Rocks containing pyrrhotite bands are sometimes used to produce concrete. These rocks oxidize and produce long-term expansive reactions that damage concrete structures, leading to important economic and risk related repercussions. The present study analyses several aspects that affect the oxidation process of the aggregate such as the existence of preferential paths for the entrance of the oxidizing agent and the conversion process of the chemical elements involved in the reaction. For that, host rock samples containing pyrrhotite were investigated by scanning electron microscopy and energy dispersive spectroscopy. The results shows that the pyrrhotite appears in bands that create planes of weakness and present cracks that serve as preferential paths for the entrance of oxygen. Furthermore, a new representation is proposed for the oxidation process.
Geological formations of endogenous and metamorphic host rocks with inclusions of pyrrhotite bands may be found in Nature. In some cases, these rocks have been used to produce aggregates that were then incorporated in the dosage of concrete. Elements cast with this material have shown severe cracking caused by an expansive phenomenon known as internal sulfate attack (ISA). In such phenomenon, the particles of pyrrhotite included in the aggregates oxidize, producing iron hydroxides and sulfates (
Several examples of structures affected by the presence of this harmful material are found in the literature. Some of these studies remark the important economic, social and risk related repercussions of using the contaminated rocks in concrete (
Detail of a) downstream face and b) crest crack at the Graus Dam (
The rate and the overall repercussions of the ISA are clearly dependent of the interaction between the particle and the surrounding host rock that forms the aggregate. Factors such as the pyrrhotite concentration in the host rock and the oxygen access (oxidizing agent) may affect the oxidation process. Therefore, the characterization of this reaction in the host rock is a fundamental step towards understanding, modeling and properly treating the degradation observed in the concrete.
The objective of this paper is to analyze three aspects that may affect the oxidation process of pyrrhotite particles inside the aggregate: the shape and the integrity of the particle, the existence of preferential paths for the entrance of oxygen and the conversion process of the chemical elements involved in the reaction. For that, host rock contaminated with pyrrhotite particles were extracted from quarries and used to produce samples. The latter were investigated by scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS).
The study shows that the pyrrhotite appears in bands that create planes of weakness in the rock. Cracks observed in these bands serve as preferential paths for the entrance of oxygen. Furthermore, a new representation is proposed for the pyrrhotite oxidation process. Both observations denote a significant contribution towards the development of more realistic models to predict the evolution of the ISA.
The oxidation process of pyrrhotite may be caused either by the activity of microorganisms or by the direct chemical reaction with the compounds present in the atmosphere under favorable conditions. Even though the former is more frequent in Nature, the latter is the focus of the present study since it is the one usually observed in concrete structures subjected to ISA. In this case, the access of the oxygen (O2) and of the moisture (H2O) are essential to initiate the reaction (
The evaluation of the oxidation process is often performed through a scanning electron microscopy with microanalysis (SEM/EDS) since the EDS can quantify the chemical elements present in the samples analyzed. The atomic ratios obtained from this analysis serve to assess the compound present at a given point and time throughout the course of the reaction.
In order to facilitate the identification of the chemical compounds involved, a graphical representation of the pyrrhotite oxidation is commonly used. These graphs define a Cartesian system with axis X and Y, each of them corresponding to a different atomic ratio between the contents of two chemical elements obtained from the microanalysis. For each analyzed portion of the sample, a pair of atomic ratios is obtained, making it is possible to define a point in the Cartesian system.
Depending on the point position with reference to the axis, the chemical compound in the analyzed zone is determined. This approach allows an easy comparison of the composition of different zones of the same sample and outlines the path and the stage of conversion of the chemical compounds during the reaction.
To represent the iron sulfides oxidation process, Schmidt et al. (
Pyrrhotite oxidation according to a) Schmidt et al. (
Mycroft et al. (
To justify the profile Mycroft et al. (
In order to obtain a clearer picture of the evolution and the factor affecting the pyrrhotite oxidation, an extensive set of rock samples were extracted from the quarry located in the right abutment of the Rumedo dam. This is a concrete dam built in Lladorre (Spain) by 1971 with a 91 m of length and 9 m of height severely affected by the ISA.
The rocks collected from the quarry were crushed and divided into 4 different grading ranges: 4 to 5 mm (A), 5 to 20 mm (B), 20 to 40 mm (C) and 40 to 50 mm (D). The physical properties testing showed that the porosity and the density of the host rock are 2.17% and 2.73 g/cm3, respectively. The chemical analysis presented in the
Chemical composition of the rocks by XRF (values in %)
Óxidos | A (4–5) | B (5–20) | C (20–40) | D (40–50) | s.d. (σ) | |
---|---|---|---|---|---|---|
Na2O | 1.38 | 1.71 | 1.99 | 1.90 | 1.75 | 0.27 |
MgO | 2.82 | 2.52 | 2.29 | 2.89 | 2.64 | 0.28 |
Al2O3 | 18.20 | 16.79 | 17.30 | 19.88 | 18.05 | 1.36 |
SiO2 | 50.62 | 58.70 | 59.33 | 56.51 | 56.29 | 3.97 |
P2O5 | 0.14 | 0.10 | 0.14 | 0.10 | 0.12 | 0.02 |
SO3 | 1.72 | 1.50 | 1.06 | 1.38 | 1.42 | 0.28 |
K2O | 3.43 | 3.01 | 2.92 | 4.21 | 3.40 | 0.59 |
CaO | 3.87 | 0.73 | 0.70 | 0.03 | 1.34 | 1.72 |
TiO2 | 0.88 | 0.80 | 0.64 | 1.06 | 0.85 | 0.17 |
Fe2O3 | 11.02 | 8.34 | 8.08 | 8.47 | 8.98 | 1.37 |
The rock samples for scanning electron microscopy SEM (FIE QUANTA 200) were prepared from the host rock by cutting, grinding and polishing to give representative micro sections of the surface. The polishing was done in several steps by using diamond suspensions from 9 to 0.25 μm (BUEHLER, Beta-vector). The polished samples were then coated with carbon to get an electrically conductive surface. The microstructure of the samples was examined by SEM using backscattered electron imaging (BSE) and energy dispersive X-ray spectroscopy (EDS). The chemical analysis using EDS was performed with a Li/Si crystal detector and an accelerating voltage of 15 kV. The pyrrhotite oxidation was studied by EDS point analysis to determine the elements iron, sulfur and oxygen.
