3202

Fig. 2. Specific silicate modulus SiO2: M2O that can be obtained using ashes of silicate dioxide during its neutralization with an organic compound to рН = 9

It is known that soluble silicate alkali systems are numerous and classed according to the following criteria [16]:

–– according to a degree of polymerization I of silicate dioxide, i.e. according to an average number of silicate atoms forming a continuous system of silicate bonds ≡Si-O-Si≡ during polymerization. Polymerization of silicate ashes is accompanied by an increase in its molecular mass М for high degrees of polymerization by increasing the size d of colloid particles of silicate dioxide. For a certain degree of polymerization I colloid silicate dioxide appears in systems of alkali silicates as ashes and highly hydrated silicate dioxide:

–– according to the chemical composition they are characterized with a molar ratio SiO2/М2О (a silicate modulus n) as alkalinity goes up; in case of alkali silicate systems there is a row that corresponds with the four above forms of silica:

Therefore similarly to silicate liquid glasses for silicates of strong organic compounds, the optimal silicate modulus SiO2:M2O ranges from 2 to 4. For some particular cases it can be reduced to SiO2:M2O ranging from 17 to 19 and even higher. Nevertheless these systems are

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difficult to obtain and they can be rather unstable. A silicate modulus SiO2:M2O cannot be less than 2. In this case there are regular molecular silicates that do not have binding properties. Any type of strong organic compounds can be employed in this process. As a silicate dioxide completely dissolves at рН from 10.7 to 11.0 and high values of рН, even silicates of organic compounds with a dissociation constant рКb < 3 can be prepared. The resulting compounds will also have a predetermined silicate modulus expressed similarly to that of alkali metals as SiO2:(NR4)2О.

2. Tetrafurfuryloxysilane as a nanostructuring agent

As specific organic silicate additives such as tetrafurfuryloxysilane are introduced, a considerable increase in the density and strength of a silicate matrix in liquid conditions can be achieved. This is due to stronger contacts between globulus of a binding gel and alkali component owing to an “injection” of furan radical [2].

During hydrolysis of tetrafurfuryloxysilane active nanoparticles of hydrated silicate oxide SiO2 and fulfuric alcohol are formed as well as oligomer nanofilms on the surface of silicate grains of the matrix. Tetrafurfuryloxysilane is a centre of nucleation of a new microcrystallic phase that blocks pores on the surface of a silicate matrix and reduces shrinkage deformation of a surfacing.

Lately there have been efforts made to develop applied silicate polymer composites that are water-soluble silicates with additives of active substances of the furan type. They operate in acid and neutral media and at extremely high temperatures. They are cheap and easy to prepare, not toxic, not combustible. The cost of polymer silicate materials is compared with that of cement concrete and is several times lower than that of polymer concrete. Silicate polymers such as concrete, construction solutions, mastics are used to prepare a variety of structures (monolith as well as facing). It is potentially promising to develop composites based on a liquid glass binder modified by substances based on furfuryl alcohol.

A considerable increase in the strength, heat and fire resistance of a silicate matrix is achieved by introducing ethers of orthosilicate acid and furfuryl alcohol (tetrafurfuryloxysilane) (Fig. 3). This is due to stronger contacts between silica gel globules and modification of the alkaline component owing to an “injection” of a furan radical. Introduction of a binder, a tetrafurfuryloxysilane additive results in nanoparticles of SiO2 and furfuryl alcohol that fills the matrix and forms a cross-linked polymer. Adding tetrafurfuryloxysilane enhances the mechanical and chemical strength of a binder and this is what is commonly done to prepare acid resistant surfacings [13].

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Fig. 3. Chemical structure of tetrafurfuryloxysilane

(tetrakis (furan-2-ylmethyl) orthosilicate)

This effect can be accounted for using the following assumptions. E.g., according to the thermal stability of oxide compounds, the relative strength of bonds of atoms of М-О and С-О in their crystallic structure can be evaluated. The lengths of bonds М-О and С-О as part of a coordination polyhedron may significantly vary, which indicates their different energy values. During dehydration and thermal impact dentancy of a particular part of ligands might change. In a forming structure they can be both ligands and a missing solvate. An increase in the dentancy of ligands leads to distortions of oxide surroundings of a matrix element or a filler with a corresponding change in the distances М-О and С-О in the structure and thus changes in their strength.

