An Isotopic and Mineralogical Investigation of Concrete Decay

MacLeod, Gordon (1990) An Isotopic and Mineralogical Investigation of Concrete Decay. PhD thesis, University of Glasgow.

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Concrete is a man made rock composed of aggregate bound rigidly in a paste composed of hydrous cementitious minerals. It is a porous and permeable medium on both the macro and microscopic scales. This study has revealed that the phases present in cement pastes, in particular portlandite, can have random or preferred orientations at the cement paste to aggregate interface. The micro-mineralogy of the cement paste to aggregate bond interface in concretes is texturally complex and concrete mixing techniques can effect the phases present at the interface and the density of the interface. In a core of concrete from a road bridge undergoing active decay, pyrite and pyrrhotine were found to have oxidised by interaction with percolating rainwater. The sulphate released by the oxidised iron sulphides has been taken into solution and eventually precipitated in large spherical cavities in the cement paste, in the form of the mineral ettringite. Water flowing through the structure has possibly been more significant in leaching gypsum from the cement paste and depositing it in the cavities as ettringite. The ettringite has a whisker crystal habit, and has nucleated heterogenetically from solution on the walls of the spherical cavities. The whisker crystal habit is propagated by a screw dislocation in the crystal, the concentration of the elements required for growth being in the solution at low enough concentrations as to limit multi-directional nucleation. The fluid that has percolated through the structure oxidising the iron sulphides has become hyper-alkaline due to the leaching of portlandite from the cement paste. This fluid has reacted with atmospheric carbon dioxide to produce a calcitic crust on the top of the road bridge and a stalactitic growth on the underside of the road bridge. Similar crusts and stalactites were collected from other concrete structures in the Midland Valley of Scotland, and analysed for delta18C and delta18O values. The delta13C values are in the range of -18.8‰ to -28.9‰ PDB. The delta18O values are in the range of +8.5‰ to +16.5‰ SMOW. The calcites formed by the interaction of the hyper-alkaline fluids that have percolated through the structures, with atmospheric carbon dioxide (delta13C = PDB, delta18O = +41‰ SMOW). The stalactites have formed by the production of successive calcite layers, by the diffusion of carbon dioxide into the bulbs of hyper-alkaline solutions as they emerge on the underside of the structure. The computer codes EQ3NR/EQ6 predict that rainwater flowing through a structure will oxidise iron sulphides present in aggregate fragments, and precipitate ettringite. They also predict that the residual fluid that emerges from the structure has a pH in the range 12-13. It is this hyper-alkaline fluid that reacts with atmospheric carbon dioxide to produce the calcitic growths.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Mineralogy
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78218
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09

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