How plaster is made. Field of use and properties of high-strength and polymer gypsum

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

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STRUCTURE

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

PROPERTIES

The color is very different, but usually white, gray, yellow, pink, etc. Pure transparent crystals are colorless. It can be painted in various colors with impurities. The color of the line is white. Crystals have a glass luster, sometimes with a pearlescent tint due to microcracks of perfect cleavage; in selenite it is silky. Hardness 2 (standard of the Mohs scale). Cleavage is very perfect in one direction. Thin crystals and cleavage plates are flexible. Density 2.31 - 2.33 g / cm 3.
It has a marked water solubility. A remarkable feature of gypsum is the fact that its solubility with an increase in temperature reaches a maximum at 37-38 °, and then falls rather quickly. The greatest decrease in solubility is established at temperatures above 107 ° due to the formation of "hemihydrate" - CaSO 4 × 1 / 2H 2 O.
At 107 ° C, it partially loses water, turning into a white powder of alabaster (2CaSO 4 × Н 2 О), which is noticeably soluble in water. Due to the lower amount of hydrated molecules, alabaster does not shrink during polymerization (it increases in volume by approx. 1%). Under p. Tr. loses water, splits and fuses into white enamel. On coal in a reducing flame it gives CaS. It dissolves much better in water acidified with H 2 SO 4 than in pure water. However, when the concentration of H 2 SO 4 is over 75 g / l. solubility drops sharply. Very little soluble in HCl.

MORPHOLOGY

Due to the predominant development of the (010) faces, crystals have a tabular, rarely columnar or prismatic appearance. The most common prisms are (110) and (111), sometimes (120) and others. Faces (110) and (010) often have vertical shading. Fusion twins are frequent and are of two types: 1) Gallic according to (100) and 2) Parisian according to (101). Distinguishing them from each other is not always easy. Both of them resemble a dovetail. Gallic twins are characterized by the fact that the edges of the m (110) prism are parallel to the twin plane, and the edges of the l (111) prism form an inward angle, while in the Parisian twins, the edges of the Ι (111) prism are parallel to the twin seam.
It occurs in the form of colorless or white crystals and their intergrowths, sometimes colored by inclusions and impurities captured by them during growth in brown, blue, yellow or red tones. Characterized by intergrowths in the form of a "rose" and twins - the so-called. "Dovetails"). Forms veins of parallel-fibrous structure (selenite) in clayey sedimentary rocks, as well as dense continuous fine-grained aggregates resembling marble (alabaster). Sometimes in the form of earthy aggregates and cryptocrystalline masses. Also composes sandstone cement.
Common pseudomorphs on gypsum of calcite, aragonite, malachite, quartz, etc., as well as pseudomorphs of gypsum on other minerals.

ORIGIN

A widespread mineral, in natural conditions it is formed in various ways. Sedimentary origin (typical marine chemogenic sediment), low-temperature hydrothermal, occurs in karst caves and solfatars. It precipitates from sulfate-rich aqueous solutions during the drying up of sea lagoons and salt lakes. Forms layers, interlayers and lenses among sedimentary rocks, often in associations with anhydrite, halite, celestine, native sulfur, sometimes with bitumen and oil. In significant masses, it is deposited by sedimentary means in lacustrine and marine saline dying basins. In this case, gypsum, along with NaCl, can be released only in the initial stages of evaporation, when the concentration of other dissolved salts is not yet high. Upon reaching a certain value of the concentration of salts, in particular NaCl and especially MgCl 2, instead of gypsum, anhydrite will crystallize and then other, more soluble salts, i.e. gypsum in these basins must belong to the earlier chemical sediments. Indeed, in many salt deposits, gypsum (and also anhydrite) layers, interbedded with rock salt layers, are located in the lower parts of the deposits and in some cases are underlain only by chemically precipitated limestones.

In Russia, thick gypsum-bearing strata of Permian age are widespread in the Western Urals, in Bashkiria and Tatarstan, in the Arkhangelsk, Vologda, Gorky and other regions. Numerous deposits of the Upper Jurassic age are established in the North. Caucasus, Dagestan. Remarkable collection 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 unique size gypsum crystals up to 11 m long were found.

APPLICATION

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

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

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

Gypsum - CaSO 4 * 2H 2 O

CLASSIFICATION

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

PHYSICAL PROPERTIES

Mineral color	colorless turning into white, often colored with minerals-impurities in yellow, pink, red, brown, etc.; sometimes there is a sectorial-zonal color or distribution of inclusions over growth zones within crystals; colorless in internal reflexes and enlightenment.
Line color	White
Transparency	transparent, translucent, opaque
Shine	glassy, ​​close to glass, silky, pearlescent, dull
Cleavage	very perfect, easily obtained from (010), almost mica-like in some samples; along (100) clear, passing into a conchoidal fracture; by (011), gives a splinter break (001)
Hardness (Mohs scale)	2
Break	smooth, conchoidal
Strength	flexible
Density (measured)	2.312 - 2.322 g / cm 3
Radioactivity (GRapi)	0

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

First of all, this is due to its properties - plasticity, natural uniformity, uniformity of color, final hardness, which allows you to create absolutely any form, whether it be a bas-relief drawing, an ornament from stucco elements or sculpture. With proper use, good storage conditions, careful restoration, the created products can last forever. An example of this is the temples around the world, which have preserved a unique interior from past centuries to the present day.

