MicroCTbased identification of double porosity in fired clay ceramics
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Abstract
Optimizing thermal and mechanical properties of clay block masonry requires detailed knowledge on the microstructure of fired clays. We here identify the macro and microporosity stemming from the use of three different poreforming agents (expanded polystyrene, sawdust, and paper sludge) in different concentrations. MicroCT measurements provided access to volume, shape, and orientation of macropores, and in combination with Xray attenuation averaging and statistical analysis, also to voxelspecific microporosities. Finally, the sum of micro and macroporosity was compared to corresponding data gained from two statistically and physically independent methods (namely from chemical analysis in combination with weighing, and from mercury intrusion porosimetry). Satisfactory agreement of all these independently gained experimental data renders our new concept for identifying the pore spaces of fired clay as a very promising tool supporting the further optimization of clay blocks.
List of symbols
 a
Slope parameter
 b
Intercept parameter
 \({\hbox {err}}^{{\mathrm{CT}}}\)
Relative error in microCTbased porosity determination, with respect to weighingbased determination
 \({\hbox {err}}^{{\text {Hgintr}}}\)
Relative error in mercury intrusionbased porosity determination, with respect to weighingbased determination
 GV
Voxelspecific attenuationrelated grey value
 \({\hbox {GV}}_{{\mathrm{Air}}}\)
Attenuationrelated grey value of air
 \({\hbox {GV}}_{{\mathrm{Al}}}\)
Attenuationrelated grey value of aluminium
 \({\hbox {GV}}_{{\mathrm{FC}}}^{{\mathrm{peak}}}\)
The most frequently occurring grey value of the fired clay matrix domain
 \({\hbox {GV}}_{{\mathrm{thr}}}\)
Grey value threshold value
 \(m_{{\mathrm{Dilatometer+Hg+Sample}}}\)
Mass of dilatometer filled with mercury and sample
 \(m_{{\mathrm{Dilatometer+Hg}}}\)
Mass of dilatometer filled with mercury
 \(m_{{\mathrm{Dilatometer}}}\)
Mass of dilatometer
 \(m_{{\mathrm{dry}}}\)
Mass of dry ceramic sample
Probability density function
 \(V_{{\mathrm{FC}}}\)
Volume of the fired clay matrix in ceramic sample
 \(V_{{\mathrm{micropores}}}\)
Volume of micropores in ceramic sample
 \(V_{{\mathrm{por}}}\)
Volume of pores in ceramic sample
 \(V_{{\mathrm{sample}}}\)
Volume of ceramic sample
 \(w_i\)
Weight fraction of ith constituent of the clay matrix
 \(w_{{\mathrm{dry}}}\)
Weight of dry ceramic sample
 \(w_{{\mathrm{sample}}}\)
Weight of ceramic sample
 \(w_{{\mathrm{sub}}}\)
Weight of submerged ceramic sample
 \(w_{{\mathrm{wet}}}\)
Weight of hydrated ceramic sample
 NIST
National Institute of Standards and Technology
 \((\mu {/}\rho )_i \)
Xray mass attenuation coefficient of ith constituent of theoretically completely dense clay matrix
 \(\Delta m\)
Difference in mass, between the dilatometer filled with both ceramic sample and mercury, and the solely mercuryfilled dilatometer
 \(\delta \)
Orientation angle of macropore
 \(\epsilon \)
Photon energy
 \(\mu \)
Attenuation coefficient
 \(\mu ^{{\mathrm{NIST}}}_{{\mathrm{Air}}}\)
Attenuation coefficient of air, according to the NIST database
 \(\mu ^{{\mathrm{NIST}}}_{{\mathrm{Al}}}\)
Attenuation coefficient of aluminium, according to the NIST database
 \(\mu _{{\mathrm{FC}}}^{{\mathrm{peak}}}\)
Most frequently occurring attenuation coefficient in the fired clay matrix domain
 \(\mu _{{\mathrm{Si}}}^{{\mathrm{NIST}}}\)
Attenuation coefficient of a theoretically completely dense clay matrix, derived from the NIST database
 \(\phi \)
Voxelspecific microporosity
 \(\phi ^{{\mathrm{peak}}}\)
Most frequently occurring value of voxelrelated microporosity
 \(\phi _{{\mathrm{macro}}}\)
Ceramic samplerelated macroporosity
 \(\phi _{{\mathrm{micro}}}\)
Ceramic samplerelated microporosity
 \(\phi _{{\mathrm{sample}}}^{{\text {Hgintr}}}\)
Ceramic samplerelated total porosity, obtained by mercury intrusion porosimetry
 \(\phi _{{\mathrm{sample}}}^{{\mathrm{weighing}}}\)
Ceramic samplerelated total porosity, obtained by weighing tests
 \(\phi _{{\mathrm{sample}}}^{{\mathrm{CT}}}\)
Ceramic samplerelated total porosity, obtained from microCT
 \(\rho _{{\mathrm{sample}}}\)
Ceramic samplerelated mass density
 \(\rho _{{\mathrm{Si}}}\)
Mass density of the theoretically completely dense clay matrix
 \(\rho _{{\mathrm{xylene}}}\)
Mass density of xylene
Introduction
Clay block masonry comfortably combines thermal and mechanical competences, making it one of the most sustainable and soughtafter building materials, in particular when it comes to the construction of small storey houses. The recent quest to extend the applicability of this material, both in scope and volume, motivates deeper scientific scrutiny into what actually lies at the origin of the aforementioned comfortable combination of material characteristics. It is well accepted that porosities which are induced in a more or less designed way at different scales into the material, are the key governing factor for both its mechanical and thermal properties [1, 2]. In this context, poreforming agents are used to increase porosity at scales ranging from micrometres to millimetres, in order to enhance the thermal insulation characteristics of the material. For the time being, the actual effect of this measure can only be determined empirically, through direct macroscopic testing. A first step towards a more scientific exploration of this effect is the quantification of the porosities themselves, and this is the focus of the present paper. Therefore, our preferred method of choice is microcomputed tomography, which, in recent years, has not only revealed microstructural features as represented by voxelbuilt patterns [3], but also basic compositional information within each and every voxel [4, 5, 6]. Accordingly, after presenting the investigated material in the “Investigated materials and microCT scanning” section, the “Methods for macroporosity determination” section is devoted to the determination of the ceramic macroporosity from thresholdingbased image analysis, while the “Methods for microporosity determination” section describes how fundamental relations from Xray physics, in combination with basic knowledge on fired clay chemistry, allow for the determination of the microporosity within each and every voxel. In order to check this new way of 3D quantification of the dualscale ceramic porosities, independent experimental access to the (average) porosities is provided by mercury intrusion porosimetry and weighing tests, as described in the “Mercury intrusion porosimetry and weighing tests” section. The corresponding results are presented in the “Results and discussion” section, and discussed thereafter.
Materials and methods
Investigated materials and microCT scanning
Ceramic specimens with dimensions \(30 \times 15 \times 125\,\hbox {mm}^{3}\) were fired at \(880\,^{\circ }\hbox {C}\). Their extrusion direction was parallel to the edge measuring 125 mm. These specimens exhibited different concentrations of different poreforming agents, namely expanded polystyrene (EPS) in mass fractions of 10 and 20%, as well as paper sludge and sawdust in mass fractions of 10, 20, and 40%, respectively. For reference purposes, we also investigated ceramic samples made of pure clay without poreforming agents, fired at 880 and \(1100\,^{\circ }\hbox {C}\). In order to image the ceramic microstructures by means of microcomputed tomography (\(\upmu \hbox {CT}40\), Scanco, Switzerland), ten smaller samples with dimensions \(6 \times 5 \times 15\,\hbox {mm}^{3}\) were cut out from the aforementioned samples, by means of a distilled watercooled lowspeed diamond saw (Isomet, Buehler, USA). The extrusion direction of the aforementioned smaller samples was parallel to the edge measuring 6 mm. The following settings were used in the scanning process: source current \(114 \,\upmu \hbox {A}\), source voltage 70 kVP, integration time 300 ms. The voxel size of the resulting microCT images was \(6 \times 6 \times 6 \,\upmu \hbox {m}^{3}\), and the voxelspecific attenuation was characterized through a 16bit grey value scale ranging from 0 to 32767.
Methods for macroporosity determination
Methods for microporosity determination
Chemical composition of clay: chemical elements in (mass %), and in parts per million (ppm), as a function of firing temperature, obtained from Xray fluorescence spectroscopy
Firing temperature  \(880\,^{\circ }\hbox {C}\)  \(1100\,^{\circ }\hbox {C}\)  

