ep 10592

Feasibility of Recycling Waste Diatomite and Fly

Ash Cosintered as Porous CeramicsKae-Long Lin and Jen-Chieh ChangDepartment of Environmental Engineering, National I-Lan University, Taiwan; [email protected] (for correspondence)

Published online 30 December 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.10592

This investigation demonstrates the feasibility of usingdiatomite and coal fly ash as alternative raw materials inthe production of the porous ceramics. The following operat-ing conditions are set for sintering process; a constant pres-sure of 5 MPa, a sintering temperature of 1000–12708C, asintering time of 2 h, and various proportions (0–20%) ofcoal fly ash in waste diatomite. This investigation concernsthe effects of heating temperature and proportion of fly ashon the characteristics of porous ceramics that are formedfrom a mixture of fly ash and waste diatomite. Heat-treatedsamples were analyzed by X-ray diffraction (XRD), scanningelectron microscopy (SEM), and mercury intrusion porosime-try (MIP), which supported the following conclusions. Whenthe heating temperature was increased above 12708C, thecompressive strength of the ceramics was between 5.85 and15.8 MPa. When the amount of coal fly ash exceeds 20%, theporosity of the porous ceramics decreases sharply with increas-ing temperature from 61.6% obtained at 11008 C to 52.9%obtained at 12708 C. When heating to 12708 C, the pore sizesof sintered samples �2.5–3 lm, and a smaller peak around0.3–2 lm. Adding coal fly ash to the porous ceramicsincreased their compressive strength, indicating that wastediatomite and coal fly ash can indeed be 100% recycled andreused as porous ceramics. � 2011 American Institute of Chemical

Engineers Environ Prog, 32: 25–34, 2013

Keywords: recycling, cosintering, compressive strength,water absorption, toxicity characteristic leaching procedure

INTRODUCTION

Waste management has become a serious social concern inmodern societies. The annual production of waste diatomiteby the food-processing industry in Taiwan is 4713 tonnes.Since this waste is neither accepted in most of the regularlandfills nor accepted in incinerators in Taiwan, much of it isdumped illegally somewhere, causing an environmental haz-ard. As living and working standards increase, more waste islikely to be generated each year in Taiwan. Accordingly, theproper treatment of such waste is becoming increasinglyimportant. Diatomaceous earth is a useful material for fabricat-ing porous ceramics because of its low cost, well-definedporosity, low density, and high thermal stability [1, 2].Diatomaceous earth is produced by diatoms, a diverse arrayof microscopic single-cell algae, which are the most familiarmembers of the phylum Bacillariophyta. The diatoms live inboth salt and fresh water, from which they extract silica tobuild their shells. When the diatoms die, their silica shellsaccumulate in layers or beds of diatomaceous earth [3, 4].

Approximately 1.8 million tonnes of diatomaceous earth aremined annually worldwide [5]. The structure of diatomite isquite complex and contains several fine microscopic pores,cavities, and channels. Accordingly, the material has a largespecific surface area, high absorption capacity, and lowdensity. The chemical composition and the physical structureof diatomite make it of great commercial value for a broadspectrum of applications, including in beer filters, the removalof textile dyes from waste water [6], and the sorption of heavymetal ions [7, 8]. Diatomite particles with high porosity andsmall particle sizes [9] have potential use in the fabrication ofhighly permeable microporous membrane filters. Their onlyshortcoming is the presence of impurities, such as calcium,which directly coats the surface of the diatomite particles andlimits the use of this material for filtration [4].

The annual production of coal fly ash in Taiwan is4,330,000 tonnes. Its major constituents are SiO2, Al2O3, andFe2O3, and its minor constituents include CaO, MgO, andother oxides. Accordingly, these oxides have been regardedas low-cost material resources for the cement industry andthe manufacture of pozzolanic material [10, 11]. Coal fly ashis also used to manufacture brick [12], fly ash mineral-basedpolymer composites [13], ceramic tableware and artware [14].Recent investigations have demonstrated alternative means ofmanaging coal fly ash in glass [15] and ceramic tiles [16–18].However, little research as been done on the use of coal flyash in the production of porous ceramics. According toSheng [19], coal fly ash with large amounts of glass networkformers (SiO2 1 Al2O3) can be utilized to fabricate durableproducts.

