Building and other gypsum materials are used in various sectors of the economy. They have not surprised anyone for a long time. But few people think about what constitutes a gypsum binder, what serves as raw materials for it and how it turns out. But for the production of all building materials (plasters, masonry mortars, plaster sheets) and other details, you must first prepare the raw materials. After all, the characteristics of the finished material to a large extent depend on the quality of the raw materials used.
Concept and composition
A gypsum binder is an airy material that consists mainly of two-water gypsum. The composition of gypsum is also supplemented by natural anhydride and certain industrial wastes, which include calcium sulfide.
The same group also includes combined substances. They include semi-aquatic gypsum, lime, blast furnace slag, cement.
Raw materials for production are rocks containing sulfates. GOST determines that for the production of a gypsum binder, only gypsum stone (which meets all the requirements that apply to it by GOST 4013) or phosphogypsum, which also meets the requirements of regulatory documents, can be used.
Characterization of gypsum binders
Gypsum mortar must be used until it is fully hardened. You can not stir it after the crystallization process has already begun. Mixing causes the destruction of the formed bonds between the crystals of the frame. Because of this, the solution loses its astringency.
Plaster products are not waterproof. But material manufacturers have found a way out of this situation. Scientists have determined that various additives of gypsum binders can increase this indicator. Therefore, various substances are added to the composition of the material: lime, crushed blast furnace slag, urea resins, organic liquids, which include silicon.
The use of gypsum materials does not require the use of additional fillers. They do not shrink, cracks on the treated surface will not appear. Gypsum binders, on the contrary, increase in volume after complete hardening. In some situations, sawdust, bonfire, pumice, expanded clay and other materials are added.
Another feature - gypsum materials accelerate the process of corrosion of ferrous metals (nails, fittings, wire and so on). This process is even faster in humid conditions.
The gypsum binder quickly absorbs moisture and loses its activity. Therefore, during storage and transportation, some rules must be observed. 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. Material is transported in bulk or packed in containers. It is important to protect it from debris and moisture.
Production
For this process, the following processes must be completed:
- crushing of natural gypsum matter;
- drying of raw materials;
- exposure to temperature.
Gypsum stone is fed into the hopper, from where it enters the crusher. There it is crushed into parts whose size does not exceed four centimeters. After crushing, the material is sent to the feed hopper through the elevator. From there, in equal parts, he enters the mill. There it is dried and ground to a smaller fraction. Drying at this stage is necessary to accelerate and facilitate the crushing of the material.
In the mill, the powder is heated to ninety degrees. In this condition, it is transported to a gypsum boiler. It is there that water is released from the substance during the firing process. This process begins with low temperatures (of the order of eighty degrees). But water from the material leaves best at a temperature range of one hundred and ten to one hundred and eighty degrees.
The whole process of temperature processing is divided into two stages. First, the material is kept in a digester for three hours. Water is removed there, and two-water gypsum turns into semi-water. All this time, gypsum is stirred for uniform heating. At the end of the specified time, the substance in a heated state is sent to the so-called languishing hopper. It is no longer heated. But due to the high temperature of the substance itself, the dehydration process continues there. It takes about another forty minutes. After that, binders are considered finished. And they are sent to the finished goods warehouse.
Material hardening
Hardening of gypsum binders occurs when the powder is mixed with water. In this case, a plastic mass is formed, which hardens within a few minutes. From a chemical point of view, the process is the opposite of what happened in the production process. It only happens much faster. That is, semi-aquatic gypsum attaches water, as a result of which a two-water gypsum substance is formed. This whole process can be divided into three stages.
At the first stage, the semi-aqueous gypsum substance dissolves in water to form a saturated solution of two-water gypsum. The hemihydrate has a high solubility index. Due to this, the process of supersaturation of the solution occurs very quickly. As a result - precipitation, which is the hemihydrate. These precipitated particles stick together, thereby starting the setting process.
The next stage is crystallization. Separate crystals of the substance as they begin to grow together and form a strong frame. As drying (moisture removal) bonds between crystals become stronger.
Setting rate change
The setting process can be accelerated or, conversely, slow down as necessary. They do this with the help of additives that are introduced into gypsum binders.
Types of additives that accelerate the setting process:
- substances that increase the solubility of the 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.
Most often, crushed gypsum stone is used. Its particles serve as crystallization centers around which the crystal will grow in the future. “Secondary” gypsum is characterized by greater efficiency. Under it is understood gypsum, which is already undergoing the stage of setting and hardening of calcium sulfide. This kind can include broken and crushed products.
The following substances slow down the setting process:
- increasing the plasticity of the dough: a solution of wood glue in water, coniferous infusion, lime-glue emulsion, LST and so on;
- crystal growth is prevented by a film that forms on grains of semi-aquatic gypsum under the influence of substances such as borax, ammonia, keratin moderator, alkali metal phosphates and borates, lilac alcohol, and others.
It is worth noting that the introduction of additives accelerating the process adversely affects the strength of gypsum. Therefore, they must be used with caution and added in small quantities.
Setting time (hardening) largely depends on the quality of the feedstock, the time and storage conditions, the temperature at which the process of combining the material with water, and even the mixing time of the solution.
Too short setting time is usually associated with the presence in the material of particles of hemihydrate, which remained there after firing. Setting time will also increase if the gypsum substance is heated to about forty-five degrees. If the temperature of the material is increased even more, then the process, on the contrary, will slow down. Prolonged mixing of the gypsum mixture will accelerate the setting process.
