دانلود رایگان مقاله پراکندگی نانولوله کربنی چند جداره و اثرات آن بر خواص کامپوزیت های سیمان

Abstract

In this study, two types of multi-walled carbon nanotubes (pristine, p-CNT and functionalized, f-CNT) were dispersed in water by sonication and then added to cement mortar. The purpose of this study was to characterize the dispersion degree of the CNTs in aqueous suspension and to investigate whether achieving dispersion in water would also result in dispersion inside mortar. Dispersion of the CNTs in water was investigated by means of UV–vis spectroscopy, using different CNT concentrations and sonication durations. Dispersion of the CNTs in cement mortar was investigated by measuring the compressive and flexural strength and fracture toughness as well as the microstructural characterizations of scanning electron microscopy and mercury intrusion porosimetry. The effects of the CNT addition on drying shrinkage and cement hydration were also investigated for cement pastes. The results of UV–vis spectroscopy showed that by increasing the sonication time to 120 min, the dispersion degree of the f-CNT suspension increased progressively, while for p-CNT, a maximum was reached with 60 min of sonication. The compressive and flexural strength and fracture toughness of mortars containing f- and p-CNTs were not significantly improved either by increasing the amount of CNT or imposing sonication in mixing water. High CNT dispersion in cement matrix was not equally obtained by utilizing highly dispersed CNT suspension. Sonication of f- and p-CNT led to a remarkable deceleration of cement hydration in the first hour of hydration and drying shrinkage of the cement composites was found to be reduced by f- and p-CNT addition.

4. Conclusions

In this study, the relationship between the degree of dispersion inside water and cement composites with sonication time was determined. The main focus was characterization of dispersion degree of functionalized- and pristine-CNTs in aqueous suspension by UVevis spectroscopy and comparing with the CNT dispersion state in mortar as verified by its compressive and flexural strength and fracture toughness. Drying shrinkage behaviour of the CNTcontaining composites and CNT effects on early cement hydration were also investigated. The results of UVevis spectroscopy showed that with increasing sonication time, the degree of dispersion in suspension for f-CNTs increased progressively up to 120 min, while p-CNTs showed a maximum with 60 min sonication. The compressive and flexural strength and the fracture toughness of mortars containing f- and pCNTs were not significantly improved either by increasing the amount of CNT or imposing dispersion in mixing water. In addition, dispersed f- and p-CNTs decelerated C3A hydration in the first hour of hydration. These results suggested that sonication was effective to reach a high degree of dispersion of f- and p-CNT in aqueous media and that the dispersion degree of suspensions containing fCNTs was higher than p-CNTs. However, utilizing the same dispersion method for mixing water to fabricate CNT-containing cement composites, a corresponding degree of dispersion inside the composite was not obtained. The reason could be reagglomeration of highly dispersed CNT suspensions after coming into contact with the solid phase. As a consequence, the agglomerates may mitigate the strengthening ability of CNTs. The drying shrinkage and total porosity volume of the cement composites in addition to pore volume related to pore sizes below 100 nm were found to be reduced by f- and p-CNT addition. Porosity analysis and microstructural observations evidenced the presence of individual CNTs inside the cement matrix, thus a limited degree of dispersion inside the cement composite could be reached by using the dispersion method. Insufficient bonding between both types of CNTs and hydration products could also contribute to the lack of mechanical improvement in addition to the negative effects of initial re-agglomeration on mechanical properties.