Chalk Wiki. Description of the physical property of chalk - strength

The main component of chalk is calcium carbonate (CaCO3) - one of the forms of limestone. Limestone deposits are formed from coccoliths - a shell of tiny flagellar plates created from the decayed skeletons of plankton. For the manufacture of pastel crayons, calcium sulfate (CaSO4) is taken as the basis, which is extracted from the evaporite mineral, which is formed from the salts of ocean water.

Chalk and dehydrated gypsum have similar properties. Pastel crayons also contain clay and oils, which bind the ingredients and make the color last. Thanks to this composition, the crayons have a velvety structure, glide smoothly over the surface and do not crumble. Although in production Special attention is given to the purification of impurities, yet some of them remain. The main ones are: silicon, aluminum, iron, phosphorus and sulfur. In smaller quantities, manganese, copper, titanium, sodium oxide, potassium oxide, fluorine, arsenic and strontium are present.

Chalk production process

A limestone quarry is being developed for the production of chalk; it is usually open source. The limestone is then crushed and ground together with water in a ball crusher (a rotating steel drum inside which water is sprayed). On the this stage foreign impurities are washed out of the limestone, and a pure powder remains.

The extraction of gypsum is exactly the same as that of limestone. The difference is that the gypsum must be dehydrated to produce calcium sulfate. This takes place in a special chamber where gypsum is heated to a temperature of 116-121 degrees Celsius. When boiling, 12 to 15 percent of its mass is evaporated. Next, the gypsum is heated to 204 degrees and in this form it is taken out of the chamber. Then the mass is placed in a vibrating screen, where large particles are screened out. After that, the powder is washed again, dried, packed in bags and sent to the chalk manufacturer.

In the crayon factory, the chalk or calcium sulfate is ground again. For the production of school crayons, water is added to the mass and brought to the consistency of clay. Then the mass is stamped and cut into bars about 60 cm long, which are placed in a special mold, five pieces each. This form is sent to the oven, where the mass is kept for four days at a temperature of 85 degrees Celsius. Then the hardened crayons are cut into bars 80 mm long. For the manufacture of colored crayons, the pigments are mixed with the base in a dry form, and only after that water is added and the production cycle described above begins.

Chalk is natural material organic origin, a rock from the category of sedimentary. If you figure out what chalk consists of, then the basis of the mineral is deposits formed many millions of years ago from fragments of animal skeletons, mollusk shells, and calcareous growths of algae. About 50 percent of the mass is made up of fine particles of crystalline calcite, the origin of which has not been established.

In chemical terms, it is officially considered that the formula of chalk is calcium carbonate CaCO3. However, in reality, the composition of chalk looks a little different:

about half of the mineral is calcium oxide CaO (47-55%);
about 43% is carbon dioxide CO2;
2% by weight is magnesium oxide MgO;
up to 4% in the composition of chalk is aluminum oxide Al2O3.

In addition to the above, chalk necessarily contains quartz inclusions in very small quantities. Some deposits give out a mineral with a reddish tint, which means that iron oxides are present in its composition.

Areas of use of chalk

It is impractical to use chalk as a masonry material due to its extremely low hardness. However, the premises carved into the massifs of Cretaceous deposits have been preserved for centuries in their original form and are quite suitable for life.

How is chalk mined? Most often, these are open pit developments. The resulting lumps are crushed and placed in water. When stirred, particles of calcium float. Subsequently, they are dried and used for various purposes. Unprocessed stones are sent for firing to obtain lime.

Construction is called chalk, which was previously used for whitewashing. internal surfaces premises. Now this sphere of its application has come to naught, as many other, more advanced finishing materials. Where is chalk used?

The mineral is included cement mixtures when you need to give them softness.
In the production of glass, paints and varnishes, rubber, plastics, rubber, when creating composite materials based on PVC.
Chalk is actively used in the carpet and linoleum industry. It helps to regulate the viscous properties of latex adhesives, give products strength and improve heat-saving properties.
Chalk has found application in the manufacture of animal feed and soil fertilizers.
Chalk powder is the basis for most cosmetics- lipstick, creams, powders.
The paper and cardboard industry cannot do without chalk.

