11.4 : Acid Attack on Concrete

When acids come into contact with concrete, they initiate a chemical reaction that dissolves the hydrated cement paste. This process leads to softening and structural weakening of the concrete. This issue is commonly observed in environments such as chimneys, sewers, and industrial settings. The severity of the damage increases as the pH of the water interacting with the concrete drops below 6.5. In particular, a pH under 4.5 can cause significant concrete damage.

The rate at which hydrogen ions diffuse through the cement gel also impacts the rate of acid attack, especially notable after the calcium hydroxide in the concrete has been dissolved and washed away. Additionally, water with carbon dioxide levels between 15 to 60 parts per million, like melted ice water, can degrade concrete, while peaty water with over 60 parts per million of carbon dioxide can be highly corrosive, possibly reducing pH to around 4.4.

Even domestic sewage can corrode concrete in sewers, particularly under warm conditions when anaerobic bacteria convert sulfur compounds into hydrogen sulfide. To prevent these types of acid attacks, It's beneficial to stabilize calcium hydroxide with diluted sodium silicate, which forms protective calcium silicates in the concrete pores. As well as this, surface treatments such as coal-tar pitch, rubber or bituminous paints, and epoxy resins have proven effective in combating general acid attacks.