Enhancing Physical-Thermal-Mechanical Properties of Fired Clay Bricks by Eggshell as a Bio-Filler and Flux

Fired clay bio-bricks were prepared by adding eggshell as a bio-filler and flux into earthenware clay compounds via an extrusion process. In this study, the suiTab. conditions for clay bricks preparation were firing at 1000 C for a period of 5 h. Adding 20 wt% eggshell powder into the clay brick yielded good physical-mechanical-thermal properties: high compressive strength and hardness, low thermal expansion coefficient, and low water absorption. The measured compressive strength, hardness, and refractory water absorption were 7.0 MPa, 6.0 HV, and less than 15 wt%, respectively. The obtained clay brick with the eggshell powder added as shown here is potential for uses as bricks for construction and thermal insulation.


Introduction
The green building materials are gaining popularity and usage is growing rapidly.The important factors influencing the growing green building materials are environmental regulations, benefits for health, de-carbonization, energy saving and friendliness [1][2].There have been various composites used as construction materials such as fly ash [3], limestone [4], and ceramics [5] based composites.Fired clay brick is one of important building materials as it is made from natural raw materials such as clay, sand and water.However, the limitation of utilization and quality of bricks need to be improved to be accepted as modern building materials: good appearance, high compressive strength, high density, high hardness, light weight, low water absorption value, low heat transfer coefficient, good thermal shock resistance, low thermal expansion coefficient and long-life using time.In general, the brick forming process is by the extrusion and compression process of the mixture between 10 -15 wt% water and earthenware clay.The suiTab.firing temperature of brick is in the range of 1000 -1250 o C [6][7][8][9].The chemical compositions and characteristics of raw materials, the process of clay brick formation, firing temperature and firing time are important factors to obtain accepTab.fired clay bricks.
One possible way for saving energy in the brick production is by adding a filler acting as a flux or sintering aid to reduce the firing temperature, firing time and energy cost.There are many fillers available: calcite, silica from rice husk, carbonate, sun-flower seed hull, wood sawdust, fly ash, craft pulp production residue, waste tea, river sediment, waste marble powder, and sugarcane bagasse [6][7][9][10][11].A suiTab.filler is expected to modify physical, mechanical and thermal properties such as density, products color, water absorption, strength, hardness, Young's modulus, thermal insulation, and heat transfer coefficient [12].
There are many agricultural wastes that can be used as a filler in clay bricks: rice husk, oat hull, barley husk, grape and cherry seed and corn husk ash [1][2].Eggshell is one kind of agricultural wastes which contains various inorganic oxides: 94 wt% calcium carbonate, 1 wt% calcium phosphate, 1 wt% magnesium carbonate and 4 wt% organic substances [13,[14][15].Eggshell can act as a bio-filler functioning as a binder, sintering aid or flux, and foaming agent.Eggshell has been shown as a ceramic filler suiTab.for clay brick preparation [14][15].There are abundant sources of eggshell waste, either from consumers or food industries.In addition, the recycled eggshell as a bio-filler can be expected to decrease the land-fill pollution and contributes as an asset to the agriculture rather than being a waste material.
The objective of this work was to improve physical-mechanical-thermal properties of clay brick made from earthenware clay as the matrix phase utilizing eggshell CaCO 3 as a biofiller or the dispersed phase.

Materials and methods
Earthenware clay was obtained from Pa Mok District, Ban Norasing, Angthong Province, Thailand.
Chicken eggshell was collected from a local cafeteria.Eggshell was cleaned with tap water and dried in air at room temperature for 1-2 days.The cleaned chicken eggshell was ground with a porcelain rapid ball mill for 120 min to obtain an eggshell powder.