Sintering is a thermal process that converts a powdercompact into a bulk material. It is conducted in the massproduction of complex-shaped components. The shapes ofpowder particles and pore channel networks are changed bydiffusion [20, 21], which is driven by variations in curvature-dependent chemical potential. Sintering is a complicatedprocess of microstructural evolution that involves bond for-mation, neck growth, pore channel closure, pore shrinkage,densification, coarsening, and grain growth. The force thatacts between particles, called the sintering force, comprisesboth surface tension and grain boundary tension. The shrink-age rate is approximately proportional to the sintering force[22, 23]. The features and properties of a porous ceramicmaterial, such as porosity, pore size distribution, pore mor-phology, and pore connectivity (commonly identified fromthe relationship open and closed porosity), depends stronglyon the composition and processing route. Recent reviewshave described the development of various replicas, sacrifi-cial templates, and direct foaming approaches for producingporous ceramics [24–26]. Porous ceramics with a well-definedmacroscopic shape and high mechanical stability can be� 2011 American Institute of Chemical Engineers

Environmental Progress & Sustainable Energy (Vol.32, No.1) DOI 10.1002/ep April 2013 25

fabricated by this novel processing route from powder,which retains its intrinsic porosity [27, 28]. Macroporousceramics can combine high permeability with good mechani-cal, thermal, and chemical stability, and therefore are attrac-tive for a wide range of industrial applications [29].

Liquid metal filters of alumina [30], diesel soot filters ofalumina and zirconia [31] are examples where porousceramics are already commercially established, whereas otherapplications including, e.g., hard tissue and bone scaffoldsare still in development [32–34]. The ceramic filters madefrom diatomite particles require relatively high temperatures(�12008C) of sintering [35], which melted the impuritieswithin the diatomite particles especially the alkaline earthand alkali metal minerals. Besides other operational advan-tages, the diatomite particles have high porosity with smallgrain size [36], so the siliceous algae skeletons are used forfabricating microporous ceramic membranes.

In this investigation, an attempt is made to test the feasi-bility of recycling coal fly ash and diatomite waste by using itin the production of porous ceramics. Waste diatomite andcoal fly ash assemblies were adopted and the particles werebonded into relatively strong porous ceramics. Themicrostructure and porosity of the diatomite monoliths weredetermined by X-ray diffraction (XRD), scanning electronmicroscopy (SEM) and mercury porosimetry.

MATERIALS AND METHODS

MaterialsThe waste diatomite was sampled from the food-process-

ing industry in Taipei County, Taiwan. The waste diatomitewas crushed and the particles sieved into sizes between 74and 300 mm for use in subsequent experiments. In an indus-trial context, fly ash usually refers to ash produced duringcombustion of coal. Fly ash is generally captured by electro-static precipitators or other particle filtration equipmentsbefore the flue gases reach the chimneys of coal-fired powerplants, and together with bottom ash removed from the bot-tom of the furnace is in this case jointly known as coal ash.The power plant produces �4,410,000 tons per year of coal-fired power plants in Taiwan.

Collected about 500 kg to make sure the waste diatomiteand fly ash samples were representative, respectively. Boththe waste diatomite and fly ash were then pulverized with aball mill until they could pass through a 100 mesh (149 lm)sieve. In this work, the fly ash-waste diatomite ratio was var-ied from 5 to 20 wt % and at 5 wt % increment. The driedand homogenized waste diatomite containing fly ash werethen stored in a desiccator until testing.

