295 / 2023-02-17 15:39:09

Recovery of vaterite CaCO3 from recycled concrete fines for use in profiting the cementitious properties of cement pastes

Recycled concrete fines,Vaterite CaCO3,Leaching-carbonation process,Fresh and hardened properties,Cement paste,Water-to-cement ratio

In recent decades, rapid urbanization in China has increased the need to demolish and replace old buildings. As a result, a considerable amount of construction and demolition (C&D) waste is generated, accounting for more than one-third of global C&D waste generation. Currently, annual C&D waste production in China is about 1.8 billion tones with a recycling rate of <10%. Concrete waste is the major constituent (approximately 65%) of C&D waste, and techniques have been developed to recycle concrete waste into recycled concrete aggregates. However, high amount of very fine particles (<150 µm) was produced during the recycling process. Hence, this study proposes a two-stage leaching-carbonation process to recover high-purity of vaterite CaCO₃ from recycled concrete fines (RCF) (Fig. 1), and utilize them in the cement paste at different water-to-cement ratios (w/c ranging from 0.4 to 0.6) so as to explore their performance at fresh and hardened stages. 



The experimental results showed that the maximum Ca²⁺ leaching rate of 65.7% can be achieved under the optimal leaching conditions (2 mol/L NH₄Cl solution, liquid to solid ratio of 10 mL/g, 85 °C, and for 60 min). Also, 615 g of spherical vaterite CaCO₃ (1-10 µm) with a purity of 97.8% (Fig. 2) can be successfully synthesized from 1 kg of RCF by injecting CO₂ into the leachate containing 1 mol/L NH₄OH at 25 °C and 0.025 MPa for 30 min. Meanwhile, the high-purity vaterite CaCO₃ could be used as an additive in accelerating the hydration rate of cement paste and improving the mechanical strength. The addition of vaterite CaCO₃ in cement paste can shorten the setting time (Table 1) by about 2 hours, but revealed a different impact on the flowability of cement (Fig. 3). The acceleration of cement hydration could be related to the increase in the nucleation sites provided by vaterite particles and the synergetic effect of Cl^- ions existed in the vaterite CaCO₃. The decrease in the flowability of the cement mixture could be related to a significant increase in the wetting surface associated with the porous vaterite particles with a large specific surface area, resulting in the increase of water demand and subsequent decrease of the amount of excess water. Moreover, mechanical test results (Fig. 4) demonstrated that the compressive strengths of cement paste increased with the addition of vaterite CaCO₃, regardless of the w/c. Increase of w/c slightly enhanced the later age strength of cement paste, reflecting that vaterite particles are more effective in densifying the pore structure of cement paste with 0.6 w/c. The thermal analysis results (Fig. 5) revealed that vaterite CaCO₃ promotes the hydration process of cement paste, i.e. more hydration products, such as C-S-H gel, ettringite (Ett) and monocarboaluminate (Mc), were formed owing to the nucleation effect of CaCO₃, which made the denser cement paste with better mechanical properties. The proposed approach provides a green solution for CO₂ fixing and RCF recycling in the construction industry and offers a promising method for improving the hydration and mechanical properties of cement paste.