Well-crystallized and amorphous calcite in concrete slabs exposed to long-term atmospheric carbonation

Abstract

It is of immense scientific and engineering interest to know the crystalline nature (amorphous and well crystallized) of calcite forming in concrete structural elements subjected to long-term atmospheric carbonation attack. The present study re-examines the crystalline nature of calcite, and also the mechanism controlling its formation in concrete slabs carbonated by exposure to an internal drying environment on a long-term basis. The influence of chloride penetration on the crystalline nature of calcite is also investigated. The crystalline nature of calcite forming in the above slabs was examined at the end of the exposure period at three different locations: at the cast face of the slabs, below the cast face, and immediately below a surface coating used on some of the slabs. Techniques such as differential thermal analysis, X ray diffraction and Volhard's method for the acid-soluble chloride content were used. The results showed that well-crystallized and amorphous calcite form in concrete slabs subjected to a long-term internal drying environment similar to the interior of a building. The concrete close to the cast face of the slabs, on long-term carbonation, showed the presence of a significant proportion of well-crystallized calcite together with well-crystallized vaterite in trace quantities. On the other hand, only amorphous calcite was found to be present on long-term atmospheric carbonation of concrete below the cast face of the slab. In contrast, all the above three carbonate phases, that is, well-crystallized calcite, amorphous calcite together with well-crystallized vaterite, were present in the concrete immediately below the surface coating on exposure to similar conditions. The formation of well-crystallized and amorphous calcite in concrete slabs subjected to long-term carbonation is shown to be related to the flux of atmospheric carbon dioxide through the concrete cover; well-crystallized calcite forms in concrete with a high carbon dioxide flux while amorphous calcite forms in concrete with a restricted carbon dioxide flux. The acid-soluble chlorides present in the concrete cover appear to have no influence on the mechanism leading to the formation of both well-crystallized and amorphous calcite.