Pathologies in slates, part IV

Gypsification

Gypsification is the phenomenon by which the carbonates that may be present in the slate is transformed into gypsum by contact with the sulfur (S) coming from the atmosphere or from the iron sulfides, following the reaction:

H2SO4 + CaCO3 –> CaSO4 · H2O + CO2

Fig02The transformation from carbonate to gypsum is potentially harmful, because the gypsum has a mineral size substantially larger than the carbonate, so a swelling occurs inside the slate (figure 1), affecting seriously the integrity of the tile. As oxidation, gypsification is very evident when occurs, since it develops a characteristic whitening along the surface of the slate tile (figures 2 and 3). The gypsification is closely linked to acidic environments, especially urban environments where sulfur concentrations are usually high.

Sin título-1

Figure 2 (left): Cover affected by gypsification
Figure 3 (right): Slate severely affected by gypsification after exposure to SO2 test

Gypsification prevention

The best way to know if a slate may suffer gypsification are the normative tests of exposure to SO2, as expressed in EN 12326, or to the test of weather resistance of ASTM C-217. Both tests submit the slate to acid conditions, and then quantify the alteration suffered by giving three degrees. EN 12326 provides three visual alteration levels (S1, S2 and S3), while ASTM performs a scraping of the slate surface after the acid exposure, and then makes three estimates of the service life depending on the depth of the scratch (S1:> 75 years, S2: 40-75 years, S3: 20-40 years).

The carbonate content test of EN also gives an idea of how susceptible to gypsification can be a slate. In theory, higher carbonate content will lead to a high susceptibility. However, this fact has to be taken with caution, as the carbonate may be present as well crystallized calcite, which resists very well against yesificación. Again, petrographic examination can help in this case, since it will determine the form in which is present the carbonate.

Carbonate crystal in a schist roofing slate

Carbonate crystal in a schist roofing slate, transmitted light microscopy, zoom 250, crossed polarizers

Further reading: Standard tests for the characterization of roofing slate pathologies

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