11.4
Acid Attack on Concrete
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Q1: How do acids damage concrete structures?
Acids dissolve the hydrated cement paste in concrete, causing softening and structural weakening. This chemical reaction occurs when water pH drops below 6.5, with accelerated deterioration below pH 4.5. The damage is particularly severe in environments like chimneys, sewers, and industrial settings where acidic conditions persist.
Q2: What pH levels cause significant concrete degradation?
Concrete begins to deteriorate when water pH falls below 6.5, but significant damage occurs at pH levels below 4.5. The severity of acid attack increases as acidity intensifies. Peaty water with over 60 parts per million of carbon dioxide can reduce pH to approximately 4.4, causing highly corrosive conditions.
Q3: How does carbon dioxide in water affect concrete?
Water containing 15 to 60 parts per million of carbon dioxide, such as melted ice water, can degrade concrete. Peaty water with over 60 parts per million of carbon dioxide is highly corrosive and can significantly reduce pH levels. The carbon dioxide dissolves in water to form carbonic acid, which attacks the cement paste.
Q4: Why does domestic sewage corrode concrete sewers?
Although domestic sewage is alkaline, it corrodes concrete sewers in warm conditions when anaerobic bacteria convert sulfur compounds into hydrogen sulfide. This hydrogen sulfide is then oxidized by bacteria into sulfuric acid, creating highly acidic conditions. The resulting acid attack weakens the sewer structure over time.
Q5: What role do hydrogen ions play in concrete acid attack?
The diffusion rate of hydrogen ions through the cement gel influences how quickly acid attacks concrete. This process accelerates significantly after calcium hydroxide has been dissolved and washed away, removing a protective barrier. Faster hydrogen ion diffusion leads to more rapid concrete deterioration and structural compromise.
Q6: How can calcium hydroxide be protected from acid attack?
Calcium hydroxide can be stabilized with diluted sodium silicate, which forms protective calcium silicates in the concrete pores. These calcium silicates provide a chemical barrier against acid penetration. This preventive treatment helps maintain concrete integrity in acidic environments.
Q7: What surface treatments prevent acid attack on concrete?
Coal-tar pitch, rubber or bituminous paints, and epoxy resins have proven effective for protecting concrete from general acid attacks. These surface treatments create a physical barrier that prevents acidic water from penetrating the cement paste. Applying these coatings is particularly important in high-risk environments like sewers and industrial facilities.