## 3 Results and Discussion
### Mechanical Properties  
The compressive strengths of the cement specimens are shown in Figure 6. The addition of flax fibers reduced the compressive strength of the cement in all cases. However, the loss of compressive strength reduces as curing age increases; F0.5 compressive strength was reduced by 22.6%, 16.82%, and 5.8% as compared to C0 at curing ages of 3 days, 7 days, and 28 days, respectively. MF0.5 demonstrates that the alkali treatment somewhat offsets the compressive strength reduction caused by flax fiber, with reductions of 21.78%, 7.89%, and 1.46% as compared to C0 at curing ages of 3 days, 7 days, and 28 days, respectively. Notwithstanding, concrete compressive strength decreased with increasing fiber content, even with alkali pretreatment. This was attributed to the presence of the lumen and pore in the wall of the flax fibers, and the fact that flax fibers display obvious hydrophilicity, this essential characteristic induced a stiff fight on the water demand between the flax fibers and cement particles in the mixing step. Hence, the fibers content clearly affect the water availability during cement hydration [17]. Moreover, water was released from the fiber towards the end of cement hydration, producing a water-cement ratio gradient near the fiber (the water-cement ratio near the fiber was higher). The higher water-cement ratio increased the porosity of the composite and thus reduced adhesion between the fiber and matrix. Furthermore, flax fibers have lower rigidity as compared to conventional fibers, reducing dispersion during mixing and promoting the formation of balls [18]. Some carbohydrates in the flax fiber (e.g., pectin) disturb the hydration reaction of the cement and delay the initial setting time of cement composites [7]. Alkali treatment dissolves the pectin and other impurities from the flax fiber, improving the progress of hydration [7]. As shown in Figure 6, increased flax fiber content decreased the compressive strength of cement specimens due to the undispersed fiber producing microfractures within the matrix (Figure 7).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multicolumn{3}{c}{**Volume Proportions (\%)**} \\ \cline{2-4}  & **Mesopores** & **Macropores** & **Voids and Microracks** \\ \hline C0 & 57.91 & 27.01 & 15.08 \\ F0.5 & 33.39 & 38.58 & 28.03 \\ F1 & 23.97 & 40.64 & 35.39 \\ MF0.5 & 42.81 & 31.05 & 26.14 \\ M1 & 32.33 & 38.81 & 28.86 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Proportion by volume of pore structures in cement specimens.  
Figure 6: Compressive strength of cement specimens for different curing ages.  
The flexural strengths of the cement specimens are shown in Figure 8. It is apparent that the flexural strengths of F0.5, F1, MF0.5, and MF1 were higher as compared to the control specimen, and strength increased with flax fiber content. Alkali-modified flax fibers appeared to further improve the flexural strength as compared to untreated fibers. After 28 days of curing, MF1 flexural strength was 10.47% stronger as compared to F1, and 41.7% stronger as compared to C0. This could be attributed to the alkali treatment increasing the roughness of the fiber surface by dissolving and removing impurities. A rougher surface would provide better adhesion between the fiber and matrix, and thus increase the flexural strength of the cement composite. Moreover, the alkali treatment results in an improvement in the flax fiber tensile strength by removing impurities and increasing density [7].