Laboratory assessment of the contribution of aggressive to concrete chemical compounds to the degradation of Portland cement-based materials during anaerobic digestion

Anaerobic digestion allows renewable energy to be produced through the degradation of bio-waste. The process, which is of economic and ecological interest, is implemented industrially in concrete digesters. Bio-waste is a complex medium with a composition that can vary in time and space. It contains several chemical compounds, including volatile fatty acids, ammonium, and CO2, which are aggressive towards concrete and compromise its durability. The individual effects of the different compounds on concrete are significantly different. To move toward a better design of concrete intended for the building of biogas digesters, this paper aims to understand the mechanisms and intensity of alteration associated with the different components of biowaste and their contribution to the total deterioration. Ordinary Portland cement pastes were immersed for 16 weeks in six synthetic solutions made of the three metabolites, taken alone or in mixes. The mass variations of the specimens, the pH, and the concentration of the chemical elements in solution were monitored over time. The microstructural, chemical and mineralogical changes of the samples were analysed by scanning electron microscopy, electron probe micro-analysis and X-Ray diffraction analyses and showed phenomena of dissolution, leaching and carbonation. The results show that the acetic acid solution was the most aggressive, in accordance with its pH value, and had a predominant effect in mixed solutions, whereas sodium bicarbonate solution induced carbonation and showed a protective effect. Interestingly, despite its reputed high aggressiveness, ammonium nitrate did not have a major impact in mixed solutions.