building materials and products

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BUILDING MATERIALS AND PRODUCTS

UDC 666.973.6 Voronezh State University of Architecture and Civil Engineering

Ph. D. in Engineering, Assistant lecturer of Dept. of Technology of Building Products and Constructions

A. A. Rezanov

Russia, Voronezh, tel.: +7-908-132-54-33; e-mail: [email protected]

A. A. Rezanov

EXTERIOR PRESSURE OF THE GASEOUS MEDIUM

AS AN ADDITIONAL TECHNOLOGICAL FACTOR

FOR OPTIMIZING THE VAPORIZATION PROCESS

IN THE PRODUCTION OF CELLULAR SILICATE CONCRET

Statement of the problem. The quality of silicate porous concrete is largely determined by vapo-

rization processes at the stage of the formation of the macrostructure of the obtained material. In

the production of cellular concrete with the use of injection molding, the existing manufacturing

technologies do not enable the expeditious handling of the vaporization process. This is why there

is a growing need to develop additional efficient methods of handling the vaporization process

thus improving cellular silicate concrete.

Results. Based on modelling and detailed examination of the balance of pressure affecting devel-

oping gas pores, mechanisms and factors governing a defect-free structure are found. An addition-

al governing factor, which is a pressure of the external gaseous medium, was discovered. The ap-

proaches to handling the vaporization process have been developed and a plant fitted with a system

of automatic control of vaporization process by conscious operative pressuring effect from the ex-

ternal gaseous phase on a poring mixture has been designed.

Conclusions. Theoretical validation along with the results of the experimental study help to arrive

at the conclusion about the efficiency of the suggested system in controlling vaporization that

could provide a good addition to the traditional injection molding and make it more susceptible

against varying characteristics of raw materials.

Keywords: cellular concrete, force balance, a macrostructure, pressure, pore formation, automatic control, ga-

seous phase pressure.

Introduction

There has been a growing interest towards cellular autoclave curing concrete as an efficient

wall material. In order to meet the growing demand, production rates in the manufacturing of

Scientific Herald of the Voronezh State University of Architecture and Civil Engineering. Construction and Architecture

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cellular silicate concrete (gas silicate) are on the rise. These days investors normally prefer

foreign equipment and production technologies. This choice is due to a high automatization,

reliability and marketing level of foreign supply firms. Now Russia becomes home to a grow-

ing number of modern enterprises that use the equipment by the European firms ITONG, HE-

BEL, WERHAN, SIPOREX, etc. [1]. All these enterprises use the injection molding the major

advantage of which is a friendly production mode that involves no vibration, a long life span

of the forms and a possibility to improve the macrostructure caused by a lengthy development

of gaseous pores. Apart from that the injection molding technology allows one to obtain low-

density products (of less than 400 kg/m3), which is a daunting task for the vibration and bump

formation technology. However, the technology regulations of the productions using the in-

jection molding method set stringent requirements for the quality of raw materials and in par-

ticular to that of lime, fineness of grinding of the raw material and its purity. It is no secret

that as the Russian manufacturing plants are plunged into moral and physical deterioration,

manufactures find themselves struggling to subscribe to high European standards and deliver

on the quality of production.

Therefore in spite of a high technical and technological level of modern enterprises the pro-

duction is largely rejected which is because of fluctuations in the properties of the raw materi-

al component (in the form of cracks, cleat of the pores, disagreement of the density and

strength of the products to the specified make and class). Changes in the characteristics of

lime identically made causes disruptions to the key processes of porous formation and grow-

ing viscous and elastic characteristics of the porous mixture which gives rise to destruction

processes [2, 3]. Even the complete automatization of a technological process does not pro-

vide hands-on handling of the injection molding formation since they target only making fac-

tors. In case they fluctuate from the optimum, there are currently no effective methods availa-

ble for emergency handling of the porous formation if the injection molding is used.

The research by A. P. Merkin, N. I. Levin, K. E. Goryaynov, D. G. Zemtsov, N. P. Sazhnev,

G. Ya. Kunnos, P. R. Taube, G. I. Knigina, A. N. Chernov, I. T. Kudryashov and others looks

at the formation of a high-quality macrostructure of cellular silicate concrete. The scientific

school based on Voronezh State University of Civil Engineering played a key part in the de-

velopment of the gas silicate technology thanks to scientific discoveries of A. A. Fedin,

A. T. Dvoryadkin, E. M. Chernyshov, E. I. Shmitko, B. M. Zuev, etc.

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1. Equilibrium of the interior and exterior forces of the formation of a perfect cellular

structure

We should start our investigation of the mechanisms according to which pore formation takes

place by modelling presentations that account for the interior and exterior forces that act on a

developing gas bubble (Fig. 1).

