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

Microbial Induced Corrosion: Part 1

You just had your concrete septic tank pumped and the pumper noticed a wet chalky mass on the concrete above the water level. He power washed the foreign material off and found that the concrete is corroding. Now your upset and wonder what is happening to your septic system. Well, before you blame your wife for using all of those household chemicals, or blame your son for consuming so much junk food that their biological waste is destructive to concrete, consider the biological activity that occurs in the septic process first.

Waste in a septic tank is partially decomposed by bacteria and other naturally occurring processes. These bacteria live in the water and do not need oxygen to survive. The term for this type of bacteria is anaerobic. One of these species is Th. Desulfovibrio, a sulfate reducing bacteria(SRB), which converts the sulfate in the wastewater to hydrogen sulfide. The hydrogen sulfide is trapped below the scum layer in the septic tank. When there is turbulence, the hydrogen sulfide gas is released into the atmosphere above the waterline.

In the mean time, the natural environment is having an effect on the natural properties of the concrete. With a pH of 12.5-13.5, concrete is very basic. Water has a pH of around 7, and acids have a very low pH. Acids are reactive with the properties of concrete that provide this high amount of alkalinity. carbon dioxide, thiosulfuric acid, and other mild acids reduce the pH of the concrete to around 9. This process can take months or even years, depending on the concrete quality.

Once the concrete pH is near 9, a strain of the thiobacillus begins to colonize that is aerobic, or requiring oxygen. These are sulfate oxidizing bacteria (SOB), and they convert hydrogen sulfide into sulfuric acid. The weak sulfuric acid produced by this strain lowers the pH of the concrete until it dies off and another strain colonizes. Each strain of aerobic thiobacillus produces a stronger sulfuric acid than the previous one.

Eventually, a strain Thiobacillus Thiooxidan is present producing a 7% sulfuric acid that rapidly destroys the concrete. This strain is called Acidothiobacillus. At this point, concrete structures will loose 1/2" of mass per year. It can take as little as 2-3 years or as long as 10-15 years for this chain of events to reach this level of destruction depending on concrete quality.

While all concrete can be susceptible to this degredation, not all installations have the same environmental conditions that trigger the chain reaction. There is a significant amount of uncertainty as to what exact conditions must be present for this to occur. Some theories suggest a high amount of sulfur in the water supply, a high iron content in the water, very hard water, and chemicals introduced into the waste stream just to name a few. Sites that have natural gas or oil wells in the area seem to have a higher potential for the conditions. Research in this area is currently being funded and will provide some beneficial information for engineers when they plan a septic tank installation.

In the future parts of this series, I will explain how the sulfuric acid destroys the concrete, how concrete quality plays an important role in improving the resistance to corrosion, and finally the use of nano technology to break the chain of events that cause this biogenic process.