--- title: "Role and quality of water in concrete" subject: "Concrete Technology and Masonry Structure" chapter: 1 key: "semester_5/concrete_technology_and_masonry_structure:water-role-quality-impurities" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/water-role-quality-impurities site: BCT Engineering --- # Role and quality of water in concrete Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials - Asked in: 2081 Baisakh, 2+4 marks; 2068 Baisakh (old course), 3 marks; 2066 Bhadra (old course), 5 marks (asked 3 times) - Video: [Concrete Technology | Water | Water Quality and Permissible Limits](https://www.youtube.com/watch?v=b2QvJP1MjP0) — The Heller's TV - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/water-role-quality-impurities - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1 ## Question 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? ## 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. ## Answer #### 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 SO$_3$) 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 | Impurity | Effect on concrete | |---|---| | Suspended solids (silt, clay) | Increase water demand, reduce strength | | Acids / alkalis | Attack cement, change setting, reduce strength; efflorescence | | Chlorides | Corrode reinforcement; may accelerate set | | Sulphates | Sulphate attack, expansion and cracking | | Sugar (even 0.05%) | Retards or prevents setting | | Oil | Coats particles, reduces bond and strength | | Organic matter / algae | Entrains air, reduces strength | | Carbonates/bicarbonates | Change setting time; reduce strength at high amounts | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is NOT a role of water in concrete? A. Hydration of cement B. Increasing tensile strength by adding excess C. Workability D. Curing Answer: B. Excess water leaves capillary pores and lowers strength. (Key point: Roles) 2. IS 456 requires mixing water to be: A. Any water that looks clean B. Sea water for RCC C. Fit for drinking (potable) and pH not less than 6 D. Acidic with pH 4 Answer: C. Potable water is generally satisfactory. (Key point: Quality) 3. Sea water should not be used for RCC because: A. It reduces workability B. It is too hot C. It contains oil D. Chlorides corrode the steel Answer: D. Chloride corrodes reinforcement; allowed only for plain concrete. (Key point: Sea water) 4. Effect of sugar in mixing water: A. Retards or prevents setting B. Accelerates setting C. Increases strength D. Corrodes steel Answer: A. Even 0.05% sugar can retard or prevent setting. 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