Concrete

A composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens over time.

Description of what it is like

Initially a semi-liquid slurry that can be poured into molds. When cured, it becomes a rock-hard substance, grey in color, with high compressive strength but low tensile strength.

Origin and where to find it

• Artificial: It must be manufactured.
• History: Used extensively by Romans (Roman Concrete) using volcanic ash (pozzolana) and lime.

Minimum processing required

1. Preparation: Gather sand, gravel (aggregate), binder (lime or cement), and water.
2. Mixing: Thoroughly mix ingredients in correct proportions (e.g., 1 part binder, 2 parts sand, 3 parts gravel).
3. Pouring (Molding): Place into forms/molds immediately.
4. Curing: Keep moist for several days to allow chemical hardening (hydration).

Tools needed to work on it

• Mixing: Shovels, hoes, mixing box or trough.
• Forms: Wood planks to create molds.
• Tamping: Rods to remove air bubbles.

Common forms of use

• Mass Concrete: For dams or large foundations.
• Reinforced Concrete: Combined with steel (rebar) to add tensile strength (modern).
• Mortar: Finer mix for bonding bricks/stones.

Possible substitutes

• Stone: Harder to shape but natural.
• Brick: Requires firing but modular.
• Wood: For structures (but rot/fire risk).

Limitations and common failures

• Cracking: Shrinkage during drying or structural shifts.
• Setting Time: Must be used quickly after mixing.
• Temperature: Freezing during curing ruins it.

Risks and safety

• Chemical Burns: Wet concrete/lime is highly alkaline and can burn skin.
• Dust: Silica dust from dry mixing is harmful to lungs.
• Weight: Very heavy.

Related materials

• Lime: The binder.
• Primitive Cement: The historical version.
• Sand/Gravel: The aggregate.
• Mortar: Similar but without coarse aggregate.