In order to increase the strength, acid resistance, thermal resistance and fire resistance of construction materials and structures, tetrafurfuryloxysilane is introduced into a composite binder. It is synthesized by interesterification of tetraethoxysilane and furfuryl alcohol.

The used binder contains the following: liquid glass — 80—95 mass %; tetrafurfuryloxysilane — 2—7 mass %; solidifier, natrium hexafluorosilicate — 13 mass %. Therefore in order to replace some of the liquid glass organic alkaline liquid glass was used. In this product the organic cation is 1.4-diazobicycle [2.2.2] octane -1.4-diumsilicate or 1.5-diazobicycle [3.3.3] undecane-1.5- disilicate with the proportion of 2—4 mass %. Natrium silicate with a cation, 1,4-diazobicycle [2.2.2] octane -1.4-diumsilicate compatible with water dispersion of polyurethane and chloroprene as well as most latexes based on synthetic rubber.

In order to control the properties of the obtained polysilicate composite surfacings and to obtain nanostructuring materials with optimal properties, profound insight into the mechanisms of processes involved in their production is necessary. This requires detailed understanding of the structure of the used materials and tetrafurfuryloxysilane in particular. For that quantum-chemical calculations of the structure, topology and properties of tetrafurfuryloxysilane were performed. The method described in the paper [15] was applied. The calculated molecular structure of tetrafurfuryloxysilane is presented in Fig. 4.

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Fig. 4. Structure of the molecule of tetrafurfuryloxysilane:

Si(ОСН2(C4H3O))4

The structure of the molecule was calculated by minimization of the total energy of the molecule by minimization of the gradient and a minimum value it reaches using both variants of the calculation basis: the original structure and following the application of molecular dynamics. Molecular dynamics included 10000 iterations and the results are thus more accurate in describing the topology and energy of the molecule of tetrafurfuryloxysilane. The results of the energy calculations are given in Table 1.

In order to calculate the topology and chemical properties of the molecule of tetrafurfuryloxysilane — Si (OCH2(C4H3O))4 — different computational methods were employed. Computational chemistry encompasses a variety of mathematical methods that are classed into two large categories:

––molecular mechanics employs the laws of classical physics to atoms in a molecule without considering electrons. For these calculations the method ММ2 and MMFF94 was used;

––quantum physics relies on the Schrödinger equation to describe molecules with a focus on the electrone structure. Quantum and mechanical methods can be grouped into two classes: ab initio (non-empirical) and semi-empirical. In the method ab initio GAMESS Interface — The General Atomic and Molecular Electronic Structure System, i.e. a common system of atomic and molecular electron structure, is used. In semi-empirical methods the detailed Hückel method is employed.

The calculation of atomic charges that were obtained using electrostatic potentials provide necessary information on chemical activity. Atomic point charges give better insight into possible attack spots during chemical interaction of a molecule with external reagents. The data on the electron settlement of particular atomic positions, charges, coordinates of the atoms and geometric topology of the molecule of tetrafurfuryloxysilane are presented in the paper [3].

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Table 1

Results of minimization of energy characteristics of the molecules of tetrafurfuryloxysilane Si (OCH2(C4H3O))4

The obtained results indicate that functional furfuryloxy groups are not homogeneous, which also suggests that there might be a stepwise interaction of the molecules of tetrafurfuryloxysilane between one another as well as water molecules surrounding them during hydrolysis. In addition, the molecule of tetrafurfuryloxysilane itself is not symmetrical, which also suggests there might be a stepwise mechanism of the formation of nano-sized phases and nanostructuring of a polysilicate composite surfacing when it is obtained.

Based on the data, an optimal composition of a material that has an extremely high strength, durability, density and crack resistance was obtained. We investigated diffusion capacity of a surfacing and its chemical strength in different aggressive media.