Preparation of Compacted Sintered Porous CeramicsSamples

The prepared porous ceramics samples were oven driedat 1058C for 24 h and ground in a ball mill to form fine pow-ders suitable for pressing. This study using waste diatomitecontaining fly ash in different proportions (0–20%), the sam-ples were compacted at 5 MPa to form a rectangular speci-mens (60 mm (L) 3 30 mm (W) 3 15 mm (H)), which werethen desiccated before testing. The compacted porousceramics specimens were put in a platinum plate and burntin an electrically heated furnace, using a ramp rate of 58Cmin21. These mixtures contain 87.93–94.51% of SiO2 and0.87–1.52% CaO. The basicity (CaO/SiO2) is 0.009–0.017. Thepouring point is 1280–12908C. The porous ceramics sampleswere then sintered at temperatures between 1000 and12708C, for 120 min. After the heating, the samples werecooled to room temperature and then stored in a desiccatorfor subsequent analyses of the physical properties and forleachability testing.

Characterization of Sintered Porous CeramicsSpecimens

The chemical composition and physical characteristics ofthe porous ceramics pellets and sintered products were ana-lyzed. The porous ceramics samples were digested usingHNO3/HClO4/HF according to NIEA R355.00C and then ana-lyzed with ICP-AES (Inductively Coupled Plasma AtomicEmission Spectroscopy) for its major elements. The NIEAR201.14C method, Toxicity Characteristic Leaching Procedure(TCLP), was used for heavy metal determination [37]. Themass loss and absorption capacities were measured usingthe NIEA R204.00T method and ASTM C556, respectively.The major analyses performed on the porous ceramicsincluded the following:

• Heavy metal concentration: The heavy metal concentra-tions in the samples were confirmed by inductivelycoupled plasma atomic emission spectroscopy (ICP-AES).The samples was crushed, and the heavy metals wereextracted by acid (HF: HClO4: HNO3 5 2:1:1).

• Toxicity characteristic leaching procedure (TCLP): SW846–1311. This extraction procedure requires the prelimi-nary evaluation of the pH characteristics of the sample,to determine the proper extraction fluid necessary forthe experiments. It was determined after testing that inthis case the #B extraction fluid (pH 2.88 6 0.05) shouldbe used for the TCLP analysis. This fluid was preparedby adding 5.7 mL of acid to 500 mL of double distilledwater, diluted to a volume of 1 L. A 25-g sample wasplaced in a 1-L Erlenmeyer flask after which 500 mL ofextraction fluid was added. The samples were agitatedfor 18 h using an electric vibrator. The slurry was filteredwith 6–8 lm pore size Millipore filter paper. The leach-ates were preserved in 2% HNO3.

• Chemical composition: The X-ray fluorescence (XRF)analysis was performed using an automated RIX 2000spectrometer. Specimens were prepared for XRF analysisby mixing 0.4 g of sample and 4 g of 100 Spectroflux ata dilution ratio of 1:10. The homogenized mixtures wereplaced in Pt–Au crucibles before being heated for 1 h at10008C in an electrical furnace. The homogeneousmelted sample was recast into glass beads 2 mm thickand 32 mm in diameter.

• Unconfined compressive strength (ASTM C39–72): Threespecimens were used for the compressive strength tests,whereas the fourth one was used for the microstructureexamination. The average strength value from the threespecimens is presented. The coefficient of variation ofthese results was less than 10%.

The weight loss and Absorption testing were measuredusing the NIEA R204.00T method and ASTM C556, respec-tively. The following formulae were used in computing theweight loss and 24-h absorption rate of sintered porousceramics specimens:

Weight loss ¼ fðweight of specimen before firingÞ� ðweight of specimen after firingÞg=fweight of specimen before firingg ð1Þ

24-h water absorption rate ¼ fð24-h saturated surface-

dry weight of specimenÞ � ðdry weight of specimenÞg=fdry weight of specimeng ¼ g water absorbed=24h ð2Þ

A Quantachrome Autoscan Mercury Intrusion Porosimeter(MIP) was used with intrusion pressures of up to 60,000 psi.By using the Washburn equation, with p 5 � 2gcosu

r , the porevolume (V), and the corresponding radius (r) could be syn-chronously plotted by an X-T plotter. The wetting angle ofmercury was assumed to be y 5 1408. In this equation, p, g,

26 April 2013 Environmental Progress & Sustainable Energy (Vol.32, No.1) DOI 10.1002/ep

r, and y, stand for the applied pressure, surface tension, poreradius, and wetting angle, respectively. The crystalline phasespresent in the sintered porous ceramics samples were deter-mined by X-ray diffraction (XRD, Seimens FTS-40) using 30mA and 40 kV Cu Ka radiation. The crystalline phases wereidentified by comparing the intensities and the positions ofthe Bragg peaks with the data files of the Joint Committee onPowder Diffraction Standards (JCPDS). A Hitachi S-800 scan-ning electron microscope was used for SEM observation andcrystal structural determination.

