The short answer
Cement is a fine gray powder, usually portland cement, made by heating limestone, clay and similar materials in a kiln and grinding the result. On its own it isn't a building material. Concrete is the hard, stone-like material you get when cement and water form a paste that binds sand and gravel together as it hardens. Saying "a cement sidewalk" is like calling a cake "flour".
| Cement | Concrete | Mortar | Grout | |
|---|---|---|---|---|
| What it is | Powder (the glue) | Cement + water + sand + gravel | Cement + sand (+ lime) + water | Cement + sand (+ fine gravel), runny |
| Used for | An ingredient only | Slabs, footings, driveways, posts | Laying bricks and blocks | Filling block cores and gaps |
| Strength | None until mixed | Strongest, carries loads | Bonds units together | Fills and locks in place |
Which bag do I buy?
- Concrete mix (cement, sand and gravel already blended): for slabs, steps, fence and deck posts. Just add water. See the concrete bag calculator.
- Mortar mix: for laying blocks and bricks. See the mortar calculator.
- Portland cement on its own: only if you're mixing concrete or mortar from scratch with your own sand and gravel.
Why concrete gets stronger over time
Concrete doesn't harden by drying out. The cement reacts chemically with water (hydration), which is why concrete should be kept damp while it cures. It reaches most of its design strength in about 28 days and keeps slowly gaining strength after that.
How much concrete do I need?
Work out the volume with the concrete calculator or the cubic yard calculator, then price it with the concrete cost calculator.
Source: Portland Cement Association: How concrete is made.
Every worked example on this page was checked with Calcist's own engine and an independent reference.