Is chalk harmful? Absolutely safe and the best evidence is its use in the creation of toothpastes and powders. Moreover, the lack of calcium in the human body causes a desire to eat a piece of chalk. In this case, you should not use whitewash chalk or school crayons. Pharmacies have special drugs, such as calcium gluconate.

Physical and chemical indicators of chalk

The natural characteristics of chalk from different deposits can vary significantly. Moreover, the properties of the mineral can be different even at different horizons of the same quarry. This is due to certain conditions of its formation and the degree of humidity. So physicochemical characteristics the chalks shown in the following table are indicative.

No. p / p	Name of indicator	Unit measurements	Indicator
	Appearance		White powder
	Mass fraction of CaCO3 + MgCO3 in terms of CaCO3
	Mass fraction of iron and aluminum sesquioxides
	Mass fraction of substances insoluble in hydrochloric acid
	Mass fraction of water-soluble substances
	Mass fraction of iron oxide
	Chalk density, in pieces
	The same, bulk
	Humidity
	Modulus of elasticity in loose state
	The same, in dense form
	Temporary compression strength

Humidity has a great influence on the material. Does chalk dissolve in water or not? When powder is mixed with water, a suspension is obtained, but not a solution. But the presence of water significantly changes the physical properties of the material. In particular, strength indicators decrease, but plasticity appears. Often this complicates the process of extraction and processing. The material sticks to the excavator bucket, car body, conveyor belt. Excessive moisture makes it impossible to extract rock from the lower horizons.

Frost resistance in chalk is extremely low. After defrosting, it breaks up into small particles. However, this quality is useful for deoxidizing the soil, where chalk is used. Pieces of chalk rock up to 100 mm in size are introduced into the soil, which, when plowing and overwintering, are themselves destroyed, and their neutralizing effect lasts for a long time.

How not to confuse chalk with lime

What is the difference between chalk and lime? Although the materials have common roots, they are completely different both in composition and in properties. Chalk contains carbon dioxide, which is not present in lime. Both are environmentally friendly. But chalk is simply mined in quarries, and lime is obtained by firing natural limestone.

How to distinguish chalk from lime on your own? You can use primitive methods. Materials have a different reaction to the action of any acid. It is enough to drop vinegar or lemon juice on the test piece of material. Chalk will hiss, and the interaction of lime with acid will not be accompanied by any sounds.

One more way. You need to take a pinch of material, grind it and try to wash it off with a thin stream of water. Chalk will leave with water immediately and without a trace, and lime is washed off for a long time, leaving the impression of fat.

The mixing of quicklime with water is accompanied by a strong reaction with the appearance of foam. Chalk just gets wet. Lime whitewash does not leave marks if you run your hand over it, unlike chalk. In addition, lime has antiseptic properties, it is a good protection, in particular trees, from fungal infections and insects.

Chalk is a sedimentary rock which is of organic origin. The structure of the material is fine-grained, crumbly and soft, weakly cemented. Natural chalk has White color. It does not dissolve in water. By mineral composition looks like limestone.

Chalk includes:

skeletal fragments;
foraminifera shells;
seaweed fragments;
fine calcite;
insoluble minerals.

A careful analysis of the Cretaceous sediments reveals impurities in the form of very small grains of quartz. Cretaceous deposits may contain fossils dating back to the Cretaceous period: ammonites and belemnites. Natural chalk is not characterized by layering and recrystallization. Numerous burrows of soil-eating animals are included in the structure of the material.

Calcite, which dominates in the complex composition of chalk, can be of both autogenic and biogenic origin. Up to 75% of the rock is composed of organic remains. In their bulk, they are represented by skeletons and shells of plankton and foraminifers. The skeletal remains in the composition of chalk are very small size- only 5-10 microns. This substance may also contain bryozoan skeletons, mollusk shells, remains sea urchins, corals, flint sponges.

Up to 10% of the volume of chalk are impurities of the non-carbonate type:

kaolinite;
glauconite;
feldspars;
quartz;
pyrite;
opal;
chalcedony.

