What is pyrrhotite?
Pyrrhotite (PEER-o-tight) is a naturally occurring iron sulfide mineral that is often found in marine sedimentary rocks. It can also can be found in some igneous rocks and some metamorphic rocks that are of a marine origin. It is a highly reactive mineral and breaks down in the presence of water and air. Pyrrhotite, even at low concentrations in concrete aggregate, is responsible for the premature crumbling of many concrete basements in northeastern Connecticut and south-central Massachusetts.
For those interested in some more scientific details…
The chemical formula of pyrrhotite is Fe(x-0.1)S and indicates that iron (Fe) can vary within the mineral’s structure and is often deficient relative to sulfur (S). This makes pyrrhotite both challenging to quantify and weakly magnetic.
Pyrrhotite can vary from a 1:1 ratio of Fe to S (trolite) to Fe11S12 (hexagonal pyrrhotite) to Fe7S8 (monoclinic pyrrhotite). All forms of pyrrhotite can potentially react in the presence of water and air. However, there are still many unanswered questions about the various forms of pyrrhotite and relative reactivity.
How does pyrrhotite in concrete cause cracking?
Pyrrhotite is an unstable mineral. In the presence of water and air, it transforms into secondary minerals that are larger in size. This space problem initiates cracking in concrete, provides further pathways for water and air to penetrate and eventually the concrete deteriorates. This deterioration can take years to decades depending on the quality of the initial concrete and environmental conditions.
Is pyrrhotite found in every concrete foundation?
No. We have tested many homes that are pyrrhotite free. The presence (or absence) of pyrrhotite depends on the rock aggregate in the concrete and where it was derived from geologically. Connecticut rocks have some formations that are more prone to higher levels of pyrrhotite than others, and there are many rock types used for concrete aggregate in which pyrrhotite is not present.
What do cracks in concrete due to pyrrhotite look like?
This answers depends a bit on how far along the reaction has progressed. Map cracking or spider cracking is the clearest indicator of pyrrhotite-laced concrete. In all the homes that we’ve tested and have observed map cracking, pyrrhotite has been detected. Other visual types of cracking can be ambiguous and isn’t always diagnostic without further testing. Generally, vertical cracks are common in concrete due to settling or shrinkage. Horizontal cracking is less common and may be an initial sign of pyrrhotite induced cracking or could be do other factors.
Who is getting tested and why?
- Home owners who notice suspect cracking and want to know for sure that it’s pyrrhotite causing the cracking.
- Homes with a recent engineer’s report suggests testing (due to ambiguous visual results, class 1 designation).
- Potential home buyers are asking for the test before a home sale since visual inspections alone may not diagnose the presence of pyrrhotite. This is especially the case if the reaction isn’t advanced enough at the time of inspection and obvious map cracking hasn’t initiated.
- Home sellers are preparing for sales and want to know if pyrrhotite will be an issue in the sale of their home.
- Homes built between 1983-2015 are believed to be most at risk and are from the towns that are most affected in both northeastern Connecticut (see map on CRCOG website) and south-central Massachusetts. Some homes show signs, other don’t. Core testing can confirm the presence or absence of pyrrhotite in these cases where a visual inspection cannot.
Who doesn’t need testing?
In Connecticut, homes that have visual classifications of a 2 or 3 by an engineer or approved inspector do not require core testing. These home are presumably seeking state assistance through the captive insurance group that was setup by the state (see Captive Insurance Group website, link found below). Class 1 designations require core testing. Class 2 and 3 show signs of deterioration where a class 1 does not or signs are ambiguous. In Massachusetts, any home looking for tax relief will require a core test result.
Is a visual engineer’s inspection needed?
This depends. If your concrete doesn’t have any signs of deterioration, an engineer cannot determine if pyrrhotite is within your walls without requesting a core test. In this case, a core test makes sense to do first because homes that return pyrrhotite-free results don’t need any further visual inspection. If your concrete does have clear signs of deterioration, then an engineer’s inspection may be all that is needed (in Connecticut) and a core test is not required. Some engineers will recommend core testing if the visual inspection is ambiguous. See question above for Massachusetts homes. Please note that any claims by CT homeowners to the captive insurance group (CFSIC) for homes that have tested positive for pyrrhotite from a core test, will still require a visual inspection to determine the level of severity. Link to CFSIC is below.
Is there a known safe level of pyrrhotite?
Not officially, but we are getting closer to this answer. Our data appears to indicate a lower risk scenario in some homes. We presented this data to CRCOG in December of 2019. You can click on this link to access this report. Trinity_Update_Dec2019. Since this 2019 report, we have accumulated more data that has helped us statistically approach the “save level” answer.
In March of 2021 we presented new data at the Northeast Section of the Geological Society of America meeting (virtual). Based on 430 homes (~900 data points), we have observed that no home to date has shown map cracking below a pyrrhotite concentration of 0.16%wt in the concrete. We derived this number from our magnetic susceptibility and total sulfur measurements. The 0.16% equates to a magnetic susceptibility of 5 (10-8 m3/kg). Above this value, the instances and predicted probability of map cracking increases incrementally.
