Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
Issue № 4 (16), 2012 ISSN 2075-0811
13
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
14
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.
Issue № 4 (16), 2012 ISSN 2075-0811
15
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
Scientific Herald of the Voronezh State University of Architecture and Civil Engineering. Construction and Architecture
16
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.
Issue № 4 (16), 2012 ISSN 2075-0811
17
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
Scientific Herald of the Voronezh State University of Architecture and Civil Engineering. Construction and Architecture
18
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
Issue № 4 (16), 2012 ISSN 2075-0811
19
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
Scientific Herald of the Voronezh State University of Architecture and Civil Engineering. Construction and Architecture
20
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
Issue № 4 (16), 2012 ISSN 2075-0811
21
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
Scientific Herald of the Voronezh State University of Architecture and Civil Engineering. Construction and Architecture
22
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
Issue № 4 (16), 2012 ISSN 2075-0811
23
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
Scientific Herald of the Voronezh State University of Architecture and Civil Engineering. Construction and Architecture
24
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].
Issue № 4 (16), 2012 ISSN 2075-0811
25
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
Scientific Herald of the Voronezh State University of Architecture and Civil Engineering. Construction and Architecture
26
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.
Issue № 4 (16), 2012 ISSN 2075-0811
27
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.