Gypsum binder: characteristic, properties, production and application

Building and other materials from gypsum are used in various branches of the national economy. They have not surprised anyone for a long time. But few people think about what actually is a plaster cement, which serves as raw material for him and how it works. But for the production of all building materials (plasters, masonry mortars, plaster sheets) and other parts, you must first prepare the raw materials. After all, the characteristics of the finished material depend to a large extent on the quality of the raw materials used.

Concept and composition

The binder of gypsum is an air material, which consists for the most part of a two-water gypsum. The composition of gypsum also complements the natural anhydride and certain industrial wastes, which include calcium sulphide.

The same group also includes combined substances. In their composition - semi-aquatic gypsum, lime, blast-furnace slag, cement.

Raw materials for production are rocks containing sulfates. GOST specifies that for the production of gypsum binder only gypsum stone (corresponding to all the requirements that are required by GOST 4013) or phosphogypsum, which also meets the requirements of regulatory documents, can be used.

Characteristics of gypsum binders

Gypsum mortar must be used until it is completely solidified. It can not be stirred after the crystallization process has already begun. Stirring causes the destruction of the formed bonds between the carcass crystals. Because of this, the solution loses its astringent abilities.

Products made of gypsum are not water resistant. But the producers of the material found a way out of this situation. Scientists have determined that various additives of gypsum binders can increase this figure. Therefore, various substances are added to the composition of the material: lime, ground blast furnace slag, carbamide resins, organic liquids containing silicon.

The use of gypsum materials does not require the use of additional fillers. They do not shrink, cracks on the machined surface will not appear. Gypsum binders, on the contrary, increase in volume after full hardening. In some situations wooden sawdust, fire, pumice, expanded clay and other materials are added.

Another feature - gypsum materials accelerate the process of corrosion of ferrous metals (nails, reinforcement, wire and so on). This process occurs even faster in wet conditions.

The astringent of gypsum quickly absorbs moisture and loses its activity. Therefore, certain rules must be observed when storing and transporting. The material can only be stored in a dry place. Even with this rule, after three months of storage, the material will lose about thirty percent of its activity. Transport the material in bulk or packed in a container. It is important to protect it from debris and moisture.

Production

For this process, you must perform the following processes:

• Crushing of natural gypsum material;
• Drying of raw materials;
• Effect of temperature.

Gypsum stone is fed to the bunker, from where it enters the crusher. There is its grinding into parts, the size of which does not exceed four centimeters. After crushing, the material is sent to the feed hopper. From there, he enters the mill in equal parts. There it is dried and crushed to a smaller fraction. Drying at this stage is necessary to accelerate and facilitate the process of crushing the material.

In the mill the powder is heated to ninety degrees. In this condition, it is transported to a gypsum-making boiler. It is there that water is released from the substance in the firing process. This process begins with low temperatures (about eighty degrees). But the water from the material is best left at a temperature range from one hundred and ten to one hundred and eighty degrees.

The entire process of temperature processing is divided into two stages. First, the material is kept in the digester for three hours. There the water is removed, and the two-water gypsum turns into a semi-aquatic. All this time, the gypsum is mixed for homogeneity of heating. At the end of this time, the substance in the warmed-up state is sent to the so-called bumper of longing. It is no longer heated. But due to the high temperature of the substance itself, the dehydration process continues there. This takes about another forty minutes. After that, the binders are considered ready. And they are sent to the warehouse of finished products.

Material hardening

Hardening of gypsum binders occurs when the powder is mixed with water. At the same time, a plastic mass forms, which solidifies for several minutes. From the chemical point of view, there is a process opposite to what happened in the production process. Only it happens much faster. That is, semi-aqua gypsum adheres water, resulting in the formation of a two-gypsum gypsum substance. The entire process can be divided into three stages.

In the first stage, the semi-aquatic gypsum substance dissolves in water with the formation of a saturated solution of two-water gypsum. The dihydrate has a high solubility index. Due to this, the process of supersaturation of the solution takes place very quickly. As a result - precipitation, which is a dihydrate. These fallen particles are glued together, thereby starting the setting process.

The next stage is crystallization. Individual crystals of matter begin to unite as they grow and form a strong framework. As the drying (removal of moisture), the bonds between the crystals become stronger.

