For hundreds of years, two types of granite (Zarzalejo and Alpedrete) from the Madrid region, Spain, have been extensively used as building stones. Fresh specimens of both stone types have been sampled from their respective quarries and subjected to sodium sulphate salt crystallization test (SCT). The resulting physical and chemical weathering patterns have been characterized by polarized light optical and environmental scanning electron microscopy. Water absorption under vacuum conditions and mercury intrusion porosimetry techniques were used to determine the pre- and post-SCT porosity and pore size distribution. The following non-destructive techniques were performed to assess stone durability and decay: ultrasound velocity (US) and surface roughness determination (SR) of intra- and inter-granular quartz, feldspar and biotite minerals at the centre as well as at the corners and edges of specimen surfaces. Before the SCT, US values were lower and SR values higher in Zarzalejo (ZAR) than Alpedrete (ALP) granite. After SCT, the US values declined while SR rose in both types of granites, with greater average differences in ZAR than ALP for both parameters. Feldspar and biotite and their inter-granular contacts were found to be the weakest and therefore the most decay-prone areas of the stone.The initial SR parameters were generally higher and rose more steeply after SCT at the corners and around the edges of the specimens.While behaviour was found to be similar in the two types of granite, variations were greater in ZAR, the less durable and more decay-prone of the two. Surface roughness measurement of mineral grains in granite stones is a very useful, in situ, non-destructive technique for quantifying salt crystallization-mediated physical and chemical weathering. The resulting quantification of decay and of related durability provides insight into the future behaviour of this type of stone, commonly used in historic buildings.
Bibliographical noteFunding Information:
This study was funded by the Government of the Community of Madrid under the MATERNAS project (Durability and conservation of traditional natural materials in heritage architecture) ( MATERNAS CM 0505/MAT/0094 ) and the Spanish Ministry of Science and Innovation as part of the Consolider-Ingenio 2010 programme ( CSD2007-0058 ). Thank you to the JAE-Doc CSIC contract for supporting P. López-Arce to develop this work. The authors are grateful to Laura Tormo of the Natural Science Museum (CSIC) for providing the ESEM photographs and analyses. Special thanks go to Inmaculada Ruiz and Iván Serrano, IGE (Institute of Economic Geology) petrophysics laboratory technicians, for their work, suggestions, ideas, and support. We also thank Margaret Clark for the English review of the manuscript, and to Nick Schiavon for his comments and for having greatly improved the manuscript. We also appreciate the help of two anonymous referees for the reviewing process that also has improved the present research.
- Natural stone
- Salt crystallization tests
- Surface roughness indexes