What the master needs to know about the properties of gypsum and products from it

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

• Environmental friendliness and naturalness. Gypsum is a completely natural material, it is still mined in an old-fashioned way. It is as environmentally friendly as possible, which puts such raw materials many steps higher than any modern building material.
• The ability to improve the microclimate. It has long been noticed that in rooms decorated with stucco molding, it is very easy to breathe, even if it is hot or raining outside. This is easily explained by the fact that the frozen gypsum solution has the ability to exchange moisture: increased moisture is absorbed by it, and with an insufficient amount of water in the air, it is released.
• Responsive to restoration. Unlike glass, leather, wood, stone and even metal, stucco molding is subject to complete restoration. With well-done renovations, she can look perfect, even if she is a hundred years old. Try re-creating a missing piece of a china or stone bowl so that it looks like new. Agree, this is impossible. But the plaster products after restoration do not contain visible traces of the master's work.
• Endless decor possibilities. In skillful hands, plaster takes any shape, even the smallest details are visible on it. It can be stained, patinated, coated with various compounds that give shine or other visual qualities. Moreover, it is not subject to shrinkage, so the finished decor will remain in its original form as much as the owner of the room wants.

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

Where can gypsum (alabaster) be applied

Plaster is used in everyday life quite often:

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

Characteristics of gypsum for building and finishing mortars

Modern building gypsum (the second name is alabaster), used for the preparation of mortar, 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 sieving, as a result, a homogeneous powder with different particle sizes is obtained - 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 has not passed through the net is weighed by 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 an 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 mortar 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 - the viscosity of the solution, is determined by the GOST 125-79 standard and depends precisely on the degree of grinding, because the particle size directly affects the water demand. It is believed that 18.6% water would be enough to hydrate semi-aqueous alabaster to the degree of dihydrate, but such a solution is not suitable for construction work, therefore, normal viscosity is achieved by adding 50-70% water (3-hemihydrate). If a thick solution is needed, then 35-45% of water is limited, receiving a-hemihydrate. The standard consistency is determined by the spreading parameter of the mass, which should not exceed a diameter of 180 ± 5 mm.

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

The stucco production process can also go in a different order: grinding-screening-firing. If it is required to make special types of this material (medical or molding), then the technology can be changed. When the gypsum stone is heated in a vacuum and the temperature drops to 100 ° C, high-strength alabaster is obtained at the outlet.

Deformability of alabaster

Plaster can change in volume when dry. But unlike many materials, its volume does not decrease, but, on the contrary, increases. Deformability can reach 1%. This quality is a big plus in the manufacture of sculptures and stucco moldings, since the solution fills the forms perfectly, allowing you to get a very clear pattern without losing small details.

Expandability depends on the amount of soluble anhydrite in the material. Gypsum that has been fired at elevated temperatures is subject to the greatest deformability. There are several ways to reduce this indicator:

• an increase in the amount of water;
• the introduction of hardening retarders;
• addition of 1% quicklime up to 0.1%.

If the solution is improperly prepared or when creating large-scale products, significant shrinkage is possible, which leads to cracking of the gypsum. The process can be leveled by using mineral additives.

If the ratio of mortar plasticity 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 high and visually noticeable. Pozzolanic hydraulic additives in combination with Portland cement can reduce plastic distortion.

