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1

Influence of CaO reactivity on the formation of low-base calcium silicate hydrates

Baltakys K., Prichockiene E.

Materials Science Poland

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2010

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Vol. 28, No. 1

295--304

EN

The influence of CaO reactivity on the formation of calcium silicate hydrates in the CaO-SiO2onH2O-H2O system has been determined when the CaO/SiO2molar ratio was equal to 0.83 and 1.0. In a pure CaO-SiO2onH2O-H2O system, with CaO/SiO2 = 0.83, C-S-H (I) is rather stable, and at 175 °C, even after 72 h, only partially recrystallizes into 1.13 nm tobermorite. Meanwhile, in the mixtures with less reactive CaO (96%), the stoichiometric composition (CaO/SiO2) of products of the synthesis varies from 0.66 to 0.83. When the CaO/SiO2 molar ratio in the initial mixture is equal to 1.0, the CaO reactivity has a significant effect on the composition of products forming in hydrothermal conditions. The products of the syntheses were characterized by X-ray diffraction analysis, simultaneous thermal analysis and Fourier transform infrared spectroscopy.

2

The sorption of copper ions by gyrolite in alkaline solution

Bankauskaite A., Baltakys K.

Materials Science Poland

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2009

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Vol. 27, No. 3

899--908

EN

Sorption of copper ions by synthetic pure gyrolite (CaO/SiO2 = 0.66; 96 h; 200 °C) in alkaline solutions was examined. When the initial concentration of Cu2+ ions is 1 g/dm3, the cation exchange capacity of the gyrolite is 100 mg Cu2+/g after 360 min of sorption at 25 °C. It has been proved that the cation exchange capacity of gyrolite depends on the concentration of copper ions in the Cu(NO3)2 solution, because a fivefold increase of the concentration (from 1.0 to 5.0 g/dm3) reduces the duration of the ion exchange reaction by a factor of three (from 15 min to 5 min). In solutions with higher initial concentrations of Cu2+ ions (10.0 and 20.0 g/dm3), the sorption proceeds more intensively and all the copper ions are adsorbed in 1 min. It should be noted that the cation exchange reactions are reversible in alkaline solution, because nearly 90% of the copper ions are adsorbed in a physical process, while the remaining part of the cation exchange process takes place in a chemical reaction. Furthermore, the crystal structure of gyrolite is stable in alkaline solution. The products of sorption were characterized by X-ray diffraction, thermogravimetry-differential scanning calorimetry and Fourier-transform infrared spectroscopy methods.

3

Physically and chemically bound H2O in the gyrolite structure

Baltakys K., Siauciunas R.

Materials Science Poland

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2009

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Vol. 27, No. 1

255--263

EN

Synthetic pure gyrolite prepared under hydrothermal conditions (32 h, 200 °C) from a stoichiometric composition (CaO/SiO2 = 0.66) of calcium oxide and a fine-grained SiO2onH2O mixture can easily adsorb water vapour. The equilibrium quantity of adsorbed water vapour was estimated from desorption thermograms. It was determined that the thermal effect of desorption is close to the steam-heat of water vapour at temperatures up to 175 °C. This part of water vapour adsorption can be described by a physical adsorption model, only a small quantity of water vapour being chemisorbed. The crystal structure of gyrolite under a pressure of water vapour and at low temperatures (-197 °C) was stable.

4

Influence of gypsum additive on the formation of calcium silicate hydrates in mixtures with C/S = 0.83 or 1.0

Baltakys K.

