Synthesis of "Yellow Cobblestones" of Sofia Using Asbestos Waste

Our plain-English summary of Synthesis of "Yellow Cobblestones" of Sofia Using Asbestos Waste, by A. Karamanov, V. Kostov-Kytin, E. Karamanova (Materials, 2026).

The study

• Authors: A. Karamanov, V. Kostov-Kytin, E. Karamanova
• Journal: Materials
• Published: 2026;19(20)
• Read it: PubMed · Publisher · Free full text

Abstract

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.

In plain English

This was a laboratory materials-science experiment, not a study of people or health. The authors tried to make a paving ceramic resembling Sofia's historic "yellow cobblestones" from a Bulgarian marl clay from a quarry near Popovo. The original is a clinker called "Keramit" that was made in Budapest. The Bulgarian marl has less magnesium oxide (MgO), so the authors added waste cords from an asbestos rope that had been used for high-temperature insulation. X-ray analysis showed the cords were made essentially of chrysotile. Working in a fume cupboard, the authors wet-milled 2 g of cords with 18 g of marl and heated the mix to 1000 degrees Celsius for 1 hour. X-ray analysis of the result showed no traces of chrysotile. This material was mixed with more marl (70% to 30%), pressed and fired. The asbestos addition widened the sintering interval from approximately 10 to 50 degrees Celsius. Samples fired at 1150, 1160 and 1170 degrees for 4 hours shrank by similar amounts. The new ceramic's composition, porosity and crystal structure were comparable to the original. The authors concluded that a very similar clinker can be made this way, but noted that firing must be carefully controlled.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory materials-synthesis experiment (ceramic production and testing); no human or animal subjects
• Who was studied: Material samples: a marl clay from a quarry near Popovo, North-Eastern Bulgaria; waste asbestos rope cords; and samples of the original Budapest-made "Keramit" paving clinker from Sofia
• Exposure: Waste cords from an asbestos rope used for high-temperature insulation, made essentially of chrysotile. 2 g of cords were wet-milled with 18 g of marl and 10 mL of water, then heated to 1000 degrees Celsius for 1 hour. The resulting grog (G*9/1) was mixed 70% to 30% with marl, pressed and fired.
• Main results:
  • After heat treatment at 1000 degrees Celsius for 1 hour, X-ray diffraction of the grog showed no traces of chrysotile, which the authors took as confirming thermal inertization
  • Compared with the marl alone, the asbestos addition (grog G*9/1) increased the sintering interval from approximately 10 to 50 degrees Celsius
  • The new ceramic C9/1 fired at 1150, 1160 and 1170 degrees Celsius for 4 hours showed similar shrinkage of 9.9 plus or minus 0.2%. At 1140 degrees sintering was not completed, and at 1180 degrees some deformation and swelling occurred.
  • MgO content (wt %) was 5.1 in the original Keramit, 2.8 in the Bulgarian marl, 46.8 in the asbestos waste and 5.7 in the new ceramic C9/1. The new ceramic's composition was closer to the original than the marl's was.
  • Water absorption was 0.4 plus or minus 0.1% for the original Keramit and 0.2 plus or minus 0.1% for the new ceramic. Open porosity was 1.0 plus or minus 0.2% (Keramit) and 0.5 plus or minus 0.2% (new ceramic). Closed porosity was 10.1 plus or minus 0.2% (Keramit) and 10.7 plus or minus 0.2% (new ceramic).
  • Electron microscopy showed fine anorthite and pyroxene crystals, mainly 1 to 5 microns in size, in both ceramics
• The authors' conclusion: The authors concluded that adding MgO-rich asbestos waste to local Bulgarian marl makes it possible to synthesize a ceramic clinker very similar to the original yellow cobblestones, with comparable chemical and phase composition and structure
• Limitations the authors state:
  • The sintering interval remains relatively narrow, so the firing must be carefully controlled

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