/ mineral Gypsum
Gypsum is a mineral, hydrous calcium sulfate. gypsum stone, mirror stone, monmartite, sand rose, desert rose, gypsum spar. The composition of gypsum includes the following elements: Ca, S, O. Calcium oxide (CaO) 32.6%, sulfur trioxide (SO 3) 46.5%, water (H 2 O) 20.9%. Thin crystals and cleavage plates are flexible. The crystal structure is layered; two sheets of 2- anionic groups, closely associated with Ca2 + ions, compose double layers oriented along the (010) plane. H2O molecules occupy the spaces between these double layers. This easily explains the very perfect cleavage characteristic of gypsum. Each calcium ion is surrounded by six SO4 oxygen ions 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 an adjacent layer. Alabaster, Mary's glass (maiden ice, maiden glass), selenite (satin spar) It has a marked water solubility. A remarkable feature of gypsum is the fact that its solubility with an increase in temperature reaches a maximum at 37-38 °, and then falls rather quickly. The greatest decrease in solubility is established at temperatures above 107 ° due to the formation of "hemihydrate" - CaSO4 × 1 / 2H2O. At 107oC, it partially loses water, turning into a white powder of alabaster (2CaSO4 × H2O), which is noticeably soluble in water. Due to the lower amount of hydrated molecules, alabaster does not shrink during polymerization (it increases in volume by approx. 1%). Under p. Tr. loses water, splits and fuses into white enamel. On coal in a reducing flame it gives CaS. It dissolves much better in water acidified with H2SO4 than in pure water. However, when the concentration of H2SO4 is over 75 g / l. solubility drops sharply. Very little soluble in HCl. Characterized by intergrowths in the form of a "rose" and twins - the so-called. "dovetails"). Forms veins of parallel-fibrous structure (selenite) in clayey sedimentary rocks, as well as dense continuous fine-grained aggregates resembling marble (alabaster). Sometimes in the form of earthy aggregates and cryptocrystalline masses. Also composes sandstone cement. Common pseudomorphs on gypsum of calcite, aragonite, malachite, quartz, etc., as well as pseudomorphs of gypsum on other minerals. A widespread mineral, in natural conditions it is formed in various ways. Sedimentary origin (typical marine chemogenic sediment), low-temperature-hydrothermal, occurs in karst caves and solfatars. It precipitates from sulfate-rich aqueous solutions during the drying up of sea lagoons and salt lakes. Forms layers, interlayers and lenses among sedimentary rocks, often in associations with anhydrite, halite, celestine, native sulfur, sometimes with bitumen and oil. In significant masses, it is deposited by sedimentary means in lacustrine and marine saline 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. Upon reaching a certain certain value of the concentration of salts, in particular NaCl and especially MgCl2, instead of gypsum, anhydrite will crystallize and then other, more soluble salts, i.e. gypsum in these basins must belong to the earlier chemical sediments. Indeed, in many salt deposits, gypsum (and also anhydrite) layers, interbedded with rock salt layers, 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 hydration of anhydrite, which in turn was precipitated by the evaporation of seawater; quite often, during its evaporation, gypsum is precipitated directly. Gypsum occurs as a result of hydration of anhydrite in sedimentary deposits under the influence of surface waters under conditions of low external pressure (on average to a depth of 100-150 m.) According to the reaction: CaSO4 + 2H2O = CaSO4 × 2H2O. At the same time, there is a strong increase in volume (up to 30%) and, in this regard, numerous and complex local disturbances in the conditions of occurrence of gypsum-bearing strata. In this way, most of the large gypsum deposits in the world arose. In cavities among continuous gypsum masses, nests of large, often transparent crystals are sometimes found. Can serve as a cement in sedimentary rocks. Gypsum is usually a product of the reaction of sulfate solutions (formed during the oxidation of sulfide ores) with carbonate rocks. It is formed in sedimentary rocks during the weathering of sulfides, when the sulfuric acid formed during the decomposition of pyrite is exposed to marls and calcareous clays. In semi-desert and desert areas, gypsum is very often found in the form of veins and nodules in the weathering crust of rocks of various compositions. In the soils of the arid zone, new formations