The mineral composition of gypsum. What can not be done with gypsum crystalline? Building materials based on gypsum

Before you start studying this article, I want to make a small introduction ... The topic of gypsum did not arise by chance for me. I was about to do. In this regard, this is my first experience. The first thing I start doing in such cases is to study the material, i.e. I tried to learn everything about building plaster.

Initially, the topic seemed simple to me, but it turned out not to be so, so I am making a preface. Let's start with what is natural. But that's not all. Gypsum is obtained as a waste of the chemical industry (for example,) and it comes with impurities and, as a rule, worsens the properties of gypsum as a binder. Yes, and in nature, gypsum comes with impurities. Impurities are removed, but they partially remain, so you need to understand that when buying gypsum from different manufacturers, you are buying different material. If you add modifying additives yourself and bought gypsum from a manufacturer that you have not worked with before, then it is better to do a trial batch and apply a test layer.

Gypsum is β-modification and α-modification. They differ only in the method of preparation (dehydration). β-modifications are made by heating gypsum dihydrate in open ovens and the water comes out as steam, forming tiny pores, which degrades strength, because at any fineness of grinding, porous particles are obtained. α-modification is done in autoclaves under pressure and water comes out in a drip way, which makes the resulting semi-aqueous gypsum monolithic, which improves strength. The α-modification is difficult to manufacture, therefore expensive gypsum is obtained and is used only in medicine and partly in sculpture.

Alabaster is the name of natural granular gypsum, which has a finer structural grain. In some places it is written that any building plaster- alabaster. This is not true. Alabaster is granular gypsum, but not all granular gypsum is alabaster. In nature, it differs from simple granular gypsum in appearance and looks like marble. Alabaster is inherently fine-grained, so it is possible to obtain a finer grain when grinding than simple granular gypsum. Powder with finer grains has large area surface of the particles, which means it reacts with water faster and hardens faster. Building Alabaster is a semi-aqueous gypsum obtained from natural alabaster.

There is one more important point. Gypsum β-modification, which is only sold in ready mixes and so it consists of porous particles, but to prepare a working solution of the desired fluidity, it is necessary to add 2 times more water than is necessary for chemical reaction. Excess water evaporates, creating additional pores and further reducing strength. So if strength is important to you, reduce the water and use flow additives and use finely ground gypsum.

Building plaster- These are binders obtained from gypsum stone or chemical industry waste.

During the firing of gypsum stone, chemically separated bound water and depending on the temperature various forms of gypsum are formed. At 100 degrees Celsius, the formation of hemihydrate gypsum begins. When it is mixed in water, calcium sulfate dihydrate is again formed. This closed cycle was discovered about 20 thousand years ago. People built hearths of gypsum stone and probably noticed how the crumbled baked gypsum turns into stone again in the rain. In Sumerian and Babylonian cuneiform writing, there are references to gypsum and its use.

The availability of raw materials, the simplicity of technology, and the low energy intensity of production (4-5 times less than for Portland cement) make gypsum a cheap and attractive binder.

Density of semi-aqueous gypsum

The density of the hardened gypsum stone is low (1200-1500 kg/m3) due to significant porosity (60-30% respectively).

Hardening expansion

Gypsum binder is one of the few binders that expands when hardened. Volume increase during setting and hardening by 0.5-1%. When dried, a decrease in volume by 0.05-0.1%. This feature of gypsum binders allows them to be used without fillers, without fear of shrinkage cracking.

combustibility

Gypsum materials are not only non-combustible materials, but due to their porosity, they slow down the transfer of heat, and when exposed to high temperatures, as a result of thermal dissociation, they release water, thereby inhibiting the spread of fire. In dry operating conditions or when protected from the action of water (water-repellent coatings, impregnations, etc.), gypsum is a very promising binder from a technical and environmental point of view.

