3.1. Physical Properties of the Mortars
The physical properties of the clay mortars were recorded and presented here after the age of 180 days to allow the carbonation mechanism to harden the soft clay structure. This decision was made after the experience recorded in previous tests [
29]. The capillary absorption was measured at the 180 days and 365 days, with the capillary coefficient indicating the water absorption trend of the specimens. In all cases, the capillary coefficient was decreased through time (
Table 2). The time intervals used for measuring the weight values were 0, 5, 10, 15, 30, 60, 90, 120, and 1440 min, respectively.
In general, WGS mortar presented the highest absorption rate due to capillary in the ages tested, without presenting any material loss (
Figure 2). The decrease of the capillary coefficient by 20.7% at the age of 365 days indicates a more stable structure (
Table 2). However, the results differ in the case of WGN mortar, since after 24 h in contact with water, the specimens suffered material loss without being able to complete the experiment at the age of 180 days. Nevertheless, the annual results showed a more stable structure, with WGN showing no material loss. Despite presenting a fast-initial absorption rate at the age of 180 days, when tested again at 365 days, the rate of absorption was reduced significantly, as can be noted by the significant difference of 63.6% between the two values of capillary coefficient (
Table 2). Overall, the mortars that were activated with sodium metasilicate and sodium hydroxide solution (WGS, WGN) presented the highest absorption rate values at the age of 180 days. However, results differ at the age of 365 days for both mortars, since WGS showed the highest absorption rate, while the WGN mortar, as mentioned, had a significantly lower absorption rate. This fact is probably justified by the density of the geopolymer gel, being in the case of WGN less dense, and in the case of WGN, much denser [
37], a fact justified by the porosity values as well. By the SEM analysis, in the case of WGN mortar, the loss of sodium through time (leaching effect) could have resulted in a less absorbent structure [
37]. PO mortar presented low values of capillary coefficient at both ages tested, with higher final absorption value at 365 days (
Figure 2). The low porosity values, as seen in
Table 2 for both ages, indicate a dense formation that resulted in lower water uptake [
12]. In both cases, SC mortar presented the lowest absorption rate through time, showing a 28% decrease in capillary coefficient values. Moreover, it is observed that the untreated mortar A was unable to complete the test until at all ages examined. Overall, the results come to an agreement with literature for alkali-activated metakaolin or natural pozzolan-based binders that are porous and present high capillary suction [
3].
The conduction of the drying test started immediately after the completion of the capillary absorption test, as a reverse capillary test. In the case of the reference mortar A, this test was not able to be conducted since, at both ages, the samples were destroyed before completing the capillary absorption test. All other specimens were weighed using the same time intervals as the capillary absorption test and after that daily up to 960 h when all the samples have reached equilibrium with the environmental conditions (stable measurement). Weight stabilization of the specimens occurred at different times for each mortar during the total duration of the experiment. The determination of the drying curve was done after calculating the residual amount of water present in the specimen per unit area referred to as Mi (kg/m
2). Since the drying index describes the resistance of the material to drying, it can be claimed that a low value of ID reflects an overall easier drying behavior [
35,
38].
In total, ID values were decreasing for all the samples tested through time, while the highest ID value was recorded for the WGN samples in the long term.
Figure 3a,b depict the drying curves of the mortars at later ages. A higher slope of the curve to the horizontal axis reflects materials with high liquid conductivity (porous materials) [
36]. The final time of the drying test at 180 days was approximately the same for all the samples tested (
Figure 3). Moreover, it can be observed that WGS mortars have a higher liquid conductivity compared to the other two mortars, a fact that is also justified by the high porosity values measured at both ages (
Table 2,
Figure 3). Presenting the lowest values of drying index at both ages tested, WGS mortars have the fastest drying behavior comparatively, with a generally distinct and long first drying phase, a fact that agrees with their high porosity values. It is also noted that WGN mortars showed the highest resistance to drying at 365 days compared to all the treated mortars tested (
Table 2). Despite presenting similarly low porosity and capillary coefficient values with PO mortar at the age of 365 days, the drying behavior of the WGN mortar is significantly different, exhibiting low liquid conductivity.
Moreover, during the conduction of the experiment, efflorescence was observed on the surface of the WGN mortars. Efflorescence indicates an excess amount of unreacted sodium oxide in the pore structure that is transferred to the surface of the sample, with the presence of water through capillary. Then, the transferred alkalis react with the atmosphere, thus causing carbonation known as efflorescence [
6,
39]. This phenomenon that also occurred in WGN mortar shows a low exchangeability, while it can lead to a further deterioration of the system.
The PO mortars presented a low resistance to drying, with a comparably high liquid conductivity, an interesting fact considering their low porosity values (
Table 2). Additionally, the second most porous mortar SC also showed a fast-drying behavior with low values of ID and a shorter first drying phase.
In
Figure 3, the final drying time of the mortars can be distinguished. The mortars PO and SC presented a more extended drying period, while all mortars previously tested showed improved drying behavior with lower ID values. Moreover, despite the reduction in porosity values through time, the ID index was not negatively affected, since the decrease of the annual values for all samples, indicates a faster drying behavior meaning a quicker elimination of moisture (
Figure 3,
Table 2).