Change in setting speed

The setting process can be accelerated or, conversely, slowed down as necessary. Do this with the help of additives that are injected into gypsum binders.

Types of additives that accelerate the setting process:

• Substances that increase the solubility of hemihydrate: sodium or potassium sulfate, sodium chloride and others;

• Substances that in the reaction will be the center of crystallization: salts of phosphoric acid, crushed natural gypsum and so on.

The most commonly used ground gypsum stone. Its particles serve as centers of crystallization, around which the crystal will grow further. Greater efficiency is characterized by a "secondary" gypsum. It is understood as a gypsum, which already passes the stage of setting and hardening of calcium sulphide. To this form it is possible to carry the broken and crushed products.

Slow down the process of setting the following substances:

• Increasing plasticity of the dough: a solution of joinery glue in water, coniferous infusion, calc-glutinous emulsion, LST and so on;

• The growth of crystals is prevented by a film that forms on the grains of semi-aquatic gypsum under the influence of such substances as: borax, ammonia, keratin moderator, phosphates and borates of alkali metals, purple alcohol and others.

It should be noted that the introduction of accelerating additives adversely affects the strength of gypsum. Therefore, use them with caution and add in small amounts.

The setting time largely depends on the quality of the feedstock, the time and storage conditions, the temperature at which the material is connected to water, and even the time for mixing the solution.

Too short setting time is usually attributed to the presence in the material of the particles of the dihydrate that remained there after calcination. The setting time will also increase if the gypsum material is heated to about forty-five degrees. If the temperature of the material is increased even more, the process, on the contrary, will slow down. Long-term mixing of the gypsum mixture will lead to an acceleration of the setting process.

Differences between theory and practice

A special feature of the hardening process is that gypsum, unlike other binders, increases during solidification in volume (up to one percent). Due to this, for hydration of the semi-aquatic substance, approximately four times more water is needed than should be in theory. The theory of water requires approximately 18.6% of the mass of the material. In practice, water is taken to obtain a solution of normal density in an amount of up to seventy percent. To determine the water requirement of the material, determine the volume of water as a percentage of the mass of the material itself, which must be added to obtain a solution of normal density (cake diameter 180 + 5 millimeters).

Another difference of practice is that when the excess water is removed during drying, pores are formed in the material. Due to this, the gypsum stone loses its strength. Eliminate this point by additional drying. Products from gypsum are dried at a temperature not exceeding seventy degrees. If you increase the temperature even more, the dehydration reaction starts.

Effect of temperature on the substance obtained

To produce a gypsum binder, gypsum stone is exposed to high temperatures. Depending on the value of this temperature, the gypsum substance can be of two types:

• Low-baked, for the production of which the processing of raw materials occurs under the influence of temperature from one hundred and twenty to one hundred and eighty degrees. The raw material in this case is most often a semi-aquatic gypsum. The main difference of this material is the high rate of solidification.

• High-burning (anhydrite), which are formed as a result of the action of high temperature (over two hundred degrees). This material hardens longer. It takes more time to grasp, too.

Each of these groups, in turn, has several different materials included in it.

Types of low-binder

The gypsum binder of this category includes the following materials:

• Building gypsum. For its production, it is necessary to select the raw materials correctly. The production of gypsum for construction works is permissible with the use of a binding grade of 5 or higher as raw materials, the rest of which on the screen is no more than twelve per cent. For the manufacture of building products, an astringent, belonging to the brand from the second to the seventh, is suitable, irrespective of the setting time and the degree of grinding. Decorative elements are made of materials of the same species. With the exception of coarse substances and slowly seizing. Gypsum plaster mixes are made of substances of 2-25 grade, except astringent with coarse grinding and fast-hardening.

• High-strength gypsum can be characterized by one of several brands (with indices from 200 to 500). The strength of this material is about 15-25 MPa, which is much higher than that of other species.

• Forming gypsum is characterized by a high degree of water demand and high strength in the hardened state. From it, products from gypsum are obtained: ceramic forms, porcelain-faience elements and so on.

Anhydrite materials

This species, in turn, forms two substances:

• Anhydrite cement, obtained by processing the temperature to seven hundred degrees;

• Estrich-gypsum, formed under the influence of calcium sulfate temperatures above 900 degrees.

