Gypsum construction and other materials are used in various sectors of the national economy. They have not surprised anyone for a long time. But few people think about what gypsum binder actually is, what serves as raw material for it and how it is obtained. 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 to a large extent depend on the quality of the raw materials used.
Concept and composition
Gypsum binder is an airy material that consists mostly of gypsum dihydrate. 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-aqueous gypsum, lime, blast-furnace slag, cement.
Raw materials for production are rocks containing sulfates. GOST defined,that for the manufacture of a gypsum binder, only gypsum stone (which meets all the requirements that apply to it in 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 hardened. You can not stir it after the crystallization process has already begun. Stirring causes the destruction of the formed bonds between the framework crystals. This causes the mortar to lose its astringency.
Gypsum products are not waterproof. But material manufacturers have found a way out of this situation. Scientists have determined that various additions of gypsum binders can increase this figure. Therefore, various substances are added to the composition of the material: lime, crushed blast-furnace slag, carbamide 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, fire, pumice, expanded clay and other materials are added.
Another feature - gypsum materials accelerate the process of corrosion of ferrous metals (nails, rebar, wire, and so on). This process is even faster in wet conditions.
Gypsum binder quickly absorbs moisture and loses its activity. Therefore, during storage andtransportation must comply with certain rules. 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. The material is transported in bulk or packed in containers. It is important to protect it from debris and moisture.
Production
The following processes must be performed for this process:
- crushing natural gypsum substance;
- drying raw materials;
- effect of temperature.
Gypsum stone is fed into the bunker, from where it enters the crusher. There it is crushed into pieces, the size of which does not exceed four centimeters. After crushing, the material is sent to the feed hopper through the elevator. From there, in equal parts, it enters the mill. There it is dried and crushed to a smaller fraction. Drying at this stage is necessary to speed up and facilitate the process of crushing the material.
In the mill, the powder is heated to ninety degrees. In this state, it is transported to the gypsum boiler. It is there that the release of water from the substance occurs during the firing process. This process begins with low temperatures (about eighty degrees). But water is best drained from the material in a temperature range of one hundred and ten to one hundred and eighty degrees.
The whole temperature treatment process is divided into two stages. First, the material is kept in the digester for three hours. Water is removed there, and gypsum dihydrateturns into semi-aquatic. All this time, the gypsum is stirred for uniformity of heating. At the end of the specified time, the substance in a heated state is sent to the so-called languishing bunker. It no longer warms up. But due to the high temperature of the substance itself, the process of dehydration continues there. This takes another forty minutes or so. After that, the binders are considered ready. And they are sent to the warehouse of finished products.
Material curing
Gypsum binders harden when 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, there is a process that is the opposite of what happened in the production process. It just happens much faster. That is, semi-aqueous gypsum attaches water, resulting in the formation of a dihydrate gypsum substance. This whole process can be divided into three stages.
In the first step, the semi-aqueous gypsum substance is dissolved in water to form a saturated solution of gypsum dihydrate. The dihydrate 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 dihydrate. These precipitated particles stick together, thereby starting the setting process.
The next step is crystallization. Separate crystals of the substance, as they grow, begin to connect and form a strong frame. As drying (moisture is removed), the bonds between the crystals becomestronger.
Change setting speed
The setting process can be accelerated or, conversely, slowed down as needed. They do this with the help of additives that are added to gypsum binders.
Types of additives that speed up the setting process:
substances that increase the solubility of the hemihydrate: sodium or potassium sulfate, table s alt and others;
substances that will be the center of crystallization in the reaction: phosphoric acid s alts, crushed natural gypsum and so on
The most commonly used crushed gypsum stone. Its particles serve as crystallization centers around which the crystal will grow in the future. Greater efficiency is characterized by "secondary" gypsum. It is understood as gypsum, which is already undergoing the stage of setting and hardening of calcium sulfide. Broken and crushed products can be attributed to this type.
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 the film that forms on semi-aqueous gypsum grains under the influence of substances such as borax, ammonia, keratin retarder, 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 conditions of storage, 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 of dihydrate particles in the material, which remained there after firing. The 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 speed up the setting process.
Differences between theory and practice
A feature of the hardening process is that gypsum, unlike other binders, increases in volume during hardening (up to one percent). Due to this, for the hydration of a semi-aqueous substance, about four times more water is needed than 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 of up to seventy percent. To determine the water requirement 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. Gypsum products are dried at a temperature not exceedingseventy degrees. If you increase the temperature even more, 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, the gypsum substance can be of two types:
Low-burning, for the production of which the processing of raw materials takes place under the influence of a temperature of one hundred twenty to one hundred and eighty degrees. The raw material in this case is most often semi-aqueous gypsum. The main difference of this material is the high speed of solidification
High-burning (anhydrite), which are formed as a result of high temperature (over two hundred degrees). Hardens such material longer. It also takes longer to set
Each of these groups, in turn, has several different materials included in it.
