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УДК 54.022+539.193
MODELLING OF HYDRATATION OF CALCIUM SULFATE HEMIHYDRATE
N.V. KHOKHRIAKOV. G.I. YAKOVLEV. V.l. KODOLOV
АННОТАЦИЯ. Физико - химические исследования показывают, что использование ионизированной воды для гидратации полуводного гипса приводит к образованию двуводного гипса различной морфологии. Так, в щелочной среде получаются аморфные образцы, в нейтральной - призматические кристаллы, в кислой - волокнистые образования[1]. В работе исследуется структура этих материалов и развиваются модели их роста.
EXPERIMENTAL
The study of the structure, composition and sizes, and morphology of new-formations is carried out using the complex of physico-chemical investigation methods including X-ray photoelectron spectroscopy and X-ray structure analysis. To study the morphology of new-formations the investigations of hardened calcium sulfate dihydrate microstructure are carried out using raster electron microscope "Stereoskan S4-10" in the condition of secondary electrons (Fig. 1).
The changes in the structure of calcium sulfate dihydrate are registered. When tempered by neutral water (Fig. la) the binder structure is shown by conventional crystal new-formations in the form of oblong prisms. When using water with pH = 10,5 the formation of the layer with non-evident structure about 0,3 mm thick is observed. The new-formation structure on the sample is different because the binder is mainly represented by new-formations with side ratio 1:2
Fig. 1. Microphotographs (* 1200) of calcium sulfate dihydrate tempered in: a) - neutral water, b) - alkaline water with pH = 10,5 c) - acid water with pH = 2,5
their surface does not have vivid contours (Fig. lb). When tempered by water with pH = 2.5 the fibrous structure is formed (Fig. lc).
The XPS spectra of samples obtained by tempering in the water of different acidity are presented in the table 1.
ХИМИЧЕСКАЯ ФИЗИКА И МЕЗОСКОПИЯ. Том 2, № 2
N. V. K.HUK.HRIAKOV, G.I. YAKOVLEV, V.I. KODOLOV
Table 1. Values of bond energy (Eb), signal intensity (1) and relative concentration of element atoms (NC|) on the surface of crystal-hydrates formed during tempering by water ionized in electric field
Chemical pH = 2,5 pH = 7,2 pH= 10,5
element Eb, eV I, imp./s Nei, % Eb, eV I, imp./s Nel, % Eb, eV I, imp./s Net, %
Cls 285 640 4 285 430 2 285 780 13
Ols 532 5040 66 532 6000 70 532,1 1020 58
S2p 169,1 620 5 169,2 680 5 169,3 150 5
Ca 2p 3/2 347,6 1400 25 347,7 1560 23 348,0 330 25
THE STRUCTURE OF TWO WATER GYPSUM OBTAINED IN ALKALINE WATER
In the XPS spectrum obtained during the investigation of sample surface tempered by water with pH=lO,5 (Table 1), the shift of bond energy for calcium atoms with 347,7 eV up to 348 eV is registered. Simultaneously, relative carbon concentration increased by 11% and oxygen concentration decreased.
To find out the mechanism of water pH value influence on the structure of compounds being formed, the calculations are done by Hartree-Fock method in TZV* basis 111. During the calculations program product Gamess is used /3/. Energies of optimised complexes, which can be present in hydrated clusters, are shown in Table 2. Some of the optimised structures are shown in l:ig.2.
^^^ Calcium
0 Sulfur
^^ Oxygen
Q Hydrogen
Ocd
Fig. 2. The complexes being present in alkaline water tempered gypsum, a) - CaS04, b) - Ca(OH)2, c) - CaOH~, d) - H2S04, e) - HS04\ f) - S04'2
mm** * wesocKonMR TOM 2, N» 2
Table 2: Energies of ions, molecules and complexes present in calcium sulfate dihydrate tempered in an alkaline media (Hartree)
Compound Energy Compound Energy
CaS04 -1373.79 SO42- -696.92
Ca(OH)2 -827.75 OH" -75.39
H20 -76.04 Ca2' -676.13
H2S04 -698,16 CaOir -752.02
HS04" -697.75 1
From the Table 2 we can see that in the medium with hydroxyl OH excess, caicium sulphate dissociates with the formation of calcium hydroxide and SO4'2 anion. This does not contradict the data present in /4/ and XPS experiment (it observes the increasing of carbon atoms concentration on the surface of new-formations due to intensive carbonisation of calcium hydroxide, see Table 1). The lack of Ca+ in the free cations can hinder the formation of proper crystal structure calcium sulfate dihydrate. Besides, according to the Table 2, total energies of the following components can be evaluated:
Ei = E(HSO'4) + E(CaOH+) + 2E(H20) = 1525,72 (1)
E2 = E(CaS04) + 2E(H20) = 1525,88 (2)
E3 = E(H2S04) + E(Ca(OH)2) = 1525,91 (3)
From (2) and (3) the exemplary equilibrium of total energies E2 and E3 can be seen. So, calcium sulfate dihydrate should be considered as complex compound of Ca2", S04'\ and OH' ions but not as a molecular crystal.
