Effect of silica-fume content on performance of CaCO3 whisker and basalt fiber at matrix interface in cement-based composites

Abstract

The bond at fiber–matrix interface is considered as one of the most important factors influencing the performance of fiber reinforced cementitious composites (FRCC). To improve such bond, adding silica fume is considered as an effective and efficient method. In modern FRCC, CaCO3 (calcium carbonate) whisker and basalt fibers witnessed increasing popularity due to their effectiveness in improving mechanical performance at multi-scales. However, systematic research still lacks for the bond improvement at their fiber–matrix interface. In this regard, silica fume of various contents was added to plain concrete (PC), CaCO3 whisker reinforced cementitious composites (CWRCC) and basalt fiber reinforced cementitious composites (BFRC). The bond strength at the fiber–matrix interface and mechanical properties of PC, CWRCC and BFRC were evaluated. Test results show that the addition of silica-fume up to 10% could enhance the interface bond and mechanical properties of CWRCC and BFRC up to 30% and 34%, respectively. SEM analysis revealed improved bond at the fiber–matrix interface with addition of silica fume as evidenced by the fact that the whisker after pulled out from the matrix was encapsulated by cement paste, as compared to that of plain composite without silica fume. Moreover, EDS energy spectrum of whisker and basalt fiber surface show that the Si content increased noticeably as a result of the increased hydration products adhering to the whisker and basalt fiber surface, thus improving the bond at the fiber–matrix interface. The incorporation of silica-fume in BFRC lead to a large number of attached hydration products to the surface of fiber after pulling out. At the initial stage of cement hydration, dispersed silica-fume particles play a role of “crystal nucleus”, which can absorb Ca + and OH– ions, and increase the number of CH nuclei. Furthermore, silica-fume continuously reacts with CH and procedure C-S-H gel which decreases the porosity of the interface layer, makes the structure more compact and improves the interfacial adhesion. The microstructure of the interface layer is enhanced by C-S-H which is generated by “secondary reaction” and increase the interfacial bond strength of fiber and matrix. Lastly, a microstructure schematic representation for the fiber–matrix interface was developed to understand the mechanism between the fiber and the cement matrix.