What is concrete made of?

Aggregate

60–75%share of mix volume

Water

14–21%share of mix volume

Portland cement

7–15%share of mix volume

Air

0.5–8%entrapped or entrained

Short version

• Four ingredients: cement, water, fine aggregate (sand), coarse aggregate (gravel or crushed stone).
• Aggregate is most of the mix by volume; cement paste is the smallest and most expensive part.
• Cement is one ingredient in concrete, not a synonym for it.
• The ratio of water to cementitious material sets the strength and durability, not the bag count.
• Admixtures and supplementary cementitious materials are in nearly every ready-mix load today.

What are the ingredients in concrete?

Four materials make up ordinary concrete, and every one of them has a specification behind it. Portland cement is manufactured to ASTM C150, or blended cement to ASTM C595. Fine aggregate is sand and coarse aggregate is gravel or crushed stone, both graded and tested to ASTM C33. Water has to be clean enough not to interfere with hydration or corrode the reinforcement.

Air is the ingredient people forget. Some air is entrapped during mixing no matter what you do, and in freeze-thaw climates a deliberate air-entraining admixture creates billions of microscopic bubbles that give freezing water somewhere to expand. Air entrainment is required for ACI 318 exposure classes F1, F2, and F3.

Typical proportions by volume look like this. Treat them as the shape of a normal mix rather than a recipe, because a mix designer moves every one of these numbers to hit a specific strength, workability, and exposure requirement.

Is cement the same thing as concrete?

Cement is an ingredient; concrete is the finished material. Portland cement is a fine gray powder produced by burning limestone and clay at roughly 2,700 °F (1,480 °C) and grinding the resulting clinker with gypsum. On its own it does nothing structural. Mixed with water, it forms a paste that hardens through hydration, and that paste coats and locks the aggregate together.

Calling a driveway a “cement driveway” is a habit rather than an error anyone gets hurt by, but the distinction matters when you buy material. A bag labeled portland cement contains only the powder and will not make usable concrete without sand and stone added. A bag labeled concrete mix contains cement and aggregate already blended, and only needs water.

The same confusion shows up in cost conversations. Cement is the expensive component per pound, so mix designs that replace part of the cement with a supplementary material can lower both the price and the carbon footprint.

What do the aggregates actually do?

Aggregates carry the volume and hold the concrete dimensionally still. Cement paste shrinks as it dries and hardens; stone does not. By filling most of the volume with graded aggregate, a mix designer limits how much shrinkage the paste can impose on the finished slab, which is the difference between a driveway with hairline cracks and one with wide ones.

Gradation matters as much as quantity. ASTM C33 sets the size distribution requirements for concrete aggregate so that small particles fill the gaps between large particles. A well-graded aggregate needs less paste to fill its voids, and less paste means less water, less cement, less shrinkage, and lower cost all at once.

Maximum aggregate size is chosen for the job. Slabs and footings commonly use ¾-inch or 1-inch stone. Thin sections, heavily reinforced members, and pumped concrete use smaller stone so the mix can move around the steel and through the hose.

Aggregate also has to be clean and durable. Clay coatings weaken the bond to the paste, and reactive aggregates can trigger alkali-silica reaction years later.

Why does the water-cement ratio matter more than the cement content?

The ratio of water to cementitious material — w/cm — is the single strongest predictor of concrete strength and durability. Hydration consumes a fixed proportion of water; everything beyond that stays in the mix as free water, eventually evaporates, and leaves behind capillary pores. More pores mean lower strength, higher permeability, and easier passage for chlorides that corrode reinforcing steel.

That is why adding water to a truck to make placing easier is such a costly habit. The extra water raises the w/cm, and the strength the mix was designed for is gone before anyone screeds it.

The right way to get a wetter, more workable mix is chemistry rather than a hose. Water-reducing and high-range water-reducing admixtures, specified under ASTM C494, increase slump without increasing water content. That is the whole reason they exist.

ACI 318 sets maximum w/cm limits for concrete in aggressive exposures, and those limits are frequently more restrictive than the strength requirement alone would be.

What are admixtures and supplementary cementitious materials?

Admixtures are chemicals added in small doses to change how concrete behaves, and supplementary cementitious materials are powders that replace part of the portland cement. Nearly every ready-mix load placed in the United States contains at least one of them.

The common admixture families under ASTM C494 include water reducers, retarders that slow set in hot weather, accelerators that speed it in cold weather, and high-range water reducers. Air-entraining admixtures are specified separately and are what make concrete survive freeze-thaw cycling.

Supplementary cementitious materials include fly ash, slag cement, and silica fume. They react with byproducts of cement hydration to form additional binder, which typically lowers permeability and improves long-term durability while cutting the amount of portland cement in the mix. NRMCA’s Concrete in Practice guidance covers how they change set time and early strength gain, which is the tradeoff a contractor feels on pour day.

Blended cements do the same thing at the plant instead of the batch. Type IL portland-limestone cement, produced under ASTM C595, is approved by all 50 state DOTs plus the District of Columbia and produces up to roughly 10 percent less carbon dioxide than ordinary portland cement.

Common mistakes

• Buying portland cement when you needed concrete mix — the bag of gray powder has no aggregate in it and will not make a slab.
• Adding water at the truck to make placing easier — every gallon raises the water-cement ratio and permanently lowers strength and durability.
• Judging a mix by its bag count or cement content — the water-cement ratio and the aggregate gradation decide the outcome.
• Skipping air entrainment in a freeze-thaw climate — ACI 318 exposure classes F1, F2 and F3 require it, and without it the surface scales.
• Using dirty or unwashed aggregate — clay and silt coatings break the paste-to-stone bond, which is where concrete gets its strength.