BUILDING MATERIALS FROM ENERGETIC WASTES AND SUITABLE ECOLOGICAL WAYS OF THEIR UTILIZATION

 

Kulísek Karel, Černý Vít

Brno University of Technology, Faculty of Civil Engineering

 

 

1.     INTRODUCTION

 

The significant social changes, which have taken place in the Czech Republic from the beginning of the past Century nineties, had a great impact even in the area of power engineering, where property changes of basic importance have taken place. The majority of great power- and heating plants was significantly modernized, practically in all cases the hydraulic transport of combustion product to the land fills was substituted by dry withdrawal of fly ashes.

The new conception of handling with fly ashes was solved in all cases by building of great storage bins for dry fly ash and by installation of mixing centers for the production of stabilizers. These products are up to now mostly utilized as building materials for blocking and technical recultivation of fly ashes land fills and they are partially used in the building industry.

The modernization of energetic equipments consisted also in the installation of quite a number of new boilers. The new boilers were mainly directly desulphurized fluid bed boilers. The volume production of fluid bed fly ashes increased relatively quickly and in the same time the number of smaller, local sources which produced classical fly ash decreased. All new fluid bed boilers are working on the base of brown coal, respecting the domestic fuel base.

The effect of modernizing processes is a local lack of classical black fly ashes, which were up to now utilized in mass in the production of concrete in central concreting plants and in the production of cellular concrete. This problem was partially managed by the utilization of brown coal fly ashes.

The high volumes of fluid bed fly ashes production need from the utilization point of view the application of new technologies. The specific properties of fluid bed fly ashes, given first of all by the rest portion of free CaO and water free calcium sulphate content, as products of the desulphurization process, limit more likely their utilization in the building industry. These fly ashes can be successfully used, in the case of correct application, for the production of fly ash suspensions and stabilizers.

The necessary attention was paid to the development and the utilization of new building materials in the Czech Republic during the last years. From the technical and technological point of view these problems can be considered as solved. The results of research and development are step by step applied in the building practice.

 

 

2.     THE FLY ASH SUSPENSIONS AND STABILIZERS

 

The new sort of building materials is the concentrated, self compacting fly ash suspension, which is produced as homogeneous mixture of fly ash and water, mostly with the addition of one or more binders and special admixtures respectively. The basic principle of fly ash suspension use is the utilization of its thixotropic properties. The pumpable semi-product, after casting into the earth body structure, is spontaneously transformed in pastelike state and subsequently into the dense state. The compacted mixture can be 24 hours after casting loaded by walk and during subsequent compacting the strength values increase similarly as in the case of concrete.

The favourable property of the fly ash suspension is the self-compacting effect which enables to omit the technological compaction process. The self-compacting effect is determined by the comparison of resulting volume mass with the volume mass of mechanically compacted suspension having the same composition and the optimal moisture content determined by the usual Proctor Standard method (ČSN EN 13286-2 “Unbound and hydraulically bound mixtures - Part 2: Test methods for the determination of the laboratory reference density and water content - Proctor compation). The results of tests proved, that the self-compacting effect applied in the first phase of suspension maturing substitutes the compaction process.

Important property of compacted and matured fly ash suspension is the compression strength achieved after 28 days of standard setting. Following the achieved strength values the suspensions are divided into four qualitative classes. In the case of the suspension with the worse quality the compression strength 0.2 MPa corresponds with natural ground. The minimal demanded compression strength in the case of next classes is minimally 1, 2, 4 MPa depending on the supposed application.

The concentrated self-compacting fly ash suspension is, when used in road building, marked as pumpable stabilizer. The fluidity of the fly ash suspension is its basic property and it is tested by the spilling of the sample from the Vicat ring on glass plate. A cake with the diameter 140 till 180 mm has to be formed during the spilling test, provided, that the customer doesn´t want a different consistency. In the case, that the spilled cake is smaller than 100 mm, the building semi-product is marked as paste. If the cake is larger than 200 mm the suspension has a poor quality.

It was generally proved during the development of this new building semi-product that for the production of fly ash suspensions the majority of classical fly ashes can be used. The achievement of necessary properties depends only on the correct formula of the raw suspension and above all on the kind and quantity of applied binder. The rule is generally that in the production of suspensions it is necessary to use a binder (cement, lime) in the quantity of at least 3 % (by weight - of the dry fly ash). Such an addition of binder secures mostly the fulfillment of ecological demands. This is proved by the analysis of the water leach and by the eco-toxicity test. Another important demand is not to exceed the limits of radio-nuclides, expressed by mass activity 226Ra (in Bq.kg-1) and by the index of mass activity.