\(\hbox {SiO}_{2}\)  (mass %)  58.4  58.7 
\(\hbox {Al}_{2}\hbox {O}_{3}\)  (mass %)  17.5  17.2 
\(\hbox {TiO}_{2}\)  (mass %)  0.9  0.9 
\(\hbox {Fe}_{2}\hbox {O}_{3}\)  (mass %)  7.0  7.1 
\(\hbox {CaO}\)  (mass %)  5.7  5.8 
\(\hbox {MgO}\)  (mass %)  5.3  5.1 
\(\hbox {K}_{2}\hbox {O}\)  (mass %)  3.0  3.0 
\(\hbox {Na}_{2}\hbox {O}\)  (mass %)  0.9  1.0 
\(\hbox {SO}_{3}\)  (mass %)  1.4  1.3 
MnO  (mass %)  0.11  0.11 
\(\hbox {P}_{2}\hbox {O}_{5}\)  (mass %)  0.11  0.13 
Ba  (ppm)  401  420 
Co  (ppm)  31  48 
Cr  (ppm)  154  162 
Cu  (ppm)  38  36 
Ga  (ppm)  23  27 
Mo  (ppm)  4  4 
NB  (ppm)  20  16 
Ni  (ppm)  84  83 
Pb  (ppm)  25  26 
Rb  (ppm)  140  139 
Sr  (ppm)  205  205 
V  (ppm)  133  136 
Zn  (ppm)  117  114 
Equation (10) constitutes a nonbijective function between \(\phi ^{{\mathrm{peak}}}\) and \(\epsilon \), with the characteristic that a specific value of \(\phi ^{{\mathrm{peak}}}\) is related to either none, one, or two values of the (average) photon energy \(\epsilon \). As only one (average) photon energy was used for the scanning process, the one value of \(\phi _{{\mathrm{peak}}}\) which is related to only one photon energy \(\epsilon \), is the only and unique physically admissible (average) energy level. This provides unique values for both \(\epsilon \) and \(\phi ^{{\mathrm{peak}}}\).
Mercury intrusion porosimetry and weighing tests
Results and discussion
Results of macroporosity determination
Characterization of the macropore space of samples with EPS and sawdust
Poreforming agent  EPS  Sawdust  

