## 2 Materials and Methods
### Characterization of Cement Composites
#### 3.2.2 Evaluation of the Hydrates Formed in Cement Mortars  
Figure 7a shows the compressive strength and bulk density of mortars aged from 28 to 90 days, with their respective degrees of hydration shown in Figure 7b. It can be seen from this figure that the compressive strength evolves constantly over time and in the same way as the bulk density, which is not the case for the degree of hydration. The EURO2G and EURO5G mortars yield a strength that is identical or close to the reference, but the maximum strength seems to be reached after 60 days and then no longer evolves. Lastly, EURO15G shows weaknesses in compressive strength despite a significant change in strength over time compared to the other samples. For the same curing time, the higher the addition of FM, the lower the bulk density. The degree of hydration is at least identical to that of the reference but higher. This indicates good hydration of the hydraulic binder over time. These elements highlight the macroscopic granular, sandy shape of the mortar (Figure 4) caused by flocculation, which is responsible for the weakness of the material.  
Figure 6: Schematic representation of the retardant barrier formation mechanism and the increase in steric hindrance. (**A**) Anhydrorous cement grain, (**B**) formation of cationic sites on the surface of the cement grain, and (**C**) bonding of retarder and formation of a semi-permeable barrier around the cement grains.  
For EURO2G and EURO5G, Figure 7b shows the quantity of hydrates developed at different curing times. The evolution of the quantity of Portlandite and CaCO\({}_{3}\) over time are the opposite, indicating a dissolution of the carbonates by prolonged hydration over time. This trend is not observed with the reference, in which the evolution of Portlandite and carbonates are linked. These results suggest that nucilage promotes carbonation at young ages and, further, the presence of Portlandite by prolonged hydration. This phenomenon has been described for certain polysaccharides in the case of self-healing concretes [43, 44]. The increase in the carbonation of cement at young ages is favored by various factors, either jointly or independently:  
* The degradation of nucilage into alcoholic saccharides, which promote the formation of calcium carbonate;
* The presence of -COO\({}^{-}\) carboxyl groups within the nucilage and the accumulation of Ca\({}^{2+}\) on the surface of the cement grains increase the conditions for the bond between CO\({}_{3}\)\({}^{2-}\) and Ca\({}^{2+}\) ions. These conditions favor the precipitation and crystallization of CaCO\({}_{3}\)[45];
* The increase in nucilage concentration leads to an increase in grain flocculation and a decrease in the compactness of the cementitious matrix. The porosity created by the macro-structure of the composite can lead to a deeper access of ambient CO\({}_{2}\) into the material and thus to carbonation at an early age. This argument is validated by the work of [PERSON] et al. [46];
* The viscosity confers a bubble trap characteristic to the nucilage. The higher the viscosity, the stronger this characteristic. During the nucilage solubilization and mixing phase, the air bubbles contained in the nucilage entrap a significant quantity of O\({}_{2}\) and CO\({}_{2}\) from the ambient air in particular. These gases find their way into the cementitious matrix at early ages, encouraging the formation of CaCO\({}_{3}\)[47, 48].  
When FM is added in the matrix, the high amount of calcium carbonates formed in the first ages is followed by a decrease. FM is able to capture water in the ambient air as a high hygroscopic material. The captured water will diffuse slowly and in a controlled manner over time into the cement matrix, which may lead to a long-term hydration. This long-term hydration induced by the FM corresponds to the rise in the amount of Portlandite in Figure 7b.