Carbon Fiber Reinforced Cement Paste Research Articles

Enhanced thermoelectric property of cement-based materials with the synthesized MnO2/carbon fiber composite

The thermoelectric effect of the cement paste with the MnO2/carbon fiber composite inside was firstly reported in this work. The MnO2/carbon fiber composite was synthesized based on the redox reaction between potassium permanganate and carbon. After the reaction, α-MnO2 particles grew on the surface of carbon fiber. The nanorod-shaped MnO2 particles exhibit a diameter of about 50 nm and a length range of 100–300 nm. The weight ratio of MnO2 to carbon fiber is 37.6:62.4 in the MnO2/carbon fiber composite. The obtained results proved the MnO2/carbon fiber composite effective for enhancing the thermoelectric effect of the cement paste as the Seebeck coefficient increased sharply with its content. The Seebeck coefficient of the MnO2/carbon fiber reinforced cement paste was about 100 times that of the pure carbon fiber reinforced cement paste, while the electrical conductivity and thermal conductivity of these two kinds of cement paste were similar. The obtained maximum ZT value of the MnO2/carbon fiber reinforced cement paste was 2.12 × 10−3, which was about 104 times that of the pure carbon fiber reinforced cement paste.

Improving the Interfacial Bond Properties of the Carbon Fiber Coated with a Nano‐SiO2 Particle in a Cement Paste Matrix

To improve the interfacial bond properties of the carbon fiber coated with a nano‐SiO2 particle in a cement paste matrix, the present study proposed a method of coating nano‐SiO2 particles on the surface of the carbon fiber by the chemical reaction of a silane coupling agent (glycidoxypropyltrimethoxysilane, GPTMS) and colloidal nano‐SiO2 sol in an alkaline environment. To verify whether a nano‐SiO2 particle was effectively modified on the surface of the carbon fiber, the surface morphology, chemical composition, and chemical structure were characterized and analyzed by several techniques such as the scanning electron microscope (SEM), energy‐dispersive spectrometer (EDS), and Fourier‐transform infrared spectroscopy (FT‐IR). Nano‐SiO2 particles were entirely covered and uniformly distributed on the surface of the carbon fiber, resulting in the formation of a thin layer of nano‐SiO2 particles. A thin layer of nano‐SiO2 particles reacted with Ca(OH)2 to form a calcium‐silicate‐hydrate (C‐S‐H) gel, which is most helpful to increase the form between the fiber and the matrix. In addition, a pull‐out test of the tow carbon fibers was performed to verify the effect of the new surface modification method on the interfacial bond properties of the carbon fiber embedded in the cement paste matrix. The experimental results showed that the frictional bond strength of the carbon fiber coated with a nano‐SiO2 particle was significantly increased compared to the plain carbon fiber. These results were expected to improve the interfacial bonding force of hardened cement paste from the formation of the C‐S‐H gel produced through the chemical reaction of nano‐SiO2 particles coated on the surface of the carbon fiber with Ca(OH)2. In particular, it was confirmed that the carbon fiber‐reinforced cement paste (CFRCP) specimens coated with a nano‐SiO2 particle and silica fume which replaced 10 wt.% of cement by mass showed the highest pull‐out resistance performance at 28 days of age. The new surface modification method developed in this study can be very beneficial and helpful in improving the interfacial bond properties of CFRCP.

Highly dispersed graphene oxide electrodeposited carbon fiber reinforced cement-based materials with enhanced mechanical properties

