Raw materials used in this study were red clay obtained from “AL BARRAGE” (a region in Safi, Morocco with a coordinates 32.324962, -9.188877) and phosphate sludge collected from the Youssofia industrial phosphate wash plant. Initially, both raw materials were dried at 105°C for 5h in an electrical oven, then they were crushed and sieved using a 100-μm sieve.

Five mixtures, M1–M5, were prepared with the mass proportions, in clay/phosphate sludge, as follows: M1: 1/0, M2: 0.75/0.25, M3: 0.5/0.5, M4: 0.25/0.75 and M5: 0/1. Table 1 shows the mass proportions of mixtures.

The pellets were prepared by dry process from raw material powders, homogeneously mixed by a co-grinding of both powders. The mixed powder was axially compressed under a one-ton charge to get pellets with 13mm diameter and 3mm thickness, which were then heated at different temperatures. The firing cycle was the following: i. heating ramp 5°C/min up to the selected firing temperature (600, 900, 1000, 1100, and 1120°C); ii. 2h dwell time at the temperature selected; iii. cooling according to the inertia of the furnace. The heat treatments were carried out in an electric furnace (Nabertherm model LH 15/12). Fig. 1 shows the the sample preparation process.

Fig. 1.

Illustrated synthesized process of ceramics samples.

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The chemical composition of clay and phosphate sludge was determined by using Panalytical ZETIUM X-ray fluorescence (XRF) analysis. An ICP-AES instrument perkinElmer Optima 8300 DV was used for the simultaneous multielement determination of major and minor elements. Sample preparation for ICP-AES analysis was performed: 20mg of sludge and clay samples were accurately weighed into Teflon digestion vessels, and 4ml of a concentrated HF solution with 1ml of a HNO3 solution were added. Samples were subjected to the microwave heating for 75min. After cooling, colorless solutions were quantitatively transferred into 100ml volumetric flasks and made up to the volume with deionised water. The mineralogical characterization of the crystalline phases formed after thermal treatment were performed by X-ray diffraction using a Bruker D8 Advance diffractometer, operating with Cu Kα radiation, an operating voltage of 40kV, a current of 40mA, a step size of 0.02° and a scanning angle 2θ ranging from 5° to 60°. The presence of minerals was identified from the position of peaks and its intensities were matched by using reference database from software and X’Pert HighScore. The enhancement or inhibition of a certain crystalline phase was mainly judged from the presence or absence of its specific diffraction lines and the increase or decrease in their relative intensities..

The heat flow and weight loss of the clay and sludge were tracked as a function of the temperature using the coupled TG-DTA (thermal gravimetric-differential thermal analysis). The unfired powders were analyzed with a Setaram 92 instrument. Data were collected under air from 30 to 1000°C at a heating rate of 10°C/min.

Table 2 lists the chemical composition of raw materials. It can be seen that the phosphate sludge is mainly composed of CaO 31.44wt%, SiO2 27.48wt% and P2O5 16.83wt%, with a small quantity of MgO and Al2O3 (respectively 2.90–2.41wt%). The elemental analyses of phosphate sludge (Table 3) were used to quantify the major and minor elements and also heavy metals and radionuclides. From the results of the analysis of the samples, phosphorus was found to be 13.52mg/l, silicon was 23.7mg/l, and calcium was 41.46mg/l, also, minor elements were detected (aluminum, iron, potassium, and titanium). There was no heavy metals and radionuclides present. The X-ray diffractogram of the sludge shown in Fig. 2 revealed the following mineralogical composition: quartz (SiO2), calcite (CaCO3), dolomite (CaMg(CO3)2) and francolite Ca5+0.5x(PO4)3−x(CO3)x(F,OH).

Fig. 2.

XRD diffraction patterns of sludge. Q: quartz; FR: francolite; C: calcite; D: dolomite.

