Lubica Kriskova

Susceptibility of mineral phases of steel slags towards carbonation: mineralogical, morphological and chemical assessment

Process limitations have thus far prevented mineral carbonation of alkaline wastes from been widely applied. These barriers are caused by inefficient processing, but also by mineralogical aspects inherent to the materials. Better understanding and predictability of the effects of mineral carbonation on alkaline materials could be obtained by studying the carbonation susceptibility of constituent minerals separately, allowing for detailed and accurate analysis of their reaction kinetics and maximal conversions and of the carbonate products formed. For this purpose, this paper presents the synthesis and carbonation of the seven most abundant alkaline minerals found in AOD, CC and BOF slags, namely: akermanite (Ca 2 MgSi 2 O 7 ), bredigite (Ca 7 Mg(SiO 4 ) 4 ), cuspidine (Ca 4 Si 2 O 7 F 2 ), β– and γ–C 2 S (Ca 2 SiO 4 ), merwinite (Ca 3 Mg(SiO 4 ) 2 ), and srebrodolskite (Ca 2 Fe 2 O 5 ). Two experimental approaches to mineral carbonation of increasing levels of process severity are utilized: (mild) incubator carbonation, and (accelerated) pressurized slurry carbonation. In addition, the slags and two free oxides (CaO and MgO) are equally carbonated and evaluated. Data regarding CO 2 uptake, mineral conversion and formed carbonate and non-carbonate products in the samples were obtained through TGA, QXRD (Rietveld refinement) and SEM techniques. Reduction in material basicity and evolution of particle morphology were also assessed. The synthesized mineral purities (>70 wt% target mineral phase) were found sufficient for more accurate assessment of carbonation behaviour of the individual minerals. Bredigite was found to be the most reactive mineral under all processing conditions; C 2 S and wollastonite were more reactive under slurry carbonation, while srebrodolskite and calcium monoferrite were found to be more reactive under moist carbonation. Merwinite and diopside had the slowest carbonation conversions. Calcite and aragonite were the dominant carbonate products formed, whereby aragonite formation was promoted in Mg-containing materials. The morphology of aragonite crystals and the packing density of its product layer were found to vary depending on the parent mineral. Characteristic slag carbonation products, not observed as extensively from synthetic mineral samples, were magnesian calcite from slurry carbonation, and monohydrocalcite and vaterite from moist carbonation. Wollastonite was the main crystalline non-carbonate product, occurring predominantly from slag carbonation, while silica-rich amorphous matter formed in all samples proportionally to CO 2 uptake. Free lime, when present, controlled material basicity above pH 12, while silicates were found to typically possess pH in the range of 11.3–11.9, and Ca-carbonates eventually controlled the pH of well carbonated samples to values under 10.

Effect of High Cooling Rates on the Mineralogy and Hydraulic Properties of Stainless Steel Slags

This article investigates the effect of chemical composition and cooling rate during solidification on the mineralogy and hydraulic properties of synthetic stainless steel slags. Three synthetic slags, covering the range of typical chemical composition in industrial practice, were subjected to high cooling rates, by melt spinning granulation or quenching in water, and to low cooling rates, by cooling inside the furnace. Both methods of rapid cooling led to volumetrically stable slags unlike the slow cooling which resulted in a powder-like material. Stabilized slags consisted predominantly of lamellar β-dicalcium silicate (β-C2S) and Mg, Ca-silicates (merwinite and bredigite); the latter form the matrix at low basicity and are segregated along the C2S grain boundaries at high basicities. Slowly cooled slags consist of the γ-C2S polymorph instead of the β-C2S and of less Mg, Ca-silicates. Isothermal conduction calorimetry and thermogravimetric analysis indicate the occurrence of hydration reactions in the stabilized slags after mixing with water, while calcium silicate hydrates (C-S-H) of typical acicular morphology are identified by SEM. The present results demonstrate that the application of high cooling rates can result in a stable, environmental-friendly, hydraulic binder from stainless steel slags, rich in β-C2S, without the necessity of introducing any additions to arrest the β polymorph.

The influence of curing conditions on the mechanical properties and leaching of inorganic polymers made of fayalitic slag

This study reports on the impact of the curing conditions on the mechanical properties and leaching of inorganic polymer (IP) mortars made from a water quenched fayalitic slag. Three similar IP mortars were produced by mixing together slag, aggregate and activating solution, and cured in three different environments for 28 d: a) at 20 °C and relative humidity (RH) ~ 50% (T20RH50), b) at 20 °C and RH≥90% (T20RH90) and c) at 60 °C and RH ~ 20% (T60RH20). Compressive strength (EN 196-1) varied between 19 MPa (T20RH50) and 31 MPa (T20RH90). This was found to be attributed to the cracks formed upon curing. Geochemical modelling and two leaching tests were performed, the EA NEN 7375 tank test, and the BS EN 12457-1 single batch test. Results show that Cu, Ni, Pb, Zn and As leaching occurred even at high pH, which varied between 10 and 11 in the tank test’s leachates and between 12 and 12.5 in the single batch’s leachates. Leaching values obtained were below the requirements for non-shaped materials of Flemish legislation for As, Cu and Ni in the single batch test.

Synthesis of Inorganic Polymers Using a CaO-Al2O3-FeO-SiO2 Slag

The focus of the present paper is to investigate the effect of the activating solution on the structure and mechanical properties of inorganic polymers synthesised from a slag resembling the vitrified residue from a Waste-to-Energy plasma installation. The slag consists of (in wt.%) 22 CaO, 12 Al2O3, 34 SiO2 and 20 Fe2O3 and the activation solution was 50:50 mass ratio NaOH and sodium silicate, with the NaOH solution molarities varying from 2 M to 10 M. The synthesised slag was almost completely amorphous due to the rapid cooling, with only traces of magnetite and quartz. The inorganic polymers were prepared by mixing the slag, sand and activation solution. In all cases, heat was generated during sample preparation and its amount increased with the activating solution strength. After 90 days, the compressive strength of the samples activated with 6 M or higher NaOH solutions was similar, approximately 88 MPa. For NaOH activation solutions with molarities lower than 6 M, the compressive strength was lower, both at early as well as late curing times. SEM and EPMA analysis revealed-between undissolved particle remnants-a distinct binder phase, composed of (in wt.%) 18.9±2.5 CaO, 11.5±0.1 Al2O3, 40.3±2.1 SiO2, 15.8±1.2 FeO, 5.1±1.9 Na2O and 3.7±0.6 MgO. In conclusion, the present study showed that the CaO-Al2O3-FeO-SiO2 vitrified residue could be converted into a stable inorganic polymer having reasonably high mechanical strength, when activated with a mixture of sodium silicate and sodium hydroxide solution with a molarity of at least 4 M.