Materials Science Poland

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2009

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Vol. 27, No. 4/1

1091--1101

EN

The influence of gypsum additive on the formation of calcium silicate hydrates and on the formation of a sequence of intermediary compounds in the CaO-SiO2onH2O-H2O system has been examined. The synthesis was carried out in unstirred suspensions. The molar ratios of primary mixtures were CaO/SiO2 = 0.83, 1.0 and SO3/(SiO2 + SO3) = 0, 0.0125, 0.025. The durations of isothermal curing at 200 °C were 4, 8, 16 and 72 h. It was established that sulfate ions improve the synthesis of 1.13 nm tobermorite in the CaO-SiO2onH2O-H2O system with CaO/SiO2 = 0.83 at 200 °C. However, a larger amount of gypsum (SO3/(SiO2 + SO3) = 0.025) stimulates the formation not only of 1.13 nm tobermorite but also of CaSO4 and xonotlite. In the CaO-SiO2onH2O-H2O system with CaO/SiO2 = 1.0, a small amount of sulfate ions prolong the persistence of intermediate compounds - 1.13 nm tobermorite and C-S-H (I). The products of synthesis were characterized by X-ray diffraction analysis, simultaneous thermal analysis and Fourier transform infrared spectroscopy.

5

Formation and stability of C-S-H (I) of various degrees of crystallinity in the Ca(OH)2/CaO-Hi-Sil-H2O system

Baltakys K., Jauberthie R.

Materials Science Poland

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2009

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Vol. 27, No. 4/1

1077--1089

EN

The stability and crystal morphology of C-S-H (I) in the CaO/Ca(OH)/2-Hi-Sil-H2O systems and a sequence of formation of the intermediary compound has been examined. Two series of samples, with different ratios of CaO to SiO2were used in the experimental tests. The molar ratio CaO/SiO2 of primary mixtures was 1.0. Hydrothermal synthesis was carried out under the saturated steam pressure at temperatures of 90, 110, 130, 150 and 175 °C; the duration of isothermal curing was 2-72 h. It was determined that an increase of temperature significantly influences the stability, morphology and degree of crystallinity of C-S-H (I). It was found that C-S-H (I) is stable in the ranhe 90-130 °C. The overall morphology of C-S-H varies from the common fibrous type to irregular grains forming a reticular network. The compositions of intermediate products and of final products can be changed by adding calcium containing components (Ca(OH)2, CaO). In the mixtures with Ca(OH)2, an intermediate compound is C-S-H (I) and the final product is xonotlite. Meanwhile, in the mixtures with CaO, the main intermediate compounds are Z phase and C-S-H (I), and the main product is gyrolite.

6

Surface microstructure and specific surface area of pure and Na-substituted gyrolite

Baltakys K., Siauciunas R., Kitrys S.

Materials Science Poland

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2008

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Vol. 26, No. 3

633--645

EN

Pure gyrolite was synthesized within 32 h at 200 °C from a stoichiometric composition (the molar ratio CaO/SiO2 was equal to 0.66 with water/solid ratio of the suspension equal to 10.0) of the initial CaO and SiO2źnH2O mixture. Meanwhile, Na-substituted gyrolite was synthesized twice quicker – within 16 h at 200 °C. These compounds were characterized by XRD, STA, FT-IR, SEM/EDS and BET analysis. It was found that gyrolite is a mesoporous material. Its specific surface area SBET = 143.15 m2/g, the radius of dominant plate pores rp = 80–90 Å, the calculated total pore volume ∑Vp = 0.661 cm3/g. Gyrolite texture changes upon introducing Na+ ions into its crystal structure: the specific surface area SBET diminishes to 27.24 m2/g, the radius of dominant cylindric pores rp = 60–70 Å, the calculated total pore volume #&8721Vp = 0.118 cm3/g.

7

Gyrolite Formation in CaO–SiO2źnH2O–gamma-Al2O3–Na2O–H2O System under Hydrothermal Conditions

Baltakys K., Siauciunas R.

