The Influence of Stirring and Γ-Al2o3 Or Na2o Additives on the Gyrolite (Ca16si24o60(OH)8 ·14H2O) Formation Process in The
Total Page：16
File Type：pdf, Size：1020Kb
Original papers THE INFLUENCE OF STIRRING AND γ-Al2O3 OR Na2O ADDITIVES ON THE GYROLITE FORMATION IN THE CAO–QUARTZ–H2O SYSTEM KESTUSIS BALTAKYS, RAIMUNDAS SIAUCIUNAS Department of Silicate Technology, Kaunas University of Technology, Radvilenu 19, LT - 50270 Kaunas, Lithuania E-mail: [email protected] Submitted September 29, 2006; accepted November 28, 2006 Keywords: Gyrolite; Z-phase; C-S-H(I); Calcium-silicate-hydrate; X-Ray diffraction The influence of stirring and γ-Al2O3 or Na2O additives on the gyrolite (Ca16Si24O60(OH)8 ·14H2O) formation process in the CaO–quartz–H2O mixture has been examined. The molar ratios of primary mixtures were CaO/(SiO2 + Al2O3) = 0.66 and Al2O3/(SiO2 + Al2O3) = 0 or 0.025. Water/solid ratio of the suspension was equal to 10.0. Hydrothermal synthesis has been carried out in rotating autoclave (10 rpm) under the saturated steam pressure at 200°C; the duration of isothermal curing was 2, 4, 6, 8, 16, 24, 32, 48, or 72 hours. It has been found that in the pure mixture the presence of stirring has not positive influ- ence on the gyrolite formation. Even after 72 h hydrothermal treatment at 200°C 1.13 nm tobermorite (Ca5Si6O15(OH)2 ·5H2O) dominates in the products. In the stirred CaO–quartz–Na2O–H2O system gyrolite formed more quickly - 24 hours of isother- mal curing at 200°C than in the unstirred suspension - 32 h. The results show that the addition of γ-Al2O3 in the CaO–quartz–Na2O–H2O suspension stabilizes gyrolite and prevents its transformation to pectolite (NaCa2Si3O8(OH)). INTRODUCTION over, there is little data (and it is not comprehensive) about the synthesis of gyrolite and the sequence of the Gyrolite (Ca16Si24O60(OH)8 ·14H2O), a calcium sili- formed intermediate/new compounds and the influence cate hydrate, rarely occurs as a natural mineral in asso- of additives on it, when a less active but the most com- ciation with zeolites [1]. Recently, the interest in gyro- mon natural form of SiO2 - quartz is used [11, 13]. lite increases because the new possibilities of applica- In order to use more broadly gyrolite in practice the tion were found: it may be used reduce heavy metal ions manufacturing of this material must be cut in price, i.e., and remove them from wastewaters [2] and it is known by reduction of the duration of hydrothermal treatment for its selectivity to DNA clearage [3]. and/or its temperature. As in the case of other heteroge- Meanwhile, hydrothermal synthesis of gyrolite is a neous chemical reactions, also calcium silicate hydrates long and complex process. Gyrolite was first synthe- formation is faster when the reaction mixture is stirred sized by Flint et al. (1938) [4] and subsequently by Tay- [14, 15]. However, no data is available about the in- lor (1964) [5] Meyer and Jaunarajs (1961) [6], Števula fluence of stirring on gyrolite synthesis in the and Petroviè (1983) [7] and Jauberthie (1996) [8]. They CaO–quartz–H2O suspension and the influence of reported the stability of gyrolite under saturated water γ-Al2O3 and Na2O additives on it. It is necessary to seek vapor pressure between 120 and 200°C. Above 200°C, the additives which can positively enhance or facilitate truscottite (Ca14Si24O58(OH)8 ·2(H2O) is the stable phase, the formation of gyrolite. although gyrolite can be matastably produced up to The aim of this work was to determine the parame- 270°C. Their formation, stability and phase relationship ters of gyrolite hydrothermal synthesis in the stirred were studied and plotted in phase equilibrium diagrams CaO–quartz–H2O suspension and to examine the influ- by Roy and Harker [9], Harker [10] and Taylor [5]. ence of γ-Al2O3 and Na2O additives. However, almost all of these tests were performed with SiO component using its active forms such as amor- 2 EXPERIMENTAL phous SiO2, finely dispersed aerosil, silicic acid. The results of the research showed that in the In this paper the following raw materials were unstirred suspension optimal temperature of gyrolite used: fine-grained quartz (99.5 % Reachim, Russia; spe- 2 synthesis is rather high (approximately 200°C) and the cific surface area Sa = 1100 m /kg), after grinding, duration is long (about 32 h) when amorphous washing off from iron impurities by hydrochloric acid; 2 SiO2·nH2O is used as SiO2 component [11, 12]. More- calcium oxide (99.5 % Reachim, Russia; Sa = 548 m /kg) 106 Ceramics − Silikáty 51 (2) 106-111 (2007) The influence of stirring and γ-Al2O3 or Na2O additives on the gyrolite formation in the CaO–quartz–H2O system produced by calcinations of calcium carbonate at hydrothermal treatment at 200°C and in the products 1000°C for 6 hours; NaOH solution (Delta Chem, dominate 1.13 nm tobermorite which is hard to disrupt Czech Republic; c = 1.04 %); γ-Al2O3 (Lachema, Czech (Figures 1a, c, e). Republic;) obtained by heating Al(OH)3 for 5 hour at The dehydration of 1.13 nm tobermorite with C/S = 2 550°C (Sa = 712 m /kg). 