## 3 Results and Discussion  
### Effect of Curing Time on Mechanical and Physical Behavior  
Figure 3 shows the relationship between the mechanical properties (MOR, MOE, and toughness) and carbonation curing period. In general, the increase of curing time considerably improved the mechanical properties of the carbonated cementitious matrix. Different mechanical behavior was found for the carbonated composites with increased carbonation time (Figure 3). The carbonated composites at any curing period showed higher average values of MOE and lower toughness than non-carbonated ones. However, only the long curing period of 9 h exhibited higher MOR, with similar values for 5 h curing and even lower MOR for 3 h curing. Figure 3 also shows an increase in MOR and MOE and decrease in toughness due to continuous hydration up to 28 days. For MOR, the carbonated composites indicated a better mechanical performance at 28-day testing, with approximately 2.4% and 6.4% increase after 5 h and 9 h curing time, respectively, compared to the non-carbonated samples. These beneficial outcomes would have resulted from the lower fiber deterioration under the less matrix alkalinity, which helped improve the fiber-matrix bond-ability and allow the dissipation of energy in the post-cracking region. However, MOR was slightly decreased at a shorter carbonation curing time (3 h) because the temperature during curing sharply dropped when  
Figure 2: The flexural stress versus flexural strain/deflection curves to characterize the mechanical properties of the cement composite materials: (**a**) Modulus of rupture (MOR) and modulus of elasticity (MOE); (**b**) toughness. removing the sample from the curing chamber. The rate of hydration and hence also the carbonation reaction rate decreased compared to the reference samples cured under semi-adiabatic conditions up to 8 h, while at longer curing time, the degree of hydration and carbonation reaction were much higher compared to that at 3 h curing. The MOE was significantly increased by 15.6%, 66.5%, and 76.5% for 3, 5, and 9 h carbonation time, respectively. This behavior is attributed to the reactions taking place during carbonation in which there was precipitation of calcium carbonate (CaCO\({}_{3}\)) in the pores [39], especially on the surface. The calcium carbonate densified the matrix and improved the fiber-matrix bond (i.e., ITZ) and consequently the mechanical behavior of the fiber-cement composite [14; 22]. Note that even at a short curing period (3 h), the MOE still increased despite the decrease of MOR because the slope of the stress-strain curve (see Figure 2) was still increased compared to the reference samples, while the strength could be lower resulting in lower MOR (see Equation (5)). The flexural toughness values were lower for the carbonated samples (Figure 3c), an approximate decrease of 38.7%, 48.7% and 85.3% for 3, 5, and 9 h carbonation time, respectively. The results show that the toughness was negatively correlated with carbonation time, the longer carbonation time, the lower the toughness. Reduction of toughness can be compensated for by the increase of MOE, but should not be less than, 5 kJ/m\({}^{2}\), which is the minimum value required for proper transportation and installation of cement fiber composites at the construction site. Figure 3: Average values of mechanical properties at different carbonation time (\(\times\)non-carbonated curing (NCC), \(\square\) 3 h accelerated carbonation curing (ACC), \(\bullet\)5 h ACC and \(\Delta\) 9 h ACC): (**a**) Modulus of rupture (MOR); (**b**) modulus of elasticity (MOE) and (**c**) toughness. Figure 4 shows the comparison of the effects of the accelerated carbonation on the physical characteristics of water absorption, apparent void volume, and bulk density for carbonated and noncarbonated fiber-cement composites at different carbonation times.