Production of supplementary cementitious material as a sustainable management strategy for water treatment sludge waste

Case Studies in Construction Materials 12 (2020) e00329

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Case study

Production of supplementary cementitious material as a sustainable management strategy for water treatment sludge waste Luis Gabriel Graupner de Godoya , Abrahão Bernardo Rohdenb , Mônica Regina Garceza,* , Silvana Da Daltc , Lucas Bonan Gomesd a Federal University of Rio Grande Do Sul, Civil Engineering Post-Graduation Program: Construction and Infrastructure, Av. Osvaldo Aranha 99, Porto Alegre, RS, 90035-190, Brazil b Regional University of Blumenau, Post-Graduation Program in Environmental Engineering, Blumenau, Brazil c Federal University of Rio Grande Do Sul, Interdisciplinary Department, Tramandaí, Brazil d Federal University of Rio Grande Do Sul, Laboratory of X-Ray Diffraction, Porto Alegre, Brazil

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 November 2019 Received in revised form 16 December 2019 Accepted 30 December 2019

This paper investigates the application of Water Treatment Sludge (WTS), the main waste generated by water potabilization activities, for the development of a Supplementary Cementitious Material (SCM). The waste has been processed by calcining at a temperature range of 600  C–800  C for one hour. Chemical, mineralogical, physical, and morphological characterization has been performed to identify the potential pozzolanic activity of the calcined WTS and validate the application as SCM. Compressive strength tests have been performed in cement mortars with 14%, 35%, and 50% replacement of Portland cement by WTS. The WTS is a non-hazardous and non-inert waste, composed of SiO2, Al2O3, Fe2O3 and contains essentially quartz and kaolinite. Results confirmed the transformation of kaolinite into reactive amorphous phase by calcining. WTS calcined at 600  C shows great potential to the production of SCM, confirmed by the chemical and physical analysis and the evidence of pozzolanic activity. The mechanical properties of mortars produced with 14% and 35% WTS calcined at 600  C suggests a promising application in the production of blended and pozzolanic Portland cement. © 2020 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Water treatment sludge Supplementary cementitious material Cement mortar

1. Introduction Water Treatment Sludge (WTS) is a heterogeneous solid waste generated in the drinking water treatment process [1–3]. Commonly irregularly dumped into drain systems, streams, and rivers, WTS can prejudice the quality of drinking water, resulting in human, aquatic, and terrestrial toxicity [4–10] due to the presence of biodegradable organic compounds, pathogens, heavy metals, and other inorganic constituents [11]. The environmental issue related to the enormous amount of WTS disposal worldwide leads to a seek of individuation smart and sustainable solutions for reusing or recycling of this waste [12]. Water treatment plants, 69.2% of them with conventional treatment process, are present in 85% of the Brazilian municipalities (7500 units in 8516 million km2) and supplies drinking water to 95% of Brazilian population [13,14]. However,

* Corresponding author. E-mail address: [email protected] (M.R. Garcez). https://doi.org/10.1016/j.cscm.2020.e00329 2214-5095/© 2020 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

