Dispersion of Sonicated Sulfated Cellulose Nanocrystals and Their Effect on the Mechanical Properties of Cement Mortars

Nanocellulose extracted from lignocellulosic biomass not only provides a promising way to deal with lignocellulosic biomass wastes but also contributes to the performance of cementitious materials. In this study, sulfated cellulose nanocrystals (CNCs) were selected as nanoreinforcement for cement mortars to improve their mechanical properties, especially the flexural strength. Three different sonication procedures were applied to CNC suspensions with CNC dosages of 0.01%, 0.05%, and 0.1% to improve their dispersion. The sonicated suspensions then were incorporated into the mortars. The effects of the sonication procedure and the CNC dosage on the particle dispersion of CNC suspension and the mechanical properties of mortars were examined in detail. The result showed that increasing the sonication energy from ∼7,835 to ∼37,587 J or lowering the CNC dosage from 0.1% to 0.01% could eliminate the aggregation of CNCs and increase the magnitude of the zeta potential of the CNC suspension, resulting in better dispersion (less aggregation in water). When continuous sonication for 12 min was used, the flexural strength of mortars with 0.01% CNC dosage improved the most, about 10%; however, an unexpectedly obvious decrease occurred when the CNC dosage was increased to 0.05% or 0.1%, which was due mainly to the reduced cement hydration degree and the more porous microstructure of the mortars. Thus, the optimum sonication energy needs to be adjusted for different dosages of CNC used; otherwise the CNC may have a negative effect on the performance of cement mortars. Given the same sonication procedure, the compressive strength of specimens did not change too much with the variation of CNC dosage; however, the flexural strength varied. For 3- and 28-day specimens, the flexural strength had maximum values for 0.05% and 0.01% CNC dosages, respectively. In addition, this study indicated that the improved dispersion of CNCs in water cannot ensure the significant strength improvement of mortars.