacks of foam concrete are its susceptibility to moisture and decreased adhesion.

When open foam blocks are exposed to atmospheric carbon dioxide, they become more brittle and experience carbonation shrinkage. Additionally, the porous material’s abrasion and compressive strengths are low. Walls therefore require protection, especially those that are external.

Plastering is one of the easiest and most accessible forms of protection currently available.

Let’s look at the methods used to plaster foam block walls.

What to plaster foam blocks with

The variety of plastering compounds available is somewhat restricted by the aforementioned drawbacks of foam concrete. In other words, foam block plastering needs to be robust, lightweight, water-resistant, and highly adhesive.

Let’s look at the various types of plasters so that you can choose the best plastering material by navigating the vast array of options available.

Plaster mixtures are separated based on their uses (functions) into:

• simple, used for rough (leveling) work;
• special, used both for leveling and for solving certain problems, for example, for noise reduction (soundproof plaster), X-ray protection (barite), insulation or waterproofing (thermal insulation and waterproofing plaster, respectively);
• decorative, used with design purposes.

The foam block’s plastering takes place at the usage location.

• internal (under sparing operating conditions should contribute to the creation of favorable living conditions in the room, for example, to be vapor permeable, environmentally friendly, and prevent the appearance of molds of mold);
• external (should have high atmosphere resistance, frost resistance, not fading under the influence of UFL, have increased strength, etc. D.);
• universal, which is used for interiors and for facades.

Plastering compounds are made using the fundamental materials (binders):

• acrylic (vapor -permeable, waterproof, easily used, plastic, durable, quite durable, highly adhesive, especially good for decor);
• mineral – gypsum, cement, etc. (used for rough finishing, t. to. they are afraid of vibrations and shrinkage, are popular due to their lower cost, availability of components for DIY);
• silicate (vapor-permeable, water-repellent, durable, very fast-hardening mixtures, perfect for plastering facades);
• silicone (vapor-permeable, durable, elastic, long-lasting, water-repellent, but expensive, so they are often used for interior finishing).

The blocks take no more than 28 days to reach their peak strength. They can be placed in walls earlier since they can be sold before this time. The blocks’ shrinkage in this instance is clearly visible. It is best to wait to use mineral plasters that are not shrinkage resistant until the shrinkage process is finished.

What to plaster foam blocks inside the house with

Speaking about plastering foam blocks from the inside, we will emphasize the compositions for dry rooms and those with moisture levels above average because there is a "big difference" from room to room. The best materials for the former are gypsum compositions and lime plasters with perlite sand. For the latter, consider waterproof materials like cement, silicone, acrylic, or cement-polymer.

Where there are sources of open flame, acrylic mixtures should not be used. Additionally, using them in living rooms and bedrooms is not recommended.

How to plaster foam blocks outside the house

For foam blocks outside the building, experts advise using silicate, cement (with specific additives), cement-lime, or acrylic plaster. Plaster mixtures are chosen based on the polystyrene coating if the home’s owner adds extra insulation in accordance with the type of wet facade. Foam block silicone plaster works well for finishing facades as well.

Necessary tools and materials

To begin plastering, get ready:

• spatulas (notched, wide and narrow);
• grater;
• mixer (for mixing the solution);
• trowel;
• level;
• roller;
• rule;
• paint brushes;
• masking tape;
• containers;
• rags.

Moreover, buy the necessary quantity of finishing supplies:

• dry plaster mix (otherwise – SS) or ready-made compositions (in buckets);
• primer for foam block under plaster;
• adhesive composition (for the device of a wet facade);
• reinforcing mesh;
• corners;
• sandpaper;
• beacon strips;
• perforated corners.

Surface preparation

Remove any coating that adheres weakly from old walls. Prior to plastering, all chips, cracks, and vacant areas of masonry joints are fixed. Grease and rust stains are removed by washing and applying the proper treatment. The same is applied to biodeterioration foci. Extrusions are ground down or removed. Sandpaper or a grater is used to treat the surface of cast foam blocks that have been cleaned of stains in order to increase the plaster coating’s adherence to foam concrete walls. Make sure all dust is gone.

Plastering foam blocks requires careful mixture selection and application to produce a smooth, long-lasting finish. Because foam blocks are porous and lightweight, they require specific plaster mixtures that are meant to bond well and not crack. Knowing the different kinds of mixes that are available and how to apply them correctly will guarantee that your foam block walls are long-lasting, well-protected, and visually beautiful.

How to properly plaster walls made of foam blocks

The entire wall’s thermal conductivity will change when the foam blocks are plastered in place. As a result, you should become familiar with ideas like the dew point.

Moisture condensation occurs when air and surface come into contact at a specific temperature differential. The temperature at which air moisture starts to condense is known as the dew point. One can locate the dew point (DP) at:

• a — on the wall surface in the room;
• b — in the thickness of the wall, shifting towards the room;
• c — closer to the outer surface.

In case "c," condensation does not form if the walls are not insulated; in cases "a" and "b," condensation on the wall of the room may occur if the outside temperature drops significantly.

The dew point "drifts" in the insulation layer and the walls stay dry if they are only insulated from the outside. The DP may be found inside the wall closer to the room and move to the boundaries or into the thickness of the internal plaster in frosts if the walls are insulated from the inside and the outside in an incorrect ratio of the thicknesses of the internal and external thermal insulation. This results in issues with mold growth, moisture, and plaster coating integrity.

The "golden rule," which states that the thickness of the external "warm" plaster layer should be twice as thick as the thickness of the room’s insulating plaster coating, can also be used.

