Assessing Pyrrhotite in Concrete

Summary

Damage to concrete structures in residential and commercial construction in central Connecticut 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 and a proposed Canadian 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.

Description

Objective - Develop a standard test method and a set of calibration reference materials to quantify pyrrhotite in concrete.

What is the technical idea?
Pyrrhotite (Fe_1-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 ranging between 4.58 and 4.65 and is magnetic, but with varying intensity depending upon the 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. The 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, residential foundations in Quebec, Canada and, Connecticut.

What is the research plan?
To meet these measurement challenges of pyrrhotite in concrete, new standardized test protocols are necessary. To facilitate development and evaluation of these test procedures, a suite of reference samples for calibration and for 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.

Existing literature on the Brimfield schist in the Connecticut region provides little detail on pyrrhotite aside from noting it as an accessory mineral. X-ray powder diffraction and x-ray microanalysis on specimens representing the different lithologies of the Brimfield schist should provide some insight on the specific forms and polytypes of pyrrhotite present, their major and minor chemistry and aid in planning the pathway to production of a reference material. The reference materials will consist of known concentrations of pyrrhotite in a matrix of concrete that reflects that used in Connecticut concrete construction. ASTM C114 test methods will be used to assess the proportions of sulfate and sulfide in addition to use of wavelength-dispersive x-ray fluorescence measurements.

All measurements are subject random error and a lab-specific systematic error (bias). Four factors contribute to test variability: 1) the operator, 2) the equipment, 3) equipment calibration, and 4) the testing environment. The within-laboratory repeatability excludes these four factors as they are generally constant for a single lab while the 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. A set of reference calibration standards will minimize lab-specific systematic bias. A refined test protocol with a documented within-lab precision statement may then be drafted and promoted within the appropriate ASTM subcommittees.