## 4 Results and Discussion
### Effect of Storage  
The effect of storage on early-age mechanical properties, specifically in terms of elastic E-modulus, was investigated on both CEM II/A- and CEM IV/B-based cement pastes, according to the scheme of Table 2. Starting from the recorded accelerations and applying  
Figure 4: (**a**) dimensionless curves of CEM II/A and CEM IV/B; (**b**) incremental ratio of dimensionless curves. the procedure described in Section 3, the fundamental natural frequency and E-modulus of the tested composite beams were obtained. Figure 5 shows the results for the pozzolanic CEM IV/B cement paste. As known, from a phenomenological point of view, after mixing cement with water, the hydration process starts. At a microscopic level, the process is characterized first by a period of almost no reaction, which is called the dormant period, with the initial formation of ettrinig; production of C-S-H (calcium silica hydrate) and calcium sulfoferrite characterizes the subsequent setting and hardening phases [37]. From macroscopic EMM-ARM results, an initial dormant period of between 1 and 2 h since the beginning of the test was observed for all tested specimens. The dormant period is characterized by nearly zero values of the Young's modulus. Setting and hardening stages took place afterward. During such a phase, the stiffness monotonically increased, as expected, regardless of the storage time. However, a considerable effect of storage time on both E modulus absolute value and hardening time was experimentally observed. The E modulus and hardening rate, i.e., stiffness value reached in a certain period, reduced as storage time increased. In particular, 30 days of storage (see specimen C325d30) caused a decrease in E equal to 8% on average during the observation period (i.e., 120 h); cement stored for 360 days (see specimen C325d360) yielded, on average, a 26% reduction in the Young's modulus over the whole observation period. For specimens C325d240 and C325d360, the elastic modulus assumed very similar values in the first 36 h of observation, whereas larger differences were observed thereafter; this result seems to suggest that prehydration completely affected the formation of the first reaction products. At the end of the observation, i.e., after 120 h, the elastic modulus reached the following values: 6.85 GPa for C325d0, 6.55 GPa for C325d30, 5.83 GPa for C325d240, and 5.57 for C325d360. Therefore, 1-, 8-, and 12-month storage caused respectively 4.4%, 14.9%, and 18.7% reduction in E modulus. It is noteworthy that, as expected, after 5 days of observation, the E vs. time curves do not show a clear plateau because the significant presence of pozzolanic delays the achievement of the long-term stiffness. Figure 5: Elastic modulus vs. time for CEM IV/B cement paste at different storage times. Two experimental tests were also carried out on CEM II/A cement paste. In the present case, a reduced number of tests have been carried out with respect to CEM IV/B because they mainly addressed existing knowledge, even if from a different (phenomenological rather than chemical-physical) point of view. Specifically, two different storage conditions were considered: new material (right after the opening) and material stored for 240 days (i.e., 8 months) in uncontrolled conditions. Figure 6 shows E vs. time curves for an observation window of 120 h after casting. In this case, the reduction in the elastic modulus is more significant with respect to the pozzolanic CEM IV/B cement paste. Unlike the pozzolanic CEM IV/B cement pastes, at the end of the observation period, the curves showed a substantial reduction in slope because of approaching the maximum long-term stiffness value. The values of the elastic modulus at 120 h were equal to 15.6 GPa and 9.7 GPa for the C425d0 and C425d240 specimens, respectively, corresponding to a final reduction of 38%.