Sewage sludge ash

Sewage sludge ash

5

Jordi Paya´, Jose´ Monzo´, Marı´a Victoria Borrachero and Lourdes Soriano Chemistry in Building Materials Research Group (GIQUIMA), Concrete Science and Technology Institute (ICITECH), Universitat Polite`cnica de Vale`ncia, Valencia, Spain

5.1

Introduction

The increase in the human population and the development of new technologies, materials and processes have caused an increase in the demand for resources. The mismanagement and extensive use of these resources cause detrimental environmental problems. To minimise the impact on the environment, it is necessary to ensure the elimination of dangerous substances into nature. The demand for water and the subsequent generation of huge quantities of wastewater, both of urban origin and of industrial origin, requires the development of diverse technologies for the elimination of contaminants present in wastewater. In remote times, nature itself could manage the self-purification of wastewater since the amount generated was small. However, industrial development and the concentration of people in large cities has resulted in the self-purifying capacity of rivers, lakes and seas being exceeded. One of the most notable characteristics of wastewater, especially those of urban origin, is its high concentration of organic matter, both soluble and insoluble. The oxygen demand (chemical oxygen demand, COD, and biological oxygen demand, BOD) caused by this organic matter decreases dissolved oxygen levels in the receiving water (river, lake or sea), causing the death of many species and limiting the possibility of using that natural resource. To minimise the impact of wastewater, various wastewater treatment processes (WWTP) have been developed, such as pre-treatments, primary treatments, secondary treatments and tertiary treatments (Ramalho, 2013; Gupta et al., 2012). These treatments allow, on the one hand, the reduction of the impact on the environment and, on the other hand, increasing the possibility of reusing treated water (for example, in the area of agriculture). Pre-treatment consists of the elimination of large elements present in the wastewater through bars and screens. Likewise, fats and oils that remain on the surface of water can also be eliminated in order to improve the exchange of air with the atmosphere in later stages. In this case, the type of waste generated can be considered as a waste similar to urban solid waste. Primary treatment consists mainly of a physical/chemical sedimentation processes in such a way that the insoluble particles, both organic and inorganic, can be New Trends in Eco-efficient and Recycled Concrete. DOI: https://doi.org/10.1016/B978-0-08-102480-5.00005-1 © 2019 Elsevier Ltd. All rights reserved.

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New Trends in Eco-efficient and Recycled Concrete

removed from the bottom of the decanter tanks as sewage sludge (SS). In many cases, the addition of chemical reagents (e.g., polyelectrolytes, iron or aluminium salts) facilitates and accelerates the settling process. There are several types of secondary treatments (biological), although the most common involve the digestion of organic matter, either by aerobic or by anaerobic processes, both based on the action of bacteria. The process results in water with a low organic content (lowering of the COD and BOD values below established limits) and mud with a high-water content. Tertiary treatments try to eliminate some compounds, or groups of specific compounds, such as those related to nitrogen (nitrates and nitrites) and those related to phosphorus (phosphates). There are also processes for the removal of some soluble organic compounds, such as pharmaceuticals. Both types of SS generated in primary treatments and secondary treatments are liquid, or semi-solid liquid, and require water removal processes (dewatering by centrifugation, plate pressing or bed drying, etc.) so that the generated SS can be transported and handled in a convenient way. The solid content ranges in 0.25%
12% by weight (Metcalf, 1991). In some cases, SS is anaerobically digested and then the chemical composition changes in terms of total solid content and volatile compounds. The processes for humidity reduction produce a sludge with humidity ranging from 80% to 20%, depending on the technology used. All of these SS contain, in addition to water, a large percentage of diverse organic matter and pathogens. They also contain inorganic components. In these inorganic components, we can find various types of salts (such as sulphates and phosphates) and minerals, such as quartz, feldspars and also clays, among other substances. Typical composition ranges for untreated and digested sludge are reported in Table 5.1 (Fytili and Zabaniotou, 2008).

