Artificial stone made of a mixture of cement, aggregate (hard material), and water. In addition to its potential for immense compressive strength and its ability, when poured, to adapt to virtually any form, concrete is fire-resistant and has become one of the most common building materials in the world. The binder usually used today is portland cement. The aggregate is usually sand and gravel. Additives called admixtures may be used to accelerate the curing (hardening) process in low temperature conditions. Other admixtures trap air in the concrete or slow shrinkage and increase strength.