Assessing the Acid Resistance of Alkali-Activated Slag Concrete Containing Microsilica

This study investigates the impact of partially replacing binder with microsilica on the long-term durability of one-part alkali-activated slag concretes (AASCs) under extreme acidic conditions, focusing on sulfuric and nitric acid environments. The research involved the substitution of 5%, 10%, and 15% of the binder with microsilica, followed by extensive evaluation of the concrete’s compressive strength, electrical resistivity, mass and volume changes, and acid penetration depth over 210 days of exposure. The results indicate that replacing 10 wt.% of the binder with microsilica improves resistance to nitric acid, with compressive strength reaching 65 MPa after 210 days, an 18% increase compared to control samples. However, in sulfuric acid, deterioration was observed, with compressive strength reducing to 22 MPa at 15 wt.% microsilica replacement. Mass loss in sulfuric acid increased from 28% (0% microsilica) to 66% (10% microsilica), while nitric acid exposure resulted in only a minor reduction (8% to 6%). Acid penetration depth increased to 16.7 mm in sulfuric acid at 10% microsilica but decreased to 2.5 mm in nitric acid at 10% microsilica. The best performance was observed at 10 wt.% microsilica content, which significantly improved resistance to a nitric acid attack. However, samples containing microsilica exhibited greater deterioration in sulfuric acid environments compared to those without microsilica. Further, microstructural analysis revealed that exposure to the nitric acid environment altered the chemical composition of the binder gel, as indicated by changes in the relative atomic ratios; however, no detrimental new phases were formed within the concrete matrix. In the sulfuric acid environment, replacing microsilica with the binder led to increased gypsum formation in the samples, a primary factor contributing to concrete degradation. These findings emphasize the dual effect of microsilica on the durability of AASCs in acidic environments, where its influence may either enhance or reduce performance depending on the type of acid.