Impact of chemical weathering on micro/macro-mechanical properties of oolithic iron ore

This article concerns the problem of collapses of abandoned underground (room-and-pillar) iron mines in Lorraine (North-Eastern France). A study on the ageing of iron ore was carried out using different analytical techniques (SEM-EDS, EMPA, X-ray diffraction, Mössbauer and ICP spectroscopy). As iron...

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Bibliographic Details
Published in:International journal of rock mechanics and mining sciences (Oxford, England : 1997) England : 1997), 2013-12, Vol.64, p.236-245
Main Authors: Grgic, D., Giraud, A., Auvray, C.
Format: Article
Language:English
Subjects:
Aging
Anaerobic conditions
Applied sciences
Building failures (cracks, physical changes, etc.)
Buildings. Public works
Cements
Chemo-mineralogical analyses
Collapse
Compressive strength
Diffraction
Durability. Pathology. Repairing. Maintenance
Earth Sciences
Exact sciences and technology
Geotechnics
Iron
Iron mine
Micro-indentation tests
Mine drainage
Minerals
Mines
Miscellaneous
Oxidation
Pillars
Reactive environments
Sciences of the Universe
Underground
Weathering
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Summary:This article concerns the problem of collapses of abandoned underground (room-and-pillar) iron mines in Lorraine (North-Eastern France). A study on the ageing of iron ore was carried out using different analytical techniques (SEM-EDS, EMPA, X-ray diffraction, Mössbauer and ICP spectroscopy). As iron ore is a reactive environment in an oxidizing atmosphere created by anthropic activity, this kind of weathering is triggered or activated by chemical processes, and differs from diagenetic evolution under reducing conditions. The weathering is characterized by the chemical alteration/oxidation of reduced minerals (siderite and berthierine) in the inter-oolithic cement, mechanical disaggregation of oolithes, recrystallization of berthierine and neo-crystallization of a honeycombed hematite crust (i.e., rust) on all mineral surfaces. This corrosion of iron ore was found to occur at a large scale in pillars, especially in very old pillars which have been exposed to the mine atmosphere for 70–100 years. The alteration of the inter-oolithic cement which ensures material cohesion was not found to have been compensated by the crystallization of new minerals, thus inducing the decrease in the micro/macro-mechanical properties of iron ore (micro-indentation tests at millimetric scale, micro-compression triaxial tests at centimetric scale and macro-compression triaxial tests at decimetric scale). This chemical weathering of anthropic-origin plays a large-scale and major role in the long-term behavior of pillars, and is probably one of the main causes of mine collapses that occured in Lorraine. Therefore, the decision to stop pumping of mine drainage, thus inducing flooding of iron mines (by groundwater), was the correct decision, since oxidation reactions in iron ore are necessarily less important under anaerobic condition than under aerobic condition. In addition, the knowledge of iron ore ageing allows for improved assessment of the long-term stability of iron mines which have yet to collapse (i.e., flooded iron mines and also iron mines which are still above the ground water-table). •We studied chemo-mineralogical transformations resulting in the ageing of iron ore.•Oxidizing atmosphere created by anthropic activity induced chemical weathering.•The reduced minerals of the cement were oxidized and new minerals crystallized.•The decrease in cohesion induced the decrease in micro/macro-mechanical properties.•The chemical weathering is one of the main causes of iron mine
ISSN:1365-1609
1873-4545
DOI:10.1016/j.ijrmms.2013.09.005