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Role and quality of water in concrete

Short answer

Water hydrates cement, provides workability, and aids curing. It must be potable, with pH ≥6 and limits on chlorides (500 mg/L for RCC), sulfates, and solids. Impurities like chlorides corrode steel, sulfates cause expansion, sugar retards setting, and suspended solids weaken concrete.

What is the role of water in concrete? What quality of water is to be used for concreting, and what are the impurities of water and their effects on concrete properties?

Role of water in concrete

  • Hydration: it reacts with cement to form the binding C-S-H gel (about 0.23 w/c is needed chemically, and about 0.38 in total).
  • Workability: it lubricates the mix so that it can be mixed, placed and compacted.
  • Curing: it keeps the concrete moist so that hydration continues and cracking is avoided.
  • Washing aggregates and cleaning equipment.

Excess water leaves capillary pores on drying, which lowers strength and durability.

Quality of water (IS 456:2000, cl. 5.4)

Water should be fit for drinking (potable) and free from harmful amounts of oils, acids, alkalis, salts, sugar, organic matter and silt. Potable water is generally satisfactory.

  • pH not less than 6.
  • Permissible limits for mixing water (IS 456 Table 2): organic solids 200 mg/L, inorganic solids 3000 mg/L, sulphates (as SO3_3) 400 mg/L, chlorides 2000 mg/L for plain concrete and 500 mg/L for RCC, suspended matter 2000 mg/L.
  • Sea water: not for RCC or prestressed concrete (chlorides corrode steel); it may be used for plain concrete only.
  • Tests in case of doubt: the average 28-day compressive strength of 150 mm cubes made with the water should not be less than 90% of that of cubes made with distilled water (IS 456 cl. 5.4.1.2); and the initial setting time with the water should not be less than 30 minutes and should not differ by more than ±30 minutes from that with distilled water (cl. 5.4.1.3).

Impurities and effects

ImpurityEffect on concrete
Suspended solids (silt, clay)Increase water demand, reduce strength
Acids / alkalisAttack cement, change setting, reduce strength; efflorescence
ChloridesCorrode reinforcement; may accelerate set
SulphatesSulphate attack, expansion and cracking
Sugar (even 0.05%)Retards or prevents setting
OilCoats particles, reduces bond and strength
Organic matter / algaeEntrains air, reduces strength
Carbonates/bicarbonatesChange setting time; reduce strength at high amounts

✓ Answer checked by Claude Opus 5.5 (high effort)

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