Wanja Wedekind

Weathering of volcanic tuff rocks caused by moisture expansion

Moisture expansion in natural building stones is considered one of the most important factors affecting their weathering and deterioration. The processes that may be responsible for the expansion under determinate relative humidity (hygric dilatation) and water-saturated conditions (hydric dilatation) are generally attributed to the presence of swellable clay minerals. In contrast to this assumption, our investigations show that moisture expansion also takes place in volcanic tuff building stones almost free from clay minerals. To provide a deeper understanding of the processes, swelling and deterioration were performed on 14 volcanic tuffs used as important building stones of different ages, compositions and weathering stages from Mexico, Germany and Hungary. The investigations undertaken include extensive chemical, petrophysical and fabric analyses. The samples show a wide range of effective porosity, microporosity, capillary water absorption, moisture expansion, and CEC values. High moisture expansion does not seem to depend on clay mineral content alone. We also observed that there is no significant effect on dilatation if clay minerals are present but only form a thin coat on the outer shell of bigger pores. Moreover, we identified a correlation between microporosity, average pore radius and moisture expansion. The investigations highlight the fact that moisture expansion cannot only be attributed to swellable clay minerals, and suggest that the presence and accumulation of micropores and their average radius and distribution play an important role for non-clay associated swelling intensity, which can most probably be attributed to the disjoining pressure.

Moisture expansion associated to secondary porosity: an example of the Loseros Tuff of Guanajuato, Mexico

The old mining city of Guanajuato in middle Mexico preserves one of the most important historical legacies in colonial buildings, the UNESCO declared the city World Heritage Site in 1988. Practically all the colonial constructions were built with natural stones from the neighbourhood, of which stands a greenish to reddish vulcanite, called Loseros Tuff. Although the Loseros Tuff is widely used in historical buildings in the city. It shows significant deterioration and weathering effects, principally in the parts where the tuff shows a coarse grain size. The petrographic, petrophysical, mineralogical and geochemical properties of the Loseros Tuff were analysed in order to determine the causes, effects, behaviour and response to deterioration of this volcanic rock. The results of the investigations suggest that in addition to the parameters like the grain size and the porosity properties, the pore radii distribution is decisive for the effectiveness of porosity and the water transport into the rock. It is recognized that once the liquid water invades the rock the dissolution of the matrix occurs, which is accompanied by a sudden moisture expansion favoured by the newly formed secondary porosity and the high content of expandable clay minerals.

Cryptogam covers on sepulchral monuments and re-colonization of a marble surface after cleaning

Re-colonization of freshly cleaned surfaces by aeroterrestrial microbial communities is up to now poorly understood. Here, we present a comparative study addressing the composition of algal and fungal communities on a marble sculpture, based upon the analysis of 18S rRNA gene clone libraries from environmental samples. The samples were taken from a blackish and greenish biofilm cover before surface cleaning and 1 year after cleaning treatment, when traces of re-colonization became visible to the naked eye. The composition of the fungal community indicated clear differences between the old grown biofilm and the treated surface. While the former was dominated by the ascomycetes Rhinocladiella, Glyphium and Capnodiales, the black yeast Sarcinomyces was clearly dominant 1 year after cleaning, but could not be retrieved from the old grown black biofilm. The green algal community was dominated by different phylotypes of the lichen algae Trebouxia, as well as the cosmopolitan green algae Apatococcus and Stichococcus. No essential differences in the green algal community before and after cleaning could be observed.

Salt weathering in granitoids: an overview on the controlling factors

Historically granite is one of the most applied building materials worldwide. Building stones should accomplish several properties required by different testing materials standards. Salt weathering affects the aesthetical properties of the stones and eventually diminish their durability. The use of weathered granites has increased in the last several decades, but their behavior under adverse environmental conditions requires continued investigation. The use of salt for the prevention of ice formation in colder climates can have harmful consequences on high-porosity stones. Twenty-eight different stones, mostly granitoids, all of them often used as dimensional building stones, were subjected to the salt bursting test. The porosity and the pore network are important parameters in salt weathering; therefore, the pore radii distribution and capillary water uptake were measured. The capillary pores and related porosity are the main factors controlling the behavior of the studied stones under salt action. However, the pore radii size and distribution also plays an important role. In some cases, the salt action is only visible after a high number of test cycles, thus making the actual salt test standards unrealistic.

Deterioration of volcanic tuff rocks from Armenia: constraints on salt crystallization and hydric expansion

Volcanic tuffs are widely used in the Armenian architecture and represent building stones of the country’s most precious cultural heritage sites. For the very first time, extensive investigations regarding their chemical and mineralogical composition as well as the influence of petrophysical properties on their weathering behavior were realized. Different groups of tuff rocks could be identified, which differ greatly in their chroma, texture, their chemical and mineralogical composition, as well as their weathering behavior. At 30% porosity and 25% micropores, the tuff rocks show a sharp limit of changing water transport and retention behavior. Swellable clay minerals, the amount of micropores and the hygroscopic sorption value show a direct relation with the hydric expansion and proved to be reliable parameters for its estimation. Zeolite minerals proved to drastically increase the sorption values of the tuff rocks and are discussed as a cause for potential disjoining pressure. Furthermore, the application of the salt bursting test by European standard on these glass-rich volcanic tuff rocks is questioned, due to considerable different mineralogy and fabric in comparison to classic ash-rich tuffs.