Impact of the microstructure of polycarboxylate superplasticizers on the dispersion of graphene

Graphene can not only toughen cement-based materials, but also give them sensing ability. However, the uniform dispersion of graphene in a cement matrix is the major problem in the fabrication process. Four polycarboxylate superplasticizers (PCEs) with different charge densities and side-chain lengths were synthesized and the effects of their microstructures on the dispersibility of graphene in deionized water and in solution in cement pores were examined by UV-Vis spectroscopy, dynamic light scattering and optical microscopy with a large depth of field. A mechanism for the dispersion of graphene in the two media was also proposed. In deionized water, PCEs with a higher charge density showed more electrostatic repulsion, which improved the graphene dispersion efficiency. Conversely, PCEs with a lower charge density and longer side-chains gave a lower graphene dispersion. In solution in the cement pores, however, a PCE with a high charge-density produced a low graphene dispersibility, due to a cross-linking Ca2+-bridging effect. This effect was insignificant in cement pores for a solution containing low charge-density PCEs. Moreover, it was found that PCEs with the longer side-chains produced the worst graphene dispersion efficiency in both media. Overall, PCEs with a low charge density and relatively short side chains are more suitable for the preparation of graphene-composited cement pastes.