Gypsum variety

Gypsum β-modification

Gypsum of β-modification is produced at a temperature of 150-180°C in apparatuses communicating with the atmosphere. The product of grinding β-modification gypsum into a fine powder before or after processing is called building gypsum or alabaster, with finer grinding, molding gypsum is obtained or, when raw materials of increased purity are used, medical gypsum.

Gypsum α-modification

Gypsum α-modification is obtained by low-temperature (95-130°C) heat treatment in hermetically sealed ovens. Gypsum is made from it.

Alabaster

Alabaster(from gr. alebastros - white) - fast-hardening air binder, consisting of calcium sulfate CaSO 4 hemihydrate. 0.5H 2 O obtained by low-temperature processing of gypsum raw materials.

Alabaster - β-modification gypsum, a powdered binder material obtained by heat treatment in open ovens at a temperature of 150-180 degrees of natural two-water gypsum CaSO 4 · 2H 2 O. The resulting product is ground into a fine powder. With a finer grinding, molding plaster is obtained. For medical gypsum, raw materials of high purity are used.

Anhydrite

Anhydrite is a natural anhydrous gypsum. The anhydrite binder is slow setting and slowly hardening, consisting of anhydrous calcium sulfate CaSO 4 and hardening activators.

Estrich-gypsum

High-calcined estrich gypsum is obtained by firing natural gypsum stone CaSO 4 . 2H 2 O to high temperatures (800-950°C). In this case, its partial dissociation occurs with the formation of CaO, which serves as an anhydrite hardening activator. The final hardening product of such a binder is gypsum dihydrate, which determines the operational properties of the material.

Technological properties of estrich gypsum differ significantly from the properties of conventional gypsum. Setting time for estrich gypsum: the beginning is not earlier than 2 hours, the end is not standardized. Due to the reduced water demand (for estrich gypsum it is 30-35% versus 50-60% for ordinary gypsum), estrich gypsum after hardening forms a more dense and durable material.

The strength of the samples - cubes from a solution of a rigid consistency of the composition - binder: sand = 1: 3 after 28 days of hardening in wet conditions - 10-20 MPa. According to this indicator, the brand of Estrich-gypsum is set: 100, 150 or 200 (kgf / cm 2).

Estrich-gypsum was used in the late XIX - early XX centuries. for masonry and plaster mortars (including for obtaining artificial marble), installation of seamless floors, bases for clean floors, etc. Currently, this binder is used to a limited extent.

Building plaster properties

Grinding degree

According to the fineness of grinding, determined by the maximum residue of a gypsum sample when sieved on a sieve with holes of 0.2 mm, gypsum binders are divided into three groups: coarse, medium, fine.

Compressive and bending strength

The brand of gypsum is determined by testing the compression and bending of standard samples - beams 4 x 4 x 16 cm 2 hours after their molding. During this time, hydration and crystallization of gypsum ends.

12 grades of gypsum have been established in terms of strength from 2 to 25 (the figure shows the lower compressive strength of this grade of gypsum in MPa). In construction, gypsum grades from 4 to 7 are mainly used.

According to GOST 125-79 (ST SEV 826-77), depending on the compressive strength, the following grades of gypsum binders are distinguished:

When moistened, hardened gypsum not only significantly (2-3 times) reduces strength, but also exhibits an undesirable property - creep - a slow irreversible change in size and shape under load.

Normal Density (Water Demand or Water Gypsum Ratio)

Normal density (standard consistency) of gypsum dough is characterized by the diameter of the gypsum dough flowing out of the cylinder when it is raised to a height of at least 100 mm. The spread diameter should be equal to (180±5)mm. The amount of water is the main criterion for determining the properties of the gypsum binder: setting time, tensile strength, volumetric expansion and water absorption. The amount of water is expressed as a percentage, as the ratio of the mass of water required to obtain a gypsum mixture of standard consistency to the mass of gypsum binder in grams.

