Concrete is a mixture of cement, sand (fine aggregate), small stone or gravel (coarse aggregate) and admixtures. When water is mixed in with this product, it activates the cement, which is the element responsible for binding the mix together to form one solid mass. Cement is crushed limestone blended with other raw materials, such as shale and sand, ground into a powder and then heated in a rotary kiln. This process produces a cement clinker which is mixed with gypsum and ground further to produce cement.
Concrete as a structural element has many applications and hence there are many different ratios or recipes for the constituents to be mixed. In example a medium strength mix, known as C20 mix, means that it will attain a strength equivalent to withstanding compression of 20 Nt/mm2 after 28 days. The proportions of materials for this mix are 1× cement, 2× fine aggregate and 4× coarse aggregate. The ideal water to cement ratio is 0.55 but, since most of the times aggregates are damp or even wringing wet, water mass must be calculated carefully to ensure that mix does not become too sloppy. In addition, the volume of water and the admixtures used can dictate the strength and the workability of the finished mix, and hence the proportioning task is very important. The mixing method and the mixer condition used to process the concrete are of high importance since they, in some measure, determine concrete's microstructure and hence its overall performance.