Mechanical behavior of carbon fiber (CF) reinforced cement-based materials greatly depends on the dispersion of CF and interfacial properties between the CF and cement matrix. In this study, graphene oxide (GO) was utilized to modify the surface properties of CF, including the roughness, wettability and chemical reactivity, and the graphene oxide/carbon fiber (GO/CF) hybrid fibers were fabricated by a newly designed electrophoretic depositing method. The scanning electron microscopy and contact angle measurement results indicated that GO/CF hybrid fibers not only had a rougher surface which was expected to improve the physical friction when CF was pulled out from cement matrix, but also had a higher wettability surface that made it easier to contact with cement hydrates as nucleation sites. In addition, GO/CF hybrid fibers were capable of high chemical reactivity due to the introduction of GO with many functional groups, which ensured them more likely to interact with cement hydrates due to the hydrogen bonding at interface and therefore benefited to strengthen the bonding between the CF and cement matrix. In terms of mechanical behavior, three-point bending test showed that compared with the CF reinforced cement paste, flexural strength of the GO/CF hybrid fibers reinforced cement paste was enhanced by 14.58%, and could be further improved by 10.53% when the GO/CF hybrid fibers were pre-dispersed in the GO solution and then mixed with cement powders. The larger electrostatic repulsion and steric stabilization led to the better dispersion of GO/CF hybrid fibers in GO solution, which were responsible for the further mechanical enhancement of cement paste. In conclusion, the research outcomes provided a novel way for utilizing GO as both of dispersant and surface modifier to improve the dispersion of CF in cement and strengthen its bonding with cement hydrates, consequently achieving a significant enhancement in the mechanical properties of cement paste.

Portland cement paste with aligned carbon fibers exhibiting exceptionally high flexural strength (> 100 MPa)

Here, we introduce a nozzle injection technique for carbon fiber-reinforced cement paste leading to unidirectional alignment of cement-embedded short carbon fibers that follow the movement direction of the guided nozzle. In comparison to non-reinforced cement pastes, this novel material exhibits a tremendous increase of its flexural strength upon admixing and aligning 1 to 3 percent (by volume) of chopped carbon fibers. Cement pastes containing carbon fibers aligned in the stress direction thus acquire high compressive and flexural strength values at the same time. Mechanical tests prove the material to withstand flexural loads larger than 100MPa in conjunction with a deflection hardening behavior resembling that of high performance fiber-reinforced cementious composites at relatively low fiber volume content. Insights into the preparation, fiber alignment, rheology and the fracture behavior of this material are presented in this study.

Study on Electrical Properties of Carbon Fiber Reinforced Cement Paste with CCCW

In this work, CCCW (cementitious capillary crystalline waterproofing materials) was added in CFRC (carbon fiber reinforced cement paste). Variations of electrical conductivity of CFRC with fiber contents (0.2%~2.0% by the weight of cement) and CCCW (3% by the weight of cement) were studied. The results showed that the resistivity of CFRC containing CCCW versus the concentration of carbon fiber curves had typical features of percolation phenomena. The percolation threshold was 1.2 wt.%. The relationship between resistance and compressive stress was repeatable in CFRC with carbon fiber content of 1.2 wt.%. The resistance decreased in nearly linearity to the compression stress during loading, and increased during unloading.

Effect of Moisture on Piezoresistivity of Carbon Fiber-Reinforced Cement Paste

Piezoresistivity, as exhibited by carbon fiber-reinforced cement-based material, allows a structural material to sense its strain or stress through electrical resistance measurement. This paper addresses the worst scenario associated with the effect of moisture on the piezoresistivity of carbon fiber-reinforced cement paste. This scenario is associated with both water saturation and a fiber content below the percolation threshold. For this scenario relative to the condition after drying at room temperature, the gauge factor (fractional change in resistance per unit strain) is decreased by at least 12% and the variability of the gauge factor with the strain amplitude and with the strain history is increased. These negative effects are attributed to the water at the fiber-matrix interface. The water interferes with the electronic conduction across the interface. Nevertheless, the piezoresistivity remains strong, the signal-to-noise ratio of the resistance remains high, and the relationship between resistivity and strain remains quite linear.

Monitoring of hydration using the electrical resistance of carbon fiber reinforced cement materials

The electrical resistance of cement paste with and without carbon fibers during hydration was measured and the relationship between hydration and compressive strength and electrical resistance was investigated.The effect of adding an early-strengthening agent or heating was also studied.Results show that the electrical resistance of the cement paste fluctuates sharply during measurement because of strong polarization.The fluctuation of electrical resistance of the carbon fiber reinforced cement paste(CFRCP)disappears when more than mass fraction 0.30% of carbon fibers is added,and the electrical resistance shows a linear relation to the extent of hydration when the carbon fiber mass fraction is 0.35%.The same results are observed for CFRCP treated by the early-strengthening agent or heat curing.Therefore,it is feasible to monitoring the hydration of cement-based materials by testing its electrical resistance.

