Formed by the accumulation of the outer shells of algae, diatomite is a siliceous sedimentary rock that is considered a cheap source of silica (SiO₂.2H₂O and crystalline silica).^1,2 Diatomite originating from salt water is mainly contaminated with sand or grit, while the main contamination in fresh water deposits tends to be of a clay nature.

Diatomite is an important industrial mineral with unique physical properties, including low density, high porosity, permeable structure, high resistance to chemicals, large filtration area and good adsorption properties.^2,3 Pure diatomite (SiO₂ > 90%) is widely used as a filter aid and supplementary cementing material, while calcareous or clay diatomite is commonly used in insulation products, mild abrasives, special filters, and absorbents.⁴

Diatomite deposits at the Fayoum depression spread from the Cairo-Fayoum desert road in the east to Kasr ElSagha and Demia in the west, over an area that is 30 km long and 3-6 km wide.³ Previous investigations have indicated that these deposits are composed of significant amounts of valuable diatomitic amorphous silica embedded in a calcite matrix, along with a relatively low percentage of clay minerals.^5,6,7

Due to their low thermal conductivity, high strength and chemical resistance, diatomite brick are used in the design of thermal insulation for furnaces. The brick are used as a heat-insulating material in electrolyzes for melting primary aluminum.⁸ It has also been proposed to build roofs, wall and kiln cars in tunnel kiln with diatomite brick.⁹ However, a relative disadvantage of diatomite is the comparatively low application temperature (900-950°C), which is due to the formation of a liquid phase in diatomite objects as a result of the reaction of amorphous silica with impurities. Some investigations were carried out for increasing the application temperature; these works are based on the addition of CaO, MgO or clay.^8,9,10 The addition increases the temperature at which the liquid phase is formed, thus increasing the operating temperature.

In Egypt, thermal insulating fireclay brick of 0.9-1.0 g/cm^-3 bulk density, 10-15 kg/cm^-2 compressive strength and 1,250°C application temperature are produced using the plastic method. The brick are made of an Aswan ball clay-grog mixture, with polystyrene used as a pore forming agent. The relatively high bulk density of the product is mainly due to the high plasticity of the Aswan ball clay. Therefore, this work aims to partly replace Aswan ball clay with Egyptian diatomite in order to improve properties and decrease costs.

For this purpose, a Masakheet diatomite sample was used because of its relatively high purity. The fired diatomite clay specimens were prepared using the plastic method (polystyrene was the pore forming agent) and fired at temperatures of 1,150, 1,250 and 1,350°C for two hours. The effects of firing temperature and batch composition on the samples’ physical and mechanical properties, mineral composition, and microstructure were studied.

Experimental Details

The chemical compositions of the Aswan ball clay and Masakheet diatomite samples are shown in Table 1. The BET surface areas of the two samples are 190 and 17.97 m²/g, respectively. A fired Aswan ball clay sample was also used as grog. In addition to these materials, a sample of fine expanded polystyrene beads was used as a pore forming agent (27.7 mass% and 68.2 mass% of employed polystyrene beads are between 0.85-1.6 and 1.6-2.0 mm, respectively).

Two groups of specimens were prepared. In the first group, the mass of grog was 25% and diatomite content increased at the expense of Aswan ball clay from 0 to 20 mass%. In the second group of specimens, the diatomite Aswan ball clay mass ratio was 20:55 and the grog content changed from 10 to 30 mass%. In all cases, polystyrene was added in an amount of 3.5%. During the mixing of the constituents, an appropriate amount of water (approximately 25 mass%) was added in parts. The resulting mass was molded and tamped under a light pressure into small cubic specimens of 2 in. long. The specimens were left to dry at ambient temperature for 48 hours and then at 110°C for 24 hours. The dried specimens were first fired at 600°C to burn out the additives, and then at temperatures of 1,150, 1,250 and 1,350°C for two hours.

