PORE STRUCTURE AND THERMAL CONDUCTIVITY OF BURNT CLAY BRICKS INTRODUCTION

Transcription

1 PORE STRUCTURE AND THERMAL CONDUCTIVITY OF BURNT CLAY BRICKS Olga Koronthalyova, Peter Matasovsky Insttute of Constructon and Archtecture, Slovak Academy of Scences, Dubravska 9, Bratslava, Slovaka. e-mal: Abstract: Commonly manufactured burnt clay brcks were analysed from the aspect of the relatonshps between ther mcrostructure and dry thermal conductvty. On the base of the analyss a smple mcrostructural model of thermal conductvty was proposed. The proposed model was compared wth the mcrostructural model developed for the calcum slcate hydrates based composte materals. Keywords: Thermal conductvty, clay brck, porosty, fractal dmenson INTRODUCTION Most of the buldng materals are compostes and ther thermal parameters represent the effect resultng from the propertes of ther partcular phases and components. Usually the composte conssts of the bondng matrx, aggregate and pore space. The thermal conductvty of the dry porous materal s gven by the propertes of the sold phase as a whole and by the pore volume. The known relatonshps among the sold phase propertes pore structure parameters and materal thermal parameters enable to model thermal propertes of the porous materals only from the knowledge of propertes of the sold phase and the pore space. In the prevous work [6] the relatonshp between the thermal conductvty and pore structure parameters for dry CSH-based mortars, plasters and nsulaton boards was analysed. It was found out that n the frst approxmaton the thermal conductvty of tested materals was proportonal nversely to the second power of ther total porosty. It was also shown that the thermal conductvty of the tested materals could be modeled by seral confguraton of the aggregate wth bulk paste and nterfacal transton zone conductvtes, weghted by the power functon of ther volume fractons. The exponents of the power functon were determned usng the partcular components fractal dmensons. Wth the am to verfy ths approach on a dfferent buldng materal the relatonshp between the thermal conductvty and pore structure parameters was analysed for burnt clay brcks. Burnt clay brcks are common buldng materal manufactured from the clay-sand mxture (the porton of the sand can be from 30 to 50%). The burnng takes place at temperatures C. Ther mcrostructure s dependent of the partcle sze dstrbuton, mneralogcal and chemcal composton, type of burnng addtves and the way of burnng [2,3]. 00

2 THEORY Macroscopc thermal conductvty of composte materal can be expressed usng the thermal conductvtes of partcular components. For the contnuous hgh conductvty components fractons wth the dscrete low contnuty components the macroscopc thermal conductvty can be modeled by the summaton of partcular components: = n ( Φ Φ ) () Where s thermal conductvty of component, Φ s component volume fracton, Φ,crt s the crtcal volume needed for a connected network to be formed through the materal, n = + FD, FD s the fractal dmenson of the component or pore volume fracton [5]. In case of parallel confguraton of the components n =. In case of dscrete hgh conductvty components n the contnuous low conductvty or pore volume fractons the followng relaton can be appled: = ( Φ Φ ) Where n =for seral confguraton of the components, n > n case of dsperson of the low conductvty component. In the prevous work [6] the followng emprcal relaton between thermal conductvty and total porosty was found out for CHS based materals:,crt,crt n (2) = 2 Φ (3) It was also shown that the thermal conductvty of the tested materals could be modeled by seral confguraton of the aggregate wth bulk paste and nterfacal transton zone conductvtes by the followng relaton: = (4) n n2 ( Φ aggregate + Φ paste Φ pasteitz ) ( Φ + Φ pasteitz ) + aggregate Where ITZ s thermal conductvty of nterfacal zone + porosty. The porosty and sold phases fractal dmensons were estmated as FD.0. ITZ EXPERIMENTAL The pore structure parameters and dry thermal conductvty were determned for four types of burnt clay brcks. 0

3 The pore sze dstrbuton, the open porosty and the specfc surface area of pores were studed usng the mercury ntruson porosmetry (MIP): the hgh-pressure porosmeter mod and macro-porosmeter mod. 20 (both Carlo Erba, Mlan). Ths system enables determnaton of mcropores wth the radus from 3.7 up to 7500 nm and of larger pores wth a radus up to 0.06mm. The porosmetry measurement was carred out by the fracton of broken dred (up to 05 C) samples. Specfc surface area of pores was determned usng the cylndrcal model. The open porosty was also determned from the sucton test. The thermal conductvty was measured by transent pulse method usng the commercal devce ISOMET 204 wth the surface probe API The used surface probe s sutable for thermal conductvtes n the range from 0.3 to 2 W/m K. The measurement s based on the analyss of the temperature response of the practcally sem-nfnte body to the heat flow mpulse. The heat flow s generated by electrcal heatng usng a resstor heater havng a drect thermal contact wth the surface of the sample. The samples were condtoned under laboratory condtons: 28 C and 36% relatve humdty and they can be practcally consdered as dry. RESULTS AND DISCUSSION The total porosty, specfc surface area and pore radus medan values, determned by MIP are presented n Tab.. The values of open porosty, determned from sucton test, bulk and true densty and the measured values of thermal conductvty are n Tab. 2. Because for the clay brcks the values of open porosty and total porosty are practcally dentcal [3], the values of open porosty were used n the followng analyss of the relatonshps between the thermal conductvty and mcrostructure. In Fg. the relatonshp between the total porosty and thermal conductvty of tested brcks s compared wth the prevous measurements of CHS materals and the emprcal relaton (3). As can be seen from the fgure the dependence thermal conductvty/porosty dependence of brcks can be n the frst approxmaton expressed by emprcal relaton (3) smlarly lke n case of CHS materals. The clay brcks structure s charactersed by confguraton of dscrete sold components n contnuous low thermal conductvty phases. Therefore t was expected that the model of thermal conductvty could be based on relaton (2). Analogously to the relaton (4) for CHS based materals the thermal conductvty of dry clay brcks was expressed usng the model of the seral confguraton of sold and low thermal conductvty phases: = (5) n n 2 ( Φsold Φ ITZsold ) ( Φ + Φ ITZsold ) + sold ITZ Table. Pore structure parameters of the brcks (MIP )[Bagel 200] Materal Total porosty Specfc surface area [m 2 /g] Pore radus medan [nm] Brck D Brck L Brck P Brck S

