*2.3. Assessment of the CRMC-Based Mortars Developed*

Compressive strength tests were carried out through uniaxial loading in triplicate, and their average was reported. A compression machine with digital readout and self-centering platens, operated at a constant loading rate of 1.35 kN/s, was used (ADR Touch 3000 BS EN, ELE International, Leighton Buzzard, UK). The tested material was collected for further microstructural investigations (TG-DTG and FTIR analyses). MIP analyses were performed using a mercury porosimeter (AutoPore IV 9500, Micromeritics, Norcross, GA, USA), with maximum and minimum applied pressures of ~34,000 psia and ~0.5 psia, respectively, thus corresponding to a minimum pore size of 5 nm and a maximum pore size of 345 μm. A mercury surface tension of 480 mM/m was applied. Mercury-solid contact angles for intrusion and extrusion were defined as 130◦ and 104◦, respectively. Specimen fragments with a mass range of 1.50–1.90 g were obtained by sawing the specimens that were not subjected to the compressive strength test to acquire more accurate data for MIP analysis [13]. Before being tested, these fragments were stored in a glass desiccator containing silica gel for seven days to ensure the removal of moisture and ensure the effectiveness of the test. TG-DTG analyses were performed from ambient temperature (20 ± 2 ◦C) to 1000 ◦C at a heating rate of 20 ◦C/min under nitrogen flow (SDT Q-50, TA Instruments, New Castle, DE, USA). The material tested consisted of ~7 mg of particles passing through a 63 μm sieve, which were first submitted to a drying stage of 24 h at 60 ◦C to avoid the overlapping effect of free water release with the dehydration step in the TG-DTG curves. FTIR data were obtained by recording the infrared spectrum from 600 to 1600 cm1 using a FTIR Spectrometer apparatus (Nicolet iS10 with a Smart ATR accessory, Thermo Fisher Scientific, Waltham, MA, USA).

### **3. Results and Discussion**

The average compressive strength results of each mixture label are shown in Figure 1 and Table 4. The specimens reached a compressive strength of 19.8 MPa to 31.2 MPa. It was noticed that a change in only one parameter of the casting process can enhance the compressive strength results by ~58% when comparing M.P-10 with M.P-70. However, it was also observed that the degree of enhancement in compressive strength reduces as the static compaction pressure of the casting process increases. Thus, when comparing the M.P-50 with M.P-70, the compressive strength gain was only 1.5%. Therefore, it must be highlighted that the static compaction pressure applied in the casting process plays an important role in the strength development of CRMC-based materials but this strength gain

may be almost negligible for static compaction pressures above 50 MPa. Similar behaviour was observed in steel slag-based CO2-cured pastes [7]. Furthermore, the calculated standard deviation of the compressive strength results indicates that the materials devised for this study exhibit a high degree of homogeneity.

**Figure 1.** Static compaction pressure (SCP) vs. compressive strength (CS).


**Table 4.** Compressive strength results.

The porosity and other parameters obtained throughout the MIP analysis are summarized in Table 5 and shown in Figure 2.



The analyses revealed that the mixture labels hold different porosity indices ranging from 15.80% to 22.34%. These results show that for the range of compaction pressure applied in the mortars' casting process of this study, higher values of static compaction pressure led to lower porosity indices and higher compressive strength results, as shown in Figure 2a. The M.P-30, M.P-50, and M.P-70 mixtures presented similar indices of mesopores, macropores, and air voids/cracks, while M.P-10 exhibited a much higher index of air voids/cracks, a lower index of macropores, and a similar index of mesopores than the rest (Figure 2c). Thus, such behaviour indicates that the compaction pressure applied in the mortars' casting process affects the pore typology up to a certain degree and acts especially on the transition of air voids/cracks to macropores and scarcely affects the increment of the mesopores index. Regarding the critical diameter (∅c), the results show that M.P-10 presented a much higher value than the other mixtures (M.P-10 = 25.6 μm; M.P-30 = 12.4 μm; M.P-50 = 5.4 μm; M.P-70 = 11.8 μm), where such behaviour seems to be related to the lower static compaction pressure applied in the casting process. Interestingly, M.P-50 exhibited a critical diameter about two times lower than the one observed in M.P-70. Such behaviour may be attributed to the BFA pores since a peak in this diameter size is also present in other mixtures. The biomass fly ash was used as it was received, so differences in the pore structure of the raw materials may be found. Therefore, MIP results demonstrated that the static compaction pressure applied during the casting process plays an important role in the mortar's porosity index and in the characteristics of its pores which, in turn, affect the compressive strength results.

**Figure 2.** (**a**) Static compaction pressure (SCP) vs. compressive strength (CS) and porosity; (**b**) Cumulative intrusion (CI) vs. pore diameter; (**c**) Pores typology (%); (**d**) Log differential intrusion (LDI) vs. pore diameter.

