Abstract
Exposure conditions are critical for the autogenous self-healing process of Portland cement based binder matrixes. However, there is still a significant lack of fundamental knowledge related to this factor. The aim of this paper was to investigate and understand the effects of various potentially applicable curing solutions on the efficiency of the crack closure occurring both superficially and internally. Four groups of exposures were tested, including exposure with different water immersion regimes, variable temperatures, application of chemical admixtures, and use of solutions containing micro particles. The self-healing process was evaluated externally, at the surface of the crack, and internally, at different crack depths with the use of optical and scanning electron microscopes (SEM). The phase identification was done with an energy dispersive spectrometer combined with the SEM. The results showed very limited self-healing in all pure water-based exposures, despite the application of different cycles, temperatures, and water volumes. The addition of a phosphate-based retarding admixture demonstrated the highest crack closure, both internally and externally. The highest strength recovery and a very good crack closure ratio was achieved in water exposure containing micro silica particles. The main phase observed on the surface was calcium carbonate, and internally, calcium silicate hydrate, calcium carbonate, and calcium phosphate compounds. Phosphate ions were found to contribute to the filling of the crack, most likely by preventing the formation of a dense shell composed of hydration phases on the exposed areas by crack unhydrated cement grains as well as by the additional precipitation of calcium and phosphate-based compounds. The micro sized silica particles presumably served as nucleation sites for the self-healing products growth. Changes in the chemical composition of the self-healing material were observed with a distance from the surface of the specimen.
Highlights
Exposure conditions are one of the crucial factors affecting the efficiency of the self-healing processes in cementitious materials
Representative images taken with the optical microscope and with scanning electron microscope (SEM) are shown in
The SEM images were taken from Surface 1 and Cross-section7 of
Summary
Exposure conditions are one of the crucial factors affecting the efficiency of the self-healing processes in cementitious materials. The autonomous systems are to some extent dependent on exposure. Full time [1,2] or at least part time water immersion [3] is a mandatory condition for the bacteria-based self-healing systems. As the presence of water might be problematic in real-life applications, the portable water-reservoirs in the form of hydrogels are used [4,5,6]. Crack widths of up to 0.5 mm are healed due to bacterial CaCO3 precipitation inside and on the surface of the hydrogel [5]. The crack closure efficiency increases by 50% with the use of hydrogel encapsulated bacterial spores [5]
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