The majority of fly ashes in the Czech Republic doesn´t exceed the admissible radioactivity limits. This specific property of the fly ash cannot be affected by the processing method and the inconvenient fly ash cannot be used.

Higher compression strength (2 till 4 MPa) in the production of suspensions from classical fly ashes can be achieved in the matured mixture only by addition of cement, mostly in quantities 5 till 10 % (by weight – of the dry fly ash). In processing of suitable fluid bed fly ash is it possible to utilize the specific properties of the material and to achieve higher compression strength.

Example of fluid bed fly ash of quality formed during burning of black coal (power plant Třinec) is obvious from the following tables. During the tests the properties of classical suspension and of pastelike stabilizer were determined.

 

Technological Tests – Fly Ash Třinec

 

Table 1:  Formulae of fly ash suspensions and pastes for 1 m3 of fresh mixture

 

 

Mixture

No.

 

 

Portion of Binder

(Mass %)

 

Fly Ash

(kg)

 

Water

(l)

 

Consistency

(loose)

(cm)

 

Note

1.

-

740

740

18,0

Suspension

2.

5

660

729

20,0

“

3.

10

643

714

19,5

“

4.

-

952

604

8,0

Paste

5.

5

926

585

8,0

“

6.

10

900

575

8,0

“

 

Applied raw materials:     - fluid bed fly ash – power plant Třinec

                                          - Slag Portland cement II/B-S-32.5, Mokrá

                                          - Drinking water

 

 

Table 1:  Results of technologic tests of hardened suspensions and pastes

 

 

Mixture

No.

 

 Compression strength  (MPa)

Storing  - days

 

 

Volume mass

in dry state (kg.m-3)

 

14

28

60

90

 

1.

1,58

2,30

3,20

3,50

999

2.

2,94

3,25

3,35

3,55

1030

3.

4,70

4,80

5,20

5,60

1050

4.

4,90

5,30

6,40

6,80

1119

5.

3,60

5,40

6,40

6,85

1197

6.

6,90

8,00

10,60

11,20

1226

 

 

 

The properties of the matured suspension prepared from the fluid bed fly ash Třinec which are presented in tables were selected as examples of high quality fluid bed fly ash formed in combustion of black coal. In processing of common brown coal fly ashes are the strength values of matured suspensions lower, after 28 days of storing the achieved values are 1 till 2 MPa.

The concentrated self-compacting fly ash suspension is in principle a specific type of fly ash stabilizer. The fly ash stabilizer is mostly produced in mixing centers in the form of optimally humidified mixture, which is suitable for following hardening. After maturing the hardened fly ash stabilizer has mostly the same properties as the hardened fly ash suspension. The deciding advantage of the suspension use as pumpable stabilizer is the possibility of its easy placing into difficult approachable spaces.

The basic raw materials necessary for the production of suspension are fly ash, water, and binder (cement, lime). Even other components can be used for the production in order to improve the properties of the building material. The compression strength of hardened suspensions can be increased by the addition of energo-gypsum. The waste carbide lime can be used also as the binder.

In specific cases the fly ash suspension can be produced by the addition of washed clay. In this way we can improve the sealing effect of the applied suspension. The filtration coefficient can reach in this case the value x.10-9 m.s-1.

It is advantageous in order to fulfill safely all demanded properties of fly ash suspensions and stabilizers as new type of building semi-products, to apply as the basic input raw material the classical fly ash. In great power plants and heating plants are the properties of produced classical fly ashes stable and they dont´t change reliably. The properties of fluid bed fly ashes are not stable especially with respect to the rest of free CaO and to the water-free calcium sulphate. These components can change in dependence on the sulphur content of the used fuel. The mentioned facts show, that it is possible to achieve good results even in the condition of combined processing of classical fly ash and fluid bed fly ash. The fluid bed fly ash enters into the processing as binder after its properties have been tested.

The development of new production technologies and of fly ash suspensions and stabilizers application have taken place under conditions in which the fly ashes were  divided according its origin as classical and as fluid bed fly ashes. In the present time the process of European Standards introduction takes place in the Czech Republic in order to substitute the old standards. The European Standards divide the power plant and the heating plant fly ashes in two groups: siliceous and calcareous fly ashes. This division is for the most part the same as the up to now applied classification as classical and fluid bed fly ashes. From the point of view of processing and problems of using the fly ash suspension and the stabilizers is the new classification not important.

Greater importance has the observance of the new classification in siliceous and calcareous fly ashes in the case that the building material prepared from the fly ash is used in road building. It is possible in accordance with the European Standards to use the fly ashes separately.

