Enhancing self-healing in ultra-high-performance fiber-reinforced concrete: The role of superplasticizer anchor groups

This study investigates the influence of anchor groups from two types of superplasticizers—polycarboxylate (PCE) and phosphate-modified PCE (PCE-P)—on the self-healing behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC). Specimens were subjected to 150 µm microcracks at 3 days of age and immersed in water at 25°C for 90 days. Self-healing was assessed using digital microscope imaging, compressive strength recovery, and scanning electron microscopy (SEM). The results showed that PCE-P exhibited superior performance compared to PCE. Additionally, combining ground granulated blast-furnace slag (GGBFS) with PCE-P resulted in synergistic effects, yielding the most favorable outcomes. Superplasticizers on crack surfaces react with water, releasing anchor group anions. Moreover, varying absorption processes on silica fume and cement lead to the desorption of superplasticizers, driven by increased alkalinity from release during concrete hydration. According to Pearson’s acid-base theory, the bond between and the carboxyl anchor group () is weak, leading to compound instability. In contrast, the phosphate anchor group () forms a stronger bond with . In specimens containing GGBFS and PCE-P, the enhanced alkalinity promotes the release of anchor group anions. These anions interact with calcium ions, facilitating the formation of calcium phosphate compounds such as hydroxyapatite. While their contribution is minor compared to primary self-healing agents like calcium carbonate and gel, they significantly improve crack healing efficacy.