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

general information

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

• reddish;
• gray;
• white;
• brown;
• yellowish.

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

Properties and varieties

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

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

Today, 3 varieties are known, among them:

• selenite;
• "Maryino glass";
• alabaster.

The first is parallel needle-shaped and has a silky sheen. Transparent thick sheet is “Maryino glass”. Fine-grained painted alabaster can be used.

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 materials are turned out. As a result, it is also possible to obtain 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 produce glazes, enamel and paint in industry and the pulp and paper industry.

The fired material is used for impressions and castings. These can be cornices and bas-reliefs. In medicine and construction, the material acts as a binder. Denser varieties perform the functions of ornamental material.

Additional information 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 combat soil salinity. This mineral was mined in karst caves. From ancient times to the present day, gypsum has been added to the soil to increase crop yields.

For many peoples he was the breadwinner. Entire cities were built from plaster. Crystal blocks were cut out of it and used to build walls. The white stone shines dazzlingly in the sun. This can be seen today, when only ruins remain of the ancient cities.

All over the world, sculptors cannot do without this mineral. It is inexpensive, weighs little and is easy to handle. Valued 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 origins. This mineral has several types, the method of formation of which differs. In some deposits, a mineral is mined that was concentrated there during the accumulation of marine sediment. In other cases, gypsum formed when various lakes dried out. The mineral could have arisen during the deposition of native sulfur and during the weathering of its compounds. In this case, deposits may be contaminated with rock fragments and clays.

Place of Birth

After reading 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 loose bulk form, the density can vary from 850 to 1150 kg/cm 3 . In compacted form, this parameter reaches 1455 kg/cm3. When reading 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.

The finished gypsum solution requires immediate use. To slow down the setting, water-soluble is added to the ingredients. Among the properties of gypsum, the melting point should be highlighted. The material can be heated up to 700 °C without destruction. Products made from gypsum are quite fire resistant. They begin to collapse 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 can even exceed this value. Well-dried samples have 3 times higher strength. The mineral complies with state standards 125-79. It has a thermal conductivity of 0.259 kcal/m*degree/hour. The temperature range 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 increased to 15 °C, the solubility increases to 2.534 g.
• This value increases 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 binding materials, then the first has a wider range of uses. With its help you can save on other components. The construction variety is used in the manufacture of gypsum parts, during plastering work and the formation of partition slabs.

It is necessary to work with gypsum mortar very quickly. The start time of polymerization can range from 8 to 25 minutes after mixing the solution. The final value depends on the variety. At the moment hardening begins, the mineral gains about 40% of its final strength. With this process, white gypsum does not become cracked, so you can dispense with various aggregates when mixing a solution with a lime composition. The construction type reduces the labor intensity and costs of work.

Area of ​​use and properties of high-strength and polymer gypsum

The chemical composition of the high-strength variety is similar to the construction variety. 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 sealed conditions.

The area of ​​use is the production of building mixtures and the construction of fireproof partitions. Molds are made from the high-strength mineral for the production of earthenware and porcelain products. The polymer type is also called synthetic and is more familiar to orthopedists and traumatologists. Based on it, they are made for applying bandages for fractures. But the area of ​​application of gypsum is not the only advantage; among others, the following should be highlighted:

• easy application;
• 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 inquire about other properties, as well as varieties. Among others, molding, sculpture and cellacast should be highlighted.

The latter is used to make bandages, and the structure allows the material to be stretched in all directions. The most highly durable is sculptured gypsum, which does not contain impurities. Among the properties of white gypsum, one can highlight its impeccable whiteness.

Introduction

Gypsum-based materials have various purposes in dental practice. These include:

Models and stamps;

Impression materials;

Foundry molds;

Fireproof 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. A casting mold is used to make dentures from metal alloys.

Stamps- These are copies or models of individual teeth that are necessary in the manufacture of crowns and bridges.

Refractory molding material for cast metal dentures is a high temperature resistant material in which gypsum serves as a binder or binder; This material is used for molds in the manufacture of prostheses from certain gold-based casting alloys.

Chemical composition of gypsum

Compound

Gypsum- calcium sulfate dihydrate CaS04 - 2H20.

