Portland cement: varieties, properties and applications

Portland cement: varieties, properties and applications

For the manufacture of concrete, inorganic binders are used, which, when mixed with water, form a pasty solution that gains strength as it hardens. Portland cement is a type of such a binder..

Features and manufacturing

Portland cement is often talked about when it comes to the need for a solution that is durable and resistant to the negative effects of the environment. Portland cement is a kind of binder for concrete mortars.

It is a dry mixture that is diluted with water. After a certain time, the product sets when interacting with air.

Portland cement is based on finely ground clinker, as well as gypsum, which accelerates the setting of the mixture. Depending on the type and brand of the product, its formula may include certain additives and impurities.

The mixture was invented back in 1824 by an American mason, and owes its name to the external resemblance to Portland limestone, which was mined in one of the English counties..

To obtain this composition, carbonate rocks (limestone, chalk, alumina and silica), as well as marl (a mixture of carbonate rocks and clay, transitional rock from limestone to clay) are used. The production process begins with carefully grinding the raw materials and mixing them in certain proportions. The next stage is the burning of raw materials in furnaces at a temperature of 1300-1400 ° C. Reflow results in a material called clinker..

The clinker is ground again and mixed with gypsum. If necessary, other elements are added to increase the performance of the finished product. This mixture undergoes quality control and, if it meets the accepted standards, receives a certificate of conformity..

There are several options for firing raw materials:

  • Wet. First, the components are crushed, then the clay is soaked until the moisture content reaches 70%. After that, it is mixed with limestone in mills..
  • Dry. The process of grinding and drying the mixture takes place simultaneously, which reduces labor costs and production costs. As a result of processing in mills, powdered raw materials are obtained.
  • Combined. With this technology, 2 types of production are combined – dry and wet. The moisture saturation of raw materials rises to 14%, after which the products are crushed and dried in special mills.

Composition and properties

As already mentioned, Portland cement is composed of clinker. In nature, ready-made granules are quite rare, therefore, clinker chips are obtained by an artificial method by mixing and firing carbon and clay mixtures.

The finished clinker is mixed with gypsum, the content of which in the composition does not exceed 5%. It is introduced in order to ensure the mobility of the solution for 45 minutes, which is necessary when molding products or performing some types of work..

The composition and percentage of the components of the mixture is regulated by GOST 10178 85 “Portland cement and slag Portland cement”. It is the observance of the state requirements during production that guarantees high technical and operational characteristics of the product..

On its packaging, there must be an indication of production in accordance with GOST. In the absence of the latter, it means that Portland cement is manufactured in accordance with TU (technical conditions), which means that its properties differ from those adopted.

To give Portland cement certain technical characteristics, mineral additives are introduced into the composition, the content of which does not exceed 20-25%.

The most popular are the following:

  • Aluminate increases the setting time of cement, but has low strength indicators (possible content in Portland cement – no more than 15%).
  • Alumoferrite has the same properties as the previous additive, but its content in the finished product is reduced to 10-18%.
  • Belit has an astringent effect, helps to increase the hardening time, however, excessive content can negatively affect the strength characteristics of the composition (the permissible content is no more than 15-37%).
  • Alit widely used (the percentage can reach 60%) in high grade compositions, since it provides their quick hardening.

Portland cement properties are determined by its composition. The main criteria by which the quality of the product is assessed are the following:

  • Setting period. The setting of the mixture, subject to the technical requirements of its dilution, should occur after 40-45 minutes. Mineralogical composition, fineness of grinding and temperature at which work is carried out – these factors primarily affect the setting speed of the product..
  • Water demand. This term refers to the amount of water required to obtain a thick, suitable for operation of the cement paste. Typically moisture should not exceed 25% of the mix. In order to reduce the required amount of water, sulphite-yeast mash or plasticizers are used.
  • Water separation. This term denotes the extraction of water in the finished solution, the occurrence of which is due to the settling of heavier cement particles. Mineral supplements allow reducing this indicator..

      • Frost resistance – the ability of the product to endure a certain number of freeze and thaw cycles without losing its performance characteristics.

      To increase frost resistance, sodium abietate or washed away wood pitch is added to the composition..

