Effect of Addition of Ca and CO Ions with Temperature Control on Self-Healing of Hardened Cement Paste.

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 (CaCO). According to previous studies, the self-healing performance can be maximized by optimizing the temperature and pH to control the crystal formation of CaCO. This study focused on the crystal form of CaCO 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 CaCO (which is a self-healing precipitate), nanosized ultrafine CO bubbles using CO gas were used, along with an adequate supply of Ca by adjusting the aqueous solution (Ca(OH), CaO + ethanol). For greater pore-filling effects by controlling the CaCO crystal forms in the cement matrix, the change in the crystal form of the precipitated CaCO 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.