Gypsum as a building and finishing material. Determination of the main indicators of the quality of stucco
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. The impurities are removed, but they partially remain, so you need to understand that when buying gypsum from different manufacturers, you buy different materials. If you add modifying additives yourself 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. It differs in nature from simple granular gypsum in appearance and is similar to marble. Alabaster is fine-grained in 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 a 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 a finer grinding, molding gypsum or, when using raw materials of increased purity, medical gypsum is obtained.
Gypsum α-modification
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 slowly sets and hardens slowly, 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 hardening product of such a binder is gypsum dihydrate, which determines the performance properties of the material.
The technological properties of estrich gypsum differ significantly from those 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 - 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 consistency (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 is required from the mass of building or molding gypsum and 35-45% of water from the mass of high-strength gypsum.
When the gypsum binder is mixed with water for the course of the chemical reaction of hydration of CaSO 4 hemihydrate, theoretically 18.6% of water is 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 the lower the value of the normal density when 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, 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 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 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 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 binding material
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 (calcining 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 intergrowth 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 the already formed crystals.
When re-wetting, the process proceeds in the 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. After hardening of such a dough, 30-40% of free water will remain in it, which is about half of 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, which has a water demand of 35-40%. Gypsum α
Modifications are called high-strength gypsum, since, 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 wastes 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.
Plaster 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, when there is a large amount of material and fluctuating 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 former quickly dissolves and turns into gypsum dihydrate, and the latter 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 use of plaster: decorative architectural details (stucco molding) and sculpture.
- Fired gypsum is used to make molds (for example, for ceramics) for castings 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 built 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, the 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 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.
Gypsum properties
Gypsum(hydrous calcium sulfate) is the most abundant mineral in the sulfate group. Its name comes from the Greek word gypsos. Gypsum is scratched with a fingernail and can be 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 cabochon cut and polished for a cat-eye effect.
Soft selenite, which is colorless and transparent, is also sometimes cut. Popular with collectors are the beautiful desert roses, twin dovetail crystals and star shapes.
The use of plaster
Gypsum is used in the production of plaster, fertilizer, Portland cement, paper, paints and pencils. It is the most common evaporite - the sediment that remains 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 the hydration of the mineral anhydrite.
Gypsum is accompanied by calcite, sulfur, quartz, dolomite, halite and clay. Sometimes gypsum is deposited as a result of the evaporation of salt water or forms soft translucent crystals in the place of dried up lakes. It also occurs as crystals in clay, as a shell of a salt dome, and in volcanic areas. 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 deteriorate. As a rule, alabaster is translucent and colored white, pinkish or brownish. The main gypsum deposits and alabaster are found in Italy and England. Pink alabaster is mined in Wales.
Origin of gypsum
There are alabaster deposits in Spain, Iran and Pakistan. "Alabaster", from which in Ancient Egypt and Ancient Rome vases, tombstones, etc. were allegedly made, is actually marble (calcium carbonate). There are rich deposits of gypsum in the USA (the states of Arizona, California, Utah, Colorado, Oklahoma, New Mexico, Ohio, Michigan, Virginia and New York), Canada and France.
Gypsum- a natural mineral from the sulfate class. Of all the natural sulphates in the construction industry, it is the most important. In nature, it is found in the form of a dihydrate - calcium sulfate dihydrate CaSO 4. 2H 2 O and in anhydrous state - anhydrite CaSO 4.
Basically, gypsum is used mainly as a raw material for the production of low- and high-calcined gypsum binders and as an additive introduced during the grinding of Portland cement clinker and its varieties in order to regulate the setting time.
Another area of using natural gypsum is the manufacture of wall and partition products, which is due to its low thermal conductivity: at 30 ° C 0.28-0.34 W / (m.K).
Natural dihydrate gypsum is a rock of sedimentary origin, composed mainly of large and small crystals of CaSO 4. 2H 2 O. Intergrowths of gypsum crystals can form plaster roses... Dense formations of gypsum are called plaster stone.
Structural differences
The appearance and structure of the rock are distinguished:
- crystal transparent plaster;
- poikilitic or sandy gypsum - crystals filled with sand.
