I have just read an interesting research paper, available on-line, about heave under houses caused by the pyritic expansion of unsuitable fill material. This is a huge ongoing problem, with tens of thousands of houses in Ireland alone affected.
The research details the results of experiments carried out to try to quantify the rate of pyritic expansion and study the influencing factors. This is important research.
The paper has the ungainly title of ‘A laboratory study of the expansion of an Irish pyritic mudstone/ siltstone fill material’ and was written by David Sutton, Bryan McCabe, Aidan O’Connell and John Cripps. It is available to download and can be read by clicking here.
I found the most interesting aspect of the paper was the description of the pyritic reaction itself. Previously I had thought the oxidation of pyritic (Iron Sulphate) was only one reaction, which as a result expanded the fill material under the building.
The process explained. The paper explains that the actual heave is caused by a process of three consecutive reactions. Which simplified are as follows:
1. Initially when disturbed, and exposed to water and air, the pyrite mineral undergoes oxidation, producing sulphuric acid and ferrous sulphate.
But the sulphuric acid produced by this reaction prevents any further oxidation from occurring.
2. However a second reaction now commences. Certain bacteria are capable, in the presence of the recently formed sulphuric acid, and water and oxygen, of converting the ferrous sulphate produced in the first reaction into ferric iron oxide and more sulphuric acid.
3. Finally where calcium carbonate is also present this reacts with the sulphuric acid produced in reactions 1 and 2 to form gypsum and carbon dioxide. As gypsum is much less dense than pyrite the volume of the fill increases causing movement/ damage to the building above.
Temperature is a factor as well as moisture. This is fascinating, prior to reading this paper, I thought that the presence of water was the determining factor for the rate of swelling of the fill material. I had read reports from Canada stating that it had taken as long as 10 years for buildings to start developing cracks from pyrite heave there. Whereas in Ireland the faults developed much faster, I had thought this was because the soil was usually damper in Ireland.
The reactions detailed in this paper indicate that while water is required to commence the reaction, for it to continue a bacterial process is required. This bacterial process is dependent on temperature; at temperatures estimated to be below 3 degrees Celsius the bacterial process ceases. So the mild Irish climate means that the bacterial reaction, and therefore pyrite heave, will occur quicker here than somewhere with a cooler climate; for example Canada.
Possible alternative solutions. Currently the only solution to serious pyrite heave is to simply to remove the affected fill material from under the floor slabs. This is a difficult and expensive process. Now that we are beginning to understand the reactions better, could more cost effective remedial methods be tried? Perhaps a possible solution would be injecting something into the affected fill material which would prevent the bacterial reaction from occurring?
Read the paper yourselves, there is much interesting information on the pyrite heave problem.