Ákos Török

Surface strength and mineralogy of weathering crusts on limestone buildings in Budapest

Abstract The oolitic limestone blocks on the buildings of Budapest show extensive crust formation, which is related to atmospheric pollution and continental climate. Crusts can be grouped according to their colour and morphology. Two types of white crusts and two types of black crusts have been studied in detail. Physical properties (Schmidt hammer rebound, Duroscope rebound, water absorption) and mineralogical composition (XRD, DTA–DTG) were measured and compared. Most crusts have higher Duroscope and Schmidt hammer rebound values than their host rock. The strength of the crust is related to the thickness, to the mineral composition, to the crust morphology and to the water absorption capacity. Calcite-rich, impermeable, thick white crusts have the highest strength, which is followed by gypsum-rich laminar black crusts alongside with thin gypsum-rich white crusts. These differences in strength and mineralogy are attributed to differences in rain/wind exposure and conditions conducive to pollution entrapment.

Observations on the factors influencing stability of building stones following contour scaling: A case study of oolitic limestones from Budapest, Hungary

Abstract Salt-induced decay of porous, granular building stones is often manifested initially by contour scaling, followed by rapid surface retreat through granular disaggregation and/or multiple flaking. It has been hypothesised that rapid retreat in polluted environments could be avoided if newly scaled surfaces can be stabilised by, for example, the re-growth of gypsum crusts. In this paper, we investigate the conditions required to ‘switch on and off’ rapid decay through a case study of oolitic limestones in a city that continues to experience high levels of atmospheric pollution. In a locally humid location adjacent to the River Danube, positive feedbacks are seen to occur following scaling that favour continuous rapid retreat linked to the synergistic salt weathering effects of halite and gypsum. In contrast, the Castle Hill overlooking the Danube is less prone to winter and autumn fogs and there is evidence that stone retreat here may be more sporadic and that episodes of contour scaling can be interspersed with periods of stability marked by the re-growth of gypsum crusts. On individual stone blocks it is therefore possible to see fragmentary evidence of several crusts. The negative feedbacks that operate to stabilise newly scaled surfaces are thought to be associated with low chloride concentrations, and a relatively dry microclimate at the block surface that inhibits the frequency and depth of surface wetting. This is linked in turn to the continentality of the meteorological climate of Hungary. It is possible, however, that particular significance attaches to the rapid deposition and effective retention of particulate pollutants rich in water-soluble sulphate and coincident biological colonisation of scaled surfaces that aids the adherence of precipitated dust.

Oolitic limestone in a polluted atmospheric environment in Budapest: weathering phenomena and alterations in physical properties

Abstract In Budapest damage due to atmospheric pollution on many public buildings is severe. Black encrustations, white crusts and other decay features of a soft oolitic limestone have been studied in detail by using field measurements and laboratory analyses. Limestone weathering was assessed by description of weathering forms, by on-site petrophysical tests (Duroscope, Schmidt hammer, water absorption) and by laboratory mineralogical assessment and thermoanalysis (X-ray diffraction, Derivatograph). There is a clear correlation between the organic carbon content in stone and location of the site, particularly in the polluted city centre. Gypsum, which is not an indigenous mineral in the limestone, can contribute up to 70% of the crust composition and indicates the importance of air-derived SO2. This mineralogical change in stone composition leads to changes in physical properties, by strengthening laminar black crusts and white case hardened crusts and weakening the host rock.

How river rocks round: resolving the shape-size paradox.

River-bed sediments display two universal downstream trends: fining, in which particle size decreases; and rounding, where pebble shapes evolve toward ellipsoids. Rounding is known to result from transport-induced abrasion; however many researchers argue that the contribution of abrasion to downstream fining is negligible. This presents a paradox: downstream shape change indicates substantial abrasion, while size change apparently rules it out. Here we use laboratory experiments and numerical modeling to show quantitatively that pebble abrasion is a curvature-driven flow problem. As a consequence, abrasion occurs in two well-separated phases: first, pebble edges rapidly round without any change in axis dimensions until the shape becomes entirely convex; and second, axis dimensions are then slowly reduced while the particle remains convex. Explicit study of pebble shape evolution helps resolve the shape-size paradox by reconciling discrepancies between laboratory and field studies, and enhances our ability to decipher the transport history of a river rock.

