Under the microscope — polarising microscope use to investigate building materials
Dr Alan Poole, Secretary of the Applied Petrography Group, London, UK, explains how a polarising microscope is used to investigate the fabric and mineralogy of concrete as a building material.
Minerals such as stone, slate, aggregates, cements, lime and concrete are one of the largest groups of materials used in civil engineering. Examining and evaluating such materials relies on the use of a petrographic polarising microscope to assess their suitability as building materials.
Modern requirements for precise specification and quality control have led to the development of numerous national and international standards, test procedures and codes of practice. These ensure that the material used meets its specification and is fit for purpose. In cases where failure occurs, diagnosing the cause is necessary to make repair both economic and effective.
Concrete illustrates the value of the polarising microscope as an investigative tool. It is a complex and difficult material to evaluate, partly because of its heterogeneity. Mineral aggregates represent approximately 75% of its volume. A cement paste matrix binds the particles together through chemical exothermic hydration reactions. Portland cement is the most commonly used, but other varieties exist.
This cement is made by burning shale or clay (aluminium silicates) with a limestone (calcium carbonate) at 1,300-1,500oC. The resulting clinker is ground to a fine powder and typically contains four principle crystal species and a glassy phase.
The aggregate particles, the anhydrous cement grains, which persist in hardened concrete, and the cement hydrates that form in the matrix all contain crystalline and non-crystalline components often of very small size. These components, and the way they interrelate, control concrete properties such as permeability, strength and durability.
Since the mineral components in concrete are small, the most important investigative tool is the petrological polarising microscope. This was developed by Samuel Highley in 1856 as a chemical microscope and differs from devices used for metallurgy, or biological materials because it applies polarised light to identify mineral materials.
If concrete or other building materials are cut and ground to 20-30µm thickness, the majority of minerals appear transparent in transmitted light. Transparent minerals resolve the plane of polarised light causing interference colour effects, which can be analysed to identify the mineral species. In concrete, materials, including the anhydrous cement minerals and their hydrates, as well as the familiar rock minerals of aggregates, can be recognised easily by using the polarising microscope.
The process of identifying, characterising and quantifying materials to comply with a specification or for quality control purposes is a simple one. It may only require inspection of representative samples by a petrographer.
A petrographic investigation of hardened concrete allows components to be identified. Alternatively, identities can be determined using a polished concrete slab, employing physical and chemical testing or staining.
However, microscopical examination provides additional information. Not only can the components be identified and quantified, including admixtures such as fly ash or slag, the original water/cement ratio can be estimated. Also, the causes of cracking, exudations or other forms of deterioration can be determined by a detailed examination of an appropriately selected thin section.
Concrete structures can have a long design life, but there are many cases worldwide that show symptoms of premature deterioration. The symptoms include surface crack development and spalling, misalignment of concrete elements within the structure, staining, and exudations which may be associated with cracking. The deterioration mechanisms are complex, and in many examples, the observed decay is found to be the result of several mechanisms working together. A linked site and petrographic investigation can identify cause of failures.
The natural pH of concrete can be reduced by carbonation of the cement matrix or leaching calcium hydroxide from it. Steel is normally passified by the surrounding alkalinity, but if chloride ions are present, corrosion may occur if moisture and oxygen reach the metal. The oxides/hydroxides occupy a substantially larger volume than the original steel, leading to cracking and spalling of the concrete.
Permeation of the material by external solutions carrying ions such as chlorides or sulphates is another cause of degradation, indeed, damage to foundations by sulphate rich ground-waters is well known. Sulphate ions react within the cement matrix to form the sulphate minerals ettringite and gypsum. They can cause cracking, spalling, exfoliation and general expansion, or a softening and degradation of the concrete.
Alkali aggregate reaction (AAR) in concrete is relatively uncommon in the UK, but it can continue for many years and disrupts the structure from within. The most common type is alkali-silica reaction, where specific varieties of silicious aggregate particles react with the alkali pore fluids to produce a hygroscopic alkali-silica gel reaction product. This gel absorbs water and expands with sufficient force to crack and disrupt the concrete.
A rarer type of AAR involves reaction between the alkalis and certain carbonate aggregate types, but the exact causative mechanisms remain unclear.
Unsound aggregate particles in concrete can also be readily identified and quantified by a petrographer. Damage resulting from frost or fire is more difficult to assess, it relies on identification of characteristic crack patterns coupled with modifications to the microfabric and formation of new minerals in the cement matrix.