Gypsum and its use in construction. Natural gypsum. Varieties and structural differences

Gypsum is a valuable stone, and not only as a building material. Thousands of years ago, people noticed that ground gypsum helped combat soil salinity. Mining mineral in karst caves, ancient miners contributed to the emergence of huge and extensive underground spaces. Their compatriots, by incorporating gypsum into the soil, increased the productivity of agricultural crops.

For many peoples, gypsum was the breadwinner. But entire cities were built from plaster! Blocks cut from crystalline gypsum were used to build the walls of the city of Risafa (Syria). White stone shines dazzlingly in the hot sun even today, when only picturesque ruins remain of the city...

Sculptors all over the world could not work if there was not a light, inexpensive and easy-to-use material called gypsum. Plaster is valued by traumatologists, plasterers, and paper manufacturers.

Physical properties of gypsum

The crystals are thick and thin tabular, sometimes very large. The aggregates are dense, granular, leafy, fibrous (selenite). The color of the crystal is white, often transparent, sometimes gray and pink due to impurities. The line is white. The luster is glassy, ​​while the fibrous varieties of gypsum have a silky luster. Hardness 2 on the Mohs scale. Density 2.3 g/cm3.

Chemical formula – Ca(SO4)2H2O.

Origin and deposits

The origin of gypsum is different. Some deposits contain a mineral that accumulated as marine sediment, chemically altered during the drying out of brine lakes. In other places, gypsum was formed as a result of weathering of compounds and deposits of native sulfur - in this case, mineral deposits are often contaminated with clays and rock fragments.
Gypsum deposits are found on all continents. Large Russian developments are underway in the Urals and the Caucasus. Gypsum is mined in the mountainous regions of Asia and America (the USA is the champion of gypsum production), in the foothills of the Alps.

The healing properties of gypsum

Official medicine widely uses the astringent properties of gypsum. The hygroscopicity of the material allows it to be used as an effective anti-sweating remedy. Gypsum-oil emulsion is used in medical cosmetology as a substance that restores skin turgor.

Not long ago, science found out: the crystalline structure of gypsum seems to be deliberately created to retain heavy metal ions. Lithotherapists responded to the discovery: today, wet wrapping in crushed plaster is becoming increasingly common. Calcium and sulfur literally pull out harmful substances from the skin and thereby gradually heal the body.

Looking at a selenite ball (selenite is a fibrous form of crystalline gypsum) helps calm people down. nervous system with simultaneous concentration.

The magical properties of gypsum

Main magical property gypsum – the ability to absorb passions. That is why ownership of plaster jewelry is recommended for nervous, hot-tempered, and hot-tempered people. Aries and Capricorns, Leos and Sagittarius can successfully use plaster talismans to optimize their own behavior.

Use gypsum crystals in magical rituals difficult: a stone knows how to show a person the vanity of his ideas, the wretchedness of his goals, the primitiveness of his actions. The magically destructive role of gypsum is useful for convinced proud people and self-confident dropouts, but it can do a disservice to a person who is not too confident in himself.

Use of plaster decorations

Apart from purely practical use, gypsum can be used as an excellent interior decoration. In this case, we are not talking about gypsum stucco, a common architectural element of premises, but about crystalline formations.

“Desert Roses” is the name given to accretions of smoothly curved gypsum plates that truly resemble flowers. The similarity is especially strong if the size of the natural aggregate does not exceed the size of a garden rose flower, the color of the plates is white to translucent, and the “petals” themselves are thin, like real petals.

Such specimens are relatively rare and therefore expensive. More often than not, “desert roses” are inconspicuous, mined by local collectors in the hundreds, and sold by weight... However, even the most modest cream-colored gypsum “rose” can become an interior object of admiration and a source of positive aesthetic impressions.

Gypsum crystals in nature can grow to gigantic sizes and at the same time have enviable optical quality. However, gypsum is rarely cut: the crystalline druses of the mineral themselves are very diverse and very decorative. A collection of gypsum crystals can take a lifetime to collect, but display all forms natural diversity It's unlikely to succeed!

Plaster in art

Colorless lamellar crystals of gypsum in the Russian linguistic tradition are called “Marya glass”. The name comes from the past. In the old days, such plaster (especially specimens with a mother-of-pearl sheen) was used to frame images. Transparent or iridescent plaster was especially often used to decorate icons of the Virgin Mary. Hence the “Maryino glass”.

