85 / 2022-04-16 14:48:25

Self-healing of Cracks Based on Aggressive-ion-bonding Agent in Cement-based Materials in Sea Water

Self-healing of cracks; Aggressive ions; Chemical binding; Thermodynamic modeling; Marine environment

Concrete is a brittle material that easily cracks during construction and service processes. Cracks in concrete can accelerate invasion of aggressive ions in marine environments. As a result, cracks can extremely shorten the service life of concrete structures. It would be useful for concrete to have a self-healing capacity so that cracks can be healed autonomously once they appear. In order to significantly improve the durability of concrete structures in a marine environment by self-healing of cracks, it is necessary to promote the crack closure and simultaneously weaken the damage of aggressive ions in cracks from sea water. In this study, novel self-healing agents that can bind with aggressive ions in cracks and simultaneously form expanded reaction products to heal the cracks rapidly was developed via thermodynamic modeling and experimental study. The promotion and the mechanism of self-healing of cracks by the developed self-healing agents were explored.

      It is found that the self-healing agent with CaO-metakaolin or CaO-NaAlO₂ as the main component can achieve the purpose of binding seawater erosion ions, and the amount of self-healing agent and the ratio of calcium to aluminum will affect the mineral phase composition of the reaction products. Besides, by adjusting the composition of self-healing agents, the volume expansion of self-healing agents can be optimized and the pH in crack solution can also increase to delay the destruction of passive film on steel bars.

The reaction kinetics of the developed self-healing agent with sea water was explored. It was found that NaAlO₂ can rapidly dissolve in water and release Al(OH)₄^- and consequently, precipitates binding aggressive ions in synthetic water formed very fast. For the CaO-metakaolin agent, portlandite mainly formed within the first 1 day, while the reaction of metakaolin occurred mainly after 3 days, resulting in the formation of Friedel’s salt, SO₄-CO₃-AFm salt, hydrotalcite and C-S-H. Therefore, reactions between CaO-NaAlO₂ agent and sea water, particularly the binding efficiency of Cl^-, is faster than that between CaO-metakaolin agent and sea water.

Crack closure tests showed that the self-healing agents developed in this study can promote the closure of cracks effectively. Moreover, the BSE/EDS results, the self-healing products tend to precipitate nearby the crack mouth and inside the broken artificial aggregates. Friedel’s salt and hydrotalcite were found inside the broken artificial aggregates, which indicates the effective chemical binding of aggressive ions in sea water.