Differences in texture, physical properties and microbiology of weathering crust and host rock: a case study of the porous limestone of Budapest (Hungary)

Ashlars of the Parliament building and Citadella fortress made of three porous Miocene limestones, a fine-grained limestone, a medium-grained oolitic limestone and a coarse-grained bioclastic limestone, were studied and compared with quarry blocks of the same lithologies. The commonest weathering forms are white (thin and thick) and black (laminar and framboidal) crusts. To assess the processes of crust formation and detachment, descriptions of lithologies and associated weathering features were combined with micro-drilling, pore-size distribution and ultrasonic pulse velocity tests. Microbiological and textural analyses were also performed. The micro-drilling resistance measurements and ultrasonic pulse velocities clearly document the presence of crusts and the degradation of underlying fineand medium-grained limestones. A textural change, with calcite recrystallization, is also marked by pore occlusion and reduction of microporosity in the crust zone. Crust detachment is initiated by the opening up of microfissures that develop below the cemented crust zones. Fine-grained limestone appears to be less durable than the coarse-grained variety and more prone to rapid crust formation and detachment. Ashlars from where the crusts were removed have lower micro-drilling resistance compared to quarry stones. Microbiological activity appears to play an insignificant role in crust formation and removal. Indeed, the combined effect of air pollution and related gypsum crystallization and more probably freeze–thaw weathering activity lead to crust detachment with rates strongly controlled by the texture and porosity of the limestone substrate. Weathering crusts found on limestone exposed to air pollution are probably some of the most thoroughly studied weathering phenomena (Kieslinger 1949; Amoroso & Fassina 1983). Previous work has described dark coloured and white weathering crusts, which are further divided according to their morphology and thickness (Smith et al. 1992; Camuffo 1995; Fitzner et al. 1995; Antill & Viles 1999; Maravelaki-Kalaitzaki & Biscontin 1999). Most of these crusts are enriched in gypsum. The influence of environmental conditions on gypsum crust formation has been thoroughly studied in the field (Amoroso & Fassina 1983; Zappia et al. 1998; Fassina et al. 2002; Smith et al. 2003; Bonazza et al. 2004) and under laboratory conditions (Rodriguez-Navarro & Sebastian 1996; Ausset et al. 1999; Primerano et al. 2000; Cultrone et al. 2004). Differences in weathering of various limestones exposed to the same pollution regime have been studied using small test blocks over periods of time (Smith 1996). However, less information is available regarding the physical changes that are triggered by pollution fluxes (Winkler 1966, 1970) or weathering (Bell 1993a), and even less information is available on the physical properties of crusts and host rocks. The sparse examples report weatheringrelated changes in physical properties of various limestones (Christaras 1991; Torok 2002a, 2003; Torok et al. 2004) and marbles (Christaras 1996). The loss in strength caused by weathering was also reported for granites (Irfan & Dearman 1978; Kahraman 2001) and for rhyolite tuffs (Topal & Sozmen 2003; Torok et al. 2005). Research generally focuses on the description of processes and decay products for one type of stone, but rarely describes the variations in physical properties and crust formation on various limestone types (Torok 2004). The aim of this paper is to analyse weathering crust formation on porous limestone and to describe the differences in physical properties of crusts From: PŘIKRYL, R. & SMITH, B. J. (eds) Building Stone Decay: From Diagnosis to Conservation. Geological Society, London, Special Publications, 271, 261–276. 0305-8719/07/$15.00 # The Geological Society of London 2007. developed on fine-, mediumand coarse-grained limestones from one stratigraphic level. The approach taken was to compare ashlars that have experienced several tens or even hundreds of years of exposure on building facades with similar or the same type of porous stone that was collected fresh from a quarry. Two monuments were studied in Budapest; the Parliament building and the Citadella fortress. Both are located in the city centre and thus experience high pollution fluxes (Torok 2002a). After the description of their lithologies and identification of crust types, microscopic textural analyses, micro-drilling resistance tests and, on core samples, porosimetric and ultrasonic pulse velocity analyses were performed to identify the nature of physical and textural changes with depth below the surface. Microbiological analyses of samples were also performed to assess the role of organisms in weathering crust formation and crust detachment. Previous work (Warscheid & Braams 2000; Krumbein 2003) indicates that organisms may be involved in crust formation, patination or roughening of stone surfaces, as well as inducing material loss by sanding, chipping or scaling. However, other work has shown that organisms may also accompany weathering processes without producing deteriorative effects (Hoppert et al. 2005), or form a protective surface on stone preventing or slowing down other weathering processes (Kurtz & Netoff 2001). The combination of these techniques was used to understand the mechanism of crust formation and to identify circumstances that contribute to crust detachment. In addition, differences in texture and physical properties of both crusts and their host rocks were investigated with specific emphasis on the role of microscale porosity differences in crust removal. Methods and environment