The mineral composition of gypsum. Mineral gypsum: description and application

Gypsum

Gypsum (eng. G ypsum) - a mineral, aqueous calcium sulfate. The chemical composition is Ca × 2H 2 O. The syngony is monoclinic. The crystal structure is layered; two sheets of 2- anionic groups closely associated with Ca 2+ ions form double layers oriented along the (010) plane. Molecules of H 2 O occupy places between these double layers. This easily explains the very perfect cleavage characteristic of gypsum. Each calcium ion is surrounded by six oxygen ions belonging to the SO 4 groups and two water molecules. Each water molecule binds a Ca ion to one oxygen ion in the same double layer and to another oxygen ion in the adjacent layer.

Properties

The color is very different, but usually white, gray, yellow, pink, etc. Pure transparent crystals are colorless. Impurities can be dyed in different colors. The dash color is white. The luster of the crystals is glassy, ​​sometimes with a mother-of-pearl tint due to microcracks of perfect cleavage; selenite is silky. Hardness 2 (standard Mohs scale). Cleavage is very perfect in one direction. Thin crystals and cleavage plates are flexible. Density 2.31 - 2.33 g / cm 3.
It has significant solubility in water. A remarkable feature of gypsum is the fact that its solubility reaches a maximum at 37-38°C with increasing temperature, and then drops rather quickly. The greatest decrease in solubility is established at temperatures above 107 ° due to the formation of a "hemihydrate" - CaSO 4 × 1/2H 2 O.
At 107 o C, it partially loses water, turning into a white alabaster powder, (2CaSO 4 × H 2 O), which is noticeably soluble in water. Due to the smaller number of hydrate molecules, alabaster does not shrink during polymerization (increases in volume by approx. 1%). Under p. tr. loses water, splits and fuses into white enamel. On charcoal in a reducing flame it gives CaS. It dissolves much better in water acidified with H 2 SO 4 than in pure water. However, at a concentration of H 2 SO 4 over 75 g/l. solubility drops sharply. Very slightly soluble in HCl.

Location forms

Due to the predominant development of (010) faces, crystals have a tabular, rarely columnar, or prismatic appearance. Of the prisms, (110) and (111) are the most common, sometimes (120) and others. Faces (110) and (010) often have vertical shading. Intergrowth twins are frequent and are of two types: 1) Gallic according to (100) and 2) Parisian according to (101). It is not always easy to tell them apart. Both of them resemble a dovetail. Gallic twins are characterized by the fact that the edges of the prism m (110) are parallel to the twin plane, and the edges of the prism l (111) form a reentrant angle, while in the Parisian twins the edges of the prism Ι (111) are parallel to the twin seam.
It occurs in the form of colorless or white crystals and their intergrowths, sometimes colored by inclusions and impurities captured by them during growth in brown, blue, yellow or red tones. Intergrowths in the form of a "rose" and twins are characteristic - the so-called. "dovetails"). It forms veinlets of a parallel fibrous structure ( selenite) in clay sedimentary rocks, as well as dense continuous fine-grained aggregates resembling marble ( alabaster). Sometimes in the form of earthy aggregates and cryptocrystalline masses. It also forms the cement of sandstones.

Common are pseudomorphs after gypsum of calcite, aragonite, malachite, quartz, etc., as well as pseudomorphs of gypsum after other minerals.

Origin

A widely distributed mineral, it is formed in natural conditions in various ways. Sedimentary origin (typical marine chemogenic sediment), low-temperature hydrothermal, found in karst caves and solfataras. It precipitates from sulfate-rich aqueous solutions during the drying of sea lagoons and salt lakes. Forms layers, interlayers and lenses among sedimentary rocks, often in association with anhydrite, halite, celestine, native sulfur, sometimes with bitumen and oil. In significant masses, it is deposited by sedimentation in lacustrine and marine salt-bearing dying basins. In this case, gypsum, along with NaCl, can be released only in the initial stages of evaporation, when the concentration of other dissolved salts is not yet high. When a certain value of salt concentration, in particular NaCl and especially MgCl 2 , is reached, instead of gypsum, anhydrite will crystallize and then other, more soluble salts, i.e. the gypsum in these basins must belong to the earlier chemical sediments. Indeed, in many salt deposits, layers of gypsum (as well as anhydrite), interbedded with layers of rock salt, are located in the lower parts of the deposits and in some cases are underlain only by chemically precipitated limestones.
Significant masses of gypsum in sedimentary rocks are formed primarily as a result of the hydration of anhydrite, which in turn precipitated during the evaporation of sea water; often, during its evaporation, gypsum is deposited directly. Gypsum results from the hydration of anhydrite in sediments under the influence of surface water under conditions of low external pressure (on average, to a depth of 100-150m.) According to the reaction: CaSO 4 + 2H 2 O = CaSO 4 × 2H 2 O. In this case, strong an increase in volume (up to 30%) and, in connection with this, numerous and complex local disturbances in the conditions of occurrence of gypsum-bearing strata. Most of the large deposits of gypsum on the globe arose in this way. Nests of large, often transparent crystals are sometimes found in voids among solid gypsum masses.
Can serve as cement in sedimentary rocks. Gypsum vein is usually the product of the reaction of sulfate solutions (formed by the oxidation of sulfide ores) with carbonate rocks. It is formed in sedimentary rocks during the weathering of sulfides, under the influence of sulfuric acid formed during the decomposition of pyrite on marls and calcareous clays. In semi-desert and desert areas, gypsum is very often found in the form of veinlets and nodules in the weathering crust of rocks of various compositions. In the soils of the arid zone, new formations of redeposited gypsum are formed: single crystals, twins (“dovetails”), druses, “gypsum roses”, etc.
Gypsum is fairly well soluble in water (up to 2.2 g/L), and with an increase in temperature, its solubility first increases, and falls above 24°C. Due to this, gypsum, when deposited from sea water, is separated from halite and forms independent layers. In semi-deserts and deserts, with their dry air, sharp daily temperature drops, saline and gypsum soils, in the morning, with an increase in temperature, gypsum begins to dissolve and, rising in solution by capillary forces, is deposited on the surface when water evaporates. By evening, with a decrease in temperature, crystallization stops, but due to lack of moisture, the crystals do not dissolve - in areas with such conditions, gypsum crystals are found in especially large quantities.