Fibrous gypsum, found in the Urals in the century before last, immediately became an object of adoration among lovers of elegant trinkets. Mineral that seems to glow inner light, received the sonorous name “selenite” and became the main material for making figurines. Some varieties of selenite, having the effect of asterism, make it possible to carve mystically shimmering sculptural miniatures.

Jewelry made from crystalline gypsum is more of a souvenir character. The fragility of the stone, which is extremely susceptible to abrasive wear, does not allow cabochons and rings carved from a gypsum monolith to retain their attractiveness for a long time.

Dehydrated gypsum, called anhydrite, resembles marble in appearance and properties. Once popular cabinets writing instruments for two centuries they were also carved from anhydrite. Today this mineral is used to make sculptural interior decorations.

However, those buyers of anhydrite figurines who place their purchases in greenhouses, winter gardens, swimming pools and other wet rooms are mistaken. In the presence of water, anhydrite absorbs moisture, gradually (not necessarily proportionally) increases in size and loses its decorative effect.

Gypsum- mineral, hydrous calcium sulfate. The fibrous variety of gypsum is called selenite, and the granular variety is called alabaster. One of the most common minerals; the term is also used to designate the rocks it consists of. Gypsum is also commonly called a building material obtained by partial dehydration and grinding of the mineral. The name comes from the Greek. gypsos, which in ancient times meant both plaster itself and chalk. A dense snow-white, cream or pink fine-grained variety of gypsum known as alabaster

See also:

STRUCTURE

Chemical composition - Ca × 2H 2 O. Monoclinic system. The crystal structure is layered; two sheets of anionic 2- groups, closely associated with Ca 2+ ions, form double layers oriented along the (010) plane. H 2 O molecules occupy spaces 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 SO 4 groups and two water molecules. Each water molecule binds a Ca ion to one oxygen ion in the same bilayer and to another oxygen ion in the adjacent layer.

PROPERTIES

The color varies, but usually white, gray, yellow, pink, etc. Pure transparent crystals are colorless. Impurities can be painted in different colors. The color of the dash is white. The luster of the crystals is glassy, ​​sometimes with a pearlescent tint due to microcracks of perfect cleavage; in selenite it is silky. Hardness 2 (Mohs scale standard). The cleavage is very perfect in one direction. Thin crystals and fusion plates are flexible. Density 2.31 - 2.33 g/cm3.
It has noticeable solubility in water. A remarkable feature of gypsum is the fact that its solubility with increasing temperature reaches a maximum at 37-38°, and then drops quite quickly. The greatest decrease in solubility occurs at temperatures above 107° due to the formation of “hemihydrate” - CaSO 4 × 1/2H 2 O.
At 107°C, it partially loses water, turning into white alabaster powder (2CaSO 4 × H 2 O), which is noticeably soluble in water. Due to the smaller number of hydration molecules, alabaster does not shrink during polymerization (increases in volume by approximately 1%). Under item tr. loses water, splits and fuses into white enamel. On coal in a reducing flame it produces 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 above 75 g/l. solubility drops sharply. Very slightly soluble in HCl.

MORPHOLOGY

Crystals, due to the predominant development of faces (010), have a tabular, rarely columnar or prismatic appearance. Of the prisms, the most common are (110) and (111), sometimes (120), etc. The faces (110) and (010) often have vertical hatching. Fusion twins are common and come in two types: 1) Gallic by (100) and 2) Parisian by (101). It is not always easy to distinguish them from each other. Both of them resemble a dovetail. Gallic twins are characterized by the fact that the edges of the prism m (110) are located parallel to the twin plane, and the edges of the prism l (111) form a reentrant angle, while in 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. Characteristic are intergrowths in the form of a “rose” and twins - the so-called. " dovetails"). Forms veinlets of a parallel-fibrous structure (selenite) in clayey sedimentary rocks ah, as well as dense, continuous, fine-grained aggregates reminiscent of marble (alabaster). Sometimes in the form of earthy aggregates and cryptocrystalline masses. Also makes up the cement of sandstones.
Pseudomorphoses of calcite, aragonite, malachite, quartz, etc. on gypsum are common, as are pseudomorphs of gypsum on other minerals.