Polish Journal of Chemistry

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2007

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Vol. 81, nr 1

103-114

EN

The influence of gamma-Al2O3 and Na2O additives on the gyrolite formation process in the CaO–SiO2źnH2O–H2O mixture has been examined. It has been proved that gamma-Al2O3 is not recommended for the synthesis of gyrolite, because Al3+ ions stimulate the formation of C-S-H(I) and 1.13 nm tobermorite (where C – CaO, S – SiO2 and H – H2O). Na2O positively affects the formation of gyrolite: this compound is formed already after 8 hours of isothermal curing at 200 graduate C. On the contrary, in the pure mixtures gyrolite formed after only 32 h at 200 graduate C. In order to compare the cation exchange activity of synthesized gyrolite with (Al+Na)-substituted tobermorite, the ion exchange experiments were employed as a test reaction.

8

The influence of gamma-Al2O3 and Na2O on the formation of calcium silicate hydrates in the CaO-quartz-H2O system

Baltakys K., Siauciunas R.

Materials Science Poland

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2007

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Vol. 25, No. 1

185--198

EN

The influence of gamma-Al2O3 and Na2O additives on the formation and sequence of calcium silicate hydrates in the CaO-quartz-H2O system with CaO/SiO2 (C/S) molar ratio equal to 0.66 was determined. The synthesis was carried out in unstirred suspensions. The molar ratios of the primary mixtures were CaO/(SiO2 + Al2O3) = 0.66 and Al2O3/(SiO 2 + Al2O3) = 0 or 0.025. The amount of NaOH, corresponding to 5% of Na2O in the mass of dry material, was added in the form of solution. Due to the low solubility rate of quartz in the CaO-quartz-H2O system with C/S = 0.66 in the temperature range 150-200°C, low-base calcium silicate hydrates (Z-phase, gyrolite, pectolite, and others) do not form even after 72 hours of hydrothermal curing. It has been proved that gamma-Al2O3 and Na2O additives change the reaction mechanisms and composition of intermediate (final) products. In mixtures with Na2O, gyrolite starts to form already at 175°C. It should be noted that in the temperature range 175-200°C the final products are gyrolite and pectolite, and the intermediate compounds are C-S-H(l) and Z-phase. The formation of low-base calcium silicate hydrates in the CaO-quartz-Na2O-H2O system is slowed down when Al 2O3 is added because Al3+ ions stimulate the formation of tobermorite and prevent the formation of pectolite and Z-phas

9

Formation of gyrolite in the CaO-quartz-Na2O-H2O system

Baltakys K., Siauciunas R.

Materials Science Poland

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2007

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Vol. 25, No. 4

1089--1100

EN

Optimizing the duration and/or the temperature of hydrothermal synthesis of gyrolite has been investigated by adding NaOH solution into an initial mixture of CaO-quartz-H2O. The molar ratio of the primary mixture was C/S = 0.66 (C - CaO; S - SiO2). An amount of NaOH, corresponding to 5 % Na2O from the mass of dry materials, added in the form of solution and additional water was used so that the water/solid ratio of the suspension was equal to 10.0. Hydrothermal synthesis of the unstirred suspension was carried out in saturated steam at 150, 175, 200 oC. The duration of isothermal curing was 4, 8, 16, 24, 32, 48, 72 and 168 h. The temperature of 150 oC is too low for the synthesis of gyrolite; the stoichiometric ratio C/S = 0.66 is not reached even after 168 h of synthesis neither in pure mixtures nor in mixtures with addition of Na2O. Na+ ions significantly influence the formation of gyrolite from the CaO-quartz mixtures in the temperature range from 175 oC to 200 oC. Gyrolite is formed at 175 oC after 168 h and at 200 oC after 16 h of isothermal curing. On the contrary, in pure mixtures it does not form even after 72 h at 200 oC. Na+ ions also change the compositions of intermediate and final products of the synthesis. In mixtures with 5% Na2O, intermediate compounds are C-S-H(I) and Z-phase, and the final products are gyrolite and pectolite. Meanwhile, in mixtures without this additive, the main intermediate compounds alfa-C2S hydrate and C-S-H(II), and the main products are 1.13 nm tobermorite and xonotlite.