A hard, strong construction material comprising a mixture of sand, gravel, crushed rock, or other aggregate, held together, typically by a hardened paste of cement and/or lime. Several types of aggregate are used such as crushed stone, slag, cinders or gravel. Ancient Romans developed pozzolan cement about the 3rd century BCE. Modern concretes use various cements such as portland or hydraulic. Concrete is durable and relatively inexpensive, used for foundations, bridges, dams, walls, and highways. Concrete is strong in compression but weak in tension so it is often reinforced with steel bars or wire netting. Once a concrete mixture is stirred with water and poured into a mold, it is allowed to cure slowly over about a week. Stresses, such as vibration, freezing, and rapid drying, will diminish the strength and durability of the concrete. As it ages, concrete is subject to erosion, spalling, and pollution. Poor mixing can cause erosion. Spalling can be due to freeze-thaw cycles of moisture and ice, salt crystallization, or corrosion of steel reinforcements. Acid rain can deplete the natural alkaline reserve of fresh concrete.Een mengsel van zand, grind, puin of andere aggregaten dat bijeen wordt gehouden door een verhard cementmengsel.Mezcla de arena, gravilla, rocas trituradas u otro agregado, que se mantiene unida por una pasta de cemento endurecida.一種堅硬、高強度的建築材料，是由沙、礫石、碾碎的岩石或其他聚合物所集結構成，一般是以硬化的水泥和/或石灰糊做為黏結劑。在製造混凝土的過程中利用了數種聚合物，例如碾碎的石頭、熔渣、煤渣或礫石。古羅馬人大約在西元前三世紀的時候發展出火山灰水泥（pozzolan cement）；現代的混凝土則運用了各式各樣的水泥，例如波特蘭水泥或水硬性水泥。混凝土經久耐用，且價格相對較低，主要用來建造地基、橋樑、水壩、牆壁及公路。混凝土很耐壓，但是受到張力拉扯時卻很脆弱，所以常用鋼筋或鋼絲網來強化。一旦拌合的混凝土加了水攪拌並注入模具，就可在大約一星期內緩慢地固化。震動、結凍以及急速乾燥等造成的應力，將降低混凝土的強度和耐用度。混凝土老化之後，很容易遭受侵蝕、產生碎裂及受到污染：侵蝕可能是因為拌合品質不良，碎裂的成因可能是由濕氣和冰的反覆凍結和融化、鹽的結晶作用，或者是補強材料腐蝕所導致，另外酸雨會損耗新鮮混凝土中的天然鹼質。