ORIGIN

A widely distributed mineral natural conditions is formed in various ways. The origin is sedimentary (typical marine chemogenic sediment), low-temperature hydrothermal, found in karst caves and solfataras. Precipitates from sulfate-rich aqueous solutions during the drying out of sea lagoons and salt lakes. Forms layers, layers and lenses among sedimentary rocks, often in association with anhydrite, halite, celestine, native sulfur, sometimes with bitumen and oil. It is deposited in significant quantities by sedimentation in lake and sea salt-bearing dying pools. 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 concentration of salts is reached, in particular NaCl and especially MgCl 2, anhydrite will crystallize instead of gypsum and then other, more soluble salts, i.e. The gypsum in these basins must belong to 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.

In Russia, thick gypsum-bearing strata of Permian age are distributed throughout the Western Urals, in Bashkiria and Tatarstan, in Arkhangelsk, Vologda, Gorky and other regions. Numerous deposits of 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 Central Asia(in Tajikistan and Uzbekistan), in the Middle Volga region, in Jurassic clays Kaluga region. In the thermal caves of Naica Mine, (Mexico), druses of uniquely sized gypsum crystals up to 11 m long were found.

APPLICATION

Today, the mineral "gypsum" is mainly a raw material for the production of α-gypsum and β-gypsum. β-gypsum (CaSO 4 ·0.5H 2 O) is a powdered binder material obtained by heat treatment of natural dihydrate gypsum CaSO 4 ·2H 2 O at a temperature of 150-180 degrees in apparatus communicating with the atmosphere. The product of grinding β-modification gypsum into a fine powder is called building gypsum or alabaster; with finer grinding, molding gypsum is obtained or, when using high-purity raw materials, medical gypsum.

During low-temperature (95-100 °C) heat treatment in hermetically sealed apparatus, α-modification gypsum is formed, the grinding product of which is called high-strength gypsum.

When mixed with water, α and β-gypsum hardens, turning back into gypsum dihydrate, with the release of heat and a slight increase in volume (by approximately 1%), however, such secondary gypsum stone already has a uniform fine-crystalline structure, color various shades white (depending on raw material), opaque and microporous. These properties of gypsum are used in various fields of human activity.

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

CLASSIFICATION

Strunz (8th edition)	6/C.22-20
Nickel-Strunz (10th edition)	7.CD.40
Dana (7th edition)	29.6.3.1
Dana (8th edition)	29.6.3.1
Hey's CIM Ref.	25.4.3

PHYSICAL PROPERTIES

Mineral color	colorless turning to white, often colored by impurity minerals yellow, pink, red, brown, etc.; sometimes sectorial-zonal coloring or distribution of inclusions across growth zones inside crystals is observed; colorless in internal reflexes and at random.
Stroke color	white
Transparency	transparent, translucent, opaque
Shine	glassy, ​​close to glassy, ​​silky, pearlescent, dull
Cleavage	very perfect, easily obtained by (010), almost mica-like in some samples; along (100) clear, turning into a conchoidal fracture; according to (011), gives a splintered fracture (001)
Hardness (Mohs scale)	2
Kink	smooth, conchoidal
Strength	flexible
Density (measured)	2.312 - 2.322 g/cm 3
Radioactivity (GRapi)	0

Gypsum has been known since ancient times and is still popular both in construction and in other industries, as well as in medicine. The formula of natural (dihydrate) gypsum is CaSO4 x2H2O. Even many modern materials do not surpass it in some respects technical specifications. If we talk about construction, then most often gypsum is used in the form of a powder, which is obtained by burning and grinding gypsum stone. It is used as a binder for the preparation of various mortars, and various decorative elements are also made from it. To work with gypsum, it must be diluted in a certain proportion with water, filler must be added if necessary, after which it will turn into a plastic composition, and you can begin working with it directly.

Properties and application of building gypsum.

A distinctive feature of building gypsum from other binding materials (cement, lime paste) is its ability to expand during hardening. The setting process occurs quite quickly - from 5 to 30 minutes, which, in some cases, is not entirely convenient. The setting time of gypsum depends on various components: properties of raw materials, manufacturing technology, storage duration, amount of water introduced, temperature of the binder and water, mixing conditions, presence of additives, etc. To regulate the setting time, various additives are added to the gypsum when mixing with water. . For example, to slow down setting, SDB, lime-adhesive and keratin retarders are added to the gypsum solution in an amount not exceeding 0.1-0.5% (in terms of dry matter) by weight of gypsum. For small volumes of work, for example during repairs, hide glue or gelatin is also used as a retarder. These additives are known as gypsum plasticizers and retarders.
To speed up the setting (which is necessary, for example, for faster removal of a cast product from a mold) of gypsum, gypsum dihydrate is most often used, table salt and sodium sulfate, introducing them in amounts from 0.2 to 3% by weight.
Based on strength, gypsum is divided into 12 grades. In construction and repairs, grades from G2 to G7 are mainly used. The letter stands for the word “gypsum”, and the number indicates the compressive strength of this material. For example, the G7 gypsum grade has a compressive strength of 7 MPa or 70 kg/cm2. This is quite high strength with a density of 1200 ÷ 1500 kg/m 3.
Among the disadvantages, one can highlight its rather low hygroscopicity, so its use is only applicable in rooms with low humidity.

