## 3 Results and Discussion
### Effect of Rolling Pressure in the Production Process  
As stated earlier, this parameter controls the water content in cement-fiber composites. Higher rolling pressure will decrease the water to cement ratio affecting the degree of hydration, which is the main factor influencing the initial microstructure of the cement-fiber composites before carbonation. As such, the carbonation process will behave differently resulting in different evolution of mechanical and also physical properties as seen in Figures 8 and 9. For non-carbonated samples, the results show that, at a high rolling pressure of 9 bar, the MOR and MOE increased up to 13.2 MPa and 12,683 MPa, respectively. However, the toughness decreased from 12.2 to 3.1 kJ/m\({}^{2}\) when the rolling pressure increased from 2.5 to 9 bar. The AVV and WA also exhibited a relative decrease with the increase of rolling pressure, while the BD increased significantly because of water loss at higher rolling pressure. When the samples were carbonated at a high rolling pressure, the mechanical properties were not significantly affected compared to the non-carbonated samples, except for the MOR, which exhibited a significant increase because of carbonation. In general, when applying higher rolling pressure, the initial pore volume is reduced resulting in a slower CO\({}_{2}\) penetration, thereby lower carbonation degree. This makes the changes in MOE and toughness at a higher rolling pressure much smaller compared to the case with a lower rolling pressure. In this instance, MOE increased by 72.3% at 3 bar of rolling pressure. However, at a higher rolling pressure of 9 bar, the MOE increase was only 9.8% compared to the non-carbonated samples. On the downside, a high rolling pressure can induce cracks in the composites. Therefore, we observed a maximum MOR at 5 bar rolling pressure. With further increase in pressure, cracks could occur and thus a lower MOR. When samples were carbonated, these cracks could accelerate the CO\({}_{2}\) penetration, which results in an increase of MOR. Despite this beneficial effect, the MOR of carbonated samples at 9 bar rolling pressure was still smaller than at 5 bar rolling pressure. Figure 7: Average values of physical properties in difference CO\({}_{2}\) pressure (\(\times\) 1 bar and \(\square\) 3 bar CO\({}_{2}\) pressure): (**a**) Apparent void volume (AVV); (**b**) bulk density (BD) and (**c**) water absorption (WA). Water plays an important role in the carbonation process as carbonation does not occur without water because of its role as a catalyst in the carbonation reaction. However, water can fill the pores and prevent CO\({}_{2}\) gas penetrating into the fiber-cement pore network. In fact, carbonation reaches the highest efficiency at an internal relative humidity close to 60-70% [9], which could never be reached in our case because it requires a much higher rolling pressure that may induce cracks in the composites as discussed above. Therefore, the samples should have low water content but remain uncracked and have a reasonable pore volume to optimize the carbonation process. Figure 8 also shows the minimum requirements of MOR, MOE, and toughness for fiber cement roof covering products [44]. We can see that whatever the rolling pressure, the MOR and MOE always meet the requirements. However, the toughness is lower than the required value if the rolling pressure is larger than 4 bar. In this sense, the optimal rolling pressure for carbonation curing would be in the range 2.5-3 bar because high rolling pressure may induce cracks resulting in an MOR drop as shown in the case of 9 bar rolling pressure. The rolling pressure significantly affected the physical behavior of the fiber-cement composites, including AVV, BD, and WA as seen in Figure 9. In general, for both carbonated and non-carbonated samples, a higher rolling pressure reduced AVV and WA and increased BD. When the rolling pressure increased from 2.5 to 9 bar, the average AVV showed a significant decrease of 12% and 9% for non-carbonated and carbonated samples, respectively.