Pyrrhotite usually appears in the host rock concentrated in bands (
Host rock from Rumedo dam quarry: a) pyrrhotite band and b) BSE image.
To identify the presence of the oxygen in the pyrrhotite bands, a total of 16 samples of the host rock were analyzed. In each sample, two zones were identified. Zone I corresponds to the border between the pyrrhotite band and the crack (pyrrhotite surface), whereas zone II represent the central part of the pyrrhotite band. This work presents the microanalysis of the two most representative aggregate samples. The analysis of the complete set of samples may be found in Jones et al. (
Mapping for the elements: a) BSE image, b) Fe-S, c) S-O and d) Fe-S -O.
In
The purple areas in
Mapping for the elements: a) BSE image, b) Fe-S, c) Fe-O and d) O.
A crack that cuts the pyrrhotite band is observed in
It is important to remark that the pyrite oxidation path of conversion is identical to the pyrrhotite, with the difference that the degradation process usually starts with an S/Fe atomic ratio slightly higher. This happens since S/Fe is equal to 2 for the pyrite and between 1 and 1.25 for the pyrrhotite.
Oxidation process of the pyrrhotite presented in
Following the same procedure used in the analysis of the pyrrhotite previously presented,
Oxidation process of the pyrrhotite presented in
In order to provide a general overview of the pyrrhotite degradation process,
Conversion points of the pyrrhotite oxidation process.
As may be seen, for low oxygen concentrations (non-oxidized points) iron contents show a decreasing linear tendency. On the contrary, the sulfur content is maintained with a steady tendency, despite its greater dispersion. This may reflect the diffusion of iron through the particle, which reduces the content of this element with respect to the sulfur content, characterizing the conversion of the pyrrhotite structure towards a disordered pyrite structure.
Once the Fe/O atomic ratio reaches 2.44, a significant increase in oxygen content is observed while the iron content has a fairly constant tendency, even though with a high scatter (see
Pyrrhotite oxidation process.
However, for values of Fe/O and O/S higher than the limits defined, the points start the oxidation process (in green) following a decreasing linear tendency characteristic of the conversion of a pyrrhotite structure into a pyrite stoichiometry. A new representation for the pyrrhotite oxidation process is proposed in
New representation for the pyrrhotite oxidation.
According to this new process, the oxidation of pyrrhotite goes through three distinct stages. First, the pyrrhotite is in the non-oxidized form, representing a very low or no presence of oxygen (Stage I). Then, oxidation starts and restructuring of the pyrrhotite into pyrite occurs due to the diffusion of iron through the pyrrhotite surface (Stage II). Finally, the products of the pyrrhotite oxidation are formed (Stage III).
Notice that the new representation also includes the conversion path for the pyrite. In this case, the conversion path starts for the non-oxidized particle at a S/Fe ratio of 2. Then, as pyrite becomes in contact of oxygen, the reaction gradually goes through stages II and III.
The predominant iron sulfide present in the analyzed host rock samples is the pyrrhotite, which generally appears in the host rock as bands with a plate-shaped geometry. These bands mark planes of weakness, which are susceptible to the appearance of cracks. It was also found that characteristic ratios of 2.44 for the Fe/O ratio and of 2.63 for the S/O ratio mark critical limits that produce the activation and acceleration of the pyrrhotite oxidation. This should lead to an increase of the expansive reactions and, consequently, of the risk of structural damage.
It was confirmed that the cracks mark a preferential path for the entry of the oxidizing agent (oxygen). Thus, the analyzed aggregates that present pyrrhotite bands with cracks show a degree of oxidation much more pronounced than the aggregates without cracks. Such consideration is of the outmost importance for the development of models to predict the evolution of the expansive reactions since it indicates that the diffusion of oxygen should be considered through the cracks rather than through the aggregate.
The microanalysis and the chemical study validate the new representation proposed for the progress of the pyrrhotite oxidation. According to this new representation, initial restructuring of the pyrrhotite into pyrite stoichiometry occurs due to the iron diffusion to the surface of the particle. After this, the surface oxidize, resulting in oxides and finally in iron hydroxides. This new representation should also be considered when modelling the kinetics of the reactions that govern the expansive mechanism and the resulting damage.
Funding and support of this research was provided by the Research Contract UPC – ENDESA (Dam Project) and IBERDROLA (Horex Project). The authors would like to thank the technical support of: Juan Manuel Buil and Felipe Rios by ENDESA and Arturo Gil and Ana Belen Martin Vacas by IBERDROLA. This study has been conducted in collaboration with the Department of Building Materials of the Ecole Polytechnique Federale de Lausanne - EPFL (Switzerland).