 

 

3.     MACHINERY FOR THE PRODUCTION OF STABILIZERS  

 

Quite a number of mixing centers for production of fly ash stabilizers was build up in the Czech Republic in last years. In most cases the technological machinery is intended for continuous processing of stabilizers, only in few cases it is intended for batch mixing. The building of first mixing plants was mostly realized in cooperation with foreign deliverers, the major part of machinery was imported (Germany, France, Austria). Gradually we succeeded in substituting the machinery by delivery of Czech manufacturers. In the present time all predominant machines and equipments are manufactured in good quality in the Czech Republic.

From the technological point of view, all mixing centers are in principle solved in the same way. It concerns in all cases exact dosing of fly ash and other dry components following the prescribed formula. Afterwards the dry mix is humidified, or liquefied in high speed horizontal mixer. This machine secures the perfect homogenization of all components into the form of fly ash stabilizer. This arrangement of the technological equipment is universal and it enables the manufacture of optimally humidified products, of pastelike mixtures and of pumpable fly ash stabilizers.

The majority of mixing centers is equipped with automatic process control which enables the long term operation of the center with adjusted parameters. In this way are operated the mixing centers for instance in the case of subsequent high pressure pipeline transport of the pumpable stabilizer or in the case of belt conveying of optimally humidified stabilizer. In smaller mixing centers the production of stabilizer is successfully performed in the way that after switching the start push button the production of the stabilizer proceeds till the adjusted consumption of materials, which corresponds with the quantity of stabilizer loaded up to the transport vehicle.

 

The dosing of dry fly ash and of other dry components from the storage bins is performed always by means of classical tourniquets equipped by frequency changer for adjusting the revolutions. A special two levels horizontal revolving feeder was developed for very exact dosing which is shown in the figure no.1. It is in both cases volume dosing, which is in the following transformed in mass dosing i.e. the raw material passes through a weighing screw fixed on tensometers. The data from the balance have electronic feedback to the frequency changer of the tourniquet drive.

The utilization of weighing screws fixed on tensometers proved to be in operational conditions very reliable (fig.no.2). The results are always better than in the case of up to now used sliding weights. Another advantage of this technical solution is the possibility to adapt the revolutions of the weighing screw with the optimization of the flow rate of the material weighed by the screw. The equipment has a revolution counter and the indication of this apparatus enters together with the indications of the tensometers into the control system which immediately evaluates the material mass flow through the equipment.

The construction of revolving feeders and weighing screws has some innovations, which secure reliable and dust-free operation, for instance special solution of packings or the protection against penetration of impurities. The indication of weighing screws is used also for the running control of the production line proper operation.

 

 

Fig. 1. Two levels horizontal revolving feeder for exact dosing.

 

 

 

Fig. 2. Detail of fixing the weighing screw on the tensometric weighing equipment

 

 

Very important part of the mixing center for the production of fly ash stabilizer machinery is the high speed horizontal mixer (fig.3). In this equipment are the dosed dry components perfectly homogenized with the mixing water. The development of the mixer has brought to gradually improvements of the equipment construction, which has in the present time super parameters.

In comparison with other foreign made machines the high speed mixer has a new solution of the mixing water dosing. One inflow of the water is through the hollow shaft of the machine into the nozzles near the mixing shovels, the second branch of the mixing water flows on into the rinsing ramps in the internal casing of the machine. This solution prevents the gluing of the material on the internal casing and the machine has silent run and low power consumption.

The fly ash is an abrasive material, in all known mixing equipments for the production of fly ash stabilizers quick abrasion of the mixer shall takes place. This problem is in the quick speed mixer solved by installation of changeable stainless steel inserts on the internal shell of the mixing chamber. In continuous operating of the mixing center the wear of the inserts takes place after approximately two years. The change of inserts can be realized during one shift.

The input of the mixing water into the mixer is measured by a flow-meter and the measured values are forwarded into the control system. The flow of water is controlled in the way which maintains the relation of by weight dosed dry components and of the dosed water on the adjusted values. If the quantity of the dry fly ash dosed into the mixer decreases the inflow of the mixing water decreases automatically too. It means that the stabilizer leaving the mixer has also in the case of smaller output the same properties.

 

 

 

Fig. 3. High speed horizontal mixer during the expedition from the production

 

4.                UTILIZATION OF SUSPENSES AND  STABILIZERS  IN THE  BUILDING INDUSTRY

 

     Mixing centers built in great power plants and heating plants produce fly ash stabilizers in volumes of many million of tons per year. For utilization of such raw materials quantities are mostly in the given region not suitable conditions. The bulk of the production is used for closing and technical recultivation of old uploading yards and extensive, artificial, scenery elements. The stabilizer is in suitable cases used for filling of adequate areas such as former brick plants and queries.

