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Petrography

What is petrographic analysis?

Petrographic analysis is the microscopic analysis of materials using thin sections or polished surfaces. The technique requires specialist equipment and experienced staff with appropriate qualifications.

The petrological microscope

For the successful analysis of materials such as concrete or aggregates it is essential that a microscope of the highest quality is used. At Geomaterials all petrographic work is carried out using thin sections and polished surfaces and a Zeiss Jenapol Photomicroscope capable of resolving objects of less than 2 micrometres in diameter and capable of operating in transmitted, reflected or fluorescent light. The microscope used at Geomaterials is pictured right.

What are thin sections?

For materials such as rock clay or concrete to be analysed in transmitted light with a petrological microscope, it is necessary to prepare the material to be examined as a thin section. Thin sections are 30 micrometre thick slices of the material to be examined mounted onto a glass slide. Considerable experience and skill is required to prepare thin sections that are of sufficient quality for detailed petrographic analysis.

How are thin sections made?

The first stage in preparing a thin section is diamond cutting followed by vacuum impregnation with a low viscosity resin capable of consolidating materials such as soils. After curing, one face of the impregnated sample is ground and polished flat and bonded onto a glass slide. The excess sample is then cut from the surface of the slide to produce a section about 1mm thick. Further stages of grinding and polishing are used to reduce the specimen thickness to its final thickness of 30 micrometres.

Special precautions in the manufacture of concrete thin sections:

Concrete is liable to be adversely affected by the heating, exposure to atmospheric carbon dioxide, and by exposure to water during thin section preparation unless precautions are taken. To avoid damage to the concrete during preparation it is essential that preparation is carried out using oil and not water as a lubricant and that excessive heating is avoided at all stages of preparation.

What types of materials can be studied using petrographic analysis?

Virtually any inorganic material can be studied with the petrological microscope. At Geomaterials Research Services Limited we have studied the following types of construction materials

  • Concrete
  • Soils and Clays
  • Aggregates
  • Armourstone
  • Flue liners and refractory concrete
  • Roof tiles
  • Fibre reinforced cement roofing sheet
  • Building Stones
  • Bricks
  • Mortar
  • Screed
  • Grouts
  • Ceramic tiles
  • Cement-based adhesives Archaeological materials
  • Clay-cement-GGBS composites
  • Steel and blastfurnace slag
  • Paints

How long does petrographic examination take?

A full petrographic examination typically takes from 2 to 3 weeks to complete. By prior arrangement it is possible to complete a full petrographic examination in as little as 5 working days.