Controls on development of Mid-Triassic ramps: examples from southern Hungary

Abstract During Mid-Triassic time the palaeogeographical-tectonic setting of the northern Tethyan passive margin and epicontinental basins favoured the ramp morphology, as broad flat areas were available for flooding and establishment of carbonate sedimentation. Throughout this time, in what is now southern Hungary, homoclinal ramps developed, which also exhibit some evidence of partial distal steepening. The nearly 1 km thick Anisian-Ladinian Muschelkalk carbonates are grouped into six depositional units. In each ramp unit facies types were identified representing coastal sabkhas, inner ramp peritidal facies, backshoal lagoons, oolite shoals, storm to fair-weather influenced mid-ramps, proximal to distal shell beds and low-energy outer ramps. Mid- and inner ramp carbonates make up the majority of the successions. The deepest outer ramp facies correspond to the peak transgression of a second-order Triassic sea-level rise. In the Hungarian Muschelkalk three major transgressive-regressive sequences were identified. In Germany, Poland and Spain much thinner and slightly coarser Muschelkalk carbonate successions were formed in the same Mid-Triassic interval. The major sediment redistribution processes were related to storms that generated coquinas, and mud flow deposits. Slow, early diagenesis and the predominance of unstable muddy substrate led to the formation of small-scale slumps, slides and plastic deformation. The fauna reflects the prevalence of carbonate mud and temporarily high-stress conditions. A non ‘cool water’ origin of this ramp is inferred from the presence of evaporites, early dolomization of inner ramp carbonates, the biota and the palaeogeographical setting. The major controlling factors of Mid-Triassic ramps were the gentle antecedent morphology, the subtropical climate, the high production of carbonate mud and the lack of reef-building organisms, as well as the passive margin setting, which resulted in a moderate and relatively uniform subsidence rate over a wide area.

The influence of lithology and pore-size distribution on the durability of acid volcanic tuffs, Hungary

Abstract Eight different types of acid tuffs of the Eger Castle (Hungary) and two tuffs from nearby quarries have been studied in detail. Mapping of wall sections reveals that tuffs show weathering forms that are similar to common sedimentary rocks, such as limestones or sandstones. Different lithologies display various weathering features. On pumice-rich tuff ashlars relief due to selective weathering, weathering crusts, multiple flakes and scales occur, while crumbling is common on layered flow tuffs. Conversely, cemented tuff types do not show deep weathering. Pore-size distribution rather than effective porosity controls the weathering susceptibility of tuffs. Frequent larger micropores are the main causes of freeze-thaw-related weathering. Besides clays, newly formed gypsum and calcite are the weathering-related index minerals. Schmidt hammer rebound values mark the weathering process when quarry stones and ashlars of historic walls are compared.

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

Abstract 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 fine- and 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.

Natural Stone Resources for Historical Monuments

Natural stone is considered to be a versatile, durable and aesthetically pleasing building material. From the beginning of civilization, important structures and monuments have been built from, or based on, natural stone. Until the end of the nineteenth century, the use of local stone resources was mostly in balance with the local environment. Strict environmental legislation has resulted in the closing of many long-standing quarries in industrialized countries, which has led to a shortage of traditional stone varieties. This has caused problems for restoration practice. Cheap, imported stone from less industrialized countries has become more widely available in recent years. Some of the issues related to built stone conservation and restoration covered by this volume are: the establishment of inventories of possible replacement stones; understanding the decay mechanism and use of preventive conservation methods for slowing down decay processes; evaluation of the properties of natural stone; and assessing the risks of using replacement stones of different qualities.

In Situ Methods of Testing Stone Monuments and the Application of Nondestructive Physical Properties Testing in Masonry Diagnosis

Rapid in situ diagnosis of monuments is a key issue in the preservation of heritage sites. Stone masonry diagnosis is aimed to analyse the condition of a stone structure or building in order to understand the causes of deterioration and to find an adequate treatment and optimal conservation method or management plan for a heritage site or building. These measurements can provide valuable data for maintenance, restoration or they could form the base line of “preventive conservation”. Due to the rapid development of measuring systems and the application of new methods and techniques from other fields (e.g., from the medical sciences) to masonry diagnosis, a wide range of techniques are available these days. The new methods such as portable spectroscopy units (Brunetti 2008 ), portable XRF (Thornbush and Viles 2006 ), combined XRD/XRF (Chiari 2008 ), X-ray tomography (Cnudde et al. 2009 ) or Light Detection And Ranging scanners ( LiDAR, Meneely et al. 2008 ) and many other methods can revolutionize monument diagnosis in the future. Nevertheless, the widespread application of some of these new techniques is now hampered by their high costs and therefore in daily practice simpler and cheaper tools and methods are applied in masonry diagnosis. This chapter will describe both these new, expensive techniques and the older and generally cheaper equipments and methods of stone masonry diagnosis, focusing on testing physical parameters in particular.

Physical changes of porous Hungarian limestones related to silicic acid ester consolidant treatments

Abstract Porous limestones are widely used in the monuments of Hungary, and are often treated using stone consolidants on site during restoration works. Two types of porous Miocene limestones from Sóskút were treated with dilute and concentrated silicic acid ester under laboratory conditions. The aim of the experiments was to assess the performance of the consolidants of different concentrations on fine- and medium-grained limestones, and to detect physical changes caused by consolidation. The pore-size distribution and fabric of the limestones were also different with initial porosities of 37 and 23%, respectively. In the open pores only a few per cent (3.2–5.2%) of silica gel was precipitated even under vacuum saturation. The loss of porosity was higher when concentrated consolidant was used. The decrease in porosity was found not to be proportional to the changes in strength. Indeed, the dilute consolidant caused a greater increase in strength than the concentrated one. The increase of tensile strength was higher for the less porous medium-grained limestone than for the fine-grained one. The differences in strength between non-consolidated and treated specimens were also detectable using ultrasonic pulse velocities. The tests have shown that the efficacy of silica acid ester treatments in terms of strength is less influenced by the concentration of the consolidant, but rather its ability to reach the micropores of porous limestones.