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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Pathologies – part III

Acting against oxidation

Above all, it must be remembered that oxidation is a purely aesthetic defect, which does not involve the loss of the roof waterproofing. Only in exceptional cases, where the size of the iron sulphide is greater than the plate thickness, the oxidation can break it.

Up: Lateral view of a slate tile, in which the thickness of the iron sulphides is lower than the thickness of the tile itselfDown: The thickness of the iron sulphides is now higher that the thickness of the tile, breaking it when the oxidation develops

Up: Lateral view of a slate tile, in which the thickness of the iron sulphides is lower than the thickness of the tile itself
Down: The thickness of the iron sulphides is now higher that the thickness of the tile, breaking it when the oxidation develops

It is also necessary to know the susceptibility of the slate to oxidation. An experienced technician will have no problem recognizing the existence, abundance and types of iron sulphides present, so it is possible to estimate the oxidizability of a slate variety quite rightly. Also, preoxidation treatments with H2O2 can be very illustrative, although the attack conditions must be carefully checked for no erroneous results.

In recent years there have been proposed two types of oxidation treatments, application of chemical products and passivation of the iron sulphides. The application of chemicals products is done in huge treatment stations located in the same slate producing factory. These products have several disadvantages to be considered, during the application stage and with the effective protection they can give to the slate. Still, there are already slate producing companies applying this type of products, albeit in a restricted way.

Experimental roof with tiles of slate treated with different products and treatments

Experimental roof with tiles of slate treated with different products and treatments

The other type of treatment involves selectively attacking the iron sulphides, weathering first their surface and then coating them with an inert mineral crust that protects against the environmental conditions. This method is effective in theory, and it has not been developed for practical use yet, so its real effectiveness can´t be known.

As a general recommendation, against oxidation on the roof, we should act calmly, first weighing the extent and type of damage, and then considering the possibility of changing the affected tiles. Each case is different, and not always the oxidized tiles are negative. In restoration of historic monuments it is common to search for oxidized tiles to replace the originals. Also in modern buildings, rusted slate offers new attractive textures and colors.

Portugal2007 (84)

Further reading:

Passivation techniques to prevent corrosion of iron sulphides in roofing slates

Oxidación de sulfuros en pizarra ornamental: tratamientos protectores con siloxanos

Protocolo de valoración de la efectividad de productos protectores de pizarra para cubiertas

Sealant composition for roofing slate

Pathologies – part II

Oxidation

Iron oxidation consists of the change of Fe2+ to Fe3+, by the gain of an electron. In roofing slates, most important iron minerals are iron sulphides, being the most abundant pyrite (FeS2), which is oxidized in the presence of oxygen according to the reaction:

FeS2 + O2 –> Fe2+ + 2SO2-4 + H+

The oxidation of these iron sulfides is favored by acid urban environments and coastal areas, where sea salts favor oxidation reactions.

However, not all the iron sulfides oxidize in the same way. There are several types of iron sulfides, such as pyrite, pyrrhotite, marcasite, arsenopyrite, etc, being the two firsts the most abundant by far. Each iron sulphide has a different structure. Thus, the oxidation susceptibility depends on the strength of this mineral structure. For example, pyrrhotite has a poorly ordered hexagonal structure, being more vulnerable to oxidation than the pyrite cubic structure. In real world, most oxidations developed in roofing slates are due to pyrrhotite, so it is very important to distinguish between these two minerals, since the oxidability of the slate depends on it.

Finally, the occurrence of organic matter in the slate favors the oxidation, due to the increase of acidity during its decomposition.

1 - Pyrrhotite, brown color, with not recognized shape. 2 - Pyrrhotite partially oxidized together with an inclusion of organic matter. 3 – Pyrrhotited fossil of a bivalve. 4 - Cubic pyrite crystal. 5 - Cubic pyrite crystals forming aggregates called framboids. 6 - Footprint of a disappeared cubic crystal of pyrite oxidized.

1 – Pyrrhotite, brown color, with not recognized shape. 2 – Pyrrhotite partially oxidized together with an inclusion of organic matter. 3 – Pyrrhotited fossil of a bivalve. 4 – Cubic pyrite crystal. 5 – Cubic pyrite crystals forming aggregates called framboids. 6 – Footprint of a disappeared cubic crystal of pyrite oxidized.

Further reading: Determination of iron sulphides in roofing slates from the north west of Spain

Pathologies – part I

Pathologies in roofing slates

The pathologies formed in slate roofs are mainly due to the presence of potentially unstable minerals (iron sulfides, carbonates and organic matter). These minerals may become altered by the effect of environmental agents, once the slate roof is finished. The pathologies are mainly associated with oxidation and gypsification processes of the cited mineral phases.

The oxidation is generated when the iron sulfides which may contain the slate became weathered, forming iron oxides. This forms reddish rust marks on the surface of slate tiles. This is mainly an aesthetic defect, as only rarely slate tiles disintegrate due to oxidation. However, it is the main fact in volume of complaints from slate customers (Figure 01). The presence of tiny fragments of organic matter may favor the oxidation processes.

Fig01

Customers complaints by volume of monetary costs

The gypsification occurs when the carbonates react with the environmental SO2, forming gypsum. In this case, gypsum has larger size than carbonates, so a swelling may occur within the slate tile, causing it to disintegrate. Despite this, the incidence that this pathology in the customer complaints is significantly lower than oxidation, maybe since it is not as striking (Figure 01).

There are also other characteristic pathologies and minor defects but also must be taken into account.

Following the criteria dictated by ICOMOS, defects and pathologies found in roofing slates can be classified into 3 groups (Table 01).

Most common  pathologies in roofing slates

Most common pathologies in roofing slates

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