It should also be noted, that comparative evaluations of energies are carried out according to energies of isolated ions and molecules. Besides, it is necessary to register their interaction in cluster system and with ligand. The interaction is changed due to pH-medium changes. These interactions can radically influence Ej, E2 and E3 energy ratio and this needs further calculations. Taking into account the calculations, it can be supposed that the formation of cluster and fine powder systems of various chemical composition and sizes follow the formation process of primary structure calcium sulfate dihydrate. Depending on the conditions of hardening, these systems can form amorphous gel structures or transfer in crystal-hydrate new-formations with different morphology.
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IN-V. K.HUK.HKIAK.UV,U.I. YAKOVLbV, V.I. K.UDOLOV
Investigations of structure of two - water gypsum obtained in an acid medium.
At calcium sulfate hemihydrate tempering in the water medium with pH = 2,5 the crystalline hydrates of filamentary structure are formed. The examinations of a structure of filaments separated from natural filamentary gypsum (Selenite) using an optical microscope show, that each filament represents the block of plates with structures similar to one of gypsum crystalline hydrate, but extended in one direction (Fig 3).
Fig.3. Structure of a filament of calcium sulfate dihydrate tempered in an acid media
To determine the structure of the plate at the atomic level the roentgenograms were obtained of gypsum samples of various morphologies (see Fig. 4). In the figures 4 a-d the angle 6 between the direction of incident radiation and the perpendicular to the surface of the sample lays off along the X - axis. The scale interval on the axis X is 5 degrees. The reflected intensity lays off along the Y— axis.
In figure 4a the roentgenogram for a single crystal of plate-like gypsum obtained in usual water is shown. Thus the reflection from the surface of the sample, parallel to layers was considered. Taking into account, that the crystal of calcium sulfate dihydrate has monocline structure with a = 0,569 nm, b = 1,521 nm, c = 0,629 nm, from the formula of Brages:
wX = 2rfsin(e) (4)
it follows, that the peaks in a Fig. 4a correspond to reflections from sequential layers of platelike gypsum with d = b = 1,521 nm and n = 2, 4, 6, 8, 10, 12. In figure 4b, the roentgenogram of a polycrystalline sample of plate-like gypsum is shown. In figures 4c, d the roentgenograms of a sample calcium sulfate dihydrate obtained in an acid medium (a fibrous sample) are shown. In Fig. 4c the roentgenogram from a sample surface parallel to direction of filaments is presented, in Fig. 4d perpendicular surface roentgenogram is presented. The comparative examination of the roentgenograms has shown that a majority of peaks in a Fig. 4a, b is present also in Fig. 4c, d. The considerable changes of intensities of peaks can be explained by texture of materials. From here it is possible to conclude, that the atomic structure of filaments of gypsum obtained in a medium with an excess of protons H+ (pH = 2,5) is the same as one of crystals of plate-like calcium sulfate dihydrate. Note also, that the interlayer distance in the filament must be the same as in the crystal, because in figure 4d there are reflections, characteristic for interlayer distances of plate-like gypsum (see Fig. 4a), the positions of the indicated peaks in both samples coincide.
For clearing up the reason of formation of plates elongated in one direction, it is necessary to consider properties of plane charged systems. Under increasing of length of uniformly charged
inn
XHMUHECKAfl OM3MKA H ME30CK0nHfl. Tom 2, № 2
plane object at a constant square, 'he energy of the repulsion of charges decreases, thus formation of the elongated charged systems appears energy favourable.
