Characterization and pozzolanic properties of calcined alum sludge
Graphical abstract
Introduction
Alum sludge continuously accumulates in drinking water treatment plants during the flocculation-clarification process using alum coagulant and further removed from the liquid phase by sedimentation/filtration processes [1]. Sludge amount and composition mainly depend on water quality, removal efficiency as well as type and dose of coagulant. The amount of sludge ranges from 1 to 5% of the total untreated water quantity [2]. Sludge contains suspension of inorganic and organic substances typically, silica, hydrated aluminum oxide, and iron oxide [3]. The moisture content of the wet sludge is generally above 80 wt%. The organic matter content of the dried sludge is about 25% and particle size distribution is under 100 μm [4], [5]. Sludge management process in European countries comprises the following stages: sludge gathering and storage, pumping to thickening area, thickening, storage of thickened sludge, pumping to dehydration area, dehydration, atomization, and final storage [6]. On contrast, sludge discharges directly into nearby stream. The traditional practice of discharging the sludge directly into a nearby stream is becoming less acceptable because this discharges can violate the allowable stream standards [7]. The discharging of sludge into water body leads to accumulative rise of aluminum concentrations in water, aquatic organisms, and, consequently, in human bodies. Some researchers have linked aluminum’s contributory influence to occurrence of Alzheimer’s disease, children mental retardation, and the common effects of heavy metals accumulation [8].
The most feasible alternatives to solve the problem of sludge discharge including several reuse options were identified globally such as; for coagulant recovery and reuse [9], as coagulant in wastewater treatment [10], as adsorbent for phosphorus [11], manganese [12] and fluoride [13] from aqueous solutions, as co-conditioning and dewatering with sewage sludge [14], as constructed wetlands substrate [15], as soil buffers [16], nutrients reduction in laden soils and runoffs [17], in brick making [18], [19], [20], in ceramic making [21], [22], in pavement and geotechnical works [23], for structural soil improvement [24] and in manufacture of cement [25], [26], [27], [28], [29], cementitious materials [30], [31], [32], [33], [34], [35], [36], and light weight aggregate [37], [38]. Replacement part of clay by sludge improves the compressive strength of produced cement paste. Moreover, the microstructure of the sludge based clinker was similar in terms of alite and belite crystal size, and composition to OPC clinker [29]. Sludge used as supplementary cementitious material and sand substitute in preparation of cement mortar and concrete [32], [33]. Sludge inhibits the setting and hardening of cement paste because aluminium sulphate delays setting of Portland cement [34] as well as the presence of sodium ions [35]. Addition of nano-SiO2 improves the engineering properties of cement paste containing sludge calcined at 800 °C [36]. Bearing in mind that the calcination conditions i.e., temperature and duration affect the microstructure and the pozzolanic properties of industrial solid wastes [39] and as a result control the mechanical properties of pozzolanic cements [40], [41]. Hence, the aim of this study is to study the microstructure and pozzolanic properties of water treatment sludge calcined at 600–900 °C for 3 h. The investigation was accomplished by XRF, XRD, FTIR, SEM, and TGA instrumental techniques as well as Chapelle test for pozzolanic activity.
Section snippets
Materials and experimental techniques
Sample of wet sludge cake was collected from nearby drinking water treatment plant during the cleaning process of one of the decanters. The conventional water treatment plant uses aluminum sulfate coagulant in water treatment. Raw sludge dried and milled in steel ball mill to fine powder. Raw sludge was calcined in electrical muffle furnace with a heating rate 10 °C min−1 at 400–900 °C for 2 h. Calcined sludge was recharged from the muffle furnace and cooled to room temperature in desiccator.
Characterization of alum sludge ash
Table 1 illustrates the chemical composition of alum sludge calcined at 900 °C as determined by XRF. Calcined sludge ash composes of SiO2, Al2O3, and small amounts of Fe2O3, CaO, MgO, Na2O, and K2O. The sum of SiO2, A12O3, and Fe2O3 satisfies the requirements stated for pozzolana [43]. Fig. 1 illustrates the XRD patterns of raw sludge and that calcined at 600–900 °C. The major crystalline inorganic matter that has been present in the raw alum sludge are quartz (SiO2), albite (NaAlSi3O8), and
Conclusions
1 – Alum sludge ash composes of SiO2, Al2O3, and small amounts of Fe2O3, CaO, MgO, Na2O, and K2O. The sum of SiO2, A12O3, and Fe2O3 satisfies requirements stated for pozzolana.
2 – XRD results showed that the major crystalline inorganic matter present in the raw alum sludge are quartz, albite, and calcite. Boehmite formed in alum sludge calcined at 600 °C and transforms to amorphous alumina polymorphs, i.e., γ-, θ- and δ-alumina in alum sludge calcined at 700 °C. The amount of quartz markedly
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