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Brazil faces a challenge in the sanitation sector regarding the WTS waste management since most of water treatment plants do not quantify the amount of generated waste and only a few dispose of properly [15], as preconized by international standards like ISO 24,510 series [16–18] and the Brazilian Federal Law 12.305 [19]. Investigations on the application of WTS in the development of construction and building materials are currently been carried out [2,4–6,12,20–42]. However, the application of WTS as Supplementary Cementitious Material (SCM) has great potential that is not enough explored [6,43–48], mainly due to the continuous increasing cost of raw materials and the scarcity of natural resources [48,49]. Since mineral content of WTS varies with location [48], the chemical, physical, and mineralogical characterization must guarantee that the calcined material becomes able to release reactive silica and alumina, meeting the requirements for application as SCM. In this context, this paper focuses on the chemical, mineralogical, morphological, and physical characterization of WTS waste samples, collected in the drying beds of a water treatment plant and calcined in laboratory scale, for the development of a SCM with characteristics comparable to that of SCM used in the Brazilian cement industry. This research is justified mainly by the need to cover the increasing demand for cement in Brazil, which, traditionally incorporates fly ashes, calcined clays, and blast furnace slags, as SCM. Apart from that, the relative scarcity and localization of the commonly used SCM, whose transport results in high economic and environmental impact [48,50], encourages the search for calcined pozzolanic materials obtained from different sources to supply the demand of the cement industry. 2. Experimental program 2.1. WTS production, collection, classification, and calcination The waste was collected from the drying beds of a water treatment plant that produces 1700 m3 per day of drinking water to supply 74% of a municipality located in Southern Brazil, with 518.9 km2 and 310,000 inhabitants. The water treatment plant produces 1.67 kg of sludge per m3 of treated water, which results in 85,414 kg per month, disposed of directly in a local river. The total amount of WTS disposed of in the river reaches around 5557 ton per month, which corresponds to the water supply of 14 municipalities with daily treated water consumption of 0.15 m3 per inhab. The water treatment comprehends conventional coagulation, flocculation, and rapid sand filtration process. Based on leaching and solubilized extracts [51–53] the WTS is classified as Class II-A: non-hazardous and non-inert waste, with low metal concentration and high amount of surfactants (1.67 mg l 1), which is a result of irregular discharges of detergents, household cleaners, and personal cleansing products. The riverbank soil composition and chemical products eventually discharged into the river result in a high concentration of Manganese (3.49 mg l 1). The use of poly-aluminum chloride (PAC) used as the coagulant in the water treatment process results in a high concentration of Aluminum (0.41 mg l 1). Additionally, WTS sludge presents 0.39% of organic matter content and total solid content of 19.36%. The WTS was prepared for calcining by drying at 110  C for 24 h, shredding, and homogenizing. The WTS was calcined in laboratory scale using a Jung LF4210 electrical muffle furnace (inner dimensions 300  350  400 mm, rate of work 4.4 kW, maximum working temperature 1.000  C) at 600, 650, 700, 750, and 800  C, with residence time of 1 h, heating rate of 10  C per minute, and 0.3 kg per feed. Calcined WTS samples were milled for 4 h to get more than 90% of particles smaller than 45 mm, using a ceramic ball mill with a ball to waste ratio of 5:1. 2.2. Chemical, physical, mineralogical, and morphological characterization of WTS waste Table 1 summarizes the tests performed to characterize non-calcined and calcined WTS samples. Statistical analysis has been performed using two-way ANOVA and Student t-test with α 0.05.

Table 1 Summary of characterization tests. Investigation

Equipment

Details

Quantitative chemical analysis

Energy Dispersive X-ray Fluorescence Spectrometer Shimadzu EDX 7000 Siemens D5000 X-ray diffractometer

–

Mineralogical characterization

Thermogravimetric and differential thermal analysis Morphological analysis Transmittance spectra in the infrared region

Shimadzu D-60 SEM JEOL JSM-6060 Fourier Transform Infrared (FTIR) spectrometer Shimadzu IR-Prestige-21

Particle size distribution BET (Brunauer, Emmett, and Teller) specific surface area

Mastersizer 2000 laser granulometer Quantachrome Nova 1000e analyzer

Range of 2.5 - 80 2u, steps of 0.05 , crystalline phases quantified through external standard method. Under Argon – Spectral range of 400 to 4000 cm 1, samples prepared through KBr pellet method – By nitrogen adsorption