The TG-DTG analyses (Figure 3) show that the mass loss of the devised materials was gradual, reaching the highest mass loss rate at a temperature range between 300 and 550 ◦C (Table 6). In this temperature range, the M.P-10 mixture had the highest index of mass loss (~15%), while the M.P-30, M.P-50, and M.P-70 lost ~12, 10, and 9% of their initial masses, respectively. Interestingly, the mass loss for this temperature interval does not follow the typical behaviour of compressive strength development in CRMC-based materials, since the mixture labels with lower compressive strength results had higher mass losses in the region of ~300–550 ◦C, thus indicating that more HMCs and carbonates were formed in these mixtures, probably due to the higher porosity index in the fresh moulded specimen thanks to the lower static pressure applied in their casting process. The DTG curves (Figure 3a) exhibit two initial peaks, which are attributed to the free water evaporation and dehydration of water bound to the HMCs [15–17]. The region between 300 and 550 ◦C presents a higher peak at ~420 ◦C, which refers to the overlapping curves of the dehydroxylation of Brucite [15,18,19] and Dypingite [15,18], as well as the decarbonisation of Hydromagnesite [20] and Nesquehonite [15,18,21], whereas the smaller peak at ~460 ◦C and the mass loss in the range of 450 to 600 ◦C correspond to the decarbonisation of Magnesite [19,22] and/or of undefined HMCs [4]. In turn, the last peak may be caused by the decarbonisation of Calcite and/or amorphous carbonates [23–25]. Finally, the TG-DTG analyses indicate that the four designed mortars were carbonated to a certain degree, in which the magnesia hydration and carbonation seem to be favourably affected by using a lower static compaction pressure in the casting process since the mass loss in the temperature range of 300−550 ◦C is higher as the applied static compaction pressure is lowered. Such a trend may occur due to a lower porosity index which negatively influences

**Figure 3.** (**a**) TG curves; (**b**) DTG curves.

**Table 6.** Mass loss (%) in TG-DTG.

of HMCs.


The recorded FTIR spectra data of the four mixtures are exhibited in Figure 4. Despite the low intensity of the observed bands that may be attributed to the low MgO content (10 vol. %), some considerations may be made as follows: the absorbance bands located at ~680, 855, 880, 1420, and 1485 cm−<sup>1</sup> indicate the presence of HMCs such as Nesquehonite, Hydromagnesite, and Dypingite [26–29], while the bands at ~720 and 1460 cm−<sup>1</sup> could be attributed to the presence of Lansfordite [28]. Therefore, these absorbance bands may indicate that part of the r-MgO in the mixture was carbonated to a certain degree. Furthermore, the unidentified absorbance bands located between 800 and 1200 cm−<sup>1</sup> may be due to the presence of MSH gels [30], which, along with the HMCs, tend to fill the cementitious matrix voids contributing to the enhancement in compressive strength [26].

**Figure 4.** FTIR curves.

#### **4. Conclusions**

This work presented how CRMC-based mortars behave when changes in static compaction pressure on their casting process are made. For this purpose, the compressive strength of the developed mortars was evaluated, and MIP, TG-DTG, and FTIR analyses were carried out. Therefore, the main contributions of this study are as follows: higher values of static compaction pressure used during the casting process resulted in higher compressive strengths for the mortars developed and lower formation of HMCs. However, as the static compression pressure increases, the strength gain decreases, thus making it almost ineffective when applied at pressures greater than 50 MPa (i.e., M.P-70). MIP results showed that the static compaction pressure applied during the casting process of

the devised mortars plays an important role in the porosity index and characteristics of the pores, thus affecting the results of the compressive strength obtained. TG-DTG and FTIR analyses indicated that the devised mortars were carbonated to a certain degree. More studies related to the microstructure, such as X-ray diffraction (XRD), and Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Analysis (EDX) on the devised mortars analyses, should be carried out to better understand these materials.

Finally, the casting and the accelerated carbonation curing processes established and used in this study limit the suitability of the created mortars to the production of pre-cast building materials only.

**Author Contributions:** E.G.S.: Conceptualization, Methodology, Investigation, Writing—original draft, Writing—review & editing. J.C.-G.: Writing—review & editing, Supervision, Project administration, Funding acquisition. M.M.: Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** Portuguese national funds partially financed this work through FCT, Foundation for Science and Technology, IP, within the research unit C-MADE, Centre of Materials and Building Technologies (CIVE-Central Covilhã-4082), University of Beira Interior, Portugal. This research was also partially funded by FCT grant number 2022.09813.BD.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors acknowledge Engineer Ana Paula Gomes from the Optical Centre of UBI (COUBI) for her contribution in carrying out the EDX analyses, and Engineer Carlos Alegria and Engineer Paulo Godinho from Magestop, Central de Biomassa do Fundão, for providing the biomass ashes used in this research. The publication cost of this paper was covered with funds from 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.

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

#### **References**


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