Ordinary Portland Cement
Portland cement is the most commonly used type of cement in the world today. Portland cement can be found in both concrete and mortar, not to mention other construction mediums such as stucco and some types of grout, where it acts as a binding agent. On a chemical level, Portland cement is a fine powder comprised of a minimum of 66% calcium silicate, with the remainder largely being a mix of aluminum, and iron. Portland cement is a hydraulic material, which requires the addition of water in order to form exothermic bonds, and is not soluble in water.
Originally designed as a cement which would set slowly, allowing enough time for it to be properly laid, and a water resistant cement which could be used in construction applications where water would come in contact with the cement, Portland cement was first patented in 1824 by an English man, Joseph Aspdin, but the mix which became truly successful, and which is still used today, was designed by his son, William Aspdin in around 1843.
Portland Cement Applications
Portland cement is most often used in concrete and mortar. Concrete is made by combining water, sand, gravel, and cement, whereas mortars are made by combining cement with water and sand only. Concrete is much stronger than mortar, and is used in most modern buildings as a durable and strong construction material capable of bearing great loads. Mortar is used to bind other substances together, such as the bricks in a house.
Portland Cement Strength
Portland cement usually takes several hours to set, and will harden in a matter of weeks. Cement is a somewhat curious material in that it continues to harden over time as long as there is water available for the components of the cement to form bonds with. One week old Portland cement has a strength of around 23 MPa, whereas three month old cement has a strength of 41 MPa. These numbers apply to standard Portland cement which has not had any additives added to it. Various treatments and additives can make cement set and harden at different rates, and various types of Portland cement also posses different properties which affect the rate of setting and hardening.
Types of Portland Cement
There are different standards used in different places in the world for classifying types of Portland cement. The ASTM C150 states that there are five types of Portland cement. These are outlined below.
Type I Portland cement is the most common type of cement and is commonly referred to as general purpose Portland cement. Unless cement is specified as being of another type, it is normally safe to assume that it is Type I Portland cement. Type I cement is suitable for most construction applications, especially those in which the cement will not be in contact with or buried in the ground.
Type II Portland cement is similar in price to Type I cement, but it has higher sulfate resistance, and is therefore more suitable for applications in which the concrete will come in contact with the ground, or be buried in the ground.
Type III Portland cement develops strength quickly, but is slightly less strong in the long run than other types of Portland cement. Type III Portland cement is used in applications where quick hardening and strength is required.
Type IV Portland cement has low heat of hydration, which simply means that it gives off less heat as it sets and hardens than other types of Portland cement. Type IV Portland cement sets very slowly, but is much stronger after curing than other types of cement. It is a rarely used cement, but is often ordered for the construction of very large projects which need high strength levels, such as Dams.
Type V Portland cement has a very high sulfate resistance, and is designed for use in applications where the cement is to be exposed to highly alkaline soil or water, which often contain large amounts of sulfates which can cause other types of cement to expand and distort.
White Portland Cement
White Portland cement can be of any type, but is a white color instead of the usual grey.
Making Portland Cement
There are three main stages involved in making Portland cement. The first stage is the preparation of the raw mixture which is then transformed into clinker during the second stage, and the third stage then involves preparing the cement powder from the clinker.
Raw Cement Mixture
Simply viewed, the raw material for cement is rock. Many cement factories simply quarry the necessary rock from local resources. In some locations the rock contains all the correct elements to successfully make Portland cement, and in other cases the rock needs to have other materials added to it such as limestone or clay in order to make Portland cement.
Of course, even though making cement seems as simple as crushing rocks down to a very fine powder, it is in fact quite a great deal more complicated than this. Various oxides and minerals must be present in the mix in quite specific quantities in order for Portland cement to be made.
Once the rocks have been quarried, rocks containing various types of raw material are crushed into very small pieces, normally no larger than half a centimeter in diameter. Once these pieces have been crushed, they are normally mixed and then ground in a raw mill.
In order for there to be correct portioning of materials, it is quite common for there to be individual silos each containing a different raw material. In order to create the raw mixture which will be fed into a kiln which produces Portland clinker, these individual silos are controlled to release the correct amounts of material onto a conveyor belt. The conveyor belt then carries all the raw materials into the raw mill, where they are ground and mixed together to form a mix of raw materials often referred to as a raw mix.
The quality of raw mix is a crucial factor in the production of Portland cement. It must be homogeneous, meaning that it is well mixed and that there are no areas of the mix where any individual raw materials are present in disproportionate amounts. It must also be ground down into particles of a similar size, as overly large particles slipping through into the kiln can cause irregularities in the clinker, and subsequently, the cement.
Dry or Wet cement production?
There are two methods of producing Portland cement. One is a dry method, in which the materials stay dry throughout the entire process, and the other is a wet method, in which water is added to the raw mix. Depending on which method is to be used, the raw mix might be dried with hot air before leaving the raw mill, or water may be added to it in order to form a slurry.
Blending Raw mix
After the raw mix has been made, it is taken to a blending chamber, where it is monitored for chemical composition. It is almost impossible to overstate the importance of controlling the composition of the raw mix, as having the correct components in the correct proportions is essential for successful firing in the kiln. Calcium and silicone must be present in large amounts in order to form the calcium silicate which comprises over 66% of Portland cement. Aluminum and iron are present in the mix because they form liquids during the firing process. These liquids act as a flux material, lowering the temperature at which calcium silicates will form. Too much flux leads to weakened cement, whereas too little flux makes the firing process ineffective. The raw mix is checked regularly using chemical analysis in order to ensure that the chemical composition is correct.
Firing The Raw mix, Making The Clinker
The raw mix is then fed into the kiln, which is a cylinder that rotates slowly and is heated along the length of the cylinder, with the end that the raw mix enters being relatively cool, and the far end being very hot indeed. As the raw mix travels through the kiln it grows hotter and hotter, releasing gases and beginning to form new compounds. At its highest temperature, the raw mix is fired at around 1450 degrees Celsius. During the heating of the raw mixture, the aluminum and iron melts, and the calcium and silicone compounds begin to fuse to create calcium silicates. By the end of the process small pebble sized nodules of hard material known as clinker are created.
Grinding The Clinker
After cooling the clinker may be stored, or it may very quickly be ground into cement. During grinding, gypsum (calcium sulfate) is added to the clinker. This helps control the speed at which the resulting cement will set. Clinker is ground until the particles are so very fine that they could pass through a sieve capable of holding water. The actual size of particles varies somewhat, but the bulk are between 4 μm and 45 μm.
Finished Product – Portland Cement
The finished product, fine powder Portland cement is usually sold in bulk. Though bags of cement can occasionally be purchased, the majority of cement sales are in the form of large truckloads of cement which are delivered to the customer by a pressure fed hose which blows the cement powder from a truck into the customer’s own silo.
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