When calcining or roasting this substance, i.e. When heated to temperatures sufficient to remove some water, it turns into calcium sulfate hemihydrate (CaS04)2 - H20, and at higher temperatures anhydrite is formed according to the following scheme:

The production of calcium sulfate hemihydrate can be carried out in three ways, allowing one to obtain varieties of gypsum for various purposes. These varieties include: fired or regular medical plaster, model plaster 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.

Burnt plaster (ordinary medical plaster)

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 HS hemihydrate. The resulting material consists of large, porous, irregularly shaped particles that 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.

Model plaster

When calcium sulfate dihydrate is heated in an autoclave, the resulting hemihydrate consists of small, regularly shaped particles that 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 produces denser packing and requires less water for mixing. Mixing ratio: 20 ml of water to 100 g of powder.

Super gypsum

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 the formation of flocs in the mixture and promoting particle separation, because otherwise the particles tend to agglomerate. The particles of the resulting hemihydrate are even more dense and smooth compared to the particles of autoclaved gypsum. Supergypsum is mixed in the following ratio: 20 ml of water per 100 g of powder.

Application

Ordinary fired or medicinal plaster is used as a material for general use, mainly as the base of models and models themselves, since it is cheap and easy to work. Expansion during solidification (see below) is not significant in the manufacture of such products. The same gypsum is used as an impression material and also in gypsum-bonded refractory molding material formulations, although for such uses the working and setting time, as well as setting expansion, are carefully controlled by the addition of various additives.

Autoclaved plaster is used to make models of oral tissue, while the stronger super plaster is used to make models of individual teeth, called dies. They are used to model various types of wax restorations, which are then used to produce cast metal prostheses.

Hardening process

When calcium sulfate hydrate is heated to remove some of the water, a largely anhydrous substance is formed. As a consequence of this, calcium sulfate hemihydrate is able to react with water and convert back to calcium sulfate dihydrate by the reaction:

It is believed that the hardening process of gypsum occurs in the following sequence:

1. Some calcium sulfate hemihydrate dissolves in water.

2. The dissolved calcium sulfate hemihydrate reacts again with water to form calcium sulfate dihydrate.

3. The solubility of calcium sulfate dihydrate is very low, resulting in a supersaturated solution.

4. This 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 amount of calcium sulfate hemihydrate dissolves again and this process continues until all the hemihydrate has dissolved. Working time and hardening time

The material must be mixed and poured into the mold before the end of working hours. Working time varies for different products and is selected depending on the specific application.

For impression plaster, the working time is only 2-3 minutes, while for refractory molding materials with a gypsum binder it reaches 8 minutes. The short working time is associated with a short curing time, since both processes depend on the speed of the reaction. Therefore, while the typical working time for impression plaster is 2-3 minutes, the setting time for refractory gypsum molding materials can vary from 20 to 45 minutes.

Materials for making models have the same working time as impression plaster, but their hardening time is slightly longer. For impression plaster, the hardening time is 5 minutes, while for autoclaved or model plaster it can last up to 20 minutes.

Changing the handling properties or performance characteristics of gypsum can be achieved by introducing 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 influence dimensional changes during solidification, as will be mentioned below.

Various manipulations when working with a powder-liquid system also affect the solidification characteristics. The powder-liquid ratio can be changed, and if more water is added, the hardening time will increase, since it will take more time to obtain a saturated solution, and therefore more time will be required for the dihydrate crystals to precipitate. Increasing the time of mixing the mixture with a spatula leads to a decrease in the hardening time, since this may cause destruction of the crystals as they form, therefore, more crystallization centers are formed.

Clinical significance

Increasing the time of mixing the gypsum with a spatula leads to a decrease in hardening time and an increase in the expansion of the material during hardening.

Increasing the temperature has minimal effect because the increased dissolution of the hemihydrate is counterbalanced by the higher solubility of calcium sulfate dihydrate in water.

