Degradation of concrete by flue gases from coal combustion

The effect of flue gases from coal combustion on the concrete shell of a power plant stack was analyzed and the damage to the concrete was evaluated. The compressive and tensile strengths of concrete were determined by rebound hammer test and pull-off test on the site. Samples of concrete taken from various zones of the stack shell were analyzed in detail. The examination techniques used included chemical analysis, water suspension analyses, XRD, thermal analysis, SEM and EDS. It was found that the flue gases and the acid condensate, acted very aggressively at an elevated temperature and caused severe degradation of the inside surface zone of the concrete shell. The binding material in this zone was converted into sulfur-bearing compounds. Gypsum was identified as the dominant compound in the degraded zone of concrete in the upper parts, while anhydrite in the lower parts of the stack. Carbonated zone was located below the clearly delimitated sulfated zone of the concrete. The aggressive environment in the stack did not affect the internal zones in the concrete.