Assessing Pyrrhotite in Concrete

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 new technical 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 ​ ​chemistry, geology, materials science, mechanical and civil engineering is critical for success.

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 w​ill​​ ​ 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. The NIST studies will 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. Two distinctive lithologies are present in the Connecticut aggregate with a coarse and a fine-grained rock, which both may contain pyrrhotite. The coarse-grained lithology is thought to be more problematic because of its higher inter-granular porosity and permeability and the coarse nature of pyrrhotite. The fine-grained lithology also contains pyrrhotite, but it is thought to be less accessible because of the relative impermeability of this lithology, although pyrrhotite exposed on the surface will likely decompose.

To match the matrix of a typical concrete, in FY23 NIST completed a set of pyrrhotite reference materials, which includes pyrrhotite containing aggregate, sand, and hydrated cement. Because material characterization of the aggregate had given variable pyrrhotite concentration, synthesized pyrrhotite from iron and sulfur powder was added to the reference material in FY24. This will allow stakeholders to add known amounts of pyrrhotite to meet their applications. NIST is advancing ​​tests methods ​​​​in ASTM C09.50​​ ​​Aggregate Reactions in Concrete (​​wavelength dispersive x-ray fluorescence (WD-XRF) and scanning electron microscopy/ energy dispersive spectroscopy (SEM/EDS)​)​ ​ ​to assess the proportions of sulfate and sulfide​ in samples​.

All measurements are subject to random error and a lab-specific systematic error (bias). Four factors contribute to test variability: 1) operator, 2) equipment, 3) equipment calibration, and 4) testing environment. Within-laboratory repeatability excludes these four factors as they are generally constant for a single lab while between-laboratory reproducibility includes these four factors. A good test method has low variation on repeated tests of identical specimens for both single-operator and multi-laboratory precision. Fractional factorial experiment design facilitates development of robust test methods through identification and control of significant factors that are potential sources of variation in test determinations. This NIST set of reference calibration standards will minimize lab-specific systematic bias. NIST is proceeding with the WD-XRF test method initially. A refined test protocol with a documented within-lab precision statement will be drafted and promoted within the appropriate ASTM C09.50.

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, (AFm1) 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. Currently, NIST does not have in-house staff with thermodynamic modeling expertise and so we will utilize academic collaborations.

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). This ​​synthesis​​​​ ​​ will be a challenge. Approaches include sintering the matrix at temperatures that will not affect the pyrrhotite. The optimum matrix should be representative of a rock type found in the CT aggregates. To begin, clay and kaolinite will be assessed. Sintering clays may not be an option as the lower limit temps may exceed that for pyrrhotite. Kaolinite, which decomposes at 550 °C, may sinter around that temperature up to 750 °C. NIST experiments will test the synthesized aggregates incorporating known mass of specific particle size distribution (PSD) pyrrhotite as a standard test to assess 1) lower limit concentrations for crack initiation and 2) to assess propensity of proposed mitigation procedure effectiveness. The combination of pyrrhotite concentration and PSD will be assessed to identify their impact on rates of reaction and expansive stresses. Specifically, the NIST Empyrean XRD with the Mo source and large diameter capillaries will be used to examine reaction kinetics using the synthesized pyrrhotite that has been size graded and incorporated into a hardened cement paste. The reaction will use O2-saturated water flow and dissolution of the pyrrhotite, development of iron hydroxides (if crystalline), sulfate reactions (crystallization of ettringite) will be monitored. This would be an additional approach in investigating efficacy of a treatment by comparing to an untreated specimen.

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. An iron chelating agent may help to migrate away rather than precipitate these iron species in place. For new concrete, solutions to the volume increase include reformulation to change the size of the AFm inclusions and/or eliminate or reduce the amount of AFm. For current concretes these items have been fixed. Another possible mitigation method would be carbonation- AFm and the concrete- to reduce/eliminate this reaction regardless of any pyrrhotite reactions. Lithium salt treatments that have been reported to treat alkali-silica aggregate reactivity will also be assessed with caution as it is challenging to permeate concrete. Although the pyrrhotite-affected foundations contain lower-quality residential concrete where the permeability will be higher, transport mechanisms in concrete are slow and there is much unknown about what reactions are useful/helpful.

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. Results using a well-defined model system compared with specimens from the field will be used to identify those coatings that seal the concrete and prevent/reduce moisture and oxygen penetration. Moreover, NIST will advance methods to test the permeability of coatings on concrete and cements to moisture and oxygen.