Physico-chemical decay of carbonate stonesthe role of atmospheric aerosols and dissolution processes

Resumen

This PhD Thesis deals with the deterioration of carbonate stones caused by mineral dissolution processes and physico-chemical weathering processes triggered by their exposure under diverse polluted environments (i.e. urban and marine conditions). To this end, different carbonate stones used to replace similar historic geological materials in monuments in Andalusia, as well as employed in present civil buildings, have been characterized, namely Escúzar calcarenite, Sierra Elvira (dolomite-bearing) limestone and Olivillo travertine. The final goal of this investigation is to evaluate the Cultural Heritage suitability of these stones and their endurability as building materials. In this regard, special attention was paid in the stone pore network modification near surface due to different commercial stone surface finishings, since they play a key role in the uptake of atmospheric aerosols and a major impact on stone mechanical and aesthetic properties. First, a detailed compositional, petrographic and physico-mechanical characterization of the fresh stones was carried out, which was indispensable for comparing with changes produced on aged stones after different ageing tests (under natural and laboratory conditions). A comprehensive examination was performed by applying on both fresh and aged stone samples a multianalytical approach comprising: X-ray diffraction, X-ray fluorescence, optical microscopy, scanning and transmission electron microscopy, hydric tests, mercury intrusion porosimetry, gas adsorption, ultrasounds and chromatic measurements, transmission-Fourier Transform infrared spectroscopy and micro-Raman spectroscopy. To address the impact of atmospheric aerosols from polluted and marine environments in the studied carbonate stones, several investigations were carried out: i) the effect of indoor and outdoor atmospheric pollutants in the deterioration of the construction and decorative materials used in the San Jerónimo Monastery (Granada), ii) an ad hoc sea-salt ageing test conducted in a controlled-atmosphere chamber, and iii) a long-term ageing tests (ca. 2 years) where stones were exposed under the Granada urban air in different polluted sites to determine their black soiling. The degree of stone weathering was estimated by comparing changes at macro-, microand nanoscale of chemical and mineralogical composition, petrophysical characteristics and chromatic variations between freshly quarried and aged carbonate stones. As well the nature and morphology characteristics of precipitated salts, and the atmospheric particles deposited on aged stone surfaces and on particulate matter filters were studied combining complementary analytical techniques. Part of the Architectural Heritage of the city of Granada is built with a dolomite-bearing limestone (Sierra Elvira limestone). Chemical dissolution and crystallization of soluble salts in building carbonate stones play a critical role in their decay. However the physicochemical process behind it is not completely understood particularly in the case of the dolomite mineral. To improve our knowledge of the nanoscale processes governing dolomite-fluid interactions, a systematic in situ Atomic Force Microscopy (AFM) study was performed in this PhD Thesis. Dissolution rate of dolomite was estimated in the pH range 3- 10 since all previous nanoscale studies were conducted at fixed pH conditions and, to date, a systematic study of the dependence of dissolution rates on pH, such as in the case of macroscopic flow-through or batch reaction experiments have been not reported. The results obtained in this PhD Thesis are discussed in the context of the Cultural Heritage Science field, with the wish to help planning suitable conservation/restoration building stone policies, and preventive conservation strategies for the protection of our invaluable Cultural Heritage. Finally, general conclusions and future work perspectives are presented.