What is a cement clinker
Everything you need to know about cement
Is cement the same as concrete?
In a nutshell: No. As a binding agent, cement is a basic component of concrete, but cannot be equated with it.
The ancient Romans already used "Opus Caementitium", a concrete-like masonry made of rubble stones with burnt lime as a binding agent. As before, cement mainly consists of calcium silicate with proportions of aluminum and iron compounds as well as proportions of sulphates. In order to meet the higher or changed demands on the material, however, special cements are produced today: the necessary physical and chemical properties are developed by adding various substances. The following additives are permitted in Austria: blast furnace slag (= granulated blast furnace slag), fly ash and limestone or a mixture of these.
What types of cement are there?
The large number of types of cement makes it necessary to subdivide them into "normal cements" and "special cements" (cements with additional or special properties). Of the normal cement types specified in the European cement standard ÖNORM EN 197-127, eleven types are used in Austria - due to the climatic conditions. There are basically five main types of cement:
Portland cement: This type of cement is named after the Portland peninsula in southern England, where limestone is mined. Portland cement clinker is made by grinding and burning the raw materials limestone or chalk and clay.
Portland composite cements: In addition to the Portland cement clinker, all other possible main components (limestone, slate, blast furnace slag, pozzolan, fly ash) can be added to this cement.
Blast furnace cement: In blast furnace cement, parts of the Portland cement (up to 95 percent) have been replaced by slag sand. The production of cement containing slag sand requires less energy and therefore causes lower CO2 emissions. CO2 emissions are reduced by around one ton per ton of blast furnace slag. Blast furnace cement is also more resistant to salts (e.g. important for buildings by / in the sea) and carbonic acid (dissolves lime). It is therefore increasingly used for the construction of tanks and systems in the chemical industry, petrol stations and sewage treatment plants.
Pozzolan cement: Pozzolans are substances containing silica or silica and alumina from natural occurrences that can only harden in connection with calcium hydroxide. The name, derived from the southern Italian town of "Pozzuoli" near Vesuvius, already indicates that pozzolans are of volcanic origin (e.g. trass, lava) or are obtained from clays, slate or sedimentary rocks (phonolite).
Composite cement: A large part of the cements produced today are composite cements in which clinker is partially replaced by other additives. This conserves natural raw materials. In addition, these additives, for example blast furnace slag and fly ash, are residues and waste materials from other branches of industry.
White and green special cements
Did you know that...
Is iron oxide responsible for the gray color of cement and concrete? However, the color of the cement is not a sign of quality and varies. Different shades of gray are the result of the raw materials used, the type of cement, the grinding fineness and the manufacturing process.
- White cement is a Portland cement and is made from raw materials that are low in iron. It is mainly used for terrazzo panels, plastered and exposed concrete. White cement creates very light surfaces and can be easily colored with color pigments. For the coloring, however, the color of the sand used is also important.
- Celitement: Cement works are real CO2 spinners, which is why environmentally friendly innovations in cement production are the order of the day. The special cement Celitement is such an innovation. Its raw materials are similar to conventional Portland cement: quicklime and sand. Calcium hydrosilicates are newly added, which make it possible to produce the cement by boiling in water at approx. 300 degrees. Portland cement is produced at much higher temperatures and therefore uses a lot more energy.
In short: how is cement made?
The most important raw materials for the production of cement are limestone, clay and marl. They are extracted in quarries and crushed in crushing plants to the size of road gravel. This raw gravel is evened out in the cement works, mixed with quartz sand and iron ore, ground and dried. The resulting raw meal is burned to clinker (= bricks) at temperatures of approx. 1,450 degrees and then cooled. Cement mills grind the clinker with the addition of gypsum, anhydrite and any other additives, such as limestone, blast furnace slag or fly ash, to finally produce cement.
When working with cement, gloves, skin-covering clothing and protective goggles must be worn! The resulting lye can irritate the skin and mucous membranes.
Cementing on your own
It can pay off to mix cement yourself, especially for smaller projects around the house and garden. To do this, you buy cement in a 25 kg sack from a builder or hardware store. Pay attention to the strength class of the cement, because this determines how quickly the cement hardens in the first few days (possible key figures: 32.5, 42.5 and 52.5). The 32.5 cement is the DIY store standard. This is ideal for all applications, you should only use a faster-hardening cement in cold weather.
The necessary sand can be bought cheaply in a gravel works. For concreting, sand with a grain size of 0/32, 0/22, 0/16 (practically all work can be done) or 0/4 is used. The larger the grain, the higher the final strength when the concrete has hardened. Keep in mind, however, that larger grain size will be harder to compact. Fine work cannot be done with it. Water and sand must be absolutely clean for processing, impurities (earth, clay, roots, etc.) considerably reduce the strength and must be removed.
Good to know:
Mixing cement and water exclusively with sand or another aggregate with a grain size of less than four millimeters is called mortar.
Mix cement properly
Optimum strength can only be achieved if the correct mixing ratios for the cement or concrete are adhered to. The ratio between cement and sand should be around 1: 4. That means: a full shovel of cement should mix with four full shovels of sand. Basically, the higher the cement content, the harder the concrete is afterwards. A commercially available mixer can be used for mixing, which can be rented from a specialist store, for example. Small quantities of less than 12 liters can also be easily mixed by hand with a trowel in a construction bucket. With the help of ready-mixed concrete screed, time and effort can be saved for very small quantities.
The mixing ratio of water to cement is more important than is often assumed. The rule of thumb four liters of water for ten kilograms of cement can be followed quite easily if you calculate a 10 liter bucket of water for a 25 kg sack of cement. The end product should be pulpy and slightly shiny. If you use too much water, it later evaporates and leaves unsightly pores on the surface. If too little water is used, the cement does not bind properly and does not harden properly. Mixing it requires experience. There are also different methods when it comes to mixing the concrete: In theory, it is taught that the mixing water is first mixed with the cement so that a homogeneous cement paste is created. Then the surcharge follows. In practice, however, cement and sand are often mixed first.
By the way: cements should not be mixed with one another. Each cement is individually optimized with regard to setting and setting. If mixing cements is technically and economically sensible for special applications, it must first be checked whether the mixture is harmless. A consistent composition of the raw material is crucial for the quality and uniformity of the cement.
If you wanted to check whether your cement could still achieve the necessary strength, crush the lumps between your fingers. If this is possible, the reduction in strength is negligible and you can still use the cement. Otherwise, you should dispose of the cement.
What is the best way to store cement?
Bag cement is best stored in a dry place. If he has to go outside temporarily, a ventilated square timber underlay is recommended. Foils used to cover the cement sacks must not touch them directly, as they will become damp in the event of condensation. Cement that is stored longer and unprotected absorbs moisture, which makes it lumpy and harder to harden.
As a rule of thumb, if properly stored bagged cement and cement in construction site silos, a strength reduction of slightly more than ten percent occurs after three months. Fast-setting cements (strength class 52.5) should not be stored for longer than a month.
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