Types of low-fired binders
Gypsum binder of this category includes the following materials:
Construction gypsum. For its manufacture, it is necessary to select the right raw materials. The production of gypsum for construction work is permissible using as a raw material a binder grade of the fifth and higher, the balance of which on the sieve is no more than twelve percent. For the manufacture of building products, a binder belonging to the grade from the second to the seventh is suitable, regardless of the setting time and degree of grinding. Decorative elements are made from materials of the same types. With the exception of coarse grinding substances andslowly grasping. Gypsum plaster mixtures are made from substances grade 2-25, except for a binder with coarse grinding and quick-hardening
High-strength gypsum can be characterized by one of several grades (with indexes from 200 to 500). The strength of this material is about 15-25 MPa, which is much higher than that of other types
Moulding plaster has a high degree of water demand and high strength in the hardened state. Gypsum products are made from it: ceramic molds, porcelain-faience elements, and so on
Anhydrite materials
This species, in turn, forms two substances:
anhydrite cement obtained by processing at temperatures up to seven hundred degrees;
Estrich-gypsum, formed under the influence of calcium sulfate over 900 degrees
The composition of anhydrite gypsum includes: from two to five percent lime, a mixture of sulfate with vitriol (copper or iron) up to one percent, from three to eight percent dolomite, from ten to fifteen percent blast-furnace slag.
Anhydrite cement has a 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:
manufacture of adhesive, plaster or masonry mortar;
concrete production;
production of decorative items;
manufacture of thermal insulationmaterials
Estrich gypsum has the following characteristics:
- Slow grasp.
- Strength up to twenty megapascals.
- Low thermal conductivity.
- Good soundproofing.
- Resistant to moisture.
- Frost resistant.
- Slightly deformed.
These are the main, but far from all, advantages that estrich gypsum has. Its application is based on these indicators. It is used for wall plastering, artificial marble production, mosaic flooring and so on.
Dividing the binder into types
The properties of gypsum binders allow us to divide them into several different groups. Several classifications are used for this.
The following groups are distinguished by setting time:
Group "A". It includes astringents that set quickly. This takes two to fifteen minutes
Group "B". The binders of this group seize in six to thirty minutes. They are called normally setting agents
Group "B", which includes slowly setting binders. It takes more than twenty minutes to set. The upper limit is not standardized
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 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 grind(second group), if no more than fourteen percent of the binder is left on the sieve
Fine grinding (third group) indicates that the residue of the substance on the sieve does not exceed two percent
The material is tested for flexural and compressive strength. To do this, bars with a size of 40 x 40 x 160 millimeters are prepared from a gypsum mortar. Two hours after manufacture, when the crystallization and hydration processes are completed, tests begin. Gypsum binders (GOST 125-79) are divided into twelve grades according to strength. They have indices from two to twenty-five. The value of the 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 recognized by its labeling. 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 by setting time (that is, quick-hardening).
- 11 - indicates the degree of grinding (in this case medium).
Field of application of gypsum materials
The technology of gypsum binders makes it possible to obtain materials suitable for use in various fields. Gypsum is most widely 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 uses less fuel by almostfour times. 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.
Gypsum is widely used for plastering. Its application does not depend on the grades of the material. A binder with fine and medium grinding particles is used, normally and slowly setting. Gypsum is added to limestone and sand plaster. This improves the strength of the solution after drying. And the layer of plaster on the surface becomes smooth and light, suitable for further finishing.
Gypsum substances belonging to grades from G-2 to G-7 are used for the manufacture of partition panels, sheets of so-called dry plaster and other gypsum concrete products. They are added to solutions to obtain compositions for interior work.
Ceramic, porcelain and faience products and parts are made with the addition of a binder of gypsum, belonging to grades from G-5 to G-25. The binder must be classified as normally setting and finely ground substances.
Gypsum binder is used to prepare mortar, which is used for caulking windows, doors, partitions. For this purpose, lower material grades are suitable.
As you can see, the characteristics of the gypsum binder make it possible to use the material for various purposes and in various fields of activity. It is durable, frost-resistant,hygienic, eco-friendly, fireproof material. Its qualitative characteristics are determined by belonging to a certain group of materials on a particular basis.
