Incompatible building materials within the stereotomic Avalos sculptures of the Valley of Fallen (Madrid, Spain)

Abstract

The examination of the recently open pit in top of head of the La Piety sculpture of Avalos confirms that the external outer of Black Limestone Calatorao (BLC) composite is made by slabs 20 cm thick joined with gypsum mortars without aggregates. The analyses of these mortars samples performed by optical microscopy (OM), X-ray diffraction (XRD) and Environmental Scanning Electron Microscopy with Energies Dispersive Spectrometry (ESEM-EDS) techniques show hydrous admixtures composed by portlandite, mirabilite, ettringite, thaumasite and gypsum, all of them are well-know dangerous phases for a suitable preservation of the architectural heritage. The huge stereotomic structures have open access to the raining waters reaching the internal core of the sculptures made by concrete with weathered alkali-feldspar aggregates providing sodium to the surrounding sulfated environment facilitating the formation of hydrous calcium and sodium sulfates such as ettringite and mirabilite which reduces the inter-grains adherence.