Gypsum strength

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

• Compressive strength. To determine the strength of semi-aqueous gypsum, a specialist makes 4x4x16 cm bars from a test solution. It takes 2 hours for solidification, 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 with a step of 3 to G-25, where the figure means the compressive strength, for example, gypsum grade G-7 will withstand pressure up to 7 kg / sq. cm.
• Comprehensive assessment. Additional marking is the hardening speed (A, B, C) and the grinding index. Highest category quality has characteristics from G-5, index III. Increased requirements are imposed on gypsum intended for the production of molds for porcelain and earthenware and ceramic products. Grade from G-10, setting 6-30 minutes, fineness of grinding - the remainder of not more than 1%, water absorption from 30%, volumetric expansion after hardening up to 0.15%.
• Porosity. The finished gypsum products are quite hard and porous, the pore volume can exceed 60%, at least 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 a solution, it is important to take into account the degree of grinding of the powder. The finer the particles, the more water the mixture can take, but this is exactly the case when with an increase in the water content (within the limits of GOST), the final strength of the products does not decrease, but rather increases. That is why, for the most durable gypsum castings, craftsmen prefer to take 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 to 300%, so many craftsmen prefer to work with raw materials that have a low water demand. A decrease in this indicator can be achieved 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 molding.
• Ultimate tensile strength. The tensile and compressive strengths of gypsum products are always different. It should be borne in mind that alabaster withstands stretching 10 times worse than compression, so it cannot be used in conditions where the characteristics of the base may change.
• Influence of moisture on strength. Another important point is the effect of moisture on strength. The higher the water content in the air, the lower the compressive strength of the gypsum. For example, moistening stucco moldings by only 1% (at a relative air humidity of 90 - 100%) can reduce the 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. Thicker products tolerate increased humidity better. At the same time, one should not think that any cataclysm can destroy the plaster casts. It is enough to carefully dry the products, as their previous qualities will return.
• Softening factor. The dependence of products 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 vary from 0.3 to 0.5 (the harder the solution, the higher). It should be borne in mind that with the use of organic additives, a deterioration in strength can be expected; mineral additives have little effect.

Terms and method of storage of gypsum

Storing dry powders requires a low level of humidity, so bags (or scattering in boxes) are usually kept on high racks (from 50 cm). The storage periods must be observed flawlessly in accordance with GOST 2226-75. Powders used in the ceramics and porcelain industries must not be stored loose.

When buying gypsum, it is imperative to pay attention to its expiration date, 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 as follows.

• Dry fresh gypsum begins to interact with moisture, as a result of which a film of dihydrate molecules forms on the surface of the semi-aqueous gypsum grain.
• When mixing a solution from such raw materials, it can be noted that it solidifies for a long time, since the film does not allow the hemihydrate to quickly bind with water.
• The water demand is reduced, and the strength of the finished casts is therefore increased.

With prolonged exposure, the process is aggravated.

• 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 making the mortar (dough), you must prepare everything for work. If you don't take care of this, then you may not get the desired result, since the mixture will solidify very quickly.

Mold filling solution recipes.

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

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

How to adjust the setting time of plaster

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 makes castings, then a high solidification rate is simply necessary, so it is worth choosing raw materials of the appropriate quality. If finishing or restoration work is being carried out, then the curing rate should be reduced to obtain the time required for the production of one or another action.

By the time of solidification, the solutions are obtained as follows.

• Quickly hardening - 2-15 minutes from the moment of preparation of the solution.
• Normally hardening - 6-30 minutes.
• Slowly hardening - from 20 minutes.

The setting time depends on several factors at once:

• fineness of grinding (the finer the particles, the faster);
• powder properties (semi-aqueous gypsum, including dihydrate elements, sets much faster);
• manufacturing technology (temperature and duration of raw material calcination affects);
• shelf life;
• temperature of raw materials and water for the seal: cold dough hardens longer than 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 goes into the state of dihydrate;
• the percentage of water and powder (the less water, the faster the hardening goes);
• quality and intensity of mixing;
• the presence of additives (sand, slag, sawdust, polymers and special chemical additives reduce the hardening period of the solution).

How to choose additives for gypsum

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

Check out the Gessostar plaster catalog

There are 5 types of additives in total.

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

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

Inhibitors... Retardant additives that react and form low-dissociating compounds. The percentage should not exceed 0.2-3%.

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

Catalysts... Crystallization accelerating additives. The percentage should not exceed 0.2-3%.

• CaHP04-2H20, CaS04-2FI20, KCl and other salts.

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

• Lime-adhesive mortar, keratin.

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

The most common accelerators are sodium sulfate, gypsum dihydrate and common table salt, and lime-glue is the retarder. The addition of a surfactant in this case compensates for the decrease in strength caused by the additives.

Die lubricants

If you decide to work with plaster, then you should purchase a special mold release agent that facilitates easy separation of the impression and matrix.

• Stearin and paraffin, dissolved in kerosene, are suitable for separating gypsum from gypsum.
• In the manufacture of reliefs with a complex pattern, you can use soap foam, copper sulfate, soda ash, potash.
• Epoxy resin dissolved in acetone is used commercially.
• There are special industrial lubricants for all types of products.

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

Check out the Gessostar plaster 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 increasing 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 boric acid (5%, temperature 30 degrees).
• Additive to water for a solution of silicic acid (up to 50%), followed by heating the casting to 60 degrees.
• Use for a solution of borax, followed by treatment of the casting with barium chloride and hot soap solution.
• Treatment of the casting with Glauber's salt solution.
• Impregnation of finished gypsum with copper or iron sulfate.
• Exposure in a solution of potassium alum (day) followed by warming up to 550 degrees.