Fundamentals of Dental Materials Science
Richard van Noort

January 19th, 2010

Gypsum(from the Greek gypsos - chalk, lime) - mineral, aqueous calcium sulfate. Gypsum crystals are lamellar, columnar, needle-shaped and fibrous. They are found mainly in the form of solid granular and fibrous masses, as well as various crystalline groups. Often associated with and. Pure gypsum is colorless and transparent; in the presence of impurities, it has gray, yellowish, pink, brown and other colors. Precipitates from aqueous solutions rich in sulfate salts during the drying out of sea lagoons and salt lakes. One of the physical properties of gypsum - fluorescence. Glow in long-wave UV light is yellow, orange, blue or green. Mineral anhydrite(CaSO4) is similar in composition to gypsum, but lacks water. Gypsum has a chemical formula - CaSO4.2H2O

Gypsum crystal

Gypsum crystals up to 11 meters long were found in the caves of Naica Mine, Mexico. The crystals grew in the extremely rare natural environment of the cave. The temperature there constantly remained around 58 ° C, and the cave was filled with water rich in minerals, as necessary for crystal growth. The largest of these crystals weighing 55 tons had age about 500,000 years . The two brothers who discovered this cave dubbed it "the queen's eyes." The length of the cave is 290 meters underground.

And this is my sample of plaster from the magazine.
Red selenite

There are the following types of gypsum:
Selenite- a colorless and transparent type of gypsum that has a pearly luster.

Another variety is a silky, fibrous form called "Satin Spar" - satin spar . This variety has a satiny, silky luster that creates a play of light up and down the surface of the crystal.
Alabaster- a type of gypsum, white or slightly tinted, consisting of compressed fine-grained masses - a decorative ornamental stone used for fine carvings for centuries, even epochs.
In arid areas, gypsum can form into flower-like forms, usually opaque with embedded grains of sand, called "desert rose". It can also have the following names: Sand rose, Stone rose, Selenite rose, Gypsum rose, Gypsum rosette.

Ordite(ordite) are actually pseudomorphoses of gypsum. The only deposit is located in Russia - Orda, Perm region, Ural.
Sugar gypsum - gypsum in the form of a granular mass, crystallizes like sugar.
Gypsum is also sometimes found in some meteorites .

Even in the Neolithic era, gypsum was used as a building material. Back in 7000 BC. in the city of Catal Guyuk, in Asia Minor, gypsum was used to decorate the interior. Around 3000 BC in Uruk and later in Egypt, gypsum was used as a mortar mixed with lime to bind stones together. For example, in the Sphinx(2700-2600 BC) for certain works, gypsum plaster was limed. In addition, the Egyptians were familiar with translucent alabaster windows. In the Minoan civilization, gypsum was made into floors or wall coverings and used as building blocks. (famous Knossos palace, 2100-1800 BC, located on the Greek island of Crete and preserved to the present day). The Romans used gypsum only for decorative plaster inside buildings, as they were familiar with much more durable materials.

Nowadays, gypsum is used for the manufacture of binding materials, gypsum of soil, and in medicine for prosthetics. It is also used for removing masks, modeling sculptures, and creating relief decorations (stucco molding) indoors. Since ancient times, gypsum has been popular as an ornamental stone. Openwork vases, figurines, ashtrays and other decorative items are still cut from it. In Asian cuisine gypsum added to tofu (traditional bean curd) as a coagulant, making it ultimately a major source of calcium.

Gypsum

Gypsum (eng. Gypsum) - mineral, aqueous sulphate of calcium. 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 o 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.

Forms of location

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. "swallowtails"). It forms veinlets of a 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 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 widespread mineral, it is formed in natural conditions 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, interlayers 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 value of salt concentration 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.
Significant masses of gypsum in sedimentary rocks are formed primarily as a result of the hydration of anhydrite, which in turn was deposited during the evaporation of sea water; Often, when it evaporates, gypsum is directly deposited. Gypsum arises as a result of the hydration of anhydrite in sediments 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: 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. In this way, most of the large deposits of gypsum on the globe arose. In the voids among solid gypsum masses, nests of large, often transparent crystals are sometimes found.
Can serve as cement in sedimentary rocks. Vein gypsum is usually a 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 into 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 (“swallowtails”), druses, “gypsum roses,” etc.
Gypsum is quite soluble in water (up to 2.2 g/l), and with increasing temperature its solubility first increases, and above 24 ° C it decreases. 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 changes, saline and gypsum-filled soils, in the morning, as the temperature rises, gypsum begins to dissolve and, rising in the solution by capillary forces, is deposited on the surface as water evaporates. In the evening, as the temperature drops, 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 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 samples 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 uniquely sized gypsum crystals 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 - interior items (vases, tabletops, inkwells, etc.) have been made from alabaster. Burnt gypsum is used for castings and impressions (bas-reliefs, cornices, etc.), as a binding material in construction and medicine.
Used to produce building gypsum, high-strength gypsum, gypsum-cement-pozzolanic binder material.

