Influences of hybrid fiber-reinforcement using wollastonite and cellulose nanofibers on strength and microstructure characterization of ultra-high-performance mortar

This paper presents the effect of incorporating wollastonite microfibers and cellulose nanofibers on mechanical properties and microstructure of ultra-high-performance mortar (UHPM). Cellulose nanofibers of 0.005%, 0.015%, and 0.030% are combined with 4.8% wollastonite microfiber by weight of binder, which consists of 10% silica fume and 90% cement. Compressive and flexural strengths of the UHPM specimens are measured at 7 days. The microstructure is characterized in terms of SEM/EDX, XRD, FTIR, and nanoindentation. The results show the effectiveness of the fiber hybridization in enhancing the compressive and flexural strength of ultra-high-performance mortar with the improvement in flexural toughness and fracture energy by as much as 10% than control mortar. Moreover, hybrid fiber-reinforcement based on the microstructural analysis provided more synergic effect and uniform distribution of the fibers, thus developing an efficient micro crack bridging mechanism as well as resulting in enhanced toughening properties and an increased load carrying capacity. Additionally, the combination of wollastonite microfibers and cellulose nanofibers promoted the hydration reaction products and contributed a denser matrix between the fibers and cement matrix, resulting in an increase of the microstructure and interface area between the fibers and cement matrix. Nano indentation analysis also confirmed the increase of UHD C-S-H from 65% to 84% and lower percentage of anhydrous phase in ultra-high-performance matrix after combining wollastonite microfibers and cellulose nanofibers. •The hybridization of cellulose nano fiber with wollastonite microfiber increased the compressive strength of unreinforced mortar.•The greatest advantage in hybrid-reinforcement systems in ultra-high-performance mortar is the improvement in flexural toughness.•Hybrid reinforcement systems develop an efficient crack bridging mechanism as well as resulting in enhanced toughening properties and an increased load carry capacity.•The addition of wollastonite promoted a higher UHD C-S-H compared to the control specimen and other hybrid reinforced mortars.•Cellulose Nano Fibers enhance cement hydration due to the high specific surface area, in addition to the promotion of adhesion with cement paste and bonding properties when combined with wollastonite.