MultiScience - XXXIII. microCAD International Multidisciplinary Scientific Conference University of Miskolc, 23-24 May, 2019, ISBN 978-963-358-177-3
EFFECT OF FIRING ON MINERAL PHASES AND PROPERTIES OF LIGHTWEIGHT EXPANDED CLAY AGGREGATES
Mohamed Abdelfattah*, Istvan Kocserha, Robert Géber.
Institute of Ceramic and Polymer Engineering University of Miskolc, Miskolc, Hungary
*E-mail: madatow@yahoo.com 1. ABSTRACT
This work is focused on studying the effect of firing on physico-mechanical, and expansion properties and mineral phases of expanded clay minerals. Samples were collected from Mályi quarry Miskolc, Hungary. They were characterized by XRF, XRD, and SEM microscopy. Expansion properties of the different clay samples were measured by heating microscope. Compressive strength and bulk density of specimens were measured according to relating standards. Results showed that changing in mineral phase had great effect on the physical and mechanical properties of the lightweight aggregates, especially the mullite mineral enhanced the mechanical properties of the aggregates after firing.
Key words: Expanded clay, Firing, Lightweight aggregates
2. INTRODUCTION
Lightweight expanded clay aggregates (LECA) are one of the most crucial materials used in the construction and building materials. Lightweight aggregates (LWA) become the center of attention of the whole world, because these types of the aggregates have the perfect properties of good thermal behaviors, acoustic insulation and good fire resistance [1]. The primary objective of lightweight aggregates is used for building materials, such as lightweight concrete, concrete for structural, in addition heaviness for railroads or road covering in combination with bituminous materials. The other objective for LWA, it can be used as filter media for bacterial or metallic ion removal [2]. The firing treatment is the most important factor in controlling the percentage of porosity. [3,4]. Increase in temperature above the pyro- plasticity range can lead to viscous flow, in addition, increase the porosity and pore size [5]. Chemical and physico-mechanical properties of the lightweight aggregates have to be controlled for appropriate application. Chemical composition of the clay is the most vital key to effect the properties of the final product and it has a role in bloating mechanism of the clay. Latter is depends on the SiO2 and flux (Fe2O3, Na2O, K2O, CaO and MgO) ratio [6,7]. Different clays are able to trap gases like CO2 in different extent which help in bloating of aggregates. Illitic clay was more effective entrapment, which generated more liquid phase than kaolinitic clays [7]
According to definition, physico-mechanical properties of lightweight aggregates are:
(1) a particle bulk density between 0.8 and 2.0 g/cm3 (according European Union regulation EN-13055-1), (2) the compressive strength above 1 MPa [8], (3) a porous
DOI: 10.26649/musci.2019.080
and sintered ceramic core [9], (4) a spheroid shape which improves concrete properties [10].
Previous research proved that there is a link between mineral phases and physical and mechanical properties of expanded lightweight aggregates [11]. In the thermal treatment, most of the initial phases are transformed to the neo-formed minerals.
Mullite phase was one of the best examples of enhancement the properties of aggregates. This phase was produced by thermal decomposition of illitic and kaolinitic clay which, enhanced the strength of aggregates [12].
3. MATERIAL AND METHOD
Three types of expanded clay samples were collected from Mályi quarry, Miskolc, Hungary. Clays were named as follows: grey clay: G, blue clay: B and Yellow clay:
Y. Samples were dried at 105 oC for 24 h in an air circulating electric furnace. They were readily ground for a few seconds to reduce particle agglomeration and milled to pass through a No. 100 mesh for further measuring. Chemical composition of the samples was determined by using an X-ray fluorescence spectrometer, in addition the results of the chemical composition after calcined basis were plotted in the Riley and Cougny diagrams [6,13]. Mineral phases were analyzed using X-ray diffraction (Rigaku Miniflex II, Cu Kα, 2θ range from 3 to 90°). The expansion of clay samples was measured by heating microscope (Camar Elettronica). Green pellets (five each) were produced by hand-rolling. Pellets were dried 105oC in a drying chamber. Dried pellets were sintered at four different temperature. For air atmosphere sintering an electrical furnace was used. Temperature values were set to 1150oC, 1175oC, 1200oC, and 1225oC respectively. Sintering time was ten minutes at maximal temperature.
Lastly, after the heating process at 1150–1225oC, sintered products were naturally cooled in the atmosphere, and their physical and mechanical properties were studied.
Uniaxial compressive strength and bulk density were measured according to the European Standard EN-13055-1.
