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      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.

       Conclusions

      Macroscopic evidences of a fire in the calcarenites employed in an historic building were confirmed by mineralogical changes, which reflects 82on strong color changes. In particular, the change from yellow-beige to reddish color of the stone is consistent with the thermally induced transformation of goethite to hematite. This transition phase indicates that temperatures around 300 °C were reached in the red stone levels during the fire. Effects on the stone microstructure were not visible under optical microscope. Nor the measurement of physical properties showed meaningful variations in this regard. On the contrary, UPV detected a decrease of the propagation velocities, which probably denotes a stone microfissuring. Nonetheless, its entity did not compromise the mechanical resistance of the stone, which remained almost unchanged. High porosity may account for a slight microstructural damage recorded for the investigated calcarenite, where pores likely behave as free spaces for expansion of calcite grain preventing in this way an extensive damage.

Table 1: Color change (ΔE), bulk density (γb), porosity accessible to water (P), water absorption (WA), ultrasonic pulse velocity (UPV) and uniaxial compressive strength (UCS) measured for the stone from the yellow-beige and red levels.