Carbon nanotubes do not provide strong seeding effect for the nucleation of C3S hydration

Understanding the fundamental role of carbon nanomaterials is the key to unlocking their potential in enhancing the macroscopic engineering properties and performance of cementitious materials. There is significant debate over the hypothesis that carbon nanomaterials with high surface areas could promote cement hydration by providing nucleation sites (i.e., seeding effect). The seeding effect of carbon nanotubes (CNTs) for cement hydration has lacked direct experimental evidence, mainly due to the inability to directly observe the surface of CNTs after their full contact with the pore solution of cement and the complexity of hydration products at the nanoscale. Different from previous studies, we developed CNT sponge (CNTSP), a macro-scale assembly consisting of randomly oriented and entangled CNTs in three-dimension space, as the platform for discriminating the potential nucleating effect of CNTs. By design, the CNTSP features nano-sized interconnected pores that allow only the pore solution to penetrate through, thus separating the tricalcium silicate (C 3 S) particles and the pore solution of C 3 S in situ. The experimental results revealed that pristine and oxidized CNTs could barely act as seeding material for the nucleation of C 3 S hydration products. Relative to pristine CNT, oxidized CNT can promote the local formation of calcium hydroxide (CH) that is only loosely connected to the CNT, while depressing that of calcium silicate hydrate (C–S–H). The CNTSP was further employed as a platform to investigate the effects of superplasticizer on C 3 S hydration, revealing that the superplasticizer improves the CNT-hydrates binding. This work not only provides fundamental knowledge about the limited role of CNT in C 3 S hydration, but also demonstrates a novel experimental platform for investigating the interaction between carbon nanomaterials and cement hydration products.