Working with plaster.

In practice, when working with gypsum, a solution of pure gypsum is mainly used, less often with filler. Depending on the type of work, the gypsum solution may have varying degrees thickness: liquid, medium or normal or thick. To prepare a liquid solution for 1 kg of gypsum you will need approximately 0.7 liters of water, a medium or normal solution - for 1.5 kg of gypsum 1 liter of water and for a thick solution - for 2 kg of gypsum 1 liter of water.
The solution is prepared as follows: the required amount of water is first poured into the prepared container and plaster is gradually poured into it with constant thorough mixing. With this method of preparation, a homogeneous mass is obtained without any admixtures of lumps of unmixed gypsum. You should not stir a gypsum solution that has already begun to set, since in this case the gypsum begins to rejuvenate and practically loses its strength qualities.
When working with gypsum, you should take into account the rapid setting of the gypsum solution and prepare in small portions. To slow down the setting time of the gypsum mortar, set retarders are used, which have already been discussed above. When using an adhesive solution as a retarder, it is poured into the water prepared for mixing, mixed thoroughly and the plaster is mixed in this water. The adhesive solution should be prepared for one day of work.

Gypsum-based building materials.

Probably the most famous and widely used gypsum building products are plasterboard sheets. They are usually used for finishing walls, ceilings and installing partitions in rooms and buildings with dry, normal and even wet conditions. Drywall sheets are available in various sizes and are divided into two types of rectangular shape: with straight longitudinal edges or with thinned edges on the front side.
Another type of gypsum-based building material is gypsum fiber extruded decorative boards. This material is widely used for interior decoration, as it has a noble and attractive appearance. In addition, the use of decorative boards reduces the labor intensity of finishing work, since putty and plastering work is eliminated. The front side of the slabs has low water absorption and a denser structure.
In addition to gypsum-based products, bulk materials are widely used. This various types dry mixes(plasters, putties, etc.). The use of dry plaster mixtures allows you to mechanize the application of the plaster layer and significantly reduce the drying time of the surfaces being finished.

Along with gypsum-based finishing materials, building materials such as gypsum concrete. And, although, due to the rather long drying period of structures, gypsum concrete is rarely used for the construction of walls, it has found quite wide application in the construction of partitions.
The undoubted advantage of gypsum concrete is its low thermal conductivity compared to traditional concrete, as well as its heat and sound insulation properties, which significantly reduces the cost of construction and provides sound insulation for each individual residential area.

Many people who are inexperienced in repair and construction matters often have a question: what is the difference between these Construction Materials like plaster and alabaster? And why do the bags say “construction gypsum” on the top and “alabaster” on the bottom?

In order not to get lost in the terms, you need to figure out what gypsum and alabaster actually are, whether there are differences between them and, if so, what they are.

Gypsum - origin, application

Gypsum is a dry composition made from the natural mineral gypsum stone. The mineral is calcium sulfate dihydrate – CaSO4 2H2O with impurities in the form of silicon, aluminum and iron oxides.

Gypsum is a mineral of sedimentary origin. In nature, it most often occurs in the form of elongated prismatic crystals, although sometimes it forms in the form of dense tablet-like or scaly aggregates. The mineral is quite soft and easy to grind.

Large deposits of gypsum stone are located in countries such as Iran, the USA, Canada, Turkey, and Spain. In Russia, deposits of this rock are located in the Kama and Volga regions, Tatarstan, on the western slopes Ural mountains and in the Krasnodar region.

A binder is obtained from a natural mineral - in fact, the gypsum that we all know. This is a white, cream or grayish powder (depending on the existing impurities), which, when mixed with water, turns into a plastic mass that hardens quite quickly in air.

