--- title: "Chemical causes of concrete deterioration" subject: "Concrete Technology and Masonry Structure" chapter: 6 key: "semester_5/concrete_technology_and_masonry_structure:chemical-causes-deterioration" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/chemical-causes-deterioration site: BCT Engineering --- # Chemical causes of concrete deterioration Concrete Technology and Masonry Structure · Chapter 6: Concrete durability - Asked in: 2081 Baisakh, 4 marks; 2076 Chaitra, 6 marks; 2076 Ashwin, 6 marks; 2074 Ashwin, 6 marks; 2072 Chaitra, 4 marks (asked 5 times) - Video: [Top 8 types of concrete deterioration and their prevention || concrete defects || #civilogy](https://www.youtube.com/watch?v=ozieevP7I5I) — civilogy - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/chemical-causes-deterioration - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-6 ## Question Describe the chemical causes of concrete deterioration. ## Short answer Chemical causes include sulphate attack (ettringite expansion), alkali-aggregate reaction (gel swelling), acid dissolution, chloride-induced corrosion, carbonation lowering pH, leaching, sea water combined attack, and biological acid production. Dense concrete with low w/c and appropriate cement type controls the rate. ## Answer Chemical causes are the reaction of cement paste or aggregate with substances from outside or inside the concrete. | Cause | Reaction | Effect and control | |---|---|---| | **Sulphate attack** | Sulphates (soil, groundwater, sea) react with $\text{Ca(OH)}_2$ and C3A to form gypsum and ettringite | Expansion, cracking, softening. Use sulphate-resistant cement (IS 12330), PPC, low w/c, dense concrete (IS 456 Table 4) | | **Alkali-aggregate (silica) reaction** | Alkalis of cement react with reactive silica of aggregate to form an expansive gel which swells with moisture | Map cracking, pop-outs. Use non-reactive aggregates, low-alkali cement, fly ash | | **Acid attack** | Acids dissolve $\text{Ca(OH)}_2$ and C-S-H into soluble salts | Loss of material, softening. Dense concrete, protective coatings | | **Chloride attack** | Chlorides destroy the passive film of steel | Corrosion of reinforcement. Limit chloride (IS 456 Table 7, 0.6 kg/m$^3$ for RCC) | | **Carbonation** | $\text{CO}_2 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}$ | Lowers pH, then corrosion. Low w/c, cover | | **Leaching and efflorescence** | Soft water dissolves $\text{Ca(OH)}_2$ and carries it out | Loss of strength, white deposits | | **Sea water** | Combined action of sulphate, chloride, crystallisation, abrasion | Use PPC/slag, w/c 0.45 or less, cover | | **Biological** | Bacteria produce sulphuric acid in sewers | Surface erosion | The rate is controlled by permeability: the denser the concrete, the slower the attack. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the mechanism of sulphate attack? A. Sulphates dissolve silica to form a gel B. Sulphates lower pH and corrode steel directly C. Sulphates react with Ca(OH)2 and C3A to form gypsum and ettringite, which expand and crack the concrete D. Sulphates leach Ca(OH)2 as white deposits only Answer: C. The expansive products cause cracking and softening; use sulphate-resisting cement, PPC and low w/c. (Key point: Sulphate attack) 2. What causes alkali-silica reaction? A. Chlorides react with aggregate to form rust B. CO2 reacts with Ca(OH)2 in aggregate C. Sulphates react with C3A in cement D. Cement alkalis react with reactive silica in aggregate to form an expansive gel that swells with moisture Answer: D. Gel swelling causes map cracking and pop-outs; use non-reactive aggregate, low-alkali cement or fly ash. (Key point: Alkali-aggregate reaction) 3. What is the carbonation reaction and its effect? A. CO2 + C3A -> ettringite, causing expansion B. CO2 + Ca(OH)2 -> CaCO3 + H2O, which lowers the pH and leads to corrosion C. CO2 + Fe -> FeCO3 which protects the steel D. CO2 + CaCO3 -> Ca(OH)2, which raises pH Answer: B. Loss of alkalinity near the bar breaks the passive film. (Key point: Carbonation reaction) 4. What is leaching and efflorescence? A. Sulphates forming ettringite in the paste B. Chloride moving to the steel C. Soft water dissolves Ca(OH)2 and carries it out, causing strength loss and white deposits D. Bacteria making sulphuric acid in sewers Answer: C. Calcium hydroxide is removed by flowing soft water, leaving white deposits at the surface. 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