Fig. 1. Model of a gas bubble

in a lime and sand rock composite

According to their direction, these are pressure forces that can be divided into the stretching

Рраст and compression Рсжим pressure. The compression pressure is determined by:

the pressure of the surface tension σ of the interface between gas and liquid Рл = 2σ/r

(the Laplace pressure);

the resistance to the extension of the bubble — 4πστ that is caused by a specific ten-

sion of the shift of the solution component στ [4];

the hydrostatic pressure of the liquid column exerted on the bubble (with the height of

h and density of ρж), Ргидр = ρжgh;

the pressure of the free surface of the liquid Р0 where a bubble is formed..

The compression pressure Рсжим is counterparted by the interior pressure in the bubble Рг that

is made up by partial pressure of the water vapour парц

пР and gas inside it парц

гР (emerging as a

hydrogen chemical reaction). For the conditions of the dynamic equilibrium we can write:

0 or 2 / 4 .парц парц парц парц

сжим п г п г жР Р Р Р Р r gh Р (1)

By shifting the balance of stretching and compression forces to this or that point, we can be in

control of the growth of a bubble and also the formation of the structure of the entire porous


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structure. From the technological standpoint, the balance of the forces in a developing bubble

at the early stages of its growth it makes sense to vary the changes of the surface tension of

the water film in the interface between the phases and at the late stages when the bubble rather

big by change in the compression pressure forces (Ргидр, Ро, σ). Only two compression pres-

sure components can potentially have an emergency effect on the formation of a cellular

structure. These are the pressure of the gas phase which is exterior in relation to the product of

the gas phase Р0 and specific pressure of the shift of the solution component στ. However the

variation of the specific shift of the tension during the formation of pores takes place only in

the vibration or bump formation technology, therefore the focus should be placed on the pres-

sure of the exterior gas phase as an additional key factor.

The most hazardous in terms of the emergence of failure processes is the final stage of the

formation of pores when interpore partitions lose plasticity and the capacity for the relaxation

of stretching tensions that are caused by the interpore hydrogen pressure and the temperature

expansion of the gas phase. If there is a continuous emission of gas, immature interpore

membranes fail and the deficiency of the macrostructure increases with the physical and me-

chanical indicators of the solidified gas silicate. The resulting cellular pores are stable if stret-

ching tensions in the partitions caused by the interior pressure of the gas phase are made up

for by the total of the compression pressures until the solution component becomes strong

enough to resist the emerging tensions. The artificial additional pressure of the exterior gas

phase which should be considered a viable technological factor that allows for emergency in-

terference with the process should be the most important of the compression pressures.

There is often another problem to deal with. At the stage of active growth of the mass due to

the instability of the properties of the raw material components the viscosity of the solution

component is not enough to maintain the large pores in the mixture, which leads to a so-called

“false boiling” effect involving a great loss of the interpore gas and increasing density of the

gas silicate. The application of excess pressure of the gas phase acting on the free surface of

the molded product helps to prevent this situation from unfolding and thus decreases the ra-

dius of the pores, their lifting force and immediately address this type of rejection [5].

Therefore based on the analysis of the factors that are central to the balance of forces in gas pores

at the stage of the gas silicate formation we have defined another factor that can potentially mas-

sively impact the formation of a cellular structure. It is the pressure of the exterior gas stage.

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In order to put the above factor to effective use we posed ourselves a challenge to experimen-

tally determine the range of exterior pressure and develop a mathematical model of the

process that would bring together all of its major parameters for the deliberate influence on

the force balance at each point of time. These outcome of these challenges is to form the ini-

tial data for the system of automatized management of the process since emergency handling

of the formation of the macrostructure is made possible only by the automatic control and

management.

2. Results of the experimental calculation of the interstitial pressure at the destructive

impact spot

The original aim of the research was to perform approximate calculations of the required val-

ue of the compensation pressure of the exterior gas phase and its application period. For this

purpose a setup was designed and an experiment was carried out to define the gas emission

kinetics and increase in a plastic strength for different contents of the optimal and non-optimal

making. The schematic of the setup is in Fig. 2.