Location

In Russia, thick gypsum-bearing strata of Permian age are distributed in the Western Urals, in Bashkiria and Tatarstan, in Arkhangelsk, Vologda, Gorky and other regions. Numerous deposits of the Upper Jurassic age are established in the North. Caucasus, Dagestan. Remarkable collection specimens with gypsum crystals are known from the Gaurdak deposit (Turkmenistan) and other deposits in Central Asia (in Tajikistan and Uzbekistan), in the Middle Volga region, in the Jurassic clays of the Kaluga region. In the thermal caves of Naica Mine, (Mexico), druze of gypsum crystals of unique size up to 11 m long were found.

Application

Fibrous gypsum (selenite) is used as an ornamental stone for inexpensive jewelry. Since ancient times, large jewelry items have been carved from alabaster - interior items (vases, countertops, inkwells, etc.). Calcined gypsum is used for castings and casts (bas-reliefs, cornices, etc.), as a binder in the construction industry, in medicine.
It is used to obtain building gypsum, high-strength gypsum, gypsum-cement-pozzolanic binder.

• Gypsum is also called sedimentary rock, composed mainly of this mineral. Its origin is evaporite.

Gypsum (eng. GYPSUM) - CaSO 4 2H 2 O

Other names, varieties

silky spar,
Ural eolinit,
gypsum spar,
girlish or marino glass.

• English - Gypsum
• Arabic - جص
• Bulgarian - Gypsum
• Hungarian - Gipsz
• Dutch - Gips
• Greek - Γύψος
• Danish - Gips
• Hebrew - גבס
• Spanish - Yeso; Gypsita; Oulopholita
• Italian - Gesso;Acidovitriolosaturata;Geso
• Catalan - Guix
• Korean - 석고
• Latvian - Ģipsis
• Latin - Gypsum
• Lithuanian - Gipsas
• German - Gips;Atlasgips;Gipsrose;Gyps;Gypsit;Oulopholit
• Polish - Gips
• Portuguese - Gipsita
• Romanian - Gips
• Russian - Gypsum
• Slovak - Sadrovec
• Slovenian - Sadra
• French - Gypse;Chaux sulfatee
• Croatian - Gips
• Czech - Sadrovec
• Swedish - Gips
• Esperanto
• Estonian - Kips
• Japanese - 石膏

Name: Gypsum

Color: colorless turning into white, often colored by minerals-impurities in yellow, pink, red, brown, etc.; sometimes there is a sectorial-zonal color or distribution of inclusions over growth zones inside the crystals; colorless in internal reflexes and translucent..

In the construction industry, gypsum is in second place after cement-sand mixtures. The unpretentiousness of the material, excellent environmental friendliness and relatively simple technology of use have led to the massive use of building gypsum for the production of safe blocks, decoration elements and even interior items.

Gypsum mass production

The raw materials for the production of gypsum for construction purposes are natural deposits of gypsum stone in the form of anhydrous anhydride - calcium sulfate, its two-water modification CaSO 4 * H 2 O, as well as a huge amount of industrial waste from the chemical and metallurgical production sectors.

Gypsum production technology consists of three successive operations:

• Purification, fractionation and preliminary grinding of raw materials;
• Heat treatment at different temperatures, from 160 o C to 1000 o C;
• Final regrinding of the heat-treated gypsum mass to a dusty state, drying and packaging of the building material in sealed packaging.

The general technology of gypsum production divides the gypsum binder into two categories - quickly setting, or semi-aqueous material, and slowly setting gypsum stone. The first group includes building and high-strength molding gypsum material, the second group includes less durable anhydrite cement and high-fired stone, called in the old fashioned way estrich gypsum.

In the process of heating to 180 ° C, the raw material - two-water gypsum stone breaks down into two modifications, after separation on sieves, high-strength α-gypsum is used to make gypsum stone, blocks and molds, β-modification is divided into several categories, the most viscous, with high bending strength, used for construction purposes, the rest as a decorative and auxiliary material.

Varieties of gypsum stone

In addition to the chemical composition, the properties and characteristics of gypsum largely depend on the structure of the raw material. For example, in addition to natural alabaster stone, which has a pronounced polycrystalline structure, a fibrous variety of calcium anhydride, selenite, is used for production.

All varieties of gypsum, from building to decorative or architectural, are obtained by varying the content of selenite, alabaster, raw gypsum stone, finely ground waste of calcium sulfate, heat-treated at different temperatures. After fractionation of the raw material according to the degree of grinding, the gypsum is divided into three groups:

• A - fast-hardening or alabaster materials;
• B and C - mixtures with a hardening time of up to 15 minutes;
• G - building gypsum materials.

The finer the grain, the faster the material hardens.

Construction or high quality gypsum

For construction work, not the most durable grades of gypsum are used, more important is the uniformity of solidification and relatively large water absorption, which provides mixtures with high plasticity. For the production of building materials from gypsum, putties, gypsum plaster mixtures, the β-modification of the average fineness of grinding is used.

Due to special wetting and retarding additives, gypsum mortar can be handled almost like a cement-sand mixture. This reduces the shrinkage of the gypsum and the risk of cracks in the building material.

