Carlos Figueiredo

Pore structure and durability of Portuguese limestones: a case study

Abstract Exposed stone surfaces containing complex systems of pores, fractures and grain boundaries provide the surfaces where chemical, physical and biological deterioration processes take place. The pore space represents the preferred area for physical, chemical and biological weathering processes. It plays a significant role in the behaviour of porous materials. A full understanding of pore-channel network morphology, size and connectivity is important in stone decay assessment and conservation works. A contribution to the understanding of the role played by the pore system in controlling fluid-related properties and resistance to salt crystallization of limestones is presented. Optical microscopy, scanning electron microscopy (SEM) and mercury injection porosimetry (MIP) were used to characterize the pore structure of two Portuguese dimension stones (‘Semi-rijo’ and ‘Moca-Creme’) widely used for pavements and the cladding of buildings. Fluid migration physical tests (open and free porosity, capillary imbibition, and Hirschwald coefficient) were also performed, according to European (EN 1925:1999; EN 1936:1999) and French (N FB 10-504:1973) Standards. The resistance to salt crystallization was determined using European Standard EN 12370:1999. An integrated analysis facilitated comparison between durability results with stone pore network characteristics, fluid transport properties and petrographical features, suggesting the influence of available porosity and bedding.

Texture Analysis of Grey-Tone Images by Mathematical Morphology: A Nondestructive Tool for the Quantitative Assessment of Stone Decay

Both a low-cost and easily handled nondestructive methodology and its validation criterion are proposed. The methodology is based on image analysis by mathematical morphology for the assessment of decayed stone surfaces in historic limestone buildings. It is adapted to follow the evolution, at macroscopic time and space scales, of stone materials used in art pieces and monument building stones. This methodology is applied to the quantitative analysis of textures of static grey-tone CCD video camera images representative of flat stone structures that cannot be handled. These structures line the walls inside an important church, Basílica da Estrela, built in the 18th century and located in the city of Lisbon. Detailed visual/tactile observation shows that these flat vertical structures are more or less damaged depending on their position inside the church. The damage is possibly associated with different environment conditions both from the atmosphere inside the church and from contact with the walls of the church. The presence of these pathologies breaks down the order and organization of sound rock textures introducing changes in the topographical and optical characteristics of the texture of the surfaces. This new methodology is based on the granulometry and covariance analysis of grey-tone images corresponding to the structures studied. The validation criterion allows the results of the proposed methodology to be compared to the results of a previous qualitative study made by experts in the field using visual assessment and monument mapping as a valid methodology to assess the degree of decay. This procedure is based on optical and topographical characteristics and type of decay of the stone surface, and the quantity of material lost from the surface of the panels during the year. As discussed in this paper, in the context of cultural heritage, the proposed methodology was validated and its results can be considered an improvement on and complementary to expert visual analysis and also to other nondestructive techniques.

Contribution to the technological characterization of two widely used Portuguese dimension stones: the ‘Semi-rijo’ and ‘Moca Creme’ stones

Abstract Stone weathering and durability are two major concerns widely recognized within construction, cultural heritage and monument stone decay assessment and conservation works. This paper aims to complement the technological data on the two commercial varieties of Portuguese dimension stones (‘Semi-rijo’ and ‘Moca Creme’), widely used in pavement and cladding inside and outside buildings. New data on the pore structures (fluid transport–related properties) and the durability (salt crystallization tests) of these limestones are presented. The pore structure was studied by a combined application of optical microscopy, scanning electron microscopy and mercury injection porosimetry. Fluid migration physical tests (open and free porosity, capillary imbibition and Hirschwald coefficient) were also performed, according to Portuguese and French standards. The resistance to salt crystallization was determined using the Portuguese standard NP EN 12370. An integrated analysis of all data has allowed a comparison of the results of the durability tests with characteristics of the pore network and properties related to fluid transport as well as petrographical features of the stones. The open porosity and freely-interconnected pores accessible to water after 48 hours (N48) seem to be the main features controlling their durability.

