## 4 Conclusions  
This study presented the results of the analysis of the damping properties of cement pastes reinforced with different quantities (0 wt %, 0.5 wt %, and 1 wt %.) of untreated and alkali-modified flax fibers. Our conclusions are summarized as follows:  
1. Alkali treatment roughened the surface and increased the crystallinity of the flax fiber, improving adhesion between the cementitious matrix and fiber by removing amorphous components.
2. Flax fiber increased the population of macropores and voids and microcracks in cement composites, which are the main causes for the decrease in compressive strength in flax fiber reinforced cement composites.
3. The addition of flax fibers reduced the compressive strength and increased the flexural strength of the cement composite regardless of whether the flax fibers were treated or not. Alkali treatment somewhat offset this reduction in compressive strength. Compared with the untreated flax fiber, alkali treated flax fibers at an amount of 1 wt % of cement improved the compressive and flexural properties of composites, i.e., 4.55% and 11.3% growth in the compressive and flexural strength, respectively.
4. The incorporation of flax fibers was an effective method to enhance cement damping properties. The loss tangent of the cementitious composites reinforced with raw and alkali treated flax fibers at an amount of 1 wt % of cement was improved by 45.83% and 37.5% at 20 Hz, respectively, that is due to the friction in the internal structure of the flax fiber, and slipping between the fiber and matrix. The damping effect was greater in untreated flax fibers when compared to alkali-modified flax fibers due to constraints on the damping system caused by the removal of pectin and lignocellulosic in treated fibers.  
There is still a lot of controversy about the durability of the flax fiber in cementitious matrix. Some literature indicated that the plant fibers can deteriorate due to the mineralization in the alkaline environment of cementitious matrices, but there is no direct evidence that the plant fibers impregnated by mineral species can cause the loss of mechanical properties in fibers. Many researchers suggested that the mineralization may not cause any reduction in the mechanical performance but merely fill the empty spaces inside the cell lumen and wall of the plant fibers [21]. Moreover, some researchers have suggested that the reason of the mechanical strength of plant fibers was decreased was due to the degradation at the fiber-matrix interface produced by biodeterioration [22]. To control composite  
Figure 11: Structural diagram of a flax fiber.  
degradation by alkaline attack, some researchers found that surface pretreatment of plant fibers can be effectively alleviated the degradation of plant fibers [13]. Nevertheless, as mentioned earlier, there are many questions about the application of flax fibers in cementitious composites, but due to its environmental friendliness, economy, and other advantages, it still has high research value. In order to further study the effect of flax fibers for the cementitious composites, we will complete the follow-up work:  
1. Study the effect of flax fibers for the cementitious composite in longer curing ages and different curing environment.
2. Seek a suitable fiber surface treatment to improve the bonding between the fiber and the matrix, thereby improve the durability of the flax fiber in the cementitious composites.  
Methodology, X.Z.; Experiment, P.L. and P.Y.; Writing, P.L., X.Z. and P.Y.; Supervision, X.Z., Z.W. and S.S.  
This research was funded by the Chinese National Key Research and Development Program of China, Program No. 2018 YFC0705703, the Chinese National Natural Science Foundation, Project designations: 51525802 and 51778183.  
The authors declare no conflict of interest.