The ACI 318 Building Code defines cementitious materials as those that have cementitious value when used in concrete either by themselves, such as portland cement, blended hydraulic cements, or expansive cement, or in combination with fly ash (ASTM specification C618), raw or calcined natural pozzolans (ASTM C618), ground granulated blast-furnace slag (ASTM C989), or silica fume (ASTM C1240). Addition to a concrete mix of fly ash, silica fume, or slag decreases permeability, protects reinforcement, and increases strength. Concrete made with polymers, plastics with long-chain molecules, can have many qualities much superior to those of ordinary concrete. See also Sec. 4.