IMAGE: Microbiologically influenced corrosion (MIC) can cause significant damage to concrete wastewater assets. Photo courtesy of TU Graz.
According to researchers from the Graz University of Technology (TU Graz) (Graz, Austria), geopolymer-based concrete (GPC) offers superior resistance to acids, abrasions, and are less permeable than OPC-based (Ordinary Portland Cement) materials, which are formed via cement hydration reactions. These calcium-rich hydration products are generally acid dissolvable, the researchers explain, thus making OPC prone to corrosion from microbial-induced acid attack. They say their research shows that concrete made from geopolymers can have a greater lifespan than conventional concretes, thus extending the service life of wastewater assets.
Microbiologically influenced corrosion (MIC) often corrodes the conventional types of concrete used in wastewater treatment plants at a rate of 1 cm or more per year,” says a professor at TU Graz’s Institute of Applied Geosciences. “Accordingly, the concrete elements can be destroyed in a matter of only a few years, causing significant damage to wastewater systems. Closing the manhole covers and looking the other way is not the answer.”
Acid attack process in OPC
With OPC, microbial-induced acid corrosion in wastewater treatment facilities usually results from a sequence of biogenic sulfate reduction reactions, followed by reoxidation. Initially, the researchers explain, sulfate in pressurized pipelines or standing water is reduced by bacteria under oxygen-free conditions, thus forming hydrogen sulfide (H2S).
From there, the H2S gas escapes into the sewer air and diffuses into sewer pipes and manholes. Autotrophic bacteria on the walls of the concrete then oxidize the H2S into sulfuric acid (H2SO4), which reacts with the concrete’s construction elements.
“This leads to the vigorous formation of a biofilm on the surface of the concrete, a reduction of the pH value to below two,” says a research team leader and professor at Tu Graz’s Institute of Molecular Biosciences. “In other words, [it is] highly acidic, and [leads to] extensive formation of new minerals, mainly in the form of gypsum. The combination of these processes results in the rapid destruction of the concrete.”
Advantages of geopolymer technology
In contrast to OPC, which is formed through hydration reactions, GPC is produced through the polycondensation of aluminosilicates. In this ion-exchange reaction process, materials like blast furnace slag, fly ash, or metakaolin are mixed with alkaline reagent solutions such as sodium- or potassium-soluble silicates to form a hardened, concrete-like texture. These geopolymer materials also offer a lower carbon footprint, potentially making them a “greener” alternative.
According to the researchers, the framework vacancies formed by the dissolution of aluminum are mostly reoccupied by the silicates. This results in an amorphous, highly siliceous framework that is relatively hard but brittle. This undissolved corroded layer can effectively inhibit the process of further corrosion by acting as a barrier to the transport of acid protons.
“Thus, compared to the neutralization capacity achieved by dissolution of hydration products, the permeability of the acidified [geopolymer] layers governs the rate of further ingress of acids,” the researchers write. In essence, they continue, the microorganisms that trigger the initial oxidation process in OPC corrosion cases and the formation of H2SO4 are unable to settle on the surface.
The alkali-activated specimens produce a layer of aluminosilicate gel, which studies have shown remains in place even after decalcification and offers chemical stability even at a low pH, they explain. X-ray diffraction (XRD), electron microscopy, and mechanical tests have been conducted on the acid resistance of a metakaolin material exposed to 0.01 M of hydrochloric acid (HCl) for 28 days, finding that the network structure remained practically intact while still showing excellent binding properties.
TU Graz believes the addition of antimicrobial cation additives could further help in reducing bacterial colonization, though it acknowledges more research is needed.
Further MIC testing needed
Going forward, the researchers say more thermodynamic data are needed on the performance of geopolymer concretes in the field before commercial advances can be made on the technology. Nonetheless, even considering the relative uncertainty with the GPC process, the researchers believe the vulnerability of traditional concrete to MIC makes further research a priority.
“While the overall process mechanisms and environmental parameters responsible for MIC are fairly well understood, to date, no commercially available concrete can satisfactorily withstand the adverse conditions in such aggressive environments over its projected operating life,” they write. “Accordingly, further research should focus on how to constrain concrete-microorganism interactions.”