The carriage of cement at sea has grown steadily in volume until today it provides a major source of merchant ship employment.There are various types of cement requiring a variety of ingredients based on supplies of chalk or limestone, clay, ores, gypsum and flyash, (once a waste product from coal-burning power stations). But from a seaborne trade perspective, the product can be divided into bagged and bulk, the latter being capable of sub-division into finished cement and clinker. Whether for bagged or for bulk, nearly all carriage terms are negotiated on adapted versions of the GENCON Charterparty Form. Many nations possess cement factories, and thus the business takes the form of a complex trade of criss-crossing shipments of raw materials to these factories; partly refined cement (clinker) transported between cement works perhaps hundreds of miles apart; and the final, vital product for construction projects being delivered around the world in bags or in bulk. This final product is a fine grey powder which contracts as much as 12% from an aerated condition following handling, to a non-aerated state once it has settled, and which can be very dusty. Obviously it should be kept scrupulously dry so as to avoid solidifying, and conventional, non-sealed cargo-handling work cannot be conducted in rainy conditions.