Influence Of Nano-CaCO₃ On Properties Of Cement-based Materials And Its Application

In recent years, with the accelerated process of urbanization and the continuous improvement and development of public infrastructure, in the construction field, the demand of cementitious material is increasing and the cement-based material is required to have good durability. But for most current concrete admixture-early strength agent, they contain harmful ions that may damage the durability of cement-based material, such as the Cl- that may cause the corrosion of reinforcement and the alkali metal ions that can cause the alkali aggregate reaction. Therefore, it is necessary to develop an early strength agent which is good for durability and has no harmful ions.Based on the above considerations, in this paper, nano-CaCO₃ was selected as the research object. Manual stirring, ultrasonic vibration with 10 min, and ultrasonic vibration with 15 min were used to study the influence of nano-CaCO₃ dispersing method on the performance of cement-based material in order to find out the best dispersion method under the conditions of this experiment. According to the best dispersion method, the effects of different dosage of nano-CaCO₃ on the properties of cement-based materials were studied. The mechanism of nano-CaCO₃ on the properties of cement-based materials was analyzed. At last, the nano-CaCO₃ was attempt to selected as one of the components of the early strength agent and combined with the light calcium powder to develop a novel water reducing early strength agent. The study results showed that:?1? By using ultrasonic dispersing method with 10 minutes, the dispersity of nano-CaCO₃ in water is better and the physical properties are much higher. Also, the amount of Ca?OH?₂ and the chemically combined water produced by the hydration of Portland cement is larger. The whole structure of cement-based materials can get more dense and homogeneous. The results indicate that the ultrasonic dispersing time is not the longer the better.?2? The apparent density of cement paste is increased, the flowbility of cement mortar is decreased, the flexural strength and compressive strength are improved due to the addition of nano-CaCO₃. Under the conditions of this experiment, the mechanical properties of cement-based materials containing nano-CaCO₃ of 1.5%?in mass? are significantly improved; however, when the dosage of nano-CaCO₃ is 2.5%,it impacts the improvement of strength.?3? The evolution rate of hydration heat is accelerated as well as quantity of hydration heat with the addition of nano-CaCO₃. Both Ca?OH?₂ content and the chemically combined water produced by the hydration of Portland cement increase with the increasing dosage of nano-CaCO₃. The results above indicate that the hydration of Portland cement can be accelerated with the incorporation of nano-CaCO₃. At the same time, the interface transition zone between aggregate and cement paste and the microstructure of hardened cement paste are largely improved so that the whole structure can get more dense and homogeneous.?4? The best recipe of water reducing early strength agent under this experiment conditions is the group M5?the mass ratio of nano-CaCO₃ and light calcium powder is 1:1 and the total content of cementitious materials was 1%?, the flexural and compressive strength for 1d of M5 are increased by 19.64% and 25.62% respectively compared with the reference group, the flexural and compressive strength for 3d of M5 are increased by 14.29% and 30.05% respectively compared with the reference group, and the later strength development is not affected. The mechanical properties of M5 have greatly improved compared with the reference group.?5? The results show that the suitable nano-CaCO₃ dispersion method and its content is the key to the early strength of the cement-based materials, and nano CaCO₃ can be used as early strength agent components.