• Gypsum is also the name given to sedimentary rock composed predominantly of this mineral. Its origin is evaporitic.

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

Other names, varieties

silky spar,
Ural eelinite,
gypsum spar,
maiden or Maryino 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 sulfatée
• Croatian – Gips
• Czech – Sádrovec
• Swedish - Gips
• Esperanto - Gipsoŝtono;Gipso
• Estonian – Kips
• Japanese—石膏

Name: Gypsum

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 to the naked eye..

For many centuries now, in the architecture of states that are based on a well-developed culture and art, that value the beautiful and extraordinary, that preserve their historical monuments and traditions in construction and decoration, a material such as gypsum has been used.

This is primarily due to its properties - plasticity, natural uniformity, uniform color, final hardness, which allows you to create absolutely any shape, be it a bas-relief design, an ornament from stucco elements or sculpture. With proper use, good storage conditions, and careful restoration, the created products can last forever. An example of this is churches around the world that have preserved their unique interiors from past centuries to the present day.

What a craftsman needs to know about the properties of gypsum and products made from it

Gypsum has so many advantages that it can be called a truly unique material.

• Eco-friendly and natural. Gypsum is a completely natural material; it is still mined using the old-fashioned method. It is as environmentally friendly as possible, which puts such raw materials many levels higher than any modern building material.
• The ability to improve the microclimate. It has long been noticed that in rooms decorated with stucco, it is very easy to breathe, even if it is hot or raining outside. This is easily explained by the fact that the hardened gypsum solution has the ability to exchange moisture: increased moisture is absorbed by it, and if there is insufficient amount of water in the air, it is released.
• Responsiveness to restoration. Unlike glass, leather, wood, stone and even metal, stucco can be completely restored. With proper repair work, it can look perfect even if it is a hundred years old. Try to recreate the lost part of a porcelain or stone bowl so that it looks like new. Agree, this is impossible. But plaster products after restoration do not contain visible traces of the master’s work.
• Limitless decor possibilities. In skillful hands, plaster takes on any shape, even the smallest details are visible on it. It can be painted, patinated, coated with various compounds that add shine or other visual qualities. Moreover, it is not subject to shrinkage, so the finished decor will remain in its original form for as long as the owner of the room wants it.

These properties were decisive when choosing an option many centuries ago, and they remain relevant to this day. Until now, the wealthiest people prefer to decorate their family estates with stucco, and public cultural buildings - churches, libraries, museums - are simply unthinkable without such decoration. Decorating a room with real stucco (not to be confused with cheap polyurethane) is a sign of excellent artistic taste and aristocracy.

Where can gypsum (alabaster) be used?

Gypsum is used quite often in everyday life:

• construction work - leveling internal and external walls, ceilings, ventilation ducts, manufacturing partitions;
• production of fireproof barriers and sound-absorbing structures;
• production - plasterboard, dry plaster, wood concrete, gypsum particle boards and gypsum fiber boards, etc.;
• decoration - interior design, landscape design, architectural elements, stucco molding, tiles, souvenir items, etc.;
• repair of damaged stucco molding and other alabaster products;
• as an element of high quality gypsum cement.

Characteristics of gypsum for construction and finishing mortars

Modern building gypsum (another name is alabaster), used to prepare the solution, is produced by the classical method of heat treatment of gypsum stone (150-180 ° C) mined in quarries. The resulting raw material goes through the stages of grinding and sifting, resulting in a homogeneous powder with different particle sizes - coarse, medium and fine grinding.

The degree of grinding is still determined in the same way as 500 years ago. The resulting powder is sieved on a fine mesh sieve (0.2 mm). The residue that did not pass through the mesh is weighed, determining its mass (as a percentage of the total weight).

• If there are a lot of large particles left - up to 23% - the resulting raw material is assigned index I, which corresponds to coarse grinding.
• Up to 14% - index II - medium grinding.
• Up to 2% - index III - high-quality fine grinding.

The finer the degree of grinding, the faster the solution will set. To establish the final verdict on quality, the resulting powder is examined on an ADP-1 (PSKH-2) device, determining its specific surface area. It must comply with GOST 23789-79.