of secondary redeposited gypsum are formed: single crystals, twins ("swallow's tails"), druses, "gypsum roses", etc. Gypsum is quite well soluble in water (up to 2.2 g / l.), And with an increase in temperature, its solubility first increases, and above 24 ° C it falls. Due to this, gypsum, when precipitated from sea water, is separated from halite and forms independent layers. In semi-deserts and deserts, with their dry air, sudden daily temperature changes, 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 the evening, with a decrease in temperature, crystallization stops, but due to a lack of moisture, the crystals do not dissolve - in areas with such conditions, gypsum crystals are found in especially large quantities. In Russia, thick gypsum-bearing strata of Permian age are widespread in the Western Urals, in Bashkiria and Tatarstan, in the 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 of 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), druses of unique size gypsum crystals up to 11 m long were found. Fibrous gypsum (selenite) is used as an ornamental stone for inexpensive jewelry. Since ancient times, large jewelry has been carved from alabaster - interior items (vases, countertops, inkpots, etc.). Fired gypsum is used for casts and casts (bas-reliefs, cornices, etc.), as a binding material in the construction business, in medicine. It is used to obtain stucco, high-strength gypsum, gypsum-cement-pozzolanic binder.Synonyms
Chemical composition
Mineral varieties
Forms of finding
Origin
Location
Application
tell about an error in the description
Mineral properties
| Color | White, reddish, monocrystals are often colorless, transparent, water-transparent (Mary's glass). |
| Line color | White |
| origin of name | From the Greek γυψοζ meaning chalk or plaster |
| Opening year | The first mention of gypsum by Theophrastus 300-325g. |
| IMA status | valid, described for the first time before 1959 (before IMA) |
| Chemical formula | CaSO 4 * 2H 2 O |
| Shine | glass pearl silky dim |
| Transparency | transparent translucent shines through opaque |
| Cleavage | perfect by (010) average over (100) |
| Break | conchoidal stepped splinter |
| Hardness | 2 |
| Thermal properties | P. tr. Decomposes with loss of crystallization water and melts into white enamel. In a closed tube, it loses water of crystallization, turning into calcium sulfate (“hard-fired gypsum”) |
| Luminescence | Gypsum crystals with inclusions sometimes show bluish-white, yellow, green fluorescence |
| Strunz (8th Edition) | 6 / C.22-20 |
| Hey "s CIM Ref. | 25.4.3 |
| Dana (7th Edition) | 29.6.3.1 |
| Dana (8th Edition) | 29.6.3.1 |
| Molecular weight | 172.17 |
| Cell parameters | a = 5.679 (5) Å, b = 15.202 (14) Å, c = 6.522 (6) Å β = 118.43 ° |
| Attitude | a: b: c = 0.374: 1: 0.429 |
| Formula units (Z) | 4 |
| Unit cell volume | V 495.15 ų |
| Twinning | twins of germination are frequent according to one of two laws: 1) twins dovetail, which are most widespread - twinning along the edges of the prism; 2) Montmartre (Parisian) twins-prism edges are located parallel to the twin seam |
| Point group | 2 / m - Prismatic |
| Density (calculated) | 2.308 |
| Density (measured) | 2.312 - 2.322 |
| Dispersion of optical axes | strong r> v oblique |
| Refractive indices | nα = 1.519 - 1.521 nβ = 1.522 - 1.523 nγ = 1.529 - 1.530 |
| Maximum birefringence | δ = 0.010 |
| A type | biaxial (+) |
| angle 2V | measured: 58 °, calculated: 58 ° to 68 ° |
| Optical relief | short |
| Allocation form | Crystals are tabular, rarely columnar and prismatic; accretion twins are characteristic. Druses of crystals, dense fine-crystalline aggregates, asbestos-like parallel-fibrous masses (selenite), veins, nodules |
| Classes on taxonomy of the USSR | Sulphates |
Gypsum - hydrated calcium sulfate, one of the most abundant minerals; the term is also used to refer to the rocks he built. It is also customary to call gypsum a building material obtained by partial dehydration and grinding of a mineral.
In the old days, the ancient Greek word "gypsum" was used to refer to both gypsum and chalk, but over time the name stuck only to the gypsum itself.
Plaster plates with a mother-of-pearl shimmer used to frame images, including the icon of the Mother of God - the Virgin Mary. Hence such ancient names of gypsum as "Mary's glass", "ladies' ice" or "girl's ice".
Other synonyms: alabaster, ribbon stone, lollipop (obsolete), marmolite, Zhiguli marble.
The dehydrated analogue of gypsum is the mineral anhydrite (Greek "anhydrous").
