When cement is mixed with water, the paste that is formed is fluid, or plastic, for a short period of time. During this time the paste material may be reformed or remolded. As the chemical reaction between water and cement continues, the paste becomes stiffer and ultimately hardens. The early period of hardening is referred to as the setting time.
Typically, cement is ground to very fine particle sizes to enhance its ability to react with water and to increase fluidity in its plastic state. To a point, a smaller particle size improves the mixing characteristics, and strength development of the paste. Finer ground cements tend to set or react quicker than coarser ground products.
Raw materials are finely ground and proportioned for the desired chemistry. The material is then blended and heated to about 1400 degrees celcius in rotary kilns. The heated product, called clinker, is allowed to cool, and is then ground to a fine powder.
Portland cement is a fine, powder material produced by burning, at high temperatures, a mixture of lime, alumina, iron, and silica in definite proportions. The material is typically mixed with water, sand, and gravel to produce concrete. Cement reacts with water to harden into calcium silicate hydrates resulting in stone-like properties.
Although all portland cements are governed by the British specification, all cements are not the same. First, cement can be manufactured to meet different criteria or different types. Second, even the same types of cement will vary from manufacturer to manufacturer and there may even be variation from a given manufacturer depending upon when the material was made. Items which can cause cement variation include raw material variations, chemical variations in kiln feed, variations in processing and variations in grinding and grinding additives used. Because consistency is one of the most important criteria for cement manufacturing, producers of cement invest ££millions in plant equipment and quality control to provide the most consistent product possible.