In the manufacture of gypsum products by casting, 60-80% of water is required by weight of building or molding gypsum and 35-45% of water by weight of high-strength gypsum.

When mixing a gypsum binder with water, the chemical reaction of hydration of CaSO 4 hemihydrate theoretically consumes 18.6% of water, and the excess amount of water remaining in the pores of the hardened product evaporates during hardening and causes a high porosity characteristic of gypsum products - 50-60% of the total the volume of the hardened product. That is, the less water is used when mixing gypsum dough and less value normal density when a good workability of the dough is achieved, the denser and stronger the gypsum product.

The normal density of a gypsum binder depends on many factors, the main of which are the type of gypsum binder, fineness of grinding, the shape and size of hemihydrate crystals.

To reduce the water demand of the gypsum binder, additives are used - thinners (plasticizers), which increase the mobility and workability of the gypsum mass without reducing the strength properties.

These supplements include:

• glucose;
• molasses;
• dextrin (introduced into a gypsum binder mixed with lime);
• sulfite-alcohol stillage (SSB) and its thermopolymers;
• bicarbonate soda;
• Glauber's salt, etc.

The addition of 0.1% Ca-Cl 2 solution to gypsum stone during the cooking process intensifies the cooking process, reduces water demand and accelerates the setting time of the gypsum binder.

When storing gypsum binders in air, their water demand is somewhat reduced (artificial aging of gypsum occurs), which leads to a distortion of the results of determining the strength in standard tests.

AT practical activities sometimes the gypsum binder is moistened with steam specifically to reduce water demand, to slightly increase the plasticity of the dough and the strength of the products. The amount of water additive in the gypsum binder is about 5%, while there is a partial hydration of the surface layers of the gypsum grains and a change in their wettability during the subsequent mixing of the gypsum binder with water. However, long-term storage of gypsum binders (more than 3 months) in the presence of water vapor is unacceptable, since its activity is significantly reduced due to premature hydration of gypsum.

Frost resistance

15-20 or more cycles of freezing and thawing.

Reinforcement

Steel reinforcement in gypsum products in a neutral environment (pH=6.5-7.5) is subjected to intense corrosion. Gypsum is moistened due to its good hygroscopicity (the ability to absorb moisture from the air).

Gypsum adheres well to wood and therefore it is advisable to reinforce it with wooden slats, cardboard or cellulose fibers and fill it with wood chips and sawdust.

Gypsum as a binder

Gypsum binders are materials based on semi-aqueous gypsum or anhydrite. They are air binders.

Depending on the method of production, gypsum binders (GB) are divided into three main groups:

• I - binders obtained by heat treatment of gypsum raw materials: low-firing (firing and cooking) and high-firing: α

  Calcium sulfate hemihydrate (or a mixture thereof), as well as soluble anhydrite (completely dehydrated gypsum or even partially dissociated anhydrite containing a small amount of free calcium oxide).

• II - binders obtained without heat treatment (non-firing): natural anhydrite, special additives are introduced to activate hardening.
• III - binders obtained by mixing gypsum binders of groups I or II with various components(lime, Portland cement and its varieties, active mineral additives, chemical additives, etc.).

Binders of groups I and II are non-water resistant (air) gypsum binders (NGB). Binders of group III belong, with some exceptions, to waterproof gypsum binders (VGV).

For the production of gypsum binders indicated in Table 1.1, natural gypsum, anhydrite raw materials or gypsum-containing waste are used.

Depending on the heat treatment temperature, gypsum binders are divided into two groups:

Low firing group

Low-calcined (actually gypsum, based on CaSO 4 . 0.5H 2 O), obtained at a temperature of 120-180 ° C. They are characterized by fast hardening and relatively low strength. These include:

• building plaster, including alabaster;
• molding plaster;
• high-strength gypsum;
• medical plaster;

High firing group

High-calcined (anhydrite, based on CaSO 4), obtained at temperatures of 600-900°C. Anhydrite binders differ from gypsum binders in slow hardening and higher strength. These include:

• estrich-gypsum (high-fired gypsum);
• anhydrite cement;
• finishing cement.

