pH sensor using aniline blue and functionalized nylon for monitoring the final stage of concrete carbonation

The construction industry is one of the most significant contributors to global warming, with 5% of the world’s CO₂ emissions coming from the manufacturing process of cement. Therefore, to slow the rate of global warming, efforts are needed to increase the lifespan of infrastructure via structural health monitoring and timely repair. Carbonation is often cited as a significant cause of the deterioration of concrete structures and is measured using a destructive testing in the current structural code. However, since this method is time-consuming, costly, and labor-intensive, the development of pH sensors has been required for monitoring carbonation. Most pH sensors developed so far were manufactured using a physical entrapment method, so they are vulnerable to leaching and cannot guarantee long-term durability. A few pH sensors ensuring durability through covalent immobilization have also been proposed, but they pose limitations since most of them can only monitor the early stage of carbonation. Therefore, this study sought to develop a pH sensor that can be used to measure the final stage of carbonation. The sensor was manufactured via functionalization of nylon using formaldehyde and chloroacetyl chloride and then covalent bonding with aniline blue. The sensor showed a significant color change in the pH ranging from 5 to 10 and exhibited sufficient response within 30 min and reversibility. In addition, the sensor was stable even in high pH environments such as concrete, and its applicability was verified inside the concrete.