Siegfried Siegesmund - Monument Future

This work was generously supported by the Volkswagen Foundation (AZ93919).

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145

Luís Sousa 1,2, Johanna Menningen 3, Siegfried Siegesmund 3

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1University of Trás-os-Montes e Alto Douro, Quinta de Prados, 5001-801 Vila Real, Portugal

2Centro de Geociências da Universidade de Coimbra, Coimbra, Portugal

3Geoscience Centre of the Georg August University Göttingen, Germany

Eleven Portuguese limestones were evaluated regarding their bowing behavior. Only one variety showed meaningful bowing values under thermohydric conditions. The slabs of this variety, named Valverde , show a constant rate of deformation and reach bowing values of about 8 mm/m. While under dry heating cooling cycles no deformation was observable, the addition of water lead to increasing deformation values. On the contrary, alternating dry and wet cycles enable the limestone samples to recover during the dry cycles. Only continuously wet conditions imprint permanent bowing.

Petrographic observations and dilatometry tests allow to identify several stylolites filled with iron oxides as a possible reason for this behavior. These irregular fractures open under thermohydric conditions. Beyond a certain opening limit, the asperities inhibit the minerals to retake the initial position and deformation will be permanent. Therefore, this stone should not be used under wet conditions combined with higher temperatures. Those conditions would cause a bowing behavior as reported in several cases.

Keywords: limestone; bowing; petrographic properties

Bowing behavior has been described for different rock types. Nevertheless, marble is the stone mostly prone to such a permanent deformation. The specific textural characteristics of marble together with anisotropic calcite crystal properties causes thermal strain within grain to grain contacts and originates bowing (Siegesmund et al. 2000; Siegesmund et al. 2008). Although this phenomena was described for many rock types, as for example 146even weathered granites can display bowing behavior (Siegesmund et al. 2018), there are only scarce cases reported for limestones (Siegesmund 2008).

Figure 1:Macroscopic appearance of Valverde limestone (honed surface; size of the photo: 10 cm × 10 cm).

Table 1:General characteristics and classification (according Dunham (1962) and Folk (1962)) of the selected limestones. The sample VAV is the Valverde variety.

For a Portuguese limestone (usually named Valverde ) several stoneworkers described bowing phenomena. In order to evaluate these observations systematically, eleven limestone varities were selected and analysed: Alpinina (ALP), Ataíja Azul (ATAZ), Ataíja Creme (ATCR), Semi-rijo Codaçal (cut pararellel to the sedimentary lamination, CODFV), Lioz (LIOZ), Moca 4M (cut perpendicular to the sedimentary lamination, MCCT), Semi-rijo Branco Mais(i) (SBM), Semi-rijo Branco Real (SBR), Vidraço Azul Valverde (VAV); Vidraço Portela Azul (VPAZ); Vidraço Portela Creme (VPCR).

The limestone samples used in this study where collected in the Maciço Calcário Estremho (MCE), a Jurassic limestone massif located in the Portuguese Lusitanian Basin, with the exception of one sample (LIOZ) collected in a Cretaceous Unit near Lisbon (Silva, 2017). MCE is one of the world’s leading regions producers of limestones for ornamental purposes. The quarries develop along the hillside, with several benches, can reach depths around 50 m and are grouped in several exploitation sites. Several lithostratigraphic units are exploited and originate different ornamental varieties, from cream laminated calcarenites to grey calciclastic micritic limestones (Carvalho and Lisboa, 2018). The samples selected from MCE show this high variability.