Irregular silica cementation in sandstones and its implication on the usability as building stone

Palaeocene sandstones outcropping in Western Spain have long been used for building purposes. Their use varies considerably: in some cities they have been the main stones used in the construction of buildings such as churches and cathedrals (Zamora), whereas in others, such as in the historic district of Salamanca, which has been a World Heritage site since 1988, they have been used in the lower part of the town's most striking monuments [two cathedrals (12th and 16th centuries), the Main Square (1755) and the House of Shells (15th century)] for protection against erosion and weathering, because they are more resistant than the Eocene sandstones used for the higher parts of the monuments. Although in some cases this solution has been successful, in others the results have been less satisfactory.Palaeocene sandstones have an important, although irregular, silica cement content. According to their uniaxial compression strength (UCS) they may be strong or very strong rocks, but under wet conditions some select samples display a significant decrease in their UCS. Furthermore, two types of opal cement can be found, one isotropic and the other birefringent, the former being the most efficient. Strength is even higher when isotropic opal cement is found together with alunite [KAl3(SO4)2(OH)6].