*3.8. Salt Crystallization Test*

Table 6 shows the ENS weight before the test and its weight after the salt crystallization cycles. In each cycle, the samples were submerged in saline water and taken to an oven for drying, respecting their respective times. It can be observed that there was a small weight variation during the cycles, which indicates little absorption of salinized water, evidencing the low porosity shown in the physical index.

**Table 6.** Weight variation after 10, 30, and 50 cycles of ENS salt crystallization test.


Figure 5 shows the engineered stone's pore distribution before and after the salt crystallization test.

**Figure 5.** Pore diameter distribution of ENS before and after the salt crystallization test.

The ENS had four pore diameter ranges after the test, large, medium, small, and micropores, whereas the intact ENS had no range of small pores. Mercury intrusion and volume increased for ENS in the micropore range, probably due to sample failures. The intact ENS obtained a 0.005 mL/g mercury intrusion volume and 0.89% porosity, and the ENS after the test had 0.0064 mL/g mercury intrusion and 1.29% porosity, indicating the appearance of new pores.

The diameter of pores and the mercury intrusion volume increased in all diameter ranges. The elevation in mercury intrusion and pore diameter can be attributed to temperature variations and the crystallization of salts in the pores that expand, producing tensions that enlarge their diameter.

The studies by Benavente et al. [35] and Yu and Oguchi [36] reported that saline solutions could come from external sources, such as natural salts or anthropogenic discharges, such as marine spray, pesticides, and wastes from fossil fuel combustion. The exposure of artificial stones to these external saline solutions could induce a capillarity and porosity increase, consequently impairing the material's mechanical properties.

The literature also reports an enormous discrepancy between the limited damage observed in the laboratory and the severe deterioration that happens when artificial stones are contaminated with NaCl from external sources. When considering the pore damage due to salt crystallization, the lower destructive potential of NaCl in the laboratory could be attributed to its tendency to supersaturate [37], which can be observed in Table 6, as, despite repetitive cycles, there was no considerable weight variation in the tested specimens.

Flat and three-dimensional micrographs of the ENS before and after the test were obtained with confocal microscopy during cycles of 10, 30, and 50, as shown in Figures 6 and 7.

**Figure 6.** Photomicrograph of ENS under the salt crystallization test: ENS before test (**a**), after 10 cycles (**b**), after 30 cycles (**c**), and after 50 cycles (**d**), with 430× magnification.

**Figure 7.** Three-dimensional photomicrograph of ENS under the salt crystallization test: ENS before test (**a**), after 10 cycles (**b**), after 30 cycles (**c**), and after 50 cycles (**d**), with 430× magnification.

As displayed in Figure 6, ENS after 50 cycles (Figure 6d) underwent a greater change on the surface, evidenced by the more whitish and yellowish color. The accelerated degradation in the laboratory brings the brightness loss, yellowing, porosity, microcracks, and the mineral alteration state caused by the joint action of factors such as temperature and sodium presence (saline water). This leads to salt crystallization in the exposed pores, generating tensions responsible for increasing the diameter of pores and physical origin surface changes [38]. At 10 and 30 cycles, there was no significant surface change.

The three-dimensional photomicrographs (Figure 7) show that the ENS after 30 cycles (Figure 7c) had a greater pore or cavity number, being more pronounced in the ENS after 50 cycles (Figure 7d), indicating the loss of surface cohesion as the number of cycles rise, leading to the detachment of the material in the form of dust and/or small particles [38].