High strength gypsum stone

Finely milled α-modifications of raw gypsum are used for the manufacture of prefabricated building finishing elements, such as artificial facing stone, gypsum boards, fire barriers and slabs for laying flooring.

High-strength gypsum mixes can be used for finishing the walls of frame buildings, ceilings, and interior details. For 100 kg of heat-treated raw mass, there is no more than 20% of the high-strength fraction, so the material is quite expensive and is rarely used in its pure form. Most often, high-strength building gypsum is the basis for the manufacture of fire-resistant or architectural material.

Polymer stone-gypsum

The idea of ​​adding polymer additives to the gypsum mass has been used for a long time. Get polymer gypsum in two ways:

• The addition of water-soluble polymer compounds that improve the fluidity of gypsum and the wetting of the grain. A water-soluble polymer, for example, a polyvinyl acetate emulsion or an aqueous solution of carboxycellulose, increases the material's resistance to impact and alternating loads;
• Saturation of the surface of the finished casting from building gypsum with volatile polymer compositions, most often based on polyurethane or polypropylene.

In both cases, a thin plate of building gypsum turns out to be quite elastic and at the same time light. From polymer gypsum, you can easily make an inexpensive finish that imitates expensive wood species in texture and pattern.

cellacast gypsum material

The widespread use of gypsum material is hindered by one of its inherent shortcomings - the high fragility of gypsum. This prevents the production of thin screeds or shells from building gypsum. Therefore, the building material is saturated with a special reinforcing microfiber, the surface of which is treated with polyurethane.

As a result, the strength of the building material increases by 40-50%, and the resistance to bending loads by 150-200%. Celacast plaster is widely used in medical institutions for applying fixing dressings for fractures and severe injuries of the extremities.

Sculptural or molding plaster material

Ordinary building gypsum, after a slight modification with polymer resins and dihydric alcohol, turns into a mass from which you can make a model, impression, bas-relief of any complexity.

Gypsum molding material must not be diluted with water, as is usually done for building gypsum. In the kit, a special solvent on a water-alcohol basis is attached to a white or beige-gray finely ground powder. Due to the use of a solvent, it is possible to achieve almost zero shrinkage of the material. Therefore, souvenir products and casts from objects with the smallest carving or engraving are often made from sculptural plaster, for example, when copying rare coins, artifacts, old awards.

Acrylic gypsum block

Building gypsum is quite simple to turn into a homemade version of homemade faience. It is enough to carry out the kneading with the preliminary addition of one-component acrylic resin. The result is a light and very hard casting, which can be processed by carving, grinding, drilling. For example, make decorative stucco molding or vases for antique porcelain out of building plaster.

In the construction industry, mixtures of acrylic and gypsum are used for the manufacture of wall cladding from gypsum blocks and the formation of a rough base for self-leveling self-leveling floors.

Polyurethane gypsum material

The use of non-woven polyurethane fabrics and fibers with a specially treated surface made it possible to create a fundamentally new material for the manufacture of immobilizing dressings, tourniquets and pads that fix limbs and body parts in case of severe injuries.

Unlike cellocast gypsum, polyurethane gypsum material has high strength and sufficient casting flexibility to reduce discomfort from its use. Polyurethane material is obtained from building materials using a special procedure for reseeding the ground mass and isolating the largest grain of the same size. As a result of the processing of the rough mass of building gypsum, a casting with huge pores is obtained, providing free access of air to the tissues of the body.

White gypsum stone

Building gypsum serves as a raw material for the manufacture of so-called white or dental gypsum materials. The white color is obtained due to the deep purification of the feedstock, sulfur oxides, sulfates of heavy metals, iron, and organic impurities are removed, which usually stain the building gypsum in a grayish-beige color.

From white finely ground stone, mixtures are made for molding impressions necessary for subsequent prosthetics or treatment. White stone differs from building material in a whole bunch of additional qualities:

• The composition of the gypsum casting must not contain irritating or toxic materials;
• No shrinkage of the white plaster mold;
• Minimum water absorption;
• Rapid setting of the gypsum matrix.

For your information! White gypsum typically provides very high impression characteristics, so it is often used to make molds for jewelry. Parts weighing at least 3 g in size are poured into a form of building gypsum.

Fine-grained gypsum

Reducing the grain size of building gypsum can significantly improve its two main characteristics:

• The strength of the material increases under the influence of bending loads;
• Higher flexibility of castings of small thickness.

Casting based on α-gypsum grains of fine grinding is capable of showing a strength of 350-400 kg/cm 2 . The only limitation to be reckoned with is high shrinkage, which is why fine-grained building gypsum is used for repair work and the manufacture of high-strength coatings.

For your information! From fine-grained gypsum, after vacuuming and high-temperature curing of the mixture, it is easy to make a thin sheet, which is almost identical in appearance and properties to packaging cardboard.

liquid gypsum material

If alcohol glycol solutions are used instead of water for mixing building gypsum, then the material can be stored unchanged for a long time. Liquid gypsum material is used to perform repair and thermal insulation work. After adding an aqueous solution of calcium chloride and sodium chloride, liquid gypsum can be pumped under pressure into cracks in walls or floor slabs. To repair the foundation, the liquid is used only in combination with polymer resins, for example, polyurethanes.

Waterproof gypsum stone

For all its merits, ordinary building gypsum remains quite sensitive to moisture or condensation. Moisture-resistant material GKVL is made using thermosetting polymer powders, and sometimes just finely divided polystyrene, added to dry building gypsum at the stage of plate molding.

After curing, the building boards are subjected to heat treatment, and the material acquires water-resistant qualities.