The porosity results signify the porous structure of the WGS mortars since the porosity values were the highest recorded compared to the other mortars at all ages (
Table 2,
Figure 4). For PO and WGN mortars, it is noted that the porosity values remained relatively low, with the annual results being close to the values of the untreated mortar A. These values indicate the compact structure of these specimens. The high porosity values of WGS mortars, agree with the high absorption rate through capillary, while the values of the SC mortars reveal a porous structure. The high porosity values justify the low values of drying index at all ages for mortars SC and WGS.
The results of water penetration through Karsten tubes indicate the increased water absorption through time, of the most porous mortars SC and WGS. In general, it is observed that all treated mortars, besides SC, showed a higher absorption rate compared to the untreated mortar A, at all ages tested. The high tendency to water absorption of WGS and WGN mortars remains unchanged, presenting, however, a reverse behavior through time. The water absorption of WGS was increased from 90 days to 365 days by 93.2%, while the WGN mortars presented a decrease in water absorption by 53.9% (
Table 3). It is noted that SC mortars showed a low water intake compared with the other treated mortars in every water absorption test conducted. PO mortars showed an average water penetration during the Karsten tube test, yet still higher than the untreated mortar A that had an overall low water intake.
Linear shrinkage and volume loss of the mortars were recorded, up until 365 days after manufacture. In
Table 3, both values at 180 days and 365 days are noted as to present their progress through time (
Table 3). The long-term measurements were decided to test the probable instability of the mortars through time regarding volume loss and shrinkage. According to DIN 18947, the linear shrinkage should not be more than 2% [
40]. Despite the reference mortar A and mortar SC, that have barely satisfied this requirement in the long term, all the other mortars meet the standard’s requirements concerning linear shrinkage. Mortars treated with potassium metasilicate (PO) present an overall stable structure. In
Figure 5, it can be detected that the percentage of volume loss through time is the lowest recorded. In general, mortars SC, WGS, and WGN presented higher values of linear and volume shrinkage, with the first showing a significant volume loss percentage in relation to the untreated mortar A, especially with the completion of one year. The total volume loss of SC mortars was 66.1% greater compared to the reference one, while PO mortars presented a 79.7% decrease in volume loss. WGS mortars presented a similar shrinkage behavior with the reference samples, showing an improvement in volume loss of 13.7%. WGN mortars presented overall good stability, with around 64% decrease in volume loss compared to A.
Overall, the mortars with the lower porosity and liquid/solid ratios presented the most stable structure also in terms of volume loss and linear shrinkage. The use of potassium metasilicate and water–glass with sodium hydroxide solution as activators have been proved beneficial in making the structure of the samples more stable.
3.2. Mechanical Properties of Mortars
In
Table 4, the mechanical properties of the mortars are presented. The values are the average of six samples in each case. Overall, both the PO and WGN mortars presented increased values of compressive strength. An impressive 302.83% increase in one year for PO and 195.93% for WGN compared to the reference A indicate the strengthening of the structure (
Table 4). In total, PO mortar presented the best performance in terms of mechanical characteristics, granting the specific activator compatible with the precursor used in this study, matching the results that derive from SEM analysis. For clay-based materials, such values of compressive strength are considered exceptionally high, since the values that are usually expected of such systems are around 1–2 MPa [
41,
42].
Moreover, concerning the mortar SC, it is also established in this study that sodium carbonate presents a slow strength development [
32]. Even after one year, the compressive and flexural strength is considerably low, granting it as the one with the weakest behavior in terms of mechanical characteristics. The porosity values of SC also agree with the results of mechanical testing. Perhaps the slow strength development could be overcome with curing at a temperature higher than 40 °C [
32] or even for an extended period, allowing a more stable and less porous structure.
The compressive test results seemed to agree with the porosity values also in the case of the WGN mortars. Notable is the fact that the WGN mortar showed the most significant compressive strength development from 180 days to 365 days by 171.19%. This fact could imply a lower strength development, especially when compared with the strong activator of potassium metasilicate. At the same time, it could also be linked to the loss of unreacted sodium through efflorescence, with an alteration of the Si/Al ratio in the structure as SEM indicates (see below). The high porosity values of WGS mortars indicate a weak performance in mechanical properties. This fact stands true for both ages, with the results at the age of 180 days being comparatively very low, while the annual values of compressive strength are comparable to the reference.
Flexural strength results presented a similar pattern, with PO and WGN mortars showing the most notable values, especially at the age of one year, with an impressive 183.33% and 39.71% increase, respectively (
Table 4). In general, the mechanical characteristics are bond to the microstructure of the mortars. Through SEM analysis, it is noted that the differences in the values of Si/Al, Si/Ca, and Na, K/Al explain whether the formation of an inorganic polymeric network of alkali aluminosilicates was realized [
3,
43,
44].
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.
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.
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].
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 (NaHCO
3) and sodium carbonate (Na
2CO
3) [
46].
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.
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.