Fig. 4. Roentgenograms of samples of calcium sulfate dihydrate (see the text)
To make a simple estimation of an energy advantage for the elongated systems, we shall use the fact, that in calcium sulfate dihydrate the majority of the charge is localised near the boundary of the plate crystal, that is explained by high mobility of charges in the system under consideration Let's consider a rectangle with side lengths a and b, uniformly charged on boundary, and complete charge of rectangle is q. Perimeter of the rectangle L=2 (a + b), whence line density of the
XMMHHECKAfl OM3HKA M ME30CK0riMR Tom 2, № 2
90Q
charge on boundary p = q/2 (a + b). Below we shall neglect the influence of the angles of the rectangle and interaction between charges localised on opposite sides. The electrostatic energy per unit length of boundary is proportional to quadrate of the line density W = kp\ where coefficient k depends only on the charge distribution near the rectangle boundary and dielectric constant of the medium. The full electrostatic energy of the system U=LW. After the substitutions and accepting, that the square of the rectangle is S, it is possible to obtain for magnitude U following expression
2 — + b
U )
If to fix the square of the rectangle, the quadrate will be the less energetically favourable, if to elongate one side in relation to another the repulsion energy will be reduced. At a large b the energy becomes inversely proportional to b and depends on square only through magnitudes of charges.
Thus, the electrostatic energy promotes increasing of length of plates to infinity. The increasing of length of a plate is restricted by a surface energy. Denoting line density of the surface energy as p, and energy density per unit square inside a sample as we shall obtain for the total energy of a crystal:
\
E = lS +
- + b b
b
Here we artificially separate the factors associated with electrostatic interactions to an individual item in order to compare the behaviour of different contributions to the surface energy with changing of crystal length. At fixed square of the rectangle the minimum of the energy is determined by the second and third items in the formula. The optimal value b may be obtained at examination of the energy derivative and depends only on the ratio p/k and charge of the crystal q (so it also depends on pH value of medium). Let's note also that as the crystal length increases the contribution of electrostatic energy becomes weak, in addition the temperature factor also can break the crystal growth.
To study the chemical processes flowing in the boundary region of the plate, the quantum chemistry calculations of energies and equilibrium structures of various complexes (see Figs. 5 - 9) were performed. For definition of structure of CaS04[H30]+ complexes we made optimisation of an energy of systems consisting of hydroxonium ion H30+ and molecule CaS04 with various initial configurations. The result of the energy optimisation depends on the initial disposition of reagents, however as a whole it is possible to make two conclusions.
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XMMMMESK&l *№№A M ME30CK0nt1R Tom 2, № 2
First, the calculations show lability of hydrogens in hydroxoniunis, and from some initial dispositions one of them passes to an oxygen of sulphate molecule (see Fig. 5, 6). Thus the hydrogen can set as to oxygen, proximate to atom Ca (Fig. 5). and to oxygen removed from Ca (a Fig. 6). Though the first variant is a little more energy favourable (see Table 3), probably more often the second variant is implemented, because the forming of the structure shown in a Fig. 5b, needs a considerable activation energy. The energy of the initial state shown on the Fig. 5a is much higher than the energy of the state in the Fig. 6a. In Fig. 5c. 6c the resulting complexes CaS04H+ are shown. At the same time, from some initial states (see for example Fig. 7a) it is possible also to obtain a stable complex CaS04 [H30]* with maintenance of hydroxonium ion. The bond energy of such complex is a little bit lower, than similar values for variants shown in a Fig. 5, 6.
Table 3. Energies of ions, molecules and complexes present in calcium sulfate dihydrate tempered in an acid media (Hartree)
Complex Energy Energy of interaction
H30+1- -76,32
CaS04[H30]+l (fig. 7b) -1450,24 -0,13
CaS04[H30]+1 (fig. 5b) -1450,28 -0,17
CaS04[H30f1 (fig. 6b) -1450,26 -0,16
CaS04[H20] (fig. 8a) -1449,89 -0.06
CaS04[H20] (fig.8b) -1449,85 -0,02
2H20 -152,10 -0.01
H20[H30]+l -152,42 -0,05
[H30]+l-3H20 -304,58 -0,13
¡j) b
Q
A
Fig. 5.
Dynamics of CaS04[H30]' complex.
Fig.6.
Dynamics of CaS04[H30]+ complex.
