Abstract
Concrete has a remarkably low ratio of tensile strength to compressive strength, and is widely used in construction. However, the occurrence of cracks in a concrete structure is inevitable. Nevertheless, in the presence of adequate moisture, small cracks in the concrete structure exhibit a propensity to self-heal by getting filled due to the rehydration of cement particles and the subsequent precipitation of calcium carbonate (CaCO3). According to previous studies, the self-healing performance can be maximized by optimizing the temperature and pH to control the crystal formation of CaCO3. This study focused on the crystal form of CaCO3 generated in the self-healing of a cement-based composite material. To evaluate the self-healing performance depending on the type of aqueous solution and the temperature, the weight change, the weight change rate, and the porosity reduction in each case were evaluated. Moreover, to increase the generation of CaCO3 (which is a self-healing precipitate), nanosized ultrafine CO2 bubbles using CO2 gas were used, along with an adequate supply of Ca2+ by adjusting the aqueous solution (Ca(OH)2, CaO + ethanol). For greater pore-filling effects by controlling the CaCO3 crystal forms in the cement matrix, the change in the crystal form of the precipitated CaCO3 in the hardened cement paste with changing temperature was analyzed by scanning electron microscopy and X-ray diffraction. As a result, the possibility of the effective generation and control of vaterite with a dense pore structure together with calcite was confirmed by adjusting the temperature to approximately 40 °C at a pH of 12.
Highlights
Concrete is the most widely used material in the field of construction and has no viable replacement in the foreseeable future
To identify changes in the physical characteristics of the hardened cement paste according to the type of aqueous solution and temperature, the differences in the absolute dry, surface-dried, and underwater weights prior to and after self-healing were used to calculate the absolute dry weight ratio
The focus was on the crystal forms of CaCO3 generated during self-healing and the possibility of modifying them in the hardened cement paste by adjusting the temperature of the aqueous solution to obtain vaterite, which shows significant pore-filling effects and has a more stable crystal structure than that of calcite
Summary
Concrete is the most widely used material in the field of construction and has no viable replacement in the foreseeable future. Since its tensile strength is remarkably low when compared to its compressive strength, the occurrence of large and small cracks in a concrete structure is inevitable [1]. The cracks serve as infiltration points for external elements such as water, oxygen, chloride, sulfate, and carbon dioxide, which catalyzes the process of deterioration [2,3,4]. It is important to prevent the formation of detrimental fine cracks in order to preserve concrete structures for longer periods of time. In Japan, cracks in Materials 2019, 12, 2456; doi:10.3390/ma12152456 www.mdpi.com/journal/materials
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