Analysis: the mica mineral has been identified as having a damaging effect on the strength and durability of concrete

Since the early 1960s the presence of free mica on concrete aggregates has been identified as having a damaging effect on the strength and durability of concrete. This has occured in multiple countries such as England, India, Nepal, Sweden, Norway and, of course, Ireland. The adverse impact of mica has also been observed in the mechanical behaviour of asphalt concrete in countries like Finland and Sweden.

What exactly is mica?

Minerals are solid substances formed naturally on the Earth. They have characteristic chemical and physical properties and usually a regular crystal structure. There are currently over 5,000 known species. Naturally occurring minerals commonly form through the cooling of magma (molten rock), and the speed of the process determines the size of the mineral crystal. The slower the process, the larger the crystals. The type of mineral formed depends on the chemical composition of the magma. The most abundant elements on the magma and Earth's crust are Oxygen and Silicon, making up close to 75% of it.

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From RTÉ Radio 1's Morning Ireland, Minister for Housing and Local Government Darragh O'Brien discusses his meeting with mica victims today

Silicates are minerals containing silicon-oxygen tetrahedra, a crystalline structure formed by the elements Silicon and Oxygen. Silicates are the most common type of minerals found on the Earth's crust. Depending on how the crystalline structure is arranged, different types of silicate mineral would be formed, including minerals such as quartz, olivine, mica, several varieties of clay minerals, feldspars and many more.

Micas are a group of silicate minerals in which the crystalline structure forms continuous sheets and have the property of forming thin flakes. Depending on the elements contained, different types of mica can occur. Muscovite mica contains aluminium and potassium and is light to white in colour. Biotite mica also contains Iron or Magnesium and tends to be brown to black in colour.

How does mica operate?

The flaky nature and small size of mica particles create large specific surface areas on this material. Specific surface area is the ratio between the surface area of a particle and the particle's mass. Furthermore, due to the chemical composition of the minerals, these large areas have a net negative charge which makes the mica minerals quite hydrophilic (i.e. very good at attracting water). The attraction of water may be due to multiple mechanisms such as ion hydration, attraction by osmosis or dipole attraction.

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From RTÉ Radio 1's Morning Ireland, RTÉ North West Correspondent Eileen Magnier speaks with homeowners in Donegal affected by the mica brick controversy

Rocks are formed by the agglomeration of multiple minerals. Mica minerals are commonly present on rocks such as granite, pegmatites, schists, gneisses, to name a few. Through natural weathering or industrial processes (i.e. crushed rock from quarries), rocks are disintegrated and form different types of soils, including materials used as aggregates for construction. During these processes, discrete (free) mica particles are released from the parent rock. In general, crushed rock tends to produce more flaky aggregates than natural weathering.

What is the effect of mica on concrete?

Aggregates containing mica minerals may have decreased resistance. When wet, sheet minerals like mica have been shown to significantly reduce the internal friction of the material. This has been explained as an increase in the mobility between the surfaces, as the water introduced separates the sheets reducing the friction between them.

The main effect of the presence of mica in concrete mixtures is to increase the water demand due to the hydrophilic nature of the mineral. This reduces what is known as the workability of the fresh concrete mix, which is how easy the mixture can be transported, placed, compacted and finished; and this, in turn, reduces the compressive strength of the finished concrete.

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From RTÉ News, report on the houses affected by mica in Donegal which have to be demolished

Another reason for the lower strength in the concrete may be the poor adhesion between the cement paste and the flat and smooth surface of the mica rich aggregates. Furthermore, the durability of the concrete has been shown to be affected by flaky particles, such as the mica mineral, as water or air tend to accumulate underneath.

The freeze-thaw response of concrete has also been shown to be affected by aggregates with mica minerals. Volume change, namely expansion, of the concrete is observed under freezing conditions. Experiments with multiple cycles of freezing-thawing of mortar blocks have shown that when the expanding forces due to freezing exceed the tension strength on the interface between aggregate and cement paste, separation of the materials occurs, and the sample collapses.

What does this mean for buildings?

Long term exposure to the elements, coupled with lower durability and strength, and a hindered response to freeze-thaw cycles, may lead to the deterioration and loss of integrity of the concrete or concrete blocks. The use of mica bearing aggregates in concrete and concrete blocks should be avoided. Mica free material deposits should be identified through adequate geological assessment for the construction industry.


The views expressed here are those of the author and do not represent or reflect the views of RTÉ