# Rheology, Hydration, and Microstructure of Portland Cement Pastes Produced with Ground Acai Fibers
## 4 Conclusions  
This work investigated the effect of acai ground fiber incorporation on the rheology, hydration, and microstructure of Portland cement pastes. In nature and chemically treated (with NaOH and HCl) fibers were used, in dry and saturated conditions. The FTIR, XRD, and SEM-EDS results revealed that the chemical treatment reduced the cellulose and lignin contents in acai fibers while increasing their surface roughness. The treatment also reduced the zeta potential (in absolute value), increasing the agglomeration trend.  
When incorporated in cement paste, 5% addition of either fiber slightly increased the yield stress and viscosity of the paste, while 10% addition drastically increased these properties, reaching yield stress and viscosity values respectively 40 and 8 times higher  
Figure 8: SEM images of the pastes at 7 days of hydration. REF: (**a**) [\(\times\)100; scale = 500 \(\upmu\)m]; (**b**) [\(\times\)500; scale = 100 \(\upmu\)m]. 5% T: (**c**) [\(\times\)100; scale = 500 \(\upmu\)m]; (**d**) [\(\times\)500; scale = 100 \(\upmu\)m].  
than those of plain paste. The highest incorporation level led to shear-thinning response at low shear rates, probably associated with the water release and fiber alignment with the increasing shear rate.  
The incorporation of 5% in nature fibers delayed the cement hydration kinetics by about 2.5 days while 10% in nature fibers delayed it by over 160 h, which was the maximum testing time. In contrast, the chemical treatment significantly reduced this retarding effect, leading to a 3 h delay when 5% treated fibers were incorporated. At 160 h, in nature fibers significantly reduced the cumulative heat (314-348 J/g compared with 432 J/g for plain paste), while the chemical treatment led to a 420 J/g cumulative heat at this age. In line with these results, XRD measurements at 160 h of hydrated pastes revealed comparable compositions for both plain and 5% treated fiber pastes, while the paste with 5% in nature fibers had lower alite consumption and low hydrates (i.e., portlandite and ettringite) formation. SEM of 7-day-old pastes revealed that the incorporation of acai fibers increased the macroporosity of the cementitious matrix.  
Overall, the combined NaOH/HCl treatment was effective for acai fiber functionalization, and the incorporation of 5% treated fibers led to fresh and hardened performances comparable to those of plain cement paste, indicating the feasibility of using such fibers in cementitious composites. As a general requirement to be used as reinforcement in a cementitious matrix, natural fibers should have a proper surface chemical composition (even if it requires chemical treatment), with controlled contents of lignin and cellulose.  
Conceptualization, A.A. and M.M.; methodology, A.A. and P.d.M.; validation, R.S.; formal analysis, L.S.; investigation, R.S., and L.S.; resources, P.G.; data curation, P.d.M. and L.S.; writing--original draft preparation, A.A., P.d.M. and M.M.; writing--review and editing, P.G. and J.d.B.; supervision, P.G., and J.d.B.; funding acquisition, P.G., and J.d.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
A.A. acknowledges the Brazilian governmental agencies CAPES, CNPq, and FAPERJ (proc. number E-26/210.150/2019) for their financial support. P.d.M., R.S., L.S., and P.G. acknowledge the Brazilian governmental agencies CAPES, CNPq, and FAPESC for their financial support. LabMat-UFSC and [PERSON] are kindly acknowledged for the SEM-EDS analysis. J.d.B. acknowledges the support of Instituto Superior Tecnico, the CERIS research unit, and the Foundation for Science and Technology. The authors declare no conflict of interest.