One of the most common types of binders for concrete mixes is Portland cement. What is it, how it is used, what properties it possesses, we will consider these and other questions below.
Composition
As with any building material, high demands are placed on the components from which Portland cement is made. 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 of not more than 5%.
- Plaster corresponding to GOST 4013-82. The presence of phosphorus, boron and fluorine compounds in an amount that does not contradict the regulatory documentation is allowed.
- Mineral additives of a certain property, necessary to create mixtures of the desired type.
All components are co-milled to a homogeneous state.
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 underway, all components undergo the necessary compliance tests, the proportions of the creation of dry and working mixtures are clearly observed.
GOST Portland cement and its designation are contained on the packaging and in the accompanying documentation. If there is none, then the product is manufactured according to TU, its properties may differ from the generally accepted ones.
Production
To understand Portland cement - what it is, it is worth considering the process of its production, which consists of several stages:
- Greenish or yellow limestone mining. It lies at a depth of 10 meters in layers, because they seize it openly in places of geological exploration.
- Training. First, the raw materials are subjected to drying and primary grinding using some additives to obtain a working mixture. To find out what size of the mass fraction of impurities contains a binder, 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. So it turns out cement clinker - the main component of Portland cement.
- Finishing At this stage, the clinker is dried and ground into a working fraction together with limestone and special additives for each type of Portland cement. Further, the mixtures are Packed in bags of 30, 40, 50 kg and sent to the warehouses of construction sites, shops.
Manufacturing Features
All types of Portland cement (PC) are made from different raw materials, the common component for all is only cement clinker. The remaining components may have various properties: humidity, 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:
- Wet method involves grinding the components in water. This is done with chalky and clay-based iron additives. For them there are humidity standards: clay - 20%, chalk - 29%. The emulsion obtained after processing has a humidity of 50% and in this form is sent to the kiln for firing. In the process of heat treatment, clinker balls are formed, which are subsequently ground into a finished binder.
- The dry method does not imply wetting the raw material: it enters the oven in a dry form. The cost of cement produced in this way is lower since it does not require energy for drying.
- The combined method combines wet and dry. Sludge (raw mix) is ground in water. Further, using a filter, they get rid of more moisture; the mixture enters the kiln with a moisture content of up to 18%.
The 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:
- The absolute density in the range of 3050-3150 kg / m 3 , bulk for different types of PC will differ.
- The fineness of Portland cement should be determined by sieve No. 008 with a powder throughput of at least 85%.
- The specific surface size after screening 2500-3000 cm 2 / year
- Setting time: start - 45 minutes, end - 12 hours. Determined by the Vita scale.
Strength is measured by testing samples with dimensions of 4x4x4 cm, made of 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.
Kinds
Portland cement can have several varieties, depending on the type of additives used:
- quick hardening;
- sulfate resistant;
- hydrophobic;
- plasticized;
- with moderate exothermy;
- slag Portland cement;
- acid resistant;
- aluminous;
- white and color;
- pozzolanic.
Each type gives concrete stone certain properties necessary for use in a particular place or area.
Quick-hardening PC contains a large number of calcium compounds and is able to intensively increase hardening strength.
Sulfate-resistant Portland cement has in its composition additives that increase the resistance of concrete to chemical aggressive substances.
The hydrophobic type of binder contains additives that contribute to the slow evaporation of water, which makes it suitable for use in dry areas where it is necessary to gradually solidify the stone without loss of strength.
The plasticized type of binder, introduced into the concrete mixture, increases its plasticity and workability.
Portland cement with moderate exothermy produces a relatively small amount of heat during hydration.
Slag Portland cement has blast furnace slag in its composition , which significantly reduces its cost.
Acid-resistant PC contains pure quartz sand and sodium silicofluoride, which are resistant to aggressive chemical environments;
Alumina Portland cement contains a high concentration of alumina, which promotes rapid hardening.
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 paint the stone in specific colors.
Strength classification
Portland cement is divided into grades on the basis of its activity indicator - tensile strength under axial compression of half of samples aged 28 days. Accordingly, there are 400, 500, 550, 600 brands.
Any type of Portland cement grade 400 is used to create conventional and standard structures that are not subject to increased loads.
Higher grades 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 for concrete. Accordingly, depending on the type of filler, the stone has some or other enhanced characteristics. For example, quick-hardening Portland cement 500 and 600 is characterized by a high hardening rate, because it is added to concrete for the production of critical and massive structures of both underground and ground structures. It is also often used in cases where there is a need for a quick set of strength, for example, for pouring the formwork of the foundation.
The most common is Portland cement 400. It is universal in application: for creating monolithic and prefabricated reinforced concrete elements with increased strength requirements, while not significantly inferior to the characteristics of the PC500, but it has a lower cost.
Sulfate-resistant type of binder is used to prepare mixtures that are involved in the creation of underwater structures. Such are most susceptible to the damaging effects of aggressive sulfate waters.
Plasticized Portland cement grade 300-600 significantly improves the plastic properties of the mortar, its strength characteristics, allows you to save 5-8% binder, compared with conventional cement.
Special types of PCs are rarely used in small construction, since such raw materials have a high cost, and not every inhabitant knows about this type of active substance. More often Portland cement is used for concrete mixtures used in especially large and critical facilities.
When not to use
Portland cement is an active component that gives concrete special properties. But none of its kind should be used in highly flowing riverbeds, in salt ponds, in water with a high content of minerals. The sulfate-resistant species in these conditions does not cope with its functions, it is designed to work in static temperate waters. For the construction of dams, dams, hydraulic structures that serve in a water pressure environment, special types of cements are used.
How to choose and buy
When buying, you should understand Portland cement - what is it? First of all, a binder to create a concrete mixture. Like regular cement, it is sold in packaging bags, paper bags of 50 kg, sometimes 30, 40.
When choosing, pay attention to the symbols on the package. Be sure to specify the type (Portland cement, slag Portland cement, etc.), compliance with GOST or TU, the amount of mineral additives. You also need to pay attention to the date of manufacture: the fresher the raw material, the better its quality.
Both the manufacturer and the reseller can influence the cost of special cement. The same manufacturing technology does not imply a big difference in price. Therefore, choose the material of the plant that 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., PTs500 (50 kg) - from 230 rubles. It would seem that the difference is small, but to create structures requires a large amount of mixture, the basis of which is a binder. As a result, an error in the selection can cost a pretty penny.