Expanded polystyrene should be used to insulate the facade for the same reasons.

How should foam blocks be plastered correctly?

Novices ask questions like this. The following guidelines must be adhered to in order to guarantee that the plaster coating holds firmly and is long-lasting:

1. Do not plaster foam block walls at temperatures outside the range of +5 °C … +30 °C.
2. After the walls have been erected, finishing work should not begin for at least a month, allowing the blocks to shrink.
3. The surface of the walls is pre-cleaned.
4. Priming of the surface of the blocks and the surface of the plaster coating is mandatory.
5. When performing multi-layer plastering, each layer is dried.
6. Reinforcement is mandatory for cast blocks.
7. Drafts are not allowed inside the premises. Fans and heaters are not used to dry the coating.
8. Air humidity – no more than 60%.
9. The plastered facade is covered with precipitation and sun rays.
10. When preparing the solution, strictly follow the manufacturer"s instructions.
11. De-energize the electrical wiring.

There are two plastering methods employed:

• thick layer plastering (some types of compositions allow applying a layer 3 cm or more thick);
• multi-layer plastering.

Primer

Prior to plastering, the block surfaces need to be primed. The primer reinforces the block material’s surface, increases the adherence of the solution to the base, and seals open pores to prevent the mixture from drying out. Certain compositions exist, such as Knauf primer designed for use with foam blocks.

When selecting pieces, take into account:

• for internal surfaces – relative humidity;
• for facades – maximum temperatures;
• for both – wall roughness, porosity level, type of finishing coating.

Experts advise using compositions like "Starateli" and "Gazobeton-kontakt" that deeply penetrate foam concrete.

Using a roller, the primer composition is applied in two or three layers over two or three passes. Every layer has dried.

Fastening the plaster mesh

Fiberglass mesh is used when plastering foam blocks to improve adhesion. Since rust spots appear on top of the plaster coating when metal corrodes, they are attempting to avoid using metal mesh. Underneath the mesh, there needs to be a plaster solution. This is accomplished by using a spatula to recess the mesh fabric into the applied solution’s initial layer. Joints: 5 cm of overlap.

The steps are as follows: apply a layer of adhesive solution (5–10 mm), level it with a spatula, add a mesh, and use a spatula to smooth the surface from the center outward. Using a notched trowel, the solution that is visible above the mesh is "lined" horizontally. In cases where the solution is absent, it is implemented in addition. The most important aspect is that a solution covers the whole mesh, making it lie flat.

Installation of beacons

Beacons are plastic, metal, wooden, or mortar guide strips whose surfaces are used to cut and level the mortar with a rule in order to level the walls to the necessary coating thickness of more than 7 mm.

Beacons are affixed to the wall in 100–130 cm intervals so that the tops of the beacons form a vertical plane within the plaster coating. A plumb line, building level, or laser level are used for this. The intermediate strips are placed along a nylon cord that is pulled between the extreme beacons once they have been mounted on the wall. They start plastering the foam block walls after putting in the beacons and getting the mortar ready.

Laying plaster in several layers

Large thicknesses of leveling coating require plastering to be applied in multiple passes, or distinct layers, with a different mixture being prepared for each layer. Applying the solution requires using a trowel or spatula. The wall is sprayed with water prior to plastering. Applying layers to plaster is similar to leveling plaster TsPS.

1. Spray – a solution of semi-liquid consistency is thrown onto the wall with a ladle or trowel without leveling with a layer of ~5 mm, the surface is “lined” with the corner of a spatula for better adhesion to the soil layer.
2. Primer (basic) – a solution of normal viscosity is applied with a spatula or trowel, then leveled with a rule (along the beacons) or a spatula.
3. Covering (grouting) – a layer up to 2 mm thick is made with a more liquid solution with fine filler. Apply with a spatula on wet soil. Immediately leveled.

Grout

Grouting should begin as soon as the foam block plaster stops sticking to your fingers. Use a grater with rounded corners to accomplish this. Applying slight pressure, move the tool in circular motions parallel to the wall on the layer’s surface. The milk that is sticking out from the float’s edge falls into the other depressions. Take out the extra from the float on a regular basis.

Grouting "in acceleration" is done in two passes after grouting "in a circle." Float movements are linear, with a vertical first pass and a horizontal second pass. If paint or wallpaper will be the final coating, grouting must be done.

Popular manufacturers

Prominent producers of plaster for foam concrete include:

• Livna – cement Profit Kontakt MN (for machine plastering);
• Ceresit – cement mixture for cellular concrete ST 24, as well as ST 24 light (for facades and interior walls);
• Osnovit – cement Belsilk, Startwell, gypsum Econsilk, etc.;
• Atlas Concern – lightweight mixture with perlite KB-TYNK;
• Knauf Company – Sevener plaster-adhesive is used as a transition layer.

Nuances of plaster

Plastering walls using foam blocks is acceptable if plastering aerated concrete walls from the inside and outside cannot be done at the same time. As a result, extra water evaporates outside the blocks rather than being absorbed.

Plastering foam blocks correctly is essential for your walls’ longevity and aesthetic appeal. Selecting the proper mixture guarantees a long-lasting, smooth finish and a strong bond. Plaster that is enhanced with polymers, gypsum, or cement each has advantages and applications unique to them.

Knowing the subtle differences between each type of plaster mixture enables you to choose the best option for your project. Even though the application process is simple, the best results need close attention to detail from surface preparation to the final coat.

You can give your foam block walls a long-lasting and beautiful finish by using high-quality materials and the suggested techniques. This improves the building’s overall aesthetic appeal in addition to maintaining its structural integrity.

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