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Synthesis of Wollastonite from Alf3-Rich Silica Gel and Its

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Synthesis of Wollastonite from Alf3-Rich Silica Gel and Its www.nature.com/scientificreports OPEN Synthesis of wollastonite from AlF3- rich silica gel and its hardening in the CO2 atmosphere Andrius Gineika*, Raimundas Siauciunas & Kestutis Baltakys This work combines some aspects of eco-friendliness: consumption of toxic waste, cutback of energy consumption during the synthesis of the binding material, reduction of CO2 emission by using less CaCO3 in the raw meal, and consumption of carbon dioxide. In the study, the kinetics of two-step synthesis of wollastonite from CaO and AlF3 production waste, namely, silica gel, its carbonisation process and the mechanical properties of obtained samples were investigated. According to XRD and DSC data, the optimal temperature in the mixture with CaO/(Al2O3 + SiO2) = 1 for the hydrothermal synthesis of the wollastonite precursors is 130 °C: F−–containing compounds were bound into katoite and cuspidine, and portlandite reacted completely within 8 h. The optimal temperature for wollastonite formation is 900 °C, but fuormayenite, cuspidine, and the traces of larnite form as well. During the curing in the CO2 atmosphere, wollastonite and larnite reacted completely and formed calcite, vaterite, and amorphous CaCO3. Cuspidine also participates in the carbonisation process and, in addition to amorphous SiO2, it releases fuorite, which contributes to the total compressive strength of the products. The values of the compressive strength (10–15 MPa) in the wollastonite-sand samples match the requirements for the belite and special low-heat cements. Wollastonite – calcium silicate CaSiO3, natural or artifcial material, is widely used in ceramics, plastics, paints, paper industries and medicine felds. Tere are many ways to synthesise wollastonite, yet two of them are most common. Te frst is a single step synthesis, when limestone or other calcium source is mixed with silicon dioxide source (e. g. quartz) and then calcined in a temperature range of 1200–1400 °C to obtain wollastonite through solid state reactions1–3. Te other way is a two-step synthesis, when calcium oxide and silicon dioxide are mixed with water to obtain suspension and then hydrothermally cured at 130–220 °C to obtain calcium silicate hydrates such as xonotlite, 1.13 nm tobermorite, and others4,5. It is also possible to synthesise these compounds by precip- itation using calcium nitrate and sodium silicate solutions6,7. Te obtained precursor is calcined at 800–900 °C when it recrystallizes into wollastonite4–7. Tis mineral has a wide range of applications in ceramics, cement, paints, biomedicine, etc. because of its desirable mechanical and chemical properties: low shrinkage, thermal stability, whiteness, hardness, and others8–10. When using amorphous silica instead of crystalline silicon for the initial mixture, calcium silicate hydrates are easily formed. However, the latter modifcation of SiO2 is quite rare in the nature, therefore, it is needed to search for alternative sources. One of them may be industrial waste such as AlF3 manufacturing by-product – silica gel. While the fuorine-rich silica-gel waste is stored in landflls, the fuorine compounds can dissolve in the rain- water, leak and pollute the ground water. To prevent leakage, the fuorine can be bound into stable compounds. Silica-gel waste can be used as a source of silicon dioxide for the synthesis of calcium silicate hydrates11–16. Tis reaction is carried out in hydrothermal conditions and can also bound the pollutants of the silica-gel into stable calcium fuoride and calcium-silicate-aluminate compounds. As calcium fuoride is thermally stable, the products of the hydrothermal synthesis can be calcined to recrystallize calcium silicate hydrates into wollastonite15,16. Te temperature of this process is ~900 °C, which is much lower than the temperature required for the single-step synthesis of wollastonite. Terefore, not only the energy demand but also CO2 emission is reduced because of lower fuel consumption. Te reduction of CO2 emission is a great challenge for the industry of binding materials as, during the pro- 17,18 duction of ordinary Portland cement (OPC), ~5% of all man-made CO2 is emitted . An alternative to the 19,20 OPC could be low lime calcium silicates such as wollastonite CaSiO3 or rankinite Ca3Si2O7 . Even though Department of Silicate Technology, Kaunas University of Technology, Radvilenu pl. 19, LT–50270, Kaunas, Lithuania. *email: [email protected] SCIENTIFIC REPORTS | (2019) 9:18063 | https://doi.org/10.1038/s41598-019-54219-6 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. X-ray pattern of silica-gel waste. Indices: A – aliuminium hydroxyfuoride, R – rosenbergite, G – gibbsite. 