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THERMAL INSULATING MATERIALS BASED ON GEOCEMENT Sergii G. Guzii1, Vít Petránek2, Konstantinos Sotiriadis3, Jan Maňák4 V.D .Glukhovsky Scientific Research Institute for Binders and Materials1, Kyiv National University of Civil Engineering and Architecture, Kyiv Ukraine Institute of Technology of Building Materials and Components2-4, Faculty of Civil Engineering, Brno University of Technology, Brno, Czech Republic Abstract: In the this work, sandwich-type thermal insulating materials (lightweight concrete – expanded coating) were developed on the basis of geocement (formulation Na2OAl2O36SiO2(2025)H2O). The optimal composition (lightweight concrete: 90% ceramsite – 10% geocement) of such materials was determined at the temperature of 373 K and was characterized by average density of 573.3 kg/m3, compressive/bending strength of 1.20/0.09 MPa and thermal conductivity coefficient of 0.192 W/mK. After heat treatment at 1073 K, the average density was 450.3 kg/m3 and the thermal conductivity coefficient was 0.134 W/mK. The expanded coating of the sandwich-type materials had an expansive coefficient of 10.5 and showed a fine pore structure of the aluminosilicate frame, including evenly distributed large pores, and had low thermal conductivity. The developed sandwich-type thermal insulating material is possible to be used to protect and to insulate metal, concrete or wooden surfaces against heat flow in the temperature range of 773-1073 K. Key words: geocement, insulating materials, thermal conductivity, pore structure 1. Introduction Lightweight concrete has a long history of usage as a structural or thermal insulating

material. However, the interest towards its use has not been quenched till nowadays. On the contrary, lightweight concrete is increasingly gaining the attention of both researchers and civil engineers. This is due to the fact that its properties and structural features become more perspective regarding the contemporary scientific and technological development, which is related to environmental protection and energy saving [1,2].

Lightweight concrete has a very heterogeneous structure because of the heterogeneity of cement and the quality and distribution of aggregates. Several researchers [35] studied some of the factors that determine structure's heterogeneity, such as failure mechanism, strength, durability and other properties of lightweight concrete. The above mentioned factors are considered for lightweight concretes and thermal insulating materials which are based on Portland cement. 1 Sergii G. Guzii, PhD (Eng)/Senior scientist, Povitroflotsky pr., 31, Kyiv, 03680, Ukraine, sguziy@ukr.net 2 Vít Petránek, Ing. PhD/Assistant professor, Veveří 95, 602 00 Brno, Czech Republic, petranek.v@fce.vutbr.cz 3 Konstantinos Sotiriadis, Ing. PhD/Post-doctoral researcher, Veveří 95, 602 00 Brno, Czech Republic, sotiriadis.k@fce.vutbr.cz 4 Jan Maňák, Student, Veveří 95, 602 00 Brno, Czech Republic, janmanak@volny.cz

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In the present work the properties of lightweight concretes, as well as of sandwich-type thermal insulating materials (lightweight concrete – expanded coating) based on geocement – alkaline hydroaluminosilicates were studied. The theoretical principles for geocement production were developed in the V.D. Gluchovsky Scientific Research Institute for Binders and Materials [613].

2. Materials and methods Lightweight concretes based on geocement were produced. The geocement used was

formulated as Na2OAl2O36SiO220H2O and prepared using kaolin, sodium water-glass (silicate modulus Мs=3.2-3.5; density =1380-1409 kg/m3), microsilica and NaOH. The necessary quantities of the above materials to produce geocement were calculated according to [14], and introduced into a mixer in the following order: initially water-glass was poured, in which NaOH was dissolved. Furthermore, microsilica was added and after that kaolin. The mixture was subjected to a mixing process of 30 minutes duration.

Ceramsite with a maximum size of 1-4 mm and bulk density of 557 kg/m3 was used as a filler to lightweight concretes. The size distribution of ceramsite is given in Fig. 1.

Fig. 1. Particle size distribution of ceramsite.

The specimens (prisms of 4040160 mm and plates of 16013640 mm) were prepared applying vibration to load-bearing moulds for 1 minute. In Tabl. 1, the composition of each thermal insulating material is summarized.

