One of the most common types of binder for concrete mixes is Portland cement. What it is, how it is used, what properties it has, we will consider these and other questions further.
Composition
As with any building material, high demands are placed on the components from which Portland cement is produced. GOST 10178-85 "Portland cement and slag Portland cement" regulates the composition and quality of ingredients for this type of binder:
- Cement clinker with a mass fraction of silicon oxide not more than 5%.
- Gypsum, corresponding to GOST 4013-82. The presence of phosphorus, boron and fluorine compounds is allowed in an amount that does not contradict the regulatory documentation.
- Mineral additives of a certain property, necessary to create mixtures of the desired type.
All ingredients are ground together until smooth.
For different types of Portland cement, various components can be used, which also have certain technical requirements prescribed in GOST.
At the factories where production is carried out, all components pass the necessary tests for compliance, the proportions of creation are clearly observeddry and working mixtures.
Produced GOST Portland cement and its designation are on the packaging and in the accompanying documentation. If there is none, then the product is manufactured according to specifications, its properties may differ from those generally accepted.
Production
To understand Portland cement - what it is, it is worth considering the process of its production, which consists of several stages:
- Extraction of greenish or yellow limestone. It lies at a depth of up to 10 meters in layers, therefore it is removed by an open method in places of geological development.
- Preparation. First, the raw material is subjected to drying and primary grinding with the use of some additives to obtain a working mixture. To find out what mass fraction of impurities the binder contains, just look at the marking: PC400 D20 has 20% improvers in its volume, and PC500 D0 does not have them at all. After the initial grinding, the mixture is fired in special powerful furnaces. This is how cement clinker is obtained - the main component of Portland cement.
- Finishing. At this stage, the clinker is dried and ground into a working fraction along with limestone and special additives for each type of Portland cement. Further, the mixtures are packaged in bags of 30, 40, 50 kg and sent to warehouses of construction sites, shops.
Production Features
All types of Portland cement (PC) are made from different raw materials, the common component for all is only cement clinker. Other components may havevarious properties: moisture, strength, mineral composition. Given these factors, it is impossible to process all types of raw materials in one way, so there are several ways to implement the second stage of production:
- The wet method involves grinding the components in water. This is done with chalk and clay iron additives. For them, there are humidity standards: clay - 20%, chalk - 29%. The emulsion obtained after processing has a moisture content of 50% and is sent to the kiln in this form. During the heat treatment, clinker balls are formed, which are subsequently ground into a finished binder.
- The dry method does not imply moistening of the raw material: it enters the kiln in a dry form. The cost of cement produced in this way is lower because no drying energy is required.
- The combined method combines wet and dry. The sludge (raw mixture) is crushed in water. Further, with the use of a filter, more moisture is removed, the mixture enters the kiln with a moisture content of up to 18%.
Properties
As already mentioned, concrete acquires certain qualities when one or another Portland cement is added to it. The characteristics of each of them are unique in their own way, but there are parameters common to all:
- Absolute density within 3050-3150 kg/m3, bulk for different types of PC will be different.
- The fineness of grinding Portland cement should be determined by sieve No. 008 with a powder passability of at least 85%.
- Specificsurface size after screening 2500-3000 cm2/g.
- Setting time: start - 45 minutes, end - 12 hours. Determined on the Vita scale.
Strength is measured by testing samples with dimensions of 4x4x16 cm, made from a cement-sand mortar in a ratio of 1:3 with a ratio of water to cement of 0.4, after 28 days of hardening. Finished prisms are subjected to bending and compression, determining their value and compliance with the brand indicators.
Views
Depending on the type of additives used, Portland cement can have several varieties:
- quick setting;
- sulfate resistant;
- hydrophobic;
- plasticized;
- with moderate exotherm;
- slag Portland cement;
- acid-resistant;
- aluminous;
- white and color;
- pozzolanic.
Each of the types endows the concrete stone with certain properties necessary for use in a particular place or area.
Quick hardening PC contains a large amount of calcium compounds and is able to intensively increase the strength during hardening.
Sulfate-resistant Portland cement contains additives that increase the resistance of concrete to chemical aggressive substances.
Hydrophobic type of binder contains additives that contribute to the slow evaporation of water, which makes its use appropriate in arid areas where it is necessary to gradually set the stone without lossstrength.
The plasticized type of binder introduced into the concrete mix increases its plasticity and workability.
Moderately exothermic Portland cement releases relatively little heat when hydrated.
Slag portland cement incorporates blast-furnace slag, which significantly reduces its cost.
Acid-resistant PC is composed of pure quartz sand and sodium silicofluoride, resistant to aggressive chemical environments;
Portland alumina cement contains a high concentration of alumina, which helps to set quickly.
Pozzolanic PC is enriched with mineral additives of volcanic and sedimentary origin (up to 40% of the total mass). They increase water resistance and do not form efflorescence on the surface of the finished concrete.
White and colored types of binder contain various metal compounds that can color the stone in certain colors.
Strength classification
Portland cement is divided into grades based on its activity indicator - the axial compressive strength of halves of samples aged 28 days. Accordingly, there are brands 400, 500, 550, 600.
Any type of Portland 400 cement is used to create conventional and standard structures that are not subject to increased loads.
Higher brand values are much more expensive, they are used for critical structures and their elements.
Application
Portland cement - what is it? This is an improved type of binder forconcrete. Accordingly, depending on the type of filler, the stone has certain enhanced characteristics. For example, fast-hardening Portland cement 500 and 600 has a high hardening rate, because it is added to concrete for the production of critical and massive structures, both underground and above ground. It is also often used in cases where there is a need for a quick set of strength, for example, for pouring foundation formwork.
The most common is Portland cement 400. It is universal in application: for the creation of monolithic and precast concrete elements with increased strength requirements, while significantly inferior to the characteristics of PC500, but has a lower cost.
Sulfate-resistant type of binder is used to prepare mixtures that are involved in the creation of underwater structures. These are most susceptible to the damaging effects of aggressive sulfate waters.
Plastified Portland cement grade 300-600 significantly improves the plastic properties of the mortar, its strength characteristics, saves 5-8% of the binder, compared with conventional cement.
Special types of PC are rarely used in small-scale construction, since such raw materials are of high cost, and not every layman knows about this type of active substance. More often, Portland cements are used for concrete mixtures used in especially large and critical facilities.
When not to use
Portland cement is the active ingredient that gives concretespecial properties. But none of its species should be used in strongly flowing riverbeds, in s alty reservoirs, in water with a high content of minerals. The sulfate-resistant species does not cope with its functions in these conditions; it is designed to work in static temperate waters. For the construction of dams, dams, hydraulic structures that serve in the environment of water pressure, special types of cements are used.
How to choose and buy
When buying, you should understand Portland cement - what is it? Primarily a binder to create a concrete mix. Like ordinary cement, it is sold in packing bags, paper bags of 50 kg, sometimes 30, 40.
When choosing, pay attention to the designations on the package. The type (Portland cement, Portland slag cement, etc.), compliance with GOST or TU, the amount of mineral additives must be indicated. You also need to pay attention to the date of manufacture: the fresher the raw material, the better its quality.
The cost of special cement can be influenced by both the manufacturer and the reseller. The same manufacturing technology does not imply a big difference in price. Therefore, choose the material of the plant you trust, which has the best reputation.
How much does Portland cement cost? The price of the most popular P 400 for a bag of 50 kg starts from 200 rubles, PC500 (50 kg) - from 230 rubles. It would seem that the difference is small, but to create structures, a large amount of a mixture is required, the basis of which is a binder. As a result, a mistake in choosing can cost a lot of money.
