An Investigation on the Effects of Cellulose Nano-Fibrils (CNF) on the Performance of Cement Paste and Concrete (3)
Publication Name: Advances in Civil Engineering Materials
Cellulose nanofibrils (CNFs) are hydrophilic biodegradable nanoparticles (typically, less than 0.2 mm in length and 20-50 nm in width) that can be extracted from various plants, trees, and recycled paper stock. Their low density, low toxicity, relatively low cost, high aspect ratio, and high specific surface enable functionalization. In this work, the effects of CNFs on selected properties of cement paste, mortar, and concrete were investigated. For the cement pastes, rheology tests using a simple rheometer showed a reduction in workability as CNF dosage increased. Free shrinkage tests showed an increase in shrinkage at high w/c ratios but a decrease at low ratios. Additional tests showed that CNFs reduced autogenous shrinkage in low w/c ratio systems. Tests of mechanical properties showed that CNF additions improve compressive strength at low w/c ratios, but reduce strength for higher ratios. Fracture tests showed that CNFs have little effect on crack initiation energy, but they produce a significant improvement in overall fracture energy as well as modulus of rupture. Isothermal calorimetry and thermogravimetric analysis tests were used to examine effects of CNFs on cement hydration. Both tests showed no significant changes in the degree of hydration for pastes with CNF after 7, and 28 days. For interpreting the results, a tunnels, reservoirs, and bridges (TRB) mechanism is proposed. This model suggests that, as proposed by others, CNFs can modify microstructure by providing tunnels for transporting water to unhydrated cement grain. However, the improvement in hydration this would bring is negated by additional barriers formed later during hydration. Because of their hydrophilicity, CNFs retain water and work as reservoirs. This stored water isreleased at later ages and works as a supplementary source of water (internal curing), which explains the improvement in properties at low w/c ratios. Significant increases in fracture energy suggest that CNFs are an effective toughening mechanism, acting as bridges that increase the energy required for crack propagation. Effective dispersion of CNFs in the cement matrix is identified as a source of both high variability and some unsystematic results.