1990 Volume 30 Issue 2 Pages 91-100
Concrete sewer pipe was heavily corroded by sulfate which was produced by sulfur oxidizing bacteria in the Ohmuta region of Japan. The corroded materials were studied mineralogically to gain a better understanding of the corrosion mechanism. The formation of jarosite KFe3 (SO4) 2 (OH) 6, as a result of reaction between sulfate and hydration products of portland cement, were found in the most corroded part of concrete sewer pipe. The sulfate-generated corrosion is grouped into several zones on the base of differences in the mineral formation of secondary products:
Zone A & B; Jarosite formation zone, Zone C; Fe-rich zone,
Zone D; gypsum formation zone, Zone E; Fe-rich zone,
Zone F; Cracking zone, Zone G; Fresh concrete zone.
Zones A-E in the corroded parts include expansion, loss strength and stiffness and disintegration, whereas Zones E and F show cracking and brown rim stained with iron oxides. In Zone G, ettringite crystals are formed under conditions of high pH. Since hydrated cements containing considerable amount of large ettringite crystals show high strength but no expansion.
In order to evaluate the relative effects of corrosion of concrete, the rates of K-feldspar dissolution have been analized by EDAX-step scanning. The dissolution process of K-feldspar was observed that Al and K contents were rapidly decreased near rim, within 2μm region, whereas Fe content was increased.
The results of these analyses indicate that the effect of K component from K-feldspar dissolution is important factor to form jarosite at pH of 3.5-4.0. The dissolution mechanism of K-feldspar and the addition of Fe component are changed significantly due to the corrosion of concretes.
