Embryonic sulphated black crusts on carbonate rocks in atmospheric simulation chamber and in the field: role of carbonaceous fly-ash

Experimental conditions characteristic of the urban pollution in many Europ ean cities over the last decades were reproduced in a simulation chamber in which samples of limestone were exposed for a period of 12 months, both na ked or sprinkled with carbonaceous fly-ash. In agreement with the findings reported in the literature the fly-ash emitted by heavy fuel combustion are highly reactive, undergoing chemical and morphological transformations whi ch give rise to crystals growth, especially gypsum. The development of gyps um crystals was observed overall in close proximity of fly-ash anchoring th em to the limestone surface. Samples of the same limestone exposed in the f ield in a polluted environment for the same period of time led to similar r esults. The preliminary mechanisms leading to the genesis of sulphated blac k crusts in polluted environments were thus highlighted. Because of their r oughness the embryonic black crusts increase the development of the crust b y trapping new particles. This trapping is also facilitated by the wetness of the stone surface leading to the development of hydrated mineral (gypsum ) in the water meniscus between fly-ash and stone surface. (C) 1999 Elsevie r Science Ltd. All rights reserved.