Читать книгу Monument Future - Siegfried Siegesmund - Страница 109

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.