The weathering and weatherability of Basílica da Estrela stones, Lisbon, Portugal

Abstract This paper presents a study of stone decay on the Basílica da Estrela, the most famous 18th century monument in the city of Lisbon, Portugal. It was built with Jurassic and Cretaceous limestones from the surroundings of Lisbon. Different approaches were used to establish the typology, causes and processes of the major weathering forms. Limestone samples from ancient quarries, salt efflorescences and disintegrated stone material from the interior of the church were characterized by chemical, mineralogical and petrographical analyses. Limestone physical properties related to fluid percolation were also determined. Detailed surveys of stone decay phenomena were carried out on the monument. Textures of grey-level images representative of the weathering forms were analysed by image analysis through covariance and granulometry operators. An HIDSPEC computational hydrogeochemical model, phase and graphical diagrams, and multivariate statistical analysis were used for water-rock interaction studies. Physical weathering forms prevail inside the church. The yellow Cretaceous limestone is the most deteriorated stone. This observation compares well with its intrinsic properties. The weathering is determined by the stone structures, such as stilolytes and fossils, and architectural features (geometry and surface finish). Soluble salts such as trona and thenardite were only found in a very small area inside the church.

Susceptibility of limestone petrographic features to salt weathering: a scanning electron microscopy study.

Salt weathering is a major erosive process affecting porous materials in buildings. There have been attempts to relate erosive mass loss to physical characteristics of materials, but in the case of natural stone it is necessary to consider the effect of petrographic features that are a source of heterogeneity. In this paper, we use scanning electron microscopy before and after salt weathering tests in cubic specimens of three limestone types (two grainstones and a travertine) in an attempt to built conceptual models that relate petrographic features and salt weathering susceptibility (represented by mass loss). In the grainstones, the most relevant feature in controlling salt weathering processes is the interface between micrite aggregates and sparry cement that constitute weakness surfaces and barriers to fluid migration. Given the small size of the heterogeneities in relation to the test sample dimension and their spatial distribution, the macroscopic erosive patterns are globally homogeneously distributed, affecting edges and corners. In the travertine specimens, there are macroheterogeneities related to the presence of detritic-rich portions that cause heterogeneous erosive patterns in the specimens. Petrological modeling helps to understand results of salt weathering tests, supporting field studies for natural stone selection.

Microtomography-Based Pore Structure Modelling of Geologic Materials Used as Building and Dimension Stones

In this paper, an initial X-ray Computed Tomography study of sulphate salt degradation of two Portuguese Dimension Stones (“Semi-rijo” and “Mocacreme”) is presented, based on the Standard EN 12370 (1999). This study was performed using a high resolution X-ray Computed Tomography, a 3D X-ray microscopy non-destructive technique, in order to get representative digital information on 3D texture of the external surface and of the interior of visible light opaque objects for imaging and eventually measuring them, at resolutions in the m range. This will produce spatial quantitative information map on the density distribution of the scanned samples, usually associated to different mineral phases and voids’ textures constituting the natural materials. Both types of stones are also being investigated by the combined application of classic methods: Optical Microscopy, Scanning Electron Microscopy and Mercury Injection Porosimetry (MICP). Most of those classic studies use mainly traditional 2D imaging techniques, none of these being able to produce the 3D resolution details that X-ray micro-tomography enables. In order to achieve future better qualitative and quantitative integrated models, it will be important to combine its non destructive and 3D characteristics results with those of 3D MICP models obtained for such complex materials. This enables to qualitatively and/or quantitatively assess the evolution and decay potential of different phases and voids (pores + fissures) textures in different environmental interaction conditions. So, in order to set-up more efficient forecasts of their engineering properties behaviour in a given environment this study is an essential initial complementary step to compare and integrate in near future studies all the advantages and disadvantages of the application of these classic and new methods, based respectively upon visual examination of the samples’ exteriors and on micro-tomographic image models of the samples’ interiors and surfaces, to these geologic materials.

The church of Santa Engrácia (the National Pantheon, Lisbon, Portugal): building campaigns, conservation works, stones and pathologies

Abstract A study of the different building campaigns, restoration works, pathologies and stones used through time in the Church of Santa Engrácia (Portuguese National Pantheon) is presented. The changing fortunes of the monument make it a useful case study in terms of building stone use and decay. This bold baroque project, begun in the seventeenth century, remained without a roof until the 1960s. At that time the leader of the conservative regime of Estado Novo (‘New State’), António de Oliveira Salazar, decided to complete the building as a national pantheon to provide a memorial to some of the personalities of Portuguese history. A complementary approach between the geosciences and history of art identified the ancient and more recent quarries, and the main stone types, used during its different building campaigns and restoration/conservation works. The local stone variety ‘lioz’, extracted in the area around Lisbon, was the main type used. An overview of the current knowledge of the chemical, physical and mechanical characteristics of these stones is now available. Besides new data on the typology, causes and processes of the major weathering forms observed inside and outside the building, the building campaigns and restoration works are also presented.

