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How does hydrogen sulphide attack concrete?

Hydrogen sulfide (H₂S) attacks concrete primarily through a chemical process known as microbial-induced concrete corrosion (MIC), or more specifically, sulfuric acid attack. This problem is especially prevalent in environments such as wastewater treatment plants, sewers, and other systems where hydrogen sulfide gas can be generated. Here’s how the process works in detail:

1. Production of Hydrogen Sulfide (H₂S)

In environments where organic matter decomposes anaerobically (in the absence of oxygen), sulfate-reducing bacteria (e.g., Desulfovibrio) convert sulfates present in wastewater into hydrogen sulfide gas. This gas can accumulate in sewer systems or treatment tanks, often under stagnant or low-oxygen conditions.

2. H₂S Gas Rises and Reacts with Water

Hydrogen sulfide is a gas that, when it comes into contact with moisture or water, can dissolve and form sulfurous acid (H₂SO₃). This weak acid in itself is not very aggressive, but it’s a precursor to a more harmful process.

3. Bacterial Conversion of H₂S into Sulfuric Acid (H₂SO₄)

Certain aerobic bacteria (e.g., Thiobacillus species) thrive in environments where hydrogen sulfide is present. These bacteria, which are found on the damp surfaces of the sewer, convert hydrogen sulfide gas into sulfuric acid (H₂SO₄) in the presence of oxygen. This occurs in the upper parts of sewer pipes, manholes, and other infrastructure exposed to H₂S.

4. Sulfuric Acid (H₂SO₄) Attacks the Concrete

Once sulfuric acid is formed, it reacts aggressively with the calcium compounds in concrete. Concrete is primarily made of calcium hydroxide (Ca(OH)₂), which is highly alkaline and provides the concrete’s durability and resistance. Sulfuric acid reacts with calcium hydroxide and other cementitious materials, forming calcium sulfate (gypsum), a weak, brittle material, and other byproducts like ettringite. These chemical reactions weaken the concrete over time.

Key Reactions:

• Sulfuric acid reacts with calcium hydroxide: H2SO4+Ca(OH)2→CaSO4(gypsum)+2H2OH₂SO₄ + Ca(OH)₂ \rightarrow CaSO₄ (gypsum) + 2H₂OH2​SO4​+Ca(OH)2​→CaSO4​(gypsum)+2H2​O
• Formation of Ettringite: When calcium sulfate (gypsum) reacts with hydrated aluminate phases in the cement, it forms ettringite, a bulky crystal that expands and causes internal stress within the concrete. This further weakens the structure.

5. Physical Deterioration of Concrete

The formation of gypsum and ettringite causes the concrete to become soft, brittle, and porous. As the sulfuric acid continues to erode the cement matrix, the concrete starts to crack, spall, and lose its structural integrity. Over time, this leads to severe surface deterioration, loss of strength, and eventually structural failure if left unchecked.

6. Other Contributing Factors

• Environmental conditions: Areas with poor ventilation or stagnant water promote higher concentrations of H₂S, accelerating the attack.
• pH of concrete: Acidic environments lower the pH of the concrete, making it more vulnerable to attack. Normally, concrete’s alkaline nature protects it from corrosion, but sulfuric acid lowers the pH significantly.
• Moisture: The presence of water is critical as it facilitates the dissolution of H₂S and subsequent acid formation. Wet-dry cycles can worsen the damage.

Summary of Effects:

• Surface degradation: Concrete surfaces become rough and eroded.
• Structural weakening: Over time, concrete loses its load-bearing capacity.
• Cracking and spalling: The internal stresses from gypsum and ettringite formation cause cracking.
• Increased porosity: The cement matrix becomes porous, accelerating further damage.

Conclusion:

Hydrogen sulfide attacks concrete through a chain of biochemical and chemical reactions that lead to the formation of sulfuric acid. This acid severely degrades the calcium compounds in the concrete, causing a reduction in strength and eventual structural failure. Effective mitigation strategies involve improving ventilation, using acid-resistant materials, and applying protective coatings to prevent hydrogen sulfide from coming into contact with the concrete.