Differences between theory and practice
A feature of the hardening process is that gypsum, in contrast to other binders, during hardening increases in volume (up to one percent). Due to this, about four times more water is needed for hydration of a semi-aquatic substance than it should be in theory. In theory, water requires approximately 18.6% by weight of the material. In practice, water is taken to obtain a solution of normal density in an amount up to seventy percent. To determine the water demand of the material, the volume of water is determined 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 in practice is that when excess water is removed during drying, pores form in the material. Due to this, the gypsum stone loses its strength. Eliminate this moment by additional drying. Plaster products are dried at a temperature not exceeding seventy degrees. If the temperature is further increased, the dehydration reaction of the substance will begin.
The effect of temperature on the resulting substance
To obtain a gypsum binder, gypsum stone is subjected to high temperatures. Depending on the value of this temperature, gypsum substance can be of two types:
- Low-roasting, for the production of which the processing of raw materials occurs under the influence of temperature from one hundred twenty to one hundred eighty degrees. The raw material in this case is most often semi-aquatic gypsum. The main difference of this material is its high solidification rate.
- High fired (anhydrite), which are formed as a result of high temperature (over two hundred degrees). This material hardens longer. It also takes more time to set.
Each of these groups, in turn, has several different materials included in it.
Types of low fired binders
Gypsum binder in this category includes the following materials:
- Building plaster. For its manufacture, it is necessary to select the raw materials correctly. The production of gypsum for construction work is permissible with the use of a cementing grade of fifth and higher as raw material, the balance of which on the sieve is not more than twelve percent. For the manufacture of building products, a binder suitable for the second to seventh grade is suitable, regardless of setting time and degree of grinding. Decorative elements are made from materials of the same types. Except for coarse and slow setting substances. Gypsum plaster mixes are made of 2-25 grade substances, except for coarse binder and quick hardening.
- High-strength gypsum can be characterized by one of several grades (with indices from 200 to 500). The strength of this material is about 15-25 MPa, which is significantly higher than other types.
- Molding gypsum is characterized by a high degree of water demand and high strength in the hardened state. Gypsum products are obtained from it: ceramic forms, porcelain and faience elements and so on.
Anhydrite materials
This species, in turn, forms two substances:
- anhydrite cement, obtained by processing temperatures up to seven hundred degrees;
- estrich-gypsum, formed under the influence of calcium sulfate temperature in excess of 900 degrees.
The composition of gypsum anhydrite includes: from two to five percent lime, a mixture of sulfate with vitriol (copper or iron) to one percent, from three to eight percent dolomite, from ten to fifteen percent blast furnace slag.
Anhydrite cement is characterized by slow setting (from thirty minutes to a day). Depending on the strength, it is divided into the following grades: M50, M100, M 150, M200. Cement of this type is widely used in construction. It is used for:
- the manufacture of adhesive, plaster or masonry mortar;
- production of decorative elements;
- manufacturing heat-insulating materials.
Estrich-gypsum has the following characteristics:
- Slow setting.
- Durable up to twenty megapascals.
- Low heat conductivity.
- Good soundproofing.
- Resistant to moisture.
- Frost resistance.
- A slight degree of deformation.
These are the main, but by no means all the advantages that estrich plaster possesses. Its application is based on these indicators. It is used for wall plastering, the production of artificial marble, for the construction of mosaic floors, and so on.
Division of binder into types
The properties of gypsum binders allow you to divide them into several different groups. For this, several classifications are used.
By setting time, the following groups are distinguished:
- Group A. It includes binders that quickly set. It takes two to fifteen minutes.
- Group B. The binders of this group set in a period of six to thirty minutes. They are called normally setting substances.
- Group "B", which includes slowly setting binders. It takes more than twenty minutes to set. The upper bound is not standardized.
The grinding fineness 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 mesh size of 0.2 mm. 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.
- Medium grinding (second group), if not more than fourteen percent of the binder is left on the sieve.
- Fine grinding (the third group) indicates that the residue on the sieve does not exceed two percent.
The material is tested for strength in bending and compression. For this, bars with a size of 40 x 40 x 160 millimeters are prepared from a gypsum solution. Two hours after manufacture, when the crystallization and hydration processes are complete, tests begin. Gypsum binders (GOST 125-79) are divided into twelve grades by strength. They have indices from two to twenty five. The value of tensile strength depending on the grades are collected in special tables. It can be seen even in the GOST itself.
The main parameters and types of material can be found by marking it. It looks something like this: G-6-A-11. This inscription will mean the following:
- G-gypsum binder.
- 6 - material grade (means that the strength is more than six megapascals).
- A - determines the type of setting time (that is, quick hardening).
- 11 - indicates the degree of grinding (in this case, average).
Scope of gypsum substances
The technology of gypsum binders allows you to get materials suitable for use in various fields. The most widely used gypsum in construction. The scope of its application can be compared with the use of cement. Gypsum binder has some advantages over the same cement. For example, its production takes up almost four times less fuel. It is hygienic, resistant to fire, has a porosity ranging from thirty to sixty percent, low density (up to one and a half thousand kilograms per cubic meter). These characteristics determined the scope of the material.
For plastering , gypsum is widely used. Its use does not depend on the grades of material. Used binder with a fine and medium grinding of particles, normally and slowly setting. Gypsum is added to stucco made of limestone and sand. Due to this, the strength of the solution after drying is improved. And the layer of plaster on the surface becomes smooth and light, suitable for further finishing.
Gypsum substances belonging to grades G-2 to G-7 are used for the manufacture of partition walls, sheets of so-called dry plaster and other gypsum concrete products. They are added to solutions to obtain formulations for internal work.
Ceramic, porcelain and earthenware products and parts are made with the addition of a gypsum binder, belonging to grades from G-5 to G-25. A binder should be classified as normally setting and fine-grained substances.
Gypsum binder is used to prepare the solution, 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, refractory material. Its qualitative characteristics are determined by belonging to a certain group of materials according to one or another attribute.