Abstract:A total of six washed French and Algerian kaolins were studied. Kaolinite, halloysite, muscovite, feldspars, anatase, rutile, gibbsite, goethite, and todorokite were present. The thermal behavior of the samples was studied and the transformation heats were determined and quantified by differential thermal analysis. Calcined samples from 900°C to 1400°C are studied by X‐ray diffraction, the results show that the crystallite sizes of mullite rises as the temperature rises. The calcined samples showed an inverse … Show more
“…11) is due to the fact that the sintering starts at different temperatures; sintering occurs later in HS clays, which means that the formation of newly formed crystals of Fe and/or rutile oxides also occurs at higher temperature and therefore the decrease in L* begins later. On the other hand, the increase in L* again also suffers a delay in this case, it would be expected that in HS at a temperature greater than 1300 °C the value of L* increases, probably due to the reduction of Fe 3+ to Fe 2+ [29].…”
Section: Thermal Propertiesmentioning
confidence: 75%
“…10). This increase can be attributed to the destruction of the structure of illite, which causes the liberation of Fe 3+ that is in substitution of Al in clays, and goes to substitute Al in the structure of the new phases formed [29] or crystallizes as hematite, which drives to the reddish colours [19,30].…”
The thermal properties and evolution of mineralogy and colour of kaolinitic clay from the Terra Alta region were studied. The mineralogy of these materials consists mainly of kaolinite (13–27 mass%) and quartz (48–86 mass%). Minor illite, hematite, K-feldspar and calcite also occur. The linear expansion and absorption curves were used to predict the optimal firing temperature of the raw clays. During firing, from 1100 °C the water absorption decreases steeply, due to an increase in liquid phase, which penetrates into the pores and close the porosity. At this temperature, the firing shrinkage increases progressively. The fired clays are mainly composed of quartz, cristobalite and mullite, with minor hematite and rutile. Mullite starts to appear at 1050–1100 °C. SEM observations show that porosity decreases with the firing temperature. The colour properties were measured in the raw clays and in the fired bricks at different temperatures. The lightness, L*, is lower in the fired test pieces respect to the natural clays. This colour varies according to the hematite content, being from white to reddish in the fired samples.
“…11) is due to the fact that the sintering starts at different temperatures; sintering occurs later in HS clays, which means that the formation of newly formed crystals of Fe and/or rutile oxides also occurs at higher temperature and therefore the decrease in L* begins later. On the other hand, the increase in L* again also suffers a delay in this case, it would be expected that in HS at a temperature greater than 1300 °C the value of L* increases, probably due to the reduction of Fe 3+ to Fe 2+ [29].…”
Section: Thermal Propertiesmentioning
confidence: 75%
“…10). This increase can be attributed to the destruction of the structure of illite, which causes the liberation of Fe 3+ that is in substitution of Al in clays, and goes to substitute Al in the structure of the new phases formed [29] or crystallizes as hematite, which drives to the reddish colours [19,30].…”
The thermal properties and evolution of mineralogy and colour of kaolinitic clay from the Terra Alta region were studied. The mineralogy of these materials consists mainly of kaolinite (13–27 mass%) and quartz (48–86 mass%). Minor illite, hematite, K-feldspar and calcite also occur. The linear expansion and absorption curves were used to predict the optimal firing temperature of the raw clays. During firing, from 1100 °C the water absorption decreases steeply, due to an increase in liquid phase, which penetrates into the pores and close the porosity. At this temperature, the firing shrinkage increases progressively. The fired clays are mainly composed of quartz, cristobalite and mullite, with minor hematite and rutile. Mullite starts to appear at 1050–1100 °C. SEM observations show that porosity decreases with the firing temperature. The colour properties were measured in the raw clays and in the fired bricks at different temperatures. The lightness, L*, is lower in the fired test pieces respect to the natural clays. This colour varies according to the hematite content, being from white to reddish in the fired samples.
“…The SiO 2 /Al 2 O 3 ratio is 4.02; this value controls the formation of zeolites, which is close to the lower limit for mordenite given by the literature at 4.15–5.15 (Hernáandez et al , 2000; Khoury et al , 2015). Biotite, hematite and feldspar are accessory minerals and affect the colour of the tuff (Bouzidi et al , 2014) because these minerals contain transition metals such as Fe 2+ , Ti 3+ or Mn 2+ . Fig.…”
This work examines a tuff from the Tinebdar deposit located in Sidi Aich (east Algeria) for possible use as an alternative material for the adsorption of Asucryl red (a textile dye). Natural tuff represents an economic and environmentally friendly alternative compared to synthetic zeolites. The starting materials were characterized by means of powder X-ray diffraction, Brunauer–Emmett–Teller-specific surface area and pore diameter analysis. Batch experiments were performed and various parameters that have an effect on the adsorption process (i.e. pH, clay amount, contact time and initial concentration) were investigated. The <125 μm grain-size fraction of the tuff contains 45 wt.% mordenite. The adsorption equilibrium was established in 10 min and the adsorption kinetics were better described by the second-order kinetic model. The adsorption isotherm of the results obtained fits better to the Langmuir and Timkin models. The adsorption capacity qt varies from 60 to 70 mg g–1 with temperature increasing from 293 to 333 K. The thermodynamic nature of the adsorption process was determined by calculating ΔH°, ΔS° and ΔG° values. The positive value for ΔH° confirms that the adsorption is endothermic.
“…Mechanical activation enhanced the reaction of BaAl 2 SiO 6 and the formation of BaAl 2 Si2O 8 at BaCO 3 contents <50 wt.%. However, because the Ba 2+ ions are reactive enough to form an immediate chemical bond with the kaolinite–mullite liquid state during sintering (Bouzidi et al , 2014), samples KB2 and KB3 became more densified. In KB4, the peak intensities of BaAl 2 O 4 and Ba 2 SiO 4 increased with increasing temperature and increasing BaCO 3 content to 60 wt.%, leading to the formation of refractory phases that required a higher sintering temperature.…”
The present study examined the microstructure and mechanical properties of ceramic composites based on a kaolin from Djebel Debbagh, northeast Algeria, composed mainly of kaolinite and halloysite with the addition of various amounts of BaCO3. The composites were prepared by high-energy ball milling and sintered at 1100°C and 1200°C for 3 h. The samples sintered at 1200°C without BaCO3were composed mainly of mullite, which disappeared with increasing BaCO3content. X-ray diffraction investigation showed the presence of hexacelsian (BaAl2SiO6and BaAl2Si2O8), which disappeared at BaCO3contents >50 wt.% in favour of barium aluminium and barium silicate phases. At 40 wt.% BaCO3content, the porosity of the composites decreased from 0.7% to 0.1% and the microhardness increased from 7 to 8 GPa, respectively, at 1100°C and 1200°C due to the amorphous phase.
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