Electric polarization in carbon fiber-reinforced cement

Electric polarization induced an increase of the measured electrical resistivity of carbon fiber-reinforced cement paste during resistivity measurement. The effect was diminished by increasing the conductivity of the cement paste through the use of carbon fibers that were more crystalline, the increase of the fiber content, or the use of silica fume instead of latex as an admixture. Intercalation of crystalline fibers further increased the conductivity of the composite, but it increased the extent of polarization. Voltage polarity switching effects were dominated by the polarization of the sample itself when the four-probe method was used, but were dominated by the polarization at the contact–sample interface when the two-probe method was used. Polarization reversal was faster and more complete for the latter.

Cement as a thermoelectric material

Cement pastes containing short steel fibers, which contribute to electron conduction, exhibit positive values (up to 68 μV/°C) of the absolute thermoelectric power. Cement pastes containing short carbon fibers, which contribute to hole conduction while the cement matrix contributes to electron conduction, exhibit negative or slightly positive values of the absolute thermoelectric power. The hole and electron contributions in carbon fiber reinforced cement paste are equal at the percolation threshold. Addition of either steel or carbon fibers to cement paste yields more reversibility and linearity in the variation of the Seebeck voltage with temperature difference (up to 65 °C).

Uniaxial tension in carbon fiber reinforced cement, sensed by electrical resistivity measurement in longitudinal and transverse directions

Uniaxial tension of carbon fiber reinforced cement paste in the elastic regime caused reversible increases in both longitudinal and transverse resistivities. Without the fibers, the resistivity increase was much smaller and less reversible.

Enhancing the Seebeck effect in carbon fiber-reinforced cement by using intercalated carbon fibers

The absolute thermoelectric power of carbon fiber-reinforced cement paste was rendered as negative as −17 μV/°C by using bromine-intercalated carbon fibers, which had a high concentration of holes. The corresponding paste with pristine carbon fibers exhibited absolute thermoelectric power as negative as −0.8 μV/°C only.

Seebeck effect in carbon fiber-reinforced cement

The Seebeck effect in carbon fiber-reinforced cement paste was found to involve electrons from the cement matrix and holes from the fibers. The two contributions were equal at the percolation threshold, with a fiber content between 0.5 and 1.0% by mass of cement. The hole contribution increased monotonically with increasing fiber content below and above the percolation threshold. The fiber addition increased the linearity and reversibility of the Seebeck effect. Silica fume and latex as admixtures had minor influence on the Seebeck effect.

Ozone treatment of carbon fiber for reinforcing cement

Ozone treatment of isotropic-pitch-based carbon fiber was found to increase the surface oxygen concentration and change surface oxygen from C–O to CO, thereby causing the contact angle between fiber and water to be decreased to zero. Thus, the bond strength between fiber and cement paste was increased and the tensile strength, modulus and ductility of carbon fiber reinforced cement paste were increased. Moreover, the degree of dispersion of fibers in mortar was increased and the effectiveness of the fibers for reducing the drying shrinkage was improved. As a consequence, the strain sensing ability of carbon fiber reinforced mortar was improved in terms of increased gage factor and better repeatability. The ozone treatment did not affect the morphology, tensile strength or volume electrical resistivity of the fiber itself.

Improving the tensile properties of carbon fiber reinforced cement by ozone treatment of the fiber

The tensile strength, modulus and ductility of carbon fiber reinforced cement paste were increased by ozone treatment of the fibers prior to using the fibers. Increases were observed whether or not the paste contained methylcellulose/silica fume/latex. The ozone treatment involved exposure to O3 gas (0.3 vol.%, in air) for 10 min at 160 °C.