ABOUT THE SINTERING OF HISTORICAL “YELLOW BRICKS” OF SOFIA

The historical “yellow cobblestones” of Sofia are a very well sintered ceramic clinker with practically zero water absorption. However, the high amount of crystal phase formed, which explains the extraordinary mechanical properties of this remarkable material, is in formal contradiction with its low final porosity. In fact, in the modern ceramics clinkers the crystallinity is similar or inferior, but the reached degree of sintering is lower; as a result, the mechanical properties are reduced. The aim of this report is to elucidate some peculiarities of the densification process of “yellow ceramics bricks” because such information is essential for the eventual successful production of replica of this emblematic pavement. The chemical and phase compositions of original “yellow paving bricks” together with those of modern clinker from “Vitosha” boulevard in Sofia were evaluated by XRF and XRD analysis, respectively. The structures of both ceramics were investigated with density measurements and SEM.Their densification behaviour was estimated by re-sintering of milled original samples by Hot Stage Microscopy (HSM) tests. The results elucidate that the sintering temperature of the historical clinker is inferior, and their sintering interval is significantly narrow. This peculiarity is explained by the rapid decreasing of apparent viscosity with temperature rise.Finally, by secondary holding at the sintering temperatures and subsequent fast quenching of a “yellow brick” sample, it is demonstrated that some phase formation occurred during the industrial cooling step. This result explains both the good degree of densification and the better properties of the “yellow cobblestones”.