Agent concentration (mass %)  10  20  10  20  40 
Macropore volume fraction (%)  4.20  6.90  4.80  9.98  19.10 
Number of macropores  2047  2496  4936  9178  6695 
Volume of largest macropore \((\hbox {mm}^{3})\)  3.41  6.88  1.13  2.11  7.51 
Statistical distribution of bounding box dimensions: percentage of bounding boxes with largest edges oriented in x, y, and zdirections
Poreforming agent  EPS  Sawdust  

Agent concentration (mass %)  10  20  10  20  40 
Percentage of bounding boxes with longest edge parallel to  
xdirection (extrusion)  51.07  45.41  62.46  61.91  67.66 
ydirection  34.02  33.21  23.87  14.32  24.88 
zdirection  0.62  5.84  1.86  12.40  1.17 
x and ydirections  12.53  9.46  10.02  4.28  5.62 
x and zdirections  0.62  2.46  0.63  3.85  0.27 
y and zdirections  0.38  2.03  0.48  1.71  0.21 
x, y, and zdirections  0.76  1.59  0.69  1.53  0.19 
Results of microporosity determination
Peak microporosities \(\phi ^{{\mathrm{peak}}}\) in all investigated samples
Sample characteristics  \(\phi ^{{\mathrm{peak}}}\) (%) 