Gypsum binder advantage:

• high setting speed;
• chemical neutrality, i.e. environmental friendliness of the material;
• satisfactory strength;
• ease of application, plasticity.

Disadvantages of gypsum binder:

• limited water resistance;
• limited scope, mainly for internal construction and finishing works;
• insufficient heat resistance;

Plaster setting

According to the setting time determined on the Vika device, gypsum is divided into three groups (A, B, C):

The hardening time of gypsum depends on the brand of gypsum, the amount of water, the temperature of the water, and the dispersion of the gypsum. With a low water content, the mixture is poorly poured, hardens quickly, releases an increased amount of heat, with a simultaneous increase in the amount of volume.

The hardening time of gypsum increases with increasing water temperature, so cold water should be used.

Slow down the setting of gypsum with additives:

• wood glue;
• sulfite-alcohol stillage (SSB);
• technical lignosulfonate (LST);
• keratin retarder;
• boric acid;
• borax;
• polymer dispersions (for example, PVA).

Plaster hardening

The chemistry of gypsum hardening consists in the transition of hemihydrate calcium sulfate, when mixed with water, into dihydrate: CaSO 4 . 0.5H 2 O + 1.5H 2 O → CaSO 4. 2H 2 O. Outwardly, this is expressed in the transformation of plastic dough into a solid stone-like mass.

The reason for this behavior of gypsum is that hemihydrate gypsum dissolves in water almost 4 times better than dihydrate (solubility, respectively, 8 and 2 g/l in terms of CaSO 4). When mixed with water, hemihydrate gypsum dissolves to form a saturated solution and immediately hydrates, forming a dihydrate, with respect to which the solution is supersaturated. Crystals of gypsum dihydrate precipitate out, and hemihydrate begins to dissolve again, etc.

In the future, the process can go along the path of direct hydration of gypsum in the solid phase. The final stage of hardening, ending after 1-2 hours, is the formation of a crystalline aggregate from fairly large crystals of gypsum dihydrate.

Part of the volume of this splice is occupied by water (more precisely, a saturated solution of CaSO 4 . 2H 2 O in water), which did not interact with gypsum. If the hardened gypsum is dried, then its strength will noticeably (1.5-2 times) increase due to additional crystallization of gypsum from the above solution at the points of contacts of already formed crystals.

With repeated moistening, the process proceeds in the reverse order, and the gypsum loses some of its strength. The reason for the presence of free water in hardened gypsum is explained by the fact that about 20% of water by weight is needed for hydration of gypsum, and 50-60% of water is needed for the formation of plastic gypsum dough. After hardening of such a dough, 30-40% of free water will remain in it, which is about half the volume of the material. This volume of water forms pores temporarily occupied by water, and the porosity of the material, as is known, determines many of its properties (density, strength, thermal conductivity, etc.).

The difference between the amount of water required to harden the binder and to obtain an easily moldable dough from it is the main problem in the technology of materials based on mineral binders. For gypsum, the problem of reducing water demand and, accordingly, reducing porosity and increasing strength was solved by obtaining gypsum by heat treatment not in air, but in a saturated steam environment (in an autoclave at a pressure of 0.3-0.4 MPa) or in salt solutions (CaCl 2 . MgCl 2 and others). Under these conditions, another crystalline modification of semi-aqueous gypsum is formed - α-gypsum, which has a water demand of 35-40%. Gypsum α

Modifications are called high-strength gypsum, because due to the reduced water demand, it forms a less porous and more durable stone during hardening than ordinary β-modification gypsum. Due to the difficulties of production, high-strength gypsum was not found wide application in construction.

Production of building gypsum

Raw materials for building plaster

The raw material for gypsum is mainly natural gypsum stone, consisting of calcium sulfate dihydrate (CaSO 4 . 2H 2 O) and various mechanical impurities (clay, etc.).