Poikilit(English Poikilite) - a crystal or grain that contains numerous inclusions of other minerals that were captured during the growth of the individual.
- gypsum spar- a lamellar mineral with flat transparent crystals of a layered structure, individuals of rather large size, transparent (Mary's eye);
- selenite- parallel-fine-fiber gypsum, yellowish in color with a silky sheen
- granular gypsum;
- alabaster
Distinguish between crystalline, fibrous, granular and sandy gypsum varieties.
Under difference means a set of mineral individuals of the same mineral species, differing in morphological characteristics. For example, gypsum differences: "Mary's glass" - plate gypsum, selenite - fibrous gypsum.
Gypsum forms continuous marble-like masses, veinous clusters, as well as single crystals and druses. The appearance of its crystals is usually lamellar, columnar and acicular.
Physical properties of gypsum
The crystal lattice of gypsum dihydrate and anhydrite
In the crystal lattice of gypsum dihydrate, each calcium atom is surrounded by six complex groups consisting of four tetrahedra and two water molecules. The structure of the crystal lattice of this compound is layered. The layers are formed, on the one hand, by Ca 2 + ions and SO 4 -2 groups, and on the other, by water molecules. Each water molecule is associated with both Ca 2+ ions and the nearest sulfate tetrahedron. Inside the layer containing Ca 2 + and SO 4 -2 ions there are relatively strong (ionic) bonds, while towards the layers containing water molecules, the bond of the layers is much weaker. Therefore, during heat treatment, gypsum dihydrate easily loses water (dehydration process). In practice, this process can be carried out to varying degrees of completion and, depending on this, obtain gypsum binders of various modifications with different properties.
In the crystal lattice of anhydrite, sulfur ions are located in the centers of tetrahedral oxygen groups, and each calcium ion is surrounded by eight ions. For the most part, anhydrite forms solid masses, but there are cubic, short-columnar and other crystals.
Heating gypsum
Under the blowpipe, gypsum loses water, splits and melts into white enamel. Three effects are observed on the gypsum heating curves:
- at 80-90 ° C, a certain amount of H 2 0 is released;
- at 140 ° C gypsum turns into a hemihydrate;
- at a temperature of 140-220 ° C, there is a complete release of water;
- at a temperature of 400 ° C, gypsum is firmly fired.
Solubility of gypsum
Gypsum has an appreciable water solubility (about 2 g / l at 20 ° C). A remarkable feature of gypsum is that its solubility with increasing temperature reaches a maximum at 37-38 ° C, and then falls rather quickly.
The greatest decrease in solubility is established at temperatures above 107 ° C due to the formation of "hemihydrate" - CaSO 4. 0.5H 2 O. The solubility of gypsum is increased in the presence of certain electrolytes (eg NaCl, (NH 4) 2 SO 4 and mineral acids).
Gypsum crystallizes from solution in the form of characteristic needle-like crystals, white or colored with impurities.
Gypsum from the Greek - plaster, easily determined by the following properties:
- low hardness;
- abundant sublimation of water in a closed tube;
- in the flame of an alcohol lamp turns white (becomes cloudy) and crumbles into powder, melts into white enamel, which gives an alkaline reaction;
- relatively poorly soluble in water and acids.
Dissolution of anhydrite is a direct interaction of water and calcium sulfate, saturation occurs when the energy of the hydrated ion becomes equal to the energy of the ion in the lattice. Usually, such dissolution is accompanied by a slight heat release (not always and not for all salts). The main influencing factor is temperature.
The process of dissolving salts also depends on the properties of the solvent (water), its mineralization, composition and pH-environment. So, the solubility of gypsum increases with an increase in the content of sodium chloride and magnesium salts in water. In distilled water, the solubility of gypsum is 2 g / l, and in highly concentrated solutions of NaCl (100 g / l) or MgCl (200 g / l), the solubility of gypsum increases to 6.5 and 10 g / l, respectively.