Fig. 2. Setup for defining

the pore formation kinetics:

1 — moulding box with the mixture;

2 — cone rheometer;

3 — thermometer;

4 — airtight vessel with the mixture specimen;

5 — level gauge; 6 — fridge with a pipe;

7 — measuring burette

In order to study the gas formation kinetics and increase in plastic strength the vessel with the

specimen of the identical mixture 4 was submerged into the molding box with the erupting

mixture 1. In this case the identical temperature conditions of the gas emission and expansion

of the gas phase were provided. The vessel was joined with the pipe of the fridge 6 where the

emitting hydrogen was cooled down to the environment temperature. The burette 7 was used

to determine the volume of the emitted gas with a liquid meniscus fit in that respond to a

change in the volume of the emitted gas by moving around.

During the course of the experiment the temperature of the mass and the fridge was controlled

by the thermometer 3 and the height of the elevation of the mass by the level gauge 5. Plastic


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strength was measured with the cone rheometer 2 and the applied effort was registered by a

tension sensor.

Fig. 1 shows two graphs that reflect the non-optimal and optimal pore formation. The follow-

ing parameters were taken control of: gas emission, erupting level, the temperature of the

process and specific tension of the shift of the mixture.

а)

b)

Fig. 3. Change of the controlled parameters in time for the non-optimal (а)

and optimal (b) of the making contents:

1 — eruption level, %; 2 — volume of the emitted gas, ml;

3 — plastic strength of the solution component, kgs/cm2; 4 — failure interstitial pressure, kgs/сm2

The graphs (Fig. 3) suggest that in case of the non-optimal content the gas emission peak the

eruption levels and plastic strength showed to be in disagreement in time. The moment of the

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loss of the plasticity system can be the time the mass ceases to grow. The graph of the non-

optimal content (а) suggests that the gas emission continues after the mass ceases to grow,

which results in a failure increase in the interstitial pressure. Knowing the amount of the gas

emitted after the mass ceased to grow as well as the temperature and average size of the pores

we can calculate the excess interstitial pressure. The design value of the failure pressure was

shown by the line 4. During the porization of the mixture of the non-optimal content it reach-

es 0.17 atm, which is accompanied by some heaving of the mixture heaving (line 1) that is

connected to a partial failure of the structure and interstitial gas output. Then the interstitial

pressure gradually drops due to a gradual cooling and diffusion of the gas through the defects.

Hence in case of the fluctuation of the mixture making from the optimal porization at a cer-

tain point of time to the system it makes sense to exert a compensation influence of the ex-

terior gas phase that is expected to increase in proportion to the growth of the failure inters-

titial pressure.

The performed preliminary experiments proved the benefits provided by the pressure of the

exterior gas phase. In certain cases we succeeded to obtain a huge rise of the strength of gas

silicate, however the solidification effect was accompanied by a small solidification but we

still were able to get a 35—40 % increase in the coefficient of the construction quality. How-

ever since the balance of the stretching and compression pressure in the pores of the develop-

ing system is in dynamics, it might be a challenge to immediately determine the parameters of

the applied pressure. The optimal counter pressure to be applied to the porous system at each

point of time is different. There is thus a need to develop the system of the automatic man-

agement of the additional exterior pressure.

3. Designing of the mathematical model for gas emission as the temperature function

and the viscosity of the mixture

The counter pressure to be applied to the porous system mainly depends on the gas emission

kinetics and temperature changes after the system loses its plasticity. Therefore in addressing

the management of pore formation a focus of attention should be on obtaining a mathematical

model that gives a full account of gas emission.

The major factors contributing to the gas emission rate were identified in the course of the

problem solution. Taking into account the major premises of the chemistry of heterogeneous


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reactions these are dispersity and shape of the particles of the aluminium powder, рН solution

of the disperse environment, temperature of the reaction, viscosity of the solution component

[6]. A рН value remained unchanged through all the experiments since the saturation of the

water solution of the molding mixture by the ОН ions reaches its maximum as early as in the

first minutes of the experiment. For all the experiments aluminium powder of the same design

was used therefore the factor “of the dispersity and shape of the aluminium powder particles”

can be neglected.

Therefore the factors that are crucial to the interaction of the powder-like aluminium with wa-

ter in the alkaline environment are the temperature of the process and viscosity of the mixture

(amount of water of hydration). In order to study the influence of these factors, the study was

performed on the working contents of cellular concrete with the use of slaked lime (in order to

rule out the exothermic heat emission during the course of the hydration reaction).

This problem involved the design and construction of the setup to define the gas emission ki-

netics that allows the automatic register of the data that describe the process at the fixed tem-

perature of the reaction (Fig. 4).

