*3.2. Linear Changes in the Cement Paste Samples*

The linear changes were defined as the change in the length of the prismatic cement paste samples. Length changes are given in Table 4.


**Table 4.** Length changes of the cement paste prismatic samples.

All the cement pastes studied showed dimensional stability for up to 7 days, as shown in Table 4. No or little shrinkage was observed due to the hydration process. These slight linear changes in the initial curing period probably resulted from the disappearance of monocarboaluminates in favor of higher amounts of ettringite.

Phase composition analysis was performed for the 60PC-20GBFS-15SFA and 40PC-40GBFS-15SF cement pastes. The XRD patterns after 2, 7, 14, and 28 days of hydration are presented in Figure 2.

**Figure 2.** XRD patterns of the (**a**) 60PC-20GBFS-15SFA and (**b**) 40PC-40GBFS-15SFA cement pastes after 2, 7, 14, and 28 days.

For the cement paste samples with a higher proportion of Portland cement (60PC-20GBFS-15SFA cement paste), more hydration products were formed (Figure 2a); mainly portlandite. Ettringite dominated among the reaction products of the aluminate phase. After 7 and 14 days the monocarboaluminate was visible. After 28 days the hydrotalcite also appeared.

### *3.4. Microstructure of Cement Paste Samples*

Figure 3a and b present the SEM images of the 60PC-20GBFS-15SFA cement paste with a magnification of 2000 times after 2 and 28 days of hydration, respectively.

**Figure 3.** SEM micrographs of the 60PC-20GBFS-15SFA cement paste after (**a**) 2 days and (**b**) 28 days.

As shown in Figure 3a, after 2 days of hydration, the microstructure of the 60PC-20GBFS-15SFA cement paste was porous, which, as is known from the literature [2], is associated with a slower hydration process of the 60PC-20GBFS-15SFA mixture in the early stages. This is due to the fact that the 60PC-20GBFS-15SFA mixture contains less CEM I 42.5R and, as a result, less C-S-H phase is formed after 2 days of hydration. As the hydration process is carried out, the microstructure of the 60PC-20GBFS-15SFA becomes denser, similar to previous results [13] (hydration products of Portland cement and granulated blast furnace slag, as shown in Figure 3b). However, after 28 days, the siliceous fly ashes were still unreacted; siliceous fly ashes do not represent hydraulic properties, but only pozzolanic properties, which become apparent over a longer period of time [2].

#### **4. Conclusions**

The following conclusions can be drawn based on the results presented above:


**Author Contributions:** E.T. and G.M.: Conceptualization, methodology, formal analysis, investigation, resources, visualization; E.T.: writing—original draft preparation, writing—review and editing, supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** Publication cost of this paper was covered with founds of the Polish National Agency for Academic Exchange (NAWA): "MATBUD'2023–Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD'2023.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This research was supported from the subsidy of the Ministry of Education and Science for the AGH University of Science and Technology in Kraków (Project No 16.16.160.557).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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