21,22 wollastonite and rankinite are non-hydraulic, they can be activated by CO2 with the presence of humidity . Wollastonite is more preferred for this process because of its better solubility in water, which means better reac- 23 tion with CO2 and lower carbon dioxide emission caused by CaO production from CaCO3 . A newly formed matrix afer the carbonation is composed of calcite CaCO3 and amorphous SiO2 provides similar mechanical properties of hydrated OPC where matrix consists mainly of calcium silicate hydrates and calcium hydroxide21,24. Bukowski and Berger24 investigated mechanical properties of a binder which contained 50% sand and 50% wollastonite. Tey determined that the binder reached a compressive strength of 35 MPa within 24 h in 1 bar of CO2 using dynamic 1.4 l/min gas fow system (from the formation of the sample to the end of carbonisation). Other researchers21 reached a compressive strength of 70 MPa when the samples contained only wollastonite and were carbonised for 65 h at 60 °C in 100% carbon dioxide. Tis proves that wollastonite can be an alternative to OPC. Another advantage of wollastonite over ordinary Portland cement is that wollastonite can be synthesised from the same raw materials as the cement clinker and at 250–500 °C lower temperature19,20,25. Te aim of this work was to investigate the kinetics of a two-step synthesis of wollastonite from CaO and silica-gel waste, its carbonisation process, and the mechanical properties of the obtained samples. Tis work com- bines four aspects of eco-friendliness: (1) consumption of toxic waste, (2) cutback of energy consumption during the synthesis of the binding material, (3) reduction of carbon dioxide emission by using less CaCO3 in the raw meal, and (4) consumption of carbon dioxide. Materials and Methods Materials. AlF3 production by-product – silica gel from chemical plant SC Lifosa (Lithuania), dried at room temperature until constant mass for 2 weeks was used. Te main elements were 39.86 wt% Si, 5.37 wt% Al (deter- mined using X-ray fuorescence analyser Bruker X-ray S8 Tiger WD (Germany)), and 8.76 wt% F− (determined potentiometrically using Mettler Toledo titrator T 70 (USA) with F− selective electrode), mass losses – 4.1 wt% at 105 °C and 20.0 wt% at 1000 °C. Te silica gel waste was milled for 2.5 min at 950 rpm using a planetary mill 2 Fritsch Pulverisette 9 (Germany) until specifc surface area Sa = 1537 m /kg by Cilas LD 1090 (France) granulom- eter and density ρ = 2354 kg/m3 (gas pycnometer Quantachrome Instruments Ultrapyc 1200e, USA). Calcium oxide was obtained by calcining calcium hydroxide (≥96%, Honeywell, Germany) at 550 °C for 1 h and milling 2 3 for 0.5 min at 950 rpm (Sa = 2076 m /kg, ρ = 2837 kg/m , free CaO – 94.22%). CEN standard sand (DIN EN 196-1 (ISO 679)) was used to press wollastonite-sand samples. Figure 1 shows that the silica-gel waste contains several crystalline compounds: aluminium hydroxyfuoride (Al2(OH)3F3·0.75H2O, PDF No. 74-0940, d = 0.5675, 0.2964, 0.2837 nm), rosenbergite (AlF3·3H2O, PDF No. 35-0827, d = 0.5460, 0.3860, 0.3299 nm) and gibbsite (Al(OH)3, PDF Nr. 07-324, d = 0.4850, 0.437, 0.4320 nm). Methods. Te mixture of silica gel and calcium oxide with molar ratio CaO/(Al2O3 + SiO2) = 1 (48.31 wt% CaO, 42.39 wt% SiO2, 4.98 wt% Al2O3) was homogenised in Turbula Type T2F (Switzerland) for 1 h at 49 rpm and mixed with the distilled water to obtain the suspension with water/solid ratio W/S = 20. Te hydrothermal syntheses were carried out in stirred suspensions (50 rpm) in Parr Instrument 4560 (USA) 600 mL autoclave at 95 and 130 °C, for 0–48 h. Te products were fltered, rinsed with acetone to reduce carbonisation, and dried at 80 ± 0.3 °C for 24 h. Te calcinations were carried out in the temperature range of 850–1050 °C (at the interval of 50 °C) for 1 h in Nabertherm B 180 (Germany) furnace. Te calcined product-sand mixtures which contained 20%, 25, 30, and 35% calcined product were prepared. Tey were homogenised for 1 h at 49 rpm in Turbula Type T2F and mixed with water to obtain water/binding material ratio W/C = 0.35. Te samples of 36 × 36 mm were pressed in a cylindrical frame under the pressure of 10, 12.5 and 15 kN at 1 kN/s speed and 20 s exposure at the maximum pressure in a Test Form Mega 10-400-50 (Germany) hydraulic press. Te samples of humidifed bind- ing material without sand were also formed to determine the changes in the mineral composition during the cur- ing process. Te curing in the CO2 environment was carried out in Parr Instruments 4600 autoclave at 45 °C and 15 bar for 8-24 h. Before curing, the autoclave was purged twice from atmospheric air by letting CO2 up to 2 bar. Te compressive strength was determined right afer the curing using a Test Form Mega 10-400-50 hydraulic SCIENTIFIC REPORTS | (2019) 9:18063 | https://doi.org/10.1038/s41598-019-54219-6 2 www.nature.com/scientificreports/ www.nature.com/scientificreports press at a loading rate of 1.5 kN/s.
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