---
title: "What is a Concrete made of?"
description: "Stone and sand glued by cement paste — and it hardens by a chemical reaction with water, not by drying. Every component, the material it uses, and the constraint tha"
url: https://atoneplace.net/made-of/concrete/
site: "At One Place"
---

# What is a Concrete made of?

**What is a Concrete made of?: Stone and sand glued by cement paste — and it hardens by a chemical reaction with water, not by drying.**

Concrete cures rather than dries, and the difference is practical: it will set perfectly well underwater, and keeping a fresh slab damp for a week makes it substantially stronger than letting it dry in the sun. It keeps gaining strength for years, with the 28-day figure being a convention rather than an endpoint. It is also enormously strong in compression and weak in tension, which is why steel reinforcement exists and why cracks appear where a slab is bent. Cement manufacture releases carbon dioxide twice over — from the fuel and from the limestone itself — and accounts for a widely cited 7-8% of global emissions, which is why blended cements using slag and fly ash are now standard.

## Component by component

*Every material is a choice, and the third column is the constraint that drove it.*

| Part | Material | Why that material |
| --- | --- | --- |
| coarse aggregate | crushed rock or gravel, typically 10-20 mm | Stone is the cheapest, stiffest and most dimensionally stable part; the goal is to use as much of it as the mix will carry. |
| fine aggregate | sharp sand | It fills the spaces between the stones so the expensive cement paste only has to fill what is left. |
| cement | Portland cement — limestone and clay burnt to clinker at about 1450 °C, ground with gypsum | Its calcium silicates react with water to grow interlocking crystals, which is the glue; the gypsum is there to stop it flash-setting. |
| water | clean water, ideally at a water-to-cement ratio near 0.45 | Water is a reactant, and any water beyond what the reaction needs leaves pores behind — which is why a sloppy mix is a weak mix. |
| admixtures | plasticisers, air-entrainers, retarders, accelerators | A plasticiser buys workability without adding water, which is the only way to get a pourable mix that is also strong. |

## Other building materials objects

- [Glass](https://atoneplace.net/made-of/glass/) — Sand melted with soda ash and limestone — three cheap minerals, floated flat on molten tin.
- [MDF](https://atoneplace.net/made-of/mdf/) — Wood pulled apart into individual fibres, mixed with resin and wax, and pressed hot into a board.
- [Road asphalt](https://atoneplace.net/made-of/asphalt/) — About 95% crushed stone and sand, glued together by roughly 5% bitumen from the bottom of an oil refinery.
- [Brick](https://atoneplace.net/made-of/brick/) — Clay and shale moulded and fired until the particles partly fuse — a manufactured rock.
- [Mirror](https://atoneplace.net/made-of/mirror/) — A sheet of ordinary glass with a microscopically thin silver layer on the back, sealed under paint.
- [Plywood](https://atoneplace.net/made-of/plywood/) — Thin wood veneers glued in alternating grain directions — the cross-lamination is the entire idea.

## Common questions

### What is a Concrete made of?

Stone and sand glued by cement paste — and it hardens by a chemical reaction with water, not by drying.

### What is the coarse aggregate of a Concrete made of?

Crushed rock or gravel, typically 10-20 mm. Stone is the cheapest, stiffest and most dimensionally stable part; the goal is to use as much of it as the mix will carry.

### What is the fine aggregate of a Concrete made of?

Sharp sand. It fills the spaces between the stones so the expensive cement paste only has to fill what is left.

## Related pages

- [Glass](https://atoneplace.net/made-of/glass/)
- [MDF](https://atoneplace.net/made-of/mdf/)
- [Road asphalt](https://atoneplace.net/made-of/asphalt/)
- [Brick](https://atoneplace.net/made-of/brick/)
- [Mirror](https://atoneplace.net/made-of/mirror/)
- [Plywood](https://atoneplace.net/made-of/plywood/)
- [Pencil](https://atoneplace.net/made-of/pencil/)
- [Tennis ball](https://atoneplace.net/made-of/tennis-ball/)
- [Banknote](https://atoneplace.net/made-of/banknote/)
- [Golf ball](https://atoneplace.net/made-of/golf-ball/)
- [Paper](https://atoneplace.net/made-of/paper/)
- [Soccer ball](https://atoneplace.net/made-of/soccer-ball/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)