Concrete

Concrete is a composite material made by binding aggregate together with a fluid cement that hardens into a solid. It is the second most used substance on Earth, after water; the most widely employed building material; and the most manufactured material globally. Cement-based concrete is distinct from asphaltic concrete, which uses a bituminous binder and is less rigid and durable.

When dry Portland cement, aggregate, and water are combined, they form a fluid slurry that can be poured and shaped. The cement undergoes an exothermic chemical reaction with water called hydration, hardening over several hours into a solid matrix that binds the aggregate into a durable, stone-like material. This setting time allows concrete to be cast in forms and worked with various tools. Ambient temperature significantly affects the hydration rate. Additives such as pozzolans or superplasticizers are often mixed in to alter the wet mix’s properties, speed up or slow down curing, or modify the final material. Most structural concrete is poured with embedded reinforcing materials like steel rebar to provide tensile strength, creating reinforced concrete.

Before the invention of Portland cement in the early 1800s, lime-based binders like lime putty were common. Today, the vast majority of concrete uses Portland cement, though other hydraulic cements, such as calcium aluminate cement, are sometimes employed. Non-cementitious concretes also exist, including asphalt concrete (with a bitumen binder, often used for roads) and polymer concretes that use polymers as binders.

Concrete differs from mortar: concrete contains both coarse and fine aggregate and is a building material itself, whereas mortar contains only fine aggregate and serves mainly as a bonding agent for bricks, tiles, and masonry units. Grout, another related material, lacks coarse aggregate and is pourable or thixotropic, used to fill gaps between masonry or around pre-placed coarse aggregate; some concrete manufacturing and repair methods involve pumping grout into voids to form a solid mass in place.

**Etymology** The word *concrete* derives from the Latin *concretus* (meaning compact or condensed), the perfect passive participle of *concrescere*—from *con-* (together) and *crescere* (to grow).

**History**

**Ancient times** Concrete floors have been found in the royal palace of Tiryns, Greece, dating to roughly 1400–1200 BC. Lime mortars were used in Greece, including Crete and Cyprus, by 800 BC. The Assyrian Jerwan Aqueduct (688 BC) employed waterproof concrete. Many ancient structures used concrete. The Nabatean traders, who controlled oases in southern Syria and northern Jordan from the 4th century BC, pioneered small-scale production of concrete-like materials. By 700 BC, they discovered the self-cementing properties of hydraulic lime. They built kilns to produce mortar for rubble masonry houses, concrete floors, and underground waterproof cisterns, keeping the cisterns secret to thrive in the desert. Some of these structures survive today.

**Classical era** The Romans used concrete extensively from 300 BC to AD 476. Their concrete (*opus caementicium*) was made from quicklime, pozzolana, and pumice aggregate. Its widespread use—a key event in architectural history known as the Roman architectural revolution—freed Roman construction from the limits of stone and brick, enabling revolutionary designs in structural complexity and scale. The Colosseum in Rome was built largely of concrete, and the Pantheon features the world’s largest unreinforced concrete dome. Roman concrete, laid in arches, vaults, and domes, hardened quickly into a rigid mass, avoiding many internal thrusts and strains that troubled builders of similar stone or brick structures. Modern tests show *opus caementicium* had compressive strength similar to modern Portland-cement concrete (about 20 MPa or 2,800 psi), but its tensile strength was far lower due to the absence of reinforcement, and its application differed: Roman concrete was hand-layered with rubble aggregate rather than poured as a fluid homogeneous mix, and it lacked integral reinforcing steel, relying solely on concrete bonding for tension resistance. The long-term durability of Roman concrete is attributed to pyroclastic rock and ash in the mix. The crystallization of strätlingite (a complex calcium aluminosilicate hydrate) during formation, merging with similar calcium-aluminium-silicate-hydrate structures, gave Roman concrete greater fracture resistance than modern concrete—sometimes described as self-healing—and made it significantly more resistant to seawater erosion than modern concrete.