Microbial Induced Corrosion: Part 2

Not all concrete corrosion is a result of the biogenic process described in Part 1 of this blog series. Corrosion can result from chloride attack, internal reactions, or as a result of other acids. So the first step in the diagnosis is determining a root cause of the concrete deterioration.

In this blog, I would like to explain how an acid attacks the concrete. There are some compounds that are formed from the concrete/acid reaction that further react to form other compounds. Having a basic understanding of this link in the chain of events will be valuable in determining the best defense for preventing concrete deterioration.

During the hydration of cement, a compound is formed that provides the desirable properties of hardened concrete. This compound is calcium silicate hydrate (C-S-H). Typical hydrated cement forms about 50% C-S-H. Another compound is also formed in this phase, calcium hydroxide, which composes 15%-25% of paste by mass (Kosmatka, et.al, 2002). Some of the chemical reactions occur within minutes of cement hydration, other reactions take days, weeks, and even months. By day 3-7 of the curing process, the mass of the concrete is primarily composed of three compounds; C-S-H, calcium hydroxide, and calcium aluminoferrite hydrates.

Calcium hydroxide is hydrated lime. It does not provide any of the desirable properties to the concrete. Calcium hydroxide will easily react with acids and other compounds. The reaction with carbon dioxide forms calcium carbonate resulting in what is referred to as concrete carbonation. When sulfuric acid reacts with calcium hydroxide, the result is calcium sulfate and water. Calcium sulfate is a naturally occurring compound also known as gypsum. If you recall the previous blog, the evidence of MIC is a whitish foamy mass on the concrete: wet gypsum. Think of what drywall looks like when it is wet.

What happens next is the process that causes the concrete to disintegrate. In the hydration process of concrete, gypsum in the clinker reacts with Tricalcium Aluminate to form ettringite. Ettringite increases in volume, which is acceptable before the concrete hardens. When gypsum is formed on the walls of hardened concrete, it reacts chemically with the aluminates present in the mass of the hardened paste. This reaction forms ettringite, the expansive gel. As the ettringite reacts with water and expands it causes cracking and spalling to occur. The chain of events allows more penetration, access to calcium hydroxide, and a snowball of destruction.

Sources cited:
Kosmatka, S., Kerkhoff, B., and Panarese, W. (2002) Design and Control of Concrete Mixtures, 4th ed., p. 40-41. Portland Cement Association: Chicago, IL