What Is Concrete Made Of?

The four core ingredients

• Cement — the binder, most commonly Portland cement, that reacts chemically with water
• Water — triggers the hydration reaction that hardens the cement paste
• Fine aggregate — typically sand, filling smaller voids and adding workability
• Coarse aggregate — typically crushed stone or gravel, providing bulk and much of the mix's compressive strength

The water-cement ratio

The ratio of water to cement (by weight) is one of the most important variables in a concrete mix. A lower water-cement ratio generally produces a stronger, less permeable concrete, but also a stiffer, harder-to-work mix. Too much water weakens the finished concrete even though it makes the wet mix easier to pour and finish — a common on-site mistake.

Admixtures

Beyond the four core ingredients, many modern concrete mixes include chemical admixtures — air-entraining agents (for freeze-thaw resistance), water reducers (allowing a lower water-cement ratio while keeping workability), retarders or accelerators (adjusting curing time), and others tailored to the project's needs.

Why proportions matter for estimating

When you're calculating how much concrete you need with BuildFactor's Concrete Calculator, you're calculating the total mixed-product volume — the cement, water and aggregate already combined. If you're mixing on-site from bags of cement plus separately purchased aggregate, you need each ingredient's quantity individually, which is a different (and more involved) calculation than ordering ready-mix.