Table 5.1 Range of chemical compositions for untreated and digested sludges Parameter

Untreated sludge

Digested sludge

Total dry solids (TS, %) Volatile solids (%) Grease and fats (ether soluble, %) Proteins (%) Nitrogen (N, %) Phosphorous (P2O5, %) Potash (K2O, %) Cellulose (%) Iron (not as sulphide, %) Silica (SiO2, %)

2
8 60
80 6
30 20
30 1.5
4.0 0.8
2.8 0
1 8
15 2
4 15
20

6
12 30
60 5
20 15
20 1.6
6.0 1.5
4.0 0
3 8.15 3
8 10
20

Note: All parameters, except total dry solids (TS), are given as a weight percentage of TS. Data taken from Fytili, D., Zabaniotou, A., 2008. Utilization of sewage sludge in EU application of old and new methods—a review. Renewable Sustainable Energy Rev. 12, 116
140. Available from: https://doi.org/10.1016/j. rser.2006.05.014.

Sewage sludge ash

123

In many countries, there is a continuous increase in the requirements concerning wastewater discharge and more restrictive control regulations about SS disposal options have been observed. In 2005, the total production of SS in the European Union was 10Mt, and in the United States it was 7 Mt (Cyr et al., 2007). Several disposal options may be developed for enhancing the management of these huge amounts. Initially, SS was applied, directly or indirectly, to the soil for agriculture and forestry activities, or alternately disposed by landfilling. Many restrictions due to the presence of heavy metals and pathogenic agents diminished the application of SS as a soil amendment or fertilisation. Additionally, many regulations tried to reduce the amount of landfilled SS. For example, according to the European directive EU 99/13/EC, it was obligatory to reduce 75% of biodegradable waste to landfills by the end of 2010 with respect to the amount produced in 1995 (Stasta et al., 2006). Certainly, the feasible SS disposal alternatives, such as thermal treatments (incineration, pyrolysis, gasification) will gain importance in the future because they offer an important reduction in volume (up to 90%) and mass (up to 70%). Additionally, the final product is sterile and odourless, and in many cases, it can be considered as an inert residue (Tay et al., 1991; Lynn et al., 2015). Cyr et al. (2007) provided a summary of the SS disposal for European countries, Canada and the United States. Fig. 5.1 depicts that some countries, such as the Netherlands, Austria, Switzerland and Canada incinerated more than 40% of SS. Also, Denmark, USA, Germany, United Kingdom and Belgium incinerated between 20% and 40% of SS.

Figure 5.1 Disposal proportions of sewage sludge ash in European countries, Canada and the United States (Cyr et al., 2007).

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5.2

New Trends in Eco-efficient and Recycled Concrete

Production of sewage sludge ashes

The thermal processing of SS is the valorisation of its energy content. In such energy recovery, it may be taken into account that SS has a considerable water content and dewatering/drying processes may strongly influence the energy balance and total cost (Samolada and Zabaniotou, 2014). Thermochemical technologies, mainly incineration, pyrolysis and gasification, were found to be promising alternative ways for the valorisation of SS (Samolada and Zabaniotou, 2014). The pyrolytic process consists of the thermal decomposition of the organic fraction into inert atmosphere. Thus, gas and liquid fractions with potential energy content are recovered and a solid residue (char) is obtained as a by-product. Pyrolysis can be considered a zero-waste process, because char can be used as an adsorbent of H2S or NOx in gaseous streams. Gasification, in contrast to pyrolysis, is a thermal treatment in the presence of a limited reactive atmosphere. Thus, a combustible gas (syngas, which is a mixture of CO, H2 and other gases) is obtained. This process has several advantages with respect to simple combustion: no supplementary fuel is required, low emissions of SOx and NOx are produced and there is a limited production of chlorinated dibenzodioxins and dibenzofurans. Finally, incineration/combustion of SS is the most-used thermal technology. Wet or dried SS is combusted in a fluidised bed reactor. In some cases, fuel is required, especially when high-water content SS is used. Dry SS has an important calorific value, ranging in 12
20 MJ/kg, which is a similar value to lignite. Usually fly ash (FA) particles are produced during thermal treatment in the fluidised bed reactor which are trapped by means of mechanical and electrostatic filters. This ash, the SS ash (SSA), is a powdered solid material, in some cases characterised as inert dust and in others as a hazardous solid waste. In some countries (China, Cyprus, Spain) (Zhang et al., 2013) the co-incineration of SS with coal as supplementary fuel in cement kilns, power plants and brick kilns has been proposed. The European production of SS (OECD Organization for economic co-operation and development, 2017) in 2012 was estimated to be 10 Mt. Taking into account (Lynn et al., 2015) that incineration processes reduce the volume of the waste by 90% and its mass by 70%, the potential production of SSA would be about 3 Mt. In 2012, 2.3 Mt of SS was incinerated, mainly from Germany, France and the Netherlands, generating 0.7 Mt of SSA. This amount will probably increase in the future (Stasta et al., 2006) because of a 50%
55% decrease in the agriculture disposal option and the increase in price for landfilling.