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2.3. Calcined WTS compliance with chemical and physical requirements for SCM The potential application of the calcined WTS as raw material in the production of SCM has been investigated through the compliance with the chemical and physical requirements of standards applicable to highly active [54–56] and normal pozzolanic material [57], presented in Table 2. Performance Index at 7 and 28 days mentioned in Table 2 have been determined based on compressive strength results of mortars produced with mix proportions presented in Table 3. Statistical analysis has been performed using two-way ANOVA and Student t-test with α 0.05. It is important to note that PI28 is not an evolution of PI7 in time [48] since the mortars have been produced in two different groups: Mortar 7A as reference and mortars 7B to each calcining temperature and residence time; Mortar 28A as reference and mortars 28B to each calcining temperature and residence time. 2.4. Incorporation of calcined WTS in cement mortars Results of Performance Index with cement at 28 days were used to select the most cost-effective calcined WTS to be applied as SCM in the production of cement mortars. Compressive strength tests were performed in cement mortars with 14%, 35%, and 50% replacement of Portland cement by WTS, according to the mix proportions presented in Table 4, used in the Brazilian cement industry to verify minimal strength requirements of blended and pozzolanic Portland cement. In the Brazilian cement industry, Blended Portland cement CPII-Z [60], equivalent to CEM II/A-M [61], is obtained by replacing a part of clinker by 6–14 % of pozzolanic material addition, while pozzolanic Portland cement CP IV [60], equivalent to CEM IV of EN 197-1 [61], is obtained replacing a part of clinker by 15–50 % of pozzolanic material addition. 3. Results and discussion 3.1. Chemical, mineralogical, and physical characterization of calcined WTS Fig. 1 presents the TGA/DTA curves obtained through the thermal analysis performed in a sample of non-calcined WTS and Table 5 summarizes physical and chemical changes that happen in the sample, related to thermal decomposition during the test. The physical and chemical changes that occur between 400  C and 600  C define the efficiency of the SCM obtained from WTS waste, identified by its amorphous state and consequent pozzolanic reactivity [47]. When calcining temperature leads to loss of hydroxyls and, consequently, a collapsed and disarranged structure, the clay achieves its most reactive state [66]. Calcining temperatures lower than the temperature of formation of the crystalline phase and higher than the temperature corresponding to the end of the last endothermic peak result in higher amount of amorphous phase and, consequently, higher pozzolanic reactivity [64]. Chemical composition of the WTS waste, presented in Table 6, confirms the presence of silica and alumina as the major components and highlights the high content of iron oxide, related to the concentration of iron in the soil from Southern Brazil that results the characteristic red color of the sludge. The quantitative mineralogical analysis of non-calcined WTS crystalline phases (Table 7) based on the diffraction patterns of Fig. 2, identifies the presence of quartz and mica as major constituents, associated with plagioclase, kaolinite, and hematite. The absence of kaolinite in calcined WTS samples means that the kaolin–metakaolin transformation (dehydroxylation) is complete. On the other hand, the absence of the crystalline and not reactive phase mullite in samples calcined at 800  C is indicative of the calcination method efficiency [10,36,63].

Table 2 Chemical and physical requirements for SCM. Chemical Requirements

Highly active pozzolanic material*

Normal pozzolanic material **

CaO + MgO SiO2 Al2O3 SiO2 + Al2O3 + Fe2O3 SO3 Na2O Na2O equivalent Moisture content Loss on ignition Physical requirements Percentage retained on 45 mm sieve B.E.T. specific surface area Performance Index with cement at 7 days Performance Index with cement at 28 days

 1.5% 44% and  65% 32% and  46% –  1%  0.5%  1.5%  2%  4%

– – – 70%  4% –  1.5%  3%  10%

 10%  15 m2 g  1.5% –

 20% – –  90%

1

*NBR 15,894 [54–56] applied to highly-active pozzolanic materials produced through calcination and milling of clay minerals, formed essentially by lamellar particles with predominantly non-crystalline structure; **NBR 12,653 [57] applied to normal pozzolanic materials defined as natural pozzolans, fly ashes from thermoelectric plants, or any other pozzolanic material; * ** Classified based on Mehta & Monteiro [58].

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Table 3 Mix proportion of mortars produced to assess performance indexes.

Cement CPII-F-32*** Calcinated Sludge Sand Water Superplasticizer

Mortar 7A

PI7* Mortar 7B

Mortar 28A

PI28** Mortar 28B

624 – 1872 300 –

530 93 1872 300 adjustable

624 – 1872 300 –

530 93 1872 300 adjustable

* NBR 15,894-2 [55]; ** NBR 5752 [59]; * ** In g, enough to produce 6 w50 mm x 100 mm samples; *** NBR 16,697 [60], equivalent to CEM II/A-M [61].

Table 4 Mix proportion of mortars produced with different percentages of WTS.

Cement CPII-F-32b Calcinated Sludge Sand Water Superplasticizer a b

Mortar STDa

Mortar 14%WTSa

Mortar 35%WTSa

Mortar 50%WTSa

624 – 1872 300 –

537 87 1872 300 adjustable

406 218 1872 300 adjustable

406 312 1872 300 adjustable

In g, enough to produce 6 w50 mm x 100 mm samples [62]. NBR 16,697 [60], equivalent to CEM II/A-M [61].