Besides the relatively rare transparent crystalline variety, there are two main forms of gypsum:
1) alabaster is a massive fine-grained variety, sometimes used as a facing material; Historians (in particular, Egyptologists) often call alabaster an antique building and ornamental material, which is actually a fine-grained crystalline calcite or "marble onyx". However, in mineralogy, the term "alabaster" is used only in relation to gypsum.
2) selenite or silky spar - fibrous gypsum, which is often used as an ornamental stone.
Composition - CaSO4 2H2O. The system is monoclinic. The actual composition of gypsum is close to the theoretical formula; only small fluctuations are characteristic.
The crystal structure is layered. It is based on double layers consisting of 2– anionic groups, which are closely related to Ca2 + ions. Water molecules are located between these layers, which determines the perfect cleavage of the mineral. Each calcium atom is surrounded by six oxygen atoms (from the SO4 groups) and two water molecules.
Gypsum crystals are predominantly tabular; columnar and prismatic forms are less common.
Gypsum; the accretion of crystals is about 9 cm; North-Svetlinskaya placer, South Urals. Photo: R. Khayryatdinov.
Characterized by very peculiar twins, their appearance resembling a dovetail. In voids, it is sometimes found in the form of druses. Forms dense fine-crystalline aggregates, as well as parallel fibrous masses with a silky sheen (selenite).
Sometimes gypsum crystals are twisted and twisted, resulting in spectacular intergrowths - "gypsum flowers".
The most famous such formation is the so-called "", consisting of gypsum and small grains of sand captured in the growth process.
In its pure form, it is colorless or white, sometimes with a grayish tint. Impurities captured during crystallization give gypsum greenish, blue, yellow and other shades. For example, iron paints it reddish brown. Luster: glassy; on the seam surfaces - mother-of-pearl. Refractive indices: 1.519 - 1.531.
Fragile. Good cleavage in three directions; the cleft fragments are rhombus-shaped with angles of 66 ° and 114 °. Hardness - 2; gypsum is one of the standards in the Mohs scale. The average specific gravity is 2.3 g / cm3. Slightly soluble in hydrochloric acid. Let's noticeably dissolve in water. When a small amount of sulfuric acid is added to water, the solubility increases significantly, but when the concentration of H2SO4 is more than 75 g / liter, it drops sharply.
Gypsum; crystal about 4 cm. © Milton Speckels
Gypsum has one unusual and very useful property for us - its solubility in water reaches a maximum at a temperature of about 40 ° C, and with further heating it decreases rather quickly. The greatest drop in solubility is achieved at about 110 ° C, which is associated with the transition of gypsum to the so-called hemihydrate (Ca 0.5H2O) - plaster gypsum (alabaster), which, when mixed with water, soon expands and hardens, releasing heat.
In total, there are five different ways of forming natural gypsum:
- deposition by evaporation of sea water;
- concentration of dispersed gypsum by flowing waters;
- changes in limestones under the influence of acidic sulphate waters;
- change (hydration) of anhydrite;
- less often crystallizes as a hydrothermal mineral in sulfide deposits.
Deposits of gypsum are so numerous that it hardly makes sense to list them. its thick strata usually occur in the single-arm estuaries of ancient rivers, off the shores of inland seas, in shallow marine sediments, or in places where salt lagoons were once located.
Such areas are widespread throughout the planet, but the most extensive deposits are located in the Mediterranean countries: Greece, Italy, Spain, Morocco, Tunisia. In Russia, gypsum is mined in many regions; among them - the Urals, the Arkhangelsk region, Dagestan, the Volga region.
Gypsum; Vodinskoe deposit, Volga region.
Gypsum is available, easy to process, easily absorbs water, and possesses a number of special chemical and physical properties. All this makes it an almost irreplaceable building material, which is very plastic - in an uncured form it is easy to give it any shape. We owe smooth walls and ceilings in our houses to gypsum - this is the most important component of cements. In addition, it finds application in the creation of various architectural details, for obtaining sculptural copies and even when removing death masks.
Ganch (also known as clay gypsum) is a natural or artificial mixture of gypsum and clay, which has been used since ancient times for plastering walls and ceilings, as well as for making decorative cladding and sculptural details.
It is the ganch that determines the appearance of the most beautiful cities of Central Asia - Bukhara, Samarkand, Khiva, where masters carved sayings from the Koran on it.
In addition, the ganch excellently retains its color and its beautifully painted plates perfectly convey the artistic intention of the carvers.