The method of using ground gypsum depends on what exactly it is planned to be used for:

• “raw” gypsum is used in medicine to fix fractures, as well as in agriculture– scattered on fields to normalize soil acidity;
• in the form of “building gypsum” it is used during repair and finishing work, for the production of wall slabs and blocks, cornices, and stucco molding.

The mineral is also widely used in the paper and chemical industries: in the production of cement, sulfuric acid, glazes and paints.

Natural gypsum is fibrous and granular. To produce alabaster, fine-grained gypsum is used - alabaster. Construction alabaster has a finer grind and is the same calcium sulfate, but not dihydrate, but semi-hydrate - CaSO4 0.5H2O. It is obtained by firing crushed natural alabaster at temperatures up to 180 degrees.

Thus, the alabaster that we purchase at a hardware store is, in a broad sense, gypsum, but not all gypsum can be called alabaster.

Construction gypsum has the following characteristics:

• Density (true) is 2.6 – 2.76 g/cu. cm. At the same time, in loose form, the density is 0.85 - 1.15 g/cu. cm, and in compacted - 1, 245 - 1,455 g / cubic meter. cm.
• Products made from gypsum have high fire resistance - they are destroyed only after 6-8 hours of exposure high temperature. The structures can withstand heating up to 600-700 degrees without destruction.
• The compressive strength of building gypsum is 4-6 MPa, high-strength gypsum is 15-40 MPa.
• Gypsum and products made from it conduct heat poorly; its heat transfer coefficient in the temperature range from 15 to 45 degrees is only 0.259 kcal/m deg/hour.
• Drying speed. After mixing with water, the gypsum solution begins to set within 4 minutes and over the next half hour it completely hardens. Therefore, you need to work with this solution very quickly.

Brands and properties of building gypsum

The regulatory document regulating the properties and quality of construction gypsum binders is GOST 125-79. The industry produces 12 grades of alabaster, differing in compressive strength.

The indicators are shown in the table:

Gypsum brand	Tensile strength of beam samples measuring 40×40×160 mm at the age of 2 hours,MPa, not less
	compression	bend
G-2	2	1,2
G-3	3	1,8
G-4	4	2,0
G-5	5	2,5
G-6	6	3,0
G-7	7	3,5
G-10	10	4,5
G-13	13	5,5
G-16	16	6,0
G-19	19	6,5
G-22	22	7,0
G-25	25	8,0

An important indicator is the setting time of the binder.

Depending on it, the following types of building gypsum are distinguished:

• A – fast-hardening (start no earlier than 2 minutes, end no later than 15 minutes).
• B - normal hardening (start of setting no earlier than 6 minutes, end - no later than 30 minutes).
• B – slowly hardening (start of setting no earlier than 20 minutes, end – not standardized).

The degree of grinding is also standardized:

Thus, by the brand of binder, all its main characteristics can be determined.

For example, the bag says: G-6 B II.

This means that we have before us a material with the following characteristics:

• strength not less than 6 and not more than 7 MPa;
• slow hardening;
• medium grind.

Types of gypsum

Gypsum binders are used not only in their pure form, but also with various additives that allow them to change their properties.

Currently, the following varieties of gypsum can be found on sale:

• Construction – for the production of gypsum building materials and for plastering work. This material is good because it does not form cracks when drying. Lime is often added to it, which gives the mixture plasticity. The material is mainly used for interior decoration of dry rooms.
• High-strength - a binder with large crystals, providing the final product with less porosity and, accordingly, greater strength. This material used for constructing fireproof partitions, molds for the production of earthenware and porcelain sanitary ware. It is also used in traumatology and dentistry.
• Polymer gypsum is a binder with the addition of polymers. Often used in traumatology. Dressings with such plaster are much lighter than conventional plaster, allow the skin to breathe, are not afraid of moisture, and are permeable to x-rays (allowing you to control the process of bone fusion).

• Sculptural gypsum is the highest strength gypsum, practically free of impurities. The material has a high degree of whiteness and is used for making figurines. Sculptures, souvenirs, as well as in the automotive and aviation industries. This binder is the basis of dry putty mixtures.
• Acrylic gypsum - obtained by adding water-soluble acrylic resin to the binder. Externally, it is practically indistinguishable from ordinary plaster, but much lighter. Due to this, it is often used for ceiling stucco. The material is frost-resistant and has low water absorption, so it can be used for work on building facades.

Thus, alabaster is one of the varieties of gypsum, which is mainly used in construction. It has greater hardness than natural gypsum, but is less widely used.