Performance of Concrete with Alkali-Activated Materials and Nanosilica in Acidic Environments

Owing to their high chemical resistance, alkali-activated materials (AAMs) may represent a promising option for repairing concrete structures affected by acidic media. However, AAMs have some technical limitations with respect to cast-in-situ applications because they require heat curing to mitigate delay in setting time, strength gain, and microstructural development at early ages. Therefore, this study investigated the performance of concrete with AAMs incorporating blends of fly ash, slag, and nanosilica cured at normal temperatures and exposed to very aggressive (10%) sulfuric acid environments. In addition to full immersion exposure for 18 weeks, a combined exposure was implemented to simulate field conditions involving alternating wetting-drying and freezing-thawing cycles combined with sulfuric acid attack. The evaluation criteria were based on visual assessment, neutralization depth, mass loss, and bond strength to substrate concrete in repair prototypes. In addition, the alterations of the microstructure of specimens were analyzed by mineralogical, thermal, and microscopy studies. The results showed that concretes prepared with fly ash-based AAMs with a small amount (6%) of nanosilica or (10%) slag without heat curing showed improved durability in sulfuric acid environments in terms of reduced penetrability of acidic media, low mass loss, and increased bond strength to substrate concrete with time. Thus, such AAMs could be a viable option for cast-in-situ repair applications of concrete elements in acidic environments.