Microscopic Features of Ground Water-Induced Sulfate Attack in Highly Permeable Concretes

摘要

A number of unusual microscopic features have been documented in studies of the alteration of permeable concretes undergoing sulfate attack in semi-arid climates. The high W/C of the concretes examined here has permitted complete penetration of sulfate-bearing ground water through the concrete, and deposition of crystallized salts on evaporative surfaces, including the upper surfaces of slabs. The microstructural alteations observed in response to this penetration of sulfate-bearing ground water include the deposition of secondary ettringite and the accompanying local expansion and cracking, and the deposition of gypsum, but are much more extnesive and complicated than that. Calcium hydroxide is often entirely removed throughout the concrete, leaving open areas, particularly in the interfacial zones around aggregates. Various new compounds are deposited in these spaces, and elsewhere within the paste. In addition to ettringite and gypsum, the substances deposited often include monosulfate; brucite and magnesium silicate hydrate may be formed where the ground water is rich in magnesium; and thaumasite may be developed where carbonation accompanies sulfate attack. Partial decalcification of the C-S-H gel is common, and magnesium silicate hydrate may locally replace it. Even the large residual unhydrated cement grains, usually stable indefinitely in most concretes, are profoundly altered. The C_2S and C_3S in such grains may be completely dissolved and the spaces thus provided within the outlines of the cement grains may also be filled by secondary deposits. It is evident that sulfate attack produces profound internal changes in the paste microstrucutre, leading to the softening, exfoliation, and other external symptoms of sulfate attack reported in the literature.