The chemical composition of gypsum includes calcium and sulfur, which makes this mineral a useful fertilizer. It also cleans the soil from heavy metals - due to the peculiarities of the crystal structure, gypsum can retain and absorb these polluting elements, preventing them from penetrating into the water. This eliminates harmful substances from the food chain and makes agricultural products environmentally friendly.
At the end of the 19th century, the commercial name "selenite" was coined for the ornamental fibrous gypsum mined in the Urals.
With its mother-of-pearl sheen, this stone really resembles the moon. In addition, some of its varieties have the effect of a cat's eye. Such plaster was used to make figurines, balls and other souvenir items. It is still widely used today as an inexpensive but beautiful ornamental and jewelry stone.
Read more about - a stone like the Moon.
Introduction
Plaster-based materials have various purposes in dental practice. These include:
Models and dies;
Impression materials;
Foundry forms;
Refractory molding materials;
Model- This is an exact copy of the hard and soft tissues of the patient's oral cavity; the model is cast from an impression of the anatomical surfaces of the oral cavity, and subsequently it is used for the manufacture of partial and complete dentures. The casting mold is used for the manufacture of a denture from metal alloys.
Stamps- These are copies or models of individual teeth that are needed in the manufacture of crowns and bridges.
Refractory molding material for cast metal dentures is a material that is resistant to high temperatures, in which gypsum serves as a binder or ligament; this material is used for molds in the manufacture of prostheses from some casting alloys based on gold.
The chemical composition of gypsum
Compound
Gypsum- calcium sulfate dihydrate CaS04 - 2H20.
When calcining or firing this substance, i.e. heating to temperatures sufficient to remove a certain amount of water, it turns into calcium sulfate hemihydrate (CaSO4) 2 - H20, and at higher temperatures anhydrite is formed according to the following scheme:
Obtaining hemihydrate of calcium sulfate can be carried out in three ways, allowing you to obtain varieties of gypsum for various purposes. These varieties include: calcined or conventional medical plaster, plaster cast and super plaster; it should be noted that these three types of material have the same chemical composition and differ only in shape and structure.
Calcined Plaster (Plaster of Paris)
Calcium sulfate dihydrate is heated in an open digester. The water is removed and the dihydrate is converted to calcium sulfate hemihydrate, also called calcined calcium sulfate or HZ hemihydrate. The resulting material consists of large, porous particles of irregular shape, which are not capable of significant compaction. The powder of such gypsum must be mixed with a large amount of water in order for this mixture to be used in dental practice, since the loose porous material absorbs a significant amount of water. The usual mixing ratio is 50 ml of water per 100 g of powder.
Modeling plaster
When calcium sulfate dihydrate is heated in an autoclave, the resulting hemihydrate consists of small particles of regular shape, which have almost no pores. This autoclaved calcium sulfate is called a-hemihydrate. Due to its non-porous and regular particle structure, this type of gypsum gives a tighter packing and requires less water for mixing. The mixing ratio is 100 g of powder for 20 ml of water.
Supergypsum
In the production of this form of calcium sulfate hemihydrate, the dihydrate is boiled in the presence of calcium chloride and magnesium chloride. These two chlorides act as deflocculants, preventing floc formation in the mixture and promoting particle separation as otherwise the particles tend to agglomerate. The particles of the obtained hemihydrate are even denser and smoother than those of autoclaved gypsum. Supergypsum is mixed in a ratio of 20 ml of water to 100 g of powder.
Application
Common calcined or medical plaster is used as a general-purpose material, mainly as the base of models and the models themselves, since it is cheap and easy to process. Expansion on solidification (see below) is not essential in the manufacture of such products. The same gypsum is used as an impression material as well as in formulations of refractory molding materials based on a gypsum binder, although for this use the working time and setting time, as well as the expansion upon setting, are carefully controlled by the introduction of various additives.
Autoclaved plaster is used to make models of the tissues of the oral cavity, while more durable super plaster is used to make models of individual teeth, called dies. Various types of wax restorations are simulated on them, which are then used to obtain cast metal prostheses.