How to increase the durability of plaster

Gypsum will last forever, provided that temperature and humidity standards are met. Long-term high humidity with sharp temperature fluctuations or exposure to wind, as well as being completely in water, can destroy an alabaster product.

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, granular slag);
• surface treatment with moisture-proof solutions (synthetic resins, barite milk, hydrophobic compounds).

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

The fire of alabaster is not terrible, the flame will destroy the gypsum only after 5 hours of exposure, which means that 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 business. Even a child can make primitive castings, but only a specialist with great experience and rich skills can produce really high-quality stucco molding that can last a very long time.

A mineral derived from calcium is its hydrous sulfate called gypsum. It has many synonymous names: monmartite, desert rose, gypsum spar (crystalline and leafy forms). The sample of the fibrous structure is selenite, the granular one is alabaster. It will focus on the varieties and properties of this stone, its prevalence in the country and its use in construction, medicine and other areas of the economy.

History reference

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

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

In England and France, starting from the 16th century, they began to cover wooden buildings with plaster, protecting them from fires. The year 1700 is considered the beginning of the use of the mineral as fertilizer. To create architectural forms in Russia in the 17th-18th centuries. plaster decor was widely used, and in 1855 the Russian surgeon N.I.

During the Crimean War, Pirogov invented and began to use a plaster cast to fix the limbs to treat the wounded. This saved many soldiers from losing an arm or a leg.

Description of the mineral

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

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

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

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

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

Properties and application

In nature, the physical features of the structure are in a variety of forms: dense and granular, earthy, leafy and fibrous, nodules and dusty masses. In the voids, they are found in the form of crystal druses. The solubility of gypsum in water increases with temperature up to 37―38 ºС, then decreases, and upon reaching 107 ºС the mineral passes into the state of CaSO4 · ½H2O hemihydrate. When a small amount of sulfuric acid is added to the water, the solubility is improved. L reacts weakly to HC.

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

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

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

Natural species

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

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

Varieties for practical use

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

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

Other uses

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

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

Deposits of stone

The distribution of gypsum in the earth's crust is observed everywhere, mainly in layers of sedimentary rocks with a thickness of 20 - 30 m. World production is about 110 million tons of stone per year. The largest producers are Turkey, Canada, USA, Spain and Iran. Among the unique ones, one can note the Naica Mine thermal caves in Mexico, where druses of giant gypsum crystals 11 m long were found.

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

Objective: Acquaintance with devices and methods of gypsum examination.

Equipment and materials: hydraulic press, Vic's device, a cup and a spatula for making plaster dough, electronic scales, Suttart device, sieve No. 02, ruler, stopwatch, gypsum.

Safety regulations: in order to protect the eyes from the ingress of a foreign body, carry out laboratory work in safety glasses.

Theoretical part

Mineral binders Artificially obtained powdery materials are called, which, when mixed with water, form a plastic substance capable of hardening as a result of physicochemical processes, that is, passing into a stone-like state. Construction mineral binders fall into three categories:

Airborne binders(lime, gypsum) are characterized by the fact that, when mixed with water, they harden and retain their strength for a long time only in air environment ... In the case of systematic moistening, they lose strength and collapse.

Hydraulic binders(Portland cement) are characterized by the fact that, after mixing with water and preliminary hardening in air are able to further harden both in air and in water, while their strength increases.

Acid-resistant binders(acid-resistant quartz fluorosilicon cement) is a finely ground mixture of quartz sand and sodium fluorosilicon, mixed aqueous solution sodium silicate or potassium. It is an initial hardening binder in air that can resist the aggressive action of inorganic and organic acids , except for fluoride-hydrogen.

1. Airborne binders. Gypsum

Gypsum binders divided into 2 groups: low-burning and high-burning.

Low firing gypsum binders are obtained by heating gypsum dihydrate (CaSO4 * 2H2O) to a temperature of 150 ... 160 ° C. In this case, there is a partial dehydration of dihydrate gypsum with its transition to semi-aqueous gypsum: CaSO4 * 2H2O → CaSO4 * 0.5H2O + l, 5H2O... Low-calcined binders include: construction, molding, high-strength and medical plaster. Natural gypsum stone (CaSO 4 * 2H2O,) as well as industrial waste containing calcium sulfate -CaSO4.

High-firing(anhydrite) binders get thermal

By light calcination of gypsum dihydrate (CaSO4 * 2H2O) at a higher temperature - 600 ... 900 ° C. In this case, gypsum dihydrate completely loses chemically bound water, resulting in the formation of aqueous calcium sulfate - anhydride CaSO4.

High-calcined binders include: anhydrite c-

cop and estrich plaster.

The raw material for the production of high-calcined binders is anhydrite CaSO4, as well as industrial waste containing calcium sulfate -CaSO4.

