Thaumasite in Deteriorated Concretes in the Presence of Sulphates


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Title: Thaumasite in Deteriorated Concretes in the Presence of Sulphates

Author(s): M. Berra and G. Baronio

Publication: Special Publication

Volume: 100


Appears on pages(s): 2073-2090

Keywords: calcite; calcium sulfates; carbon dioxide; chemical analysis; concretes; deterioration; humidity; low temperature; pyrite; thaumasite; sulfate attack; tunnel linings; Materials Research

Date: 4/1/1987

Some different degrees of alteration have been found in a concrete tunnel lining affected by dissolved calcium sulfate in water due to the oxidation and solubility of the pyrites contained in the rock. In some cases the damage has been so extensive that the concrete has changed into a plastic mass. Measurements taken inside the tunnel over many years showed a range of temperature from 4 to 6 C. The chemical analysis of the concrete, the diffractometric analysis, and the scanning electron microscope analysis performed on some alteration products all clearly showed the presence of thaumasite and calcite, while no ettringite was found. The recorded thermal conditions, the presence of silica due to feldspar alteration, and the free carbon dioxide in the water lead to the conclusion that the ettringite has been transformed completely into thaumasite. To understand this transformation, some laboratory tests were carried out on mortar immersed in a water mixture containing calcium sulfate, calcium carbonate, calcium hydroxide, and amorphous silica in an environment rich in carbon dioxide, at 5 and 20 C, respectively.