Weathering of building stones of the Medieval Sasov Castle in Slovakia: Indication and impact of the mineral alteration

The ruin of the medieval Sasov castle, a cultural heritage object in the Middle Slovakia, suffers from a strong deterioration. Besides the weathered binder, a reasonable part of the damages is caused by the intensive weathering of the building stone. The walls are builtmainly of Neogene andesite, which is common in this area of the volcanic Stiavnicke Vrchy Mts. The sound rock is of dark grey colour, but most of the building stones have yellowishbrown coatings of iron oxyhydroxides on the surface and in the fissures. Highly macro-porous stones are most affected by the alteration. Weathering of the andesite was studied within the project VEGA 1/0413/09 of the Ministry of Education of the Slovak Republic.
   The X-ray diffractometry (XRD) of the powdered weathered building stone showed that the mineral assemblage is dominated by the plagioclase andesine (63 wt%) and the main alteration product smectite (up to 22 wt%), the rest are amorphous phases/volcanic glass (13 wt%), augite (1 wt%), magnetite (1 wt%) and traces of mica. The building stone material was compared with the rock material from an old local quarry, which was assumed to be the source of the building material for the castle walls. The study with polarized light microscopy (PM) of thin sections revealed the glomeroporphyritic character of this rock, i.e. phenocrysts of the same type (here plagioclase) are partly grouping into distinct clusters. There are three types of plagioclase: 1. sound plagioclase with regular shape and typical crystal twinning, 2. phenocrysts exhibiting growth zoning, and 3. plagioclase disintegrated due to magmatic corrosion. The corrosion creates alteration rims around the plagioclase grains or intrudes the whole mineral. A secondary mineralisation can be seen in some fine cracks cutting the phenocrysts, as well. The alteration of mafic minerals (augite, magnetite, mica) resulted in opaque phases. The rock matrix is built of volcanic glass, fine-grained plagioclase and some not identified brown phases. These are probably products of a post-magmatic alteration, i.e. weathering and/or post-volcanic hydrothermal alteration processes, very common in Neovolcanites.
   The results from XRD, PM and tests on physical properties confirmed a very good match of the building stone material with the most altered parts of the rock mass in the local quarry. Therefore, a comparison with the sound rock from the deeper parts of the quarry was used for the illustration of the alteration impact on the rock properties. The sound rock consists of andesine (≈ 62 wt%), the rest are amorphous phases (27 wt%), augite (8 wt%) and magnetite (3 wt%), no smectite is present. It means that amorphous phases are the most unstable components of the rock. With the alteration, smectite appears and its content is increasing (up to 22 wt%), while amorphous phases, augite, and magnetite are decreasing. A considerable part of the macro-pores is the result of selective leaching of weathering products. Total porosity increased from 2.4 % to 21.2 %, the effective porosity from 2.2 % to 15.0 % when compared with the sound rock. The uniaxial strength (tested by the point load test) dropped from 270 MPa (sound rock) to 51 MPa in the weathered, but still coherent macroporous stone. However, the most altered building stones are almost white and disintegrate into sand. Cyclic volume changes due to smectite swelling and water freezing in the effective pores probably weaken the structure and enhance the deterioration. Both processes are supported by the high water uptake due to the high smectite content, reflected also in the results of the Enslin-Neff tests. Therefore, ruins should be prevented from the infiltrating water precipitations (rain, snow) by roofing of the wall crowns. Missing or deteriorated building stones could be replaced by material taken from the deeper parts of the local andesite quarry.