# Cement Pastes with Hygyroscopic Polymeric Additions for Potential Building Applications
## 3 Results and Discussion
### Mechanical Tests  
To gain further insight into the response of cement pastes under shear loading/ unloading cycles, tests were carried out on small prismatic specimens. Each test consisted of five cycles; Figure 6a-c shows the load-displacement curve (maintaining the same  
Figure 4: SEM micrographs of 3-day-aged (**a,b**) and 28-day-aged (**c,d**) SA-PA samples. Figure 5: SEM micrographs of 3-day-aged (**a,b**) and 28-day-aged (**c,d**) CS samples. abscissa). The presence of a hysteresis loop suggests the occurrence of anelastic phenomena. The area of each hysteresis cycle is proportional to the energy dissipated during each loading-unloading cycle. All the curves started from zero (stress-strain), but none returned to zero, maintaining a permanent strain (non-recoverable strain); this depended on the curing time, additions, and of course, the previous loading history [26]. After three days (Figure 6a), the amount of energy dissipated in each cycle was very small. This suggests that the stiffness of each sample was maintained high for the entire stress-strain test. The stiffest materials were those with fibers and the most deformable were CS and SA-PA after seven days of aging. These plots are not shown. This behavior is neither strange, nor was it unexpected. In a previous paper [20], it was reported that cement paste prepared with a crystallization mixture is more permeable after seven days than after three days of aging, due to the evaporation of water when the crystallization of cement phases is far from complete. On the other hand, polyacrylate acts as a plasticizer in cement paste, absorbing water and always retaining at least a small amount of it. After 14 days of aging, the deformability of all the samples increased. SA-PA/Ny not only showed an increase in permanent strain with each loading and unloading cycle, but also exhibited an increasing maximum strain: every time the sample was unloaded, the next cycle moved to higher maximum deformation, even if the sample regained almost the first permanent deformation. In Figure 6b, the unloading and the reloading paths of the SA-PA/Ny sample are significantly different, exhibiting large hysteresis loops. The curves of the 14-day-aged samples showed an inflection point on unloading. This could be  
Figure 6: Loading–unloading shear stress–shear strain (%) curves at different curing times: 3 days (**a**), 14 days (**b**), and 28 days (**c**). an indicator of crack development in the material [42]. Degradation of the concrete was reflected in the decrease in the slopes of the reloading curves. Reloading curves were nearly linear, up to the beginning of the unloading curve, after which softening could be observed. The unloading curve softened gradually while the stress decreased and the stiffness of the unloading curve became smaller. After 28 days of aging, all the materials appeared to be stiff (see Figure 6c), except for NY. The latter exhibited considerable permanent deformation which increased every cycle, while the hysteresis loop did not increase at all. The number of microcracks in the paste was expected to increase sharply. As a result, undamaged portions that carried the load were reduced and the stress-strain relationship became even more non-linear. The residual strains were a function of the strain at unloading. An increase in unloading strain caused approximately the same increase in the accumulated residual strain. The differences from the other samples were mainly due to the extent of crystallization of the cement paste, which, in turn, depends on the degree of porosity and viscous deformation. In Figure 7a-d, the evolution of the shear stress/shear strain curves during aging (3, 14, and 28 days) for each type of sample is shown. As the aging increased, the viscous deformation of all the samples decreased progressively, but this could not occur at the same rate or achieve the same final properties.