Curing process
When calcium sulfate hydrate is heated to remove some of the water, a largely dehydrated material is formed. As a consequence, calcium sulfate hemihydrate is able to react with water and convert back to calcium sulfate dihydrate by the reaction:
It is believed that the gypsum hardening process occurs in the following sequence:
1. Some calcium sulfate hemihydrate dissolves in water.
2. Dissolved calcium sulfate hemihydrate reacts with water to form calcium sulfate dihydrate.
3. The solubility of calcium sulfate dihydrate is very low, so a supersaturated solution is formed.
4. Such a supersaturated solution is unstable and calcium sulfate dihydrate precipitates as insoluble crystals.
5. When calcium sulfate dihydrate crystals precipitate out of solution, the next additional calcium sulfate hemihydrate dissolves again, and this process continues until all the hemihydrate is dissolved. Working time and curing time
The material must be mixed and poured into the mold before the end of working hours. The working time is different for different products and is chosen depending on the specific application.
For impression plaster, the working time is only 2-3 minutes, while for refractory molding materials on a gypsum binder, it reaches 8 minutes. A short working time is associated with a short solidification time, since both of these processes depend on the reaction rate. Therefore, if the normal working time for impression plaster is within 2-3 minutes, then the setting time for refractory gypsum molding materials can vary from 20 to 45 minutes.
Model materials have the same working time as impression plaster, but the hardening time is slightly longer. For impression plaster, the setting time is 5 minutes, while for autoclaved or plaster cast it can take up to 20 minutes.
Changes in the handling properties or performance of gypsum can be obtained by adding various additives. Additives that accelerate the hardening process are the powder of the gypsum itself - calcium sulfate dihydrate (<20%), сульфат калия и хлорид натрия (<20%). Эти вещества действуют как центры кристаллизации, вызывая рост кристаллов дигидрата сульфата кальция. Вещества, которые замедляют процесс затвердевания, это хлорид натрия (>20%), potassium citrate and borax, which prevent the formation of dihydrate crystals. These additives also affect dimensional changes during solidification, as will be discussed below.
Various manipulations when working with the powder-liquid system also affect the hardening characteristics. You can change the powder-liquid ratio, and if you add more water, the hardening time will increase, since it will take longer to obtain a saturated solution, respectively, it will take more time for the dihydrate crystals to precipitate. An increase in the mixing time of the mixture with a spatula leads to a decrease in the solidification time, since this can cause the destruction of crystals as they form, therefore, more centers of crystallization are formed.
Clinical significance
Increasing the stirring time of the gypsum with a spatula results in a shorter cure time and an increase in the expansion of the material during curing.
The increase in temperature has minimal effect, since the acceleration of dissolution of the hemihydrate is counterbalanced by the higher solubility of calcium sulfate dihydrate in water.
Fundamentals of Dental Materials Science
Richard van Noort
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 refer to the rocks he built. It is also customary to call gypsum a building material obtained by partial dehydration and grinding of a mineral. The name comes from the Greek. gypsum, which in ancient times meant both gypsum 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. The system is monoclinic. The crystal structure is layered; two sheets of 2- anionic groups, closely associated with Ca 2+ ions, compose double layers oriented along the (010) plane. Molecules H 2 O occupy the 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 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 an adjacent layer. PROPERTIES
The color is very different, but usually white, gray, yellow, pink, etc. Pure transparent crystals are colorless. It can be painted in various colors with impurities. The color of the line is white. Crystals have a glass luster, sometimes with a pearlescent tint due to microcracks of perfect cleavage; in selenite it is silky. Hardness 2 (standard of the 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 a marked water solubility. A remarkable feature of gypsum is the fact that its solubility with an increase in temperature reaches a maximum at 37-38 °, and then falls rather quickly. The greatest decrease in solubility is established 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 a white powder of alabaster (2CaSO 4 × Н 2 О), which is noticeably soluble in water. Due to the lower amount of hydrated molecules, alabaster does not shrink during polymerization (it increases in volume by approx. 1%). Under p. Tr. loses water, splits and fuses into white enamel. On coal in a reducing flame it gives CaS. It dissolves much better in water acidified with H 2 SO 4 than in pure water. However, when the concentration of H 2 SO 4 is over 75 g / l. solubility drops sharply. Very little soluble in HCl.
