Synthesis of “Yellow Cobblestones” of Sofia Using Asbestos Waste

The „yellow cobblestones“ of Sofia are considered as one of city’s symbols. This high-quality clicker was developed in Budapest in the late 19th century by using a local marl. Since this paving was declared part of Sofia’s cultural heritage, studies to synthesize similar materials were carried out. Unfortunately, the appropriated Bulgarian clays are characterized by a lower MgO amount, which results in a very narrow sintering range. However, some increase for MgO in the parent batch, bringing the composition closer to that of the original, has a positive effect on the sintering, phase composition and structure. This work discusses the possibility to obtain similar clicker by adding asbestos waste containing high percentage of MgO to a selected Bulgarian marl. The process involves pre-calcination of a part of the marl together with the asbestos waste, followed by mixing the resulting grog with the remaining clay, homogenization, pressing and final firing. The studies with Hot Stage Microscopy and Optical Dilatometry demonstrate that the asbestos addition has a positive effect on the widening of sintering interval, while XRD, SEM and pycnometric results elucidate that the samples obtained in this way are characterized by phase composition and structure comparable to the original material.