According to GOST 4013 - 82 gypsum stone for the production of gypsum binders must contain:

Impurities: SiO 2, Al 2 O 3, Fe 2 O 3.

Gypsum-containing industrial wastes can also be used as raw materials, for example, fluorogypsum, borogypsum, which are formed during the processing of corresponding raw materials with acids, for example

Ca 5 (PO 4) 3 F + H 2 SO 4 → H 3 PO 4 + HF + CaSO4. nH2O

All this indicates that there are no problems with raw materials for gypsum binders.

Construction gypsum dehydration schemes

The production of any gypsum binder is based on the dehydration of raw materials during heat treatment. Depending on the conditions, as the temperature increases, various dehydration products are formed.

The general scheme for the dehydration of calcium sulfate dihydrate can be represented schematically:

The diagram shows the transition temperatures under laboratory conditions; in practice, under a large number material and fluctuations in chemical composition, higher temperatures have to be applied to accelerate firing.

Depending on the temperature and firing conditions, calcium sulfate hemihydrate (hemihydrate) α can be obtained

And β-modifications, α

And β-soluble anhydrite, insoluble anhydrite.

Today it is generally accepted that education α

Or β-modifications of hemihydrate gypsum (they are similar in the structure of the crystal lattice) depends on the conditions of heat treatment: α-hemihydrate is formed at a temperature of 107-125 ° C and above, provided that water is released in a drop-liquid state, for which autoclave treatment is provided ; β-modification of semi-aqueous gypsum is obtained by heating to 100-160°C in open apparatuses (rotary kilns or digesters) with the removal of water in the form of steam.

High-strength α-hemihydrate crystallizes in the form of well-formed large transparent needles or prisms; ordinary building gypsum - β-hemihydrate - consists of the smallest, poorly expressed crystals that form aggregates.

This is the reason for the different properties of the product: β-hemihydrate is characterized by a higher water demand, a higher rate of interaction with water, a lower density and strength of the resulting gypsum stone. Despite this, β-hemihydrate is significantly cheaper and makes up the bulk of gypsum binders.

For practical purposes, the conditions for obtaining modifications of hemihydrate calcium sulfate (hemihydrate) are of particular importance. The dehydration reaction of gypsum dihydrate with the formation of a hemihydrate proceeds with the absorption of heat and has the form:

2(CaSO 4 . 2H 2 O) => 2CaSO 4 . H2O + 3H2O

This reaction is often written in a somewhat conventional form:

CaSO4. 2H 2 O => CaSO 4 . 0.5H2O + 1.5H2O

Factory building gypsum, fired at temperatures higher than theoretically necessary for the formation of hemihydrate, contains, in addition to hemihydrate gypsum, also soluble and even insoluble anhydrite, which affects the properties of the product. Soluble anhydrite in the air absorbs moisture and turns into a hemihydrate.

Consequently, the quality of slightly burnt gypsum increases during aging, while the admixture of unburned gypsum with insufficient firing is ballast and adversely affects the mechanical strength of the hardened binder, as well as the setting speed.

The simultaneous content of soluble anhydrite and raw gypsum in building gypsum causes a very rapid setting, since the first quickly dissolves and passes into gypsum dihydrate, and the second creates crystallization centers.

Industrial production of gypsum binder

Building gypsum is produced using digesters, rotary kilns and combined grinding and firing installations. The most common production of building gypsum with the use of digesters.

Production stages:

• Crushing of gypsum stone (jaw and hammer crusher).
• Grinding combined with drying (shaft mill).
• Heat treatment at atmospheric pressure or in an autoclave (cooking in a gypsum kettle).
• Languishing (laying out in a bunker).
• Secondary grinding (ball mill).