Construction gypsum... Plaster of paris or alabaster

(GOST 125-79) is called an air binder obtained by heat treatment natural gypsum dihydrate - calcium sulphate CaSO4 * 2H20 at a temperature of 150 - 180 ° С until it turns into semi-aqueous gypsum - calcium sulfate CaSO 4 * 0.5H2O, followed by grinding into a fine powder:

Production stucco consists of crushing, ton-

whom grinding and heat treatment of gypsum stone.

There are 2 ways to produce plaster of paris:

When firing in open apparatus communicating with the atmosphere at a temperature of 150-160 ° C, when water is removed from the raw material in the form of steam, and gypsum binders consist mainly of small crystals β - modifications.

In mine or aerobic mills, followed by firing at a temperature of 100 ° C of the crushed product in gypsum boilers or ovens.

Construction (semi-aqueous) gypsum is a white or gray powder. The color of the gypsum depends on the amount of impurities in the gypsum stone and the purity of the firing. In the production of gypsum,

It is suggested to introduce additives in order to regulate the setting time and improve the physical and mechanical properties of gypsum.

Remember! - Formula of stucco - CaSO4* 0.5H2O. Formula of natural gypsum dihydrate (from which building gypsum is obtained): CaSO4 * 2H2O.

The reaction of obtaining stucco:

CaSO4 * 2H2O → CaSO4 * 0.5H2O + l, 5H2O.

Assessment of the quality of stucco

The quality of plaster of paris is determined by the following indicators:

By fineness of grinding;

According to the normal density of the gypsum dough;

By the time of setting;

Compressive strength.

Depending on the quality, there are two types of stucco, see table 4.1.

Table 4.1 - Varieties plaster quality

Depending on the degree of grinding, stucco has three groups (table 4.2).

Table 4.2 - Groups of gypsum according to the degree of grinding

Depending on the setting time, stucco has three groups (table 4.3).

Table 4.3 - Groups of building gypsum depending on the setting time

Depending on the ultimate strength, gypsum has the following grades (table 4.4).

Table 4.4 - Grades of gypsum depending on the ultimate strength of the sample in compression and bending

Gypsum grade	Ultimate strength in MPa, not less	Gypsum grade		Gypsum grade	Strength limit in MPa, not less
when compressed	bending	when compressed	bending	when compressed	bending
G-2		1,2	G-6		5,0	G-16		6,0
G-3		1,8	G-7		3,5	G-19		6,5
G-4		2,0	G-10		4,5	G-22		7,0
G-5		2,5	G-13		5,5	G-25		8,0

Setting and hardening of stucco. The setting and hardening of stucco consists in the fact that when mixed with water, gypsum forms a plastic dough, which then turns into a solid stone-like body with a certain strength. The main reaction of the process is as follows:

CaSO4 * 0.5H2O + l, 5H2O = CaSO4 * 2H2O.

At the same time, hy-

sa and their intergrowth. The gypsum hardening process can be accelerated by drying at temperatures below 65 degrees.

The beginning of the setting of the gypsum should occur no earlier than 6 minutes. and no later than 30 minutes after the start of mixing with water. The setting and hardening times can be adjusted by introducing NaCl, KCl, NaNO and other substances that change the solubility CaSO4 * 0.5H2O in water .

Molding plaster ... This gypsum differs from construction

gypsum with a finer grinding, greater strength. Get it from

Gypsum stone containing at least 96% CaSO4 * 2H2O (i.e. impurities no more than 4%) in digesters at a certain cycle time and a given temperature . Its quality is higher than stucco. It consists, like stucco, of β-modifications CaSO4 * 0.5H2O ( β-hemihydrate) and is characterized by the following data:

The fineness of grinding is characterized by a residue on sieve No. 02 of not more than 2.5%;

The beginning of setting - not earlier than 5 minutes;

End of setting - no later than 25 minutes;

Ultimate tensile strength after 1 day not less than 1.4 MPa, and after 7 days - not less than 2.5 MPa (differs from stucco plaster in a smaller grinding thickness, increased strength and does not contain impurities).

Molding gypsum is used for the manufacture of molds, models and products in building ceramics, engineering and other industries. Articles made of porcelain and ceramics are cast in molds from molding plaster. The plaster mold must be strong enough and at the same time porous to suck water out of the slip without being destroyed.

High strength plaster are obtained by heat treatment of high-grade gypsum stone in sealed apparatus under a pressure of 0.2 ... 0.3 MPa at 124 0C within 5 hours.

It consists of α-modifications of CaSO4* 0.5H2O. Its strength reaches 15-40 MPa. High-strength gypsum is produced in small quantities and is used in the metallurgical industry for the manufacture of molds.