Instruments
X-ray diffraction patterns (XRD, Bruker, D8 Discover) were taken and analyzed using an analyzer with a VANTEC-1 detector and a double-crystal wide-angle goniometer.Scans were obtained from 10 to 80°, with a 2θ increment of 0.02° using CuKα radiation (λ = 0.154 nm).Peak positions were compared with standard JCPDS files to identify the crystalline phases.
Cumulative mass percent and fractional distributions were measured by a particle size analyzer (Mastersizer S long bed, Polydisperse 2.19).Each sample was dispersed in a water medium and vibrated in an ultrasonic cleaner for 20 min.
True density of samples was measured according to the ASTM B212-72 by a gas pycnometer (Quantachrome, Ultra Pycnometer 1000) by boiling in hot water for 5 h, and cooling in water for 24 h, according to the ASTM C 373-72 in Eq. 2: where ρ is the bulk density, D is the weight of dry sample (g), and true volume is the volume of the solid component only.It was determined by crushing each sample into a powder form so that all pores were destroyed.
Bulk density of samples was measured according to the ASTM B212-82 according to Eq. 3: where ρ is the bulk density, D is the weight of dry sample (g), and apparent volume is the volume of sample including open and closed pores.
SEM micrographs were obtained using a scanning electron microscope (SEM, JEOL, 5200) for the raw materials (eggshell powder and earthenware clay) and the clay products with eggshell added.Each sample was mounted on a stub using a carbon paste and was sputter-coated with ~0.1 µm gold to improve conductivity.An acceleration voltage of 15 kV with the magnification of 2,000 times were used.
The specific surface areas, pore sizes, and surface distributions were measured using an AUTOSORB-1 (QUANTACHROME) by determining the quantity of gas adsorbed onto or desorbed from the sample solid surface at some equilibrium vapor pressure by the static volumetric method.The specific surface area, S, of the solid was calculated from the total surface area and the sample weight, according to Eqs. 4 and 5: where S is the specific surface area of the solid, S t is the total surface area, W is the sample weight, N is Avogadro's number (6.023 × 10 23 molecules/mol), M is the molecular weight of the adsorbate, and A cs is the area occupied by one adsorbate molecule (16.2 × 10 -20 m 2 for N 2 and 19.5 × 10 -2 m 2 for Kr).There are three types of porosity classification by gas adsorption: (i) pores with openings exceeding 500 Å in diameter (macropores); (ii) pores with diameters not exceeding 20 Å (micropores); and (iii) pores of intermediate size (meso-pores).
The % water absorption was measured according to the Thai Industrial Standard Institute TIS 601/2547.Each sample was boiled in hot water for 5 h, and then allowed to cool in water 24 h.The excess water on each sample was removed by a dry cloth and the %water absorption was calculated according to Eq.6: where m 2 is the weight of dry solid sample after boiling in hot water for 5 h and cooling in water for 24 h.m 1 is the mass of the sample including open and closed pores.The color of the obtained clay refractory after firing was compared with the Munsell soil color charts for the reference colors.
A dilatometer (TL130052_1) was used to measure the thermal expansion coefficient of clay refractory after firing.Each clay brick sample was prepared into a rectangular rod shape and measured from 25 to 1000 o C.
A thermal gravimetric analyzer (TGA, Perkin-Elmer) was used to measure the thermal reaction, weight loss, and residue mass in the range of 25 to 1000 o C. A micro-Vicker hardness tester (Shimadzu, HMV-2000) was used to measure the hardness.
Compressive strength was measured according to the Thai Industrial Standard Institute TIS 243/2520.Each sample was prepared into a circular disk-shape with a thickness of 1.0 cm and a diameter of 2.5 cm.The maximum load was 25 N/m 2 (Pa).Each formula was measured with 5 samples and the average value of the compressive strength was calculated from Eq.7: where the compressive strength is the ability of samples to resist the compressive stress; ultimate load is the maximum load before sample cracking (Pa); and cross-sectional area is the cross-sectional area (cm 2 ).
An extruder was used for the clay sample preparation.The extruder was composed of a screw to extrude the mixture from the hopper to the die.
An oven (WTB Binder, 78532 Tuttlingen) was used to dry the green products at 100 o C for 24 h.The highest temperature used was 200 o C.

Clay samples preparation by extrusion
The clay green products were prepared according to the compositions in Tab.I.The raw materials were mixed and kneaded into homogenous mixtures and fed into the hopper of the extruder.Each mixture was pressed through the cylindrical shape die with the diameter of 5.00 cm.After the mixture passed through die, it was cut with a nylon wire into a 1.00 cm thickness sample of a uniformly disk-like shape.The obtained clay green products were dried at 100 o C for 24 h, and then fired at 800, 900, 1000, and 1100 o C for 1, 3, and 5 h, respectively.

