When fungus and then bacteria colonize the surface of new concrete, the acid attack begins first by a chemical neutralization reaction from the calcium aluminate phases. Calcium aluminate hydrates (Ca + Al + H2O) are dissolved and react with H2SO4 to precipitate both calcium sulfate (CaSO4.2H2O) and hydrated alumina (AH3). As shown in the graph, the basic neutralization capacity of a unit weight of calcium aluminate is significantly greater than Ordinary Portland Cement by around 40% for low pH, i.e. more acid is needed to dissolve the same quantity of Calcium Aluminate Cement (CAC). Keep in mind that SewperCoat® is made up of 100% Calcium Aluminate chemistry, with the final result that 100% of the product is reactive, not just the cement binder portion (generally around one third of the solid content for an average repair mortar),but the calcium aluminate aggregate as well. This first barrier is capable of providing a several times longer service life.
With neutralization capacity around 40% higher than OPC, each gram of SewperCoat® neutralize more biogenic acid
The result of the first acid attack, the hydrated alumina AH3, precipitates at the surface of SewperCoat® in the form of a dense amorphous layer. This hydrated alumina layer is very important because it is chemically stable down to a pH around 3-4. In fact, AH3 precipitation creates a physical barrier of a few hundred microns to the sulfuric acid penetration as long as the pH remains higher than 3-4. This is the second line of defense of SewperCoat®, effective as long at the pH of the biofilm remains higher than 3-4.
Once biogenic corrosion starts, alumina gel precipitates to form an acid resistant solid barrier down to pH 3-4
Once the continuous bacterial activity has generated enough sulfuric acid to lower the pH below the hydrated alumina solubility point, AH3 dissolves and liberates alumina ions onto the biofilm adhering to the surface. Because the biofilm is very thin, the local Alumina ion concentration rises rapidly. The presence of free alumina ions have a direct "bacterio-static effect", i.e. it drastically slows down the bacterial activity down to a near "dormant state". Bacteria metabolism is impaired in such a way that the acid production is drastically reduced. This third barrier is the most important because stopping the sulfuric acid production means that the whole corrosion process is stopped or drastically reduced. SewperCoat® outstanding durability is not due to some "acid-proof" properties, but rather to its unique interaction with the bacteria ecosystem.
When finally alumina gel begins to dissolve, aluminum ions released have a "bio-static" effect on Thiooxidans bacteria, stopping the production of biogenic sulfuric acid.
The essential outcome of the bacterio-static effect of SewperCoat® is that the surface pH stabilizes around the hydrated alumina solubility threshold, i.e. around 3-4. This means that once the hydrated alumina barrier is created, it will last for a very long time. Even as some hydrated alumina is dissolved over time to maintain the bacterio-static conditions, the acid concentration remains 100 to 1000 times lower compared to typical corroding sewer surfaces where a pH of 1 is often measured. A higher pH implies much less aggressive conditions and a much longer life of the hydrated alumina barrier.
"SewperCoat® Total Barrier" is put in evidence by the surface pH that is never getting as low as epoxy or OPC surfaces