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THERMODYNAMIC ANALYSIS OF TOBERMORITE FORMING REACTION IN THE LIME-SAND MATERIALS OF AUTOCLAVED SOLIDIFICATION
M.V. Kaftayeva, I.Sh. Rakhimbayev
ТЕРМОДИНАМИЧЕСКИЙ АНАЛИЗ РЕАКЦИИ ОБРАЗОВАНИЯ ТОБЕРМОРИТА В ИЗВЕСТКОВО-ПЕСЧАНЫХ МАТЕРИАЛАХ АВТОКЛАВНОГО
ТВЕРДЕНИЯ
М.В. Кафтаева, И.Ш. Рахимбаев
Abstract. The thermodynamic analysis of the Tobermorite ligament synthesis of Hydrous silicates of autoclaved solidification has been made. Using the Born-Haber cycle, the activities of calcium ions, silicic acid and hydroxyl have been calculated in the liquid phase of the system CaO -SiO2 - H2O at 25 and 190 ° C, and also their verification by comparison with experimental data has been made. It has been shown, that the limiting stage of the process of Tobermorite synthesis is the calcium hydroxide and silicon dioxide dissolution. On this basis, the substantiation of methods of its intensification has been given.
Keywords: tobermorite; autoclaving; ions activity; chemical thermodynamics.
Аннотация. Произведен термодинамический анализ синтеза тоберморитовой связки гидросиликатов автоклавного твердения. С использованием цикла Борна-Габера рассчитаны активности ионов кальция, кремнекислоты и гидроксила в жидкой фазе системы СаО - SiО2 -H2O при 25 и 190 °С, а также произведена их верификация путем сравнения с экспериментальными данными. Показано, что лимитирующей стадией процесса синтеза тоберморита является растворение гидроксида кальция и диоксида кремния, на этой основе дано обоснование методов его интенсификации.
Ключевые слова: тоберморит; автоклавная обработка; активность ионов; химическая термодинамика.
Dense and cellular silicate materials of autoclaved solidification are widely used almost 100 years in the construction industry all over the world, including in Russia. In our country, the producing of energy-efficient autoclaved solidification porous concrete of new generation by D 300 - D 500 of average density is increasing the most intensively [1].
The phase composition of interporous partitions has great influence on the physical and chemical characteristics and durability of cellular concrete. The researches of a number of domestic and foreign scientists are devoted to its analysis, but on this issue there are some contradictions and ambiguities. One of the reasons is the data that the most important hydrous silicates of calcium, that make the material of interporous partitions in autoclaved aerated concrete - tobermorite and xonotlite - during external conditions changings are prone to mutual transformations.
At the first time, results of thermodynamic calculations for the reactions of tobermorite, xonotlite and their mutual transformations formation at 25 - 175 ° C were presented in the book [2]. In the work [1] it is noted, that experimental data have been provided, which indirectly confirmed the possibility of converting xonotlite into tobermorite at 25°C and the reverse reaction at 180 -200°C. However, the authors' calculations [2] have been carried out on the basis of outdated initial data, and in the work [1] there are no results of X-ray diffraction analysis of cement stone samples.
This work is devoted to a further comprehensive review of the issue.
The thermodynamic analysis of the system CaO - SiO2 - H2O is made by authors by the method described in works [1, 3, 4] using initial data [3, 5].
Let's perform the thermodynamic analysis of reactions of Tobermorite synthesis from a mixture of calcium hydroxide and quartz sand at temperature 25° C and 190 ° C using the BornHaber cycle [3].
AGi
5Ca(OH)2(soiid) + SiÜ2(soiid). + 0,5H2Ü(iiquid) ^ 5CaO • 6SiO2-5,5H2O(soiid).
AG2 \ ^ AG3 (1)
2+ • - -
5Са (solution) + 6HSiO3 (solution) + 4OH (solution) + 0,5H2Ü(liquid)
Let's calculate the thermodynamic effects of the separate stages of the process, keeping in mind that AGi is the free energy change at the synthesis of Tobermorite from calcium hydroxide and quartz, AG2 is the free energy of these materials' dissolution with calcium ions, metasilicate ions and hydroxyl groups forming in the solution, AG3 is the free energy of Tobermorite crystallization from the noted ions and water.