1. Reactor with the injection and mixing system

2. Thermal couple

3. Turbine mixer

4. Thermostat body

5. Lid of the body

6. Pressure gauge

7. Gas accumulator

8. TAN, power 0.4 KWatt

9. Eight-channel gauge

10. Interface converter RS232 AC2 (Oven)

11. Two-channel gauge ТРМ 200 (Oven)

13. Laptop Lenovo B450

Fig. 4. Temperature-stabilized setup to define the gas emission kinetics

The analysis of the resulting graphs suggests that the temperature has a massive influence on

the gas emission rate and a slowdown of the reaction rate in an increasing amount of water of

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hydration in the mixture. Based on the obtained data and analysis of possible ways of their

mathematical presentation, we have approximated the graphic dependencies of the exponen-

tial function in the following way:

(1 )kV a е , (2)

where V is the gas emission rate; τ is the time after the outset of the reaction; k, е are the coef-

ficients.

In order to determine the reaction rate using the interval temperature values of the studied

range of Lagrange polynomials was used that entails a polynomial description of the depen-

dence using the interpolation points. Based on this, we have obtained the mathematical de-

scription of gas emission in the numerical and analytical form (Formula (2), Table 1) which

allows one to define the amount of the emitted hydrogen knowing the current characteristics

of the mixture.

Table 1

Numerical form

of obtaining the mathematical model of gas emission

Reaction temperature t, 0С Coefficient values

а k

Suttard spread— 10 сm; Water/Solid — 0.87

41 0.92 -0.056

… … …

65 … …

Suttard spread — 15.5 сm; Water/Solid — 0.78

41 0.894 -0.048

… … …

65 0.891 -0.130

Suttard spread — 25.5 сm; Water/Solid — 0.7

41 0.832 -0.028

… … …

65 0.888 -0.123


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The results of defining the gas emission kinetics depending on the reaction temperature and

Water/Solid ratio are presented in Fig. 5.

Fig. 5. Gas emission kinetics depending on the reaction temperature and Water/Solid of the mixture

4. Exterior pressure of the gas environment as an additional technology factor in the au-

tomatic management of porous formation

As was noted, in the process of porous formation the interstitial pressure constantly changes

reaching the maximum value after the mass ceases to grow. Therefore for a proportional

compensation with the exterior pressure it is necessary to know its value at each point of

time. Automatic software adaptive management with the fluctuation of feedback seems to

be most promising in the implementation of the problem under investigation. This variant of

the system of the automatic management develops a managing signal based on the mea-

surement of the basic parameters and evaluation of their fluctuations that describe the per-

fect management mode.

In order to implement these principles an automatic system of the management of porous

formation was developed under the pressure based on the hardware and software complex. In

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order to implement the hardware part the setup was designed and constructed (Fig. 6) that in-

cludes the airtight molding box with a retractable bottom the system of pressure application,

the system of the collection of data on the parameters of the process, the system of data

processing and generation of the managing signal.

Sohematic

Photo

Fig. 6. Setup to mold the cellular silicate concrete under pressure

with the system of automatic management of porous formation:

1 — Airtight molding box with a retractable bottom; 2 — Outlet electromagnetic valve; 3 — Pressure gauge;

4 — Lazer source of a light beam; 5 — Magnetic condensate purifier; 6 — Web camera;

8 — Chromel copple thermal couple ДТПL011-0.5/1.5; 9 — Inlet electromagnetic valve; 10 — Compressor;

11 — Eight-channel gauge ТРМ138; 12 — Interface converter RS232 AC2 (Oven); 13 — Two-channel gauge;

14 — Interface converter RS485 AC4 (Oven); 15 — Laptop Lenovo B450; 16 — Digital discrete converter


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In order to implement the software part the SCADA system was developed with the use of

functional tools of the MatLab package and the system of quick application development

Delphi.

The system includes the feedback according to the following parameters:

the temperature of the mixture and exterior gas phase,

eruption level of the mixture,

the current pressure inside the molding box.

In order to measure the temperature flexible chromel coppel thermal couples were (8). The

eruption level is determined by a non-contact optical register of the lifting height of the mass

using the optical and digital system that consists of a laser source of coherent radiation (4) and

video camera (6) that registers a shift of the angular light beam. The pressure in the moulding

box (1) is determined by a precision pressure gauge (3) with the precision of ±0.001 atm. In or-

der to transform the measuring and managing signals the setups 11—14 were used, 16. The

processing of information and generation of the managing digital signal is provided by a porta-

ble computer. Further the managing digital signal is transformed into an identical from using the

device 16 thus providing the timely opening/closing of electric and magnetic valves (2, 9).