Fig. 3 shows the transmittance spectra in the infrared region obtained through Fourier Transform Infrared (FTIR) spectrometry. Peaks are reported based on percentage transmittance to given wavelengths. At the low-frequency region, the 694 to 788 cm 1 bands observed in all samples suggest vibrations of Si-O-Si related to quartz in crystalline form, which corroborates with the DRX patterns presented in Fig. 3 [68,69]. The calcined samples present a shift in the characteristic band between 1000 and 1078 cm-1, which is probably related to a crystalline structure rearrangement caused by the α-β phase transformation that changes the Si-O bonds orientation in the trigonal space. Calcined WTS samples presents changes in Si–O characteristic bands of kaolinite, which is a characteristic of the amorphous silica [69] and can be exemplified by the transformation of kaolinite Si–O characteristic bands at the region 1000 m-1 to a single absorption slightly shifted to higher wavenumber [36,70]. The absorption band at 1650 cm-1 corresponds with bending vibration of water molecules chemically associated with Al(OH)3 [8], which intensity decrease as the calcination temperature increases, due to dihydroxylation. It is worth noting that raising calcination temperature shifts the Si-O-Si stretching bands to higher wavenumbers giving an indication of changes in bonding structure of silicate network that could, depending on the temperature, accompany crystallization of amorphous silica and adversely affect the pozzolanic activity of the calcined WTS [8].

Fig. 1. TGA/DTA curves of non-calcined WTS.

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Table 5 Physical and chemical changes related to weight loss with temperature increasing. Temperature 



23 C to 200 C 200  C to 400  C 400  C to 600  C 600  C to 800  C

480  C to 1000  C

a

Changes in weighta

Physical and Chemical changes

References

weight loss of 4.8% endothermic peak at 79  C weight loss of 5.39% exothermic peak at 338  C weight loss of 5.45% endothermic peak at 482  C weight loss of 1.06%

release of the physically adsorbed water no changes in the WTS structure decomposition of organic matter

2 10 31 36

dehydroxylation of Kaolinite metakaolin phase generation decarbonation CO2 dissipation production of hematite structural rearrangement to form mullite loss of pozzolanic activity

no weight loss exothermic peak at about 926  C

47 63 64 66

Total weight loss of the sample: 16.13%.

Table 6 Chemical composition of WTS sludge.

SiO2 Al2O3 Fe2O3 K2O MgO TiO2 CaO MnO Loss on ignition SO3

non-calcined

600  C

650  C

700  C

750  C

800  C

52.2 26.7 12.4 4.2 1.5 2.2 0.5 0.3 – –

54.1 28.2 10.2 4.2 1.4 1.2 0.5 0.2 4.50 0.23

57.4 24.9 10.2 4.3 1.4 1.2 0.5 0.2 4.70 0.26

54.9 26.1 11.6 4.0 1.3 1.3 0.5 0.3 4.60 0.18

57.4 25.2 9.9 4.0 1.4 1.2 0.6 0.3 3.60 0.13

58 24.5 9.9 4.3 1.4 1.2 0.5 0.3 1.90 0.06

Table 7 Mineralogical composition of WTS.

Quartz Mica Plagioclase Kaolinite Hematite

non-calcined

600  C

650  C

700  C

750  C

800  C

48.6 29.5 14.6 5.5 1.8

52.7 36.2 9.7 – 1.4

55.1 30.3 13.0 – 1.6

44.5 33.3 19.2 – 3.0

48.5 33.3 16.4 – 1.8

53.6 33.4 11.1 – 1.9

Table 8 shows the BET specific surface area, average particle size (D50), and percentage of WTS retained on 45 mm sieve of WTS samples after grinding. Grinding has been used to get more than 90% of particles smaller than 45 mm. Average particle size (D50) of non-calcined WTS is 350 mm before grinding. Because BET specific surface area is highly influenced by the internal porous structure and the amorphous content of the particles [15,45], WTS samples do not follow the inverse relationship between D50 and BET specific surface. Depending on the porosity and pore size distribution of the particles, the specific surface area is influenced by size, shape, and roughness [71].

