We are legit trader, supplier and exporter of Ordinary Portland Cement (OPC) in Maryland City, Maryland, United States. Portland cement (often referred to as OPC is the most common type of cement in general use around the world because it is a basic ingredient of concrete, stucco and most non-specialty grout. It usually originates from limestone. It is a fine powder produced by grinding Portland cement (more than 90%), a limited amount of calcium sulfate (which controls the set time) and up to 5% minor constituents as allowed by various standards such as the European Standard. In order to achieve the desired setting qualities in the finished product, a quantity (2-8%, but typically 5%) of calcium sulfate (usually gypsum or anhydrite) is added to the clinker and the mixture is finely ground to form the finished cement powder. This is achieved in a cement mill. The grinding process is controlled to obtain a powder with a broad particle size range, in which typically 15% by mass consists of particles below 5 Î¼m diameter, and 5% of particles above 45 Î¼m. The measure of fineness usually used is the \"specific surface area\", which is the total particle surface area of a unit mass of cement. The rate of initial reaction (up to 24 hours) of the cement on addition of water is directly proportional to the specific surface area. Typical values are 320â380 m2Â·kgâ1 for general purpose cements, and 450â650 m2Â·kgâ1 for \"rapid hardening\" cements.