Concrete is made of four things: portland cement, water, sand (fine aggregate), and gravel or crushed stone (coarse aggregate). Cement and water form a paste that coats the sand and stone and, through a chemical reaction called hydration, hardens into solid rock. Aggregate makes up the bulk — about 60 to 75 percent of the mix.
A common mix-up: cement is not concrete. Cement is the gray powder that acts as the glue; concrete is the finished material you get after you combine cement with water and aggregate. See the concrete vs cement guide for more on the difference.
Cement: the binder
Portland cement is a fine powder of ground, kiln-fired limestone and clay. It is only about 10 to 15 percent of the mix by volume, but it is the active ingredient: mixed with water it forms a paste that chemically bonds everything together. More cement (relative to aggregate) generally means stronger concrete.
Water: the trigger
Water is not just there to make the mix workable — it drives hydration, the reaction that hardens the paste. The water-to-cement ratio is the single biggest strength lever: too much water leaves the cured concrete porous and weak, which is why pros keep it low (around 0.5) and add only what they need to place the mix.
Aggregate: the bulk and strength
Sand fills the small voids; gravel or crushed stone provides the structural mass. Together they make up most of the volume and most of the strength — the hardened paste simply glues this stone matrix together. Well-graded aggregate (a range of sizes) packs tightly and uses less cement to fill the gaps.
Air and admixtures
Real-world concrete also contains a little trapped air, and often admixtures — small chemical additions that tune the mix. Air-entraining agents create tiny bubbles that help concrete survive freeze-thaw; accelerators speed setting in cold weather; retarders slow it in heat; water reducers keep it workable with less water. To skip the proportions entirely, use bagged mix and the bag calculator below.
Common questions
What are the four ingredients in concrete?
Portland cement, water, sand (fine aggregate), and gravel or crushed stone (coarse aggregate). Admixtures and trapped air are usually present too.
Is cement the same as concrete?
No. Cement is the powdered binder; concrete is the finished material made by mixing cement with water, sand, and stone.
What makes concrete strong?
A low water-to-cement ratio, enough cement, well-graded aggregate, and proper curing. Too much water is the most common cause of weak concrete.