Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time.

Concrete is the second-most-used substance in the world after , and is the most widely used building material.

Its usage worldwide, ton for ton, is twice that of steel, , plastics, and aluminum combined.

Globally, the ready-mix concrete , the largest segment of the concrete market, is projected to exceed $600 billion in revenue by 2025.

This widespread use results in a number of environmental impacts. Most notably, the production process for cement produces large volumes of greenhouse emissions, leading to net 8% of global emissions.

Other environmental concerns include widespread illegal sand mining, impacts on the surrounding such as increased surface runoff or urban heat island effect, and potential public health implications from toxic ingredients.

Significant research and development is being done to try to reduce the emissions or make concrete a source of sequestration, and increase recycled and secondary raw materials content into the mix to achieve a circular economy.

Concrete is expected to be a key material for structures resilient to climate disasters, as well as a solution to mitigate the pollution of other industries, capturing wastes such as coal fly ash or bauxite tailings and residue.

When aggregate is mixed with dry Portland cement and , the mixture forms a fluid slurry that is easily poured and molded into shape.

The cement reacts with the through a process called concrete hydration that hardens over several hours to form a hard matrix that binds the materials together into a durable -like material that has many uses.

This time allows concrete to not only be cast in forms but also to have a variety of tooled processes preformed.

The hydration process is exothermic this means ambient temperature plays a significant role in how long it takes concrete to set.

Often, additives (such as pozzolans or superplasticizers) are included in the mixture to improve the physical properties of the wet mix, delay or accelerate the curing time, or otherwise change the finished material.

Most concrete is poured with reinforcing materials (such as rebar) embedded to provide tensile strength, yielding reinforced concrete.

In the past, lime based cement binders, such as lime putty, were often used but sometimes with other hydraulic cements, ( resistant) such as a aluminate cement or with Portland cement to form Portland cement concrete (named for its visual resemblance to Portland ).

Many other non-cementitious types of concrete exist with other methods of binding aggregate together, including asphalt concrete with a bitumen binder, which is frequently used for road surfaces, and polymer concretes that use polymers as a binder.

Concrete is distinct from mortar. Whereas concrete is itself a building material, mortar is a bonding agent that typically holds bricks, tiles and other masonry units together.