The use of gypsum

• It is widely used in industry and construction as a building material. In its pure form, it is rarely used, mainly used as an additive, as a binder. The main area of ​​application is the device of partitions.
• In repairs, they are used as the main finishing or leveling material. For leveling, prefabricated panels, gypsum stones, gypsum boards are used.
• Acoustic slabs are made from gypsum.
• It is used in various ways for fire retardant coatings metal structures.
• Small in size but important direction gypsum use: decorative architectural details(stucco) and sculpture.
• Fired gypsum is used to make molds (for example, for ceramics) for castings and casts (bas-reliefs, cornices, etc.). It is used to make durable molds for filling figures.
• In dentistry, they are used to make impressions of teeth.
• In medicine, for fixing fractures (medical plaster).

The history of the use of gypsum

Gypsum is one of the oldest mineral binders. In Asia Minor, gypsum was used for decorative purposes as early as 9,000 BC. During archaeological excavations in Israel, floors covered with plaster for 16 thousand years BC were found. Gypsum was also known in ancient Egypt, it was used in the construction of the pyramids. Knowledge of the production of building gypsum from Egypt spread to the island of Crete, where in the palace of the king of Knossos, many of the outer walls were built of gypsum stone. The seams in the masonry were filled gypsum mortar. Further information about gypsum through Greece came to Rome. From Rome, information about gypsum spread to central and northern Europe. Gypsum was especially skillfully used in France. After the Romans were ousted from central Europe knowledge about the production and use of gypsum was lost in all regions north of the Alps.

And only from the 11th century the use of gypsum began to increase again. Under the influence of the monasteries, a technology spread, according to which the voids inside half-timbered buildings were filled with a mixture of gypsum with hay or horsehair. In the early Middle Ages in Germany, especially in Thuringia, the use of gypsum for floor screeds, masonry mortars, decorative items and monuments. In Saxe-Anhalt, the remains of gypsum floors from the 11th century have been preserved.

Masonry and screeds, made in those ancient times, are distinguished by their extraordinary durability. Their strength is comparable to that of normal concrete.

The peculiarity of these medieval gypsum mortars is that binders and fillers consisted of identical materials. Gypsum stone, ground to round grains, not pointed and not lamellar, was used as fillers. After the solution hardens, a bonded structure is formed, consisting only of calcium sulfate dihydrate.

Another feature of medieval mortars is the high fineness of gypsum grinding and extremely low water demand. The ratio of water to binder is less than 0.4. The solution contains few air pores, its density is approximately equal to 2.0 g/cm3. Later gypsum mortars were produced with a much higher water requirement, so their density and strength are much less.

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

general information

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

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

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

Properties and varieties

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

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

To date, 3 varieties are known, among them:

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

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

Application

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

• inkwells;
• countertops;
• vases.

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

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

More about application

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

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

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

Origin

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

Place of Birth

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

Specifications

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

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

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

White gypsum dissolves in water in small quantities:

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

If further heating occurs, the solubility decreases.

Description, scope and properties of building gypsum

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

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

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

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

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

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

Finally

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

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

Gypsum as a material has been known since ancient times, but it has not lost its popularity and demand to this day. In addition, even the newest and most advanced materials could not compete with him. The use of gypsum 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 gypsum stone. They are fired in furnaces of different temperatures, and then they are broken until a powder mixture appears. Gypsum-treated surfaces can absorb unwanted moisture from the air and release it when the air is very dry. This material is referred to as sulfates. Exists two types of plaster: selenite and alabaster. The first is fibers and the second is grains.

What are the technical characteristics of building gypsum?

Almost all gypsum mixtures have similar characteristics. These include:

1. Density. The building material is a fine-grained structure. On average, the density varies from 2.6 to 2.8 g per cm.

2. Drying period. It sets up in just a matter of minutes. Experience shows that in the fourth minute after mixing it, 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, so the specific, volumetric and bulk weight are almost the same.