3.2. Morphological characterization of calcined WTS Fig. 4 shows morphological characteristics of non-calcined and calcined WTS. Sample of non-calcined WTS presented in Fig. 4(a) displays small clusters of agglomerates with flaky morphology. In Fig. 4(b) aglomerates with rough texture [10] and non-homogeneous size are coupled into conglomerates. Fine particles are attached to the surface of larger particles and there are hollow areas among the coupled particles, related to the high porosity [2,9] on the sludge surface. In Fig. 4(c) conglomerates present a more uniform size if compared to samples of Figs. 4(a) e 4(b), probably due to the higher calcining temperature. Fragments of pseudo-hexagonal microstructures with scaly morphology, compatible with kaolin [2]. Fig. 4(d) suggests the presence of aggregates as a result of the higher sintering temperature that leads to a decrease in the relative amount of finer particles [9]. Formation of rounded and aggregated particles, which are typical characteristics of sintering processes, is detailed in Fig. 4(d).

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Fig. 2. Diffraction patterns of WTS waste.

Fig. 3. FTIR of WTS samples.

3.3. Compliance of calcined WTS with chemical and physical requirements for SCM The application of calcined WTS as a raw material in the production of supplementary cementitious material presupposes the compliance with the chemical and physical requirements presented in Table 2, applied to highly-active and normal pozzolanic materials. Results of Table 9 show that all samples comply with the requirements related to the use of calcined WTS as SCM with properties equivalent to those of a highly active pozzolanic material concerning SiO2, Na2O, and SO3 contents, percentage retained on 45 mm sieve, and B.E.T specific surface area. On the other hand, CaO + MgO content of samples calcined in electrical muffle furnace is between 1.8 and 2%, higher to the required maximum of 1.5%, Al2O3 content is between 24% and 28%, lower than the required minimum of 32%, and Na2O equivalent (Na2O + 0.658 K2O) is between 2.63% and 2.83%, higher to the required maximum of 1.5%. Maximum loss on ignition requirement of 4% was reached only for 750  C and 800  C. Only the mortar produced with WTS calcined at 800  C reached the minimum PI7 of 1.5%, however, results of PI7 for 750  C and 800  C do not show any significant difference at a confidence level of 95% (Fig. 5).

Table 8 BET, D50, and % retained on 45 mm of WTS samples after grinding.

D50 (mm) % Retained on 45 mm sieve BET (m2 g 1)

non-calcined

600  C

650  C

700  C

750  C

800  C

23.35 – –

21.59 7.77 28.81

22.06 7.05 33.20

31.26 9.66 28.61

26.48 7.20 29.97

17.69 7.38 24.84

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Fig. 4. SEM observation of non-calcined and calcined WTS: (a) non-calcined, 10,000X; (b) 650  C, 10,000X;(c) 750  C, 10,000X;(d) 800  C, 10,000  .

Table 9 Chemical and physical properties of calcined WTS. Chemical Requirements

Highly active pozzolanic material

Normal pozzolanic material

600  C

650  C

700  C

750  C

800  C

CaO + MgO SiO2 Al2O3 SiO2 + Al2O3 + Fe2O3 SO3 Na2O Na2O equivalent Moisture content Loss on ignition Physical requirements Percentage retained on 45 mm sieve B.E.T. specific surface area PI7 PI28