The specific surface areas and chemical compositions of the materials were determined using Brunauer, Emmett and Teller (BET) surface area analysis and X-ray fluorescence analysis, respectively. The physical properties of the fired specimens (e.g., bulk density and open porosity) were measured with the water displacement method after boiling for two hours to eliminate residual air bubbles.

The measurement of the specimens’ compressive strength was carried out in a testing machine (L 10 k Plus 10 KN, 2248 ibf) at a cross-head speed of 0.01-5.08 mm/min. After the strength test, the samples were used for the studies of mineralogical and morphological composition using X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM), respectively. X-ray diffraction patterns were obtained using a Philips X-ray diffractometer (model PW/1710 Cu kαradiation with a Ni – filter, λ= 1.5405 Å). Care was taken to keep the X-ray unit constant at 40 KV and 30 mA. The scanning speed was chosen to be 2θ= 2 o/min. The obtained X-ray patterns were correlated and compared with the ASTM cards for the expected phases. The fracture surface of some fired specimens were cleaned and then coated with thin layer of gold under vacuum. The SEM photographs were carried out using a model Philips XL 30 with accelerating voltage of 30 KV and magnification of 100,000 x.

Physical and Mechanical Properties

In order to optimize the composition of diatomite-clay grog mixture, two groups of specimens were prepared using the plastic method and fired at temperature of 1,150, 1,250 and 1,350°C for two hours. In the first group of specimens, the amount of grog was 25 mass% and the diatomite content was changed from 0 to 20 mass% at the expense of Aswan ball clay content.

Figure 1 shows the effect of firing temperature on the bulk density of the diatomite-Aswan ball clay specimens. As expected, the addition of diatomite to Aswan ball clay led to a decrease in the bulk density of green diatomite-clay specimens, thus reducing the fired specimens’ bulk density. This appears more clearly in the specimens fired at relatively low temperature (1,150°C). However, a higher firing temperature led to an increase of bulk density. As shown, an increase in firing temperature from 1,150 to 1,350°C led to an increase of 3% and 10% in the bulk density of specimens containing 0 and 20 mass% diatomite, respectively. This may be due to coalescence of diatomite and sintering of diatomite clay composite. These led to a decrease of porosity and hence increased density.

It has been reported that there are three different stages in diatomite sintering behavior.¹¹ In the first stage, a drop of open porosity from 71.6 to 68.1% is observed between 850 and 1,000°C. In the second stage, a decrease of porosity from 68.1 to 36.7% occurs between 1,000 to 1,250°C. Finally, a decrease to 5% of the open porosity was reached at 1,350°C. It was also found that the numerous small pores of diatomite will close due to sintering and coalescence of diatomite.¹² In addition, an increase of density is expected due to the phase transformation of clay and diatomite, as will be discussed later.

Figure 2 shows the effect of diatomite content on the open porosity and compressive strength of the diatomite-Aswan ball clay specimens fired at 1,350°C for two hours. By increasing the diatomite content, the compressive strength of fired specimens increased as a result of sintering and coalescence of the diatomite-Aswan ball clay composite. However, the diatomite and clay shrink differently, leading to a slight increase of the inter-particle porosity.¹²

The results indicate that specimens containing 20 mass% of diatomite and 55 mass% of Aswan ball clay, along with 25 mass% of grog, have the best properties among all of the investigated specimens (bulk density, open porosity and compressive strength of 0.78 g.cm^-3, 61% and 20 kg.cm^-2, respectively). The diatomite-Aswan ball clay mass ratio in these specimens was approximately 0.35. The amount of grog in these specimens ranged from 10-30 mass% with an additional proportion of 5 mass%. Polystyrene in the amount of 3.5 mass% was used as pore forming agent. The specimens were prepared as normal and fired at temperature of 1,150, 1,250 and 1,350°C for two hours.