3. Nanostructured polysilicate composite surfacing

We developed a nanostructured polysilicate composite surfacing that consists of a binder, solidifier, polymer additive, thin and rough filler. A water-soluble natrium or potassium silicate glass with the density 1.38—1.4 g/сm3 is used as a binder foundation. Technical silicate fluoric natrium is mostly used as a solidifier. Fillers are natural or artificial materials with high acid resistance, particularly quartz, zircon, diabase, basalt, granite, andesite, etc. [8, 10, 14].

Polysilicate composite surfacing has a number of important operational characteristics (high strength, fire resistance, acid resistance) and is thus widely used as a facing material for chemical equipment and setups.

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Nevertheless a major drawback of these surfacings is their low strength and high shrinkage. By introducing special organic silicate additives such as tetrafurfuryloxysilane, a considerable increase in the strength and density of a silicate matrix in aggressive media can be achieved due to stronger contacts between structural elements of a composition [6].

An optimal composition of a composite surfacing was obtained. It has a high strength, durability, density and impact strength. Diffusion properties of a surfacing and its chemical resistance in different aggressive media were studied.

An optimal composition of a polysilicate composite surfacing was determined. The criteria for an optimal composition of a composite surfacing were conditions when the smallest amount of a binder is necessary, easy processing, high density and strength. Prior to optimization, the optimal amounts of a silicate binder and extra monomer were identified. The results of the experiments are in Table 2.

Таble 2

Effect of the content of a binder on the mobility and rigidity of a mix while preparing a polysilicate composite surfacing

The above data suggests that even a small change in the amount of a silicate binder causes dramatic changes in the technological characteristics of a composite surfacing. A decrease in the amount of a binder reduces the mobility of a mix by 2.5 times and increases its rigidity by 5 times. The influence of a monomer tetrafurfuryloxysilane additive was investigated using the example of a plastic mix containing 11 % of a binder (Table 3).

The results of the study show that introduction of a tetrafurfuryloxysilane additive increases the rigidity of a mix. An effect of the content of a silicate binder on the strength of a composite surfacing was identified for a composition with a monomer with and without an additive. In the first case the original sample was a plastic mix with a minimum content of a binder (10—13 %) as well as a tetrafurfuryloxysilane binder in the amount of 3 % of the binder mass. The results of the experiments are given in Fig. 5.

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The experiments showed that the strength of the samples of the surfacing material goes up as the content of the liquid glass binder decreases within the entire range of changes. A decrease in the amount of the binding substance only by 3 % causes an increase in the strength of the surfacing material by about 25 %. We can assume that this is due to the thickness of the film of the binding substance that covers grains of the filler. Hence as the thickness of the film decreases, the adhesive force goes up, which contributes to an increase in the strength and density of the mix.

Note that the introduction of tetrafurfuryloxysilane in the amount of 0,3 %, into the composite surfacing increases the strength and density of the material by about 50 % in the entire range of the investigated content of organic water-soluble silicates [12, 18].

Fig. 5. Changes in the strength of a polysilicate composite surfacing depending on the consumption of the liquid glass

The service life of a polysilicate nanocomposite surfacing in aggressive media depends on the diffusion rate of chemically active reagents that is affected by the molecular structure of the

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binder due to the temperature and pressure of the environment. Therefore determining the diffusion coefficient in a composite surfacing at a certain point in time is vital for evaluating the effect of monomer additives and identifying a maximum acceptable concentration of the substances causing corrosion.

We investigated the diffusion penetration into the composite surfacing in a neutral water medium that is most aggressive for compositions based on liquid glasses. The results of the tests of the samples of the polysilicate nanocomposite surfacings with different compositions are shown in Fig. 6.

Changes in the mass, mass %

Time, days

Fig. 6. Change in the mass (1, 2) and rate of change in the mass (3, 4) of the samples of the polysilicate nanocomposite surfacings in water

It is obvious that diffusion penetration into the material modified by the tetrafurfuryloxysilane additive is significantly lower than for the composition containing the furfuryl alcohol. The rates of the processes for these samples also differ considerably, particularly in the initial period. The diffusion coefficients obtained by the sorption method were calculated within two periods of water exposure — 7 and 30 days (Table 4). Therefore we can conclude that the introduction of modifiers of the furan range into silicate composite surfacings reduces the rate of diffusion penetration of aggressive media.

