What is the mechanism of sulphate attack?

Sulphates react with Ca(OH)2 and C3A to form gypsum and ettringite, which expand and crack the concrete

Sulphates dissolve silica to form a gel

Sulphates lower pH and corrode steel directly

Sulphates leach Ca(OH)2 as white deposits only

What causes alkali-silica reaction?

Cement alkalis react with reactive silica in aggregate to form an expansive gel that swells with moisture

Chlorides react with aggregate to form rust

CO2 reacts with Ca(OH)2 in aggregate

Sulphates react with C3A in cement

What is the carbonation reaction and its effect?

CO2 + Ca(OH)2 -> CaCO3 + H2O, which lowers the pH and leads to corrosion

CO2 + C3A -> ettringite, causing expansion

CO2 + Fe -> FeCO3 which protects the steel

CO2 + CaCO3 -> Ca(OH)2, which raises pH

What is leaching and efflorescence?

Soft water dissolves Ca(OH)2 and carries it out, causing strength loss and white deposits

Sulphates forming ettringite in the paste

Bacteria making sulphuric acid in sewers

Asked 5/32
2081 Baisakh · 4 marks
2076 Chaitra · 6 marks
2076 Ashwin · 6 marks
2074 Ashwin · 6 marks
2072 Chaitra · 4 marks

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.

Describe the chemical causes of concrete deterioration.

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.

✓ Answer checked by Claude Sonnet 5.5

Physical and chemical causes of deterioration · Concrete Technology and Masonry Structure, chapter 6 · asked 5×
Physical causes of concrete deterioration · Concrete Technology and Masonry Structure, chapter 6 · asked 6×
Causes of deterioration; effects of steel corrosion · Concrete Technology and Masonry Structure, chapter 6 · asked 2×
Mechanical and physical causes of deterioration · Concrete Technology and Masonry Structure, chapter 6 · asked 6×
Causes of deterioration of stone · Civil Engineering Materials, chapter 2 · asked 4×

All Concrete Technology and Masonry Structure chapter 6 questions →

Chemical causes of concrete deterioration

Related questions