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Francolite and fluorapatite are two phases presenting almost the same diffractogram, the only difference is at the level of the peaks between 30 and 35 2theta, francolite presents three peaks and fluorapatite presents four peaks. Fig. 3 illustrates the difference between the theoretical diffractogram of francolite and fluorapatite. Many studies have demonstrated that fluorapatite Ca5(PO4)3F is the major phase in the Yousoufia sludge [1,2,5,6]. However, the results of the present study showed the absence of the fluorapatite while the francolite is the major phase contained in this sludge. Francolite which is a carbonate fluorapatite with the common formula of Ca5+05x(PO4)3−x(CO3)x(F,OH)(s), appears when the PO43− within the structure of fluorapatite is substituted by CO32−. CO32− content of francolites vary from 1.4% to 6.3% [12,13]. The maximum level of carbonate substitution is limited by the disruption of the francolite structure at high contents of CO2 (>6.3%). Many studies have shown that PO43− is replaced by (CO32−+F−) and not by CO32− alone [9,10]. Other studies have reported that PO43− and Ca2+ are replaced by CO32−[11].

Fig. 3.

Theoretical XRD patterns of francolite and fluorapatite phases.

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The value of LOI 15.64wt% for sludge is due to the content of carbonate, dehydroxylation and water release.

The phase evolution of phosphate sludge during heat treatment was studied by XRD analysis. Fig. 4 shows the XRD patterns of phosphate sludge powder after sintering at 600, 900, 1000, 1100 and 1120°C.

Fig. 4.

XRD patterns of sludge sintered at different temperatures. Q: quartz; C: calcite; D: dolomite; FR: francolite; FL: fluorapatite; AK: akermanite.

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After the first heating at 600°C dolomite decomposes to CaCO3 and MgCO3. However, at 900°C, calcite is not detected and the francolite phase transformed into fluorapatite Ca5(PO4)3F. At 900°C, a few major changes can be seen, including calcite phase disappearance and the decrease in quartz peaks intensity as well as the increase in the intensity of two new phases akermanite and monticelitte. The weakening of SiO2 and disappearance of CaCO3 peaks at 900°C is due to the monticelitte formation according to the reaction shown in Table 4. Monticelitte CaMgSiO4 is considered intermediate compounds, which became unstable at high temperatures and react with SiO2 and CaO to give akermanite (Ca2MgSi2O7). Above 1000°C, the major phases found are the fluorapatite, akermanite and quartz [14,15].

Table 4.

Sludge phases evolution as a function of temperature.

To confirm the presence of francolite and how it transforms into fluorapatite, a zoom in the region between 2θ=30–35° was placed in Fig. 4. The diffractogram corresponding to this area is shown in Fig. 5. The three peaks of francolite phases in 2θ=30–35° change their position and display a doublet characteristic of fluorapatite phase before 900°C.

Fig. 5.

Zoom in region of 30–35° XRD patterns of sludge sintered at different temperatures.

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The thermal behavior of the sludge analyzed by TG-DTA is presented in Fig. 6. Five endothermic peaks are clearly shown in the DTA curve; the first peak at 93°C is probably attributed to the removal of weakly bound water. The endothermic peak detected at 575°C could be assigned to the decomposition of dolomite. No discernable mass loss was observed by the TGA curves at the same temperature, while the peak shown at 700°C is corresponding to the transformation of francolite to fluorapatite. Peaks at 736 and 746°C could be attributed to the decomposition of calcite and magnesium carbonate, respectively. TGA curve indicated that these changes were responsible for the mass loss.

Fig. 6.

TG-DTA analysis of sludge.

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To confirm the above transformations and for understanding better the reactions that occurred during the thermal treatment, calcination has been carried out at three additional temperatures: 720°C, 750°C and 800°C. The results of the XRD analysis in the region 2θ=28–35° are presented on the diffractogram in Fig. 7. They show the presence of francolite and calcite where their peaks are detected at 600°C. At 720°C we notice the appearance of fluoroapatite and the disappearance of francolite, which occurred between 600 and 720°C. XRD diffraction patterns of sludge sintered at 750°C revealed the missing of calcite. The results of the XRD analyses are in good agreement with the findings from the DTA, which confirms that the transformation of francolite to fluoroapatite and the decomposition of calcite are occurring at around 700 and 746°C respectively.

Fig. 7.

Zoom in region of 30–35° XRD patterns of sludge sintered at intermediate temperatures.