Anhydrite cement consists predominantly of anhydrite CaSO4 ("dead-burned"). It is "revitalized" by the addition of catalysts that increase its solubility and create conditions for its hydration. Such catalysts are CaO - 3 ... 5% and other anhydrite cements are used for the preparation of masonry and plaster mortars, concretes, the production of heat-insulating materials, artificial marble and other decorative items.

Estrich plaster(high-calcined gypsum) is formed at a temperature of 800 ... 1000 0C, it consists of anhydrite CaSO4 and CaO (3, .. 5%), formed during the decomposition of CaSO4 ( CaSO4 → CaO + -SO3) and perform-

which plays the role of a hardening catalyst. This element sets slowly and hardens.

High-calcined gypsum is a type of anhydrite cements. It is used for masonry and plaster mortars, mosaic floors, etc. Products made from this gypsum, in comparison with stucco, are more frost-resistant, have increased water resistance and less tendency to plastic deformation.

The use of plaster

Plaster of paris - white, environmentally friendly, fast-setting and fast-hardening binder. It is used for the manufacture of building parts and products, for self-leveling floors, adhesive compositions, stucco decorations, mold making for casting artistic ceramics, as well as for plastering. Gypsum is not water-resistant and not suitable for external work but with the addition of cement, it becomes waterproof. Gypsum is widely used in medicine. Gypsum panels and partitions absorb sound well. Gypsum is fire resistant and keeps warm well. In addition to stucco, other gypsum binders are used (in limited volumes): molded gypsum, high-strength gypsum.

Water demand of gypsum binders

Water demand gypsum binder is determined by the amount of water (as a percentage of the weight of the binder) required to obtain a standard consistency of gypsum dough.

Theoretically, hydration of semi-aqueous gypsum requires 18,6% water from the mass of gypsum binder, In practice, to obtain a formable plastic mixture, stucco requires 50 ... 70% water and high-strength - 30...40%. Excess water evaporates, forming pores, therefore gypsum products have a high porosity.

Before you start studying this article, I want to make a short introduction ... The topic of gypsum arose for me not by chance. I was going to do. In this regard, this is my first experience. The first thing I start to do in such cases is to study the material, i.e. I tried to find out everything about stucco.

Initially, the topic seemed to me simple, but it turned out not to be so, and therefore 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, impairing the properties of gypsum as a binder. And in nature, gypsum comes with impurities. Impurities are removed, but partially they remain, so you need to understand that when buying gypsum from different manufacturers, you buy different materials. If you add modifying additives on your own and bought gypsum from a manufacturer with whom you have not worked before, then it is better to do a test batch and apply a test layer.

Gypsum can be β-modification and α-modification. They differ only in the method of preparation (dehydration). β-modifications are made by heating the dihydrate gypsum in open furnaces and the water comes out with steam forming the smallest pores, which deteriorates the strength, because at any fineness of grinding, porous particles are obtained. The α-modification is done in autoclaves under pressure and the water comes out in a drip method, which makes the obtained 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 given to natural granular gypsum, which has a finer structural grain. In some places they write that any gypsum is alabaster. This is not true. Alabaster is granular gypsum, but not every granular gypsum is alabaster. In nature, it differs from simple granular gypsum in appearance and is similar to marble. Alabaster is fine-grained by nature, therefore it is possible to obtain finer grain when grinding than that of simple granular gypsum. A powder with a finer grain has a larger particle surface area, which means it reacts faster with water and hardens faster. Building Alabaster is a semi-aquatic gypsum obtained from natural alabaster.

There is one more important point. Gypsum β-modification, which is only sold in ready-made mixtures, and so consists of porous particles, but to prepare a working solution of the desired fluidity, you have to add 2 times more water than is needed for chemical reaction... Excess water is released by evaporation creating additional pores and further reduces strength. Therefore, if strength is important to you, reduce water and use additives to increase flow and use finely ground gypsum.

Construction gypsum are binders obtained from gypsum stone or chemical waste.

When gypsum stone is fired, chemically bound water is separated and, depending on the temperature, various forms of gypsum are formed. At 100 degrees Celsius, hemihydrated gypsum begins to form. When mixed in water, calcium sulfate dihydrate is again formed. This closed cycle was discovered about 20 thousand years ago. People built hearths from gypsum stone and probably noticed how the scattered burnt gypsum turns into stone again in the rain. In Sumerian and Babylonian cuneiforms, there are references to gypsum and its use.

The availability of raw materials, simplicity of technology and low energy consumption of production (4-5 times less than for the production of 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 / m 3) due to significant porosity (60-30%, respectively).

Expansion during hardening

Gypsum binder is one of the few binders that expand upon hardening. Increase in volume during setting and hardening by 0.5-1%. When dry, a decrease in volume by 0.05-0.1%. This feature of gypsum binders allows them to be used without aggregates, without fear of cracking from shrinkage.