Refractory block

A heat-resistant or even refractory gypsum block on an industrial scale is made on the basis of conventional building gypsum and fire-resistant additives. Such material can even be made with your own hands according to the following recipe:

• 30% by weight of high-quality building gypsum and the same amount of water;
• 15% ground ash or fireclay dust;
• 4% alumina, you can take washed skinny white clay;
• 2% quicklime and ground iron dioxide.

For your information! If building gypsum is needed according to fire safety class G1, then a complex composition can be replaced with finely ground quartz sand, however, such a gypsum stone will not withstand heating above 600 ° C.

Architectural

Most often, building gypsum for architectural work means ordinary molding gypsum modified with polyurethane fibers or polystyrene. This is a relatively soft material, and you can easily make a model or cast the simplest elements of stucco molding from it without any problems.

Real architectural gypsum for construction work is made on the basis of gypsum stone, fired at a temperature of 800-1000 ° C. It turns out a very hard, viscous building gypsum that does not absorb water well. If you withstand the technology of preparing the batch, you will get a gypsum casting with a very hard and at the same time wear-resistant surface.

Unlike polystyrene architectural plaster, from which craftsmen now like to assemble decoration in the style of the 17th century, real stucco for exterior walls was cast from highly fired building plaster. The difference is impressive. Polystyrene stone stands at most 10 years, old red-hot gypsum in the climate of St. Petersburg withstood almost two hundred years.

Grades of gypsum mixtures

In the production process, the heat-treated mass after grinding is fractionated according to the density and particle size. In accordance with GOST No. 125-79, the material is divided into four groups or twelve grades.

The first group includes ordinary gypsum materials G2-G7, with a strength of 20-70 kg / cm 2, the second group - low-shrink mixtures G10, G13-16. The third group is high-strength G22-25, the fourth includes gypsum mixtures with special properties, for example, fire-resistant or highly porous blocks and stones.

Building plaster properties

An ordinary gypsum block used for building purposes is a highly porous mass, the volume of air channels can reach 50-55%. The density of building plaster stone is 2.6-2.75 g/cm 3 , for a bulk mass of 900-1000 kg/m 3 in a pressed but uncured state, the building mixture can be compacted up to 1400 kg/m 3 .

Dry hard gypsum stone easily withstands heating up to 450-500 ° C, after 100-120 minutes after the onset of thermal exposure, the surface begins to peel off until gradual destruction. The thermal conductivity of the gypsum block is 0.259 kcal/m deg/h at room temperature.

Grinding degree

Raw building gypsum obtained in the process of processing with superheated steam under a pressure of 1.5-2.5 atm is conditionally divided into three grades

• The first grade of material corresponds to the fraction that leaves on a sieve with an opening density of 918 units. per cm 2 no more than 15% of the initial volume. This is the most active and durable fraction of building gypsum;
• To the second grade include more viscous masses with a residual moisture of not more than 0.1% of the mass, after passing the sieve test, no more than 25% should remain on the grid;
• The third grade, building plaster of especially fine grinding, leaves no more than 2% of the mass on the sieve.

It is clear that the finer the calcium anhydride grain, the faster water absorption occurs and the more hydraulic bonds are formed between the individual grains of building gypsum, the stronger and harder the gypsum stone is taught.

Compressive and bending strength

The tensile strength for building gypsum of the first category is defined as 55 kg / cm 2. The second category after the completion of the hardening process must withstand a static load of 40 kg/cm 2 . After about four hours, the hardened building stone after drying must withstand up to 200 kg / cm 2.

The flexural strength for dried stone is 30% of static compression for unreinforced material and 65% for reinforced mass. Increasing the stone's moisture content by as little as 15% can reduce strength by 40-60%.

Normal Density, Water Demand or Water-Gypsum Ratio

The amount of water required to form internal bonds between grains depends on the chemical composition. For hemihydrate-based α-gypsum, 35-38% of water is required by weight of the building gypsum stone, for a weaker viscous β-hemihydrate, from which the bulk of the building gypsum material is made, 50-60% of an aqueous solvent is needed.

The density of the gypsum mixture in the first minutes corresponds to wallpaper glue, after 10 minutes. it is already thick sour cream, and after another 5 minutes. - viscous, crumbling mass. By introducing additives based on FFA, alum gels or even lime, the density can be stabilized, and the total water consumption of the building material can be reduced by 10%.

Reinforcement of gypsum boards and blocks

Despite the internal homogeneity of the hardened gypsum mass, the bending strength of blocks and slabs is considered insufficient. It is especially difficult to work with thin plates and sheets. Often, the fall of building gypsum cladding from the wall to the floor means the destruction and chipping of the material.

Building gypsum blocks are reinforced with chopped polyester fiber, thin-sheet panels are reinforced with the introduction of fiberglass and fluff pulp.

Gypsum as a binder

Dry gypsum mix has a high water absorption capacity, for example, hemihydrate α-gypsum has a surface area of ​​up to 6000 cm 2 /g, and a weaker β-modification is twice as large. A small amount of 3-5% gypsum mixture added to lime or cement mortar can increase the viscosity by 15%.

A relatively simple and effective way to correct the viscosity of any mortar, but it should be borne in mind that the process of water absorption develops in progression, so the residual viscosity of the mixture will be formed no earlier than 15 minutes after the addition of the material.

Plaster setting

High-quality gypsum has a high hardening rate, in practice for a freshly fired building material of the first category, the setting process should begin as early as 4 minutes after dilution with water. For gypsum material of the second category, the curing process according to the standard should begin no earlier than after 6 minutes. It is clear that due to the absorption of water vapor from the air, gypsum, even when carefully packed in a waterproof shell, loses activity, therefore, by the standards for gypsum material, the maximum time for the onset of hardening is limited to 30 minutes. Anything more than that is already considered unusable. The total setting time from the start of mixing to the transition to a solid state should not exceed 12 minutes.