      • Corrosion resistance. This characteristic is associated with the fineness of the grinding of the mixture and the degree of porosity of the finished concrete..
      • Heat dissipation. This refers to the ability of concrete to generate heat during the hardening process. The composition that quickly releases heat is optimized by adding active mineral components to it.

      Specifications

      Portland cement has higher strength characteristics in comparison with other types of cement, which is due to the peculiarities of the composition. These or those additives can enter into reactions, changing the technical properties of the material. The latter are associated with its mechanical resistance and operational capabilities..

      It cannot be said that one of the technical characteristics is of higher priority. For example, strong, but too slow hardening Portland cement can increase the construction time. A frost-resistant, but corrosive composition can only be used to solve a narrow range of tasks..

      Today, manufacturers strive to create universal compositions in which the properties most important for cement are equally strongly manifested..

      At the same time, there are specialized formulations with a special purpose. This can be considered pozzolanic Portland cement, which has maximum corrosion resistance and moisture strength, but rather low strength indicators at the initial stages of work (in the first days of setting).

      Technical

      Among the technical characteristics should be highlighted:

      • Specific gravity product – 1100 kg / m³ for bulk mixes, 1600 kg / m³ for compacted.
      • Grinding fineness the average is 40 microns (determined by the ability of the mixture to pass through a sieve No. 008), which provides the necessary cement strength and hardening time, and also affects its performance.
      • Water consumption, the optimal liquid content in the composition should not exceed 25-28%, since this indicator affects the strength of the composition (with excess, the concrete dough delaminates, with a shortage, cracks appear on the finished product).

      • Density depends on the brand and the presence of certain additives in the composition. In a loose state, the mixture has a density of 1.1 t / m³, in a compacted state – 1.5-1.7 t / m³.
      • Setting time after mixing with water does not exceed 40-45 minutes, further hardening depends on the characteristics of the composition and environmental conditions (in winter, the process slows down), but does not exceed 10-12 hours (measured with a Vic’s device).
      • Volume change when solidified, it means a decrease in the cement body in volume by 0.5-1 mm / m in the open air and its swelling to 0.5 mm / m in water. An important point is the uniformity of changes throughout the volume of the solution..

      Physical

      • Corrosion resistance is achieved due to the introduction of hydroactive materials into the composition, which prevent the chemical activity of salts, as well as the addition of impurities that reduce the porosity of concrete.
      • The storage period is no more than 12 months, provided that the original packaging is preserved (3-4-layer, hermetically sealed paper bags), since after 3 months of storage, up to 20% of the activity of the composition is lost, after a year – up to 40%. It is possible to return such cement to its former qualities only by secondary grinding..
      • Compressive strength. In accordance with this characteristic, 4 strength classes are distinguished – 22.5; 42.5; 42.5; 52.5. This indicator is directly related to the setting speed of the solution..

      Mechanical

      The indicators of the mechanical strength of Portland cement are at least 42.5 MPa on the 28th day after pouring. The determination is carried out in laboratory conditions using a sample as an example. In accordance with the results obtained, the cement is marked (for example, M 500). The coefficient in this case indicates what pressure the sample can withstand (measured in kg / cm³).

      The higher this coefficient, the greater the strength indicators of the composition. Strength characteristics depend on the degree of grinding (the finer it is, the more active the solution has), the presence of additives and additives.

      Strength indicators, in turn, affect the degree of setting of the mortar (determined using a Vicat needle).

      Differences from simple cement

      Portland cement is considered to be the type of cement that is most commonly used in concrete pouring. The latter, in turn, is used in monolithic and reinforced concrete construction, during the construction of objects to which increased strength characteristics are presented..

      Due to the presence of clinker granules and other additives, Portland cement has a large margin of safety, has higher frost resistance, resistance to aggressive environments. it makes Portland cement a popular material in the construction of oil and gas facilities.

      It is suitable for the construction of foundations on difficult unstable soils, in which case it is recommended to use a sulfate-resistant mixture. Such a composition almost does not shrink buildings, no cracks form on its surface..