Gypsum dissolves well in alkalis and hydrochloric acid. With an increase in the concentration of alkali solution from 0.1 N. up to 1 n. the solubility of gypsum increases sharply. Thus, depending on the mineralization and composition of the solvent, the dissolution rate of gypsum can vary within wide limits, which must be taken into account when leaching it from the rock.
CaSO 4 + NaCl = NaSO 4 + CaCl 2
CaSO 4 + MgCl = MgSO 4 + CaCl 2
A kind of plaster
Selenite
Selenite is a fibrous variety of gypsum, a translucent mineral, stronger than alabaster. Soft, Mohs hardness 2 (scratches easily with a fingernail). As inclusions, it can contain clay, sand, rarely - hematite, sulfur, organic impurities.
Has a silky sheen. After polishing, thanks to the parallel fibers, it has a beautiful iridescent optical effect, similar to that of a cat's eye.
The color scheme is presented in pink, blue, yellow and reddish-pearl shades. You can also find crystal white selenite.
It is used as an ornamental stone for the manufacture of jewelry, figurines, carved art and household items. It is easy to sand with sandpaper and polishes well. Products made of selenite are easily rubbed and lose polish due to their low hardness and, after use, require re-processing.
Alabaster
The name "alabastrites" comes from the name of the city of Alabastron in Egypt, where the stone was mined. Alabaster was highly prized and used to make small vessels for perfumery and vases for ointments. Cut into thin sheets, alabaster is quite transparent so it was used for "glazing" windows.
Today, alabaster is the main raw material for the production of gypsum - a powdered binder obtained by heat treatment of natural dihydrate CaSO 4 gypsum. 2H 2 O at temperatures from 100 ° C and above.
Let me remind you that alabaster- the purest fine-grained gypsum, resembling marble in appearance, white or light-colored.
Anhydrite
Anhydrite (from ancient Greek. "Deprived of water") - anhydrous calcium sulfate. Anhydrite can be white, bluish, grayish, less often reddish.
When water is added, it increases in volume by about 30% and gradually turns into gypsum dihydrate.
Anhydrite deposits form in sedimentary strata mainly as a result of dehydration of gypsum deposits.
Anhydrite is sometimes used as a cheap decorative and ornamental stone, occupying an intermediate position in hardness between jasper, jade and agate, on the one hand, and soft selenite and calcite, on the other.
Today it is used for the production of non-fired and high-fired gypsum binders, as well as an additive for the production of cement.
The class of sulfates, CaSO 4 .2H 2 O. In its pure form contains 32.56% CaO, 46.51% SO 3 and 20.93% H 2 O. Mechanical impurities mainly in the form of organic and clay substances, sulfides, etc. Crystallizes in monoclinic. The crystal structure is based on double layers of (SO 4) 2- anionic groups linked by Ca 2+ cations. Crystals are tabular or prismatic, form twins, the so-called dovetail. very perfect. Aggregates: granular, leafy, powdery, nodules, fibrous veins, radially needle-like. Pure gypsum is colorless and transparent, in the presence of impurities it has a gray, yellowish, pinkish, brown to black color. Glass luster. 1.5-2. 2300 kg / m 3. We will noticeably dissolve (2.05 g / l at 20 ° C). Mostly chemogenic in origin. It precipitates at t 63.5 ° С, and in solutions saturated with NaCl - at a temperature of 30 ° С. With a significant increase in salinity in drying out sea lagoons and salt lakes, instead of gypsum, anhydrous calcium sulfate begins to fall out - in a similar way, anhydrite occurs when gypsum is dehydrated. Also known is hydrothermal gypsum formed in low-temperature sulfide deposits. Varieties: - translucent fibrous aggregates, casting a beautiful silky sheen in reflected light; gypsum spar - lamellar gypsum in the form of transparent crystals of a layered structure, etc.