Physical properties of raw materials and clay bricks
The colors of the clay green products (before firing) as prepared by the extruder are tabulated in Tab.I.The main color of the clay green products after kneading and extrusion was a dark yellowish brown.
Tab.I Formula of sample compositions.The physical properties namely average particle size, true density, and specific surface area of raw materials used (earthenware clay and chicken eggshell) are shown and reported in Fig. 1 and Tab.III.The average particle size, true density, specific surface area, and pore diameter of the chicken eggshell powder are 98.03 µm, 2.25 g/cm 3 , 4.06 m 2 /g and 196.90 Å, respectively.From the particle size distribution, the d 10 , d 50 and d 90 of the chicken eggshell are at 2.65, 65.51 and 258.95 µm, respectively.For the earthenware clay, the average particle size, true density, specific surface area and pore diameter are 16.04 µm, 2.40 g/cm 3    From Tab.I, the clay compound has a dark yellowish brown color.When the clay was mixed with the eggshell powder at 0, 5.26, 11.11, 17.65 and 25.00 wt% the color changed, according to the Munsell soil color chart, from the dark yellowish brown to a very pale brown.Each formula was prepared with 5 samples.Totally 300 samples of the clay brick were prepared by the extrusion into a disk-like shape having 5.00 cm in diameter and 1.00 cm in thickness.All clay samples were then fired at 800, 900, 1000, and 1100 o C for 1, 3 and 5 h.The color of the clay brick samples changed to a reddish yellow or yellow as reported in Tab.IV due to the presence of iron oxide (Fe 2 O 3 ), consistent with the result as to be verified by XRF.

Samples
The thermal gravimetric analyzer (TGA) was used to measure the thermal reaction of the raw materials used (earthenware clay and chicken eggshell) as shown in Figs.2a and 2b, respectively.The TGA profile of the earthenware clay shows two sharp peaks at 78 o C due to moisture content and 480 o C due to organic matter contents within the earthenware clay contributing to the 11 wt% weight loss or the 89 wt% residue mass at 1000 o C. The TGA profile of the chicken eggshell powder shows only one sharp peak occurring at 817 °C due to transformation from calcium carbonate (CaCO 3 ) to calcium oxide (CaO) along with the CO 2 release into the air.The weight loss of the chicken eggshell powder from 25 to 1000 o C is equal to 48 wt%, consistent with the result reported by Murakami et al [15].The TGA profiles of both raw materials were useful in choosing the firing temperature of the clay samples to be at 800, 900, 1000, and 1100 o C for 1, 3, and 5 h.The calcium oxide (CaO) from the chicken eggshell powder acted as a bio-filler and a flux producing the phase formation and affecting the physical-thermal-mechanical properties of the obtained clay bricks.The physical properties (bulk density, true density, color and water absorption) of the clay brick samples with 0, 5, 10, 15 and 20 wt% chicken eggshell powder added and fired at 800, 900, 1000 and 1100 o C for 1, 3 and 5 h, are data tabulated in Tab.V.The higher firing temperature and longer firing time are used, the higher strengths of the clay bricks are obtained.If the firing temperature is less than 1000 o C (800 or 900 o C), the clay bricks obtained are of low strength.However, if the firing temperature is much higher than 1000 o C, cracking on clay bricks appeared.Therefore, the suiTab.firing temperature of the clay bricks with the eggshell powder added is 1000 o C for 1, 3, and 5 h, as data are tabulated in Tab.V, consistent with the schematic diagram of the mineralogical transformations of calcite rich clay in the temperature range from 25 o C to 1000 o C Trindade et al [17].Furthermore, if the percentage of the chicken eggshell powder added in the clay bricks is high, the percentage value of water absorption is low with a good appearance.The obtained clay brick sample with 20 wt% eggshell powder added has the average water absorption equal to 14.36 -14.88 %, consistent with the results obtained by Raut et al [18].This result suggests that the high densification with low porosity content from the obtained clay samples yield high mechanical and thermal properties.Furthermore, the obtained water absorption percentages of the clay brick samples are of low values, less than 15 wt%, consistent with the accepTab.refractory value according to the TIS 601/2547.

