Vasilios Skliros

A detailed textural, mineralogical, and geochemical investigation of beach sands and seabed sediments from Thorikos and Oxygono bays of the eastern coast of Lavrion is performed, with the objective the provenance of the ore types exploited, the processing and beneficiation types employed, and the respective exploitation periods. The Oxygono Bay beach and seabed sands are highly heterogeneous, predominated by lithic clasts originating from surrounding lithologies. Examination of the fine-grained fraction from the seabed core revealed that only the upper 50 cm was affected by recent and ancient mining activity. Combining the mineralogy and geochemistry of Oxygono Bay sands with the radiochronological model of Pappa et al. (2018), four periods of recent exploitation (mid-19th—late 20th century) are distinguished: (1) The “1860–1875 A.D.”, involving exploitation of the ancient smelter slags, (2) the “1875–1900 A.D.”, with ongoing ancient smelter slag processing and the commencement of underground sulfide ore exploitation, (3) the “1900–1930 A.D.”, where heavy mining of the carbonate-hosted Pb–Zn–Ag ore occurs, (4) the “1930–1980”), where the implementation of flotation-type processing assisted in the exploitation of the poor skarn and porphyry-type ores. The latest “1980 A.D.—to date” period depicts the cessation of all mining and processing activities. The southern Thorikos Bay beach sands are homogeneous and fine-grained, are mainly composed of gangue and pyrite, and show elevated Fe, As, Pb, Zn, and Mn content. The southern Thorikos Bay beach sands clearly point to exploitation and processing by flotation of the carbonate-hosted Pb–Zn–Ag sulfide ore, and the tailings were disposed of from the nearby facilities to southern Thorikos Bay without any environmental concern during the “1930–1980 A.D.” period.

Synthetic nesquehonite with a Mg(HCO₃)OH·2H₂O chemical formula is a solid product of CO₂ mineralization with cementitious properties. It constitutes an “MHCH” (magnesium hydroxy-carbonate hydrate) phase and, along with dypingite and hydromagnesite, is considered to be a promising permanent and safe solution for CO₂ storage with potential utilization as a supplementary material in “green” building materials. In this work, synthetic nesquehonite-based mortars were evaluated in terms of their compressive strengths. Nesquehonite was synthesized by CO₂ mineralization under ambient conditions (25 °C and 1 atm). A saturated Mg²⁺ solution was used at a pH of 9.3. The synthesized nesquehonite was subsequently studied by means of optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Impurity-free nesquehonite formed elongated fibers, often around a centerpiece, creating a rosette-like structure. The synthesized nesquehonite was mixed with reactive magnesia, natural pozzolan, standard aggregate sand and water to create a mortar. The mortar was cast into 5 × 5 × 5 silicone mold and cured in water for 28 days. A compressive strength of up to 22 MPa was achieved. An X-ray diffraction study of the cured mortars revealed the formation of brucite as the main hydration crystalline phase. Carbon dioxide mineralized nesquehonite is a very promising “green” building material with competitive properties that might prove to be an essential part of the circular economy industrial approach.

Nesquehonite synthesized by mixing MgCl₂·6H₂O and Na₂CO₃ at T 25°C, was thermally treated at various temperatures (170°C, 200°C, 295°C, 350°C, and 390°C), based on thermal analysis study of raw nesquehonite. The mineral phases formed at each temperature were studied by means of Raman microspectroscopy, Fourier transform infrared spectroscopy, and X‐ray diffraction. After thermal treatment of raw nesquehonite above 295°C, its dominant Raman peak at 1,100 cm⁻¹, which corresponds to the v₁ CO₃²⁻ antisymmetric stretching vibrations, exhibits broadening and upshift of up to 10 cm⁻¹, whereas the Raman peaks at 707 cm⁻¹, at 770 cm⁻¹, at 1,425 cm⁻¹, and at 1,710 cm⁻¹ disappear, and new Raman peaks arise at 1,282 cm⁻¹ and at 1,386 cm⁻¹. The latter have been attributed to the decomposition of HCO₃⁻, in accordance with the Fourier transform infrared spectroscopy study. According to the differential thermal analysis study, water of crystallization is lost in two steps until 295°C; therefore, the phase studied above this temperature corresponds to Mg (HCO₃)(OH). The Raman peak at 3,550 cm⁻¹ assigned to the existence of OH⁻, exhibits weakening after thermal treatment. Our results corroborate a Mg (HCO₃)OH·2H₂O nesquehonite formula.

MINERALOGICAL AND SPECTROSCOPIC STUDY OF NESQUEHONITE SYNTHESIZED BY REACTION OF GASEOUS CO2 WITH MG CHLORIDE SOLUTION