Table 1. Composition of insulating materials Lightweight concrete components ratio, % Material ceramsite geocement hardener*

I 60 40 - II 80 20 - III 90 10 - IV 85 15 - V 80 15 5

Note: * A calcareous reactant has been used as a hardener.

After molding, the specimens were thermally cured in a chamber at 333 K for 24 h, in order to accelerate the hardening process of geocement and to increase the specimens resistance against water absorption.

The expansive coating, applied to the sandwich-type materials (lightweight concrete – expanded coating) was based on a geocement, formulated as Na2OAl2O36SiO225H2O.

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Aluminosilicate granules with a maximum size of 0.63-2.0 mm were used as filler to expansive coating. The granules were prepared by granulating geocement of the Na2OAl2O36SiO225H2O formulation in a calcium chloride solution of 1350 kg/m3 density. Additionally, grinded limestone with a specific surface of 350 kg/m2 (Blaine method) was also used as a filler to expansive coating.

The components used to produce the protective expansive coating were subjected to a mixing process of 5 minutes duration. The material obtained was applied with a spatula on the surface of the thermal insulating plates.

The physical and mechanical properties of the prisms were determined in accordance to the Czech Standards.

The thermal insulating plates covered with the expansive coating were subjected to heat treatment at 1723 K for 1 h. The calculation of the coating's expansion coefficient is [5]:

(2.1) 1

2exp V

VK ansive or 1

2exp h

hK ansive ,

where: V2 and h2 are the volume (cm3) and the height (cm) of the specimen after heat treatment; V1 and h1 are the volume (cm3) and the height (cm) of the specimen before heat treatment.

The thermal conductivity coefficient was determined according to a formula suggested by V.P. Nekrasov [15]:

(2.2) 16.022.00196.016.1 2 d ,

where: ρ is the average density of lightweight concrete (kg/m3); d is the relative density of

lightweight concrete (kg/m3), derived as

2d in relation to water's density.

The microstructure of the hardened coatings was studied by SEM using secondary electron imaging. For this purpose, samples were obtained from the non-polished rough surface of the coatings, and they were coated with Cu. The hydration products were identified according to the data provided by [16].

3. Results As it was mentioned above, geocement was used as a binder for both thermal

insulating materials and expansive coatings. Its production was based on the formation in its structure of heulandite type phases which are similar to zeolite [68].

When heat is applied (temperature above 423 K), the geocement of heulandite composition is getting expanded, forming a porous anhydrous aluminosilicate frame characterized by low thermal conductivity (Fig. 2а). The same approach is also considered in the case of expansive coatings, in which, when heat is applied, both geocement and geocement based filler are simultaneously expanded (Fig. 2b).

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(a) (b)

Fig. 2. SEM images of (a) hardened geocement (composition Na2OAl2O36SiO225H2O),

and (b) expansive coating of the same composition, after heat treatment at 773 K.

The structure of the coating becomes more porous since the quantity of the pores of various sizes formed is increased in the structure of the expanded material. The pores are distributed in a chaotic way, however a kind of regularity does exist: alternation of big sized pores with the small sized ones. The areas between the pores have also a porous structure.

In Fig. 3, the dried prisms and plates of lightweight concrete with applied coating are presented.

(a) (b)

(c) (d)

Fig. 3. Top view (а, c) and side view (b, d) of prisms (a, b) and plates (c, d) made by lightweight concrete after a drying process at 333 K for 24 h.

The basic physical and mechanical properties of lightweight concretes before the

drying process are given in Tabl. 2. The sandwich-type materials (lightweight concrete – expanded coating) III, IV and V possess the best characteristics, concerning strength and thermal conductivity coefficient.

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Table 2. Physical and mechanical properties of insulating materials

Material Average density, kg/m3

Thermal conductivity coefficient, ,

W/mK

Compressive strength, MPa

Bending strength, MPa

I 790.0 0.2995 2.0 0.14 II 707.0 0.2575 1.7 0.12 III 573.3 0.1917 1.2 0.09 IV 589.1 0.1993 1.5 0.10 V 616.7 0.2128 1.9 0.13

Note: dimensions of prisms of material I: 16.063.964.11 cm; material II: 16.134.034.07 cm; material III; 16.034.004.01 cm; material IV 16.414.023.86 cm; material V: 15.91х3.99х4.10 cm.

