Effects of a novel carbon nanomaterial on hydration, mechanics, and chloride binding of cement composites

Nanomaterials have made significant progress in the field of cementitious materials, but suffer from primary problems of high cost and poor dispersity. Carbon dots (CDs) as a novel low-cost and high-dispersity nanomaterial preliminarily demonstrate their potential for cement. Nevertheless, it lacks systematic research on their effects on cement properties. For the first time, this work comprehensively investigates the effects of CDs on the hydration, mechanics, and chloride binding of cement composites. More importantly, the relevant mechanisms are revealed in depth. Specifically, cost-efficient ($0.013/g) and well-dispersed (dispersed in simulated concrete pore solution for 7 d) CDs are directly prepared by a facile microwave approach. The CDs dramatically retard the early hydration, improve later mechanical performance, and obviously enhance chloride binding. In detail, 0.2 wt% CDs reduce the second exothermic peak value by 75%, but improve the chloride binding capacity of paste by 51%. And 0.1 wt% CDs enhance the compressive/flexural strength of mortar by 17%/21%. Crucially, related influence mechanisms are uncovered. Namely, CDs create a protective barrier to retard hydration through the complexation of their abundant surface groups with Ca2+; While CDs exert the nucleation effect to boost the formation of C–S–H and Friedel's salt, thereby playing a positive impact on chloride binding; Meanwhile, CDs with suitable dosages carry out nucleation effect to counteract their air-entraining effect, thus improving later mechanical properties of cement composites. This study is expected to provide a novel low-cost and high-dispersity CDs nanomaterial to advance the development of high-performance cementitious materials.