AG1 - AG(Tobermorite) - 5AGCa(OH)2 - 6AGSi02 - 0,5AGH2O -
= -2361,5 + 5-214,4 + 6^240,7 + 0,5-56,7 - -32,65 kcal - - 136,699 kJ; AG2 - 187,99 kJ; AG3 - - 324,69 kJ.
The accuracy of calculation in the limits of adopted initial data is confirmed by the fact that AG2 = AG3 - AGi, and AG3 = AGi - AG2.
The result of calculation shows that the reaction (1) is possible at room temperature. In practice, the reaction occurs only in a monomolecular superficial layer, and diffusion limitations in the volume at temperature 25°C are superimposed on its going.
To calculate the thermodynamic effect of the reaction (1) at 463 K (190°C) - the temperature at which the new generation of hydro silicate products are mainly produced in the Russian Federation, we use the method described in [1, 3].
To do this, we calculate the enthalpy of reaction at temperature 298 K.
AH1 = AH(Tobermorite). - 5AHCa(OH)2 - 6AHSi02 - 0,5AHH2O = - 163,285 kJ
Further calculations are made in kcal.
We accept the following values of the heat capacity Cp of the reaction components (1) keeping in mind that Cp = a + b 10-3E 105E-2.
C5S6H5,5 : 110,6 + 189,0-10-3E;
Ca(OH)2 : 19,07 + 10,08-10"3E;
H2O : 12,65 + 11,38^10"3E + 1,7-105E"2;
SiO2: 11,22 + 8,20^10"3E + 2,7^ 105E-2.
Using these data, we'll calculate the change of heat capacity for reaction (1):
Aa = 110,6 - 5^19,07 - 6 11,22 - 0,5-12,65 = -58,43; Ab = (189 - 5^ 10,8 - 6^8,2 - 0,5 • 11,22) 10-3 = 80,19a0"3; Ac = (6-2,7 - 0,5^1,7)105 = 15,35^ 105. AH0 = AH0298 - AaE + Afr0,5^10"3E2 +Ac^ 105E"1 = -20,0064 kcal/mol = -83,76 kJ/mol
The next calculation is shown above: of AG0298 = -3 9 0 00 kcal. This value can also be calculated by the formula:
AG°298 = ДН0 - Aа•298•ln298 - 0,5Ab 10"3-2982 - 0,5Ас^ 103^298-1 + х^298,
where x is a constant of integration.
AG0298 = -20006,4 + 99146,0 - 3557 - 2575,5 + 298-х; х = -375,86.
Now we can calculate the free energy change for the Tobermorite synthesis reaction at temperature 463 K.
AG0463 = АН0 - Да•463•ln463 - 0,5-80,19-10"34632 - 0,5^15,35^105^298-1 -- 375,86463 = -37,8 kcal = - 158,26 kJ
This value is greater than at 25°C (-136,699 kJ), but not by much.
Tobermorite synthesis is also possible by other scheme, forming not ion HSiO3-(metasilicate), but H2SiO42- (orthosilicate) during dissolving SiO2 in the liquid phase:
AG1
5Ca(OH)2(solid) + SiO2(solid) + 0,5H2O0iquid) ^ 5CaO • 6SiO2-5,5H2O(solid)
AG2 \ AG3 (2)
5Са2+ + 5H2SiO42- + SiO2 + 0,5H2O0iqUid)
AG1 = -136,699 kJ; AG2 = 68,035 kJ; AG3 = -204,735 kJ.
Absolute values AG2 and AG3 by schemes (1) and (2) are different, but they do not provide unambiguous information to speak about the advantage of any scheme. More information about the process can be obtained by calculating the activity of ions in the liquid phase of the system and the degree of Tobermorite supersaturation in them.
The analysis of numerical values AG2 and AG3 of reactions (1) and (2) indicates that the limiting step of the processes is the Ca2+solution cations and hydrosilicate anions formation in the liquid phase, since during this there is a growing of isobaric-isothermal potential of the system, which is then compensated by a sharp decrease of the last one, during interaction of these ions with the formation of calcium hydro silicate in the solid condition.
