Cement-sand plaster: composition and scope

The application of universal plaster is one of the stages of finishing work and performs a number of tasks. Plaster masks external defects of the wall and levels the surface for a "finishing" finish. Serves as a solid foundation for subsequent finishing work, and also reduces costs, allowing you to reduce the amount of work and limit yourself to minimal finishing: plastering and painting. Plaster improves the waterproofing of the surface and enhances the heat and sound insulation of the wall.

Cement-sand plaster is used for such works:

• building facade decoration;
• leveling the walls inside the premises for further decoration (rooms with high humidity or without heating);
• concealment of screeds and cracks both on the inside and on the front side;
• elimination of significant surface flaws.

The positive qualities of plaster include the following features:

• high strength;
• immunity to temperature changes;
• excellent moisture resistance;
• durability;
• good frost resistance;

• good adhesion (ability to stick) with certain types of surfaces: concrete, brick, stone, cinder block;
• the simple formula of the solution allows you to find all the necessary components in any hardware store;
• affordability, especially when preparing the solution on your own.

The negative aspects of working with cement-sand plaster include the following:

• working with the solution is physically difficult and tiring, it is difficult to level the applied layer;
• the hardened layer is very rough, it is not suitable for direct painting or gluing thin wallpaper without additional finishing;
• the dried surface is difficult to grind;

• increases the mass of the walls and, as a result, makes the structure as a whole heavier, which is especially important for small buildings where there are no powerful bearing supports and a massive foundation;
• poor adhesion to wood and painted surfaces;
• strong shrinkage of the layer requires at least two layers of finishing and cannot be applied in a layer thinner than 5 and thicker than 30 millimeters.

A standard solution consists of the following components:

• cement, depending on the brand of which the strength of the composition varies;
• sand - you can use only coarse (0.5-2 mm) sifted river or quarry;
• water.

When using fine-grained sand, the chance of cracking increases. The presence of impurities in the form of clay or earth reduces the strength of the hardened layer and increases the chances of cracking. If the grain size is larger than 2 mm, the surface of the solidified layer will be too rough. A sand fraction of 2.5 mm or more is used only for brickwork and is not suitable for plastering work.

The cement-sand mixture has a number of basic parameters that determine its properties.

• Density. One of the main characteristics determines the strength and thermal conductivity of the solution. The standard composition of the plaster, without the presence of impurities and additives, has a density of about 1700 kg / m3. Such a mixture has sufficient strength for use in facade and interior work, as well as for creating a floor screed.

• Thermal conductivity. The base composition has a high thermal conductivity of about 0.9 W. For comparison: a gypsum solution has three times less thermal conductivity - 0.3 W.
• Water vapor permeability. This indicator affects the ability of the finishing layer to pass the air mixture. Vapor permeability allows moisture trapped in the material under the layer of plaster to evaporate, so that it does not damp. Cement-sand mortar is characterized by vapor permeability from 0.11 to 0.14 mg / mhPa.

• Drying speed of the mixture. The time spent on finishing depends on this parameter, which is especially important for cement-sand plaster, which gives strong shrinkage, and therefore is applied several times. At an air temperature of +15 to + 25 ° C, the complete drying of a two-millimeter layer will take from 12 to 14 hours. With increasing layer thickness, the hardening time also increases.

The normal consumption of a cement-sand mortar with a standard composition at a layer of 10 millimeters is approximately 17 kg / m2. If a ready-made mixture is purchased, this indicator is indicated on the package.

When manually creating a mortar with a mixture consumption of 17 kg / m2 with a layer of 1 cm, one should take into account a water consumption of 0.16 liters per 1 kg of dry components and a ratio of cement to sand 1: 4. Therefore, to finish 1 m2 of surface, the following amount will be required ingredients: water - 2.4 liters; cement - 2.9 kg; sand - 11.7 kg.

To ensure a reliable base for plastering work, the wall must first be prepared. Depending on the thickness of the applied layer, the type of work surface, additional plaster reinforcement and other conditions to obtain a quality result, the following actions are performed:

• A special glue is applied to the wall in a thin layer, it has excellent adhesion (adhesion to the coating material), strength and will serve as a base for plaster. On top of the applied layer, a plaster mesh is applied - so that the edges of adjacent fragments overlap 100 millimeters. After that, using a notched trowel, the mesh is leveled and pressed into the applied adhesive. The dried layer will be a solid base for the cement-sandy plaster mortar.

