# Rheology, Hydration, and Microstructure of Portland Cement Pastes Produced with Ground Acai Fibers
## 3 Results and Discussion
### Hydration and Microstructure  
Figure 6 presents the isothermal calorimetry curves of the pastes. Incorporating 5% of either in nature fiber delayed the occurrence of the main heat flow peak from 11.3 h (REF) up to 71.8 and 75.5 h, respectively, for 5% IN and 5% IN-S. In turn, the incorporation of 10% of either in nature fiber (10% IN and 10% IN-S) delayed the main heat flow peak for over 160 h, which was the maximum testing time. [PERSON] et al. [56] characterized acai fibers, observing that they are usually composed of about 9% moisture, 2% incombustible (i.e., ash), and 89% volatile matter, the latter corresponding mainly to sugars. The volatile fraction was composed of 30% glucose, 19% xylose, 45% polysaccharides (27% cellulose + 18% hemicellulose), and 35% lignin. In this regard, [PERSON] et al. [57] investigated the effect of different saccharides on the hydration of cement pastes. The authors observed that the addition of 0.2% and 0.5% glucose delayed the main heat flow peak by about 2 and 5 days, respectively, compared with plain cement paste; 0.2% sucrose addition was enough to delay it for 5 days. According to these authors, saccharides change the surface of the hydrating cement particles and hydration products, promoting the formation of a temporary barrier and therefore hindering further reaction. In addition, the authors observed that the increase in saccharide content reduces the concentration of Ca\({}^{2+}\) ions in pore solution. It is well accepted that the Ca\({}^{2+}\) supersaturation (and consequent precipitation of Ca(OH)\({}_{2}\)) is the trigger for the main cement reactions [58; 59]. Thus, a reduction in the Ca\({}^{2+}\) concentration delays cement's hydration kinetics. Similar trends were reported when different natural microfibers were added to the cementitious paste. [PERSON] et al. [60] incorporated 0.3-2.0% of cellulose microfiber over the cement weight in paste, observing that the main heat flow peak delayed from 16.6 h (control mix) up 28.9 h (2% fiber). [PERSON] et al. [61] incorporated ground hemp shives in 1-10% over the cement weight, observing that 5% addition delayed the main peak of heat release by 24 h, while 10% addition delayed it by over 144 h (the maximum testing time), similarly to that observed in the current work. One can expect that the reduction in the cement content caused by its replacement would delay the hydration due to the lower release of Ca\({}^{2+}\) ions in the pore solution. However, the incorporation of the quartz inert filler did not significantly affect the occurrence of the main heat flow peak for either replacement levels, indicating that the delay observed in the mixes with the fibers was not related with the cement reduction. The chemical treatment considerably reduced the delay in cement hydration caused by acai fiber incorporation, leading to the occurrence of the main hydration peak at 14.2 h when either treated fibers were added at 5% (5% T and 5%-S, Figure 6a). This behavior can be attributed to the reduction in lignin and cellulose contents discussed in Section 3.1. In addition, isothermal calorimetry curves showed that the hydration kinetics of pastes with 5% of either treated fibers was slower than that of plain cement paste within the first days (Figure 6a) but practically matched its cumulative heat at 160 h (with a difference of 2.8%, Figure 6b). These results indicate that the combined NaOH/HCl treatment used was effective in reducing the delay in cement hydration caused by acai fiber incorporation. Figure 7 shows the XRD patterns of the hydrated pastes at 160 h. Regarding the mixes with acai fibers, the incorporation of 5% in nature fibers led to low portlandite (18.1, 34.1  
and 47.1\({}^{\circ}\) 20) and ettrinigite (8.9, 15.8 and 23.0\({}^{\circ}\) 20) peak appearances, while the main alite peaks (29.3, 32.1, 41.2 and 51.7\({}^{\circ}\) 20) were much more intense than in the other samples.