Utilization of Water Treatment Plant Sludge and Coal Fly Ash in Brick Manufacturing

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2.3. Characterization of Materials

The chemical compositions of materials were analyzed by energy dispersive XRF (X-ray ﬂuorescence spectrometry) using (RX Axios Advanced, PANalytical, Phillips spectrometer). Phase and crystallinity were confirmed by powder XRD (Rigaku Multiflex) with nickel filter CuKα radiation (λ = 1.54060 Å) scanning from 5 to 80° at a rate of 1°/min with current 40 kV and 20 mA. The Fourier transform infrared spectroscopy (FTIR), spectra were recorded on Nexus 670 Thermo Nicolet using KBr pellet method. Scanning Electron micrograph was obtained by using XL-30 Philips scanning electron microscope (SEM). The particle size of the materials was measured using a laser based particle size analyzer, namely a Malvern MSS Mastersizer 2000 Ver. 5.54. The real density and the specific surface area of fly ashes and zeolite samples were determined by a helium picnometer (Micromeritcs Instrument Corporation - Accupyc 1330) and by a BET Surface Area Analyser (Quantachrome Nova – 1200), respectively. Prior to determination of the specific surface area, samples were heated at 423.15 K for 12 h to remove volatiles and moisture in a degasser (Nova 1000 Degasser). The BET surface areas were obtained by applying the BET equation to the nitrogen adsorption data. Thermogravimetric and differential thermogravimetric analyses (TG/DTG) were carried out using a Mettler Toledo STAR SW 9.30 thermogravimetric analyzer with heating rate of 10 °C min-1 between 25-1400 °C, under nitrogen flow rate of about 50 ml min-1. Other relevant characteristics of SWTP such as organic matter content, pH, cation exchange capacity, the loss on ignition and total solids content, fixed solids, volatile solids were also determined. Physicochemical characterizations of fly ash are reported in previous study ^[20].

3. Results and Discussion

3.1. Characterization of SWTP

The X-ray diffraction pattern of the waterworks waste sample is shown in Figure 1. The sample exhibited peaks that are characteristics of kaolinite (Al₂Si₂O₃(OH)₄), quartz (SiO₂), goethite (FeO (OH)), and muscovite (KAl₃Si₃O₁₀(OH)₂), also known as mica, with predominance of kaolinite and quartz.

Table 2 presents the chemical composition of the waterworks waste sample. Although the properties of the sludge samples are highly variable and dependent on both the type of the raw water and the chemical composition of coagulant ^[21], it can be seen from Table 2 that aluminum is the dominant component in the dewatered alum sludge with ~39 % in mass expressed as Al₂O₃. The other principal chemical components were SiO₂ and Fe₂O₃.

Table 2. Chemical composition of the waterworks waste

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This result is consistent with the X-ray diffraction data (Figure 1). The alumina does not occur in its free form in the waterworks waste sample, being bounded to the clay minerals (kaolinite and muscovite). The quartz is present in the structure of clay minerals and free silica particles. The iron oxide is present in the structure of the goethite. The contents of alkaline oxide (K₂O), TiO₂, alkaline earth (CaO and MgO), SO₃ and P₂O₅ are between 0.2 -1.4 %.

The presence of these elements is attributed to the use of aluminium sulfate as coagulants in water treatment plants and due to the composition of the water, which contains suspended materials, such as sand and clay materials.

Figure 1. X-ray Diffraction pattern of the waterworks waste sample

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Some important physicochemical characteristics of waterworks waste sample are shown in Table 3. Usually, the moisture content of the waterworks sludge is at least 80 %. Loss on ignition (LOI) implied in high weight loss of about 52.0 wt. %, is mainly attributed to the presence of clay minerals, hydroxides, and organic matter in the waste sample ^[22].

Table 3. Chemical composition of the waterworks waste

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The pH (in water) of the waste sample was 5.57, which can be considered as being of a medium acidity. The sample presented a value of a cation exchange capacity (CEC) of 9.04 meq/100 g within the CEC range of kaolinitic clays (3 – 15 meq/100 g) ^[22], corroborating the kaolinitic character of the investigated waterworks waste.

It was also found that the waterworks waste presented a high organic matter content (24.3 %). This result suggested that the organic matter significantly contributed to the high value of loss on ignition. The waste sample presented real density of 2.27 g cm^-3, which reflected its mineralogical composition. The surface area of water treatment plant sludge was 106.9 m² g^-1 indicating a porous material.

Water treatment plant sludge was characterized by the following granulometric composition: a sand fraction, 58.2 % (particles with diameters between 0.06 and 2.0 mm); a silt fraction, 6.83 % (particles with diameters between 0.002 and 0.06 mm); and a clay fraction, 35.2 % (particles with diameter lower than 0.002 mm).

Figure 2 shows the scanning electron micrograph of SWTP. As shown by the result of the SEM analysis, the sludge has irregular shapes which are probably flakes of fine kaolinite clay particles ^[23]. In addition, a wide particle size range can be observed, in accordance with the particle size data.

Figure 2. Morphology of the waterworks waste particles

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The TG and DTG curves for the 10°C min^-1 heating rate under a N₂ atmosphere are shown in Figure 3. The profile of SWTP weight loss exhibited three stages during the degradation process (Figure 3). The first occurred rapidly between 30 and 202 °C with loss of mass of around 12 %, while the second occurred between 202 and 397 °C with mass loss of 8.4 %. Finally, the third stage occurred between 399 and 906 °C with mass loss of 9.9 %. The remaining solid residue was 68 % of the original mass.