XMMMHECKAfl OH3HKA H ME30CK0nUfl. Tom 2, № 2
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Fig. 7.
Dynamics of CaS(MH3Of complex.
Fig.8.
Dynamics of CaS04lH20] complex.
a
at
The second conclusion is considerably higher stability of all complexes of calcium sulphate with hydroxonium, than similar complexes with water. This fact is demonstrated by calculations of two types of complexes CaS04[H20] shown on a fig. 8 (see Table 3).
The presence of hydroxonium ion affects also on the structure of tempering water. The calculations of energy of interaction between an ion H3CT with a molecule of water show, that this energy exceeds the interaction energy between two molecules of water in 5 times. A stable and energy favourable complex of hydroxonium ion with three molecules of water is also shown on a Fig. 9. Let's note, that the isolated hydroxonium ion has flat structure with angles between bonds equal to 120°, where as the hydroxonium in the complex H30[H20]3 has a pyramidal structure with angles between bonds near to 116°.
The high interaction energy of hydroxonium ion with a calcium sulphate promotes the increasing of the plate crystal length for two reasons. First, it reduces a surface energy because of the high concentration of hydroxonium ions near the boundaries. Second, the maximal charge will accumulate on the ends of a filament, forming an area of increased attraction for sulphate calcium and water molecules and accelerating crystal growth along filament's axis.
4
/
«2 «
Fig.9.
a) - H20-[H30]+complex,
b) -miH^0r-3H?0 comDlex.
010
XMMUHECKAfl OM3MKA H ME30CK0riMfl. Tom 2 № 2
Conclusion
In an inference we shall formulate the basic conclusions of the article. It is experimentally found that depending on pll factor of water used for gypsum tempering, the various morphologies of calcium sulfate dihydrate are formed.
In an alkaline media the amorphous materials are formed. The quantum chemical calculations show that a probable reason of such appearance can be intensive formation of calcium hydroxide in excess of OH" ions, which lead to violation of a crystal growth This conclusion is verified indirectly by a high carbonisation degree of the samples obtained.
In an acid medium the filamentary structures are formed. Complex experimental and theoretical examinations have shown, that their crystalline structure basically is the same as the structure of plate-like gypsum, however the plates are strongly elongated in one direction. It is possible to explain such shape of a crystal by influence of an electrostatic energy of charges, peculiarities of their distribution and lowering of the surface energy of crystallites by means of a strong interaction of polar calcium sulphate molecules and hydroxonium ion.
References
1. G. Jakowlew, A. Lasis, W. Kodolov, Yu. Rats. Structure der mit ionisiertem Wasser ange-machten Gipsashen - kompositionen. In: Internationale Baustofftagung "13.1bausil", Tagungsbericht - Band 2. Weimar, 1997. P. 461 - 467.
2. T.H. Dunning, J. Chem. Phys. 55 (1971) 716 - 723. A.D. McLean. G.S. Chandler II J. Chem. Phys. - 1980. - 72. - P.5639-5648. (A.J.H. Wachters, J. Chem. Phys. - 1970. - 52. - P. 1033-1036).
3. GAMESS. General Atomic and Molecular Electronic Structure System. Original program assembled by the staff of the NRCC: M. Dupuis, D. Spangler, and J. J. Wendoloski. National Resource for Computations in Chemistry. Software Catalog, University of California: Berkeley, CA (1980), Program QG01. This version of GAMES S is described in the Quantum Chemistry Program Exchange newsletter: M.W. Schmidt, K.K. Baldridge, J.A. Boatz, J.H. Jensen, S. Koseki, M.S. Gordon, K.A. Nguyen, T.L. Windus, S.T. Elbert QCPE Bulletin. -1990. - 10.-P. 52-54.
4. Babushkin V.I., Matveev G.M., Mchedlov - Petrosjan O.P. Thermodynamics of Silicates. Moscow: Strojizdat, 1986. 408 p.
SUMMARY. The results of physico - chemical investigations show that when using water ionized in static electric field, for hemi - water gypsum hydratation, two water gypsum of different morphologies is obtained. So the tempering in alkaline water leads to forming the amorphous substances, the using of neutral water leads to forming the prism crystals, if one uses acid water the fibrous structures are formed /1/. In the article we research the structures of these materials and develop models of their growth.
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