In order to study the effect of a weakly acid medium on the properties of polysilicate nanocomposite surfacings, optimal compositions including monomer additives of furfuryl alcohol and tetrafurfuryloxysilane were used (Table 5). An average concentration corre-

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sponds with water solutions of sulphuric and hydrochloric acids used for etching of metals. Corrosion resistance of nanocomposite surfacings was evaluated by changing the compression strength of the material after 3—18 months of exposure in media of up to 3 months. The effect of the type of an additive monomer on the strength of polysilicate nanocomposite surfacings is described in Table 5.

Таble 4

Diffusion coefficients of the samples of the polysilicate nanocomposite surfacings in the water medium

The data suggest that a small change in the content of liquid binder dramatically changes the technological characteristics of polysilicate nanocomposite surfacings. As the amount of a binder decreases, the strength and density of the material goes up.

Таble 5

Compressive strength limit of the samples of the polysilicate nanocomposite surfacings, МPа, following the impact of an aggressive medium

The optimal composition of the material includes 11.23 % of liquid glass and 0.34 % of monomer additives (furfuryl alcohol or tetrafurfuryloxysilane). The composition of the polysilicate nanocomposite surfacings modified by the tetrafurfuryloxysilane additive has a high compressive strength and deformability. The introduction of tetrafurfuryloxysilane additives into the material increases the rigidity of the mix and significantly reduces shrinkage deformations. Additives of substances of the furan range reduce diffusion penetration of an aggressive medium in the structure of polysilicate nanocomposite surfacings and increase their corrosion resistance.

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Conclusions

The optimal composition of protective surfacings based on organic water-soluble silicates with extremely high strength, durability and crack resistance was obtained. Diffusion penetration of surfacings and their chemical resistance in different aggressive media was investigated.

The introduction of the tetrafurfuryloxysilane additive into a composite surfacing in the amount of 0.3 % increases the strength and density of the material by about 50 % in the entire range of the studied content of organic water-soluble silicates. The plastic composite mix allows items of any geometric shapes to be manufactured. Note that the compressive strength and deformability of the samples of the composite surfacing modified by the tetrafurfuryloxysilane additive were found to be maximum. The study showed that the introduction of monomer additives results in a sharp decrease in shrinkage deformations. Shrinkage deformations of the composite surfacing aged 28 days were 0.06 % in total when there was 3 % of tetrafurfuryloxysilane in the mix.

The formation of the structure of a composite surfacing is accompanied by intense squeezing of a binding gel by capillary forces of an intermicellar liquid. For a mix with no monomer additives this liquid is water. Squeezing of the gel results in maximum shrinkage deformations from the outset of the solidification of the mix. The introduction of the furfuryl alcohol or tetrafurfuryloxysilane additives causes a significant decrease in the effect of capillary forces due to a reduction in the surface tension of the liquid in the capillars.

1. A small change in the content of organic water-soluble silicates leads to a dramatic change in the technological characteristics of a composite surfacing. As the content of organic watersoluble silicates decreases, the strength and density of a composite surfacing go up. The optimal composition of a composite surfacing includes 11.23 % of a silicate binder and 0.34 % of monomer additives (furfuryl alcohol or). The composition of a composite surfacing modified by the tetrafurfuryloxysilane additive has a high compressive strength and deformability. The introduction of a composite surfacing of a monomer tetrafurfuryloxysilane additive leads to an increase in the rigidity of the mix and a significant decrease in the shrinkage deformations. Additives of compoudns of the furan range contribute to lower diffusion penetration of an aggressive medium into a silicate polymer concrete and enhance its corrosion resistance.

References

1. Korneev V. I., Danilov V. V. Zhidkoe i rastvorimoe steklo. Proizvodstvennoe izdanie [Liquid and soluble glass. Production edition]. St. Petersburg, Stroyizdat Publ., 1996. 216 p.

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