10 (mass %) EPS; \(880\,^{\circ }\hbox {C}\)  37.29 
20 (mass %) EPS; \(880\,^{\circ }\hbox {C}\)  37.29 
10 (mass %) paper sludge; \(880\,^{\circ }\hbox {C}\)  37.51 
20 (mass %) paper sludge; \(880\,^{\circ }\hbox {C}\)  39.74 
40 (mass %) paper sludge; \(880\,^{\circ }\hbox {C}\)  43.06 
10 (mass %) sawdust; \(880\,^{\circ }\hbox {C}\)  37.61 
20 (mass %) sawdust; \(880\,^{\circ }\hbox {C}\)  40.60 
40 (mass %) sawdust; \(880\,^{\circ }\hbox {C}\)  41.87 
No agent; \(880\,^{\circ }\hbox {C}\) clay  35.37 
No agent; \(1100\,^{\circ }\hbox {C}\) clay  28.15 
Comparison of microCTderived porosities, with those obtained from mercury intrusion and weighing
Sample characteristics  \(\phi _{{\mathrm{micro}}}\) (%)  \(\phi _{{\mathrm{macro}}}\) (%)  \(\phi _{{\mathrm{sample}}}^{{\mathrm{CT}}}\) (%)  \(\phi _{{\mathrm{sample}}}^{{\text {Hgintr}}}\) (%)  \(\phi _{{\mathrm{sample}}}^{{\mathrm{weighing}}}\) (%)  \({\hbox {err}}^{{\mathrm{CT}}}\)(%)  \({\hbox {err}}^{{\text {Hgintr}}}\)(%) 

Equation (13)  Equation (1)  Equation (14)  Equation (17)  Equation (19)  Equation (20)  Equation (21)  
10 (mass %) EPS; \(880\,^{\circ }\hbox {C}\)  34.03  4.20  38.23  38.36  38.55  − 0.83  − 0.50 
20 (mass %) EPS; \(880\,^{\circ }\hbox {C}\)  34.76  6.90  41.66  36.41  42.16  − 1.19  − 13.65 
10 (mass %) paper sludge; \(880\,^{\circ }\hbox {C}\)  38.51  0  38.51  37.22  41.80  − 7.87  − 10.96 
20 (mass %) paper sludge; \(880\,^{\circ }\hbox {C}\)  41.24  0  41.24  42.08  42.76  − 3.54  − 1.58 
40 (mass %) paper sludge; \(880\,^{\circ }\hbox {C}\)  44.04  0  44.04  49.68  49.84  − 11.64  − 0.33 
10 (mass %) sawdust; \(880\,^{\circ }\hbox {C}\)  34.56  4.80  39.36  38.58  40.71  − 3.33  − 5.25 
20 (mass %) sawdust; \(880\,^{\circ }\hbox {C}\)  34.07  9.98  44.05  41.60  44.7  − 1.66  − 7.13 
40 (mass %) sawdust; \(880\,^{\circ }\hbox {C}\)  33.36  19.10  52.46  50.27  52.66  − 0.37  − 4.53 
No agent; \(880\,^{\circ }\hbox {C}\) clay  36.01  0  36.01  36.06  37.76  − 4.62  − 4.50 
No agent; \(1100\,^{\circ }\hbox {C}\) clay  28.83  0  28.83  28.49  28.21  2.22  0.98 
Conclusion

The pore size distribution depends critically upon the used poreforming agent. EPS and sawdust induce macropores with sizes ranging from many micrometres up to millimetres, which paper sludge does not. Hence, paper sludgeprocessed ceramic samples only exhibit micropores of many nanometres to a few micrometres in size, such pores also contributing to the total porosity of EPS—and sawdustprocessed ceramic samples.

The macropores are larger and less numerous in the EPS as compared to the sawdustprocessed samples, and the pores are typically elongated in shape and oriented towards the extrusion direction.

The voxelspecific microporosity (measured per \(6 \times 6 \times 6\,\upmu \hbox {m}^{3}\) reference volume) depends strongly on the firing temperature and moderately on the sawdust and paper sludge content; which it is not affected by the EPS content.
Notes
Acknowledgements
Open access funding provided by TU Wien (TUW). The authors are thankful for the support by the ‘Klima und Energiefonds’ through the programme ‘Energie der Zukunft’. Funding for research was provided by Österreichische Forschungsförderungsgesellschaft (Grant No. 843897).
Compliance with ethical standards
Conflict of interest
All authors declare that they have no conflict of interest.
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