Is 0.16% pyrrhotite in concrete safe? That question remains to be tested by us and others, but as of March 2021 it’s the best approximation we have.
What if a trace of pyrrhotite is found in my foundation? Is this a problem?
So far, no homes that we have tested that show clear signs of deterioration due to pyrrhotite (i.e. map cracking) contain only a trace of pyrrhotite. Again, please refer to our .pdf report from December 2019 (linked above) to better understand our data at the low end of the spectrum.
How does the Trinity Concrete Analysis for pyrrhotite work?
We conduct two independent tests that approach the same answer. This will get a bit technical so hang on, but we will try to keep it simple…
- A physical thermo-magnetic susceptibility analysis
- A chemical total sulfur analysis
Pyrrhotite is a weakly magnetic mineral due the arrangement of iron atoms in the mineral’s structure. This property can be used to semi-quantify the amount of pyrrhotite in concrete samples to very low concentrations. It’s a method that has been widely known and used in the rock magnetism and mining fields. When a sample containing pyrrhotite is heated to the prescribed temperature of 325⁰ C, the magnetism due to pyrrhotite is lost. Essentially, it’s too hot for pyrrhotite to maintain its magnetism at this temperature. Other common magnetic minerals found in rocks, such as magnetite, have a different prescribed temperature which makes it relatively straight forward to uniquely identify pyrrhotite in a heated sample. This drop in magnetism during the heating process is recorded and the size of the drop is roughly equivalent to how much pyrrhotite is in the sample. We use two AGICO magnetic susceptibility instruments for our magnetic analyses.
Total sulfur is also important because pyrrhotite is an iron sulfide mineral. While there is always a background level of sulfur in all concrete samples (typically ranges from 0.1% to 0.2% by mass), an elevated concentration of sulfur indicates that pyrrhotite is present for Connecticut rocks. We use a purge and trap elemental chromatography instrument (Elementar vario EL cube) that is calibrated to measure sulfur gas when samples are combusted at temperatures over 1000⁰C.
For a full technical explanation of our applied method, you can read our peer-reviewed paper that was published in the Journal of Cement and Concrete Research. (Geiss and Gourley, 2019). To request a copy, you can email Prof. Gourley directly (address is below).
The Trinity Concrete Analysis detects the presence of pyrrhotite through the thermo-magnetic measurement and quantifies the amount of sulfur through elemental chromatography. The two tests combined can estimate pyrrhotite in a sample to as low as 0.5% pyrrhotite. Assuming all the sulfur in the concrete sample is bound in pyrrhotite, the concentration of pyrrhotite is approximately 2.5 times the value of the concentration of sulfur. Since pyrrhotite is unlikely the only sulfur mineral present, this value should be considered a maximum estimate of pyrrhotite concentration. This test simply confirms the presence of the mineral pyrrhotite in the sample(s) provided, estimates the maximum concentration and compares it (them) to the results obtained from other similar samples. The test cannot predict what will happen to the concrete in the future.
How much does the test cost?
Currently, the cost of testing 2 concrete cores from an average sized home costs $1800-$2000 ($1000 for analysis at Trinity and anywhere from $800 to $1000 for the coring job.) In Connecticut homeowners can be reimbursed 50% of this cost. In Massachusetts, 75% can be reimbursed. Additional cores (for home additions or larger structures) cost an additional $800-$975 ($500 for the analysis, coring prices may vary).
How long will it take for me to get my results?
From the time of coring, we estimate reports will be generated within 2 weeks. This time could vary depending on seasonal work flow, although, we’ve consistently been able to send out reports within this time frame.
Are my results shared?
We only send the final report and receipt to the party or parties that pay for the test and/or to the emails are listed by the purchaser(s) on our questionnaire. We compile all previous home data and include the results on our graphs but never share names and addresses on any presentation without your permission. On our questionnaire (that you will fill out at the time of coring), we ask if you are willing to have your data shared as part of our on-going research. If you choose to get reimbursed for the test from the state, you will be sending your results to the state. Test results are also subject to disclosure in home sales.
Where can find more information about the pyrrhotite problem?
There are several good sources of information that can help home owners navigate this issue:
Capitol Region Council of Government (CRCOG) has several pages of information and is where to go for reimbursement for testing.
Connecticut Foundation Solutions Indemnity Company Inc. (CFSIC) – Captive insurance group who allocates CT state funds for basement replacements.
Massachusetts Residents Against Crumbing Concrete – MA home owner advocate group
Facebook page @MassRACC
Can I talk to someone at Trinity about the testing method?
Absolutely! Prof. Jonathan Gourley is available for questions via phone or email and will return your call promptly if he happens to be in the lab or teaching class when you call.
Prof. Gourley can be reached directly at: (860) 297-4128. You can also email him at: firstname.lastname@example.org