Mechanical and thermal properties of clay bricks
The mechanical properties (compressive strength and hardness) of the clay bricks were measured according to the ASTM C 373-72.The data of clay brick samples without and with 0, 5, 10, 15 and 20 wt% eggshell powder added and fired at 1000 o C for 1, 3 and 5 h are tabulated in Tab.VI.The high percentage of amount eggshell powder added in clay bricks, the better and higher compressive strength and hardness values of the clay bricks are   The thermal expansion coefficient values of the clay bricks with 20 wt% eggshell powder added and fired at 1000 o C for 3 and 5 h, are 3.2605 × 10 -6 and 2.3652 × 10 -6 ( o C) -1 , respectively (Tab.VII).While the thermal expansion coefficient values of the clay bricks without the eggshell powder added and fired at 1000 o C for 3 and 5 h, are 4.7846 × 10 -6 and 3.6566 × 10 -6 ( o C) -1 , respectively.Thus, the lower thermal expansion coefficient values were obtained with the addition of the eggshell powder.This suggests that the obtained samples have high thermal resistance and thermal shock resistance suiTab.for use in construction and thermal insulation.

Conclusion
Adding chicken eggshell powder into the clay bricks increased physical-mechanicalthermal properties of products.The higher amount of adding eggshell powder yielded better physical-mechanical-thermal properties of the clay bricks due to the presence of calcium oxide causing the calcium feldspar, calcium silicate, and wollastonite phase formations.The firing temperature and soaking time were critically important factors towards the densification or vitrification and the completion of glassy phase formation.The best condition for the clay bio-bricks was firing at 1000 o C for a duration of 5 h.The clay bio-brick with 20 wt% eggshell powder added and fired at 1000 o C for 5 h.possessed the high compressive strength and hardness (8.28 MPa and 8.79 ± 0.60 HV), low thermal expansion coefficient of 2.3652 x 10 -6 ( o C) -1 , good true density of 1.96 ± 0.02 g/cm 3 and low water absorption of 14.36 ± 0.80 wt%.The accepTab.compressive strength and hardness values of refractory (ASTM C 373-72) are 7.0 MPa and 6.0 HV, respectively, and the absorption value (TIS 601/2547) to be less than 15 wt%.Therefore, the obtained clay bio-brick with 20 wt% eggshell powder added and fired at 1000 o C for 5 h is potential to be used as a clay brick for construction and thermal insulation.

Fig. 1 .
Fig. 1.Particle size distributions of: a) eggshell powder after grinding for 120 min; and b) earthenware clay raw material.

Fig. 2 .
Fig. 2. The percentages of residue mass as measured by TGA: a) clay raw material; and b) eggshell powder after grinding for 120 min.

Tab. V
Physical properties of clay bricks sintered at 1000 o C. means not measured.a means 5 clay samples for each condition.b means the density including open and closed pores within the clay samples according to the ASTM B212-82.c means the density without open and closed pores within the clay samples from boiling in water for 5 hrs and cooling in water for 24 h before the calculations according the ASTM B212-72 and ASTM C 373-72.d means the water absorption according to the ASTM C 373-72.Encoded sample x-xxxx-x means the amount of eggshell powder added-firing temperature-firing time: 0-1000-3 (no eggshell added (0 wt%)-firing at 1000 o C-firing time of 3 h).The XRD patterns of the raw materials (earthenware clay and chicken eggshell powder) and the clay brick samples 0 and 20 wt% chicken eggshell powder added and fired at 1000 o C for 1, 3 and 5 h, are shown in Fig. 3.The XRD peaks of the earthenware clay appear at 2 (26.832°, 50.079° and 60.026°), consistent with the JCPDS numbers 01-089-8936 (quartz), 00-003-0447 (aluminum phosphate), and 00-019-0615 (iron oxide, Fe 2 O 3 ) suggesting the hexagonal phase formation of SiO 2 -Al 2 O 3 -Fe 2 O 3 .The XRD peak pattern of the chicken eggshell powder indicates the rhombohedral of calcium carbonate (CaCO 3 ) or calcite consistent with the JCPDS numbers 01-072-1937 at 2 (29.369°, 48.452°, 47.450°, 39.370° and 43.118°).The XRD peak patterns of the clay bricks without adding eggshell powder and fired at 1000 o C for 1, 3 and 5 h, show only the hexagonal crystalline phase formation of SiO 2 -Al 2 O 3 -Fe 2 O 3 , consistent with the JCPDS numbers 01-089-8936 (quartz, SiO 2 ), 00-049-0134 (aluminum oxide, Al 2 O 3 ) and 00-019-0615 (iron oxide, Fe 2 O 3 ).The XRD peak patterns of the clay brick samples with 20 wt% eggshell powder added and fired at 1000 o C show two sharp peaks of the anorthic phase formation consistent with the JCPDS number 00-003-0559 of calcium aluminum silicate (CaAl 2 Si 2 O 8 or calcium feldspar) and calcium silicate (CaSiO 3 or wollastonite) at 2 equal to 26.881°-28.776°,23.205°, 36.650° and 43.473°.In addition, the XRD peak pattern of the clay brick with 20 wt% eggshell powder added shows the cubic phase formation of calcium oxide (CaO), consistent with the JCPDS number 00-048-1467, at 2 equal to 37.360°, 53.860° and 32.200°.All obtained results are consistent with the previous results as reported by Gorhan et al. [6], Cultrone et al. [11], Jordan et al. [19], Xia et al. [20] and Wan et al [21].Furthermore, the XRD peak pattern of the clay brick with 20 wt% eggshell powder added and fired at 1000 °C shows the hexagonal phase formation of SiO 2 -Al 2 O 3 -Fe 2 O 3 consistent with the JCPDS numbers 01-089-8936, 00-049-0134 and 00-019-0615.The sharp XRD peak at 2 equal 28.776° belongs to calcium aluminum silicate (CaAl 2 Si 2 O 8 or calcium feldspar) occurring when the clay bricks were fired at 1000 o C for 5 h, consistent with the result obtained by Jordan et al [19].

