Plastering of Gas Silicate Blocks

Despite all the advantages of gas silicate as a material for external walls and partitions, one major drawback of many construction systems remains unavoidable: the need for wet processes such as plastering and finishing. This article discusses how to apply plaster correctly and with minimal waste of materials and time.

Required Tools and Equipment

For high-quality plastering work on gas silicate blocks, you will need the following tools:

• Rule – used for leveling mortar between guides;
• Guide profiles – for wall leveling;
• Float – for finishing the applied plaster layer;
• Float trowel – for removing excess mortar from large surfaces;
• Plumb bob – for checking wall geometry;
• Bucket or trowel for applying mortar to the wall;
• Drill with mixing attachment for stirring the prepared mix;
• Plastic container for mixing mortar.

Surface Preparation

Before starting the plastering process, any wall surface must be cleaned of dust and dirt. If necessary, it should not only be cleaned but also washed with water. To improve plaster adhesion, you may create grooves on the gas silicate block using a grinder. This increases the contact area between the plaster layer and the wall, thereby enhancing adhesion.

Before applying plaster, check and assess wall flatness for protrusions and depressions. This can be done using a plumb bob or laser level. If the inspection reveals significant differences (1 cm or more) on the plastered surface (common in gas silicate block masonry), install guide profiles from floor to ceiling based on laser level marks or plumb bob lines. The space between them should be filled with the same plaster mix or cement-sand mortar. The mortar between guides is leveled with a straightedge to achieve a flat surface. After this step, walls should be even and ready for plastering.

Mortar Preparation and Plaster Application

Standard plaster mixes are designed for application up to 2 cm thick. A thicker layer becomes brittle and excessively crumbly due to uneven moisture evaporation. If a thicker layer is required for any reason, apply it in two or three stages. Each subsequent layer may only be applied after the previous layer has fully dried.

An alternative solution is using high-strength plaster mix, though it is significantly more expensive than traditional plasters.

For internal plastering of gas silicate, gypsum-based mixes (or gypsum-lime) are used. For external work and areas with high humidity, cement-based mixes are preferred. The key difference is that cement mortar becomes water-resistant after drying, while gypsum-based mix does not.

Plaster mix is prepared in a special plastic container (a regular bucket may be used, but a shallow tray or trough is better for easier mixing). Dry mix is combined with water, and a drill with a mixing attachment is used to stir the mixture to a thick, creamy consistency.

Plaster is applied to the wall using a trowel, a special plasterer’s bucket, or a regular trowel. It is most convenient to apply the mix and immediately smooth it with circular motions using a float. Excess material is removed with a plaster float.

The first layer applied is a base coat. Then, two or three main layers are applied, ensuring full drying of the previous layer before applying the next. Applying plaster in one thick layer (1 cm or more) may cause it to slide off due to its weight, and after drying, it may crack.

The working time for gypsum-based plaster is approximately 20 minutes; for cement-based, slightly longer. This means that the mix should be prepared gradually, matching the area you can realistically finish within its usable lifespan, before it starts to set. Plaster that has already started to set and harden before application must be discarded.

All tools must be rinsed immediately after use, as removing hardened residues—especially cement-based plaster—is labor-intensive and not always feasible.

Quality of the applied layer is checked using a long straightedge (from ceiling to floor), placed vertically and horizontally across different wall sections. Any protrusions or dents will be visible relative to the straight edge. Acceptable deviations are within 5–7 mm, as they are not visible to the naked eye.