Greater volumes of optimally humidified stabilizers are used in the area of classical building production especially in road building. This concerns the formation of roadbeds, earthworks in building new circumferential high ways in towns and villages.

From technological point of view technologies which utilize concentrated fly ash suspensions as pumpable stabilizer under special conditions are especially progressive. Among the first successful applications in the Czech Republic belongs the use of suspensions for building of great industrial halls. The difficult accessible spaces between great concrete machine fundaments were filled by suspension of quality transported to the place of application by pump. The sufficient strength of ripe suspension enabled the building of industrial floors for very heavy operations.

Very good results and a great economical effect was achieved by the use of fly ash suspensions in the casting of open cast excavation of underground stations and lines in Prague. In this case the suspensions were used as the substitute for up to now used low strength pumped concrete. The economical gain was not only the great saving of concrete but also the shortening of operation time.

The fly ash suspensions were used in similar principle for circumfusing large scale containers for crude oil and for crude oil products, which were built in connection with the oil pipe line Ingolstadt. For this purpose suspensions of high quality were delivered from a special plant. These suspensions contained following the wish of clients higher content of cement.

Another example of the utilization of these pumpable suspensions is the filling of subterraneous spaces such as unused drifts and sewage conduits. Sand-cement mixtures were earlier used for this purpose, these mixtures were expensive and the application was time consuming. These works were mostly performed in parallel with building of new savage conduits where the suspensions were used as substitute of sewage conduit thrust block. Great consumption of this material was applied also for circumfusing of concrete shafts.

An example of rather special utilization of this building semi-product is the use of pastelike and optimally humidified stabilizers for realizing of inert interlayer in landfills on refuse sites for communal wastes. The good availability of this cheap material was utilized, owing to the near production plant. The economical effect was the substitution of suitable soils, which were mined especially for this purpose.

One of the technologically progressive ways to use the fly ash suspensions in the building industry is the building of ground layers of “KAPS” type roads. This semi-dense structural roadway layer consists of a skeleton from coarse aggregates overflowed by high-quality fly ash suspension. The technologic principle of the ground layers realization is the coarse aggregate (mainly 32 till 63 mm) spreading in a layer with optimal thickness (mostly 20 cm). The aggregate layer is overflowed with the fly ash suspension and subsequently vibrated by the surface vibrator. The aim is to achieve a homogeneous ground layer of the road with favourable parameters i.e. carrying capacity, E-modulus and heat conduction coefficient.

Natural aggregate can be used (eventually even slag) with the demanded grain size composition. The aggregate is overspread by a grader on the protective gravel sand layer and pre-compacted   by two travels of the static cylinder. The aggregate skeleton is subsequently overflowed with the fly ash suspension directly from the conveyance (tanker, mobile mixer). The travels of the vibrating roller (4 till 6 travels) fill in the spaces of the stone skeleton. In the same time the fluid, fly ash suspension forms a sliding surface for the aggregate grains and the vibration effects arrange the grains into minimal volume with maximal bind of edges and tips. The cemented skeleton can after vibration practically immediately transfer the loads of technological transport.

The design and evaluation of the KAPS layer in the road structure assigns to individual qualitative classes calculation methods in conformity with  the Czech standard ČSN 73 6127 “Road Building – Grounted Courses“. In these calculations the Poisson´s ratio value 0.25 and the heat conduction coefficient 1.0 W.m-1. K-1 are used for all classes. The demands for the ground layer for the three basic qualitative classes are in the following table.

 

Table 3: Demands for the KAPS – Arrangement

 

 

 

Arrangement Class

 

 

E-Modulus

(MPa)

 

Minimum quantity

of binder in the suspension

(Mass %)

 

 

Minimum

compression

strength

Rc 28/90

(MPa)

 

Consistency

(loose)

(cm)

 

Fraction

of admixture

recommended

(mm)

 

KAPS I

 

2000

 

15

 

3,5

 

min. 15

22-63

(32-63)

 

KAPS II

 

1200

 

10

 

2,0

till 18

max. 26

32-63

(22-63)

 

KAPS III

 

800

 

5

 

Not tested

 

 

32-63

 

The doses of the binder (cement) in the table 3 can be in individual cases smaller,

if the composition of suspensions is confirmed by necessary tests.