The reasoning behind the system is as follows. At the moment the mixture is poured into the

molding box, the time countdown starts accompanied by the calculation of the emitted gas

according to the programmed model (see Formula (2), Table 1) and register of the eruption

level. During the register of the mass ceasing to grow, the actual volume of the interstitial gas

phase determined considering the eruption level and original density of the mixture is com-

pared to the volume designed according to the mathematical model. The temperature expan-

sion of the gas phase is also taken into account. The difference between the volumes defines

the design value of the interstitial pressure that needs to be made up for. Therefore the devel-

oped system in the iteration mode allows one to maintain the necessary counter pressure that

prevents the defects emerging in the interpore partitions.

The first results proved a huge efficiency of the suggested method of porization management.

The cellular silicate concrete with enhanced technological characteristics (Table 2, Fig. 7—8)

was obtained. The characteristics of the macrostructure were assessed according to the pre-

viously developed methods [7].

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Table 2

The characteristics of the gas silicate quality obtained according

to the classic injection molding technology and injection molding

with the system of emergency management of the pressure of the exterior gas phase

Name of the characteristics

Values of the characteristics

for the injec-

tion molding

technology

for the injection molding technology

with the system of emergency man-

agement of the pressure

of the exterior gas phase

Dry density, kg/m3 320

Compression strength, МPа 1…1.5 2.4

Construction quality coefficient,

kgsec∙cm4/g2 150…180 234

Average diameter of the pores, mm - 1.75

Average thickness

of the interpore partitions, mm - 0.28

Deficiency coefficient (the ratio

of the area of the adjoining pores

to the area with a whole partition)

0.4…0.7 0.31

Sphericity of the pores 0.6…0.7 0.78

Fig. 7. The view of the pores

of the cellular silicate concrete


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Fig. 8. Size distribution of the pores

of the cellular silicate concrete

Conclusions

For the first time another factor of the emergency management of porization of cellular sili-

cate concrete was discovered. This is the pressure of the exterior gas phase the application of

which enables a conscious influence on the balance of the interior and exterior forces that de-

termine the quality of the structure being formed.

The suggested method offers an opportunity to optimize the injection molding of gas silicate

and adjust the widespread foreign technological lines to the fluctuating properties of raw ma-

terial components.

During the course of the research the process of gas emission was given undivided attention.

We have also elaborated a setup and the method of its study that enabled us to get an insight

into the effect of the key factors and obtain a mathematical description of the gas emission

process.

We have elaborated the system of emergency automatic management of porization. The fun-

damental difference of it is that the key management factor is the pressure of the exterior gas

phase.

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The current management system was implemented that allows for the regulation of the pres-

sure of the exterior gas phase both in the automatic and manual modes. The developed me-

thod was experimentally proved to be efficient.

The level of gas silicate quality that was obtained according to the developed molding tech-

nology surpasses the characteristics of the specimen made according to the classic injection

molding technology. This allows us to obtain a low density gas silicate and use it in efficient

construction and heat insulating applications.

References

1. N. P. Sazhnev (ed.), Manufacturing cellular concrete items: Theory and practice

(2nd ed., Minsk, 2004) [in Russian].

2. A. A. Fedin, Scientific and Technical Bases for Manufacturing and Application of Si-

licate Cellular Concrete (Мoscow, 2002) [in Russian].

3. G. Ya. Kunnos (ed.), Elements of Technological Mechanics of Cellular Concrete

(Riga, 1976) [in Russian].

4. D. I. Shakelberg, V. E. Mironov, “Thermodynamic Study of Gas Concrete Heaving”,

Building Materials, 1979, vol. 8, pp. 29—30.

5. Ye. I. Shmitko, A. A. Rezanov, A. A. Bedarev, “The Study of the Process of the

Structure Formation of Cellular Silicate Concrete of Pressure Hardening and Role of

Exterior Pressure of the Environment in the Formation of Defect-Free Structures”, in

Achievements and Problems of Material Science and Modernization of Construction

Industry, Conf. Proc., vol. 1 (Kazan, 2010), pp. 369—374.

6. Ye. M. Chernyshov, “Management of the System of Formation of Cellular Porosity

and Gas Silicate Technology”, in Effective Composites, Constructions and Technolo-

gies, Coll. Paper (Voronezh, 1991), pp. 123—128.

7. A. A. Rezanov, A. A. Bedarev, “Methods of Morphometric Identification of Macro-

structure of Cellular Concrete”, in Achievements and Problems of Material Science

and Modernization of Construction Industry, Conf. Proc., vol. 1 (Kazan, 2010),

pp. 352—356.

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