The composition of gypsum anhydrite includes: two to five percent lime, a mixture of sulfate with vitriol (copper or iron) to one percent, three to eight percent dolomite, ten to fifteen percent blast furnace slag.

Anhydrite cement differs by a slow grasp (from thirty minutes to a day). Depending on the strength it is divided into the following brands: M50, M100, M 150, M200. Cement of this type is widely used in construction. It is used for:

• Manufacturing of glutinous, plaster or masonry mortar;

• Manufacturing of concrete;

• Production of decorative elements;

• Manufacturing of heat-insulating materials.

Estrich-gypsum has the following characteristics:

1. Slow grasping.
2. Durability up to twenty megapascals.
3. Low heat conductivity.
4. Good sound insulation.
5. Resistant to moisture.
6. Frost resistance.
7. A small degree of deformation.

These are the main, but not all, advantages that estrich-gypsum possesses. Its application is based on these indicators. It is used for plastering walls, producing artificial marble, when erecting a mosaic floor and so on.

Binding of astringent species

Properties of gypsum binders allow us to divide them into several different groups. Several classifications are used for this.

By setting time, the following groups are distinguished:

• Group "A". It includes astringents, which quickly seize. This takes from two to fifteen minutes.

• Group "B". Knitting materials of this group grasp for a time of six to thirty minutes. They are called normally grasping substances.

• Group "B", which includes slowly seizing binders. It takes more than twenty minutes to grasp. The upper limit is not normalized.

The fineness of grinding is determined by the particles remaining on the sieve. This is due to the fact that gypsum binders always remain on a sieve with a cell size of 0.2 millimeters. GOST indicates the following groups:

• Coarse grinding or the first group indicates that up to twenty-three percent of the material remains on the sieve.

• Average grinding (the second group), if on the sieve there are not more than fourteen percent of the binder.

• Fine grinding (third group) indicates that the remainder of the material on the sieve does not exceed two percent.

The material is tested for strength in bending and compression. To do this, gypsum mortar is prepared bars with a size of 40 x 40 x 160 millimeters. Two hours after manufacturing, when the crystallization and hydration processes are completed, tests are started. Gypsum binders (GOST 125-79) for strength are divided into twelve grades. They have indices from two to twenty-five. The value of the strength limit, depending on the brands, is collected in special tables. It can be seen even in the GOST itself.

The main parameters and types of material can be identified by its labeling. It looks something like this: G-6-A-11. This label will mean the following:

• G is a gypsum binder.
• 6 - grade of material (meaning that the strength is more than six megapascals).
• A - determines the type of setting time (i.e., fast-hardening).
• 11 - indicates the degree of grinding (in this case, the average).

Scope of gypsum substances

The technology of gypsum binders makes it possible to obtain materials suitable for use in various fields. The most widely used gypsum is used in construction. The scale of its application can be compared with the use of cement. Gypsum binder has some advantages over the same cement. For example, its production consumes less fuel almost fourfold. It is hygienic, resistant to fire, has a porosity ranging from thirty to sixty percent, with a low density (up to 1,500 kilograms per cubic meter). These characteristics and determined the scope of the material.

For plastering is widely used is gypsum. Its application does not depend on the marks of the material. Used binder with a fine and medium grinding of particles, normally and slowly setting. Gypsum is added to plaster of limestone and sand. This improves the strength of the solution after drying. A layer of plaster on the surface becomes smooth and light, suitable for further finishing finish.

Gypsum substances related to brands from G-2 to G-7 are used for the production of partition walls, sheets of so-called dry plaster and other gypsum-concrete products. They are added to solutions for the preparation of compositions for internal work.

Ceramic, porcelain and earthenware and details are made with the addition of astringent from gypsum, belonging to the brands from G-5 to G-25. The binder should belong to the category of normally grasping and finely ground substances.

Gypsum binder is used to prepare mortar, which is used for caulking windows, doors, partitions. For this purpose, lower grades of material are suitable.

As you can see, the characteristics of the gypsum binder allow you to use the material for various purposes and in various fields of activity. It is durable, frost-resistant, hygienic, environmentally friendly, fireproof material. Its qualitative characteristics are determined by belonging to a certain group of materials on one or another trait.