No construction site is complete without cement, no matter what kind of house is being built – brick or wooden. The only difference is the amount of mixture required. This building material has been successfully used for more than one century due to its high quality and excellent performance characteristics..
Features of production
As soon as people began to build houses, they had a need for such a composition that could hold together stones and other building materials. At first it was ordinary clay, but such dwellings were short-lived, and the building itself looked very unpresentable.
A little later, the builders discovered that the lime mortar also has some binding characteristics – this discovery was made by the ancient Greeks and Romans, and the latter noticed that if volcanic ash is added to the lime, the masonry becomes truly monolithic.
Lime was also used in Russia, mixing it with clay and water. At the beginning of the 19th century, a building mixture was experimentally created, which in its parameters is close to modern compositions.
So, cement is the definition of a material consisting of clay and limestone components. They are mixed with each other and subjected to prolonged heat treatment, after which the resulting mass is ground to a powdery state.
The mixture has a gray tint and, when interacting with water, forms a solution, which, when dried, becomes solid like a stone. Its exclusive feature is that the mass hardens only in high humidity conditions, moreover, this process can occur even under water. This, by the way, is the main difference between cement and many other binders..
For example, gypsum and air lime gain strength only in air. And in the composition of concrete, it hardens not so much because the water evaporates, but due to its reaction with the powder: at this moment heat is released and solid or crystalline substances are formed. That is why the process of stirring the mixture itself is often called a gate, and not at all dissolution..
A monolithic mass is formed due to the hydration of cement, concrete should not be dried in the scorching sun – it will simply “break”, it will crack and begin to crumble very quickly.
To avoid such unpleasant situations and save money, it is recommended to moisten the concrete solution until it dries completely..
Let’s dwell a little more on the features of production.
At the first stage, limestone is harvested, chalk, marl, as well as limestone tuffs are considered the best, but all kinds of gypsum and dolomite are not recommended – the strength of the cement based on them leaves much to be desired. Then the raw material is mixed with clay in a ratio of 3: 1 and the resulting mixture is sent to a rotary kiln heated to 1400-1500 degrees.
It is this temperature that ensures maximum sintering, at which the components of clay and limestone melt and diffuse among themselves.
After 3-4 hours, the composition leaves the furnace, at this moment it is a set of different-sized lumps – this is cement clinker, then it is crushed to particles less than 100 microns in size, at this stage 6% gypsum is added to the composition – this is necessary for cement did not absorb moisture from the air.
In addition to gypsum, other active components are introduced into the composition, which improve the performance of cement and allow it to be used in a wide variety of construction industries: all additives give cement certain parameters, for example, water-repellent or fast-hardening building materials, and in conjunction with ASG they produce concrete.
Properties and characteristics
Consider the main properties of cement.
Grinding fineness
This parameter determines the rate of hardening of the cement slurry and the strength of the resulting stone. The dependence here is the opposite – the smaller the grinding, the faster the solidification process and the stronger the resulting monolith. Accordingly, the prices for fine-grained cement are higher.
The fineness of grinding is determined using the indicator of the specific surface area and the composition of the grains, for this the material is sieved through an ultrafine sieve, the mesh size of which does not even reach 8 microns, while the bulk of the cement should be easily sieved.
On the other side, the smaller the grind, the more water is required to dilute the cement. That is why it is considered optimal to introduce into the structure both large and small particles, presented in the range from 40 to 80 microns..
To make a fast-hardening type of cement, up to 25% of particles must be added; ultrafine grinding – they significantly increase its strength and reduce the risk of corrosion.
Corrosion resistance
During operation, the cement stone is constantly affected by water and acid-base solutions, which leads to corrosion of reinforced concrete structures. To avoid this, special hydroactive materials are introduced into the composition, which significantly slow down, or even completely neutralize the activity of chemical salts, and also contribute to the waterproofing of concrete goods..
As a rule, additives based on polymers are used in this capacity, which significantly reduce the porosity of the substance and increase its resistance to unfavorable external factors..
It is thanks to the introduction of such components that cement becomes a truly durable and reliable material..
Frost resistance
To make high-quality concrete, it is necessary to use only high-quality cement, since during storage any composition absorbs moisture from the air in micro doses. In frosts, it freezes and increases in volume by 7-10%, and with repeated repetition, it causes serious destruction of concrete in the future.
