Concrete is a composite: cement and water form a reactive paste that binds sand and gravel or crushed stone. In typical mixes, aggregate occupies most of the volume; cement is only one ingredient, not another name for the finished material. The exact proportions vary with the mix’s intended use.
What is concrete made of?
Concrete combines a cementitious binder, water, fine aggregate and coarse aggregate. The binder and water form a paste; the aggregates make up the bulk of the material and are bound together as the paste hardens.
- Cementitious binder: Usually portland or blended hydraulic cement, the reactive powder in the paste.
- Water: Starts the hydration reaction and makes the fresh mix workable enough to place.
- Fine aggregate: Usually natural or manufactured sand.
- Coarse aggregate: Usually gravel or crushed stone. Its size and type are chosen for the concrete element and its purpose; grading and cleanliness matter.
Some mixes also include supplementary cementitious materials (SCMs), chemical admixtures, fibers or entrained air. These additions serve particular performance or placement needs and are not required in every mix. Fly ash and slag, for example, can supplement or partially replace portland cement in a designed mix.
Concrete and cement are not the same thing
Cement is a binder ingredient. When mixed with water, it forms the paste that binds the aggregates. Concrete is the complete composite of that paste and aggregate, sometimes with other additions. The American Cement Association describes concrete as cement-and-water paste mixed with aggregates in its concrete FAQ.
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What proportions are typical?
The American Cement Association gives these broad typical proportions by volume; they describe a range of mixes, not a universal recipe:
| Ingredient or component | Typical share by volume |
|---|---|
| Cement | 7–15% |
| Aggregate | 60–75% |
| Water | 14–21% |
| Entrained air | Up to an additional 8% |
Other sources express the proportions differently. The Federal Highway Administration’s 1997 technical guidance puts aggregate at 60–75% of concrete volume and 80–85% by mass. The American Cement Association’s undated FAQ says cement typically makes up 10–15% of concrete by volume. These figures are broad descriptions, not mix-design instructions.
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How does concrete harden?
Concrete hardens through hydration, a chemical reaction between cement and water—not simply because water evaporates. Hydraulic cements form solid hydration products. As those products develop around cement particles and connect with one another or adhere to aggregate, they create a hardened structure.
Why water content matters
Water has two roles: it enables hydration and helps the fresh mix flow during placement. The water-to-cement ratio is an important mix-proportioning variable. A mix needs enough workability to be placed and consolidated, but unnecessary water can compromise the paste and the concrete’s eventual performance. Water quality matters too: excessive impurities can affect setting, strength, staining, corrosion, volume stability and durability.
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Why curing matters
Curing helps maintain moisture so hydration can continue after placement. Water curing or moisture-retaining methods help prevent premature drying. Most hydration and strength gain occurs during the first weeks or month, but hydration can continue more slowly for much longer. There is no universal rule that concrete is “fully cured” at 28 days: strength development depends on cement composition and fineness, mix proportions, moisture and temperature.
What is cement made from?
Cement itself is manufactured from raw materials that commonly include limestone, marl and clay, which supply calcium, silicon, aluminum and iron. The materials are heated to form clinker, then the clinker is ground with gypsum or other ingredients to make cement. These are inputs to cement manufacturing; they are not a description of all the materials in concrete.
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Why concrete mixes differ
A concrete mix is designed around the element being built and the required placement and performance. Aggregate properties and binder composition affect performance alongside the water-to-cement ratio. SCMs, fibers and chemical admixtures may be selected for a particular need; they are not universal ingredients. Placement and consolidation help reduce voids and defects, while curing supports hydration after placement.
For deeper technical detail on materials, mixture proportioning, curing and testing, the American Cement Association lists Design and Control of Concrete Mixtures, 17th edition.
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