Effect of carbon dots with different sizes on chloride binding of cement

Carbon dots (CDs) as a novel and green nanomaterial exhibit a promising application in enhancing the chloride binding capacity of cement to extend the service life of reinforced concrete (RC) structures. However, their size influence on the enhancement of chloride binding is still a research gap, enormously hindering the further employment of CDs toward cement system. Herein, their size-induced effect on chloride binding actions of cement is comprehensively unveiled using CDs with various sizes for the first time. Specifically, three kinds of CDs with different sizes are controllably prepared through simply altering the calcinating time of citric acid. And the relationship between sizes of CDs and chloride binding capacity of cement is confirmed through the equilibrium tests. More importantly, the mechanism for the effect of CDs sizes on chloride binding is reasonably proposed according to the alteration analyses of phase compositions. The consequences demonstrate that the chloride binding capacity in cement modified by CDs with average sizes of 1.73, 5.79, and 12.37 nm increases by 41%, 20%, and 14% respectively, compared with that of the blank group after 28-d immersion in 3 mol/L NaCl solution. This testifies that the chloride binding capacity of CDs-incorporated cement is negatively correlative with CDs sizes. The related mechanism is that CDs with smaller sizes can provide more nucleation sites to promote cement hydration, further advancing the production of C–S–H gels and monosulfate (Ms), thus more markedly enhancing the physical adsorption and chemical binding of chloride ions in cement pastes. This work would guide the preparation of CDs with more suitable sizes to improve the chloride binding ability of cement, thus anticipated to extend the service life of RC structures in coastal environment, ultimately reducing the CO2 emission from cement industry.