Nanostructure and Fracture Behavior of Carbon Nanofiber-Reinforced Cement Using Nanoscale Depth-Sensing Methods

Abstract

In recent years, carbon nanofibers have been investigated as a suitable reinforcement for cementitious composites to yield novel multifunctional materials with improved mechanical, electrical, magnetic, and self-sensing behavior. Despite several studies, the interactions between carbon nanofibers and Portland cement hydration products are not fully understood, with significant implications for the mechanical response and the durability at the macroscopic lengthscale. Thus, the research objective is to investigate the influence of carbon nanofibers on the nanostructure and on the distribution of hydration products within Portland cement paste. Portland cement w/c = 0.44 specimens reinforced with 0.0 wt%, 0.1 wt%, and 0.5 wt% CNF by mass fraction of cement are cast using a novel synthesis procedure. A uniform dispersion of carbon nanofibers (CNF) via a multi-step approach: after pre-dispersing carbon nanofibers using ultrasonic energy, the carbon nanofibers are further dispersed using un-hydrated cement particles in high shear mixing and mechanical stirring steps. High-resolution scanning electron microscopy analysis shows that carbon nanofibers fill nanopores and connect calcium–silicate hydrates (C–S–H) grains. Grid nano-indentation testing shows that Carbon nanofibers influence the probability distribution function of the local packing density by inducing a shift towards higher values, η = 0.76–0.93. Statistical deconvolution analysis shows that carbon nanofibers result in an increase in the fraction of high-density C–S–H by 6.7% from plain cement to cement + 0.1 wt% CNF and by 10.7% from plain cement to cement + 0.5 wt% CNF. Moreover, CNF lead to an increase in the C–S–H gel porosity and a decrease in both the capillary porosity and the total porosity. Based on scratch testing, adding 0.1 wt% CNF yields a 4.5% increase in fracture toughness and adding 0.5 wt% CNF yields a 7.6% increase in fracture toughness. Finally, micromechanical modelling predicts an increase of respectively 5.97% and 21.78% in the average Young’s modulus following CNF modification at 0.1 wt% CNF and 0.5 wt% CNF levels.

Вязкость Разрушения Цементных Материалов, Модифицированных Углеродными Нанотрубками

“…3. Similar to our values of Young's modulus were obtained as a result of studies [7], where it was also established that the presence of carbon nanofibers affects the stochastic distribution of the macroscopic Young's modulus, causing a shift towards higher values (29.66 GPa for simple Portland cement, 31.43 GPa for cement + 0.1 wt% CNT and 36.12 GPa for cement + 0.5 wt% CNT). 4.…”

“…2. Our position on the containment of cracking by carbon fibers at the nano-level coincides with the studies of Ange-Therese Akono [7,16], which show that carbon nanofibers fill nanopores and connect grains of calcium hydrates (C -S -H), which leads to the effect of bridging and facilitating load transfer, individual CNTs are embedded in hydration products and delay nanocracks. Also in [17], SEM images are demonstrated that confirm the effects of the formation of bridges that transfer the load along nano-and microcracks.…”

“…A decrease in capillary porosity and a decrease in total porosity are observed, followed by an improvement in the pore structure. Reverse micromechanical analysis shows a shift from micropores and mesopores with a size of 100 nm in ordinary Portland cement to small gel pores with a size of 5-12 nm in cement modified with CNTs [7].…”

“…In the views of Ange-Therese Akono [7], the model of the modified CNT cement composite consists of 4 scale levels. At the nanoscale, calcium silicate hydrate products are packaged in various structural units, resulting in low C -S -H densities, high C -S -H densities, and ultrahigh C -S -H densities.…”

Dielectric Model of Carbon Nanofiber Reinforced Concrete

“…Carbon nanofiber reinforced concrete is a composite of good performance, with ordinary concrete as the base material and carbon nanofiber as the reinforced material [20][21][22][23]. Compared with ordinary concrete, carbon nanofiber reinforced concrete was greatly improved in not only the mechanical properties and durability [22][23][24][25][26] but also the dielectric properties [27][28][29][30][31].…”