Deterioration of cement-based materials in wastewater treatment plant facilities- A comparison between sewage gases and sewage liquid exposure environments

Cement-based materials are preferred in constructing various infrastructures including wastewater treatment plants (WWTPs) due to their durability, low processing cost, and watertightness. However, the increasingly reported maintenance in many cement-based WWTP facilities is raising concerns about their performance in aggressive environments such as WWTPs. Wastewater contains different chemical substances (e.g., sulfates, organic compounds, and nitrates) which have known deterioration effects on cement performance. Still, less is known about the deterioration process occurring on cement-based materials exposed above sewage line and submerged in sewage liquid.

This study aimed to understand the difference in cement deterioration between submerged and above sewage structures in WWTP facilities. This was an in-situ experiment that involved 23 specimens of ordinary Portland cement. The specimens were exposed to above sewage in the pumping station and below sewage in sand-trap structures. The specimens were exposed for different durations:30, 75, and 24 days. After exposure, specimens were analyzed. The analysis involved material physical observation (using stereo microscopy), morphology (SEM), and mineralogical analysis (using XRD).

The results of our study show that (a) specimens exposed to sewage gases had a notable physical change compared to those submerged in sewage liquid in sandtrap locations. (b) SEM-SE images show that specimens from sewage gases had massive spongy and prismatic needle crystals, whereas, in sewage liquid, specimens showed little or no such morphologies. (c) These crystals observed in samples from sewage gases in the pumping station were confirmed by XRD to gypsum (CaSO₄.2H₂O), ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O), and thaumasite (CaSiO₃·CaCO₃·CaSO₄·15H₂O) minerals. These minerals are secondary minerals in cement and are characterized by high volume expansion and their presence in hydrated concrete results in volume expansion crack formation. These results suggest that cement-based concrete above sewage line are more prone to deterioration than those submerged in sewage liquid.