Degradation - Mica

In the last 10 years, a rise in notable cracks in properties from the Donegal and surrounding reasons has been noticed forcing investigation and research as to why. The wide use of metamorphosed mudstones and siltstones resulting in defective aggregates including phyllite and micaceous quartzite, amongst others, has caused an abundance of “free mica” (that which is <63µm in length) which is generated by abrasion of fine micas in these aggregates. The loose mica tends to fray out into the cement binder like confetti and upon concrete manufacture absorbs a higher volume of water causing an increase in viscosity and lower workability meaning this is often overcompensated by the further addition of water or a general lack of cement.

The metamorphosed phyllite and micaceous quartzite are also hosts of the reactive sulphides pyrrhotite (and marcasite) which when exposed can generate acids and sulphates which can lead to expansion of the aggregate, subsequent fracturing of the concrete and, therefore, overall degradation of the block itself.

Petrolab has to date tested over 1000 properties known to contain defective mica-bearing (and sulphide-bearing) concrete and these properties have been subject to the IS465 testing regime. Petrolab has 3 (and counting) competent concrete petrographers as well as 4 chartered geologists/engineers making us qualified to conduct testing to IS465.


Cross polarised light image of phyllite fragment


Degradation - Reactive Sulphides

In the last 10 years, a rise in notable cracks in properties from County Mayo, Clare and Limerick has been noticed forcing investigation and research as to why. The wide use of black argillaceous limestone and calcareous mudstones have been used in concrete production. These lithologies contain abundant black bands of clay which can be seen using a microscope, and when these are viewed in reflected light can be seen to contain a high abundance of small reactive “framboidal” pyrite, as termed from their raspberry-like formation derived from the French word “framboise”. These reactive framboids (due to their increased surface area to volume ratio) when exposed generate acid and sulphates which when produced cause expansion of the aggregate, fracturing of the concrete, and then lead to to loss of service of the blocks themselves.

Petrolab has to date tested over ~500 properties known to contain defective pyrite-bearing mudstones/limestones in concrete and these properties have been subject to the IS465 testing regime which Petrolab is qualified to perform.





Framboidal pyrite


Negligible classification is achievable with <1% mica, a minor proportion (<10%) argillaceous limestone, and a total sulphur content of <1% or 0.1% for aggregates containing reactive pyrrhotite or framboidal pyrite.


A broad range of aggregates containing sedimentary, igneous and metasedimentary lithologies, commonly present in concrete as blended aggregate products or as dredged gravel samples accounting for extreme heterogeneity of the lithologies. Typically contain ~5% mica, varying abundances of sulphide mineralisation, or a mix of clean bioclastic and argillaceous limestones.

High - Mica

Typically the Donegal phyllites and micaceous quartzites or the argillaceous black Mayo limestones. The phyllites always contain high abundances of free mica (>>7%) and the presence of common pyrrhotite.

High - Pyrite

The Mayo argillaceous limestones contain abundant reactive framboidal pyrite commonly coupled with high chemical sulphur values.

Critical -

Those which have suffered extreme degradation and are no longer sound. This is most frequently observed in the two principal High Risk concrete types (phyllite or black limestone), but has been observed rarely in a few other concrete types.


Critical -

Training Day Example

Photos from Petrolabs recent in-house training day on recognising typical IS465 concrete types of which we have identified ~30 to date. These images are examples of of High and Low/Medium Risk concrete including examples of sub-types within the main groups. Broadly there are 6 main High Risk aggregate types and 20 Low/Medium Risk aggregate types.

Our IS465 Engineer Partners

Classification Scheme

The principal purpose of the staged examination procedure (Suite A, Suite B, Suite C) is to classify the concrete material into one of the defined risk categories (Negligible, Low/Medium, High, Critical).

Aggregate Types

The principal purpose of the staged examination procedure (Suite A, Suite B, Suite C) is to classify the concrete material into one of the risk groups defined in the IS 465 Guidance Note (Negligible, Low/Medium, High, Critical). This classification requires: a) identification of the aggregate type, and b) assessment of the condition of the concrete. There are some 30 separate aggregates now regularly identified as part of regular testing which will, in the fullness of time, be detailed below. For now the two dominant high risk aggregates and examples of Low/Medium aggregates are displayed.


Crushed Phyllite & Micaceous Quartzite

The aggregate making up the concrete is dominated by crushed phyllite (metamorphosed low-grade schist from a protolith of mudstones and siltstones) and micaceous quartzite (metamorphosed impure sandstones, including metamorphosed greywacke with irregular sized quartz clasts). There are also rare calcareous quartzite fragments, which have probably been derived from metamorphosed calcareous sandstones. The rocks have not been sufficiently metamorphosed to form schist, as the “mica” crystals (interlayered muscovite and chlorite) are too fine grained to be visible to the naked eye. The phyllite aggregate contains a strong crenulated cleavage which frequently splays along exposed surfaces meaning that abundant platy muscovite and chlorite crystals <10 µm long become abraded from the aggregate during processing and are disseminated throughout the binder. High amounts of liberated mica and other phyllosilicates are undesirable in aggregate since they increase water demand of concrete due to the hydrophilic nature of the cleavage planes.