RESULTS AND DISCUSSION

Characteristics of Waste Diatomite and Coal Fly AshThe densities of the waste diatomite and fly ash are 1.41

and 1.61 g cm23, respectively. The pH values of waste diato-mite and coal fly ash are 7.36 and 6.46, respectively. Themoisture contents of the waste diatomite and fly ash are36.46% and 0.08%, respectively. Figure 1 shows the particlesize distribution of the waste diatomite and fly ash. It can beobserved that 13.15 (wt %) of the particles in the waste diato-mite have a median diameter of less than 37 lm and 86.71%of particles have a median diameter of less than between 37and 210 lm. About 80% of the particles in the fly ash have amedian diameter from 20 to 40 lm.

Table 1 shows the composition of the waste diatomiteand coal fly ash. The XRF analysis shows that the major com-ponents in the waste diatomite were SiO2 (94.5%). The XRFanalysis shows that the major components in the coal fly ashwere SiO2 (50.6%), Al2O3 (23.0%), and Fe2O3 (9.7%). Thenext most abundant components were CaO (4.1%) and MgO(1.7%). This result was expected since the SiO2 1 Al2O3 con-tent of coal fly ash sample were 76.3%. As we have known,

SiO2 1 Al2O3 behave as a glass former which increase themelting temperature, whereas the alkali oxides decrease themelting temperature. X-ray analysis of the waste diatomiteand coal fly ash is given in Figure 2. X-ray diffraction analysisrevealed that the waste diatomite mainly consisted of christo-balite. Coal fly ash mainly consisted of quartz, aluminumoxide, hematite, and calcium oxide.

Figure 3a shows that the diatomite powder consists ofplates, shells, and broken particles. The unbroken diatomiteparticle exhibits the inherently intricate and highly porousstructure typical of diatomite. Obviously, the excavation ofthe diatomaceous earth and the post treatment results in asignificant fraction of broken particles. Figure 3b showsmorphological features of the coal fly ash. It also observeddifferent morphologies of fly ash particles size, with the pre-dominance of cenospheres and coal fly ash particles showapproximately spherical shapes.

The TCLP test results are shown in Table 2. The concen-trations of copper, zinc, and nickel were 225, 53, and 168 mgkg21 was observed in the coal fly ash samples. The TCLPleaching concentrations for the target metals in the diatomiteand the coal fly ash met the EPA’s current regulatory thresh-olds and are presented in Table 2.

Physical and Mechanical Properties of SinteredSamples

The minimum density is related to the maximum volumeof closed pores in the sample. Densification is a pore-fillingprocess that occurs by pore shrinkage during liquid phaseflow. Figure 4 presents the porosity of specimens that arefired at various temperatures. The porosity of sintered diato-mite samples declines gradually from 1000 to 12708C (from66.4% to 62.3%). However, the rate of change is not uniform.During sintering, open and closed pores are typically formed.Figure 4 plots the measurements of the porosity of sampleswith various proportions of coal fly ash, fired at four temper-atures. The results reveal that increasing the temperaturereduces the porosity (Figure 4). The heating temperature canalso affect the porosity of the porous ceramics. Accordingly,temperature is the most important parameter in controllingthe densification of diatomite powder and in producingmonoliths with a well-defined porosity. When the amount ofcoal fly ash exceeds 20%, the porosity of the porous ceramicsdecreases sharply with increasing temperature from 61.6%obtained at 11008C to 52.9% obtained at 12708C.

