*3.3. Durability Properties and Microscopic and Stereoscopic Observation*

After reaching the age of 90 days, durability tests were carried out. These tests included freeze–thaw and wet–dry cycles, where the final percentage of mass loss, compressive strength values, porosity, and surface alteration through stereoscopic observation were recorded. The values given in Table 5 are averaged from three specimens. A full cycle in freeze–thaw durability tests consists of four hours in a chamber of −18 ± 2 ◦C, 10%RH, and the rest 20 h in ambient conditions (20 ± 2 ◦C, 65%RH). Moreover, a full wet–dry cycle includes wetting of the mortars by spraying approximately 4–5 mL of water per sample (4 cm × 4 cm × 16 cm prism) and letting them dry both in ambient conditions for three hours and then exposed them for 21 h at 40 ◦C. These conditions were decided based on experience due to the lack of regulations on clay-based materials. The conduction of the durability tests was until the completion of 50 cycles or until the destruction of the samples. The addition of potassium silicate in earth-based mortars proved disadvantageous concerning the wet–dry cycles since it has suffered the most significant dissolution, leading to a high amount of mass loss. Despite this deterioration, however, notable is the fact that the compressive strength value for wet–dry cycles was the highest one recorded. It is observed that both PO and WGN mortars suffered more significant mass loss than the reference mortar A regarding the wet–dry cycles, while SC mortars proved the most efficient. It is worth noting that the PO samples experienced deterioration after the completion of the 21st cycle. Thus, in Table 5, the mass change recorded is referred to as the mass of the samples after

the cycle. In total, the freeze–thaw cycles showed a low mass-change effect, while the compressive strength results indicated an inadequate response to compressive strain, apart from the PO and WGN mortars. Generally, significant is the fact that despite the high deterioration of PO and WGN samples, their load-bearing capacity after the completion of the cycles was efficient.


**Table 5.** Mass change and compressive strength of the mortars after durability tests.

The morphological characteristics of the mortars are presented at the age of one year, and after the durability cycles. The mortars examined at the age of one year showed similar characteristics with the reference sample A. Shrinkage cracks were detected, and a porous structure was evident in all surfaces observed (Figure 6). The reference mortar showed a rough, porous surface, while PO mortar presented the most compact structure and smoother surface, with few shrinkage cracks and with pores of a mean diameter 300 μm. Efflorescence was also observed microscopically on the surface of the WGN samples and inside the mass of PO mortar. Moreover, surface shrinkage cracks were detected in all mortars, with WGS mortar presenting the more significant amount. Their width was ranging from 20–110 μm, while a darker color was detected.

**Figure 6.** Stereoscopic observation of the mortars at the age of one year (scale 1000 μm).

After the conduction of freeze–thaw cycles, all the mortars presented a more cracked and rougher surface (Figure 7a). Additionally, the surface of the mortars after the conduction of the freeze–thaw cycles showed cracking and scaling that is explained by the expansion of the inner pore water. Thus, the mass increase of most of the mortars tested is justified, since the PO, SC, and WGS presented mostly cracks and not significant scaling. For WGS mortar, the width of the cracks was between 50–160 μm after the freeze–thaw cycles. Furthermore, for all the other mortars, the range of the cracks was 40–60 μm. A disruption between the binder and the aggregates led to the mass loss of the samples WGN and A [45].

(**b**)

**Figure 7.** Stereoscopic observation of the mortars after the (**a**) freeze–thaw and (**b**) wet–dry cycles (scale: 1000 μm).

Moreover, the color alteration was apparent in WGS mortar, while in SC, some coloring spots were detected (Figure 7a). Both WGS and WGN mortars developed tarnishes and white agglomeration spots. The least porous mortar PO presented an excellent behavior in freeze–thaw cycles without showing any significant reduction in compressive strength after the completion of the cycles. The low porosity indicates a more stable mass, justifying the high values of compressive strength. The low porosity of the WGN mortar, however, is not consistent with higher strength development, presenting moderate mechanical characteristics after the freeze–thaw cycling.

The loose cohesion of the aggregates and the increase of cracks were evident after the completion of wet–dry cycles (Figure 7b). Tarnishes were again developed on the surface of the WGN mortar, while the SC mortar presented an overall good behavior against weathering. The compressive strength and porosity of the SC mortar after the wet–dry cycles are very close to the values of the annual

results for the same mortar. That fact indicates the stability of the mortar against wet–dry cycles, presenting a water-resistant behavior, with almost no mass loss (Table 5). WGS mortar also presented low mass loss, yet the mechanical strength and porosity values were not improved. Significant is the fact that despite suffering mass loss, the compressive strength of the PO mortar was not significantly reduced. The results after the completion of the cycles are compared to the equivalent values of 180 days. Concerning WGN mortar, the results indicate a deterioration in mechanical strength results after the conduction of both weathering cycles. The WGN mortar suffered scaling and moderate mass loss after the wet–dry cycles, presenting, however, a very slightly improved behavior in mechanical characteristics compared to the freeze–thaw results.

To determine the nature of the efflorescence of WGN mortars, differential thermal, and thermogravimetric analysis (DTA/TG) was performed through a TA Instruments SDT 2960 analyzer (Thessaloniki Greece) (Figure 8). The results indicated the presence of sodium hydrogen carbonate (NaHCO3) and sodium carbonate (Na2CO3) [46].