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Chemical analysis (Table 2) shows that Safi clay is essentially composed of SiO2 (52.79wt%), Al2O3 (1744wt%), Fe2O3 (5.85wt%). Besides, there are a little amount of other oxides such as Na2O, MgO, and P2O5. Table 3 lists the elemental analysis of raw materials. It can be seen that the clay is mainly composed of silicon 98.30mg/l, aluminum 35.52mg/l, iron 17.57mg/l and potassium 14.45mg/l with a small quantity of phosphorus and sodium (respectively 0.26–1.10mg/l). These results are in good agreement with the X-ray fluorescence analyses. The crystalline phase composition and the phase evolution of clay were obtained by XRD analysis. The main detected minerals (Fig. 8) were illite K0.5 (Al,Fe,Mg)3(Si,Al)4O10(OH)2, kaolinite (Al2Si2O5(OH)4), dolomite (CaMg(CO3)2), calcite (CaCO3) and quartz (SiO2). The value of LOI for clay (11.7wt%) is due to the content of carbonate, dehydroxylation and water release.

Fig. 8.

XRD diffraction patterns of Safi clay. Q: quartz; C: calcite; D: dolomite; IL: illite; KA: kaolinite.

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Fig. 9 shows the XRD patterns of clay powder after sintering at 600, 900, 1000, 1100 and 1120°C, After heating of clay at 600°C, kaolinite decomposes to amorphous metakaolinite (Al2Si2O7) by removing of the hydroxyl groups of the silicate lattice according to Eq. (1). While dolomite was decomposed to calcite and magnesium carbonate as shown in Eq. (2).

Fig. 9.

XRD patterns of clay sintered at different temperatures. Q: quartz; D: dolomite; IL: illite; KA: kaolinite; AN: anorthite; DP: diopside.

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As the temperature increased to 900°C, calcite was decarbonized, the intensity of illite decreased and the peaks in the region 2θ=26–29° increased in intensity, which indicates the formation of anorthite. This may be attributed to the reaction between Metakaolin produced following the dehydroxylation of the kaolinite and CaO produced following the decomposition of the calcite [17,18]. The content of anorthite phase increases as the temperature increases, after 900°C the free silica reacts with free CaO and alumina, the last being derived from the decomposition of illite, forming anorthite.

As a matter of fact, wollastonite CaSiO3 and gehlenite Ca2Al2SiO7 are considered intermediate compounds which became unstable in the presence of SiO2 and react to give anorthite (CaAl2(SiO4)2). However in this study, gehlenite and wollastonite were not detected, this is due to the large SiO2 content of clay [19,20].

From 1000°C onwards, remarkable changes were noticed. Accordingly, the traces of a new phase called diopside, start to appear after reaction between three oxides: CaO originated from calcite and dolomite, MgO produced following the decomposition of dolomite and SiO2 present in the initial composition of the clay. The quartz is present in all the raw and fired samples, with contents decreasing progressively with temperature. Table 5 summarizes all the reactions associated with the thermal treatment of clay as reported in the literature [22,23].

Table 5.

Clay phases evolution as a function of temperature.

The DTA–TGA thermograms of clay given in Fig. 10 show the major transformation that clay undergoes as a function of temperature. A first weight loss was detected at 95°C corresponding to the evaporation of free water contents. The dehydroxylation of the kaolinite mineral as a result of the increase in temperature induces a large endothermic peak at 508°C, which hides the peak corresponding to the decomposition of dolomite. The small peak at 568°C corresponds to the transformation of quartz α to quartz β, while the peak at 736°C is attributed to the decomposition of calcite and magnesium carbonate.

Fig. 10.

TG-DTA analysis of Safi clay.

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The phase evolution of clay/sludge mixture as a function of the clay addition percentage (25wt%, 50wt%, 75wt%) during heat treatment was studied by XRD analysis. Figs. 11 and 12 show the XRD patterns of mixtures after sintering at 900 and 1120°C respectively. These temperatures were chosen with a view to study the evolution of the mixture at low temperatures (900°C) and high temperature (>1100°C).

Fig. 11.

XRD patterns of sludge–clay mixtures sintered at 900°C.

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Fig. 12.

XRD patterns of sludge–clay mixtures sintered at 1120°C.