The quality of porous ceramics can be further evaluatedby examining the shrinkage of samples. Figure 5 plots theamount of shrinkage upon firing at various heating tempera-

Figure 1. Particle size distribution of the waste diatomiteand fly ash.

Figure 2. X-ray diffraction patterns of waste diatomite andfly ash.

Table 1. Composition of the waste diatomite and fly ash.

Composition (%) Waste diatomite Fly ash

SiO2 94.51 50.61Al2O3 0.87 22.97Fe2O3 0.92 9.74CaO 0.78 4.14MgO 0.61 1.73SO3 0.85 0.02Na2O — 0.63K2O 0.58 1.13


tures. The shrinkage of porous ceramics without coal fly ashupon heating to 1000, 1100, 1200, and 12708C, is 2.0, 4.3,5.2, and 6.3%, respectively. When the coal fly ash content inthe mixture is increased from 0% to 20%, the shrinkage ofthe porous ceramics upon heating to 1000, 1100, 1200, and12708C changed from 1.1% to 0.63%, 3.6% to 3.4%, 4.9% to5.7%, and 6.4% to 8.3%, respectively, indicating that the addi-tion of coal fly ash should increases shrinkage upon sinteringat 12708C. Therefore, the initial gain in shrinkage that is pro-vided by the high content of coal fly ash was cancelled bythe drop in the ratio of the diatomite-to-fly ash content.When heating to 12708C, the high fluxing oxide (MgO,Fe2O3, and K2O) content in the coal fly ash is thus suggestedto promote the formation of glassy phases that fill the pores,increasing shrinkage upon firing. The result also demon-strates that combining fly ash and diatomite provides an

opportunity for balancing the increase in shrinkage that iscaused by a high Fe2O3 content in the coal fly ash. The den-sification of the coal fly ash is influenced by the sources offluxing agents such as K2O, Na2O, and Fe2O3, which favorthe formation of a vitreous phase [38]. The higher mechanicalstrength capacity of porous ceramics makes them suitable formaterials use in pavements.

The weight loss that occurs in a monolith upon sinteringis related to the development of porosity and densification. Iteventually affects the compressive strength of thermallytreated samples. Figure 6 shows plots the weight loss ofporous ceramics upon ignition, and the amount of coal flyash that is added to the mixture at various heating tempera-tures. The weight loss of porous ceramics without coal flyash upon heating to temperatures of 1000, 1100, 1200, and12708C that is attributed to organic matter in diatomite is

Figure 3. Scanning electron micrographs of waste diatomite and fly ash. (a) Waste diatomite and (b) coal fly ash.

Table 2. Total metal and leaching concentrations of waste diatomite and coal fly ash.

Total metal (mg kg21) Pb Cr Cu Zn Cd Ni

Waste diatomite N.D. N.D. N.D. N.D. N.D. N.D.Fly ash N.D. N.D. 225 53 N.D. 168TCLP (mg L21) Pb Cr Cu Zn Cd NiWaste diatomite N.D. N.D. N.D. N.D. N.D. N.D.Fly ash N.D. N.D. 0.11 0.29 N.D. N.D.Regulatory limits 5 5 15 — 1 —

N.D.: Pb < 0.015 mg L21; Cr < 0.009 mg L21; Cd < 0.021 mg L21; Zn < 0.074 mg L21; Cu < 0.089 mg L21; Ni < 0.112 mg L21.

Figure 4. Porosity of porous ceramics samples. Figure 5. Shrinkage of porous ceramics samples.


1.91%, 1.93%, 1.96%, and 1.97%, respectively. The resultsindicate that adding coal fly ash did not apparently reducethe weight loss upon ignition. As the temperature wasincreased, the weight loss of the porous ceramics is assumedto have declined. This result reveals that diatomite and coalfly ash are compatible with each other, so coal fly ash can beused as a diatomite substitute.