Fig. 4 .
Fig. 4. The SEM micrographs of samples: a) earthenware clay; b) eggshell powder ground for 120 mins; c) clay brick without eggshell powder added and fired at 1000 o C for 1 h; d) clay brick with 20 wt% eggshell powder added and fired at 1000 o C for 1 h; e) clay brick without eggshell powder added and fired at 1000 o C for 3 h; f) clay brick with 20 wt% eggshell powder added and fired at 1000 o C for 3 h; g) clay brick without eggshell powder added and fired at 1000 o C for 5 h; and h) clay brick with 20 wt% eggshell powder added and fired at 1000 o C for 5 h, respectively.
. The highest compressive strength and hardness clay belongs to the clay brick with 20 wt% eggshell powder added and fired at 1000 o C for 5 h, encoded 20-1000-5.The corresponding compressive strength and hardness values (20-1000-5) are 8.28 MPa and 8.79 ± 0.60 HV, respectively.The clay brick without the eggshell powder added and fired at 1000 o C for 5 h possesses the compressive strength of 7.42 MPa and the hardness value of 7.63 ± 0.30 HV.The theoretical compressive strength and hardness values according the ASTM C 373-72 are 7.0 MPa and 6.0 HV, respectively.
Tab. VII Thermal expansion coefficients of clay bricks sintered at 1000 o C. x-xxxx-x means the amount of eggshell powder added-firing temperature-firing time: 0-1000-3 (no eggshell added (0 wt%)-firing at 1000 ºC-firing time of 3 h).b means measuring the thermal expansion coefficient values in the range of 25-1000 ºC.Encoded sample x-xxxx-x means the amount of eggshell powder added-firing temperature-firing time: 0-1000-3 (no eggshell added (0 wt%)-firing at 1000 ºC-firing time of 3 h).
chemical compositions of the clay and the chicken eggshell were measured by XRF and the data are tabulated in Tab.II.The main composition of the clay consists of 26.46 wt% SiO 2 , 22.32 wt% Fe 2 O 3, 20.66 wt% Al 2 O 3 , and other oxide compounds.While the main composition of the chicken eggshell used as a bio-filler, a flux or a sintering agent is composed of 98.10 wt% CaCO 3 and other oxide compounds.Tab.II Chemical composition of earthenware clay and chicken eggshell by XRF.
* wt% means the amount of eggshell weight divided by the total weight of clay and eggshell powder multiplied by 100.The * NA means not available.
199.92 m 2 /g and 183.30Å, respectively.From the particle size distribution, the d 10 , d 50 and d 90 of the earthenware clay are 0.91, 4.88 and 41.43 µm, respectively.Both earthenware clay and chicken eggshell are thus fine powders suiTab.for the clay brick preparation by extrusion.
Color of clay bricks before and after firing.
Tab. VI Mechanical properties of clay bricks sintered at 1000 o C.