What is mixing water?
Mixing water is the water added to cement, mortar or concrete mix to make it workable. The amount of mixing water is critical: too much makes the mix weak, too little makes it unworkable.
Why is mixing water important?
Water plays a dual role in concrete and mortar:
- Hydration — water reacts chemically with cement, causing the mix to harden. This is the bound water.
- Workability — additional water makes the mix fluid enough to pour, cast or spread. This is the free water.
The catch: water that does not react with cement evaporates later, leaving voids. More water means more voids, and more voids means weaker concrete.
Application
Water-cement ratio (w/c ratio)
The relationship between water and cement is expressed as the water-cement ratio:
| Application | w/c ratio | Result |
|---|---|---|
| High-strength concrete | 0.40-0.45 | Hard, dense, durable |
| Standard concrete | 0.50-0.55 | Good workability |
| Weak concrete | 0.60+ | Porous, less strong |
A w/c ratio of 0.50 means: 50 litres of water per 100 kg of cement.
Mixing water for mortar
For bricklaying mortar and pointing mortar, the water quantity is less critical than for structural concrete, but still important:
- Too wet — the mortar slumps and produces weak joints
- Too dry — the mortar doesn’t bond well and is hard to work
- Just right — the mix stays on the trowel but spreads easily
Water quality
Mixing water must be clean:
- No salt water — salts attack reinforcement
- No organic matter — can interfere with cement hydration
- Tap water — is always suitable as mixing water
Related terms
- Concrete
- Mortar mix
- Reinforcement
- Cement
- Water-cement ratio
Learn more about building terms in our knowledge base at fredsdiyplans.com.