In the context of constantly growing prices for building materials, the property of maintaining its performance characteristics under the influence of low temperatures acquires special value..
Modern cement compositions are designed in such a way as to repeatedly withstand the freeze and thaw cycle..
To achieve good frost resistance, sodium abietate, wood pitch and other components are introduced into pure cement., which determine the resistance of the composition to temperature extremes and increase the durability of the concrete as a whole.
Special air-entraining additives are also introduced into the structure, thanks to which the structure is enriched with air bubbles, it is they who effectively solve the problems of liquid expansion during solidification.
Water demand
The physical density of cement grains is several times higher than the density of water, therefore, with its excessive saturation, capillary forces retain only part of the moisture inside, and the rest is removed to the surface of the mixture. This causes the settling of particles, which, in turn, has an effect on reducing the strength of the entire concrete structure and contributes to its rapid destruction..
The ability of cement to water demand is determined by the amount of liquid required to prepare the working solution. It is measured as a percentage, for example, in Portland cement this figure is 22-28%. If the water demand is low, then a more durable and frost-resistant concrete is obtained from such a material., and if this indicator is overestimated, then the materials obtained from the composition will be distinguished by special porosity and will not last long.
Setting speed
Ideally, the mortar should set and neither quickly nor slowly. Moreover, additives, as well as the volume of liquid and air temperature, have an effect on time. The accepted standard indicates that hardening should begin 35-40 minutes after the preparation of the cement slurry..
For complete hardening, take about 10 hours.
Strength parameters
The cement strength index is calculated by the compression method of the sample, which is carried out for 28 days. When testing different brands of cements, the compressive strength is also different. Modern industry produces cements, for which this figure is 300, 600, as well as 700 and 1000.
Many consumers believe that it is strength that is the main characteristic of cement, which must be paid attention first of all. This is not so – all the parameters of the composition have the same meaning and equally affect the strength and durability of concrete structures..
Views
There is no official classification for cements. In the most general terms, several groups of compositions can be distinguished, the criteria of which are based on various parameters.
So, depending on the purpose of the composition, general construction and special compositions are distinguished. If the composition is taken as a basis, then you can designate options with min. additives and pure cement.
If you classify cement depending on the rate of hardening, then the following options are most common:
- quick-setting mixtures – those, the curing speed of which is less than 45 minutes;
- normally setting – harden within 45-120 minutes;
- slowly hardening – freeze more than 2 hours.
Stressed, refractory, self-expanding cements and many others are also distinguished..
Let’s dwell on the most popular types:
- White cement – This is a material that is made from low-iron clinker, which undergoes bleaching. It is a material with unique properties, it can be used not only for bonding surfaces, but also for making sculptural elements and columns, as well as for cladding the facade of a building. Due to its color, the use of white cement allows to improve the aesthetic component of the surface. With the addition of certain pigments, colored cement can be obtained..
- Waterproof Expanding Cement Is a substance with a high setting and hardening rate. It is produced in the process of simultaneous grinding of cement, gypsum and calcium hydroaluminate. Such cement begins to set after 4 minutes from the moment of mixing, finally dries up after 10 minutes.
This material is widely used for waterproofing and capping joints..
- Waterproof nonshrinking cement – the main composition of the components of such a cement is the same, however, the processing and production technology makes the hardening rate even greater – it takes no more than 5 minutes for the composition to completely dry. Such a composition is used to form a waterproofing shell of concrete goods..
- Fast hardening portland cement Is a cement that is finely ground, making it more intense than standard Portland cement. The material gains special strength in the initial stage of hardening – after 1–3 days. Most often it is used when carrying out repair work..
- Plasticized Portland cement – a composition, during the production process of which 0.25% mash on a sulfite-alcohol basis is introduced into the grinding of clinker, due to which the frost resistance of the mixture and its plasticization significantly increase.
- Hydrophobic cement – are made by joint grinding of Portland cement clinker and gypsum, as well as by introducing axilol or another hydrophobizing additive into the composition. The water absorption of such cement is low, therefore it can be stored for a long time even in conditions of high humidity and this will in no way worsen its physical and technical characteristics. Such a mixture does not form lumps, and concrete based on it does not crack over time and does not delaminate.