Growing of -AG system according to scheme (2), when the orthosilicate-ion H2SiO42"solution is formed in the liquid phase, is less than according to scheme (1) with forming the metasilicate-ion HSiO3"solution. At the same time, the decreasing AG3 - free energy of Tobermorite crystallization from the liquid phase according to scheme (1) is greater.
In work [4] the correlation between the decrease of free energy during dissolving anhydrous (source) binder and the speed of structure forming (setting) of cement paste was shown. From that it follows that according to scheme (2), this figure will be higher.
For more additional information about the features of Tobermorite synthesis according to schemes (1) and (2) we'll calculate the ions activity in the liquid phase of lime-sand binder.
The calculation of free energy of the Tobermorite synthesis process at temperature 488 K can be done another way. To do this, we need to calculate the values AG0488 of all components of Tobermorite synthesis reaction and then to calculate AG0P of this reaction according to scheme shown above.
First we'll calculate AG0463 for Ca(OH)2.
AG0298 = - 8 97,65 kJ/mol; AH%8 = -985,28 kJ/mol, C„ = 19,07 + 10,8^0"3E;
AH0 = -235330 - 19,07^298 - 0,5-10,8-298M0"3 = -241492,4;
AG0298 = -241492,4 - 19,07-298-5,697 - 10,8-0,5-2982-10"3 + 298-x = -241492,4 - 32375,25 -
479,5 + 298^ = -27437,1 + 298-x;
298 • х = 274347,2 - 214400 = 59947,1; х = 201,16. AG0463 = -211492,4 - 19,07 •463- 6,14 - 5,4• 4632 •Ш"3 + 20116 •463 = = -203,7 kcal / mol = -852,85 kJ/mol.
Initial data for calculation AG0463 for P-quartz:
AG0298 = -8 5 7,25 kJ/mol; AH0298 = -911,68 kJ/mol Ср = 11,22 + 10,8 •10-3E - 2,7 •105E-2;
AH0 = -2223,6 - 3343,6 - 364,0 + 453 + 298х = -241771,8;
AG0298 = -2223,63 - 19048,3 - 364 + 453 + 298х = -241771,8; х = 124,233;
AG0463 = AH0 - Aa • 463 • ln463 - 0,5Ab -10"3 • 4632 - 0,5Ac • 105 •463-1 + 124,25 • 463 = = -222363,6 - 976,4 + 60634,0 = -198,6 kcal/mol = -831,5 kJ/mol.
For water at temperature 463 K on the reference data [5] we accept AG0T = -51,0 kcal/mol (213,53 kJ/mol), and for hydroxyl group -25.5 kcal / mol (106,76 kJ/mol).
AG°463 of Tobermorite requires the special calculation [2]. We are based on the following reference data for 298 K, kcal / mol: AG0(Tobermorite) = -2361,5;
AH0 = AH0298 - a • 298 + 0,5b • 2982-10"3 = -2594897,8 cal;
AG0298 = -2361500 = -2594897,8 - 110,6 •298 • 5,627 - 94,5 -10"3 • 2982 + 298 • х;
х = 1441,4.
AG0463= 2262,2 kcal/mol = 9471,38 kJ/mol.
The above provided numerical values of isobaric-isothermal potentials of Tobermorite, quartz and calcium hydroxide at elevated temperature, are not only needed for further calculations, but could help to refill the reference base by calcium hydro silicates.
Now we can calculate the thermodynamic effect of Tobermorite synthesis reaction at temperature 463 K.
AG0463 = AG0(Tobermorite) - 5AG%2 - 6AG0SiO2 - 0,5AG0h2o = -2262,2 + 5 •203,7 + 6 198,3 +
+ 0,5 • 51 = -28,4 kcal/mol = 118,91 kJ/mol.
As a result of the previous calculation the received value is - 126,44 kJ/mol, which is satisfactory in agreement with the given AG0463 of Tobermorite.
