Bound water is water that is chemically or physically attached to cement or soil particles, while unbound (free) water can move freely between the particles. The distinction matters in both concrete technology and soil mechanics, because only unbound water evaporates, flows away, or can be squeezed out under pressure.
Bound and unbound water in concrete
When concrete is mixed, mixing water is added. Part of it reacts chemically with the cement:
- Chemically bound water — reacts with the cement during hydration and becomes part of the hardened cement paste. This water never leaves the concrete.
- Physically bound water — sits in the fine pores (gel water) and is difficult to remove.
- Unbound water — the surplus mixing water not needed for hydration. It evaporates as the concrete cures, leaving capillary pores behind.
Too much unbound water (a high water-cement ratio) leads to more pores, lower compressive strength, and greater shrinkage. That is why a low water-cement ratio is preferred.
Bound and unbound water in soil
The same distinction exists in the ground:
- Bound (adsorbed) water — clings as a thin film to clay and soil grains and does not move freely
- Unbound (free) water — the groundwater that can drain away under gravity and determines the water table
During the settlement and consolidation of soft soil, it is the unbound water that is squeezed out under the load; the bound water largely remains.
Related terms
- Hydration
- Mixing water
- Water-cement ratio
- Water table
- Consolidation
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