The setting time of building gypsum is limited to a period of 3 hours. The exception is anhydrite cement, for which the setting time limit is set at 24 hours. - sand mixtures, 28 days. The hardened anhydrite gypsum binder sample must withstand a compressive load of 50-150 kgcm2.

Plaster hardening

The process of binding water and building strength with gypsum may be accompanied by the expansion of the hardening mass. The more anhydride in the chemical composition in soluble form, the greater the degree of expansion. For example, the hemihydrate is able to increase the size by 0.5%, and for the β-modification, the casting material increases by all 0.8%.

This leads to self-hardening of the building mass, but it is not very convenient if you need to maintain the maximum accuracy of the cast, so the effect is combated with the help of 1% lime additives or Pomazkov materials. In the process of drying, building gypsum shrinks, so stone masses of great thickness are always loaded with internal stresses.

Building plaster: application

A high degree of versatility and a very simple preparation technology have become the reason for the huge popularity of gypsum stone. The material is perfectly processed, cut, drilled, glued. At the same time, in the mass of building stone, there are practically no aging and degradation processes, as in plastic or polymer-mineral plates.

Gypsum blocks and plasterboard sheets have become one of the most popular options for wall cladding in residential premises. First, the high porosity of gypsum makes it possible to regulate moisture naturally. Secondly, building gypsum has good sound insulation and low thermal conductivity.

The material is easy to paint and plaster, if necessary, with the help of wax mastic, the walls can be made waterproof to water and condensate, but relatively transparent to water vapor.

Preparing the mixture

The process of preparing a gypsum mortar begins with sifting the dry mixture through a sieve, it is best to use DK0355, this is about 400 holes per square centimeter. Next, the required amount of water is heated to 40 ° C and poured into the mixer container. Gypsum is added in small portions to the water, and then a thin film formed on the water surface is broken with a trowel.

In theory, the strength of the casting of a building gypsum block depends on the consistency of the batch. The thicker the solution, the smaller the size of the pores and anhydride crystals. With an excess of water, the crystals rapidly increase in size, which leads to intense pore formation.

Material storage

The only reliable way to store dry gypsum material well is to use glass jars with a sealed lid. Dry calcined gypsum can be used to drain containers or floors, but to restore the initial qualities, the material must be deoxidized with an aqueous solution of sulfuric acid, removed by calcination of water and re-ground into dust to a grain size of 0.01-0.003 mm. Industrial polyethylene packaging provides reliable storage of the dry mixture only during the first two months. Dry plasters based on gypsum material in paper bags should be used within 3 days after opening.

Gypsum substitute

The only material that can replace building gypsum is considered to be alabaster, both in its pure form and with the addition of lime or polymer emulsions. Dry lime in an amount of up to 1% must be applied at the stage of preparing the building mixture for kneading. The material is intensively rubbed on a metal or stone surface to make the batch as homogeneous as possible. If it is necessary to prepare a mold, then white clay and flake graphite can be added to the alabaster at the rate of 2% and 1%, respectively.

What is the difference between gypsum and alabaster

Both materials are the product of roasting natural sulfuric anhydride, but due to the large amount of impurities of iron oxide and aluminum oxide, the alabaster material is obtained with a slight reddish tint. Unlike gypsum, alabaster sets in 3-5 minutes, so any alabaster stone castings have a high surface hardness. Alabaster perceives mechanical loads worse and gives a high degree of expansion with subsequent shrinkage.

The mineral derived from calcium is its aqueous sulfate, which is called gypsum. It has many synonymous names: montmartite, desert rose, gypsum spar (crystalline and leaf forms). A sample of the fibrous structure is selenite, granular - alabaster. We will talk about the varieties and properties of this stone, its prevalence throughout the country and its use in construction, medicine and other areas of the economy.

Historical reference

As a result of the evaporation of the seas 20-30 million years ago, gypsum was formed - a mineral that ancient civilizations began to use. The stone is still in great demand, despite the emergence of many modern materials.

It happened almost 10 thousand years ago. Evidence that gypsum was used in ancient Egypt, Assyria, Greece and the Roman state is:

In England and France, starting from the 16th century, wooden structures were covered with gypsum, protecting them from fires. The year 1700 is considered the beginning of the use of the mineral as a fertilizer. To create architectural forms in Russia XVII-XVIII centuries. plaster decor was widely used, and in 1855 the Russian surgeon N.I.

During the Crimean War, Pirogov invented and began to use a plaster cast to treat the wounded, fixing the limbs. This made it possible to save many soldiers from losing an arm or leg.

Description of the mineral

A mineral from the class of sulfates arising from sedimentary rocks is called gypsum. Its chemical formula looks like this: CaSO4 2H2O. In appearance, a non-metallic luster is noted: silky, mother-of-pearl, glassy or matte. The stone is colorless or colored with white, pink, gray, yellowish, blue and red hues. Description of other indicators:

• density 2.2–2.4 t/m3;
• hardness on the Mohs scale 2.0;
• cleavage is perfect, thin plates are easily separated from crystals of a layered structure;
• the line drawn on the stone is white.

This is what gypsum consists of: calcium oxide CaO - 33%, water H2O - 21%, sulfur trioxide SO 3 - 46%. Impurities are usually absent.

If we consider the stone as a rock, then the composition contains calcite, dolomite, iron hydroxides, anhydrite, sulfur and gypsum itself. Sedimentary origin, according to the conditions of creation, primary forms are distinguished, which were formed by chemical precipitation in salt water bodies, or secondary derivatives - they arose as a result of anhydrite hydration. It can accumulate in zones of native sulfur and sulfides: wind erosion produces gypsum hats contaminated with impurities.