An important parameter is the viscosity of the solution, determined by the GOST 125-79 standard and depends precisely on the degree of grinding, because the particle size directly affects water demand. It is believed that 18.6% water would be enough to hydrate hemihydrate alabaster to the degree of dihydrate, but such a solution is not suitable for construction work, so normal viscosity is achieved by adding 50-70% water (3-hemihydrate). If you need a thick solution, then limit yourself to 35-45% water, obtaining a-hemihydrate. The standard consistency is determined by the mass spreading parameter, which should not exceed a diameter of 180±5 mm.

The bulk density of gypsum powder in its natural form is 800-1100 kg/cubic. m, compacted - 1250-1450 kg/cubic. m. The density of the finished alabaster is 2.6-2.75 g/cu. cm.

The process of producing building gypsum can also proceed in a different order: grinding-screening-firing. If it is necessary to produce special types of this material (medical or molding), then the technology can be changed. When gypsum stone is heated in a vacuum and the temperature drops to 100°C, the output is high-strength alabaster.

Deformability of alabaster

Gypsum may change in volume as it dries. But unlike many materials, its volume does not decrease, but, on the contrary, increases. Deformability can reach 1%. This quality is a big plus when making sculptures and stucco, as the solution perfectly fills the molds, allowing you to get a very clear design, without losing small details.

The ability to expand depends on the amount of soluble anhydrite in the material. Gypsum that has been fired at elevated temperatures is subject to the greatest deformability. This indicator can be reduced in several ways:

• increasing the amount of water;
• introduction of hardening retarders;
• adding 1% quicklime to 0.1%.

If the solution is prepared incorrectly or when creating large-scale products, significant shrinkage is possible, which leads to cracking of the plaster. The process can be mitigated by using mineral supplements.

If the ratio of the plasticity of the solution to bending loads is incorrectly calculated, plastic deformations are also possible, the probability of which is reduced to zero when the stucco molding is well dried. At high humidity, the creep of gypsum can be quite large and visually noticeable. Plastic distortions can be reduced by using pozzolanic hydraulic additives in combination with Portland cement.

Strength of gypsum

Gypsum is considered to be a fragile material. In fact, it splits really easily if a targeted blow is applied to it. At the same time, gypsum is capable of withstanding high compressive loads, which is very important for materials used in construction. The properties of modern gypsum are determined by GOST 23789-79 and GOST 125-79. To understand how to properly handle this material, you need to become familiar with a number of concepts and characteristics that directly affect strength.

• Compressive strength. To determine the strength of semi-aqueous gypsum, a specialist makes bars measuring 4x4x16 cm from the experimental solution. 2 hours are allotted for hardening, after which the samples are tested for bending and compression. The tensile strength of finished products is divided into 12 grades: from G-2 to G-7, from G-10 in increments of 3 to G-25, where the number indicates compressive strength, for example, G-7 grade gypsum will withstand pressure up to 7 kg/ sq. cm.
• Comprehensive assessment. Additional markings include hardening speed (A, B, C) and grinding index. The highest quality category has characteristics from G-5, index III. There are increased requirements for gypsum intended for the production of molds for porcelain, earthenware and ceramic products. Grade G-10, setting time 6-30 minutes, grinding fineness - residue no more than 1%, water absorption from 30%, volumetric expansion after hardening up to 0.15%.
• Porosity. Finished gypsum products are quite hard and porous, the pore volume can exceed 60%, minimum - 40% (dense alabaster). The more water, the more porous and less durable the product will be, so the norms cannot be violated. When determining the amount of water for the solution, it is important to consider the degree of grinding of the powder. The smaller the particles, the more water the mixture can take, however, this is exactly the case when with an increase in water content (within GOST limits), the final strength of the products does not decrease, but slightly increases. That is why for the most durable gypsum castings, craftsmen prefer to use powder with a minimum particle size.
• Water-gypsum ratio. By reducing the water-gypsum ratio to 0.4, the strength of alabaster can be increased by up to 300%, which is why many craftsmen prefer to work with raw materials that have low water demand. This indicator can be reduced by using special additives - set retarders, for example, water-soluble polymers or synthetic fatty acids. This technique allows you to reduce the density of the mixture to 15%, which increases the strength of the finished stucco.
• Ultimate tensile strength. The tensile and compressive strengths of gypsum products are always different. It should be borne in mind that alabaster withstands tension 10 times worse than compression, so it cannot be used in conditions where changes in the characteristics of the base are possible.
• The effect of humidity on strength. Another important point is the effect of humidity on strength. The higher the water content in the air, the lower the compressive strength of gypsum. For example, moistening stucco by just 1% (at a relative air humidity of 90 - 100%) can reduce strength by up to 70%. Moisture saturation up to 15% leads to a decrease in strength by half. Water saturation up to 40% (full) threatens the destruction of the sample if it had a water-gypsum ratio of 0.5. Denser products withstand high humidity better. At the same time, you should not think that any cataclysm can destroy plaster casts. It is enough to carefully dry the products, and their former qualities will return.
• Softening coefficient. The dependence of products made from this material on moisture content is determined by the softening coefficient. It is calculated in the following order: first, the samples are saturated with moisture, then dried, calculating the ratio of the obtained indicators. The final result, as already mentioned, directly depends on the density of the sample and can range from 0.3 to 0.5 (the harder the solution, the higher). It is worth considering that with the use of organic additives, a deterioration in strength can be expected; mineral additives have a minor effect.

