## 4 Discussion on Optimal Carbonation Curing Conditions and Enhanced ITZ  
The carbonation curing period plays an important role in the changes of physical and mechanical properties of composites. A curing period of less than 3 h resulted in significant effects on the bulk density, modulus of rupture, modulus of elasticity, and toughness. Under semi-adiabatic conditions, the temperature in the curing chamber gradually increased to 60 \({}^{\circ}\)C during this period (see Figure 5). If we stop the carbonation directly after 3 h curing, heat and moisture loss could occur resulting in less hydration and degree of carbonation. However, at longer curing times, the results show that the  
Figure 13: Scanning electron micrographs on a fractured sample of fiber-cement composites (a) non-carbonated; (b) 1 h carbonated sample; (c) 9 h carbonated sample; (d) refined fibers containing a large number of filaments.  
carbonation improved the mechanical and physical properties, except for toughness because of its inverse relationship with MOE as described earlier (see also Figure 11a). In addition, the water content has a significant impact on the carbonation curing process. The higher the rolling pressure, the better the physical and mechanical properties. However, increasing rolling pressure higher than 5 bar induced a significant drop in toughness, which results in the composites not meeting the minimum requirement for toughness. Higher rolling pressure up to a certain value slowed down the carbonation rate because of the retention process resulting from the decrease of apparent void volume and density, which lowered the diffusion and permeability of CO\({}_{2}\). As a result, the carbonation efficiency decreased as shown in Figures 8 and 9.  
Increasing CO\({}_{2}\) pressure indeed has a positive impact on the carbonation process because it increases the concentration of CO\({}_{2}\) in both gas and aqueous phases to carbonate the cementitious matrix. We see in Figure 6a that the MOR does not improve when the carbonation curing time lasts longer in the case of 1 bar CO\({}_{2}\) pressure. However, the MOR significantly increased when the CO\({}_{2}\) pressure increased to 3 bar.  
The SEM microstructural analysis helps to understand how the accelerated carbonation process improves the contact between filaments of cellulose fiber and cement matrix by favoring a better adhesion between them. However, note that higher fiber-matrix bond could have negative effects on the toughness and flexibility of fiber-cement composites. The matrix around the fiber has a porosity gradient with lower and higher density regions than the bulk paste matrix as illustrated in Figure 14. This is because of the fact that (cellulose) fiber can absorb water and thus increase the water to cement ratio surrounding the fiber, which results in a higher porosity region during cement hydration. Therefore, the fibers are easily pulled out under mechanical forces during the fractured sample preparation for SEM analysis as shown in Figure 13a. These weak regions have been improved thanks to carbonation. The development of the microstructure as the carbonation advanced results in a reaction zone containing a considerable volume of calcium carbonate crystals, which tend to deposit in large cavities or regions with higher porosity. Analysis of SEM images (Figure 13c) could prove the presence of a reaction zone characterized by the precipitation of calcite in the pores in the interfacial transition zone (ITZ) between the fibers and cement matrix (Figure 14).