Natural stone – a building material with history Constructions with natural stone – state of the art since thousands of years 2 Natural stone has been an important building material since the time of Ancient Egypt, a fact borne out, for example, by the pyramids and the sphinx of Giza made from limestone, obelisks from granite or the statues of Ramses from Nubian sandstone. A contemporary of Ramses wrote: „The whole of Egypt is nothing more than a gigantic stonemason‘s yard.“ In antiquity, apart from being used in the construction of temples, baths and stadia, natural stone was also used for sculptures and as a building material for civil and hydraulic engineering. One example of such architecture is the Colosseum in Rome, which is built of travertine. With the exception of the High Gothic period, from the Middle Ages until the modern era natural stone was primarily reserved for the construction of religious buildings, monasteries, castles, town halls, mansions, town houses, merchants‘ houses and town fortifications. In the Romanesque period, ashlar masonry was the principal method of construction. When it came to selecting materials, little distinction was drawn between stone for masonry and that for sculpture, mainly due to the limited availability of stone, which had to be obtained from local quarries. In the Gothic period, greater thought began to be put into the selection of materials, due to the greater demands placed on the natural stone by the large proportion of filigree and complex workpieces, leading to a preference for lighter stones, such as calcareous tufa. The Renaissance, Baroque and Rococo periods are characterized by a previously unknown profusion of decorative features (columns, statues, ornamental figures). There was a willingness to select the best stone for sculptures, but there was a sharp decline in the use of natural stone in masonry. At this time, buildings were generally rendered. In the second half of the 19th century, many facades were removed in the course of restoration and conversion work – the Romantic fashion was to expose the „natural“ (unfinished) facade. The masonry which had until then been protected for centuries by render was now exposed to weathering, which meant that many of these buildings have, due to the action of atmospheric pollutants, required constant renovation during the 20th century.
Weathering processes What you need to know about the weathering of natural stone From time immemorial, sun, wind, rain and frost have been natural factors which act on natural stone and, depending on the type of natural stone, lead to different weathering processes. The onset of industrialization brought with it additional harmful man-made factors. Weathering processes may fundamentally be divided into three major groups: physical, chemical and biological corrosion. The wide variation in the composition, structure and pore volume of natural stone is reflected in the variety of their weathering processes and profiles. Restoring damaged natural stone requires appropriate materials and methods in order as far as possible to retain and protect the original fabric of the stone and to reproduce its original appearance as well as possible. Options for restoring natural stone NATURAL STONE WEATHERING THE MOST IMPORTANT MEASURES IN OVERVIEW Patterns of damage... Dusting, crumbling, flaking, crusting, encrustation, cracking Stone consolidation Making good loss of strength by targeted addition of binder preservative ... as a consequence of different weathering processes ... Physical corrosion Chemical corrosion frost-thaw cycles, wind erosion, salt crystallization, salt hydration, moisture-related swelling/shrinkage conversion of sparingly soluble binders into soluble salts, usually associated with increase in volume (for example lime => gypsum) Stone reinstatement Stone replacement Reinstating missing areas or parts of stone with suitable restoration mortars. Replacement of entire stones or workpieces restorative restorative Biological corrosion Chemical attack by aggressive metabolites (acids) due to colonization by microorganisms (algae, fungi etc.) Water repellent finish Reducing absorption of water and pollutants as preventive corrosion protection preservative/ preventive ... initiated by Uptake of water and pollutants dissolved in water (salts and acidic gases => SO 2 , NO x ) Coating Recreating original appearance Protection from weathering Protection from water absorption restorative/ preventive Natural stone restoration encompasses a whole range of different measures and processes which may have a preservative, restorative and/or preventive action. The essential cause of weathering is most usually water or the harmful salts transported by water. The various weathering processes or types of corrosion ultimately result in a loss of strength, which may occur in surface zones or also in deeper zones. This results in patterns of damage such as dusting or crumbling, flaking, encrustation, crusting or cracking. 3