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Παραγωγή πορωδών κεραμικών υλικών από πυριτικά τέλματα επίπλευσης

Gourdomichali Chrysoula-Pigi

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Summary

The valorization of industrial wastes for the production of ceramics is in several cases a feasible option and in line with the principles of circular economy. In the present experimental study, porous ceramics were produced by mixing silicate tailings from Kevitsa mine, Finland, with two different pore forming agents namely calcium carbonate (CaCO3) and calcium phosphate (Ca3(PO4)2): five different addition %, varying form 0-12 % were used. The effect of binary mixture compositions and calcination temperatures on porosity, water absorption, density, compressive strength and thermal conductivity of the produced porous ceramics was also investigated. The binary mixtures were prepared using the powder processing technique and uniaxially pressed (40 MPa) into cubic specimens (5 cm edge). All specimens were calcined at 900 oC, 950 oC, 1000 oC and 1020 oC respectively.The results indicated that the addition of CaCO3 resulted in a noticeable increase of porosity and water absorption from 28% to 51%, and 12.5% to 38.6 % respectively, while density decreased from 2.16 g/cm3 to 1.29 g/cm3. CaCO3 acts as flux and reacts with the silicate tailings, promoting the release of gas (CO2) and the development of larger pores, resulting in higher values of porosity and water absorption. The addition of CaCO3 also lowered thermal conductivity values, from 0.64 W/m·K to 0.48 W/m·K, as more pores were formed during sintering. Furthermore, the addition of CaCO3 reduced structural integrity and compressive strength (from 6 MPa to 2MPa) of the produced ceramics even when higher calcination temperature was used (1020 oC).On the other hand, the addition of Ca3(PO4)2 resulted in a slight increase of both porosity and water absorption, from 28.4 % to 31.9 % and from 13% to 14% respectively. Density remained practically unaffected (2.1 gr/cm3). Thermal conductivity increased with increased % of Ca3(PO4)2 addition, from 0.26 W/m·K to 0.32 W/m·K, due to the fact that grain growth and densification were promoted during sintering, and a more porous structure was obtained. The compressive strength increased significantly from 5 MPa to 15.6 MPa.Based on the results of the physical, mechanical and thermal properties, it is deduced that the produced ceramics are suitable for several uses such as insulating bricks, roof tiles for thermal insulation panels and lightweight aggregates for concrete.The leaching of potentially hazardous elements (PHEs) from porous ceramics was also investigated with the use of Toxicity Characteristics Leaching Procedure (TCLP) and EN 12457-2 tests and the results indicate that they exhibit no toxicity. Several analytical techniques namely X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscope (SEM) were also used for the characterisation of raw materials and the produced ceramics.

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