Llandough Hospital the date of this cracking in the wall of the eastern ward is unknown. The cracks were hidden with battens faced with formica sheet
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Llandough Hospital - the date of this cracking in the wall of the eastern ward is unknown. The cracks were hidden with battens faced with formica sheet.
Llandough Hospital was written up in the 1980s. Other sites with sulphate problems which I was involved with in the 1980s/90s were at Swindon, Hucclecote, Camarthen and piles in the London Clay. At these sites, the material was in situ and had a lower calcite content than the Dublin rocks - bacteria clearly played a significance role.
Working on the basis that this is a briefing, not a lecture, in the time available I’m only going to mention some key facts and offer you a simplified approach to a complex problem.
Pyrite? What’s the problem?
Pyrite – iron sulphide – is present in most rocks, particularly such sedimentary rocks as the mudstones and limestones found in the Dublin Basin.
Framboid and octahedral pyrite on a fractured surface in the London Clay
As long as the pyrite is isolated from moisture and oxygen, there is no problem. The difficulties start when the overburden stress is reduced and/or the rock is quarried and the pyrite exposed.
Cubes/lumps of pyrite have a relatively small specific surface (surface area : volume) compared with a cluster of microcrystals (sugar lump/sugar grains) hence the chemical reaction is slower. Unbroken pyrite cubes are often displayed in museums – in Dublin some can be seen in the walls of Christchurch.
Fractured pyrite lump
When oxidation of the iron sulphide begins, the ferrous sulphate formed has a volume some 200-300% greater than the initial components. In mudstones, the creation of the ferrous sulphate rim may crack open the rock fragment.
When mudstones are quarried, stress relief results in a dilation of the material along the bedding planes or clay-rich bands (laminations). This facilitates both the ingress of the oxidising agents (moisture and oxygen) and the movement of sulphate-rich solutes.
The presence of the argillaceous bands facilitates the splitting of the rock with stress relief and when crushed/compacted. In addition, the clay minerals help the ingress of the oxidising agents by capillary movement.
Typical gypsum crystal from the London Clay as seen in museums and handled by students.
From the initial formation, new growth extends outwards in the form of rosettes which become increasingly thick and push the rock apart.
Particularly in dark rocks, it is very difficult to distinguish calcareous mudstone from argillaceous limestone by looking at a hand specimen.
Dark coloured material should raise alarm bells……..
If aggregate fragments break down during compaction, the higher percentage of fines will accelerate the rate of oxidation of any pyrite present.
Check the total sulphur values – ensure that adequate testing has been carried out on the material YOU are purchasing. See a range of results, not just an average. Judge the likelihood of problems on the average of the two highest TS results.
Acid soluble and water soluble sulphate values tell you about what is happening/has happened rather than what may happen in the future. They also are more likely to reflect the oxidation of the fine fraction rather than the pyrite within the fragments, which causes the main heave.
In samples from 1.5 m depth below Llandough Hospital, stored in a laboratory for 17 months, the ASS content increased from 0.43 to 1.92% SO4.
??? Use suspended floors
Sulphate-rich solutes moving from deleterious fill into concrete may result in deterioration of the sub-structure concrete/concrete block walls.
Thaumasite exposed on a broken block
Typically, before building commences, a site investigation is undertaken.
NRA Series 800 was widely used in the building industry as well as for roadworks.
Appreciating the development of sulphates in crushed aggregate, the 2013 editions of both the 600 and 800 Series now recommend that material within 0.5 m of cement-bound materials should have a
SR 21:2014 recommends that if the TS content is
SR 21 also states that petrographic assessment should identify if pyrrhotite is present.
IS 398 was produced to assist in determining whether or not a building has been damaged by reactive pyrite or is likely to be in the future.
A pass category in Table 3 does not preclude the potential for both heave and damage to concrete/concrete blocks.
Most sub-structure walls are formed of concrete blocks founded on strip footings.
Most concrete blocks consist of a rock aggregate, quartz sand and cement. In some cases, the inert quartz sand is replaced with fine limestone particles/dust.
BRE notes that in concrete the presence of more than 4% sulphate by weight of cement does not automatically imply sulphate attack has taken place; it may only be a warning of potential attack in the future.
When pyrite oxidises, ferrous sulphate and sulphuric acid are formed. This is an ongoing process.
4. Expansion results from the formation of ferrous sulphate and the reaction between sulphuric acid and calcium carbonate to form calcium sulphate (gypsum). Heating accelerates these reactions.
Concrete/concrete blocks and pipes in contact with the ground, may be affected by sulphates in the adjacent material.
10. Always ensure the supplier is aware of the end use of the product you are purchasing. They should have the expertise to supply you with a reliable material.
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