Abstract
While carbon nanotubes (CNTs) have been increasingly employed in cementitious composites, there is a lack of direct evidence at the microscopic scale that explains what contributes to the interfacial bond strength (IBS) between CNT and cement paste. This study reports a new approach, i.e., the use of CNT fiber (CNTF), a macro-scale assembly of parallelly oriented CNTs, as a platform to study CNT/cement interactions. The single-fiber pullout experiments revealed that the IBS between pristine CNTF and cement paste (Type 42.5 portland cement, w/c of 0.40) is 2.076 MPa, much lower than the previously reported simulation results, suggesting the absence of the seeding effect of CNTs. Plasma oxidation can increase this IBS by 42%, whereas a graphene oxide (GO) nanocoating (GO@CNTF) greatly enhanced the IBS by 109%, outperforming all previously reported experimental results. Remarkably, a continuous crystalline domain of calcium-silicate-hydrate (C–S–H) with a width of ∼10 nm was found directly anchored at the GO nanosheets precoated on the CNTF surface, providing direct evidence for the seeding effect of GO on cement hydration. This observation also suggests the importance of significantly increasing the oxidation degree of CNT, in order to greatly promote CNT-cement interfacial bonding via the seeding effect mechanism. Our findings uncover that there is a great potential for GO to nano-engineer the interfaces in concrete.