The advantage of the second method of calculation is that using known and original data of numerical values of the isobaric-isothermal potentials of quartz, calcium hydroxyl, Tobermorite, and also of a number of ions in aqueous solutions at elevated temperatures [5] it is possible, by applying the Born-Haber cycle, to calculate the activity of ions in a liquid phase, which is in equilibrium with the system calcium hydroxide + quartz, and with Tobermorite. These calculations are below.
From cycle (1), we find:
AG2(463) = 5 • 203,7 + 6 198,3 - 5 • 132,7 - 6 •226,8 - 4• 25,5 = 82,0 kcal = 343,32 kJ;
AG3(463) = 5 • 132,7 + 6 •226,8 + 4•25,5 + 25,5 - 2262,2 = -110,4 kcal = - 462,22 kJ;
AG1 = AG2 + AG3 = -118,9 kJ.
We'll calculate now the ionic compositions of solutions, which are in equilibrium with the source materials (on the basis of AG2(463)) and with Tobermorite using AG3(463).
AG2 = 343,32 kJ; lg K2 = -AG2/RE = -38,7; K2 = [Ca2+]5 • [ШЮ3"]5 • [OH-]5 = [Са2+]15;
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^[Са 2+] = -2,95; [Ca2+] = 1Д22-10"3 mol/l = 0,063 g/l CaO. [HSiO3-] = [Ca2+] = 1Д22-10"3 mol/l = 0,67 g/l SiO2. [OH-] = [Ca2+] = 1,22-10"3 mol/l; рН = 11,0
where K2 is the equilibrium Constanta.
Next, we'll calculate the composition of the solution, which is in equilibrium with Tobermorite.
AG3 = -119,7 kcal = 501,16+ kJ; lg K3 = -AG3/RT = -56,46; lg[Ca2+p] = -3,76; [Ca2+] = 1,74-10"4 mol/l = 0,01 g/l CaO.
[HSiO3-p] = 1,74-10"4 mol/l = 0,01 g/l SiO2.
[OH"p ] = 1,74-10"4 mol/l; pH = 10,2
where K3 is the equilibrium Constanta.
The obtained numerical value of the calcium ions' activity, which are in equilibrium with the mixture of Ca(OH)2 + SiO2 (0,063 g/l CaO), is in good agreement with the value of Ca(OH)2 solubility at temperature 463 K, which equals 0.08 g/l, considering that the activity a is somewhat lower than the concentration of solution, because c = a/f, where f < 1.
On the question of Tobermorite's solubility value at high temperatures there are only the data of such scientists as Flit, Mac-Nerd and Walles (0.033 g/l CaO) at temperature 150 °C [7]. This value satisfactory agrees with the calculation result, if to consider that at 190 °C the solubility of Ca(OH)2 is three times lower than at 150 °C.
A similar calculation will be made for the scheme (2), where it is assumed that in the liquid phase instead metasilicate the ion H2SiO42"solution is formed.
AG°1 = AG0(Tobermorite) - 5AG°Ca(OH)2 - 6AG0SiO2 - 0,5AG0h2o = -2262,2 + 203,7-5 + 198,3-6 +
57-0,5 = -28,4 kcal = 118,91 kJ;
AG2 = 376,81 kJ; AG3 = -495,72 kJ.
Now we'll calculate the activity of ions, which are in equilibrium with a mixture:
Ca(OH)2 + SiO2 at 463 К.
AG2 = 376,81 kJ; lg K2 = -AG2/RE = -42,4; K2 = [Ca2+]5 • [H2SiO42-]5 = [Са2+]10; ^[Са2+] = -4,24; [Ca2+] = 5,75-10"5 mol/l = 0,00322 g/l CaO. [H2SiO42-] = [Ca2+] = 0,00345 g/l SiO2.
where K2 is the equilibrium Constanta.
Then we'll calculate the activities of Ca2+ ions, which are in equilibrium with Tobermorite.
AG3 = -495,72 kJ; lg K3 = -AG3/RE = -55,8; ^[Са2+] = -5,58; [Ca2+] = 2,6340"5 mol/l = 0,00147 g/l CaO
where K3 is the equilibrium Constanta.
The obtained values are not in agreement with known experimental data, therefore, we accept the reaction scheme of Tobermorite formation, according to which the meta silicate-ion HSiO3- is formed in the liquid phase.
