Research On The Preparation And Performance Of Carbon Nanofibers Reinforced Cement-based Composites

Cement is the most common and widely used construction material for its low cost and high compressive strength. Nevertheless, in general, cement exhibits extremely brittle failure, and characterized by a very low tensile strength and strain capacity, which increases the risk of cracks formation and reduces the durability of the matrix. Conventional fibers are added to overcome these weaknesses and it has a significantly improvement of the mechanical, electrical and electromagnetic properties of cement-based materials. However, conventional fibers delay the development of forming cracks, they do not stop their initiation. Hence, the research about nano-material reinforced cement-based composites has become hotspot in recent years.Carbon nanofibers (CNFs), is a kind of noncontinuous nano-size graphite fiber which formed by chemical vapor growth method. They are multi-wall graphitic structures as they combine microscopic length (0.5-100μm) with nanoscopic diameters (50-100nm). CNFs have an average tensile strength of7GPa and a Young’s modulus of400GPa.Because of their remarkable mechanical, thermal, electronic and optical properties, CNFs have a wide range of promising applications in areas such as composites, biomedical science and field emission et al.Due to their high aspect ratio and the strong Van der Waals forces, CNFs easily agglomerate and bundle in the form of entangled clumps and it has become the main obstacle in front of the application in composite materials. Aiming at the dispersion of CNFs, the main work and achievements are as following:(1) Six surfactants, methylcellulose (MC), hydroxypropyl methylcellulose (HPMC), sodium dodecyl sulfate (SDS), dodecylamine (DDA), N, N-dimethyl formamide (DMF) and cetyltrimethyl ammonium bromide (CTAB) were used individually and combinatorially in a certain concentration to disperse the CNFs in aqueous solution. To achieve a good dispersion of the CNFs, a method utilizing ultrasonic processing was employed. The ultrasonication-driven dispersion of CNFs in aqueous solutions was monitored by UV-vis spectroscopy. The experiments reveal that the method of high temperature annealing treatment purified the CNFs and the maximum achievable dispersion of CNFs corresponds to the maximum UV absorbance of the solution. All results show that the surfactants mixture of MC and SDS in a certain concentration of0.4g/L and2.0g/L has the maximum dispersion effect on CNFs in aqueous solution, the optimum concentration ratio of MC, SDS and CNFs was2:10:1.(2) Stable homogeneous suspensions of CNFs were prepared by using methylcellulose (MC) as a surfactant. The sonication-driven dispersion of CNFs in aqueous solution was investigated by measuring ultraviolet absorbency (UV absorbency), Zeta potential, adsorption isotherm of MC on CNFs and surface tension of CNF suspensions. In these CNF suspensions, the zeta potentials of CNFs progressively increase with the increasing MC concentration, in which the zeta potentials change from-15.4mV to0. The surface tension of CNF suspensions decrease from38.87mN/m to36.54mN/m. The adsorption isotherm of MC on CNFs shows an adsorption platform (namely "L" style), and the adsorption isotherm reaches the saturation plateau at MC concentration of about0.4g/L. All results show that the optimum concentration ratio of MC to CNFs is2:1for dispersing.Stable uniformly distribution of CNFs suspensions were prepared and incorporated into cement-based materials. The workability, mechanical behavior and microstructure of the prepared CNFs/cement composites were researched. The main work and achievements are as follows:(1) Compared to the reference paste, CNFs/cement composite pastes had a similar setting time, and no loss was gained in expansion and slump. The stability of CNFs/cement composites stayed well.(2) The addition of CNFs improved the mechanical properties of cement-based material composites. Basically, compared to the reference sample, the mechanical properties of prepared CNFs/cement composites were improved with the increase of CNFs until it reached to the optimal amount of0.1%, and then started to drop. After28days curing, the flexural strength of prepared CNFs/cement composites (CNFs amount of0.1%and the water/cement ratio of0.25,0.30,0.35and0.40) were15.2MPa,14.1MPa,12.9MPa and12.3MPa, respectively, which increased by16.0%,29.4%,24.0%and28.1%, respectively. Similar trend was obtained from correspondingly compressive strength. Moreover, the splitting tensile strength of prepared CNFs/cement composites was improved with the increase of CNFs until it reached to the optimal amount of0.1%, and then started to drop. The splitting tensile strength of CNFs/cement composites incorporated0.1wt.%CNFs increased to6.63MPa, which increased by28.5%. The correspondingly toughness improved to the maximum and all toughness indices of the prepared composites (fracture energy, initial cracking point, failure cracking point, maximum loading strength and cruve area) was higher than the reference sample. The maximum toughness indices were obtained by sample J35-3.(3) The addition of CNFs improved the mechanical properties of cement mortar composites. Basically, compared to the reference sample, the mechanical properties of prepared CNFs/cement mortar composites were improved with the increase of CNFs until it reached to the optimal amount of0.1%, and then started to drop. After28days curing, the flexural strength of prepared CNFs/cement mortar composites (CNFs amount of0.1%and the water/cement ratio of0.25,0.30,0.35and0.40) were19.9MPa,16.5MPa,15.6MPa and11.6MPa, respectively, which increased by41.1%,38.7%,54.5%and20.8%, respectively. Similar trend was obtained from correspondingly compressive strength. Moreover, the splitting tensile strength of prepared CNFs/cement composites was improved with the increase of CNFs until it reached to the optimal amount of0.1%, and then started to drop. The correspondingly toughness improved to the maximum and all toughness indices of the prepared composites (fracture energy, initial cracking point, failure cracking point, maximum loading strength and crave area) was higher than the reference sample. The maximum toughness indices were obtained by sample J35-3. Fracture energy, initial cracking point, failure cracking point and maximum loading strength were increased by118.11%,118.07%,117.86%and50.51%, respectively.Micro measuring and testing was applied to study the mechanism of CNFs on cement-based composites. The results showed that the CNFs acted as bridges and networks across voids and cracks, which improved the load-transfer efficiency. The absorbed fracture energy was provided by the mechanisms of matrix cracking, CNFs/matrix interface debonding, fiber pull-out and fiber rupture. Meanwhile, the embedded CNFs are wrapped a surface coating of hydration products, which indicates the high bonding strength is obtained between the CNFs and the matrix. The incorporation of CNFs in cement pastes accelerates the hydration reaction of cement paste, which increases the amounts of C-S-H content and finally improves the mechanical properties of hardened CNFs/cement composites. The addition of CNFs improves the pore parameters, the porosity decreased and the composites became compacted, which enhanced the microstructure of CNFs/cement composites.