Flint and phosphorite are much less common.

Cretaceous strata often cross large cracks filled with chalk flour. The network of such cracks usually thickens closer to the surface. On the different levels horizontal layers of chalk differ in their mechanical properties and chemical composition.

According to the structural properties and physical characteristics, three types of chalk are distinguished:

white writing;
marly;
chalky limestone.

Chalk chemical properties

The chemical composition of chalk is determined great content calcium carbonate with inclusions of magnesium carbonate. The chalk may also contain a non-carbonate part, including metal oxides. It is generally accepted that chemical formula this substance corresponds well-known formula calcium carbonate (CaCO3). But the real composition of chalk is more complicated. This mineral contains about half of calcium oxide. Up to 43% of the chalk composition is carbon dioxide; he is in bound state. Approximately 2% of the total mass of the substance is magnesium oxide. Mandatory, although not too significant, inclusions of quartz. Chalk with a relatively high silicon content has a higher density. Chalk contains a small amount of aluminum oxide, and iron oxides quite often color chalk layers red.

The carbonate part of the chalk is soluble in hydrochloric and acetic acids. The non-carbonate part includes quartz sand, clays, and metal oxides. Some of these components do not dissolve in acids. In small quantities, the chalk includes particles of magnesian calcite, as well as dolomite and siderite.

The molecular formula of chalk corresponds to several types of crystalline compounds that contain ions in the lattice sites.

Physical properties of chalk

Chalk is considered a semi-rock of a hard type. The strength of this mineral is determined by moisture. When exposed to chalk water strength characteristics the chalk is decreasing. Changes often occur already at 2% humidity. At 35% humidity, the compressive strength increases by about 2-3 times, the chalk becomes plastic. This physical property makes it difficult to process the substance. Chalk begins to actively stick to the working parts of the machines. The viscosity and plasticity of chalk often do not allow it to be extracted from the lower horizons.

The density of chalk reaches 2700 kg/cu. m; porosity - up to 50%. Humidity in vivo environment ranges from 19 to 33%. If the chalk is moistened, its strength is markedly reduced. At a moisture content of about 30%, chalk exhibits its plastic properties. Chalk found in nature is not resistant to frost. After multiple cycles of freezing and thawing, the chalk usually breaks into small pieces.

When analyzing the physical properties of chalk, special attention is paid to the behavior of the rock during grinding. AT technological process It is customary to set the rate of chalk blooming in a humid environment with controlled mechanical action. The modulus of elasticity of chalk for a loose state is 3000 MPa, for compacted - 10000 MPa. Temporary compressive strength: 1000-4500 MPa.

Calcium carbonate, being in a crushed form, has a high dispersion. The presence of chalk in the product reduces its abrasiveness. The physical properties of this substance help to increase the thermal resistance of products, their mechanical strength, resistance to weathering and exposure to reagents.

Previously, it was believed that the chemical and physical properties of chalk are the same for all deposits. However, practice has shown that this is not the case. The properties of Cretaceous deposits differ even within the same deposit. Therefore, when mining a mineral industrial way technological mapping is carried out. Chemical properties chalk and his physical characteristics studied in different areas of deposits. Places of accumulation of high-quality chalk rocks are put on the maps.

Chalk deposits

The richest deposits of chalk are located in Europe. It can be found from Western Kazakhstan to the British Isles. The thickness of the chalk layers reaches hundreds of meters. In the Kharkov region, deposits with a thickness of up to 600 m were discovered. A huge chalk belt stretches across Europe, capturing the northern part of France, the south of England, Poland, Ukraine, and Russia. Part of the deposits is displaced to Asia; chalk reserves are found in the Libyan desert and in Syria.

In the United States, chalk deposits have been noted only in the southern and central states. However, the chalk there is of poor quality; for this reason it has to be imported into the US from Denmark, the UK and France.

Chalk reserves are distributed very unevenly. Up to half of the quality chalk with a good content of calcium carbonate is concentrated in Russian Federation. In absolute terms, chalk reserves in Russia are estimated at 3,300 million tons. Unlimited forecast chalk deposits are located in the Belgorod region. Very high quality chalk with a low content of non-carbonate impurities is mined in the Voronezh region.