      The difference between cement and Portland cement is somewhat incorrect, since the latter is a type of cement. In other words, cement is a general name, Portland cement is a type of cement with a certain set of strength.

      It is more logical to make the distinctions based on the brand strength of cements. For example, Portland cement M 400 is inferior in its strength to cement M 600. By itself, Portland cement is not much distinguishable from cement (in terms of installation method, setting technology, features of use), the difference in certain distinctive characteristics is due to the presence of additives.

      Views

      All types of cement are divided into non-additive and additive. Additive composition does not contain mineral additives other than gypsum. He suitable for above-ground, underground and underwater objects of a monolithic nature, as well as prefabricated concrete and reinforced concrete structures, which are operated in the absence of an aggressive environment.

      The presence of mineral additives improves the technical properties of Portland cement, due to which they can be used in aggressive conditions, with prolonged contact of the structure with water. Among the most common additives of mineral origin, there are: blast-furnace slag, active mineral additives and natural active mineral additives.

      Due to the introduction of one or another additive, such indicators as water resistance, corrosion resistance are improved, but their presence contributes to a decrease in frost resistance.

      Depending on the characteristics of the composition, the following types of Portland cement are distinguished:

      • Quick dry. The mixture hardens already in the first 3 days of pouring thanks to the slags and special minerals included in the composition. It is important that the degree of grinding of the mixture is minimal. Produced in the M400 and M500 brands. The use of this composition allows to reduce the holding time of the mixture in the formwork and significantly increase the pace of construction work..

      It is mainly used for pre-fabricated and reinforced concrete objects.

      • Normal curing. It does not contain special additives, it is not so picky about the degree of grinding of the mixture. Produced in accordance with GOST 31108-2003.

      • Hydrophobic the variant is characterized by the ability not to absorb moisture and a reduced setting time. Similar properties are provided by the mylonft and asidols included in the solution. Finds application in the construction of objects operated in conditions of high humidity, as well as located in flooded areas.
      • Plasticized. A characteristic feature of the product is the presence of plasticizers in it, which provide the necessary mobility, a decrease in water absorption, and heat resistance. Plasticizers are put in during the grinding of the mixture, due to which they seem to envelop the cement particles, preventing them from sticking together. The result is a mobile, easy-to-apply composition, which is widely used for the construction of architectural structures with complex shapes..

      • Grouting. It has the ability to plug, that is, to protect wells from the effects of groundwater. It is widely used in the oil and gas industries, since it is independent of pressure and temperature and reliably holds strings in wells even at the initial stages of solidification. There is another type of this type of cement – backfill lightweight Portland cement, which has “lightweight” additives in its composition..
      • Expanding. Such mixtures can have different compositions, but all of them are united by the ability to increase in volume when mixing the solution. This is due to the fact that a chemical reaction begins between the main and additional components, which gives an increase in volume.

      As a rule, such compositions are used to fill joints and cracks on surfaces exposed to high humidity..

      • Sulfate resistant. Such concrete resists the effects of sulphate waters, which cause corrosion. As a rule, frost-resistant cement of the M300,400 brand is made sulfate-resistant, sometimes – M 500.

      It is used to create pile and other types of foundations on marshy and acidic soils.

      • Slag Portland cement. The product contains blast-furnace slags, which is the reason for the high content of metal particles in it. The mixture is used to obtain heat-resistant concrete, as well as in the construction of objects underground, water, at a height. Has low frost resistance.

      • Slag-alkaline the composition is distinguished by higher, compared to Portland cement, characteristics. It is resistant to aggressive media, temperature extremes, high frost resistance and low moisture absorption. Such abilities are achieved due to the inclusion in the composition of ground slag and alkali, sometimes clay.
      • Portland white cement. The scope of application of the mixture is finishing and architectural work, it also acts as a base for colored cements. A snow-white shade is obtained due to the production of the product from pure limestones and white clays, as well as by additional cooling of the clinker with water.

      • Magnesian – a composition based on magnesium oxides (heated to a temperature of 800C) and a 30% aqueous solution of magnesium chloride. Thanks to the components included in it and the peculiarities of the production technology, it is possible to obtain a strong white mass that is easy to process (easily polished, not susceptible to mold, mildew).

