5.1.5. SEM Images Analysis

Micrographs of RCM are shown in Figures 10–14. The shape of RCM particles (Figures 10 and 11) was defined as irregular polyhedral [37], and they were very similar to cement grains. On the surface of the RCM particles, crystalline inclusions have also been observed, which may probably be the remains of hydrates from the primary hardening process, e.g., parts of non-hydrated cement, free calcium, as well as calcium silicates, as indicated by the results of the phase composition test presented in Section 5.1.3.

**Figure 10.** SEM micrograph of RCM particles, mag. 2000×.

**Figure 11.** SEM micrograph of RCM particles, mag. 10,000×.

Micrographs of the microstructure of cement composites with RCM are presented in Figures 12 and 13. Observations of the microstructure of cement composites (Figures 12 and 13) confirmed that RCM can be perfectly embedded in the cement matrix, forming a compact microstructure. The compact C-S-H phase can be observed in Figures 12 and 13. The compact microstructure may result from strong bonding of high-quality recycling mortar containing C-S-H phases to the components of new paste, as well as the developed surface of the recycled fraction due to applied thermal and mechanical treatment [20].

As it has been noted, the rough surface of the aggregate (in this case also RCM particles) deteriorates the portlandite orientation [39]. The arrangement of portlandite plates (CH) and the C-S-H phase surrounding them are presented in Figure 14.

**Figure 12.** SEM micrograph of cement composite with RCM addition, 1—C-S-H gel (mag.10,000×).

**Figure 13.** SEM micrograph of cement composite with RCM, 1—C-S-H gel (mag. 16,000×).

**Figure 14.** SEM micrograph of microstructure of composite with RCM after thermal and mechanical treatment, 1—portlandite, 2—C-S-H gel (mag. 8000×).

The orientation of portlandite crystals is additionally disturbed by the presence of Ca(OH)2, which, being the nuclei of crystallization of this phase, causes the growth of calcium hydroxide crystals in various directions, improving mortar strength properties [39].
