# Rheology, Hydration, and Microstructure of Portland Cement Pastes Produced with Ground Acai Fibers
## 2 Materials and Methods  
### Materials and Mixes  
Ordinary Portland cement and deionized water were used in all mixes. Table 1 presents the chemical composition (obtained by X-ray fluorescence), the mineralogical composition (obtained by X-ray diffraction and Rietveld analysis, with \(R_{\text{up}}\) of 7.4%), and the physical properties of the cement used. A quartz powder (\(>\)98 wt.% SiO\({}_{2}\)) was used as an inert filler. Acai fibers were extracted from açai waste (Figure 1a). The waste was ground in a planetary ball mill (PULVERISETTE 6, Fritsch) for 30 min at 300 rpm with 10 agate balls of 10 mm in diameter, and then was sieved in a 150 \(\upmu\)m-opening mask to separate the seeds from the fibers (Figure 1b). Then, the fibers were ground in the same ball mill for another 180 min to reach a powder-like size (Figure 1c). The fibers were used in four ways: in natura and chemically treated, each one in a dry and saturated condition. The chemical treatment was based on that adopted by [PERSON] et al. [17]. For this purpose, about 10 g of fibers were immersed in 300 mL of a previously prepared NaOH solution (with 10 wt.% concentration) for 30 min while stirring. Then, the fibers were washed with distilled water and subsequently immersed in HCl solution (1:10 in volume) for another 30 min while stirring to neutralize the pH of the fibers. Finally, the fibers were washed with distilled water again and dried in an oven at 60 \({}^{\circ}\)C until reaching constant weight. For the saturated application, about 10 mg of fibers were immersed in 200 mL of water, homogenized, rested for 1 h, and then kept in a room at 23 \({}^{\circ}\)C and 50% relative humidity. Fourier-transform infrared spectroscopy (FTIR) was conducted using a Cary 600 Series (Agilent, Santa Clara, CA, United States) spectrometer, with an analysis range of 500-4000 cm\({}^{-1}\), resolution of 16 cm\({}^{-1}\), and 64 accumulations. X-ray diffraction (XRD) tests were conducted using a Miniflex II Desktop (Rigaku, Tokyo, Japan) diffractometer operating at 30 kV and 15 mA with CuK\(\alpha\) radiation and wavelength of 1.5406 A. Zeta potential tests were conducted using a Malvern Zetasizer Nano, at a size range of 3.8 mm-100 \(\upmu\)m, temperature of 25 \({}^{\circ}\)C, and pH of 7. SEM images of the fibers and the hydrated pastes were acquired using a VEGA3 (TESCAN) microscope operating at 15 kV. Energy-dispersive X-ray spectroscopy (EDS) analysis was also conducted in these samples using an x-art (Oxford) detector. The characterization results are presented in Section 3.1. A total of nine cement pastes were prepared, with a constant water-to-solid ratio of 0.50 by weight. A reference (i.e., control) paste was produced containing only cement and distilled water, while the fibers replaced cement at 5% and 10% by weight. Fibers are usually incorporated in amounts related to the overall composite; however, since ground acai fibers were used here in a particle size distribution similar to that of a filler, they were classed in terms of cement replacement. Additionally, pastes with 5% and 10 wt.% replacement with inert quartz powder were produced. Table 2 details the composition of the pastes investigated, where the sample name is given by the content (5% and 10%) and type of fibers (\"IN\" for in nature, \"T\" for treated, and \"S\" for saturated) or quartz powder (QTZ), besides REF for the control mix. The percentages of addition of natural fiber were determined based on other studies in the literature [31; 32].