**Figure 8.** TG-DTA analysis of the efflorescence appearing on WGN mortars.

When observing the produced mortars under SEM (Figure 9a), the reference mortar A showed a loose crystal structure at an early age, while a smoother surface was observed through time (Figure 9b). In PO mortar, rod-like crystals were detected of a potassium-based compound, with a noticeable decrease of potassium in later age (Table 6). In the case of both PO and WGN mortars, a continuous structure with small pores and few cracks was observed, also showing excellent structure cohesion. As to compare the differences in the inner structure, SEM analysis was performed at an early age (28 days) and after one year. A rougher surface with formation of leaf-like crystals was detected through SEM for WGN mortar at an early age, however, in time, a decrease of sodium content by 94% was remarked (Table 6). The presented spectrums are the average of many images, where a whole area was analyzed.

**Figure 9.** SEM images of the mortars (**a**) at an early age and (**b**) at 365 days (scale: 60 μm for A, 60 and 30 μm for PO, and 60 and 40 μm for WGN mortars).

(**b**)


**Table 6.** EDS spectrum analysis of (**1**) mortar PO and (**2**) mortar WGN (all results in atomic %).

The annual results of the SEM analysis are reported, together with the results of the 28 days for comparison reasons. From Table 6, the indicative atomic ratios of the modified compositions can be calculated. The atomic ratios calculated at 28 days were Si/Al = 3.06, Si/Ca = 20.32, and K/Al = 0.73, while at 365 days the ratios were Si/Al = 4.54, Si/Ca = 14.55, and K/Al = 0.50. The increase in the mechanical properties of the PO mortar can be explained by the increase of the Si/Al ratio through time [47,48]. For the WGN synthesis, the atomic ratios at 28 days were Si/Al = 4.57, Si/Ca = 4.89, and Na/Al = 2.10, while at 365 days, the ratios were Si/Al = 3.51, Si/Ca = 5.29, and Na/Al = 0.28. These results indicate the decrease of Si/Al ratio through time by 23.16%, as well as the higher decrease of Na/Al ratio by 95.4%, facts that may explain the lower compressive strength development of the

WGN mortar through time. The unbound quantity of sodium by the clay minerals in the structure probably justifies this fact. Thus, the efflorescence of WGN mortar can be explained.

#### **4. Conclusions**

The activation of earth materials rich in calcium was performed using different activators. Physical, mechanical, and durability properties were examined after one year. Overall, the most notable result is the high performance of potassium metasilicate (PO) in terms of mechanical properties, even after the conduction of durability tests. The achieved mechanical properties after durability tests at the age of 365 days (6.13 MPa in compressive and 3.28 MPa in flexural strength) constitute these mortars capable of different construction needs (keeping in mind their low mechanical properties as neat materials). The shrinkage trend of PO mortars was limited, judging by the low values of linear and volume shrinkage, up to the age of 365 days.

Furthermore, the improvement of their drying behavior without suffering material disintegration indicated the development of a more compact structure. The inadequate behavior of PO mortar at wet–dry cycles is a concern that should be further examined. However, the maintenance of its load-bearing capacity even after the completion of durability tests grand this mortar as a suitable building material and the specific activator very promising in order to allow further development of new advanced materials.

The water intake of sodium carbonate mortars (SC) in terms of capillary and water absorption by Karsten tubes was proven beneficial. Therefore, this water-resistant behavior of sodium carbonate shows great potential for special applications. While presenting a water-resistant behavior, the high values of porosity and, consequently, the low values of compressive strength diminish the value of sodium carbonate as an activator. Moreover, the high percentage of volume shrinkage of SC indicates an unstable structure, yet its high resistance against durability tests, especially in terms of wet–dry cycles, grand it as a promising water-resistant agent. Further study is required for the sodium carbonate as an activator that presented slow strength development and low values of compressive strength compared to the other mortars. This fact could be managed with the use of a combination of different activators or higher curing temperatures, as also literature suggests [38].

Concerning the behavior of the specimens mixed with water–glass (WGS), the results of mechanical and physical properties tested at different ages revealed a low performance. However, the fact that the specimens presented excellent stability in volume and linear shrinkage and low mass loss in durability tests, marks water–glass as a promising treating agent regarding mass stabilization.

The mechanical characteristics of the mortar with water–glass and NaOH solution (WGN), have been proven exceptional even in the long term. Therefore, this solution stands as another promising activator for clay mortars. Despite the efflorescence that was forming during the drying stage, it displayed a compact structure with low shrinkage tendency and satisfactory physical properties. Different methods of curing and perhaps a reduction in the dose of the activator are factors that could control efflorescence and should be further studied. The high tendency of these mortars to absorb water was evident during the capillary absorption test that caused the disintegration of the samples tested at a certain age.

In total, the combination of these activators with the specific earthen material to create advanced mortars proved satisfactory. Each agent has proven to act differently and enhance different properties. Further research is proposed to investigate the matter of the application of those agents at various earth-based materials and different percentages thoroughly. XRD analysis should be performed to establish the mineralogical composition of the newly formed materials. Moreover, further research on the reaction mechanism, the appropriate activators, and treatment should be withheld, to gain a better understanding of the conditions necessary to reach the optimum results.

**Author Contributions:** Conceptualization: M.S.; Methodology, Supervision, Review: S.K. and E.P.; Validation and Analysis: A.K.; Investigation, Writing, Draft Preparation, Resources: All authors have read and agreed to the published version of the manuscript.

**Funding:** The General Secretariat for Research and Technology (GSRT) and the Hellenic Foundation for Research and Innovation (HFRI), with grand number 347, have funded the research through the scholarship funding program for Ph.D. candidates of the author Karozou Aspasia.

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