The quality of raw materials for the production of gypsum depends on the content of calcium sulfate dihydrate CaSO4 2H2O, which varies in the range of 70–90%. The final form for application is a mineral powder, it is obtained by grinding gypsum stone burned in rotary kilns.

Properties and application

In nature, the physical features of the structure are in a variety of forms: dense and granular, earthy, leafy and fibrous, concretions and dusty masses. In voids, they are found in the form of druses of crystals. The solubility of gypsum in water increases with temperature up to 37–38ºС, then decreases, and upon reaching 107ºС the mineral passes into the state of CaSO4 ½H2O hemihydrate. By adding a small amount of sulfuric acid to water, the solubility improves. On NS l reacts weakly.

In ready-made building mixtures, the properties of gypsum are transferred to the powder itself. Products acquire the qualities of the main substance with the following characteristics:

• bulk density 850–1150 kg/m3, lower values ​​for finer grinding;
• fire resistance is high: alabaster has a melting point of 1450ºС;
• setting - the beginning after 4-7 minutes, the end - after half an hour, to slow down the hardening, animal glue, soluble in water, is added;
• compressive strength of ordinary specimens 4–6 MPa, high-strength specimens 15–40.

Poor thermal conductivity - at the level of a brick (about 0.14 W / (m·deg)) allows the use of gypsum-based products in fire hazardous structures. The first examples of the use of stone in this capacity were found in Syria - they are more than 9 thousand years old.

natural views

Geologists have established several dozen varieties of gypsum, but three are the main ones. These include:

Few people know about other varieties: gypsum spar (large-crystalline and leaf), intestinal or snake stone of gray color with white, worm-like curved veins. Another little-known form is earthy gypsum.

Varieties for practical use

The use of aqueous calcium sulfate in conjunction with other binders allows you to get significant savings on more expensive materials. Alabaster that has passed the processing stage is divided into the following classes:

There are other varieties, but in practice they use a limited list. The analogue is a fine grayish-white dust - alabaster powder, which is obtained from gypsum by heat treatment.

Other uses

In its raw form, the stone is used as an additive in the production of Portland cement, the manufacture of sculptures and handicrafts. List of additional directions:

Non-traditional direction - magic. It is believed that gypsum attracts well-being and good luck, suggests the actions of a person in a difficult situation. Astrologers recommend amulets from this mineral to persons born under the signs of Leo, Aries and Capricorn.

Stone deposits

The distribution of gypsum in the earth's crust is observed everywhere, mainly in layers of sedimentary rocks 20–30 m thick. World production is about 110 million tons of stone per year. The largest producers are Türkiye, Canada, USA, Spain and Iran. Of the unique ones, the thermal caves of Naica Mine in Mexico can be noted, where druses of giant gypsum crystals 11 m long were found.

Numerous deposits of the Upper Jurassic period are located on the territory of neighboring countries: the North Caucasus, the Central Asian republics. There are 86 commercial deposits in Russia, but 90% of production is accounted for by 19 deposits, of which 9 largest can be distinguished: Baskunchakskoye, Bolohovskoye, Lazinskoye, Novomoskovskoye, Obolenskoye, Pavlovskoye, Pletnevskoye, Poretskoye, Skuratovskoye. Their share in production is 75% of the all-Russian. Most deposits are represented by a mixture of gypsum and anhydrite in a ratio of 9:1. In Russia, 6 million tons are mined annually, which is 5.5% of the world volume.

Gypsum has been known since antiquity, but still has not lost its popularity, even many modern materials cannot compete with it. It is used in the construction, porcelain, ceramics, oil and medicine industries.

Description of building material

Gypsum is made from gypsum stone. To obtain gypsum powder, the stone is fired in rotary kilns and then ground to form a powder. Most of all, gypsum is used in construction.

Walls plastered with gypsum mortar are able to absorb excess moisture and release it when the air is too dry.

Gypsum formula

The name gypsum comes from the Greek word gipsos. This material belongs to the class of sulfates. Its chemical formula is CaSO4?2H2O.

There are two types of gypsum:

1. Fibrous - selenite;
2. Grainy - alabaster.

Photo varieties of gypsum

Selenite Alabaster

Specifications and properties

For all gypsum mixtures, the technical characteristics are very similar, let's dwell on the properties and features of building gypsum.

These include:

• Density. Gypsum has a dense fine-grained structure. The true density is 2.60-2.76 g / cm?. In a loosely poured form, it has a density of 850-1150 kg / m?, and in a compacted form, the density is 1245-1455 kg / m?.
• How much to dry. The advantages of gypsum include fast setting and hardening. Gypsum seizes in the fourth minute after mixing the solution, and after half an hour it completely hardens. Therefore, the finished gypsum mortar must be used up immediately. To slow down the setting, water-soluble animal glue is added to the plaster.
• Specific gravity. The specific gravity of gypsum is measured in kg/m? in the ICSC system. Since the ratio of mass is equal to the volume it occupies, the specific, volumetric and bulk density of gypsum is approximately the same.
• What temperature can it withstand melting temperature). Gypsum can be heated to t 600-700°C without destruction. The fire resistance of gypsum products is high. Their destruction occurs only six to eight hours after exposure to high temperature.
• Strength. Building gypsum in compression has a strength of 4-6 MPa, high-strength - from 15 to 40 MPa or more. In well-dried samples, the strength is two to three times higher.
• GOST. State standard for gypsum 125-79 (ST SEV 826-77).
• Thermal conductivity. Gypsum is a poor conductor of heat. Its thermal conductivity is 0.259 kcal/m deg/h in the range from 15 to 45°C.
• Solubility in water. R dissolves in small quantities: 2.256 g dissolves in 1 liter of water at 0 °, 2.534 g at 15 °, 2.684 g at 35 °; further heating decreases the solubility again.