The use of plaster
Gypsum is used raw and fired. 50-52% of gypsum mined in gypsum is used for the production of gypsum binders for various purposes (GOST 195-79), obtained by burning natural gypsum, 44% of gypsum - in the production of Portland cement, where gypsum is used as an additive (3-5%) to regulate the timing setting cement, as well as for the production of special cements: gypsum-alumina expanding cement, stress cement, etc. 2.5% of gypsum is consumed by agriculture in the production of nitrogen fertilizers (ammonium sulfate) and for gypsum saline soils; in non-ferrous metallurgy, gypsum is used as a flux, mainly in smelting; in papermaking - as a filler, mainly in the highest grades of writing papers. In some countries (and others), gypsum is used for the production of sulfuric acid and cement. The ability of gypsum to be easily processed, to perceive polish well and its usually high decorative properties allow it to be used as an imitator in the production of facing slabs for interior decoration of buildings and as a material for various crafts.
In the southern regions of the USSR, in the national economy, clay gypsum is used with a CaSO 4 .2H 2 O content of 40 to 90%. Loose rock, consisting of gypsum, and is called earthy gypsum, and in Transcaucasia and Central Asia - "drywall" or "ganch". These rocks in their raw form are used for gypsum soils, in burnt ones - for plastering, as an astringent.
Gypsum deposit
In the USSR, the largest deposits are located in the Tula, Kuibyshev, Perm regions of the RSFSR, in the Caucasus and Central Asia. At 150 gypsum deposits and 22 deposits of gypsum, drywall and ganch, reserves of 4.2 billion tons have been explored in industrial categories (1981). There are 11 deposits with gypsum reserves exceeding 50 million tons (including Novomoskovskoye - 857.4 million tons).
Gypsum is mined by quarries (Shedoksky, Saureshsky combines, etc.) and mines (Novomoskovsky, Artyomovsky, Kamskoye Ustye, etc.). In the USSR, 42 deposits of gypsum and anhydrite and 6 deposits of gypsum-bearing rocks are exploited with an annual production of about 14 million tons (1981), of which 60.2% are in the territory
Construction guide "Megastroyki.biz"
What is gypsum made from?
At construction sites, it is often necessary to have a fast-setting binder in the composition of cements and building mixtures so that the solutions do not have time to "float". For the preparation of such a bindermainly natural gypsum and gypsum-containing rocks are used. But now the waste of many industrial production also contains calcium sulfate - the main component of gypsum. There are already about 50 types of such waste, therefore it is advisable to use many of them to obtain gypsum.
Natural gypsum (CaSO 4 2H 2 0) is a crystalline sedimentary rock. If the formations of natural gypsum are large and dense, they are called gypsum stone. Coarse-layered gypsum stone is called gypsum spar, fine-fiber - selenite (moonstone), granular white (and gypsum due to impurities can have different shades) - alabaster, which is translated from Greek as "white".
Gypsum-containing rocks include anhydrite, gypsum-containing clays and loesses.
Anhydrite Is calcium sulphate that does not contain bound water. Usually it underlies gypsum from below, it is represented by small crystals.
Drywall- bog clay containing calcium carbonate, calcium sulfate and clay substance itself. In principle, all of its constituent parts are binding materials. And since drywall contains 15-90% CaSO 4 , then it is advisable to use it to obtain gypsum.
Ganch, arzyk- gypsum-bearing loess rocks. Along with carbonates and sulfates, they contain loess, that is, a substance consisting of particles much smaller than clay particles. These rocks are found in Central Asia in very large deposits.
Waste from the chemical and food industries, waste from other industries are rich in gypsum. Why are they not being used to their full potential? Some of them must be freed from harmful impurities by washing, drying, neutralizing, which is often not cost-effective. The other part requires the removal of excess moisture or high processing costs. But in any case, this direction is promising, since the annual amount of such waste is estimated at hundreds of millions of tons, and the earth's interior is not unlimited.
For the production of binders from recyclable materials, waste from the chemical industry is most often used:
- - borogypsum remaining after the production of boric acid and borax;
- - phosphogypsum, remaining after receiving phosphorus fertilizers (after the production of 1 ton of fertilizers, 4.5 tons of phosphogypsum remain);
- - fluorogypsum, derived from the production of hydrofluoric acid and its salts;
- - titanium gypsum obtained from the decomposition of titanium-containing ores.
More about lime and gypsum and products made from them:
