The history of cement can be traced back to a mixture of lime and volcanic ash used by the ancient Romans in their building works, while at that time, this technology was not widely used around the world. In 1796, the British used the ancient Roman technology to produce a kind of brown cement from marls, which was named Roman cement because of its similar appearance to the ancient Roman cement and was widely used in buildings because of its good properties. In 1824, Joseph Aspdin, an English man, invented the Portland cement by calcining limestone and clay. Because it had a similar color to the natural stone in island Portland, the cement was named Portland and got a patent. Nowadays, with the improvement of people’ living standard, our requirements for construction projects are increasing. Portland cement has become the most commonly used construction material in our cities. With the continuous research and improvement of Portland cement, a batch of cement suitable for various special construction projects has gradually appeared in our field of vision. So far, there are more than 100 varieties of cement have been developed around the world.
Cement Classifications
In the above content, we mentioned that we have developed more than 100 varieties of cement. In addition to the ordinary Portland cement, there are many other types of special cement, such as white cement, colored cement, rapid hardening cement, low heat cement, etc. They are produced by adding different materials in the process of clinker grinding, such as blast furnace slag and fly ash.
White Cement: this kind of cement is also called white Portland cement that famous for its white appearance. It is totally free from oxides of iron, manganese and thorium. The white cement is generally used as decorative materials for various buildings, such as sculpture, floor finishes, etc.
Colored Cement: the colored cement is manufactured by mixing the pigment with the ordinary Portland cement. Its production mainly adopts the staining method and calcination method, the latter of which has a durable color but a high production cost. This cement is widely used in the surface coating of floor, wall and various buildings.
Rapid Hardening Cement: the characteristic of the rapid hardening cement is that it can reach the maximum strength within three days. It is produced by adding more limestone than that of the ordinary Portland cement, and its grinding granularity is also finer so that increasing the speed of the hydration of cement particles. This cement is suitable for emergency repair engineering, low-temperature construction and the production of high-grade concrete precast parts.
Low Heat Cement: low heat Portland cement is a kind of cement containing a large amount of dicalcium silicate and a little tricalcium aluminate. It releases very little heat in the hydration process and has incomparable advantages of high strength in the later solidification period, high durability and corrosion resistance. It is often used in airports, dams and other large concrete structures.
What is Ordinary Portland Cement
The ordinary Portland cement is also known as silicate cement. At the beginning of the article, we said that Portland cement is so called because the color of concrete made from it is similar to that of natural rocks on the British island of Portland. The raw materials for Portland cement are mainly composed of three components: calcium oxide, silicon dioxide and alumina, which account for 60%, 20% and 10% of the total components respectively. The calcium oxide comes from limestone, while silica and alumina come from shale, clay and bauxite. In addition, most of the raw materials contain iron oxide, magnesia, calcium sulfate, etc.
Functions of Ingredients
Lime: lime accounts for more than half of the cement ingredients. A sufficient amount of lime is the basis of producing enough silicate and calcium aluminate needed for cement. However, an excessive amount of lime will make cement not firm.
Silica: silicon is the basis of the formation of dicalcium silicate and tricalcium silicate. The silicate gives cement strength, which usually accounts for one-fifth of the cement ingredients.
Alumina & Iron Oxide: alumina makes cement has the characteristic of fast solidification, and it reduces the temperature of clinker, accounting for 1/15 of cement ingredients. Iron oxides are the basis for the formation of tricalcium alumino-ferrite, which improves the hardness and strength of cement.
Magnesia & Calcium Sulfate: magnesium oxide exists in cement with very weak content. Once the content exceeds the upper limit, it will affect the performance of cement. The calcium sulfate is mainly from gypsum and has an effect of slowing down the solidification speed. In cement plant, gypsum is usually added in the clinker grinding process.
Different Types of Portland Cement
Portland cement mainly has five types from I to V. In terms of performance, they are mainly different in the rate of early hydration and their resistance to sulfate attack.
Type I: type I cement is the ordinary Portland cement. Its early cement strength performance is good, widely used in railway construction, bridge construction, military and general building construction, accounting for about 90 % of cement usage.
Type II: type II cement has the characteristic of moderate sulfate resistance. It produces less hydration heat, but its initial strength is slower than that of the type I cement. The type II cement is suitable for the construction of bridge piers and large dams.
Type III: type III cement has a high early strength. It can reach the strength of ordinary cement after 28 days in 3 to 5 days, so all kinds of projects that need to be completed in a short time will use this kind of cement, such as military construction, road laying and underwater engineering.
Type IV: the hydration heat during the hardening process of type IV cement is only about 70% of the ordinary cement, and its hydration heat development rate is kept within a very low limit, which can reduce the probability of concrete collapse. It is suitable for the construction of mega-projects.
Type V: this kind of cement has high sulfate resistance and suitable to be adopted in the construction of the sewer, basement, hot spring area and other special environment projects.
Portland Cement Manufacturing Processes
The Portland cement manufacturing is a complex process which can be mainly divided into three parts: raw materials preparation, clinker calcination and cement grinding.
Raw materials preparation: after materials are quarried, they need to undergo crushing, blending, grinding and storage four processes to finish the raw materials preparation before they are sent into the rotary kiln. In the first step, raw materials are sent into the crushing equipment in order to get an appropriate size, and mixed with each other to meet the requirements of clinker ingredients. Then we need to use the grinding mill to further pulverize them as long as they reach the qualified fineness that can be sent into the rotary kiln. After that, they will be stored in the silo waiting for further processing.
Clinker Calcination: clinker calcination is the most important stage during the whole Portland cement manufacturing process. It can be classified into the dry method and the wet method. The biggest difference is that in the dry method, raw mix is existing in the form of fine powder. While in the wet method, the raw mix is combined with water to form slurry before being sent into the rotary kiln. Anyway, they all need to be heated at a high temperature in the rotary kiln to produce chemical reactions with each other and finally forming clinker. After sintering, the clinker will have a high temperature. They will cool down at the end of the kiln firstly and then enter the cooler placed behind the rotary kiln for further cooling. When clinker drops to indoor temperature, it will go to the next step.
Cement Grinding: cement grinding is the final stage in Portland cement manufacturing. In this step, clinker will be ground in grinding mill to reach the qualified fineness of cement product. During the grinding, we usually add gypsum, fly ash and other raw materials into the cement to realize the different usage of cement. After all the above processes are completed, the cement will be packaged and sent to its destination.
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