Figure 3 shows the effect of firing temperature on the bulk density of the fired specimens. As expected, the bulk density increased with increasing amounts of grog. This appeared in the specimens fired at 1,150 and 1,250°C. However, on firing the diatomite clay specimens at 1,350°C, the opposite relationship was seen. The specimens containing 10 mass% of grog had higher density values than their 30 mass% of grog counterparts. This is due to the effect of sintering on the diatomite clay content (90%).

Figure 4 shows the effect of grog content on open porosity and compressive strength of the diatomite-Aswan ball clay specimens fired at 1350°C. As shown, the increase in the amount of grog, as expected, led to an increase in compressive strength, with the maximum value achieved with the 20 grog mass% specimens. A slight increase of inter-particle porosity also resulted. The results indicated also that the samples with 25 mass% of grog had the best properties (bulk density, open porosity and compressive strength of 0.78 g.cm^-3, 61 % and 20 kg.cm^-2, respectively). The permanent linear change of these specimens was determined by heating them at 1,300°C for eight hours; the obtained value was 0.42%.

Mineral Composition

Figure 5 shows the X-ray diffraction patterns of diatomite clay specimens fired at 1,150, 1,250 and 1,350°C for two hours. This specimen contains 20 mass% of diatomite, 55 mass% of clay and 25 mass% of grog. The results indicated that mullite and cristobalite were formed from the decomposition of the clay. The clay’s free silica (in the form of quartz) partly converted to cristobalite. Cristobalite reacted with fluxing impurities to form a glassy phase, which accelerated the samples’ sintering.

A mullite phase appeared in specimens fired at 1,150°C for two hours. The amount of mullite increased as firing temperatures increased up to 1,350°C. This increase was at the expense of cristobalite content. In spite of the relatively low CaO content of the investigated mixture (3.90% CaO), anorthite (CaO.Al₂O₃.2SiO₂), which forms as a ternary compound from the reaction between calcium oxide and both alumina and silica, was detected in the X-ray pattern of the fired samples. The amount of anorthite increased when the firing temperature was raised from 1,150 to 1,350°C.

Microstructure Characteristics

Figure 6 shows SEM micrographs of specimens containing 20 mass% of diatomite, 55 mass% of clay and 25 mass% of grog that were fired at 1,150, 1,250 and 1,350°C. The study of the specimen fired at 1,150°C showed that the diatom frustules occurred as assemblages surrounded by a clay groundmass (see Figure 6A). The surface of the diatom frustules is nearly weathered due to the effect of the heat treatment. On firing the specimens at 1,250°C, the diatom frustules become nearly imperceptible due to the coalescence of diatomite and sintering of the diatomite-clay composite (see Figure 6B).

Large pores are visible on specimens fired at 1,350°C (see Figure 6C). This is due to the different shrinkages of diatomite and clay. An increase in pore size of the diatomite-clay specimens fired at 1,300°C was also apparent and attributed to different the sintering behaviors of diatomite and clay. This observation confirms the slight increase in open porosity of specimens fired at 1,350°C (Figure 2).

Conclusions

The addition of diatomite decreases the bulk density of fired clay-grog specimens. Its effect decreases with rising firing temperature up to 1,350°C. Conversely, the bulk density of fired specimens increases with increasing grog content. An opposite relation results in specimens fired at 1,350°C.

Mullite and cristobalite are formed from the decomposition of clay and the transformation of quartz. Anorthite is formed from a reaction between calcia and both alumina and silica. The amount of mullite and anorthite increases with rising firing temperatures at the expense of cristobalite content.

The degree of preservation of diatomite frustules decreases with increasing firing temperatures. This is due to the coalescence of diatomite and its reaction with clay. Large inter-particle pores are created in specimens fired at 1,350°C.

The 20 mass% of diatomite, 55 mass%of clay and 25 mass% of grog-containing specimens fired at 1,350°C achieved the best properties among the investigated specimens. They are classified according to their bulk density (0.78 g.cm^-3) and permanent linear change (0.42%) as chamotte group 130 according to ISO No. 2245-1990.  

References

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