Petrophysical properties modifications of Strasbourg's Cathedral sandstone by black crusts

Abstract

A glossy-black thin crust adheres to the stones of sandstone monuments in Alsace, particularly in Strasbourg's Cathedral. The crust makes the stone impermeable and hydrophobic. Under these, crusts, stones are coherent and don't show visible signs of weathering. This chapter focuses on the petrophysical parameter variations on sandstone covered or not by black crust and the influence of these variations in weathering developments. Two types of sandstone, covered with black crusts, were taken for works on the Cathedral of Strasbourg. Yellow sandstone with oxide and hydroxide brown aureoles and 19% water porosity, and pink sandstone with 20% porosity and same granulometry were taken. The petrophysical examinations were conducted using microscope, chemical analysis, X-ray, and scanning electron microscope. In each case, these kinds of sandstone have normal petrophysical parameter values for Buntsandstein sandstone. These same types of sandstone show modifications of some petrophysical values, when covered with black crust. The variations include: weaker mercury porosity and pore access distribution is more widely spread without threshold; slower and weaker capillary absorption with a relative increase of the dynamic trapped porosity; quicker drying kinetics; and decreases water vapor adsorption kinetics through crusts. These modifications result from a weak moistening of the crust and the sandstone beneath it. According to the study of above variations, it is observed that thin crust causes slab fracture. Therefore, the crust should be kept slightly because their hydrophobia limits their alimentation and accretion.