## 3 Results
### Durability of Cementitious Composites Incorporating SS-PCM
#### 3.2.1 Volume Stability: Drying Shrinkage  
The drying shrinkage of cement-based materials can lead to structural cracking. In the literature, drying shrinkage is considered to be a consequence of changes in disjoining pressure, capillary pressure, and surface free energy or combinations thereof, accompanying  
Figure 11: Thermal conductivity according to density [2; 29; 30; 31].  
Figure 12: Specific heat of SS-PCM composite mortars.  
a decrease of saturation and internal relative humidity [32; 33; 34; 35]. The capillary pressure in the pore network is the mechanism that is usually employed to describe the hydric strains in porous materials [35]. It is related to the formation of water-air menisci in the partially empty pores, which induce isotropic compressive stress within the rigid solid skeleton that leads to bulk shrinkage. The essential cause of drying shrinkage is, of course, the evaporation of the capillary water from the material surface exposed to the ambient air. Evaporation occurs as soon as the relative humidity of the ambient air is lower than that prevailing in the capillary network. The tension forces developed inside the concrete lead to its contraction. The SS-PCM effect on drying shrinkage of SS-PCM mortars is investigated.  
The specimens of 24 h (4 \(\times\) 4 \(\times\) 16 cm\({}^{3}\)) are removed from the molds, and placed in a climate chamber (20 \({}^{\circ}\)C and 50% RH). The mass evolution of each sample is followed during the water evaporation. Figure 13 shows the mass loss over time for each mix-design mortar. The water desorption kinetics was rapid in the short term, then the kinetics slowed down for all mortar samples (M0, M5, M10 and M15) after 20 days. The mass loss of water into the mortars increased with the SS-PCM content. This was related to the pore diameter and porosity which increased with SS-PCM content.  
Moreover, the capillary pressure, is related to the internal relative humidity according to the Kelvin-Laplace law [32]. So, the drying shrinkage amplitude should increase with the water loss [36]. However, it is noted that the SS-PCM addition generated more water loss (Figure 13), but less drying shrinkage (Figure 14). The incorporation of 5%, 10% and 15% of SS-PCM into the mortar allowed reducing the drying shrinkage amplitude by 7%, 42% and 53%, respectively. This should be related to the higher compressibility of the SS-PCM compared to cementitious materials able to amortize the drying shrinkage [19]. Therefore, the SS-PCM (grains size 300-600 \(\upmu\)m) into the mortar matrix would amortize local stress due to the capillary pressure (generated by water loss) into the porous network, leading to less measurable bulk deformation of the skeleton. Hence, the addition of SS-PCM has the advantage of reducing macroscopic deformation linked to drying shrinkage.  
Furthermore, it should be noted that shrinkage is probably the most common cause of structural cracking [37; 38]. If not controlled, the deformation due to shrinkage can lead to durability problems and even shear stress [39; 40]. The SS-PCM could be used as a mitigation agent to reduce the drying shrinkage of cementitious materials.  
Figure 13: Mass loss of mortars over time.