According to the above data, such a construction may be functional under a variable thermal load in the temperature range of 773-1073 K for a quite long period of time.

In order to confirm the previously mentioned claim, plate specimens of the materials III, IV and V were produced, on the surface of which an expansive geocement coating was applied. The coating's composition was [13]: geocement – 65.0 %; granules – 27.0 %; CaCО3 – 8.0 % (Fig. 3b).

The expansion of the sandwich-type thermal insulating materials took place in electric furnace at 1073 K (Fig. 4). After expansion, the average density, coating's expansion coefficient and thermal conductivity coefficient were determined for each of the materials III, IV and V (Tabl. 3).

Table 3. Physical and mechanical properties of sandwich-type materials

Average density, kg/m3 Thermal conductivity coefficient, , W/mK

Materials before drying process

after heat treatment at Т=1073 K

before drying process

after heat treatment at Т=1073 K

III 791.2 450.3 0.3001 0.1339 IV 843.6 535.6 0.3269 0.1736 V 846.8 538.2 0.3513 0.1748

(a) (b)

Fig.4. External view (a) and sectional view (b) of a sandwich-type thermal insulating

material after heat treatment in electric furnace at 1073 K for 1 h. It should be noted that after heat treatment a fine porous structure is formed in the

expansive coating, characterized by low thermal conductivity, which includes evenly distributed large pores. The structure of the thermal insulating material consists of tightly

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packed spherical particles of ceramsite, which are related to the geocement's porous matrice (Fig. 4b).

After heat treatment, the maximum value of the expansion coefficient (Kexpansive=10.5) was recorded for the plates of the sandwich-type material III. The results obtained show that it is possible to use the developed material in order to protect metal, concrete or wooden structures against the effect of heat flow in the temperature range of 773-1073 K (Fig. 5).

(a) (b) (c)

Fig. 5. External view of sandwich-type thermal insulating materials, intended to protect

metal (a), concrete (b) and wooden (c) surfaces against the effect of heat flow in the temperature range of 773-1073 K: 1 – metal; 2 – concrete; 3 – wood; 4 – glue based on geocement formulated as Na2OAl2O36SiO220H2O; 5 – lightweight concrete based on

geocement formulated as Na2OAl2O36SiO220H2O; 6 – expanded coating based on geocement formulated as Na2OAl2O36SiO225H2O.

Further work will be aimed at determining the optimum thickness of both the

expansive coating and thermal insulating material, as well as the calculation of thermal resistance.

Conclusion In the this work the results related to the development of sandwich-type thermal

insulating materials (lightweight concrete – expanded coating) based on geocement (formulation Na2OAl2O36SiO2(2025)H2O) were presented. The optimal composition (lightweight concrete: 90% ceramsite – 10% geocement) of such materials was determined

1 2 3

4 4 4

5 5 5

6 6 6

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at the temperature of 373 K. This composition is characterized by average density of 573.3 kg/m3, compressive/bending strength of 1.20/0.09 MPa and thermal conductivity coefficient of 0.192 W/mK. After heat treatment at 1073 K, the average density is 450.3 kg/m3 and the thermal conductivity coefficient is 0.134 W/mK. The expanded coating of the sandwich-type materials has an expansive coefficient of 10.5 and is characterized by a fine pore structure of the aluminosilicate frame, including evenly distributed large pores, and by low thermal conductivity. The developed sandwich-type thermal insulating material is possible to be used in order to protect and to insulate metal, concrete or wooden surfaces against heat flow in the temperature range of 773-1073 K.

Acknowledgement This work was financially supported by the research project MSM 0021630511, by

the research project TIP, number FR-TI2/340, financed from the Czech Republic's state budget via Ministry of Industry and Trade, and by the project CZ.1.07/2.3.00/30.0005 – Support for the creation of excellent interdisciplinary research teams at Brno University of Technology.

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