The phyllite also contains appreciable amounts of pyrrhotite, with additional pyrite, and occasional occurrences of chalcopyrite, cobaltite and marcasite. The presence of sulphides, particularly the more unstable pyrrhotite and rare marcasite, is cause for concern due to the potential for oxidation and subsequent production of expansive cracking and associated secondary sulphate minerals. 

Black argillaceous limestone & calcareous mudstone

The aggregate used for this concrete is made with an all-in crushed calcareous mudstone / argillaceous limestone containing abundant fine framboidal pyrite. Pyrite present in this form within this type of aggregate is known to be reactive and has been widely implicated as a source of sulphate in previous cases of concrete degradation. Potentially deleterious argillaceous limestone and calcareous mudstone may comprise >70 of the total aggregate (by modal analysis). In cases where degradation is observed there is evidence of local in situ oxidation of pyrite, with associated gypsum production and local spalling of mudstone into voids along fractures. Internal fractures will also radiate into the binder, from the expansion of mudstone aggregate.


Other High Risk Aggregates

Whilst phyllite aggregate is the principal deleterious aggregate used in Co.Donegal and argillaceous limestone / calcareous mudstone is the principal deleterious aggregate used in Co.Mayo there are other High Risk aggregate’s in use.

Within Co.Donegal this picture is complicated by the varieties of metamorphic grades that have been quarried ranging from low-grade surficial weathered aggregate to higher grade amphibolite facies aggregate, many of which have been intermixed with other aggregate, or with fines. Whilst some of these have turned out to be Low/medium risk there are sub-types which are High Risk and associated with properties showing signs of degradation (BCA 3 or 4). Petrolab hopes to speak more on this at conferences in the coming year (2023).

Within other Irish Counties that have observed defective block (e.g. Co.Clare, Co.Limerick) the deleterious aggregate in use is very similar to Co.Mayo and consists of varying degrees of calcareous argillaceous sediments containing framboidal pyrite.

As with Co.Mayo the presence of framboidal pyrite is problematic due to the risk of internal sulphate attack when hosted in weak aggregate. An example of that reaction is observed above, with microscale framboids oxidising to goethite and producing a by-product of sulphate (e.g. ettringite, thaumasite and gypsum) which degrades the concrete.




Low/Medium & Negligible

Heterogeneous Gravel

The aggregate used to make this concrete is composed of a heterogeneous meta-sedimentary gravel. The metasediments include mainly well annealed micaceous quartzite (metamorphosed impure sandstones, including metamorphosed greywacke with irregular sized quartz clasts), with minor amounts of schist, weakly laminated altered siltstones, and mudstones (metapelite). The presence of biotite in the quartzite indicate a slightly higher grade of metamorphism than the typical phyllite aggregate seen in Donegal blocks. The gravel may include minor quantities of phyllite (metamorphosed mudstones and siltstones), as well as other assorted lithologies such as granite, gabbro and grey bioclastic limestone.


Bioclastic grey limestone

The aggregate used to make this concrete is made with an all-in crushed sparry limestone, with minor amounts of siliceous limestone. Some of the sparry limestone shows evidence of dolomitisation with the formation of characteristic dolomite rhombs. There are rare examples of argillaceous limestone, containing stylolites, which may host rare pyrite. This limestone is significantly different from the typical black argillaceous limestone / calcareous mudstone encountered within Co. Mayo and is derived from a different lithological unit. Potentially deleterious argillaceous limestone comprise <10.0% of the total aggregate (modal analysis).




Due to the prevalence of quarried metasedimentary aggregate of varying metamorphic grades within Co.Donegal there are a number of Low/Medium Risk aggregate types in use that have been testing under I.S.465. For gravels these normally produce free mica <5% and don’t contain sulphides in concentrations that are a cause for concern. As well as gravels there are also examples of crushed stable aggregate (e.g. gabbro) or crushed micaceous poor aggregate (e.g. quartzite).

Within Co.Mayo and the other Irish Counties, there are some Low/Medium risk gravels in use which are typically a mix of stable metasedimentary aggregate and dark grey bioclastic limestone without framboidal pyrite.


Concrete Condition

The condition of the concrete is assessed during each stage

Concrete Condition

The condition of the concrete is assessed during each stage and is based on any evidence of deterioration including, inter alia, matrix cracking, aggregate cracking, internal or external sulphate attack and physical incoherence. Evidence of any ‘non-I.S. 465’ forms of deterioration are included. At the conclusion of any stage, the condition of the concrete samples is designated as ‘Sound’ or ‘Unsound’ according to the following definitions.



Currently showing no, or only rare evidence of deterioration.

Concrete which is sound remains intact when sampled and cannot be pulled apart by hand.

Samples noted as sound display negligible susceptibility or low/medium susceptibility to deterioration due to the rare or minor presence of potentially problematic lithologies/minerals.










Lacking physical coherence and/or showing common or abundant evidence of deterioration, also concrete too deteriorated to be sampled intact.

Concrete which is unsound is often well fractured and can sometimes be pulled apart by hand.

Samples noted as unsound usually display high susceptibility to or show evidence of deterioration due to the presence of potentially problematic lithologies/minerals.