The influence of fly ash and neutral sodium silicate on concretes submitted to sulfuric acid attack

Abstract

The service life of wastewater treatment plants is often impaired by the biogenic deterioration caused by sulfuric acid (H2SO4). This study used fly ash (as a Portland cement replacement) and different contents of neutral sodium silicate (0%-10%) as mitigating solutions. One group was immersed in lime-saturated water for 182 days, and the other was submitted to a 0.5% H2SO4 solution for the same interval. The microstructural analysis (SEM/XRD) confirmed the consumption of calcium-based compounds during the pozzolanic reaction, the Na2SiO3 hydration, and the H2SO4 attack. The neutralized depths measured after spraying phenolphthalein reiterated this trend. The H2SO4 front progressed inwards, promoting gradual peeling and the formation of gypsite. The slow diffusion of H2SO4 did not impair the early-age compressive strength, but the depletion of the alkaline reserve hindered the results in the long term. Therefore, Na2SiO3 was not a viable mitigating solution against the H2SO4 attack, contrary to fly ash.