Siegfried Siegesmund - Monument Future

Figure 2:Detail of the stone from the collapsed vaults, which shows a yellow-beige-color (Y) passing to a reddish one (R) across the thickness of the masonry unit.

In 2018, after a strong rainstorm, part of the structure collapsed (Fig. 2), involving in a first time one vault and part of the two adjacent ones and in a second time (about 7 days later) the remaining part (five vaults) of the room. The collapse evidenced the inner structure of the vaults and external walls. The latter were two leafs walls without horizontal connections, filled with incoherent material (pieces of rocks and debris).

The vault collapse revealed traces of an historic fire, which were hidden by the presence of plasters, in the form of fumes and a diffuse reddish color across the thickness of the masonry units up to some centimeters from the surface (Fig. 2).

In the framework of a diagnostic activity supporting a restoration project in view of a building reuse, the study of the fire damage on the stone was undertaken.

Collapsed blocks measuring 21x20x50 cm were taken from the site and samples were obtained from both the unaltered and altered portions, having yellow-beige (Y) and reddish colour (R), respectively (Fig. 2).

The following analyses and tests were performed.

— Thin-section samples were observed in plane-polarised and cross-polarised transmitted light by means of an optical microscope (Eclipse LW100 Nikon) at magnifications of 50x e 100x.

— X-Ray Diffraction analyses (XRD) were performed on both the whole rock and insoluble residue. The insoluble residue was separated by a chemical attack of the grinded stone with HCl-3N in order to remove the carbonates by dissolution. A Philips 1742 diffractometer (APD – 3.6j version) was used for the analyses (CuKα, 40 kV, 20 mA, 2ϑ step size of 0.02 °, counting time 1.25 s, scan interval between 3 ° and 60 °). The diffraction data 79were processed with a X’Pert software — Philips Analytical.

— Simultaneous thermal analyses by Differential Scanning Calorimetry and Thermogravimetry (DSC-TG). A Netzsch STA 449 F3 Jupiter ®was used. Both samples of the whole rock and of the insoluble residue were analyzed. Approximately 25 mg of each powder sample were heated in air from the ambient temperature to 1,000 °C, at a heating rate of 10 °/min.

— Colour changes were recorded by colorimetric measurements. These were taken by light absorption in diffuse reflection using a Konica Minolta CM700d spectrophotometer. They were carried out with a D65 illuminant and under a 10 ° standard observer. L*, a* and b* colour coordinates in the CIELab system were measured and the colour variation (ΔE*) was calculated.

— Measurements of bulk density, porosity accessible to water and water absorption amounts of the stone samples were performed through saturation and buoyancy techniques, following the ISRM recommendation [ISRM, 1981].

— Ultrasonic Pulse Velocities (UPVs) were measured on specimens (cubes 70 mm side obtained from masonry blocks coming from the site) after drying at 70 °C, according to ASTM D2845-05 (ASTM 2005). In particular, three visible unaltered (Y) and three colored specimens (R) were taken from the collapsed portion of the building. Velocities were measured by direct transmission method using a TDAS 16 (Boviar) instrument and probes with a frequency of 55 kHz. They were recorded in each direction (x, y, z) of the cubic specimens and expressed as mean values.

— Compressive strength tests were performed according to UNI EN 772-1 (UNI 2011) on the same specimens used for UPV test, after drying at 70 °C. A universal testing machine (Metrocom Engineering spa), with a load capacity of 200 kN and a speed of 0.2 mm/min, was used for the test.

The petrographic characteristics, as observed by optical microscopy under polarized transmitted light (Fig. 3a), show that the investigated stone is a medium grainstone. It is almost exclusively made of calcareous fossil remains, which mainly consist of coralline algae and, at lower extents, of benthic foraminifera, echinoids, bivalves and bryozoans. The average dimensions of the bioclasts fall between 0.3 and 0.4 mm with a maximum size of 0.6 mm. The stone contains sporadic quartz and feldspar crystals. The micrite is nearly absent and the cement is made of calcite, with a texture varying from microsparitic to sparitic type. It is in poor amount and fills only partially the interparticle porosity, which results very high. At large extents the cement is in the form of a thin level surrounding the grain borders. In some areas it is present in larger spots and exhibits a well-developed sparitic texture.

Figure 3:Microscopic features of the stone (N//). a: yellow-beige level; b: red level.

No damage in the form of microfissuring affecting the stone microstructure was observed in the red portions, compared to the yellow-beige ones (Fig. 3b). On the contrary, there was an increase of red pigmented bioclasts and iron rich agglomerations, which may be related to an effect of the high temperature on the iron rich components.

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Figure 4:XRD spectra of the whole rock (on the top) and insoluble residue (on the bottom) from the yellow (Y) and red (R) levels.

The mineralogical composition, as determined by XRD analyses of the whole stone samples coming from the levels having the different colours (Fig. 4, top) does not show any differences. In all cases, almost exclusively calcite was detected. A diffraction peak at low angles was visible, as relating to the presence of clay minerals.

To detect the presence of other minerals, masqued by the preponderant CaCO 3in the whole stone composition, the insoluble residue, after removing all carbonates by dissolution in HCl-3N, was also analysed. The XRD powder patterns of the insoluble residue obtained from the yellow-beige and red levels in the stone after this chemical attack are reported in Fig. 4, bottom. Different mineralogical compositions were found. The presence of quartz, goethite, along with some feldspars and clay minerals was detected in the yellow-beige level. Goethite was absent in the red level, instead hematite was detected. The transformation of goethite to hematite comes from a dehydroxylation process. Such a transition takes place at temperature of 300 °C (Földvári 2011).

Results of the simultaneous TG and DSC analyses performed on the whole stone from the yellowbeige and rel levels are illustrated in Fig. 5.

TG curves well recorded the calcite decomposition between 670 °C and 840 °C with a mass loss of about 40 %.

Figure 5:TG/DSC curves of the whole rock from the yellow (Y) and red (R) levels.

Figure 6:TG, DTG and DSC curves for the insoluble residue from the yellow (Y) and red (R) levels.