- Alumina cement – a binder hydraulic substance, which is made from raw materials rich in alumina, usually limestone or bauxite. The material hardens in low temperatures, so it is widely used for repair work in the winter season..
- Magnesia cement – obtained from raw materials rich in magnesium oxide. It has found application in the arrangement of magnesia floors..
- Portland cement – obtained by grinding Portland cement clinker with gypsum and special additives, the firing technology is built in such a way that a high content of calcium silicates is preserved in the resulting product.
- Pozzolanic Portland Cement – these building materials include components of volcanic origin in their structure. This composition quickly hardens both in the open air and in water, while mixtures based on such cement do not give any efflorescence and are highly hygroscopic. As a rule, such material is used in the equipment of swimming pools..
- Acid resistant cement uses in the manufacture of quartz sand and sodium fluorosilicate, such a composition is resistant to most mineral and organic acids, it does not change its characteristics in water. However, upon reaction with alkalis, it begins to break down.
Marking
All types of cement are distinguished by special designations that reflect their scope of use. Marking consists of letters and numbers.
- BOC – Portland cement.
- SS – sulfate resistant cement.
- SHPC – slag portland cement.
- GF – hydrophobic composition.
- BC – White.
- PL – plasticized cement.
- VRC – waterproof.
The degree of strength of the binder is indicated by the letter “M” and a number, for example, M500 – this means that such a material can withstand a load of up to 50 kg / sq. see It happens that this characteristic is simply indicated by a number – 22.5, 42.5 or 52.5 – this is not a brand, but a class. It shows that the material can withstand a pressure equal to, for example, 22.5 MPa.
Letters A and B indicate the number of additional components: A means that their percentage is 6-20%, and B – from 21 to 35%.
Scope of application
Cement is the most demanded and popular building material, its scope of application is practically unlimited.. It is used during construction work at all stages: foundation construction, laying a basement, laying walls, arranging roofs, installing plumbing fixtures, the composition cements floor structures. Blocks are produced using the material.
The main function of cement is to bond the structural elements of buildings under construction, as well as to level surfaces of all types. Not a single, even the most insignificant repair can do without cement..
The scope of application of specialized cements deserves separate consideration:
- PC500 – is widely used for the production of reinforced concrete and concrete products, as well as in industrial and civil construction – it is in these industries that the highest requirements for hygroscopicity, frost resistance and durability of building mixtures are formed. In addition, cement of this category is often used during emergency and repair and restoration work – this is due to the extremely high initial strength of concrete..
- PC500D20 – this composition is widely used in all types of construction: residential, industrial and agricultural. The material is used for the production of precast concrete structures, all kinds of elements of beams and floors.
In addition, this cement is the base material for the dilution of concrete mortars, which are used in plastering and masonry work..
The main consumer qualities of this cement are frost resistance and high resistance to corrosion, which is why the brand is in greater circulation than ordinary Portland cement..
- PC400D0 – this type of building composition is required in the manufacture of reinforced concrete structures, as well as for the installation of possible underwater and ground structures that are significantly affected by saline and fresh waters.
- PC400D20 – necessary for the production of prefabricated reinforced concrete structures, beams, wall panels and other materials necessary for construction.
Cement is used for arranging country paths, isolating basements and creating a monolithic basement and many other types of work, its scope is wide.
However, each of them requires certain grades and types of cement, so it is necessary to clarify in advance all the requirements for the materials used..
Shelf life
The shelf life of any cement is several months. Subject to certain rules, you can use an expired composition. However, keep in mind that this is permissible only for the arrangement of irresponsible structures. Many builders with little professional experience buy cement in large quantities, for future use, but this is not quite the right approach..
According to GOST 10178-85, each type of cements has a certain shelf life, you should especially focus on the fact that this period is permissible only when certain conditions for storing the material are created:
- for quick-hardening compounds packed in bags or plastic containers – 45 days;
- for cement for general use, packing in soft containers – 60 days;
- for bulk quick-hardening cement – 45 days;
- for bulk cement – 60 days.
Despite the rather short shelf life of cement according to GOST, in practice it is possible to increase the shelf life of the M200, M500, M600, M700 and M400 grades for six months or even a year. This is possible if a number of conditions are met, namely, the cement is packed in an integral soft package and stored in a dry room..