Flammability

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 (hydrophobic coatings, impregnations, etc.), gypsum is a very promising binder from a technical and environmental point of view.

A kind of plaster

Β-modification gypsum

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

Α-modification gypsum

Gypsum α-modification is obtained by low-temperature (95-130 ° C) heat treatment in hermetically sealed furnaces. High-strength gypsum is made of it.

Alabaster

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

Alabaster - β-modification gypsum, a powdery binder 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 finer grinding, a molding plaster is obtained. For medical plaster, raw materials of high purity are used.

Anhydrite

Anhydrite is a natural anhydrous gypsum. Anhydrite binder sets slowly and slowly hardens, consists of anhydrous calcium sulfate CaSO 4 and hardening activators.

Estrich plaster

High-fired estrich gypsum is obtained by firing natural CaSO 4 gypsum stone. 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 activator of the hardening of anhydrite. The final product of hardening of such a binder is gypsum dihydrate, which determines the performance properties of the material.

The technological properties of estrich gypsum differ significantly from the properties of ordinary gypsum. Setting time for estrich plaster: start no earlier than 2 hours, end - 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 denser and more 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 humid conditions - 10-20 MPa. According to this indicator, the brand of estrich gypsum is established: 100, 150 or 200 (kgf / cm 2).

Estrich gypsum was used in the late 19th and early 20th centuries. for masonry and plastering mortars (including for the production of artificial marble), installation of seamless floors, bases for clean floors, etc. Currently, this binder is used to a limited extent.

Properties of stucco

Grinding degree

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

Compressive and flexural strength

The grade 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, the 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 ultimate strength in compression 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 ultimate compressive strength, the following brands of gypsum binders are distinguished:

Binder grade	Minimum tensile strength of sample beams with dimensions of 40x40x160 mm at the age of 2 hours, MPa (kgf / cm 2), not less
	when compressed	bending
G-2	2(20)	1,2(12)
G-3	3(30)	1,8(18)
G-4	4(40)	2,0(20)
G-5	5(50)	2,5(25)
G-6	6(60)	3,0(30)
G-7	7(70)	3,5(35)
G-10	10(100)	4,5(45)
G-13	13(130)	5,5(55)
G-16	16(160)	6,0(60)
G-19	19(190)	6,5(65)
G-22	22(220)	7,0(70)
G-25	25(250)	8,0(80)

When moistened, the 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 spread of the gypsum dough flowing out of the cylinder when it is raised to a height of at least 100 mm. The spreading diameter should be equal to (180 ± 5) mm. The amount of water is the main criterion for determining the properties of a gypsum binder: setting time, ultimate 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 standard consistency gypsum mixture to the mass of gypsum binder in grams.

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

When the gypsum binder is mixed with water for the course of the chemical reaction of hydration of CaSO 4 hemihydrate, 18.6% of water is theoretically consumed, 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 volume of the hardened product. That is, the less water is used when mixing the gypsum dough and less value normal density, while achieving good workability of the dough, 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, the fineness of the grind, the shape and size of the crystals of the hemihydrate.

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 of the properties.

These additives 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 the 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 distortion of the results of determining the strength during standard tests.

In practice, the gypsum binder is sometimes moistened with steam specifically to reduce water demand, to somewhat 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 gypsum grains and a change in their wettability with 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 due to the premature hydration of gypsum, its activity is significantly reduced.

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 subject to intense corrosion. Gypsum is moistened due to its good hygroscopicity (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 shavings and sawdust.

Gypsum as a binder

Gypsum binders are materials based on semi-aqueous gypsum or anhydrite. Refers to airy binders.

Depending on the method of obtaining, gypsum binders (HS) substances are divided into three main groups:

• I - binders obtained by heat treatment of gypsum raw materials: low-calcined (roasting and cooking) and high-calcined: 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-fired): 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 (NGV). Group III binders belong, with some exceptions, to waterproof gypsum binders (HBV).

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

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

Low firing group

Low-fired (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:

• plaster of paris, 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-calcined gypsum);
• anhydrite cement;
• finishing cement.

Advantage of gypsum binder:

• 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 interior construction and finishing works;
• insufficient heat resistance;

Gripping plaster

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, on the temperature of the water, on the dispersion of the gypsum. With a low water content, the mixture is poorly poured, hardens quickly, emits 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 the help of additives:

• joiner's 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 semi-aqueous gypsum dissolves in water almost 4 times better than dihydrate (the solubility is 8 and 2 g / l, respectively, in terms of CaSO 4). When mixed with water, semi-aqueous gypsum dissolves to form a saturated solution and immediately hydrates, forming a dihydrate, in relation to which the solution is supersaturated. Crystals of gypsum dihydrate precipitate, and semi-aqueous gypsum begins to dissolve again, etc.

