OPC 42.5
OPC 42.5 is a general use grade of ordinary Portland cement. 
Replacing the once popular OPC 33 in general cement work whilst maintaining its duties as a medium strength cement, OPC 42.5 is commonly used for both construction and pre-cast work. OPC 42.5 can therefore be found in a multitude of locations, from residential buildings to underground pipes.
OPC Grades
There are several different OPC grades, one of which is OPC 42.5. Ordinary Portland cement grades are determined by the compressive strength which is obtained on the 28th day after pouring. OPC 42.5 must have a minimum compressive strength of 42.5 MPa, or 425 kg/cm2 on the 28th day after it is poured in order to qualify as being OPC 42.5. It should be noted that the 42.5 MPa minimum requirement is just that, a minimum requirement, and that most cements exceed this strength.
The two most commonly utilized grades of OPC on the market today are OPC 42.5 and OPC 52.5. There is no chemical difference in composition between these two grades of cement, however OPC 42.5 powder is more coarse than OPC 52.5 powder.
Manufacturing OPC 42.5
OPC 42.5 is made in the same way that all Portland cement is made. First a raw mix of lime, silica, iron, and aluminum is prepared from rock sources which have been crushed and pounded to the size of small pebbles.
Rock is usually mined locally, and any components that are lacking for the making of Portland cement are then added afterwards. Though cement is essentially powdered rock, the various materials must be present in precise quantities in order for high quality Portland cement to be produced. In order to ensure that the correct mix is obtained at the beginning of the cement making process, controlled amounts of individual raw materials are fed from silos onto a conveyor belt, which takes them to the raw mill.
Once in the raw mill, the raw materials are ground up to create what is called the ‘raw mix’. Once the raw mix has been formulated, one of two manufacturing processes can follow. The raw mix is either dried by hot air if a dry process is to be used, or mixed with water to make a raw mix slurry if a wet process is to be used.
Once made, the raw mix is then blended in a blending chamber, and chemically analyzed to ensure that the precise ratios of raw materials are present. It is important that the raw mix is mixed thoroughly to form a completely homogeneous mixture, and that all elements in the mix are as close to being the same size as is possible. Calcium and silicone make up the bulk of the mix because they are the ingredients which later form the calcium silicates that comprises a 66% of Portland cement. Smaller amounts of iron and aluminum are present to act as flux during the firing process, lowering the temperature at which calcium silicates will form.
After blending, the raw mix travels to the kin, a large rotating cylinder which is heated gradually over the length of the kiln, with the temperature rising to between 1400 and 1450 degrees Celsius at its hottest point. As the raw mix grows hotter, aluminum and iron melt, and calcium silicates begin to form. By the end of the firing process, small nodules of an entirely new compound are formed. These nodules are called ‘clinker’.
The clinker is cooled after it leaves the kiln, and then ground with gypsum, an additive which controls the speed at which the cement will set. The grinding process continues until the clinker nodules are a very fine powder. If the clinker is ground relatively coarsely, (relatively coarsely being in the neighborhood of around 40 microns in diameter) the result of this grinding process is OPC 42.5
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