Use of marble sludge waste in the manufacture of eco-friendly materials: applying the principles of the Circular Economy

The marble industry generates enormous amounts of waste every year, whose incorrect management can lead to environmental problems. The Circular Economy model can provide an effective solution to this issue, an aspect that has been addressed in this research. The suitability of a marble sludge generated in the ornamental rock industry of Andalusia (Spain) has been assessed as raw material in ceramic bricks. Mixtures containing a clayey base and 0, 2.5, 5, 7.5, and 10 wt% of marble powder have been shaped into 60 × 30 × 10-mm³ prismatic specimens and sintered at 950 °C in a muffle. The main technological properties of the bricks related to color, shrinkage, porosity, water absorption, suction, and compressive strength have been determined. The addition of marble sludge has fostered the development of a lighter color, together with a significant increase of the open porosity. This aspect has implied, on the one hand, higher water absorption and suction results and, on the other hand, a decrease of the density and the mechanical strength. The correlations obtained after applying Shapiro-Wilk normality tests and r Pearson coefficients endorse the clear relationship between the addition of marble powder and the abovementioned effects on the technological properties of the sintered bricks. All the measured properties fully meet the brick standardized requirements, which would indicate that the recycling of marble wastes could be a promising alternative to obtain eco-friendly lightweight ceramic materials.

Ternary diagrams as a tool for developing ceramic materials from waste: relationship between technological properties and microstructure

The present work proposes the manufacture of ceramic construction materials using raw materials which are only industrial wastes from different production processes (electric arc furnace steel slag, coal bottom ash, carbon fly ash, and oil-filtering diatomaceous-earth). From them, the SiO₂-Al₂O₃-CaO equilibrium phase diagram has been used to formulate two materials compositions with the objective of obtaining ceramics whose chemical composition is located in the same area as a traditional clay-based material, but with two different technological behaviors: refractoriness due to calcium phase's development and densification due to a greater melting capacity. The waste-based pieces have been sintered at three firing temperatures (1000 °C, 1050 °C, and 1100 °C) and the mineralogical composition has been quantified by Rietveld refinement-XRD in order to establish the agreement between the phases foreseen by the formulations in the diagrams and the mineralogical compositions actually developed after the sintering treatment. In addition, fired materials have been subjected to a complete microstructural characterization by means of SEM-EDX and the pore size distribution has been determined by means of mercury intrusion porosimetry and helium pycnometry. In general, all obtained materials display adequate technological properties for their use as building materials so this characterization has allowed to show the suitability of the ternary diagram SiO₂-Al₂O₃-CaO for the design of ceramic compositions from industrial wastes. Therefore, extending the use of phase diagrams as a design tool is still little explored for waste-based ceramics, and those that have been mainly used for studying Mg-based phases can have an important contribution to a more sustainable construction sector.