0.66 proceeds in a different way than 1.13 nm tober- The molar ratios of primary mixtures were C/(S + A) morite with C/S = 0.83. The latter has characteristic = 0.66 and A/(S + A) = 0 or 0.025 (C–CaO, S–SiO2, sharp endothermic effect at 220°C, which is related to A–Al2O3). The amount of NaOH, corresponding to 5 % the loss of 4.5 molecules of water. Meanwhile, in the of Na2O from the mass of dry materials, added in the sample with C/S = 0.66, this effect was not observed form of solution and additionally added necessary quan- and analogical quantity of water (~6-7 %) is loosed in tity of water that water/solid ratio of the suspension was the temperature range from 125 to 280°C (Figure 1c). γ equal to 10.0. Hydrothermal synthesis has been carried In the presence of 2.6 % -Al2O3 in the out in rotating autoclave A-08 (10 rpm) under the satu- CaO–quartz–H2O mixture, already after 72 h of isother- rated steam pressure at 200°C temperature; the duration mal curing, a smaller quantity of 1.13 nm tobermorite of isothermal curing was 2, 4, 6, 8, 16, 24, 32, 48, or 72 forms, because in XRD pattern basic reflection with hours. The products of the synthesis was filtered, rinsed d-spacing-1.13 nm was less intensive as compared with with ethyl alcohol to prevent carbonization of material, a pure system (Figure 1b). Furthermore, the low-inten- dried at a temperature of 100°C ± 5, and sieved through sity basic reflection with d-spacing-2.2 nm shows the a sieve with a mesh width of 50 µm. beginning of gyrolite gel formation. The X-ray powder diffraction (XRD) data were co- The DSC analysis confirmed the results obtained llected with DRON-6 X-ray diffractometer with Bragg- by XRD: the DSC curve exhibits the exothermic peak at Brentano geometry using Ni-filtered Cu Kα radiation 861°C, characterizing the transformation of gyrolite gel and graphite monochromator, operating with voltage of into wollastonite and the endothermic effect at 224°C, 30 kV and emission current of 20 mA. The step-scan characteristic to 1.13 nm tobermorite (Figure 1d). covered the angular range 2-60° (2θ) in steps of 2θ = As far as we know it is the first evidence of positive = 0.02°. For diffraction profile refinement under the influence of γ-Al2O3 on gyrolite synthesis because in the pseudoVoigt function and for description of the diffrac- unstirred suspensions this additive stimulates the forma- tional background under the 3rd degree Tchebyshev tion of C–S–H(I), calcium aluminum silicate hydrates, polynom, we used a computer program X-fit [16]. 1.13 nm tobermorite and xonotlite [11, 12]. Simultaneous thermal analysis (STA: differential Already in 1974, Taylor [17] noted that at tempera- scanning calorimetry-DSC and thermogravimetry-TG) tures higher than 150°C tobermorite is thermodynami- was also employed for measuring the thermal stability cally unstable in the mixtures with C/S > 0.8 and trans- and phase transformation of synthesized products at a fers to xonotlite. The results of the tests performed by us heating rate of 15°C/min, the temperature ranged from showed [11, 12] that in the unstirred suspensions this 30°C to 1000°C under air atmosphere. The test was car- tendency proves out also in the mixture with lower C/S ried out on a Netzsch instrument STA 409 PC Luxx. The ratio according to the stoichiometry of gyrolite: tober- ceramic sample handlers and crucibles of Pt-Rh were morite transfers into xonotlite when the synthesis is pro- used. ceeding for 48 h. Scanning electron microscopy (SEM) (Oxford ISIS Moreover, in the stirred suspensions and after pro- Leo 440 i) coupled with energy dispersive X-ray spec- longed synthesis (72 h), 1.13 nm tobermorite becomes trometer (EDS) was performed using a accelerating unstable and transfers into a compound with the stoi- voltage of 20 kV and a working distance of 10 mm for chiometry corresponding to the molar ratio of the pri- SEM observation and a 200 s accumulation time for mary mixture - gyrolite. EDS analysis. Scanning electron microscopy (SEM) showed that The specific surface area of the raw materials was in the products with γ-Al2O3 are much shorter, needle- determined by the Blaine's method with air permeabili- shaped tobermorite crystals comparing with the pure ty apparatus (Model 7201, Toni Technik Baustoffpruf- system (Figures 1e,f). Meanwhile, between and near systeme GmbH). these crystals, there are very small plate crystallites characteristic to gyrolite gel.