Terms and method of storing gypsum

Storing dry powders requires a low level of humidity, so bags (or bulk in boxes) are usually kept on high racks (from 50 cm). Storage periods must be strictly observed in accordance with GOST 2226-75. Powder used in the ceramic and porcelain industry should not be stored in bulk.

When buying gypsum, you must pay attention to its shelf life, since during storage of semi-aqueous gypsum its properties, even if all standards are observed, change. This is especially noticeable in the first month, when due to the influence of air humidity its water demand decreases, and when the storage period is exceeded.

The process can be represented like this.

• Dry fresh gypsum begins to interact with moisture, resulting in the formation of a film of dihydrate molecules on the surface of the semi-aqueous gypsum grain.
• When mixing a solution from such raw materials, it can be noted that it takes a long time to harden, since the film prevents the hemihydrate from quickly contacting water.
• Water demand is reduced, and the strength of the finished casts is therefore increased.

With prolonged exposure, the process worsens.

• The thickness of the dihydrate film increases, leading to overhydration of the powder.
• Water demand increases, plasticity, setting time and strength decrease.

In other words, fresh alabaster with a shelf life of 1-2 months is ideal for work.

How to make a plaster solution

Before you make the solution (dough), you must prepare everything for work. If you do not take care of this, you may not get the desired result, since the mixture will harden very quickly.

Recipes for mortar for filling molds.

• You will need to prepare 2 parts by weight of alabaster and 1 part of water. First, pour water into the container, then slowly add the dry powder, stirring vigorously with a wooden spatula or a construction mixer. This solution can harden in 4-30 minutes (depending on the fineness of the grind).
• Add up to 2% animal glue to the finished solution (after dissolving it in water) or lime mortar - this will extend the hardening time.

Keep in mind that alabaster practically does not expand when it hardens, the maximum increase in volume is up to 1%, but this must also be taken into account.

How to regulate the setting time of gypsum

As mentioned above, gypsum mortar tends to harden quickly, but this process can be controlled. First of all, the master must understand what exactly he needs. If he performs castings, then a high solidification rate is simply necessary, so it is worth choosing raw materials of appropriate quality. If finishing or restoration work is carried out, then the hardening rate should be reduced to obtain the time necessary to perform one or another action.

Based on the drying time, the solutions are obtained as follows.

• Quickly hardening - 2-15 minutes from the moment the solution is made.
• Normal hardening - 6-30 minutes.
• Slow hardening - from 20 minutes.

The setting time depends on several factors:

• grinding fineness (the finer the particles, the faster);
• properties of the powder (semi-aqueous gypsum, which includes dihydrate elements, sets much faster);
• manufacturing technology (affected by the temperature and duration of calcination of raw materials);
• shelf life;
• temperature of raw materials and water for mixing: cold dough hardens longer than dough heated to 40-45°, overheated to 90° does not set at all due to the loss of solubility of semi-aqueous gypsum, it no longer turns into the dihydrate state;
• percentage ratio of water and powder (the less water, the faster hardening occurs);
• quality and intensity of mixing;
• the presence of additives (sand, slag, sawdust, polymers and special chemical additives reduce the hardening time of the solution).

