The difference between concrete and cement is that concrete contains cement plus sand and gravel. It is the cement that binds the sand and gravel or crushed rock together to form what we call concrete. Concrete then, is more than just cement.
Cement is made from limestone, calcium, silicon, iron and aluminum, plus lesser amounts of other ingredients. This mixture is heated in large kilns to about 2,700 degrees Fahrenheit (1,482 degrees Celsius) to form a product called clinkers, which roughly resemble marbles. These are ground to a powder and gypsum is added, creating the gray flour-like substance known as cement. When water is added to cement, a chemical process occurs as it hydrates, allowing it to harden.
Cement was a building material of both the Greeks and the Romans, but when Rome fell the widespread use of cement was lost for hundreds of years. Then in the 1700's Joseph Aspdin, a British citizen, discovered that if he added clay to limestone then superheated it, the resulting mixture would set anywhere, even under water. He patented what he called Portland cement, which is still the cement in use today. Dams, oilrigs, dock supports, bridges, lighthouses and various other structures are made with Portland cement.
Concrete is a masonry material that uses cement to bind together crushed stone, rock and sand. Cement makes up about 10% - 15% of the total mass of concrete. Although concrete is very strong, it requires reinforcement if used as a building material because of its poor tensile strength, or ability to withstand the horizontal forces or sway buildings must endure from wind or earthquakes. In short, concrete doesn't stretch or extend very well in reaction to cross-tensions. Concrete can be reinforced by pouring it over long steel rods, then letting it set. An even more effective method is called prestressed concrete which follows the same basic method as reinforced concrete except that the steel bars are stretched taut with cables before pouring the concrete. Once the concrete dries, the tension is released and the rods compress the concrete for even greater strength.
Concrete sets faster or slower according to how much gypsum is added to the mixture. Therefore if a contractor schedules concrete to be placed (poured) at a certain time, make sure you are available! The set time of concrete can be accelerated by adding calcium chloride, or retarded by adding sugar. These compounds affect the development of the hardening crystals that form as concrete sets.
Due to the high content of rock, concrete is extremely durable, attractive and is often used around pool areas, for sidewalks, driveways, roads, and of course skyscrapers among many other uses like subways, silos and lamp posts. The ingredients in both cement and concrete are among the most abundant on earth, and neither contains byproducts. Both extend renewable resources due to long life, and can be recycled to make future cement or concrete. These factors make concrete and cement environmentally friendly, though the industry surrounding the manufacture and delivery of cement contributes significantly to carbon dioxide emmissions.
Of all construction materials, concrete is the most frequently used. About 6 billion tons of concrete is produced every year -- approximately one ton for each person on the planet. It is estimated that one-half to two-thirds of our infrastructures are made of concrete. With an extensible life of 50,000 years, concrete is sure to be around for as long as we need it.