Abstract
Self-healing microcapsules were synthesized by in situ polymerization with a melamine urea-formaldehyde resin shell and an epoxy resin adhesive. The effects of the key factors, i.e., core–wall ratio, reaction temperature, pH and stirring rate, were investigated by characterizing microcapsule morphology, shell thickness, particle size distribution, mechanical properties and chemical nature. Microcapsule healing mechanisms in cement paste were evaluated based on recovery strength and healing microstructure. The results showed that the encapsulation ability, the elasticity modulus and hardness of the capsule increased with an increase of the proportion of shell material. Increased polymerization temperatures were beneficial to the higher degree of shell condensation polymerization, higher resin particles deposition on microcapsule surfaces and enhanced mechanical properties. For relatively low pH values, the less porous three-dimensional structure led to the increased elastic modulus of shell and the more stable chemical structure. Optimized microcapsules were produced at a temperature of 60 °C, a core-wall ratio of 1:1, at pH 2~3 and at a stirring rate of 300~400 r/min. The best strength restoration was observed in the cement paste pre-damaged by 30% fmax and incorporating 4 wt % of capsules.
Highlights
Concrete is one of the most widely used construction materials
The microcapsules for self‐healing of cementitious materials were prepared by an in situ
The microcapsules for self-healing of cementitious materials were prepared by an in situ method method using malmine urea‐formaldehyde as a shell and epoxy resin as an adhesive
Summary
Concrete is one of the most widely used construction materials. It is susceptible to crack formation when exposed to certain environmental conditions and external loads. The cracks undoubtedly endanger the performance and potential service life of concrete structures in terms of their mechanical and/or transport properties [1,2,3,4]. Immediate cracks repair in concrete is a property that is strived for. A bio-inspired self-healing capability, like natural healing of wounds or cuts in living species, has become the focus of increasing attention because it potentially enables restoration of the structural integrity of materials [5,6,7,8]. Similar ideas have been incorporated to endow concrete materials a built-in self-healing ability since it was originally proposed for cement matrix by Dry [9,10,11,12,13,14,15,16]. A variety of approaches have been developed based on experimental explorations, utilizing bacteria spores [17,18,19,20,21,22,23], mineral admixtures [24,25,26,27,28,29,30,31,32], tabulated capsules or microencapsulation, etc. [33,34,35,36]
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