The video tells about building gypsum, how you can improve its properties, giving additional strength:

Varieties of gypsum

Gypsum has the greatest variety of objects of application among other binders. It allows you to save on other materials. There are many varieties of gypsum.

Building

It is used for the production of gypsum parts, partition boards for plastering. Work with gypsum mortar must be carried out in a very short time - from 8 to 25 minutes, it depends on the type of gypsum. During this time, it must be completely used up. At the beginning of hardening, gypsum is already gaining about 40% of its final strength.

Since cracks do not form on gypsum during hardening, when mixing the mortar with lime mortar, which gives it plasticity, various aggregates can be omitted. Due to the short setting time, hardening retarders are added to the gypsum. Building gypsum reduces labor intensity and construction costs.

At deposits by undermining gypsum-containing rocks. Further, the ore is transported to the factories in the form of gypsum stones.

high strength

In terms of chemical composition, high-strength gypsum is similar to building gypsum. But building gypsum has smaller crystals, while high-strength gypsum has large ones, so it has less porosity and very high strength.

High-strength gypsum is made by heat treatment in a sealed apparatus, where a gypsum stone is placed.

The scope of high-strength gypsum is extensive. Various building mixtures are prepared from it, fireproof partitions are built. It is also used to make various forms for the production of porcelain and faience sanitary ware. High-strength gypsum is used in traumatology and dentistry.

Polymeric

Orthopedic traumatologists are more familiar with synthetic polymeric gypsum; gypsum bandages are produced on its basis for applying bandages for fractures.

Advantages of polymer plaster bandages:

1. three times lighter than conventional gypsum;
2. easy to apply;
3. allow the skin to breathe, as they have good permeability;
4. resistant to moisture;
5. allow you to control the fusion of bones, as they are permeable to x-rays.

cellacast

Bandages are also made from this plaster, their structure allows the bandage to stretch in all directions, so very complex bandages can be made from it. Cellacast has all the properties of a polymer bandage.

Sculptural or molding

This is the most high-strength gypsum, it does not contain any impurities, it has a high natural whiteness. It is used for making molds for sculptures, plaster figurines, molding souvenirs, in the porcelain and faience, aviation and automotive industries.

This is the main component of dry putty mixtures. Molding gypsum is obtained from building gypsum, for this it is additionally sifted and ground.

Known for several centuries, in our time it is still relevant. The most common rosettes of their gypsum, they are easy to make with your own hands.

Acrylic

Acrylic gypsum is made from water-soluble acrylic resin. After hardening, it looks like ordinary gypsum, but much lighter. Stucco molding on the ceiling and other decorative details are made from it.

Acrylic gypsum is frost-resistant, has a slight moisture absorption, so it can be used to finish building facades, creating interesting design solutions.

Working with acrylic plaster is very simple. If a little marble chips or aluminum powder or other inert fillers are added to the solution, acrylic gypsum products will very much resemble marble or metal ones.

It looks like acrylic gypsum

Polyurethane

Gypsum stucco can also be made from polyurethane or polystyrene gypsum. It costs much less than ordinary gypsum, and in terms of its qualities it almost does not differ from it.

White

With the help of white gypsum, seams, cracks are sealed, stucco molding is made and other types of construction and repair work are carried out. It has compatibility with various kinds of building materials. Hardening time of white gypsum 10 min.

fine-grained

Fine-grained gypsum is also called translucent. They fill the seams, joints in the plates, etc.

Liquid

Liquid gypsum is made from gypsum powder.

It is prepared according to the following technology:

• Pour water in the required amount.
• Gypsum is poured and immediately mixed.
• The density of the solution can be made different. A liquid solution is made to fill molds

Waterproof (moisture resistant)

Waterproof gypsum is obtained by processing raw materials using a special technology. To improve the properties of gypsum, vinasse is added to it - a waste from the production of ethyl alcohol.

Refractory

Gypsum is a non-combustible material, but gypsum boards made from it are quite combustible. To give them fire resistance, tongue-and-groove gypsum is used. It is used wherever it is required to increase fire resistance.

Architectural

Architectural gypsum does not contain toxic components, it is very plastic. Its acidity is similar to that of human skin. Classical plaster molding is very popular with designers, the demand for it is very high.

It requires certain knowledge, so you should first carefully study the features of such work, and only then move on to practice.

Stamps

Gypsum marking is carried out after testing standard samples of sticks for bending and compression two hours after their molding. According to GOST 129-79, twelve grades of gypsum are established, with strength indicators from G2 to G25.

Gypsum substitute

An analogue of gypsum is a fine powder of grayish white color - alabaster. It is also popular in construction. Alabaster is obtained from natural gypsum dihydrate by heat treatment at a temperature of 150 to 180 ? Externally, alabaster and gypsum are no different from each other.

Alabaster plaster walls and ceilings with low humidity in the room. Gypsum panels are produced from it.

What is the difference between gypsum and alabaster

Gypsum and alabaster have the following differences:

1. Alabaster is more limited in application, as it is used only in the construction industry. Gypsum is also used in medicine.
2. Alabaster dries instantly, so without the addition of special substances it is not suitable.
3. Gypsum is safer for the environment and human health.
4. Alabaster has a greater hardness than gypsum.

If you wondered what gypsum is, then you should know that it is a mineral belonging to the class of sulfates. Two varieties of this material are known, one of which is called fibrous, and the other is called granular. The latter is alabaster.

general information

Gypsum has a silky or vitreous luster, the former of which is characteristic of the fibrous variety. Cleavage is perfect in one direction. The material is split into thin sheets. Color can be:

• reddish;
• grey;
• white;
• brown;
• yellowish.

Fibrous varieties give a splintered fracture. The density of the material is 2.3 g/cm 3 . The gypsum formula is as follows: CaSO4 2H2O. The texture is massive.

