Synthesis of “Yellow Cobblestones” of Sofia Using Asbestos Waste

The “yellow cobblestones” of Sofia are considered one of the city’s symbols. This high-quality clinker 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 Bulgarian clays used are characterized by a lower MgO amount, which results in a very narrow sintering range. However, an increase in MgO in the parent batch, bringing the composition closer to that of the original, has a positive effect on sintering, phase composition and structure. This work discusses the possibility of obtaining similar clinker by adding asbestos waste containing a high percentage of MgO to a selected Bulgarian marl. The process involves pre-calcination of part of the marl together with the asbestos waste, followed by mixing the resulting grog with the remaining clay, homogenization, pressing and final firing. Studies with Hot Stage Microscopy and optical dilatometry demonstrate that the asbestos addition has a positive effect on the widening of the sintering interval, while XRD, SEM and pycnometric results elucidate that the samples obtained in this way are characterized by a phase composition and structure comparable to those of the original material.