The water absorption rate, which refers to the weight ofmoisture in the pores divided by the weight of the sinteredspecimen, is an effective index of the quality of a porousceramics. The infiltration of less water into a porous ceramicsupports expectations of greater durability and resistance tothe natural environment. Figure 7 plots the results of thewater absorption tests for various coal fly ash-diatomite mix-tures fired at four temperatures. The water absorption rateswere from 89.4% to 78.3%, 86.0% to 72.6%, 82.3% to 65.4,and 79.8% to 58.9% at temperatures of 1000, 1100, 1200, and12708C, respectively. The specimen without coal fly ash washeated to a temperature of 12708C, as the heating tempera-ture increased, the amount of water absorbed in the porousceramics decreased. The results indicate that when the coalfly ash content decreased, the water absorption of the porousceramics increased. However, when the amount of coal flyash exceeded 20%, even when the specimen was fired at12708, the water absorption (58.9%) of the formed porousceramics was lower than that of the other specimens withdifferent amounts of fly ash. The lower water absorption rateafter heating to a higher temperature (12708C) suggests thatlocal liquid-phase sintering had occurred, reducing the pore

volume and, thereby, the water absorption rate. Apparently,the bonding ability of the mixture is related to the proportionof coal fly ash.

The compressive strength is the most important engineer-ing quality index of building materials. Figure 8 displays theresults of compressive strength tests of the porous ceramicsthat were produced from mixtures of both diatomite andcoal fly ash. When the heating temperature was increasedfrom 1000 and 12708C, the compressive strength of the po-rous ceramics gradually increased. When the heating temper-ature was increased above 12708C, the compressive strengthof the ceramics was between 5.85 and 15.8 MPa. When up to20% coal fly ash was added to the porous ceramics that wereheated to 12708C, the strength was always apparently higherthan that of the porous ceramics without any coal fly ash.These results reveal that the heating temperature that maxi-mized compressive strength was 12708C. Coal fly ash fromMgO-Al2O3-SiO2 systems exhibited similar crystallizationbehavior. Coal fly ash can be blended with clay to generateporous ceramics. Coal fly ash with high Al2O3 1 SiO2 con-tents is suitable for the production of porous ceramics,because Al2O3 and SiO2 are known network formers, andadding coal fly ash to a system increases its capacity to formnetworks. Accordingly, the porous ceramics samples hereinwere stronger than the control specimens. Therefore, thestrength of porous ceramics samples that are formed from amixture of both diatomite and coal fly ash can be controlled.

Distribution of Sizes and Total Volume of Pores inCeramics

Figure 9 plots mercury intrusion data that were obtainedupon heating to 11008C. Figure 9a includes one strong peakwith a range of pore sizes of around 1.5–2 lm and a smallerpeak at 0.2–1 lm. Pores with sizes of 1.5–2 lm are linked tothe voids among the powder particles, whereas those withsizes of �0.2–1 lm are inherent to the diatomite in theporous ceramics samples (Figure 10a). The main peak shiftsto a smaller pore size as the heating temperature increases.Figure 11 plots the mercury intrusion data obtained uponheating to 12008C. Figure 10a includes one strong peak atpore sizes of �2–2.5 lm and a smaller peak at 0.3–1 lm. Fig-ure 12 plots mercury intrusion data obtained upon heating to12708C. Figure 11a includes a strong peak at pore sizes ofaround 2.5–3 lm and a smaller peak around 0.3–2 lm.Therefore, the sizes of the interparticle pores decline as thepeak temperature increases, and the interparticle voidsbecome partially filled with the melt and/or collapse.

Figures 9–11b present the evolution of the porosity ofsintered bodies that contained coal fly ash. They plot the

Figure 8. Compressive strength of porous ceramics samples.Figure 6. Weight loss on ignition of porous ceramics.

