Concrete definition (Entry 3 of 3) A mass is generated by the concretion or coalescence of distinct matter particles into a single substance. 2: a firm, strong construction material created by combining a cementing substance (such as Portland cement) and a mineral aggregate (such as sand and gravel) with enough water to make the cement to set and bond the entire mass. The resulting material is hard, durable, and flexible. Concretes are a class of building materials that can be molded into any shape and used in the construction of buildings and bridges. The three main types of concretes are plain concrete, mortarless brick, and stone masonry. All concrete must contain a minimum amount of water to be useful; the more water, the longer it takes to dry and the more likely it is to be contaminated with weeds, insects, or other organisms if it comes into contact with soil. Concretes are made by mixing together a cementing agent and an aggregate (the small pieces of rock or rubble that make up the bulk of the material) with enough water to form a paste. This mixture is then poured into a mold and allowed to harden into a stable structure. If you try to remove the mold after this process has taken place, the concrete will be very fragile and may break apart. The two main types of concretes for use in building projects are plain concrete and mortarless brick.