4. Melting point. This material can be heated up to 700 degrees Celsius! And it will not change its form 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 is from 10 to 50 MPa.

6. Gypsum meets GOST i.e. government regulations.

7. Thermal conductivity and solubility. It is a very weak conductor of heat. And practically does not dissolve.

What are the types of gypsum?

one. . The use of this type of gypsum extends to the creation of gypsum parts and slabs for plaster work. All work with it must be done in 10-20 minutes, as it freezes very quickly. It is during this period of time that the material must be fully used. Only at the initial moment of hardening, gypsum gains 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 slow down hardening. This mortar reduces the complexity of work and material costs in general. It is mined by blasting gypsum-containing rocks. After that, gypsum is transported to manufacturing plants in the form of stones.

2. High strength. In its structure and composition, it practically does not differ from the previous species. However, in the building type, the crystals are smaller, and in the high-strength type, they are larger, so it has less porosity and great strength. It is produced by heat treatment in a special device. The use of gypsum of 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, and more specifically about dentistry and traumatology.

3. Polymer. This type of gypsum 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 plaster ones, they are applied without difficulty and with minimal cost time, 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 almost the same as polymer, only its composition allows you to stretch the bandage in all directions and in different directions.

5. Structural or molding. The most environmentally friendly, as it does not contain any additives. They are used to create molds for sculptures, various figurines, sculpting, etc. also used in automotive and aircraft construction activities. It is the main element of dry putties. This type 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 type completely hardens, the material is similar to simple building material, but it is lighter. Various decorative moldings - full credit acrylic material. Gypsum can withstand different temperatures, has little moisture absorption, so it can also be used to create beautiful and unusual facades building. It is very easy to work with him. If aluminum powder is added to the mixture or marble chips, then the gypsum will accordingly resemble marble or metal.

7. Polyurethane. Also used in stucco. In terms of cost, it is much more profitable than the building type. But in terms of performance, there is no difference.

8. White plaster. It is a great help in various repair work. They put everything in order. White gypsum can be combined with different building materials - this is its main advantage. Freezes for about 7 minutes.

9. Fine-grained or translucent. They fill the seams.

10. Liquid gypsum. Made from gypsum powder. The manufacturing algorithm is as follows: 1 - water is prepared, 2 - gypsum is poured into it and mixed, 3 - stirred until a liquid substance is obtained.

11. Waterproof or moisture resistant. Obtained by processing the material according to a special algorithm. To improve its qualities, bard is added to it.

12. Fireproof. All gypsum is fire resistant, but this type is made from tongue-and-groove gypsum, which can withstand extreme temperatures. Used in all areas, especially where it is necessary to increase the fire resistance.

13. Architectural. It is very plastic, and does not contain toxic elements. The acidity of gypsum of this type is the same as the acidity of human skin. Modeling from this gypsum is very popular, and therefore the demand for it is high.

Can something replace gypsum?

Yes maybe. And that material is alabaster. It is also known in the construction world, it is obtained from two-water gypsum by processing at high temperatures. According to 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, without limitation, alabaster is known only in the construction field.

2. If special components are not added to the alabaster, then 1 - it will dry out very quickly, 2 - it will simply be unusable.

3. Gypsum is more environmentally friendly than alabaster.

4. Alabaster is more durable than gypsum.

Gypsum properties

Gypsum(hydrous calcium sulfate) - the most common mineral belonging to the group of sulfates. Its name comes from the Greek word gypsos. Gypsum scratches with a fingernail and is easily cut with a knife. Several varieties of gypsum used as collection stones, in particular fine-grained alabaster. silky spar, fibrous gypsum and white plaster They have a silky sheen and are often cut into cabochons and polished to produce a cat's-eye effect.

Soft selenite, which is colorless and transparent, is also sometimes cut. Popular among collectors are the beautiful "desert roses", dovetail twin crystals and star shapes.