MORPHOLOGY
Due to the predominant development of the (010) faces, crystals have a tabular, rarely columnar or prismatic appearance. The most common prisms are (110) and (111), sometimes (120) and others. Faces (110) and (010) often have vertical shading. Fusion twins are frequent and are of two types: 1) Gallic according to (100) and 2) Parisian according to (101). Distinguishing them from each other is not always easy. Both of them resemble a dovetail. Gallic twins are characterized by the fact that the edges of the m (110) prism are parallel to the twin plane, and the edges of the l (111) prism form an inward angle, while in the Parisian twins, the edges of the Ι (111) prism 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. Characterized by intergrowths in the form of a "rose" and twins - the so-called. "Dovetails"). Forms veins of parallel-fibrous structure (selenite) in clayey sedimentary rocks, as well as dense continuous fine-grained aggregates resembling marble (alabaster). Sometimes in the form of earthy aggregates and cryptocrystalline masses. Also composes sandstone cement.
Common pseudomorphs on gypsum of calcite, aragonite, malachite, quartz, etc., as well as pseudomorphs of gypsum on other minerals.
ORIGIN
A widespread mineral, in natural conditions it is formed in various ways. Sedimentary origin (typical marine chemogenic sediment), low-temperature-hydrothermal, occurs in karst caves and solfatars. It precipitates from sulfate-rich aqueous solutions during the drying up of sea lagoons and salt lakes. Forms layers, interlayers and lenses among sedimentary rocks, often in associations with anhydrite, halite, celestine, native sulfur, sometimes with bitumen and oil. In significant masses, it is deposited by sedimentary means in lacustrine and marine saline 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. Upon reaching a certain value of the concentration of salts, in particular NaCl and especially MgCl 2, instead of gypsum, anhydrite will crystallize and then other, more soluble salts, i.e. gypsum in these basins must belong to the earlier chemical sediments. Indeed, in many salt deposits, gypsum (and also anhydrite) layers, interbedded with rock salt layers, 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 widespread in the Western Urals, in Bashkiria and Tatarstan, in the 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), druses of unique size 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 powdery binder obtained by heat treatment of natural two-water gypsum CaSO 4 · 2H 2 O at a temperature of 150-180 degrees in devices communicating with the atmosphere. The product of grinding β-modification gypsum into a fine powder is called stucco or alabaster; with finer grinding, molding gypsum or, when using raw materials of high purity, medical gypsum is obtained. At 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.
In a mixture with water, α and β-gypsum hardens, turning again into dihydrate gypsum, with the release of heat and a slight increase in volume (by about 1%), however, such a secondary gypsum stone already has a uniform fine-crystalline structure, the color of various shades of white (depending on raw material), opaque and microporous. These properties of gypsum are used in various fields of human activity.
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 into white, often colored with minerals-impurities in yellow, pink, red, brown, etc.; sometimes there is a sectorial-zonal color or distribution of inclusions over growth zones within crystals; colorless in internal reflexes and enlightenment. |
| Line color | White |
| Transparency | transparent, translucent, opaque |
| Shine | glassy, close to glass, silky, pearlescent, dull |
| Cleavage | very perfect, easily obtained from (010), almost mica-like in some samples; along (100) clear, passing into a conchoidal fracture; by (011), gives a splinter break (001) |
| Hardness (Mohs scale) | 2 |
| Break | smooth, conchoidal |
| Strength | flexible |
| Density (measured) | 2.312 - 2.322 g / cm 3 |
| Radioactivity (GRapi) | 0 |
Gypsum as a material has been known since ancient times, but it has not lost its popularity and relevance to this day. In addition, even the newest and improved materials could not compete with it. The use of plaster very wide, ranging from the porcelain field of activity and ending with medicine. However, the most demanded is construction.
What is gypsum as a material?
It is made from plaster stone. They are fired in ovens of different temperatures, and then they are broken until a powder mixture appears. Surfaces treated with gypsum can absorb unnecessary moisture from the air and also release it in very dry air. This material is referred to as sulfates. Exists two types of plaster: selenite and alabaster. The first is the fibers and the second is the grains.
What are the technical characteristics of gypsum plaster?
Almost all gypsum mixes have similar characteristics. These include:
1. Density. The building material is fine-grained. On average, the density varies from 2.6 to 2.8 g per cm.
2. Drying period. He grasps literally in a matter of minutes. Experience shows that in the fourth minute after mixing the solution sets, and after 30 minutes it completely hardens. It is for this reason that it is necessary to dilute gypsum in small portions, otherwise it will harden, and nothing can be done with it. However, there is a way to slow down this process. Water-soluble animal glue is added to the solution. Its use will not affect the quality of the gypsum in any way.
3. Specific gravity. The weight ratio is equal to the volume occupied by the gypsum, therefore the specific, volumetric and bulk density have practically the same indicators.