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The X-ray diffraction analysis, of the pellets sintered at 900°C, made from the mixture with 25wt% clay (Fig. 11), revealed the presence of quartz, fluorapatite and traces of anorthite. The akermanite and monticelitte vanished and diopside appeared, these evolutions are discussed hereafter. At the same temperature and for the mixture with 50wt% clay, diopside and fluorapatite phases increased and become the major phases. Yet, a gradual decrease of quartz and anorthite phases have been noticed. At high percentage of clay, a decrease in the amount of fluorapatite and diopside phases was observed, while the anorthite phase started to be the major phase.

As shown in Fig. 12, increasing the firing temperature to 1120°C, led a decrease in the amount of quartz and an increase in the diopside within the mixtures 25wt% and 50wt% (wt% clay). The formation of the diopside is favored by increasing the firing temperature as well as the high percentage of phosphate sludge. It can be observed that akermanite and monticelitte phases completely disappeared in all mixtures following the formation of the diopside phase.

Akermanite, monticelitte and diopside are three varieties that have simple chemical compositions mainly constituted by SiO2, MgO and CaO [24]. The system CaO–MgO–SiO2 (Fig. 12) is very useful in providing a framework for the understanding of the equilibrium between akermanite and diopside phases within the sludge–clay mixture [25]. The firing behavior of no-calcareous and calcareous samples is considerably different because of the initial mineralogical composition. Moreover, the presence of dolomite, calcite or both, with a large percentage can influence the formation of different minerals after firing behavior at high temperatures [19].

The competition between the formation of diopside and akermanite during the heating was influenced by the composition and thermodynamic of the system. The above experimental results indicate that the amount of CaO and SiO2 may play a key role on the formation of diopside phase in the investigated system.

The main difference between the raw materials (clay and sludge) is the composition of the carbonate. As has been shown in the intensities of the most intensive diffraction peaks for the crystalline phases in sludge and clay, calcite and dolomite are lightly present in clay, unlike in sludge.

Another difference is the percentage of quartz, the chemical composition of clay has a higher contribution of SiO2. The quartz concentration was strongly enhanced during the enrichment of mixtures by clay. So, the addition of clay corresponded to a higher contribution of SiO2. Hence, the ratio MO/SiO2 (MO=CaO+MgO) decreased, which indicates that the formation of Metasilicate is significantly favored with sufficient silicate ions. On the other hand, unlike clay, the chemical composition of sludge has a higher contribution of CaO derived from the decomposition of calcite, dolomite and the transformation of francolite to fluorapatite. Therefore the ratio MO/SiO2 is greater than 1, in this case, the formation of Pyrosilicate was significantly favored. This leads to the crystallization of akermanite phase which is a pyrosilicate (contain Si2O76− ions which are formed by joining two tetrahedral SiO44− which share one oxygen atom at one corner). However, The composition reported by a ternary diagram (Fig. 13) indicates that the natural diopside contains higher proportions of SiO2 and minor proportions of CaO and MgO, which fits with the theoretical composition of metasilicate (MO/SiO2=1). From the result above we can conclude that the main crystalline phase changed from akermanite (Ca2MgSi2O7) to diopside as the (CaO+MgO)/SiO2 ratio decreased. These results are in accordance with the study of Liu et al. [26].

Fig. 13.

Ternary diagram of system SiO2–CaO–MgO [16,26].

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From the thermodynamic point of view, the temperature of the formation of diopside is lower than that of akermanite, which suggests that diopside is more likely to form [24,25].

The XRD pattern of the mixtures samples after heat treatment at 900 and 1120°C shows that when the clay content was less than 75wt%, the main crystalline phase formed in samples was the diopside. However, anorthite started to form with clay contents of more than 75wt%. With decreasing sludge content, the intensity of anorthite became stronger while that of diopside became weaker. This result indicates that the increase in CaO/SiO2 ratio seems to reduce the crystallinity of anorthite but favors the formation of diopside. This is in accordance with literature [26,27]. Remarkable change was noticed, including the decrease in quartz peaks in XRD patterns of the mixture, which means it reacted to form diopside phase. The presence of diopside and fluorapatite phases is suitable for the manufacture of glass–ceramic [19–21]. It is claimed that the fluorapatite–diopside glass–ceramic shows a combination of high chemical resistance, good biocompatibility and good mechanical strength. Such materials are used as artificial implants in orthopedic surgery [28–30]. Consequently, the mixture of phosphate sludge and clay can be used as raw materials in the production of glass–ceramics.