4 Table 2. Open porosty (sucton test), true densty and dry thermal conductvty of the brcks Materal Bulk densty [kg/m 3 ] Open porosty [-] True densty [kg/m 3 ] Thermal conductvty [W/m K] standard devaton [W/m K] Brck D ±0.08 Brck L ±0.033 Brck P ±0.05 Brck S ±0.04 Effectve thermal conductvty [W/K.m] CHS based compostes brcks approx Total porosty [m3/m3] Fg. Thermal conductvtes of clay brcks and CHS based materals vs. total porosty. Comparson of the measured values and emprcal approxmaton () Fg. Optcal mcroscopy of brck L 03

5 Fg. 3 Bnary mage of mcroscopy brck L The fractal dmensons were determned from SEM pctures (Fg. 2, 3) of the consdered materals by a box countng method [8]. The fractal dmensons and correspondng exponents of the brcks components are n Tab. 3. The consdered thermal conductvty of the clay brck body wth densty equal to ca 2500 kg/m 3 (Tab. 2) was about.5 W/m K [7]. As the thermal conductvty of nterfacal zone (ncludng ts porosty) the value W/m K, correspondng to the value measured for the pure CHS xonotlte [4], was consdered. It was also supposed that the nterfacal zone volume fracton represented 0% of the total porosty n average. The comparson of the measured thermal conductvtes/total porosty relaton for clay brcks and CHS based materals wth the relaton (5) s shown n Fg. 4. As can be seen from the Fg. 5, the measured values of clay brcks are n a satsfactore agreement wth the relaton (5). It was also found out that the value W/m K could qualfy the thermal conductvty of the pore space + nterfacal zone fracton also n case of burnt clay brcks. Wth the am to evaluate the senstvty of the relaton (5) to the used materal parameters, the nfluence of the values of thermal conductvty of clay brck body and pore space + nterfacal zone fracton was analysed. The changes of the thermal conductvty of clay brck body wthn the nterval ± 30% of the orgnal value had neglgble nfluence on the resultant thermal conductvty/porosty relaton for the consdered porostes. On the other hand the changes of the thermal conductvty of pore space + nterfacal zone fracton had notcable nfluence. In Fg. 5 the effect of the thermal conductvty of pore space + nterfacal zone fracton changes wthn the nterval of ± 9% of the orgnal value s presented. Table 3 Fractal dmensons and correspondng exponents of tested brcks Component Fractal dmenson Exponent Sold phase FD = 0.0 n = Porosty FD =.7 n 2 =

6 Effectve thermal conductvty [W/K.m] CHS based compostes brcks seral n=, n2 =2.7 lamitz = seral n=, n2=2,7 lamitz = seral n=, n2 =2.7 lamitz = Total porosty [m3/m3] Fg. 4 Thermal conductvtes of clay brcks and CHS based materals vs. total porosty. Comparson of the measured values and relaton (4) CONCLUSIONS The relaton between mcrostructure and effectve thermal conductvty was nvestgated for four types of burnt clay brcks. The effectve thermal conductvty of dry clay brcks can be smlarly, lke thermal conductvty of CHS based mortars, plasters and nsulaton boards, modelled by seral confguraton of sold and porous - low thermal conductvty - phases. In the model the degree of the dsperson of low thermal conductvty phase s expressed by a power low. The exponent contans the pore space fractal dmenson. The fractal dmenson can be determned wth the use of mage analyss. ACKNOWLEDGEMENTS Authors wsh to thank the APVT and VEGA (Grant No 2/500/27) for the fnancal support of ths work. REFERENCES Bagel, L. (200) Structural parameters of clay brcks. Internal report. Insttute of Constructon and Archtecture, SAS, Bratslava (n Slovak). Cultrone, G. et al. (2004) Influence of mneralogy and frng temperature on the porosty of brcks. Journal of the European Ceramc Socety. Vol. 24, pp

7 Dond, M. et al. (2005) Water vapour permeablty of clay brcks. Constructon and Bulldng Materals. Vol. 7, pp Koronthályová, O., Matašovský, P. (2003). Thermal Conductvty of Fbre Renforced Porous Calcum Slcate Hydrate-based Compostes. Journal of Buldng Physcs, Vol. 26, No. 4, pp Meng, B. (994) Calculaton of mosture transport coeffcents on the bass of relevant pore structure parameters. Materals and Structures Matašovský, P., Koronthályová, O. (2005) Thermal Conductvty of Mortars and Fractal Propertes of ther Mcrostructure. In Proceedngs of Thermophyscs 2004, USTARCH SAV, ISBN Bratslava, 2005, pp Raznjevć, K. (984) Thermophyscal Tables, ALFA Bratslava, (n Slovak). Zmeskal, O. et al. HarFa Harmonc and fractal mage analyss. 06