In the United Kingdom a number of cases of chemical attack on buried concre
te bridge foundations have occurred, involving the sulfate-bearing mineral
thaumasite. All of the affected concretes were in contact with a large volu
me of clay backfill. It is postulated that oxidation of pyrites within the
backfill led to the formation of sulfuric acid, reducing the pH of the grou
ndwater and resulting in attack on the foundations and in reactions that ra
ised the groundwater sulfate level. It is shown that the observations made
on thin sections taken from the affected bridge foundations are in accordan
ce with sulfuric acid attack being the probable primary cause of deteriorat
ion. (C) 2000 Elsevier Science Ltd. All rights reserved.