3.2.2. Compressive Strength

Figure 3 shows the effect of CDW dosage and powder fineness on the compressive strength of the cement-based specimens. Based on the results, it can be concluded that the replacement of cement with CDW reduced the strength of the specimens. This correlates well with the results of other studies [16,20,25,26]. However, it is also seen that the ground CDWs showed better results compared to the unground (0 min) sample, at 20 and 30% cement replacement. Furthermore, specimens prepared using CDW ground for 5 and 10 min gave similar results at 20 and 30% cement replacement, which can be explained by their similar particle size distribution (Table 2). Generally, no clear correlation can be observed between the grinding time and replacement ratio. This can be explained by the heterogeneity and the high quartz content of the recycled cement stone [16,27].

**Figure 3.** Results of compressive strength measurements.

#### 3.2.3. FTIR

Figure 4 shows the FTIR spectra of the samples in the case of 0 min CDW dosing at different ratios. Based on Figure 4, it can be stated that the intensity of some bands decreased as a result of CDW dosing. The intensity of the bands around 1420 cm−<sup>1</sup> and 874 cm<sup>−</sup>1, which can be attributed to calcite and other carbonates, decreased to the greatest extent. In addition, the intensity of the band at 3640 cm−<sup>1</sup> (which indicates the presence of portlandite (Ca(OH)2) [28]) and the S–O stretching vibration of [SO4] <sup>2</sup><sup>−</sup> at 1150–1100 cm−<sup>1</sup> also decreased with the addition of CDW.

**Figure 4.** FTIR spectra of samples with different CDW content.

Figure 5 shows the FTIR spectra of the samples containing CDW with different powder fineness, produced with 20% cement replacement. Figure 5 shows that the same bands appeared for each sample. The intensity of the bands at 1420 cm−<sup>1</sup> and 874 cm−<sup>1</sup> belonging to calcite and other carbonates increased with the fineness of the CDW. The broad band at around 3400 cm−<sup>1</sup> is attributed to the symmetric stretching vibration of the H2O molecule, while the sharp, narrow band at 3640 cm−<sup>1</sup> can be assigned to O–H stretching vibration (portlandite) [26,28], which showed a slight decrease by 10 min milling. The band at around 960 cm−<sup>1</sup> implies the Si–O stretching vibrations, indicating a wide range of C–S–H. The band at 960 cm−<sup>1</sup> is assigned to Si–O stretching vibrations, which is the result of the C–S–H phase with Ca/Si ≈ 2. When it reaches 1080 cm−1, it indicates the formation of silica gel [29]. Higher intensity C–S–H related bands were observed with longer CDW cement stone milling times, indicating the effectiveness of mechanical activation.

**Figure 5.** The FT-IR spectra of specimens (cement replacement with 20% CDW).