The practical value of chalk

The practical use of chalk is determined by its chemical and physical properties. In industry, it is used for the production of cement, lime, soda, glass and school crayons. Chalk also serves as a filler for plastics, paper, rubber, paints and varnishes. It is included in the formulation of toothpastes and powders.

Finds chalk application in agriculture: it is used for liming the soil and as a top dressing for animals, to protect tree trunks from sunburn.

Chalk is necessary component in the production of coated paper. It is widely used in the printing industry for the manufacture of illustrated publications. Chalk is successfully used as the main filler and pigment in the manufacture of cardboard.

Chalk also finds application in construction. Cheap ground chalk is used for whitewashing, priming, wall painting.

YULIA KARPENKO

An object

Thing research - chalk.

Target

Hypothesis

1. Study the literature on chalk.

2. Study information about the use of chalk in everyday life and at work.

3. Conduct a survey.

4. Process the results of the survey.

5. Take stock.

Download:

Preview:

Research

" Chalk "

Work completed

4th grade student

MOU "Secondary school r.p. Ozinki"

Karpenko Julia

Head: Panitskaya T.I.

Ozinki 2015

Introduction ……………………………………………………………………...

Relevance of the work, problem, research methods………………….

Purpose, objectives……………………………………………………………………..

Origin, properties and application of chalk .............................................. .

Questionnaire for teachers and students…………………………………………...

Analysis of questionnaires……………………………………………………………………

Conclusion…………………………………………………………………...

List of sources…………………………………………………………..

Introduction

Who among us does not know chalk? Whose pockets and fingers were not soiled by a piece of light stone the color of snow in childhood? Who does not know happiness artistic creativity"Cretaceous" period? Who, as a teenager, did not investigate the properties of chalk in "bubble" experiments, did not examine a chalk smear under a microscope?

What secrets does chalk hold? Sometimes a piece of chalk suddenly began to scratch the board, and Tatyana Ivanovna told us that these were the remains of ancient shells. And here is a simple piece that interested me.

An object my research is minerals.

Thing research - chalk.

Target research: study chalk as a mineral, find out its origin, where it occurs in nature, where it is used in everyday life and in production.

Hypothesis my project is that chalk plays an important role in our lives.

For myself, I have defined the stages of research activity:

Study the literature on chalk.
To study information about the use of chalk in everyday life and at work.
Conduct a survey.
Process the survey results.
To sum up.

Tasks:

1. Collect reliable information about the origin, composition, properties and use of school chalk.

2. Conduct a sociological survey to identify the effect of chalk on the human body.

Modern lifestyle has a huge impact on human health. Increasingly, you can meet people suffering from beriberi and insufficient amounts of useful substances in the body. That is why chalk for food has recently become very popular.

I want chalk, what does that mean?

Many consider the need for chalk to be very strange phenomenon, abnormal for an ordinary human body.

In everyday life, the desire to gnaw chalk is usually associated with pregnant women, they really often have such taste preferences. This is due to the fact that during the formation and growth of the fetus female body double servings of vitamins and microelements are required.

Especially important is a sufficient amount of calcium, which contains lump chalk for food. He serves building material for most fabrics nerve cells, skeleton, cartilaginous tissues, hair, nails, internal organs and the skin of a small person.

However, calcium deficiency can accompany absolutely any person, it is usually characterized by the following symptoms:

Brittle and dull hair;
Caries, bone fragility (frequent fractures);
Brittle nails prone to delamination;
Dull skin tone;
Loss of skin elasticity;
Muscle cramps;

Such conditions characterize a pronounced violation of calcium metabolism, which natural chalk for food will help to cope with.

What is chalk made of

The composition of chalk for food is represented by a combination of inorganic salts (carbonates). The main element of such compounds is Ca (calcium). Many are sure that natural lump chalk is nothing more than CaCO3 carbonate known to everyone from school.

