The growth of carbon nanotubes & carbon nanofibers on cement admixture particles

Cement-based material is a quasi-brittle material according to are weak. Under the effect of the load, temperature, humidity and other factors, the cement-based material is easy to crack and fracture. The traditional solution is mainly to use steel or millimeter/ micron fiber (such as steel fiber, carbon fiber, and polyvinyl alcohol fiber) to reinforce the cement-based materials. However, researchers recognized that micro-sized reinforced materials can only limit the expansion of internal microcracks of cement-based materials instead of preventing the microcracks from engendering. The addition of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) can address the problems mentioned above. CNTs/CNFs can transfer the reinforcement and modification behavior on the cement-based materials from microscale to nanoscale. CNTs/CNFs possess better mechanical property, whose elastic modulus, tensile strength and ultimate deformation are respectively 10, 20 and 18 times that of microscale carbon fiber. The inter-laminar shear strength of the CNTs/CNFs and the epoxy resin layer is an order of magnitude higher than that of micro carbon fiber and the epoxy resin layer [1–3]. Li et al. firstly observed that the addition of 0.5 wt.% CNTs can respectively improve the flexural, compressive strength and the failure strain by 25%, 19% and 27%. Raki et al. reported that CNTs can improve the Vivtorinox hardness of the early hydration of cement-based material by 600%, the Young modulus by 227% and the flexural strength by 40% [2]. Veedu incorporated 0.02 wt.% CNTs into the cement-based materials to make its flexural and compressive strength increase by 30% and 100% [3]. Chaipanich et al. firstly used CNTs of 0.5% and 1% by weight in a fly ash cement system to produce carbon nanotubes-fly ash composites in the form of pastes and mortars and found that the use of carbon nanotubes results in higher strength of fly ash mortars [4]. Shah et al. only added 0.048–0.08 wt.% CNTs into cement-based materials to increase its flexural strength and elastic modulus by 8–40% and 15–55% [5,6]. Gay and Sanchez founded that the split tensile strength of the cement-based materials with the addition of 0.2 wt.% CNFs is 26% higher than that of the cement-based materials with fly ash [7]. Kumar et al. discussed the effect of multiwalled CNTs on strength characteristics of the hydrated Portland cement paste [8]. Gao et al. founded that the compressive strength of the CNFs reinforced concrete with the addition of 0.16 wt.% CNFs was 42.7% higher than that of ordinary concrete. Researches pointed out that CNTs/CNFs achieves enhancement effect by nucleation, increasing the amount of C–S–H gel of high hardness, improving pore structures, controlling nanoscale cracks, improving the early strain capacity and reducing autogenous shrinkage, etc. [1–3]. These mechanisms also would improve the durability of the cement-based materials. Han et al. have founded that the addition of CNTs could improve the transport properties of the cement mortar. In addition, due to the excellent electrical, thermal, electromagnetic properties of the CNTs/CNFs, it could make the cementbased materials possess electrical, thermal, electromagnetic and sensing properties, and then make the cement-based materials possess multi-functional properties [1,3].