Properties and varieties

The specific gravity of the material can reach 2.4 g/cm 3 . Gypsum is quite dense, it can be granular and foliated, as well as fibrous. Some of its counterparts resemble. Sometimes it is confused with anhydride, which has an average hardness.

When you study the question of what gypsum is, you will find out that when heated, the material turns into CaSO4.1 / 2.H2O. The temperature limit is 107 °C. When wetted with water, it hardens and seizes, and dissolves in hydrochloric acid.

To date, 3 varieties are known, among them:

• selenite;
• "marino glass";
• alabaster.

The first is parallel needle-shaped and has a silky sheen. Transparent thick sheet is "marino glass". Painted fine-grained may be alabaster.

Application

Selenite, which is fibrous, is used for inexpensive jewelry. But the large ones are based on alabaster, which has been used since ancient times. The raw material is churned out. As a result, you can also get interior items, including:

• inkwells;
• countertops;
• vases.

If you are interested in the question of what gypsum is, then you should know: the material is used in its raw form as a fertilizer, as well as to obtain glazes, enamels and paints in industry and the pulp and paper industry.

The fired material is used for casts and castings. It can be cornices and bas-reliefs. In medicine and construction, the material acts as a binder. More dense varieties serve as ornamental material.

More about application

Gypsum is a valuable stone and is widely used in construction. Thousands of years ago, it was noticed that when ground, it helps fight soil salinization. This mineral was mined in karst caves. From antiquity to this day, gypsum has been applied to the soil to increase crop yields.

For many nations, he was the breadwinner. Entire cities were built from plaster. Crystal blocks were sawn out of it, which went to the construction of walls. The white stone shines dazzlingly in the sun. This can be seen even today, when only ruins remained of the ancient cities.

All over the world, sculptors cannot do without this mineral. It is inexpensive, weighs little and is easy to handle. Appreciated by painters, plasterers, traumatologists and paper manufacturers.

Origin

If you are trying to understand what gypsum is, then you should also familiarize yourself with its origin. This mineral has several types, the method of formation of which is different. In some deposits, a mineral is mined, which was concentrated there in the process of accumulation of marine sediment. In other cases, gypsum was formed when various lakes dried up. The mineral could have arisen from the deposition of native sulfur and from the weathering of its compounds. Deposits in this case can be contaminated with rock fragments and clays.

Place of Birth

After reviewing the description of gypsum, you should also learn about the main deposits that are found on all continents. Russian developments are carried out mainly in the territories of the Caucasus and the Urals. The mineral is mined in the mountainous regions of America and Asia. The United States is the champion of gypsum production. There are also deposits in the foothills of the Alps.

Specifications

The described mineral has a rather dense fine-grained structure. In a loose bulk form, the density can vary from 850 to 1150 kg/cm 3 . In compacted form, this parameter reaches 1455 kg/cm 3 . Getting acquainted with the description of gypsum, you will pay attention to one of its advantages, which is expressed in rapid hardening and setting. At the fourth minute after mixing the solution, the first stage of drying begins, and after half an hour the material hardens.

Ready gypsum mortar requires immediate consumption. To slow down the setting, water-soluble is added to the ingredients. Among the properties of gypsum, the melting point should be distinguished. The material can be heated up to 700 °C without destruction. Gypsum products are quite fire resistant. They begin to break down only 6 hours after exposure to high temperature.

The strength of gypsum is also often taken into account. During compression, this parameter can vary from 4 to 6 MPa. If we are talking about high-strength material, then it reaches 40 MPa and may even exceed this value. In well-dried samples, the strength is 3 times higher. The mineral complies with state standards 125-79. It has a thermal conductivity, which is equal to 0.259 kcal / m * deg / hour. The temperature range in this case is equal to the limit from 15 to 45 ° C.

White gypsum dissolves in water in small quantities:

• At 0 °C, 2.256 g can dissolve in one liter.
• If the temperature is raised to 15 °C, the solubility increases to 2.534 g.
• This value rises to 2.684 g at 35°C.

If further heating occurs, the solubility decreases.

Description, scope and properties of building gypsum

If we compare gypsum with other binders, then the first one has a wider area of ​​​​use. With it, you can save on other components. The construction variety is used in the manufacture of gypsum parts, during plastering and the formation of partition boards.

It is necessary to work with gypsum mortar very quickly. The polymerization start time can be from 8 to 25 minutes after mixing the solution. The final value depends on the variety. At the moment of the beginning of hardening, the mineral gains about 40% of the final strength. In this process, white gypsum is not covered with cracks, so it is possible to refuse various aggregates when mixing the solution with a lime composition. The construction variety reduces the complexity and cost of work.

Scope of use and properties of high-strength and polymer gypsum

In terms of chemical composition, the high-strength variety is similar to the construction one. However, the latter has smaller crystals. High-strength has coarse particles, therefore it has less porosity and high strength. This material is obtained by heat treatment under tightness conditions.

The area of ​​​​use is the manufacture of building mixtures and the construction of fireproof partitions. From a high-strength mineral, molds are made for the production of faience and porcelain products. The polymer type is also called synthetic and is more familiar to orthopedic traumatologists. Based on it, they are made for applying bandages for fractures. But the scope of gypsum is not the only advantage, among others it should be highlighted:

• easy overlay;
• moisture resistance;
• lighter weight compared to conventional plaster casts.

Finally

The formula of gypsum should be known to you if you are interested in this mineral. It is important to take an interest in other properties, as well as varieties. Among others, it is necessary to single out molding, sculptural and cellacast.

The latter is used to make bandages, and the structure allows the material to be stretched in all directions. The most high-strength is sculptural gypsum, which does not contain impurities. Among the properties of white gypsum, its impeccable whiteness can be distinguished.