 

 

5.     NEW TECHNOLOGIES FOR UTILIZATION OF ENERGETIC WASTES

 

The preceding part of the paper discussed the problems of fly ash suspensions and stabilizers utilization in the building industry. In the present time two quite new innovation subjects concerning the processing of energetic wastes are developed. The first subject is the heat treatment method of fly ashes which have excess content of combustible components, with the aim to decrease the content of combustible components under 6 %. The second innovation subject is the development of technology for artificial aggregates production from main wastes based on the principle of self burning of the raw material charge.

The first innovation subject is based on the knowledge, that great quantities of fly ashes are produced in the world with the residual combustibles content greater than 10 %. Such fly ashes are regarding the demands of valid standards practically non-utilisable. The task of this newly solved project is, based on actual experiences from USA, where the similar technology has been already solved. The result of the pretreatment is fly ash containing about 4 % (by weight) of combustibles and it is used as high quality pozzuolana admixture in the production of concrete.

The aim of the solved project is to design and to verify the process of fly ashes heat treatment on the principle of annealing it using the actual content of combustible components, to determine the optimum parameters of the process and to design model equipment using these new principles. The part of the task is the evaluation possibility of flue ashes energetic utilization by annealing the residual content of combustible components.

The up to now received results of the second innovation subject enable the preliminary conclusion that the production of artificial aggregates by clinkering of mining wastes is possible. Fairish results were achieved with tests of black coal mining wastes which contain about 7 till 12 % of combustible components. The optimum range for the self-burning of the charge was determined as 12 – 14 %. The eventual correction of the combustible portion was realized during the tests by addition of coal sludge which is in the treatment plant for coal produced together with coal wastes. The artificial aggregates produced during the tests were applied in orientation tests of concrete. The concrete samples showed after 28 days of storing compression strength values in the range 25 till 32 MPa depending on the composition of the mixture.

 The tests with production of artificial aggregates from brown coal wastes showed less favourable results. These coal mining wastes have significantly higher content of combustible components (25 till 30 % by weight). The concrete produced from these wastes had the compression strength 10 till 20 MPa. The processing of these wastes can be more advantageous in the case of the process energetic utilization.

The development of the artificial aggregates technology from coal mine wastes is formulated with has the assumption of very simple raw material treatment (crushing and separation of the suitable fraction 10 till 20 mm). The raw charge prepared in this way is solid, coherent and permeable. It contains mostly the nearly optimal portion of  combustible necessary for the self-burning of the charge.

The work on both innovation subjects continue and the results will be presented in the summary research report in the second half - year 2009.

 

 

5.     ACKNOWLEDGMENTS

 

This paper was supported by Czech Grant Agency, Project No. GA 103/06/1829, by Project No. MPO FI-IM2/183 and No. GA 103/05/H044. The authors thank for the support.

 

 

6.     REFERENCES

 

[1]   KULÍSEK, K.; ŠTĚPÁNEK, R.; ZAJÍČEK, M. Beitrag zur Problematik der Ausnutzung von Wirbel- schichtflugaschen im Bauwesen. In FISCHER, H-B. 13. ibausil-Internal. Baustofftagung, 1st ed. Tagungsbericht-Band I, HfAB Weimar 1997, S.825-830.

 

[2]   SEBÖK, T.; ŠIMONÍK, J.; KULÍSEK, K. The compressive strength of samples containing fly ash with high content of calcium sulfate and calcium oxide. Cement and Concrete Research, 2001, vol.31, No.7, p.1101-1107.

 

[3]   BÍLÝ, M. Inovační vývoj kontinuálního mixéru na výrobu stabilizovaných směsí. In:Sborník „Ekologie a nové stavební hmoty a výrobky“, VÚSTAH, Telč, 2003, S 170-175.(in Czech)

 

[4]   Fridrichová, M.; DVOŘÁK, K.; VEHOVSKÁ, L.; KULÍSEK, K. Verwertung der Fließbettflug-aschen zur Herstellung von hydraulischen Bindemitteln. In FISCHER, H-B. 16. ibausil-Internal. Baustofftagung, 1st ed. Weimar: F. A. Finger          Institut Bauhaus-Univ. Weimar, 2006, vol. 1, p.1135-1142.

 

[5]   ČERNÝ, V.; KULÍSEK, K. Možnost použití korekčních přísad na bázi odpadů při výrobě  agloporitu. (Possibility of correction admixtures based on wastes use in the production of aggloporite). In Construmat 2008. 1st.ed. Brno, Vysoká škola technická v Brně, Fakulta stavební. 2008. 8 s. ISBN 978-80-214-3660-2. (in Czech)