It's simple. Concrete is the finished product, such as sidewalks, foundations, and the surface of many roads. Concrete contains sand, gravel and cement. Cement is the special hardening ingredient (the gray powder) that makes concrete harden. Cement is usually made of 60% lime (limestone), 25% silica, 5% alumina, and 10% other materials, such as gypsum and iron oxide. (Content provided by the Mineral Information Institute, © 2002 www.mii.org)
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.
Raw materials are finely ground and proportioned for the desired chemistry. The material is then blended and heated to approximately 2700 degrees fahrenheit in rotary kilns. The heated product, called clinker, is allowed to cool, and then is ground to a fine powder.
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.
Although all portland cement is governed by the ASTM C150 specification, all cements are most definitely 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 pyro processing and variations in finish grinding and grinding additives used. Because consistency is one of the most important criteria for cement manufacturing, producers of portland cement invest millions of dollars in plant equipment and quality control to provide the most consistent product possible.
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.