(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.
Results and Discussion
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 CaCO3 in 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.
Calcite decomposition in the same range of temperature is evident in the DSC curves through the 81presence of an endothermic peak. A slight endothermic peak at about 80° C is also present, along with a larger one at 550 °C. Both are better shown in the DSC curves of the insoluble residue. TG/DTG curves (Fig. 6) of the insoluble fraction from the yellow-beige levels show a first mass reduction, with a peak in the DTG curve at 84 °C. In the R sample this mass loss is shifted at 95 °C and it is less pronounced. These thermal variations are consistent with dehydration due to the evaporation of the adsorptively bound water from the specimen (Földvári 2011). A second mass loss with a peak in the DTG curve at 287 °C is observed in the sample from the yellow-beige stone, which can be attributed to the goethite dehydroxylation (Földvári 2011). This peak is absent in the DTG curve of the R sample according to the XRD findings, which did not detect goethite in this sample, but hematite as a product of its transformation. For temperatures higher than 400 °C, the pattern of the DSC curve evidences an endothermic-exothermic process. It corresponds to a solid-phase structural decomposition of organic matter and clay minerals, which is more pronounced in the Y sample compared to the R one. It is followed by a crystallization of new phases, whose evidence is given by a subsequent exothermic bump.
Colour changes, bulk density, porosity accessible to water and water absorption measured for the stone from the yellow-beige and red levels within the blocks are reported in Table 1.
A strong colour variation was recorded in the red level, which may be attributed to the transition of goethite to hematite detected through the mineralogical and thermal analyses. High porosity and water absorption, as well, were measured in the yellow level and not significant decreases of 6 % and 5 %, respectively, were measured in the red level. Also the bulk density showed a slight reduction, namely 3 %. These variations are comparable with those reported for limestones with high porosity and notably lower than the decreases recorded for compact stones (Gomez-Heras et al., Brotons et al. 2013). They were in the range of variability of the measurements, thus they could be due to the intrinsic stone heterogeneity.
However, a decrease of UPV in the samples from the red level was recorded. It was 21 % (Table 1).
UPV