Specific surface area and salt weathering of limestones: a laboratory study

Specific surface area (SSA) results are used to discuss results of salt weathering tests on two grainstones and a travertine. SSA was found to be especially useful in the understanding of the travertine’s heterogeneous behaviour (with higher SSA in the more clay-rich zones) and the conclusions can be considered transferable to travertines in general. The transferability of conclusions applies insofar as in these continental limestones the terrigenous clay-rich zones, whose composition was verified by their infrared spectra, would promote mesoporous portions that favour the migration of solutions and that are, hence, more susceptible to salt weathering. SSA did not distinguish the grainstone types (Semi-rijo and Moca Creme), which show different weight loss in the salt weathering tests (higher in Moca Creme), a result explained by the predominant macroporous (as defined by IUPAC) character of these rocks. These results suggest that petrographic studies can be employed in the definition of groups of rocks for which SSA could be used as estimator of mass loss under salt weathering conditions. Other aspects of the stone cycle are also considered for the travertine, related to the presence of a certain amount of pores of nanometre scale in this rock and the possible higher specific values of presumably more clay-rich residues from its cutting.

Salt weathering of limestones: susceptibility of petrographical features (SEM study)

Scanning electron microscopy (SEM) studies of stone decay can give important contributions in the understanding of the susceptibility of petrographic features of rocks to salt weathering. Here are presented results of SEM studies in limestones (grainstones with the commercial designations of Semirijo and Moca Creme and a travertine, geological details in [1-4]) submitted to salt weathering tests according to the EN 12370 using sodium sulphate. The SEM observation before and after the tests allows the discussion of the petrographical characteristics of these rocks that contribute to erosive decay under salt weathering. In the case of the grainstones the texture can be described in general as allochemical particles cemented by sparry calcite (Fig. 1a) and it is visible from the preparation of the polished surfaces the interface between the sparry cement and the allochemical components. After the salt weathering tests, SEM studies show (Fig. 1b) that the sparry cement presents extensive fissuring and separation between the grains and sparry cement. The observations of disaggregation products of the grainstones showed an important amount of apparently intact grains [1,2]. These results are similar to those observed by [5] with the sparry cement being affect by salt crystallisation and lesser impact on the grains. Fragments from small cubes of the grainstones also subjected to salt weathering cycles without the final water cleaning were also studied allowing to observe the presence of sodium sulphate in the interface between allochemical grains and the sparry cement (Fig. 1c,d). In the case of the travertine, specimens show heterogeneous patterns of erosion that seem related to the heterogeneous texture of this rock, with more terrigenous (specially clay-rich) portions favouring higher erosive susceptibility [1,2]. The presence of clays has frequently being associated with salt weathering susceptibility in limestones [6]. In Fig. 1e can be observed the heterogeneity of the polished surface, with the more irregular portions showing a chemical spectra (Fig. 1f) indicative of the terrigenous component and showing also the presence of sodium and sulphate related to the weathering solutions.

Alteration Features of Stones Applied in Underground Metro Stations

Several types of stones (igneous, sedimentary and metamorphic) are found applied in pavements, stairs and walls of the underground stations of the Lisbon Metropolitan System, Portugal. A field study based solely on non-destructive and non invasive visual inspection of these stations is presented, aiming to identify different transformations of applied stones and discuss its relation with the stone characteristics. This field study has pointed to relevant issues regarding durability of stone materials namely when used in pavements and stairs. The most striking aspect is the frequently marked variable behaviour of stones from a given rock type under the same use conditions, with heterogeneities and discontinuities that seem to cause irregularity from the early stages and further increase with time, having more accentuated consequences (in terms of intensity of surface irregularity) in some types of limestones. The observations collected could be relevant for the discussion of characteristics relevant for the quality control of these materials (namely at the quarry level), including the selection of stones for future substitutions. There is, therefore, the need for a more complete study comprising detailed petrographic studies and laboratory tests both from the applied stones and from the supplying quarries.