Assessing Pyrrhotite in Concrete

Summary

Damage to concrete structures in residential and commercial construction in Connecticut and Massachusetts have been attributed to the iron sulfide mineral pyrrhotite. Iron sulfides in concrete aggregate are not desirable as their relative instability results in decomposition with associated staining, expansion, and pop-outs near concrete surfaces or, in severe cases, cracking of the structure. There are no standardized test methods to assess pyrrhotite occurrence and abundance in aggregate or in concrete. Suggested limits on aggregate include 1 % S by mass in Europe, a Canadian and now Connecticut and Massachusetts limit of 0.1 % S or 0.23 % pyrrhotite, are very challenging to meet using current standard guides for petrographic analyses. Developing a standard test method, including a set of calibration reference standards will provide a means for accurate, consistent analysis of pyrrhotite in concrete.  Moreover, the pyrrhotite/aggregate/concrete reactions and rates will be documented so that the most deleterious reactions can be efficiently reduced or eliminated.  This reaction and rate data in combination with the evaluation of proposed mitigation strategies, on both materials and structural levels, will be used to manage the deleterious effect of the presence of the pyrrhotite mineral as a component of the aggregate in concrete.

Description

Objective
Develop a standard test method and a set of calibration reference materials to quantify pyrrhotite in concrete; document pyrrhotite/aggregate/concrete reactions and rates so that the most deleterious reactions can be efficiently reduced or eliminated; and evaluate proposed mitigation strategies to manage the deleterious effect of the pyrrhotite mineral as a component of the aggregate in concrete.

Background
Some residential foundations in Connecticut and Massachusetts cracked and crumbled due to the presence of the mineral pyrrhotite in the crushed stone aggregate used in their concrete costing homeowners more than $150,000 to replace (Fig 1) [GAO 20-649, Crumbling Foundations, 2020].  The rate at which concrete containing pyrrhotite deteriorates is variable and depends on various factors, including water and oxygen exposure. However, the extent to which these factors contribute to the rate of pyrrhotite-related damage is unknown and cracking damage may take decades to appear.

Figure 1. Cracked foundation walls from houses in the Stafford Springs, CT areas that were sampled during NIST field studies.

Credit: NIST

Pyrrhotite (Fe1-xS, where x = 0 – 0.2) is a common sulfide mineral exhibiting a brownish-bronze color with a metallic luster. Physical properties include a hardness of 4 on the Mohs scale, a specific gravity between 4.58 and 4.65 and is magnetic, but with varying intensity depending upon iron content. Pyrrhotite may occur as an iron-rich hexagonal high temperature form or an iron-poor monoclinic low temperature form. While not conclusive, studies have suggested that the monoclinic form oxidizes more rapidly than the hexagonal form. Oxidation of pyrrhotite in concrete aggregate results in the formation of ferrihydrite with an accompanying volume increase. Sulfate released from pyrrhotite oxidation initiates a secondary reaction with aluminosulfate phases in the hardened cement paste matrix resulting in a more substantial volume increase. Examples of pyrrhotite damage to concrete may be found in large dams in Europe and residential foundations in Quebec, Canada, Connecticut and now Massachusetts. Currently, the only remedy for this problem, is an expensive foundation replacement.  The scientific community and standards bodies need to address the long-term risk management and mitigation strategies of crumbling foundations, which continues to be a costly challenge to building and homeowners in the northeastern area of the United States and elsewhere. 

Technical Idea
To meet the measurement challenges of assessing pyrrhotite occurrence in concrete and damage from cracking from the volume expansion from pyrrhotite and related reactions, new standardized test protocols and exploration of mitigation methods are necessary. NIST will develop research in each of the three categories:

1. Pre-Concrete Materials (aggregate testing to reduce/eliminate pyrrhotite)
2. Concrete Mix and Placement (water/cement ratio, additives, aggregate treatment, surface treatments on finished slab)
3. Pre-Existing Foundations (structural assessments and treatments: surface coating, membrane application, permeable chemical solutions)

Simply, the idea is to address mitigation strategies for crack damage caused by pyrrhotite at three phases of the concrete life cycle: before mixing, during placement, and existing structures.  The approach for each phase is to first understand which reactions should be targeted by developing a model system and/or examining field specimens from which different mitigation strategies can be targeted for each phase in the concrete life cycle.  A multiscale methodology from materials to structures that includes physics- based modeling of reactions and resulting expansion with a multi-disciplinary project team, including physics, chemistry, geology, materials science, mechanical and civil engineering is critical for success.

