Artificial weathering of Spanish granites subjected to salt crystallization tests: Surface roughness quantification

Abstract

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.