4. Melting point. This material can be heated up to 700 degrees Celsius! And it will not change its shape or quality. Its destruction will begin only after 6 hours of continuous exposure to high temperature.
5. Strength. When compressed, it is 5 MPa, and high-strength material - from 10 to 50 MPa.
6. Gypsum meets GOST, that is, government regulations.
7. Thermal conductivity and solubility. It is a very weak heat conductor. And practically does not dissolve.
What types of gypsum are distinguished?
one. . The use of this type of gypsum extends to the creation of gypsum parts and boards for plastering work. All work with it must be done in 10-20 minutes, as it freezes very quickly. It is for such a period of time that the material must be fully used. Only at the initial moment of hardening does the gypsum gain approximately half of its strength. When hardened, cracks do not appear on it, so there is simply no need to add any special components. But this does not apply to substances that retard the hardening. This building mixture reduces the difficulty of work and material costs in general. It is mined by blasting gypsum-containing rock. The gypsum is then transported to production plants in the form of stones.
2. High strength. In terms of its structure and composition, it practically does not differ from the previous species. However, the crystals of the construction type are smaller, and the high-strength ones are larger, therefore it has less porosity and great strength. It is produced by heat treatment in a special device. The use of plaster this species is quite diverse. Various solutions are made from it, partitions are erected that do not burn. It is also worth giving preference to porcelain plumbing fixtures, they are made of high-strength gypsum. Do not forget about the fields of medicine, or rather about dentistry and traumatology.
3. Polymeric. This type of plaster is very popular in traumatology; bandages are made on its basis, which will later be used to apply bandages. The advantages of using polymer dressings include: they are several times lighter than simple gypsum dressings, they are applied without difficulty and with a minimum amount of time, they allow the skin to breathe, because they have excellent permeability, do not absorb moisture, with their help you can observe the process of bone fusion.
4. Cellacast gypsum. It is practically the same as the polymer one, only its composition is allowed to stretch the bandage in all directions and in different directions.
5. Structural or molded. The most environmentally friendly, as it does not contain any additives. Used to create forms for sculptures, various figurines, sculpting, etc. also used in automotive and aircraft construction. It is the main component of dry fillers. This type of gypsum is obtained from building gypsum by sifting and grinding it. They even make sockets out of it!
6. Acrylic. Made from acrylic resin that dissolves in water. When this look completely solidifies, the material is similar to a simple building material, but it is lighter. Various decorative stucco moldings are the full merit of the acrylic material. Gypsum can withstand different temperatures, has little moisture absorption, so it can be used to create beautiful and unusual building facades. It is very easy to work with him. If you add aluminum powder or marble chips to the mixture, then the gypsum will accordingly resemble marble or metal.
7. Polyurethane. It is also used in stucco molding. At cost, it is much more profitable than the construction type. But according to its indicators, it is no different.
8. White plaster. He is an excellent assistant in various repair work. They put everything in order. White gypsum can be combined with various building materials - this is its main advantage. Freezes for about 7 minutes.
9. Fine-grained or translucent. The seams are filled with it.
10. Liquid gypsum. Made from gypsum powder. The manufacturing algorithm is as follows: 1 - prepare water, 2 - pour gypsum into it and mix, 3 - stir until a liquid substance is obtained.
11. Waterproof or moisture resistant. They are obtained by processing the material according to a special algorithm. To improve its qualities, bard is added to it.
12. Refractory. All plaster of Paris does not tolerate fire, but this look is made from tongue-and-groove plaster, which can withstand extreme temperatures. They are used in all areas, especially where it is necessary to increase the refractoriness.
13. Architectural. It is very flexible and does not contain toxic elements. The acidity of this type of gypsum is the same as the acidity of human skin. Modeling from this plaster is very popular, therefore, the demand for it is high.
Can something replace plaster?
Yes maybe. And that material is alabaster. It is also known in the construction world, it is obtained from gypsum dihydrate by processing with high temperatures. According to their external characteristics, they do not differ from each other. It is used if there is little humidity in the room.
Differences between alabaster and gypsum
1. Gypsum is used in many fields of activity, without limitation, alabaster is known only in the construction field.
2. If no special components are added to the alabaster, then 1 - it will dry out very quickly, 2 - will be simply unusable.
3. Gypsum is more environmentally friendly than alabaster.
4. Alabaster is more durable than gypsum.