Figure 7. Water absorption of the porous ceramics.


cumulative volume of pores as a function of pore size. Thesintered samples that were prepared from diatomite that con-tained more coal fly ash had smaller pores. The figures indi-cate that the porous diatomite ceramics samples contained agreater volume of pores. However, for various types of sam-ple, the dependence of total volume on coal fly ash content

was strongest between 15 and 20 wt % (Figures 9–11b).Figures 9–11b present the evolution of relevant morphologi-cal parameters, such as total Hg intrusion volume (porosity)and average size of interconnecting pores such as the porenecks among the particles of coal fly ash, in the sampleswith coal fly ash contents from 10 to 20 wt %. However, as

Figure 9. Distribution of sizes and total volume of pores in ceramics of porous ceramics sintered at 11008C (a) distribution ofsizes and (b) total volume of pores. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]




the concentration of coal fly ash increases, the coal fly ashbegins to interact with diatomite particle and an open struc-ture that interconnects the fine pores is slowly formed. Asexpected, porosity and average pore size depend directly onthe coal fly ash content. Increasing the amount of coal flyash particles tends to reduce the average distance between

diatomite particles in samples that contain coal fly ash, pro-moting diffusion during sintering.

XRD Patterns of Porous CeramicsFigure 12 presents XRD patterns of diatomite porous

ceramics that were sintered at 1000, 1100, 1200, and 12708C.As the sintering temperature increases, the intensity of thenewly emerging crystalline quartz peaks increases, and boththe amorphous SiO2 content and the amount of other impur-ities declines as the temperature increases above 11008C.When the temperature reaches 12708C, all of the amorphoussilicon dioxide is converted to the crystalline phase and cris-tobalite becomes the dominant phase (Figure 13).

SEM Microphotographs of Ceramic SamplesFigures 14a–e presents the microstructures of the fractured

surface of samples fired at 12708C. Figure 14 shows that theneck growth rate and neck size increased greatly upon firingat 12708C. The microstructure does not significantly change(Figures 14a and 14b). Cylindrical diatomite particles areidentified easily and some micropores are distributed in theirwalls. The micrographs indicate that crystalline materialswere embedded in a glassy matrix and that the microstruc-tures of the ceramic samples were not thoroughly homoge-neous. The fracture surface of the sample that contained 10%

Figure 12. XRD patterns of diatomite porous ceramics.

Figure 13. XRD patterns of porous ceramics.


fly ash had a denser well-sintered microstructure with a uni-form distribution of pores, than the sample that contained0% fly ash (Figure 14c). Figure 14e shows that the ceramicsample that contained 20% coal fly ash contained moregrains than the control sample. The fracture surface of theceramic sample that contained 20% fly ash differed markedlythose of the others with denser well-sintered microstructures,which were associated with the formation of elongated cav-ities. As the amount of fly ash increased, fly ash particlestended more to coalesce, forming necks among the particles,increasing the density and, under some conditions, improv-ing the mechanical properties (Figure 14e). From these

results, this dense microstructure is suggested to be responsi-ble for the improvement in the mechanical properties of thesample. These observations suggest that the increase in therate of sintering that is caused by Fe2O3 causes fewer poresto be formed in the ceramic samples.

CONCLUSIONS

This investigation demonstrated the feasibility of using di-atomite and coal fly ash as alternative raw materials in theproduction of porous ceramics. The results herein supportthe conclusions drawn below. Specifically, the results con-

Figure 14. SEM microphotographs of samples at 12708C.


cerning the sintering process demonstrated that the proper-ties of the produced materials depend on the sintering tem-perature and the amount of coal fly ash. Higher Al2O3 1SiO2 contents in the coal fly ash resulted in better propertiesof the sintered materials. In porous ceramics that contain flyash, when the temperature reaches 12008C or 12708C, all ofthe quartz is converted to cristobalite, which thus becomesthe major phase. The experiments revealed that the sinteredceramic samples that contained coal fly ash the best physical,mechanical, and microstructural properties. When the heatingtemperature was increased above 12708C, the compressivestrength of the ceramics was between 5.85 and 15.8 MPa.Samples that were treated at 12708C had the best sinteredstructure, with the most necks among the particles, resultingin the best mechanical properties, high compressive strength,and low water absorption. Accordingly, waste diatomite andcoal fly ash can indeed be 100% recycled and reused to pro-duce porous ceramics.

ACKNOWLEDGMENTS

The authors like to thank the National Science Council ofthe Republic of China, Taiwan, for financially supporting thisresearch under Contract No. NSC 98-2621-M-197-001. TedKnoy is appreciated for his editorial assistance.

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