Dispersion of carbon fibers in cement-based composites with different mixing methods

Mixing methods can significantly influence dispersion of carbon fibers in cement-based composites. In this work, pre-mixing method (carbon fibers were added before cement) and after-mixing method (carbon fibers were added after cement) were adopted to prepare carbon-fiber-reinforced cement-based composites (CFRC). X-ray computed tomography (CT) was used to obtain the cross-sectional images of successive layers in the composites. Scanning electron microscope (SEM) was adopted to capture microstructure images of the composites. The CT images were continuously identified through their grayscale peaks. The sub-mm pores were separated from carbon bundles to improve the accuracy of the identification results. Two-dimensional (2D) dispersion characteristics of two different mixing methods were quantitatively analyzed by extracting pixels area of each component. Meanwhile, three-dimensional (3D) models were established by means of optimized CT images to obtain the volume fraction of carbon fiber bundles. The results show that the average areas of uniformly dispersed area, which obtains through pre-mixing method, are higher than those with after-mixing method. The dispersion effect of pre-mixing method is superior than that with after-mixing method. In addition, the volume fraction of carbon fiber bundles in specimens prepared through pre-mixing method was lower than that prepared by after-mixing method. Thus, the contrast resolution of X-ray CT system can recognize materials with over 0.3% density difference in CFRC; and the pre-mixing method is recommended in the preparation of carbon-fiber-reinforced cement-based composites.