Paper Title
Hydrophobicity and Microstructure of Carbonated Concrete with Biomimetic Mineralization Technology

The hydration of cement and water in concrete will produce calcium hydroxide. Calcium hydroxide solution is in contact with carbon dioxide, carbonization will occur under normal temperature and pressure, and white calcium carbonate crystals will be deposited on the concrete surface, which is called carbonized concrete; this phenomenon is also called "Efflorescence". Because the pH value of carbonized concrete will change from strongly alkaline to neutral, so it is also called "neutralization". The biomimetic mineralization technology mainly modifies calcium carbonate to a nanometer scale, making the concrete surface such as lotus leaves and abalone shells into hydrophobic structures. This research aims to introduce biomimetic mineralization technology for concrete with different degrees of carbonation, so as to make concrete more durable and sustainable. First of all, the mixtures of high-performance concrete with a water-binder ratio of 0.65 is designed and making concrete test specimens. Secondly, the test specimens were carbonized so that the neutralization depths were controlled at 5 mm, 15 mm, and 25 mm, respectively. Afterwards, biomimetic mineralization technology was introduced to the carbonized concrete (mineralization temperature is 90°C, mineralization pressure is 2.5 atm). Finally, the engineering properties of the test specimens are tested, including hydrophobicity, water absorption, water permeability, compressive strength, microstructure, etc. From the test results, we can understand the changes in the hydrophobicity and microstructure of concrete. To explore the effect of biomimetic mineralization technology on carbonized high-performance concrete. The test results show that the contact angle test results of the control group concrete (without carbonation or biomimetic mineralization treatment group) and the carbonation group concrete are close to 0 degrees, which belongs to the hydrophilic surface. The water absorption and water permeability of the concrete specimens with different degrees of carbonation ranged from 2.07% to 2.34% and 7.10% to 8.01%, respectively; the control group was 1.53% and 5.78%, respectively. After the carbonized concrete specimen is treated with biomimetic mineralization, their contact angles are greatly increased to 108°~118°, which is a superhydrophobic surface. Because the concrete surface is hydrophobic, its water absorption and water permeability values are both 0%. The compressive strength test results show that after carbonization, the compressive strength of the concrete decreases with the increase of the neutralization depth, and the maximum decrease is about 11%. The increase was about 16%, making its compressive strength even slightly higher than that of the control group. Through microstructure analysis such as SEM, EDS and XRD, it was observed that nanoscale calcium carbonate existed in the biomimetic mineralized concrete specimen. In summary, the application of biomimetic mineralization technology to carbonized concrete specimens can successfully improve surface hydrophobicity and microstructure, and also have positive effects on compressive strength properties. Keywords - Carbonated concrete, Biomimetic mineralization, Nano calcium carbonate, Hydrophobicity, Sustainable development.