# Rheology, Hydration, and Microstructure of Portland Cement Pastes Produced with Ground Acai Fibers
## 3 Results and Discussion  
### Fiber Characterization  
Figure 2a shows the FTIR spectra of in nature and treated acai fibers. The broad band at 3600-3200 cm\({}^{-1}\) is associated with the O-H stretching of both lignin and cellulose: while the band attributed to lignin is placed around 3560 cm\({}^{-1}\), those associated with cellulose are placed at around 3280-3220 cm\({}^{-1}\)[33]. The band at 2920 cm\({}^{-1}\) is associated with C-H starching of methyl and methylene groups of cellulose [33,34]. The 1735 cm\({}^{-1}\) band is associated with C=O carboxyl and acetyl groups from (hemi)cellulose and the 1619 cm\({}^{-1}\) band is associated with C=C starching of groups in lignin [34]. The bands at 1380 and 1045 cm\({}^{-1}\) are associated with stretching vibrations of different carbohydrate groups, and the band at 1253 cm\({}^{-1}\) is associated with C-O stretching of lignin [33]. Figure 2b presents the XRD patterns of in nature and treated acai fibers. Both fibers showed a broad peak at around 22.1\({}^{\circ}\)20 associated with the combined amorphous contribution of lignin [35] and cellulose [36], in addition to minor peaks at 16.8 and 34.3\({}^{\circ}\)20 associated with the latter.  
It is worth noting that lignin removal with alkaline treatment has already been reported in the literature. [PERSON] et al. [37] treated sugarcane bagasse with 0.5-2.0% of NaOH for 10-120 min for this purpose. [PERSON] [38] partially removed lignin from Eucalyptus and Norway spruce woods with 0.3% NaOH for 1 h. In turn, acid treatment is known for promoting cellulose hydrolysis [39], as observed by [PERSON] et al. [40] when treated rice hulls with 0.3-2.4% H\({}_{2}\)SO\({}_{4}\) solution for 33 min. Considering that a coupled treatment with NaOH and HCl was used in this work, it can affect both lignin and cellulose from the acai fibers. The FTIR results revealed that the chemical treatment reduced the bands at 3280-3220, 2920, 1735, and 1253 cm\({}^{-1}\) associated with cellulose, while evidenced the \"shoulder\" band at 3560 cm\({}^{-1}\) and kept the band at 1619 cm\({}^{-1}\) associated with lignin. These results indicate a cellulose removal with the treatment while no major changes in lignin content seem to have occurred. As for the XRD results, the chemical treatment narrowed the main diffuse peak at 22.1\({}^{\circ}\) 20, while evidencing two minor peaks at about 16.8\({}^{\circ}\) and 34.3\({}^{\circ}\) 20. It is stressed that XRD evaluates the long-range order (i.e., the crystalline structure) of the material, while spectroscopy techniques (such as FTIR and Raman) evaluate the local order of the material on the scale of atomic bonds [41; 42]. Thus, XRD suggests that the chemical treatment disrupted the long-range order of lignin structure, but apparently without substantially reducing its content (from FTIR results).  
Figure 2c shows the zeta potential of the fibers before and after the treatment. The chemical treatment reduced the absolute zeta potential value from \(-\)36.2 to \(-\)25.7 mV, indicating an increased agglomeration trend with the chemical treatment.  
Figure 3 shows the SEM-EDS results of the acai fibers characterization. The EDS results correspond to the average wt.% and standard deviation of 6-8 area measurements randomly selected, while one representative spectrum is shown for each sample. The chemical treatment made the fiber surface less smooth, increasing its roughness on a scale of a few micrometers (Figure 3b,e). In addition, the EDS results showed that the carbon content was significantly reduced (from 75.3 to 60.4 wt.% on average) with the chemical treatment. These trends can be attributed to lignin and cellulose removal.