Research Plan

Fundamental reactions involving pyrrhotite and other iron sulfides in the aggregate and secondary reaction products with the surrounding cement matrix must be better understood to more effectively mitigate its deleterious effects.  NIST will document the reactions and rates both experimentally and through physics-based modeling so that the most deleterious reactions can be efficiently reduced or eliminated.

NIST will develop plans for field studies to examine the extent of damage in the CT foundations. Fields studies are essential to characterize the extent of damage and minerology and better identify critical reactions on which to focus for mitigation. A field study for this project would benefit from both structural assessments and material/specimen collections. NIST will work with the UConn research group, who have established connections to the affected community, collected foundation specimens in the affected CT area, and completed some characterization. NIST will advance studies to include foundations with varying extents of deterioration, report structural assessments and materials collections. Field samples will also be used to test the use of coatings or other protection methods on the foundations. New test methods would be developed at NIST to examine foundation core specimens to determine the extent of reaction through the thickness of the core and efficiently quantify these changes at specific locations. Information from field samples will be used to determine a shortlist of mitigation strategies to investigation.

To facilitate development and evaluation of these test methods, a suite of reference samples for calibration and validation will be invaluable to ensure consistent, accurate testing. Reference standards for quantitative assessment of pyrrhotite in concrete in the Connecticut region should be representative of the pyrrhotite, the host rock, and the concrete matrix.

Research Plan
A standard test method and a set of calibration reference materials to quantify pyrrhotite in concrete and minimize lab-specific systematic bias will be developed. To match the matrix of a typical concrete, in FY24 NIST completed a set of pyrrhotite reference materials, which included synthesized pyrrhotite, pyrrhotite containing aggregate, sand, and hydrated cement. This reference material will allow stakeholders to add known amounts of pyrrhotite in a simulated concrete matrix to meet a concentration range for their applications. NIST is advancing wavelength dispersive x-ray fluorescence (WD-XRF) and scanning electron microscopy/ energy dispersive spectroscopy (SEM/EDS) tests methods to assess the proportions of sulfate and sulfide in ASTM C09.50 Aggregate Reactions in Concrete.

Mitigation strategies will be optimized by focusing on the two deleterious reactions found in the pyrrhotite containing crumbling foundations that result in volume expansion. The first expansion is from the reactivity of pyrrhotite leading to iron hydroxide formation. The second expansion is from the sulfate reaction, which is well documented.  The presence of sulfate in solution reacts with aluminosulfate, a layered hydroxide, (AFm [1]) phases in concrete (roughly 10 % by volume) to create ettringite, which can cause up to a 150 % volume increase. Theoretical thermodynamic reaction modeling in combination with experimental reaction kinetic data will be used to document these reactions and the resulting volume expansion.

To understand the efficacy of any treatment, a “model/reference” synthesized aggregate will be developed to follow specific reaction mechanisms so that potential self-healing processes could be realized. These NIST model aggregates, or reference aggregates, with known masses of pyrrhotite and known PSD or surface area could be the basis of a standard test to evaluate the suitability of a treatment and/or to evaluate the upper limit concentrations of pyrrhotite.  To synthesize the model aggregate, pyrrhotite must be incorporated consistently into a rock matrix that is both porous and permeable and to solidify the mass in a way that the pyrrhotite is not altered (oxidized), all of which is a challenge.

