ABS’s geologists’ team has over 20 years of experience with the petrographic microscope, a valuable and effective tool for detecting and identifying sulphides in aggregates. Its use is essential during the detailed examination of aggregates meant for concrete production.

Sulphide-Induced Concrete Deterioration Process

When exposed to water and oxygen, sulphides, notably pyrite (FeS2) and pyrrhotite (Fe1-xS), can oxidize and cause concrete degradation.

In the presence of these elements, the iron in the sulphides can undergo an oxidation reaction, forming iron oxyhydroxides, such as goethite (FeO(OH)). Simultaneously, the released sulphur can interact with oxygen, calcium, and water in the cement paste or the aggregates, forming sulphates, such as gypsum (CaSO4·2H2O)[1].

Sulphates typically occupy more volume than the original sulphides, resulting in concrete swelling.

This swelling can cause internal stresses in the concrete structure, leading to cracking and a decrease in the material’s durability.

Moreover, sulphates can induce further chemical reactions with certain components of the cement paste, exacerbating the concrete’s durability issues.

Identification of Sulphide Types

While chemical analyses can provide total sulphur and sulphate contents, they do not allow for differentiation of the various types of sulphides present in the aggregates. This is why using the petrographic microscope is essential for this specific identification.

This distinction is paramount when deciding on aggregates in concrete production, as different types of sulphides react differently when exposed to oxygen and water. For example, framboidal pyrite exhibits much higher reactivity than well-crystallized pyrite, while pyrrhotite displays significantly higher reactivity than pyrite.

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Therefore, knowing which type of sulphide is present in the aggregates is essential to determine whether they can be safely used in concrete construction or if they should be rejected to avoid potential issues.

Although currently there is no Québec standard governing the interpretation or quantification of results aiming at determining the risks of harmful reactivity of aggregates containing sulphides, Annex P of the CSA A23.1 standard indicates that if the sulphur content from sulphides (i.e., total sulphur – sulphur from sulphates) thus determined is less than 0.15%, the aggregate can be used without further examination.

If the sulphur content from sulphides is equal to or greater than 0.15%, the nature of the sulphides present should be determined through petrographic examination or another appropriate method.

Finally, Article P.12 states that aggregates with a total sulphur mass content greater than 1.00% should not be used as concrete aggregates.

If the presence of pyrrhotite is detected, the aggregates should not be used; however, this can only be determined using a petrographic or electron microscope, with the former being less expensive.

Petrographic Microscope Usage

While using the petrographic microscope, differentiation between pyrrhotite and pyrite begins with their base colours.

Pyrite exhibits a yellowish-white hue, while pyrrhotite displays a salmon-pink colour.

Geologists can identify the minerals by observing the optical properties of their structures.

For example, pyrite crystallizes in the cubic system[2], meaning its optical properties are identical in all directions, making it isotropic. On the other hand, pyrrhotite crystallizes in the monoclinic system[3], resulting in different optical properties in different directions, making it anisotropic.

Using reflected light with an upper polarizer and manipulating the stage, the geologist will observe that pyrrhotite exhibits a colour variation ranging from yellow to bluish-grey or greenish while pyrite remains dark. These observations highlight distinct optical properties between these two sulphides, allowing for their effective differentiation under the microscope.

Pyrite also has higher reflectance, meaning it shines more than pyrrhotite.

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In addition to optical properties, other distinctions between these two minerals can be observed under the microscope. For example, the cubic crystalisation of pyrite is easily identifiable and is rarely confused with other minerals.

Similarly, pyrrhotite tends to oxidize faster than pyrite, resulting in more pronounced alteration when sulphides are .

Additionally, it is sometimes possible to observe the relative hardness of metallic minerals under the petrographic microscope. Polishing is necessary to observe these minerals in thin sections, sometimes leaving visible streaks on their surface. Harder minerals, such as pyrite, exhibit fewer streaks than others, allowing for further distinction between the two minerals.

In conclusion, using the petrographic microscope in civil engineering to identify sulphides in aggregates is crucial.

For over 20 years, ABS’s experienced geologists have been using this technology to detect sulphides, allowing for precise differentiation between different types, such as pyrite and pyrrhotite, which can cause concrete degradation.

This precise identification is essential to guarantee the safe use of aggregates in concrete construction so they comply with quality standards. Integrating the petrographic microscope into civil engineering analysis practices is essential to ensure the durability of infrastructure.

by Pablo Gabriel Rivera, geologist at Groupe ABS.

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[1] It should be noted that this chemical reaction can involve different processes and components depending on the environmental conditions and minerals present, and it can be more complex in some cases.

[2] The cubic system has three axes of equal length, but pyrite or other minerals crystallizing in this system will not necessarily appear as cubes, as there may be variations on the edges of the crystals, etc.

[3] In the monoclinic system, the three axes are of different lengths and one of these axes forms an angle ≠ 90°.