The bulk density was measured using the Archimedes method after the LECAs were placed in water for 24 h. The bulk density was calculated using equation 1 [14] -
Bulk density= WD/ (WS –WI) [1]
Where: WD: dry weight of the LECA, Ws: 24 h saturated surface-dry weight, WI: immersed weight in water.
The compressive strength was measured by an INSTRON universal tester with a cross-head speed of 0.1 mm/s. The compressive strength [15] of the LECA was calculated using equation 2.
Compressive strength= F/A [2]
Where: F: load belonging to fracture, A: area of the LECA
Microstructure of LECAs were observed with a scanning electron microscopy (SEM) under high vacuum mode (Carl Zeiss EVO MA10). Carbon tape was used to hold
samples on sample holder and they were coated with gold for (SEM) morphology scanning.
4. RESULTS
4.1. Expansion, mineral and chemical composition
Chemical composition of clay and expanded clay samples were summarized in Table 1. The most important oxides in the chemical composition of these technological samples were silica, alumina oxide and total flux. Order of oxides range: SiO2 range (62.23-64.97 wt%), followed by Al2O3 range (17.31-17.94 wt%) and total flux range (17.43-19.31 wt%). These ranges were acceptable for bloating of the aggregates.
According to Riley (1951) and Cougny (1990), the five patches of expanded clay were located in the bloating area, which means that the five batches can be expanded upon firing below 1300 °C [16] as exhibited in Fig. 1a and Fig. 1b. According to the Riley theory, these samples can obtain a convenient viscosity to trap a significant amount of the gaseous components, resulting in the formation of the expanded structure of the LECA [17]. XRD pattern of the three clay samples can be seen on Fig. 2. The mineral phases of the three clay sample were quartz, illite, vermiculite and illite-Montmorillonite group.
Table. 1. Chemical composition of the clay samples
Sample SiO2
wt%
Al2O3
wt%
MgO
wt%
CaO
wt%
Na2O
wt%
K2O
wt%
Fe2O3
wt%
MnO
wt%
TiO2
wt%
P2O5
wt%
FX
wt%
B 62.23 17.73 2.65 4.47 0.39 3.74 6.99 0.13 1.05 0.11 19.31 Y 64.97 17.36 2.01 2.66 0.54 3.40 7.74 0.10 1.07 0.12 17.43 G 62.91 17.94 2.54 4.13 0.57 3.76 6.3 0.08 1.00 0.10 18.34 B+G (50-50) 62.36 17.77 2.6 4.4 0.46 3.76 6.84 0.11 1.03 0.10 19.11 B+Y (50-50) 63.60 17.31 2.31 3.57 0.47 3.61 7.57 0.13 1.07 0.12 18.62
FX: total flux (Fe2O3 + MgO + CaO + Na2O + K2O); B: blue clay; Y: yellow clay and G: grey clay
Fig. 1. Chemical composition of the technological sample as plotted on: a) Riley's diagram (1951) and b) Cougny (1990) on calcined basis.
Fig. 2. XRD patterns of raw materials
In addition, the melting behavior was examined using a heating microscope. Image analysis revealed the characteristic temperatures (T) and height expansion (H) of the samples, which was shown in Fig. 3a. The dimensions of the clay samples were increased when the temperature increased (Fig. 3b). The height of the clay samples was expanded with temperature. The maximum height expansion of the blue, grey and yellow clay sample were increased to by 38%, 23 % and 22 % respectively. Blue clay sample started expanding at 1225oC, in addition to, this sample had the highest expansion of height 38%, compared with the other Mályi clay samples (Fig. 3b).
Fig. 3. a) The relation between height expansion of clay samples with the temperature.
b) Heating microscope measurement of blue clay samples 4.2. Lightweight aggregate characterization
The physical and mechanical parameters of the expanded clay aggregates were measured according to the requirements specified in the European standards EN-
13055-1 (Table 2). The bulk density of the aggregates in this investigation was found to be less than 1.2 g/cm3. Bulk density range of the Mályi clay samples was (0.57- 1.2) g/cm3 at 1225 oC. The bulk density of the bloated aggregates decreased with increasing the firing temperature (Fig. 4a). The lower density of blue clay sample was related to its higher liquid content which is characterized by its lower viscosity during firing. Whilst, the compressive strength of these aggregates at 1225 oC was (1.54- 4.82) MPa, and this range is higher than 1 MPa. So these values are acceptable according to Liao and Huang [8]. The relation between bulk density, compressive strength was indirect relation with the firing temperature (Fig. 4b). So that the aggregates were classified as lightweight aggregates according to this standard.