Mitigation treatments will be explored for the three phases of the concrete life cycle (before mixing, during placement, and existing structures). Treatments could include reagents to alter the pyrrhotite reactions from within aggregate, through cement formulations used in concrete, or a coating barrier after placement or on the existing structure to prevent reactions within the concrete. Given all mitigation treatment options, a coating barrier will be the initial focus as there are viable commercial products used for other applications that could apply to this pyrrhotite issue and there is an urgent need to find an alternative to the current practice of foundation replacement. To achieve a scientifically sound assessment of coating barriers, NIST will utilize newly prepared control and model/reference reactive aggregate concrete, respectively, to compare with field pyrrhotite containing foundation core specimens with no visible damage.   

Major Accomplishments

A four-component (aggregate, hydrated cement, sand, and synthesized pyrrhotite) NIST pyrrhotite reference material was completed and ready for distribution to stakeholders, initially using ASTM C09.50 Aggregate Reactions in Concrete.  In FY23, a three-component (aggregate, hydrated cement, and sand) pyrrhotite reference material was completed, but material characterization of the aggregate component gave variable pyrrhotite concentration. Synthesized pyrrhotite from iron and sulfur powder was added to allow stakeholders to accurately customize their calibrations.  The reference materials were bottled, formally labeled, and packaged to reduce atmospheric exposure.  SDS documents were also created for these unique materials. 

Revisions to an x-ray fluorescence spectroscopy standard test method for measuring the concentration of pyrrhotite in aggregates and in concrete foundations was completed to account for known differences in instrument response for specific iron sulfide/sulfate minerals through calibration corrections. 

A ceramic-based model aggregate systems was synthesized using a common clay body and additives to mimic the chemistry like aggregate. Synthesis parameters are being modified to reduce pyrrhotite oxidation during heating and create a compatible material to use with a cement matrix.  This model aggregate will be used to validate characterization methods for pyrrhotite and secondary products from pyrrhotite reactions with cementitious materials.

Control concrete materials (aggregate, sand, and cement) were procured and delivered.  Choice of control concrete was vetted with stakeholders and quantities (tons) sourced locally for various longer term project experiments, ranging from lab-scale (tens of centimeter prisms) to larger foundation subsection (meter sized slabs). Sourcing reactive (pyrrhotite rock) has been a challenge.

A finite element-based model of cracks due to expansion related to the presence of pyrrhotite was developed with the goal to predict the probability of failure due to amount of pyrrhotite in cement. This current model illustrates preliminary simulation of crack growth and accounts for the diffusion of sulfates in cement matrix and determines change of modulus as a function of crack growth.

Studies to assess the effectiveness of concrete sealant mitigation strategies to manage deleterious effects of the presence of the pyrrhotite mineral in the aggregate in concrete have begun.  Four commercial sealant treatments of varying chemical compositions were chosen, an aging protocol was developed, concrete specimens prepared, and new air permeability instrumentation is in operation. Control measurements to monitor transport of external conditions (e.g., moisture and oxygen) through concrete system are in progress.

Advanced instrumentation procured in FY23 was installed and training and standard operation procedures for applications used in this project completed.  This instrumentation allows: 1) small spot aggregate and concrete foundation analysis using non-destructive sample preparation techniques (micro-XRF), 2) field work that includes handheld and portable devices to directly compare to laboratory measurements (handheld and portable XRF, and 3) combustion carbon sulfur analysis of materials commonly used in the industry.

Field studies of Connecticut (CT) foundations were completed by a NIST team including members from IMG and the Earthquake Group.  CT is replacing foundations with a degree of degradation severity level 3 (1 mm cracks).  The goal of the field study was to obtain specimens with varying extents of degradation and other structural engineering assessment information.  In April, NIST collected > 32 core specimens (4 in. OD entire foundation thickness, ≤ 10 in.) and several larger (> 12 in.) specimens and completed a structural assessment that included a site walk, documentation of foundation damage, soil sample collection. Field specimens were tagged/tracked using a system and database like that used in the Physical Evidence Database for the Champlain Towers South Investigation.  Another field study is planned for August.