Читать книгу Monument Future - Siegfried Siegesmund - Страница 108

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 decrease as an effect of the heating is reported in the literature (Yavuz et al. 2010, Andriani 2014), although at entities depending on the temperature and stone structure, as well. It mainly relates to a thermal micro fissuring, which causes the reduction of the propagation velocities.

The microfissuring detected by the UPV test slightly affected the above mentioned physical parameters measured by stauration and buoyancy techniques. This finding suggests that the recorded decreases of the wave velocities may be relevant to the generation of a microporosity which has no effect on the water penetration, as reported in previous studies (Franzoni et al., 2013; Freire-Lista et al., 2016). Microfissuring recorded by UPV had a negligible effect also on the mechanical performance of the stone. Very close values of the compressive strength were measured in both yellow and red levels, corresponding to a strenght loss of 4 % in the discoloured level (Table 1). Similar entities of decrease have been recorded for porous limestones by Franzoni et al., 2013.