• For additional strengthening of the plaster, a reinforced mesh is used. It attaches to the wall with self-tapping screws, creating a solid base for thick plastering or providing a quality plaster finish on wood and clay surfaces. Alternatively, wire can be used. It is wrapped between nails or screws driven into the wall. This method is cheaper, but a large amount of manual labor is costly in time and effort. Sheathing is more often used in small areas, where its ability to cover any area without cutting the mesh has its advantages.

• An adhesive primer is used to strengthen the bond to the concrete wall. Before applying it, notches and small chips are knocked out on the working surface using a perforator or ax.
• When applying new layers of plaster on top of existing ones, the older ones should be checked for reliability by carefully tapping them with a hammer. The exfoliated fragments are removed, and the formed cavities are cleaned with a brush from small pieces.

• When working with porous concrete materials, the surface is treated with a hydrophobic primer before plastering.This is done to reduce the absorption of moisture into the work surface from the plaster solution, which leads to its dehydration, quick hardening and a decrease in strength.

The ready-made mixture is easier to use, it is advisable to buy it for small-volume work. But if it is necessary to cover large areas, the difference in price grows into a significant amount. In order for the solution to meet all standards and give the desired result, you need to correctly select the proportions of the ingredients. The main indicator here is the brand of cement.

There are such options for plastering mortar:

• "200" - cement M300 is mixed with sand in a ratio of 1: 1, M400 - 1: 2, M500 - 1: 3;
• "150" - cement M300 is mixed with sand in a ratio of 1: 2.5, M400 - 1: 3, M500 - 1: 4;
• "100" - cement M300 is mixed with sand in a ratio of 1: 3.5, M400 - 1: 4.5, M500 - 1: 5.5;
• "75" - cement M 300 is mixed with sand in a ratio of 1: 4, M400 - 1: 5.5, M500 - 1: 7.

To mix the cement-sand mortar, you need to perform a number of tasks:

• Sift the sand even if it seems clean.
• If the cement has caked, it is not recommended to use it, but it is possible that it can also be sieved to remove the lump. In such a mixture, the sand content is reduced by 25%.
• First, cement and sand are combined dry, then they are mixed until a relatively homogeneous dry mixture is achieved.

• Water is added in small portions, in between, the solution is thoroughly mixed.
• Next, additives are added - for example, plasticizers.

Correct application of the putty in compliance with all recommendations is one of the components of high-quality finishing work.

To do this, you need to do the following:

• Before applying the plaster, the surface is treated with a primer - this will provide a stronger adhesion to the mortar. Then the wall is allowed to dry.

• Guide beacons are placed on the surface, along which in the process you can determine the boundaries of the plane being created. Their height is set according to the level, in shallow areas they are replaced with putty slaps. The material for the lighthouses is often a metal profile, fixed to a mortar or slats, or wooden bars on self-tapping screws. The spacing between the beacons is the length of the leveling rule minus 10-20 cm.
• To apply a standard layer (10 mm) of plaster, a trowel is used, a thick one - a ladle or other volumetric tool.

• A new layer is applied 1.5-2 hours after the completion of the previous one. It is applied from bottom to top, completely overlapping the previous one. It is more convenient to work by breaking the wall into sections of a meter and a half. Further, the plaster is stretched and leveled by the rule. This is done by tightly pressing the tool against the beacons, with a rise and a slight shift to the left and right. Excess plaster is removed with a trowel.
• When the mortar has set, but has not yet hardened, it is time for grouting. It is carried out in a circular motion with a float in places with irregularities, grooves or protrusions.
• For interior work, final hardening occurs within 4-7 days after application under normal humidity conditions. For outdoor work, this interval increases and can reach 2 weeks.

To improve plastering work, it is worth delving into various subtleties, for example, machine application. To prevent cracks during fast setting, the layer is moistened from time to time with water from a spray bottle or covered with a film. Also, there should be no drafts, the temperature should not be elevated or fluctuate. When small cracks appear, additional grouting of problem areas is performed.

It is inconvenient to use in curved places, recesses or in the presence of various obstructing objects, for example, pipes.For such purposes, a suitable template is made, and the beacons are set according to its dimensions at the required interval. A corner is used for working with corners; it can be factory or manual.

In the next video, you can clearly see how to prepare a solution for plastering walls.