      The material is used as a finishing coating, as well as to create structures that are complex in terms of shapes. Magnesia cement concrete is essentially a type of artificial stone..

      • Colored Portland cement also used for decorative work. It is obtained by mixing a white modification and a pigment. The latter can be red lead, ocher, chromium oxide. The main thing is that the pigments are light- and alkali-resistant..

      • Pozzolanic. The mixture contains colored Portland cement, gypsum and additives of volcanic or sedimentary origin. The resulting solution has increased water resistance, it solidifies not only in conditions of high humidity, but also under water. This allows it to be used in the organization of hydraulic structures, lining of pools and other reservoirs for storing water, surfaces (including inclined ones) in contact with sea or chlorinated water. The hardened surface is characterized by strength, chemical inertness, absence of efflorescence.
      • Aluminous the composition is a fast-hardening and durable cement based on clinker and molten limestone. The finished mixture contains a large amount of low-basic calcium aluminates.

      To ensure high-quality adhesion and set the required strength, solidification should be carried out at a temperature below 25% C. Otherwise, up to 50% of concrete strength is lost..

      Another feature of alumina is the inadmissibility of mixing it with other cements and lime, compositions containing even small amounts of alkali. Portland alumina cement is suitable for the manufacture of acid-resistant concrete solutions, filling acid-resistant rocks (granite, beshtaunite). Setting speed – about 8 days.

      Stamps

      The grade is defined as the strength of a specimen when tested for bending and compression. For the manufacture of the sample, Portland cement and sand are used, taken in a ratio of 1: 3. From this solution, a sample of 4x4x16 cm is made, which solidifies within 28 days, hardening occurs in conditions of high humidity. To accelerate solidification, it is allowed to use the sample steaming technique..

      The most common today are the brands of Portland cement M 400, 500, 600:

      • M 400 – the most demanded brand of cement. The technical characteristics (strength, frost resistance) laid down in it are suitable for the construction of most objects.
      • M 500 – cement, which has a somewhat large margin of safety, which makes it possible to use it in reconstruction or restoration of facilities after an accident, to use it for road repairs, construction of military-technical facilities, asbestos-cement structures.

      • M 600. The composition has increased strength, which makes it possible to use it in the construction of critical reinforced concrete structures, engineering structures.
      • M 700 – Portland cement of maximum strength, used for concrete mix for the construction of pre-stressed structures. Its use in conventional construction (for example, in private households) is irrational due to the high cost.
      • M 900 – heavy-duty cement used only for military installations, for example, for the creation of bunkers.

        There are also “intermediate” brands of cement, for example M 550 (in terms of their technical characteristics they are close to M500, but differ in slightly higher strength).

        When is it not suitable?

        Due to the variety of Portland cement compositions, it is suitable for almost any type of building. The main thing is to correctly determine the most important property of the cement and select the appropriate additive. Some of them, when used together, neutralize the properties of each other. This happens, for example, with the simultaneous addition of components to improve moisture resistance and frost resistance. The first (increasing moisture resistance) significantly reduce the frost resistance of the composition.

        In other words, Portland cements with additives are not suitable for conditions in which there is a significant decrease in operating temperature. In this case, Portland cements should be used without additives. For humid climates, the standard mixture of Portland cements is not suitable, it is better to choose slag Portland cement.

              It is also important to consider the purpose of the material. So, for the construction of monolithic objects and structures for civil purposes (for example, bridges, high-voltage lines), Portland cement M400 is not suitable. To solve these problems, it is permissible to use a mixture with a brand strength of at least M 500.

              None of the Portland cement types are suitable for use in flowing waters, salt ponds, flowing rivers, waters with a high mineral content.

              Even the sulfate-resistant variety of Portland cement, which has increased moisture resistance, is used only in static temperate waters. In other cases (for example, for the organization of dams, dams and other engineering structures), special types of cement are used.

              The prefix “Portland” denotes the presence of a large amount of calcium silicates in the mixture, therefore it is not recommended for special-purpose blocks and structures. Pozzolanic cement is not suitable for use in conditions of significant temperature drop.

              See how Portland cement is obtained in the next video..

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