Table 2. Bulk density and compressive strength of clay aggregates fired at different temperature
1150 oC 1175 oC 1200 oC 1225 oC samples BD,
g/cm3
UCS, MPa
BD g/cm3
UCS, MPa
BD g/cm3
UCS, MPa
BD g/cm3
UCS, MPa
B 1.77 9.34 1.20 6.00 0.72 2.96 0.57 1.54
Y 2.00 63.68 1.60 31.00 1.35 6.00 1.20 4.82 G 1.92 50.20 1.35 20.00 1.07 2.50 0.86 2.22 B+G
(50-50)
1.90 52.89 1.40 25.00 0.80 4.10 0.63 1.11 B+Y
(50-50)
1.95 56.69 1.50 27.00 0.95 2.46 0.84 2.09 BD: bulk density; UCS: uniaxial compressive strength; B: blue clay; Y: yellow clay;
G: grey clay.
Fig. 4. a) Relations between temperature-bulk density. b) Relations between temperature-compressive strength.
4.3. Mineral phases after firing
The mineral phases after firing at 1225 oC were studied by XRD. The mineral phases formed at 1225 oC were quite different from the pre-firing stages. The mineral phases of blue clay sample were quartz, hematite, and anorthite whilst the mineral phases of yellow clay sample were quartz, hematite, anorthite and mullite mineral. (Fig. 5).
Fig. 5. XRD patterns of lightweight expanded clay aggregates fired at 1225 oC 5. DISCUSSION
The preferred marketable application of the aggregates formed from expanded clay, considering low density, good compressive strength within a high porosity, is the prefabrication of aggregates for highly specialized applications, even for environmental exposure. Therefore, after studying the expansion, the physical and mechanical properties of the clay sample it can be shown, the three Mályi clay samples can be used as lightweight aggregates.
From the previous measurement, the blue clay sample had the highest expansion and lowest density at 1225 oC (38% and 0.57 g/cm3) respectively. In addition, the lower density of blue clay sample was related to its higher liquid content which is characterized by its lower viscosity during firing. The lower compressive strength of this sample was related to the mineral phase formed during the firing. And this value was low Due to, there is no enhanced strength phase can be help in increasing the compressive strength. The microstructure of the blue clay aggregates was determined using a scanning electron microscope. The cross-section of the aggregate pellet revealed that the material was composed of two layers (Fig. 6a). The SEM micrograph showed the inner layer had large pores, whilst, the pores in outer layer were not only larger but also more irregular (Fig. 6b and 6c). The large pores can affect the properties: the high percentage of porosity effect on the compressive strength of this aggregate.
On the other hand, the yellow clay sample had the lowest expansion, highest density (22%, 1.2 g/cm3) and the compressive strength (4.82 MPa) at 1225oC. The higher compressive strength of yellow clay was related to the mineral phases formed during the firing. The thermal transformation of the illite- montmorillonite group to mullite was observed 1225 oC. The compressive strength of yellow clay sample was high due to the formed the mulllite phase. On the other side, the mullite phase can decrease the expansion of the aggregates.
Fig. 6. a) Cross-section of blue clay sample at 1225 oC. b). SEM micrograph of blue clay aggregate at 1225 oC (magnification =19X) c) SEM micrograph of blue clay aggregate at 1225 oC (magnification =40X).
6. CONCLUSION
The changing in mineral phase had great effect on the physical and mechanical properties of the lightweight aggregates, especially the mullite mineral enhanced the mechanical properties of the aggregates after firing. The compressive strength and bulk density of lightweight aggregates are the most crucial keys affected by the change in mineral phases. Mullite phase can be enhanced the strength of the lightweight aggregates, but can decrease the expansion of the lightweight expanded clay aggregates. Relation between the compressive strength, bulk density with the firing temperature was indirect. As the expanded Malyi clay samples comply with the corresponding specifications of the European Standard EN-13055-1, they can be used as lightweight aggregates.
7. ACKNOWLEDGEMENT
The described study was carried out as part of the EFOP-3.6.1-16-2016-00011
“Younger and Renewing University – Innovative Knowledge City – institutional development of the University of Miskolc aiming at intelligent specialization” project implemented in the framework of the Szechenyi 2020 program. The realization of this project is supported by the European Union, co-financed by the European Social Fund.
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