# Rheology, Hydration, and Microstructure of Portland Cement Pastes Produced with Ground Acai Fibers
## 3 Results and Discussion
### Rheological Tests  
Figure 4 shows the flow curves (i.e., shear stress vs. shear rate) of the pastes. The flow curves can be divided into two groups: the REF mix, the mixes containing 5% of either fiber and the mixes with 5% and 10% quartz powder (marked a) presented shear stresses within 10-90 Pa, while the mixes containing 10% of either fiber (marked b) showed significantly higher shear stresses, within 180-600 Pa. In addition, two flow curve profiles can be noted. In the (a) group, viscosity (visually assessed by the slope of the flow curve) was approximately constant at low shear rates (marked (i) in Figure 4), followed by a shear thickening region (marked (ii)) that is characterized by an increase in viscosity as the shear rate increases. In contrast, the mixes of the (b) group showed a shear-thinning response at low shear rates (marked (iii)) that corresponds to a viscosity decrease with the shear rate increase, followed by a shear thinning behavior at higher shear rates as for (a) group. The shear thickening response at high shear rates has already been reported for cement-based materials [43; 44; 45; 46]. This corresponds to the transition from a shear-ordered structure (e.g., in (i)) to a state of hydrodynamic clustering: as the shear rate increases, the hydrodynamic forces become enough to remove the particles from the layer-like flow, disturbing the ordered state and consequently clustering the particles [47]. Since these  
Figure 2: Characterization of the acai fibers. (**a**) FTIR spectra; (**b**) XRD patterns; (**c**) Zeta potential (average \(\pm\) standard deviation of three measurements). In turn, the shear-thinning response at low shear rates for the (_b_) group may be associate with two phenomena. The increasing shear rate can release the water absorbed by the fibers during mixing (for dry fibers) or previously added (for saturated fibers). The occurrence of water release with shearing was previously reported by [PERSON] et al. [48] for cement mortar containing superabsorbent polymers (SAP). Also, the fibers that are initially randomly oriented can align with the increasing shear, facilitating the flow and thus reducing viscosity. Figure 5 shows the rheological properties of the pastes. The incorporation of 5% of either replacement material increased the yield stress of the paste from 6.2 Pa (REF) to 24.3 Pa (5% T) and its viscosity from 0.3 Pa.s (REF) to 0.8 Pa.s (5% T). In turn, the incorporation of 10% fibers drastically improved the rheological properties of the paste, reaching a yield stress of 250 Pa and a viscosity of 2.8 Pa.s for the mix with 10% in nature fibers. This can be associated with the elongated shape of the fibers (Figure 3a,d), which hinders the flow of cement particles during shear. These results are consistent with those reported by [PERSON] and [PERSON] [49], which incorporated 0-2% of cellulose microfibers in cement paste and observed progressive yield stress increases with the increase in fiber content. Moreover, for the same addition ratio, incorporating treated fibers slightly increased the yield stress of paste compared to the in natura fibers: 24.3 Pa for 5% T vs. 17.1 Pa for 5% IN, and 14.8 Pa for 5% T-S vs. 13.5 Pa for 5% IN-S. This can be associated with the partial removal of cellulose and the structural order disruption of lignin promoted by the chemical treatment, as per the FTIR and XRD results (Figure 2a,b). Lignin is one of the main constituents of lignocellulosic-based plasticizer admixtures, which promotes the dispersion of cement particles through electrostatic repulsion [50]. [PERSON] et al. [51] found that cellulose nanocrystals can act like water-reducing agents by adsorbing on cement surfaces and improving the dispersion of the particles. Thus, the removal/disruption of these compounds may negatively affect the rheology of the fresh paste.