 1.5% 44% and  65% 32% and  46% –  1%  0.5%  1.5%  2%  4%

– – – 70%  4% –  1.5%  3%  10%

1.9 54.1 28.2 92.5 0.23 0.00 2.76 – 4.50

1.9 57.4 24.9 92.5 0.26 0.00 2.83 – 4.70

1.8 54.9 26.1 92.6 0.18 0.00 2.63 – 4.60

2.0 57.4 25.2 92.5 0.13 0.00 2.63 – 3.60

1.9 58.0 24.5 92.4 0.06 0.00 2.83 – 1.90

 10%  15 m2 g-1  1.5% –

 20% – –  90%

7.77 28.8 92 105

7.05 33.2 92 92

9.66 28.6 94 100

7.20 29.9 104 86

7.38 24.8 107 91

Calcined WTS

Regarding the use of calcined WTS as an SCM with characteristics of normal pozzolanic material, all calcined WTS samples attend the requirements related to the minimum content of SiO2, Al2O3, and Fe2O3, the maximum content of SO3, the maximum loss on ignition, the BET specific surface area, and the percentage retained in 45 mm. Regarding PI28, only the WTS sample calcined at 750  C do not reach the minimum of 90%, however, samples calcined at 650  C, 750  C, and 800  C present equivalent means at a confidence level of 95% (Fig. 5). The maximum Na2O equivalent of 1.5% is not attended by calcined WTS samples, but, in fact, it does not mean that the material is unsuitable for application in cementitious composites unless unstable phases are identified through mineralogical analysis [48]. The K2O content in the WTS samples is related to the presence of surfactants and fertilizers irregularly discharged in the river. Such results ratify the ones presented in Figs. 2 and 3 that confirm the transformation of kaolinite into reactive amorphous phase by heat treatment [69]. Considering environmental and economic aspects related to energy consumption, WTS calcined at 600  C shows great potential to the production of an SCM with characteristics equivalent to those of normal pozzolanic material.

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Fig. 5. Performance Index with cement at 7 and 28 days.

Fig. 6. Compressive strength of mortars produced with calcined WTS.

3.4. Incorporation of calcined WTS in cement mortars Fig. 6 shows the compressive strength of tested mortars produced with 14%, 35%, and 50% of WTS calcined at 600  C. Dashed lines show minimum compressive strength requirements for blended and pozzolanic Portland cements with strength classes 25 MPa and 32 MPa in Brazil. Results of Fig. 6 show that all replacement percentages lead to higher strength development up to 7 days, reaching 66%, 58%, and 70% of the compressive strength at 28 days for 11%, 35%, and 50% replacement, respectively. In general, the compressive strength depends on the percentage of cement replacement and decreases the higher the percentage of calcined WTS. The use of 14% of calcined WTS meets compressive strength requirements for the production of blended Portland cements CP II-Z (25 MPa) and CP II-Z (32 MPa), equivalent to CEM II/A-M of EN 197-1 (25 MPa and 32 MPa) in all ages [60,61]. On the other hand, 35% calcined WTS meets compressive strength requirements for pozzolanic Portland cement CP IV (25 MPa), equivalent to CEM IV of EN 197-1 (25 MPa) in all ages [60,61]. Such results confirm the potential for developing a new SCM through the valorization of sludge waste from water treatment plants. 4. Conclusions WTS waste produced by a water treatment plant located in the southern Brazil has been investigated in this paper, through an extensive chemical, mineralogical, physical, and morphological characterization, to verify its potential application as a raw material in the production of a SCM. The water treatment sludge is a non-hazardous and non-inert waste, mainly composed of SiO2, Al2O3, Fe2O3 and contains essentially quartz and kaolinite. The mineralogical characterization identified the absence of kaolinite in calcined WTS samples, an indicative of the complete dihydroxylation. Calcination temperatures of 750  C and 800  C lead to the production of an SCM with properties equivalent to those of highly active pozzolanic materials, though CaO + MgO, Al2O3, and Na2O equivalent content do not attend the selected standard requirements, which does not mean that the waste is unsuitable for the intended application. The use of 14% and 35% WTS calcined at 600  C meets compressive strength requirements for the production of blended Portland cement

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equivalent to CEM II/A-M (25 MPa 32 MPa) of EN 197-1 in all ages, while the use of 35% of WTS calcined at 600  C attends compressive strength requirements for the production of pozzolanic Portland cement equivalent to CEM IV (25 MPa) of EN 197-1 in all ages. Particular attention must be paid in future studies to the durability assessment of cementitious composites produced with calcined WTS. Finally, it is important to address the stability of the WTS physical-chemical composition to assure its safe and reproducible applicative function, as well as the characterization of sludge waste from different sources, and the viability of producing calcined WTS in a commercial scale. Declaration of Competing Interest None. Acknowledgments Authors would like to acknowledge funds received from the National Council for Scientific and Technological Development (PIBIC/CNPq Program) and the Regional University of Blumenau (PIBIC/FURB Program). References [1] M. 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