How to choose gypsum additives

Today there are many different additives for solutions, they all have different principles of action and composition. If you decide to make the mixture yourself, do not forget that the proportions should be ideal. Violation of this requirement leads to a deterioration in the quality of the finished products: a decrease in hardness, an increase in the ability to absorb moisture and retain dampness, a decrease in the plasticity of the solution and other negative aspects.

Check out the Gessostar gypsum products catalog

In total, there are 5 types of additives.

Electrolytes. This group includes additives that affect the solubility of raw materials without undergoing chemical reactions. The percentage should not exceed 0.2-3%.

• Accelerate: Na2S04 KC1.
• Reduces: ethyl alcohol, ammonia, etc.
• Can serve as an accelerator and retardant: NaCl.

Inhibitors. Additive retarders that react and form low-dissociation compounds. The percentage should not exceed 0.2-3%.

• Boric acid, sodium phosphate and borax;
• 5-10 percent wood glue;
• C6H5OH;
• 5 percent – ​​sugar, etc.

Catalysts. Accelerator additives that enhance crystallization. The percentage should not exceed 0.2-3%.

• CaНР04-2Н20, CaS04-2FI20, KCl and other salts.

Surfactant. Surfactants that reduce crystallization and increase dough plasticity. These additives significantly affect the hardness of finished products, increasing it. The percentage depends on the quality of the raw materials and can be adjusted experimentally by the master (0.1-0.3%).

• Lime-adhesive solution, keratin.

Complex Supplements. Experienced craftsmen rarely use any one substance and have their own recipes for preparing the solution, so the quality of the products varies very noticeably. Most often, experts combine two, or even three, elements from different groups, which allows them to initially increase the plasticity of the dough, and then, when the element is ready, speed up hardening and increase the strength of the finished stucco molding.

The most common accelerators are sodium sulfate, gypsum dihydrate and ordinary table salt, and retarders are lime-glue mortar. The addition of surfactants in this case compensates for the decrease in strength caused by additives.

Lubricants for dies

If you decide to work with plaster, then you should purchase a special lubricant for molds that facilitates easy separation of the cast and the matrix.

• Stearin and paraffin dissolved in kerosene are suitable for separating gypsum from gypsum.
• When making reliefs with complex patterns, you can use soap suds, copper sulfate, soda ash, and potash.
• On an industrial scale, epoxy resin dissolved in acetone is used.
• There are special industrial lubricants for all types of products.

At home, lubricant (calcium soap) for molds is prepared as follows: 7 parts water mixed with 1 part oil and 2 parts soap.

Check out the Gessostar gypsum products catalog

How to increase the hardness of alabaster

Hardness is a very useful quality that allows you to protect products from accidental scratches and destruction. Each master has his own recipe for enhancing hardness. Here are some of them.

• Adding lime to gypsum, followed by drying at room temperature.
• Impregnation of a fresh product with a solution of ammonium borate (5%, temperature 30 degrees).
• Adding silicic acid solution to water (up to 50%) followed by heating the casting to 60 degrees.
• Using borax for the solution, followed by treating the casting with barium chloride and a hot soap solution.
• Treatment of the casting with Glauber's salt solution.
• Impregnation of finished gypsum with copper or iron sulfate.
• Soaking in a solution of potassium alum (24 hours), followed by heating to 550 degrees.

How to increase the durability of plaster

Plaster will last forever, provided that temperature and humidity standards are met. A product made of alabaster can be destroyed by prolonged high humidity with sharp temperature fluctuations or exposure to wind, as well as complete exposure to water.

The water resistance of products can be adjusted in several ways:

• compaction of the mixture;
• the use of additives (resins, silicon, Portland cement, pozzolanic additives, granulated slag);
• surface treatment with moisture-protecting solutions (synthetic resins, barite milk, hydrophobic compounds).

Another dangerous element that can affect durability is low-quality metal used for the base. When exposed to moisture, such iron begins to rust, increasing in volume as a result of corrosion and destroying the entire structure from the inside. It is allowed to use only stainless materials or iron elements treated with special anti-corrosion agents.

Alabaster is not afraid of fire; the flame will destroy gypsum only after 5 hours of exposure, which means this factor can be ignored.

As you can see, working with gypsum requires a huge amount of knowledge in the field of chemistry, which is why, despite the availability and cheapness of raw materials, there are only a few true masters of this craft. Even a child can make primitive castings, but only a specialist with extensive experience and rich skills can produce truly high-quality stucco molding that can last for a very long time.