Petrophysical properties, composition and deterioration of the Calatorao biogenic stone: case of the sculptures masonry of the Valley of the Fallen (Madrid, Spain)

The huge sculptures placed outdoors in the Valley of the Fallen Memorial Park (El Escorial, Madrid) made with blocks of Black-Limestone from Calatorao-Zaragoza, Spain (BLCZ) and disposed on a concrete core exhibit weathering traces, flaking, saline efflorescence and falling fragments, currently represent a danger for visitors. Frost action is important in the Valley of the Fallen by the large number of freeze–thaw cycles produced during Sculptures‘live under a temperate Mediterranean climate with severe seasonality. The formation of fissures facilitates the water transport within the rock and the salt- and ice-induced deterioration. Temperate climates with frequent freezing and thawing cycles can be the most effective drivers of the visible physical weathering. In order to propose a suitable weathering model, collected black-limestones from sculptures and Calatorao quarries were analyzed by optical microscopy, environmental scanning electron microscopy with energy dispersive spectrometry (ESEM-EDS), inductively coupled mass spectrometry (ICP-MS) and X-ray diffraction. Mercury intrusion porosimetry (MIP), nitrogen absorption and helium pycnometry techniques were used for pore analyses of the BLCZ micro-blocks (10 × 10 × 10 cm) described in terms of pore size distribution, pore volume and specific surface area. The appreciable amount of organic matter was isolated by solvent extraction, acid treatment, flotation and perborate degradation followed by Gas Chromatography–Mass Spectrometry (GC–MS), Analytical Pyrolysis (Py-GC/MS), Fourier Transformed Infrared Spectroscopy and Raman techniques. Both weathered and fresh BLCZ samples contained more than 90 % calcite shells with circa 10 % of pyrite (fresh samples) or iron hydroxides (weathered samples), quartz grains, claystone and fossil organic matter consisting of a condensed matrix with polyalkyl chains and polycyclic methoxyl-lacking aromatic structures. The petrophysical analyses revealed volumes of pores, sized <0.025 μm obtained by N₂ adsorption, of 3.18 × 10⁻³ cm³ g⁻¹ while the measured porosity by MIP in the pore range from 0.005 to 200 μm was 3.30 × 10⁻³ cm³ g⁻¹. These data could be explained by the existence of clay minerals and organic matter in the pore system less than 50 nm of diameter. Concerning BLCZ deterioration it was found that the porous framework of BLCZ was filled with sulphates formed from artificial cement observed in the sculptures inside trough a testing hole and from its intrinsic pyrite. The results suggested that although biological processes were not major agents in rock deterioration, there was also weak compatibility between sculptures‘constituents, (limestone, concrete and oxidized iron clamps) which under, continental Mediterranean conditions, were continuously releasing weathering compounds accelerating disruption of the cut-stone sculptures.