The thermal events observed on the DTG curve are according to mass losses evidenced on the TG curve. The event which occurred at 76.1°C (T peak) was the major reaction area and represents the elimination of free water between the particles. The T peak = 275.2°C is probably related to water loss from hydroxides and to the combustion of organic matter. The T peak = 490.9°C is attributed to the dissociation of constitution water (or structural water), i.e., comprising the hydroxyl group of the kaolinite clay mineral.

Figure 3. TG and DTG curves of waterworks waste

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Figure 4 shows the FTIR spectrum for SWTP. The FTIR spectrum of the raw sludge showed the presence of kaolinite, quartz, goethite and muscovite identified by XDR (Figure 1) and some organic matter. The assignments for FTIR spectrum has been reported elsewhere ^[24]. The SWTP sample had an FTIR spectrum consistent with previous reports in the literature ^[25].

Figure 4. FTIR spectra of waterworks waste

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3.2. Characterization of CFA

The XRD analysis of CFA powder revealed the presence of quartz crystallite and mullite as a major component of ash and some amounts of hematite, anhydrite and albite. The presence of amorphous phases are identified as a broad diffraction ‘hump’ in the region between 18 to 30 degrees 2θ (Figure 5). The intensity of quartz is very strong, with mullite forming a chemically stable and dense glassy surface layer.

Figure 5. X-ray diffraction pattern of cyclone fly ash

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The sample of coal fly ash collected by the cyclone from Figueira Power Plant was characterized and some physico-chemical characteristics of this material are shown in Table 4. The chemical composition of CFA sample reveals that the major components are SiO₂, Al₂O₃, and Fe₂O₃. Some minor components like Fe₂O₃, SO₃, CaO, K₂O and trace elements (< 1.2 wt %) are also present in CFA sample. The characteristics of Brazilian coal fly ashes have been previously discussed in the literature ^[14].

The FTIR spectrum of CFA is shown in Figure 6. The band at 3433 cm^-1 is attributed to the asymmetric and symmetric stretching vibrations ν(O–H), suggesting the presence of an amorphous silicate material (glass) or possibly hydrated aluminum silicates. The band at 1622 cm^-1 is attributed to the bending mode of H₂O molecules. A broad band at 1069 cm⁻1 is due to Si–O–Si asymmetric stretching vibrations of silica. The bands at 794 and 460 cm^-1 could be assigned to quartz and silica, respectively.

Table 4. Physicochemical properties of cyclone fly ash

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Figure 6. FTIR spectra of cyclone fly ash

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Laser granulometric analysis results for CFA are reported both as absolute and cumulative volumetric distributions (Figure 7). From the cumulative distributions, the following values for d10, d50 and d90 can be observed: 4.994, 15.575 and 52.526 µm, respectively.

Figure 7. Particle size distribution of cyclone fly ash

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The SEM image of CFA demonstrates particles of different shapes and sizes, which majority consisted of solid spheres (Figure 8A). Different physical states of silica are responsible for the particles of irregular size. Hollow microspheres (cenospheres), plerospheres (smaller spheres inside big sphere) and irregularly shaped unburned carbon particles tended to be in the upper end of the size distribution (Figure 8B).

Figure 8. SEM micrographs of cyclone fly ash (a) particles of irregular size and spherical morphology; (b) hollow sphere (plerosphere) containing smaller hollow and empty spheres (cenospheres)

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3.3. Characterization of Bricks

In this study, sludge of the treatment water plant stations (SWTP) and coal fly ash (CFA) were used for the manufacture of soil-cement and soil-lime bricks. The quality of the bricks after curing for 28 days was determined on the basis of compressive strength and water absorption. According to NBR standard 10836 ^[19], water absorption and compressive strength are properties that deﬁne the soil-cement brick quality after curing. The speciﬁed values for water absorption and compressive strength are ≤ 20 % and >2 MPa, respectively.

The compressive strength of the bricks is shown in Table 5. The brick pieces with 0 wt. % wastes (sample BW0, 100 % soil-cement body) were used as reference materials. As it can be observed, the compressive strength of the bricks presented decrement with wastes. Such a behavior is mainly related to the following factors: i) An inadequate granulometry of the SWTP; and ii) A small amount of Portland cement or lime in relation to other components of bricks. The values of compressive strength of BW3 sample are in accordance with the limit values of the Brazilian specification used for industrial production of soil-cement bricks.

Table 5. Average resistance to compression and average absorption of water

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Table 5 shows the water absorption of the bricks. Water absorption is related to the volume of the open pores (i.e., the pores that are connected with the specimen surface). In addition, water absorption is closely related to densiﬁcation and the microstructures of the cementitious matrix. It can be observed that the reference bricks BW2, BW4 and BW5 showed higher water absorption values than the reference bricks (BW0 sample). This is related mainly to the presence of a high amount of clay minerals (kaolinite and illite/mica) and an organic matter in the waterworks waste sample that tends to influence the cement hydration reactions ^[11].

The results obtained from compression resistance tests, for the case of BW1 bricks showed unfavorable results. They presented the average values of resistance to compression less than 1.0 MPa. Therefore, they were disregarded and the study of water absorption only included mixtures showing a favorable behavior. For BW3 samples the values of water absorption are according to the NBR standard 10836 ^[19].

The brick appearance is another important parameter of brick quality. The BW3 samples were reddish brown color, free of surface stains, efﬂorescence (soluble salts), and defects (Figure 9).

Figure 9. The appearance of the brick specimens (BW3 samples) after curing at 28 days

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