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

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

When re-moistening, the process takes place in reverse order, and the gypsum loses some of its strength. The reason for the presence of free water in the hardened gypsum is due to the fact that for the hydration of gypsum about 20% of its mass is needed, and for the formation of a plastic gypsum dough, 50-60% of water is needed. 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 a 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 a formable 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 saturated steam (in an autoclave at a pressure of 0.3-0.4 MPa) or in salt solutions (CaCl 2 . MgCl 2, etc.). Under these conditions, another crystalline modification of semi-aqueous gypsum is formed - α-gypsum, with a water requirement 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 conventional β-modification gypsum. Due to the difficulties of production, high-strength gypsum has not found widespread use in construction.

Plaster of paris production

Raw materials for stucco

The raw material for gypsum is mainly natural gypsum, 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:

1st grade	not less	95 %	CaSO 4. 2H 2 O + impurities
ІІ grade	not less	90%	CaSO 4. 2H 2 O + impurities
ІІІ grade	not less	80%	CaSO 4. 2H 2 O + impurities
IV grade	not less	70%	CaSO 4. 2H 2 O + impurities

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

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

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

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

Stucco 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 rises, various dehydration products are formed.

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

The diagram shows the transition temperatures in the laboratory; in practice, in conditions of a large amount of material and fluctuations chemical composition, higher temperatures have to be used to speed up the firing.

Depending on the temperature and firing conditions, it is possible to obtain hemihydrate calcium sulfate (hemihydrate) α

And β -modifications, α

And β -soluble anhydrite, insoluble anhydrite.

Today it is generally accepted that education α

Or β-modifications of semi-aqueous 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 apparatus (rotary kilns or digesters) while removing water in the form of steam.

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

This is due to 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 heat absorption and has the form:

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

This reaction is often written in a somewhat conventional form:

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

Factory stucco, fired at temperatures higher than those theoretically required 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 air absorbs moisture and turns into a hemihydrate.

Consequently, the quality of a somewhat calcined gypsum increases during aging, while an admixture of unburnt gypsum with insufficient calcination 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 stucco causes a very rapid setting, since the first quickly dissolves and turns into gypsum dihydrate, and the second creates crystallization centers.

Industrial production of gypsum binder

Plaster of paris is obtained using digesters, rotary kilns and combined grinding and firing installations. The most common production of plaster of paris with the use of digesters.

Production stages:

• Crushing of gypsum stone (jaw and hammer crusher).
• Combined grinding with drying (shaft mill).
• Heat treatment at atmospheric pressure or in an autoclave (boiling in a gypsum boiler).
• Simmering (maturing in the bunker).
• Secondary grinding (ball mill).

The use of plaster

• It is widely used in industry and construction as a building material. It is rarely used in its pure form, mainly used as an additive, as a binder. The main area of ​​application is the device of partitions.
• In the repair they are used as the main finishing or leveling material. For leveling, prefabricated panels, gypsum stones, plasterboard sheets are used.
• Acoustic boards are made of gypsum.
• In various versions, it is used for fire retardant coatings of metal structures.
• Small in volume, but an important area of ​​using plaster: decorative architectural details (stucco molding) and sculpture.
• Fired gypsum is used to make molds (for example, for ceramics) for casts and casts (bas-reliefs, cornices, etc.). Strong molds are made from it for filling figures.
• In dentistry, they are used to make dental impressions.
• In medicine for fixation in fractures (plaster cast).

Plaster history

Gypsum is one of the oldest mineral binders. In Asia Minor, gypsum was used for decorative purposes for 9 thousand years BC. During archaeological excavations in Israel, floors covered with plaster were found 16 thousand years BC. Gypsum was also known in ancient Egypt, it was used in the construction of the pyramids. Knowledge of the production of plaster of paris from Egypt spread to the island of Crete, where in the palace of King Knossos, many of the outer walls were erected of plaster stone. The joints in the masonry were filled with plaster mortar. Further information about gypsum came to Rome through Greece. From Rome, information about gypsum spread to central and northern Europe. Gypsum was used especially skillfully in France. After the displacement of the Romans 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 plaster began to increase again. Under the influence of monasteries, a technology spread, according to which the voids inside half-timbered buildings were filled with a mixture of plaster of Paris with hay or horsehair. In the early Middle Ages in Germany, especially in Thuringia, the use of gypsum was known for floor screeds, masonry mortars, decorative items and monuments. In Saxe-Anhalt, there are remains of plaster floors from the 11th century.

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 the binders and fillers consisted of identical materials. As fillers used gypsum stone, crushed to round grains, not pointed and not lamellar. After the solution has hardened, 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 water to binder ratio is less than 0.4. The solution contains few air pores, its density is approximately 2.0 g / cm3. Later gypsum solutions were produced with a much higher water demand, therefore their density and strength are much lower.