AN APPLIED MINERALOGICAL INVESTIGATION OF CONCRETE DEGRADATION IN A MAJOR CONCRETE ROAD BRIDGE

A core of concrete taken from a major road bridge in the Strathclyde Region, Scotland, has been subjected to an applied mineralogical investigation, which involved stable isotope analysis, petrography, X-ray diffraction and scanning electron microscopy. The structure is actively undergoing severe degradation due to mineral growth which is related to chemical reactions between the concrete and pore fluid. The physical growth of minerals causes disfigurement and structural weakening. Pyrite and pyrrhotine hosted by dolerite aggregate appear to have been oxidized, providing sulphate for the deposition of ettringite and minor gypsum, in spheroidal cavities within the cement paste. The rainwater which passes through the structure mobilising sulphate from original gypsum in the paste and oxidizing the iron sulphides is also involved in the further leaching of elements from the cement paste and in the deposition of calcite. The isotopic values of calcites forming a crust on the concrete and a stalactite under the bridge are similar with delta-C-13 = 19 parts-per-thousand PDB and delta-18 parts-per-thousand = +16 parts-per-thousand SMOW. We suggest that atmospheric carbon dioxide was the carbon source. The carbon isotopic fractionation of -12 parts-per-thousand from atmospheric carbon dioxide of delta-C-13 = -7 parts-per-thousand. (O'Neil and Barnes, 1971) can best be explained as due to a kinetic fractionation related to the hyper-basicity of the pore water. The equilibrium formation temperature of about 45-degrees-C calculated from the oxygen isotope values and assuming a delta-O-18 value of meteoric water of -8 parts-per-thousand SMOW, is considered unreasonable. The exceptionally low delta-O-18 values are attributed mainly to reaction kinetics and the calcite inheriting its oxygen, two-thirds from atmospheric carbon dioxide and one third from the meteoric formation water (O'Neil and Barnes, 1971). A delta-O-18 value of atmospheric carbon dioxide of +41 parts-per-thousand value for the calcites and a delta-O-18 value of meteoric water of -8 parts-per-thousand SMOW, lead to a calculated delta-O-18 value for the calcites of +10 parts-per-thousand SMOW. The calcites analysed have a value of +16 parts-per-thousand and this may be due to partial re-equilibration towards a calculated value of +21 parts-per-thousand for calcite in equilibrium with the meteoric water at 20-degrees-C.