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The confirmation of this conclusion is the point, that the activity of calcium ions, which are in equilibrium with the mixture of Ca(OH)2 + SiO2 equals 0,067 g/l CaO, and the solubility of Ca(OH)2 at temperature 190 °C is 0.08 g/l of CaO. This can be considered a good match, because a = c/f, where fD 1 is the activity coefficient of ions in solution.
The analysis of the calculations' results leads to the following conclusions:
- The reaction of calcium hydroxide and quartz interaction with Tobermorite formation is accompanied by a decrease of Isobaric-isothermal capacity of the system Ca(OH)2 - SiO2 - H2O, already at room temperature.
- Raising the temperature up to 190°C does not lead to additional reduction of AG0. This result is in agreement with the hypothesis that one of the reasons of sharp acceleration of the reaction between Ca(OH)2 with fine-grinded quartz is the Headwall effect, due to the process quartz ^ low-temperature tridymite [1].
- The stage, controlling the reaction of Tobermorite formation, is the dissolution of the solid source components of the system, which is accompanied by increasing of its AG0. In connection with this, the additives that increase the content of Ca2+ ions and silicate anions in the pored fluid, are the accelerators of silicate products' hardening, and as a result, the total thermodynamic reaction's effect has a negative sign. They include some fuel ashes, blast furnace slags, pozzolana, electrolytes.
- The reaction of Tobermorite synthesis through the solution is possible with the formation of anions HSi03 or H2Si042- in liquid phase. The principles on the basis of which we can choose one of the schemes, are not quite clear yet. Some considerations show that the first variant is more likely, however, it apparently is an intermediate stage, and the finishing stage corresponds to the scheme (2).
1. Kaftaeva M.V. Theoretical Substantiation of Improving the Autoclave Technology of Energy Efficient Gas Silicates Production: The Doctor's of Technical Sciences Dissertation. Belgorod: BSTU, 2013. 296 p.
2. Babushkin V.I., Matveyev O.P., Mchedlov-Petrosyan G. M. Thermodynamics of Silicates. M.: Stroyizdat, 1962. 352 p.
3. Rakhimbaev Sh.M. About the Calculation of Effective Charges of Ions by the Thermochemical Data // Journal of Physical Chemistry, XXXIX, 1965. №. 2, P. 352-355; XL, 1966, No. 12, P. 3089-3091.
4. Rakhimbaev Sh.M. The Regulation of the Technical Properties of Grouting Mortars. Tashkent: Nauka, 1976. 168 p.
5. Naumov G.B., Ryzhenko B.N., Khodakovsky I. L. Reference of the Thermodynamic Values. M.: Atomizdat, 1971. 240 p.
6. Volzhensky A.V., Ivanov I. A., Vinogradov B. N. The use of Ashes and Fuel Slags in Building Materials Industry. M.: Stroyizdat, 1987. 256 p.
Кафтаева Маргарита Владиславна ТОО «Базальт-А», г. Кандыагаш, Актюбинской обл., д-р техн. наук, доцент, Зам. директора по науке.
E-mail: [email protected]
LITERATURE
ИНФОРМАЦИЯ ОБ АВТОРАХ
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2016, Т 2, №4
Рахимбаев Игорь Шаркович ФГОУ ВО «Белгородский государственный технологический университет им. В.Г. Шухова», г. Белгород, Россия, Центр высоких технологий, канд. техн. наук, инженер-исследователь.
E-mail: [email protected]
Kaftaeva Margarita Vladislavna LLP «Basalt-A», Dortor of Technical Sciences, Associate Professor, Deputy Director on science.
E-mail: [email protected]
Rakhimbayev Igor Sharkovitch FSEI HE «Belgorod State Technological University, named after V.G. Shukhov», Belgorod, Russia, High Technologies Center, PhD, Research Engineer. E-mail: [email protected]
Корреспондентский почтовый адрес и телефон для контактов с авторами статьи: 308012, РФ, г. Белгород, ул. Костюкова д. 36а, кв.38, +7 702 505 28 16
Address: 308012, St. Kostyukova, Building 36а, Apartment 38. Belgorod, Russia.
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