Packaged materials should be placed on wooden stacks or plastic pallets, while maintaining a distance from the pallets to the floor level of at least 10 cm.Then the cement packages are carefully covered with plastic wrap, or even better, each package should be pre-packed in a plastic bag.
If all conditions are met, then the shelf life of the cement can be increased to several months. For a better understanding of the consequences of improper storage, let us dwell in more detail on what happens to cement in conditions of increased moisture and carbon dioxide, which is contained in the atmosphere..
As already mentioned, any mineralized chips and cement composition are hygroscopic substances that absorb moisture well, when moisture is absorbed from the air, the hydration process begins, in which cement reacts with water and, as a result, crystalline hydrates are formed. Such materials are absolutely unsuitable for use..
In the manufacture of cement, all basic clinker components are heat treated at temperatures reaching 1000 degrees and above, while decarbonization occurs, that is, the starting material is split into lime, carbon dioxide and calcium oxide. The latter, in turn, undergoes a series of metamorphoses, as a result of which strong cement is formed, and carbon dioxide escapes into the air..
During prolonged storage, a reverse reaction occurs – carbon dioxide is absorbed by the main components of the cement and, as a result, a decrease in the strength and operational parameters of the composition.
Manufacturers
It is difficult to say unequivocally which brands produce the highest quality cement mixtures. The construction market is filled with offers of a variety of domestic and foreign manufacturers, each of which implements compositions of various markings and parameters..
A few years ago, the absolute leader of the cement market was considered Holderbank concern (Switzerland), it includes enterprises in 70 countries of the world, and the volume of cement produced exceeds 100 million tons – this figure is equal to half of the global production per year. Holderbank produces the highest quality cement using state-of-the-art technological equipment. All products sold comply with international standards EN 197-1.
However, the leader has recently changed. The French industrial Lafarge group, which has its factories in Europe, Asia and America, and closes the top three CEMEX (Mexico).
Among Russian enterprises, the greatest demand is for products of three brands:
- “Eurocement group” Is the leader of the domestic segment, the production facilities of which include factories in 16 regions of our country. Users note that the cement of this brand is the best in terms of value for money.
- “Novoroscement” Is the second largest domestic manufacturer of cement mixtures in terms of sales, its products are sold in the largest construction hypermarkets in various Russian cities in 50 kg packaging.
- “Siberian cement” – another well-known manufacturer that has established a full cycle of cement production, is one of the three leading Russian manufacturers that release only the highest quality and most practical material on the market.
Advice
Recently, the volume of construction has increased sharply, and the demand for cement has also increased. This gave rise to the emergence of a large number of unscrupulous manufacturers who produce cement with the addition of additives that worsen its performance properties..
In order not to be mistaken, you should know exactly how good cement should look like and what you should pay attention to when purchasing it..
Professionals give several relevant recommendations:
- As a rule, cement is sold in bags or in bulk. It is preferable to purchase the mixture in packaging, since at the stage of storage it was protected from the adverse effects of the external environment and, accordingly, retained its consumer characteristics..
- On the packaging, in accordance with the requirements of the legislation, information about the manufacturer, date of issue, GOST is mandatory. Any composition can be hidden under loose cement.
- If cement is purchased in large quantities, then each package should be checked for the release date, since unscrupulous sellers can mix in stale cement in the total volume, and as already mentioned, the activity of the composition decreases several times after six months after release..
- You can check the freshness of the cement yourself – for this you just need to hit the bag with your hand, if the cement is fresh, then the sensations do not resemble a hit on a stone. By the way, first of all, the cement turns to stone in the very corners of the package, so be sure to check these areas..
- If possible, make sure the cement is stored in dry locations with good ventilation..
- Experts recommend to carry out an “experimental” check of the cement, for this you need to take bicarbonate-sodium water (for example, the usual “Borjomi”), release all the gas from it and knead the cement paste, from which to mold small cakes so that the thickness in the center is about 5 cm, and at the edges – 1 cm.High quality cement should begin to harden no later than after 10 minutes, and its thickened part should heat up during this time.
If neither one nor the other happened within half an hour, refuse to buy this composition.
In the next video you can watch the process of cement production.