However, an in-depth study of this issue makes it possible to make sure that chalk for food includes other elements of the periodic table.:

Of course, calcium oxide occupies almost half of the component composition of the substance. The content of CaO in the mineral reaches 55%.
The second position in the list of components is occupied by carbon dioxide or CO2. Its share in the composition of chalk sometimes reaches 43%.
The oxide of a metal such as magnesium (MgO) occupies a very small volume, which usually does not exceed 2%.
The quartz component affects the density of food chalk in a direct proportion. The more SiO2 a substance contains, the higher its density. The mass fraction of quartz from the total volume sometimes reaches 6%.
Al2O3 is quite a bit inferior in volume content to quartz inclusions. The total volume of aluminum oxide in limestone corresponds to 4%.

Sometimes you can find natural chalk with a pinkish or even reddish tint. This feature of the breed is due to elevated content iron oxides. In ordinary white chalk, the content of Fe2O3 does not exceed the threshold of 0.5%.

Where is chalk mined

Those who wish to compensate for calcium deficiency should know what chalk is for food and where it comes from.

More than 100 deposits of natural chalk are located on the territory of the Russian Federation. Three zones stand out among them, in which the concentration of carbonate reserves reaches high levels.

The Volgograd region occupies a huge share in this figure, its quarries account for up to 26% of the country's reserves. Lump chalk from these deposits is of excellent quality, it contains a minimum amount of impurities. The number of foreign inclusions does not exceed 2%. However, the Volgograd chalk is quite water-saturated, the water content in dry chalk exceeds the specified standards by almost 10-15%. This fact makes it difficult to extract it.

The next position is occupied by the Belgorod region, up to 24% of the Cretaceous reserves are concentrated here. The tasty chalk mined in the area is 99% carbonate, which is an excellent testament to its high quality.

The Saratov region owns an 11% share in the total volume of the Cretaceous wealth of the Russian Federation.

The harm and benefits of chalk when eaten

Lumpy chalk for food is gaining more and more popularity, the harm and benefits of this substance are being discussed by experts to this day. It is not possible to unambiguously evaluate the sedimentary mineral, because it has both negative and positive influence on the human body.

The benefits of natural chalk for food are as follows:

Strengthens fragile bones and serves as a prophylaxis against caries;
Helps maintain the health and beauty of hair, nails, teeth and skin;
In small amounts, it helps to improve the functioning of the digestive system;
Provides joint mobility and muscle elasticity;
Promotes correct work of cardio-vascular system;
Participates in metabolic processes at the intercellular level;
Affects blood clotting and hemoglobin levels (indirectly).

Delicious chalk for food shows a negative effect only when it is overabundant in the body.

Excessive intake of calcium leads to the following side effects:

Increased blood clotting;
Decreased strength of bone tissue and tooth enamel;
The emergence of the risk of diseases of the endocrine system;
angina;
Decreased muscle tone;
The appearance of kidney stone disease;
Increased acidity of gastric juices, which contributes to the development of diseases gastrointestinal tract(gastritis and ulcer);
Gout;
Calcification (deposits of Ca salts in the tissues of internal organs).

Talking about negative consequences consumption of chalk for food, it can be concluded that they are caused by the quantity and quality of the consumed mineral.

Which chalk is best for eating

People with a lack of calcium in the body are often interested in where to buy chalk for food, which one is better. They are also studying the question of what is the most delicious chalk for food.

If you want to chew on lump chalk, school chalk for the board usually comes to mind. However, this product not only will not bring benefits, but is also capable of causing harm due to the presence of chemical additives in it. Such substances poison the body, the resulting toxins disrupt the functioning of internal organs and spoil health in general.

It has been proven that school crayons dry the airways, settling on the epithelium of the larynx. They also clog blood vessels, liming occurs, which leads to malfunctions. cardiovascular systems.

The hard particles that this type of chalk contains can scratch the enamel of the tooth, which will inevitably lead to caries. Similarly, they act on the oral mucosa, creating microscopic wounds in it that are invisible to the human eye. These cracks are a direct path to the appearance of inflammatory processes and the penetration of infection into the body.

Once in the stomach, school chalk causes a reaction similar to the extinguishing process. Such chalk is less tasty compared to its natural counterpart.