Acid Attack - Mechanism
Concrete being very alkaline in nature, is extremely susceptible to acid attack. The mechanism for this process is very simple. The products of cement hydration are shown below.
CS + H C-S-H + CH
Calcium Silicate + Water Calcium Silicate Hydrate + Calcium Hydroxide
Acid attack is caused by the reaction of an acid and the calcium hydroxide portion of the cement paste which produces a highly soluble calcium salt by product. These soluble calcium salts are easily removed from the cement paste thus weakening the pastes structure as a whole. This basic reaction is shown below.
Acid X + CH CX + H
Acid + Calcium Hydroxide Calcium Salt + Water
More aggressive acids such as hydrochloric, acetic, nitric, and sulfuric acids produce calcium salts that are very soluble. Less aggressive acids such as phosphoric and humic acids produce calcium salts with a lower solubility. These low soluble salts can act as a partial inhibitor to the overall process by blocking tiny passage in the cement paste through which water flows. This reduces the amount of calcium salts that enter into solution and retard the overall process.
A more aggressive and destructive case of acid attack occurs when concrete is exposed to sulfuric acid. The calcium salt produced by the reaction of the sulfuric acid and calcium hydroxide is calcium sulfate which in turn causes an increased degradation due to sulfate attack. This process is illustrated below.
H2SO4 + Ca(OH)2 Ca(SO)4 + 2H2O
Acid + Calcium Hydroxide Calcium Sulfate+ Water (calcium sulfate product contributes to sulfate attack)
The dissolution of calcium hydroxide caused by acid attack proceeds in two phases. The first phase being the acid reaction with calcium hydroxide in the cement paste. The second phase being the acid reaction with the calcium silicate hydrate. As one would expect the second phase will not begin until all calcium hydroxide is consumed. The dissolution of the calcium silicate hydrate, in the most advanced cases of acid attack, can cause severe structural damage to concrete.