The use of gypsum

Gypsum is used in the manufacture of plaster, fertilizer, Portland cement, paper, paints and pencils. It is the most common evaporite - the sediment remaining after the evaporation of water. Gypsum occurs as massive deposits in sedimentary rocks along with limestone and shale. It is formed as a result of hydration of the anhydrite mineral.

Gypsum is accompanied by calcite, sulfur, quartz, dolomite, halite, and clay. Sometimes gypsum is deposited as a result of evaporation of salt water or forms soft translucent crystals in place of dried-up lakes. It also occurs as crystals in clay, as shells of salt domes, and in volcanic zones. Alabaster, both dense and fine-grained, is used to create statues and moldings.

However, due to the extreme softness of alabaster, products made from it break easily and quickly collapse. As a rule, alabaster is translucent and colored white, pinkish or brownish. Main gypsum deposits and alabaster are found in Italy and in England. Pink alabaster is mined in Wales.

Origin of gypsum

There are deposits of alabaster in Spain, Iran and Pakistan. "Alabaster", from which in ancient Egypt and Ancient Rome they allegedly made vases, tombstones, etc., in fact, marble (calcium carbonate). There are rich gypsum deposits in the USA (Arizona, California, Utah, Colorado, Oklahoma, New Mexico, Ohio, Michigan, Virginia and New York), Canada and France.

January 19th, 2010

Gypsum(from the Greek. gypsos - chalk, lime) - a mineral, aqueous calcium sulfate. Gypsum crystals are lamellar, columnar, acicular and fibrous. They occur mainly in the form of continuous 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. precipitated from aqueous solutions, rich in sulfate salts, during the drying of sea lagoons, salt lakes. One of the physical properties of gypsum - fluorescence. Glow in long-wave UV light - yellow, orange, blue or green tones. Mineral anhydrite(CaSO4) is similar in composition to gypsum, only devoid of water. Gypsum has 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 an extremely rare natural environment caves. The temperature there constantly remained around 58 ° C, and the cave was filled with water rich in minerals, as is necessary for the growth of crystals. The largest of these crystals weighing 55 tons had age about 500,000 years . The two brothers who discovered this cave dubbed it "Queen's Eyes". The length of the cave is 290 meters underground.

And this is my cast sample from the magazine.
Red selenite

There are such varieties of gypsum:
Selenite- a colorless and transparent grade of gypsum, which has a pearly sheen.

Another variety is a silky, fibrous form called "Satin Spar" - satin spar . This variety has a satiny, silky sheen that gives a play of light up and down the surface of the crystal.
Alabaster- a kind of gypsum, white or slightly tinted, which is a compressed fine-grained mass - a decorative ornamental stone used for fine carving for centuries, even eras.
In dry areas, gypsum can form into flowery forms, usually opaque with embedded grains of sand, called "desert roses". It can also have the following names: Sand rose, Stone rose, Selenite rose, Gypsum rose, Gypsum rosette.

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

Even in the Neolithic era, gypsum was used as a building material. Back in 7000 BC. in the city of Chatal-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 join stones. For example, at the Sphinx(2700-2600 BC) for certain works, gypsum plaster was limed. In addition, translucent alabaster windows were known to the Egyptians. In the Minoan civilization, gypsum was used to make floors or wall coverings and used it as building blocks. (famous Knossos palace, 2100-1800 BC, located on the Greek island of Crete and preserved to this day). The Romans used gypsum only for decorative plaster inside buildings, as they were familiar with much more durable materials.

In our time, gypsum is used for the manufacture of binders, soil gypsum, and in medicine - for prosthetics. It is also used for removing masks, modeling sculpture, creating relief decorations (stucco moldings) in rooms. Since ancient times, gypsum has been popular as an ornamental stone. Even now openwork vases, figurines, ashtrays and other decorative items are cut out of it. AT Asian cuisine gypsum is added to tofu (traditional bean curd) as a coagulant, making it the ultimate source of calcium.