Geopolymer Solution for Sulfuric Acid Corrosion at Wastewater Plants

WASTEWATER · MIC · CONCRETE · SULFURIC ACID ATTACK

Concrete in wastewater systems can face an unusually aggressive form of deterioration: microorganisms help create sulfuric acid directly on the concrete surface.

Researchers at Graz University of Technology studied whether geopolymer-based concrete could offer better resistance to microbiologically influenced corrosion (MIC) in wastewater infrastructure than conventional ordinary Portland cement concrete.

Why Wastewater Concrete Can Deteriorate So Quickly

In wastewater collection and treatment systems, sulfate-reducing microorganisms can generate hydrogen sulfide under oxygen-deficient conditions.

The hydrogen sulfide can leave the wastewater and enter the air space above it. On damp concrete surfaces, sulfur-oxidizing microorganisms can then convert the hydrogen sulfide into sulfuric acid.

Biogenic sulfuric acid corrosion can develop when hydrogen sulfide is oxidized by microorganisms growing on moist concrete surfaces.

The researchers reported that heavily affected surfaces can develop very low pH conditions—below pH 2 in severe cases—along with biofilm growth and extensive formation of secondary minerals such as gypsum.

Together, acid dissolution, mineral alteration, moisture, and microbial activity can cause rapid loss of concrete material.

Why Ordinary Portland Cement Is Vulnerable to Acid Attack

Conventional Portland cement concrete hardens through hydration reactions that produce calcium-containing phases responsible for much of the material’s structure and binding capability.

Those calcium-rich hydration products can be vulnerable to strong acids. When sulfuric acid reaches the concrete surface, the acid can dissolve or chemically alter portions of the cement paste.

As deterioration progresses, the surface can soften, lose material, form expansive or weak reaction products, and become increasingly rough and damaged.

What Is Geopolymer Concrete?

Geopolymer and other alkali-activated concretes use a different binder chemistry from ordinary Portland cement.

Materials containing aluminosilicates—such as certain fly ashes, slags, or metakaolin—can be activated with alkaline solutions to form hardened binder systems based largely on aluminosilicate networks rather than conventional Portland cement hydration products.

The TU Graz researchers investigated whether this different chemistry could improve resistance to the aggressive acidic conditions found in some wastewater environments.

Why Geopolymers May Resist Acid Differently

According to the researchers, acidic attack on some geopolymer systems can leave behind a silica-rich altered layer instead of simply dissolving the binder in the same manner as conventional cement paste.

That altered layer may remain relatively stable and can slow additional acid transport into the material. In that case, permeability through the acid-affected layer becomes an important factor controlling the rate of continuing attack.

The researchers also reported that aluminosilicate gel could remain after decalcification in some tested alkali-activated materials, helping preserve portions of the network structure even under low-pH exposure.

Laboratory Testing Showed Promising Acid Resistance

The researchers referenced laboratory studies using techniques such as X-ray diffraction, electron microscopy, and mechanical testing to examine the acid resistance of geopolymer materials.

In one reported example, a metakaolin-based material exposed to dilute hydrochloric acid for 28 days retained much of its network structure and binding capability despite chemical attack.

Could Geopolymers Also Reduce Microbial Colonization?

The research team suggested that the altered chemistry of geopolymer surfaces may make bacterial settlement more difficult under some conditions.

They also proposed that antimicrobial cation additives might eventually provide another tool for limiting microbial colonization.

The researchers were clear, however, that additional research would be required before those approaches could be treated as established field solutions.

Why the Researchers Were Not Ready to Declare Victory

The researchers emphasized that additional field durability data and standardization would be needed before broad use of geopolymer concrete in wastewater infrastructure.

Despite the promising acid-resistance results, the researchers identified several unresolved issues that could affect practical field use.

Alkali Leaching Could Create a Different Durability Problem

One concern identified in the source was the ability of geopolymers to exchange ions with their environment.

Under MIC exposure, alkalis may leach from the geopolymer and contribute to formation of salts at the surface. This type of surface accumulation is commonly described as efflorescence.

The researchers also noted that losing alkalis could reduce internal pH. If the material contains conventional reinforcing steel, that raises an important question: whether long-term changes in alkalinity could eventually affect the passive condition that normally helps protect the reinforcement.

Why Lower Permeability Matters

The researchers also associated geopolymer materials with lower permeability than some conventional cement systems.

That can be important because aggressive species must move through the material before they can attack deeper regions. A denser microstructure can therefore reduce transport rates even when the surface is exposed to aggressive chemistry.

However, permeability depends strongly on the actual formulation, curing, water content, activator chemistry, workmanship, cracking, and service conditions. It should be measured or specified for the material being considered rather than assumed from the binder category alone.

Potential Sustainability Benefits

The research also highlighted the potential environmental appeal of some geopolymer systems. Certain formulations can incorporate industrial byproducts or alternative aluminosilicate feedstocks and may reduce reliance on conventional Portland cement.

The actual carbon footprint, however, depends on the full material formulation, activator production, transportation, curing, service life, maintenance requirements, and end-of-life assumptions. Sustainability therefore needs to be evaluated on a project-specific lifecycle basis.

What the Research Really Suggests

The 2018 research did not establish geopolymer concrete as a universal replacement for Portland cement in wastewater systems.

Instead, it identified a promising materials-development direction: use binder chemistry that is less vulnerable to biogenic sulfuric acid and investigate whether the resulting material can maintain both chemical resistance and structural durability over the required service life.

That requires balancing acid resistance against constructability, reinforcement performance, mechanical behavior, dimensional stability, field variability, standardization, and lifecycle cost.