5.3

Characteristics of sewage sludge ash

SSA is a material which has been widely researched by the scientific community. Of all the research, five uses stand out and these depend fundamentally on the physico-chemical characteristics of the ashes used:

Sewage sludge ash

G

G

G

G

G

125

As a pozzolanic addition in mortars and concrete with Portland cement (PC). For the manufacture of light aggregates. In the manufacture of bricks and ceramic materials. For the manufacture of PC clinker. As a source of phosphorus extraction for organic amendment.

This chapter will deal briefly with some of the characteristics of these ashes, focusing mainly on its uses, such as pozzolanic addition. Tay (1987) used a SSA as a filler in concrete. The sludge was collected in the Jurong sewage treatment (Singapore) and was incinerated in the furnace with temperatures above 550 C. The chemical composition is summarised in Table 5.2. The specific gravity of the SSA was 2.81 g/cm3, the loose bulk density 1345 kg/m3 and the pH 9.0. In 1996
2003, Monzo´ et al. (1996, 1999a, 2003) studied the physico-chemical characteristics of a SSA from the Pinedo sewage wastewater treatment plant (Spain). In this plant, part of the SS was incinerated at a maximum temperature of 800 C, in a silica sand fluidised bed reactor. The ash was collected by means of electrostatic precipitators. The researchers separated the original SSA in several sized fractions by sieving ( . 80 μm SSAC, between 80 and 40 μm SSAM, and between 40 and 20 μm, SSAF). In Fig. 5.2, the particle size grading parameters are shown and in Table 5.3, the chemical compositions of the original ash and the sieved fractions are summarised. The increase of SO3 content and irregular content of SiO2 is shown in the fractions. In this case, the critical parameter was the SO3 content. The authors, in a later publication (Monzo´ et al., 1999a), studied the influence of this parameter, using different types of cement with different C3A contents. Paya´ et al. (2002) also used this ash to compare the pozzolanic activity of the original SSA (with a particle mean diameter of 30.70 μm) with other pozzolans as a silica fume (SF), spent fluidised bed catalyst and rice husk ash (RHA). Finally, Monzo´ et al. (2003), studied the influence of the different size fractions of the ash in the mortars with different substitutions of cement by these fractions. Later, this original Spanish ash was used in corrosion studies (Garcı´a-Alcocel et al., 2006) or for analysis of mixtures with different types of cements (Cyr et al., 2007). It was also characterised by X-ray diffraction where the crystalline phases found were γ-anhydrite, quartz, magnetite, calcite and hydroxyapatite. Also, the minor phases detected were corundum, gypsum and lime. SSA particles had a mean diameter value of 33 μm and the specific gravity was 2.62 g/cm3. The high-water Table 5.2 Chemical composition of sewage sludge ash (g/kg) Si

S

Fe

Al

Ca

Zn

Cu

102.8 Cr 3.0

80.8 Na 2.5

80.6 Pb 2.5

50.3 Ni 1.3

29.2 Mn 1.1

26.8 K 6.1

18.8 Mg 7.9

Adapted from Tay, J.-H., 1987. Sludge ash as filler for Portland cement concrete. J. Environ. Eng. 113 (2), 345
351. Available from: https://doi.org/10.1061/(ASCE)0733-9372(1987)113:2(345).

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Figure 5.2 Size distribution of the original SSA and their sieved fractions (Monzo´ et al., 1996). SSA, Sewage sludge ash.

Table 5.3 Chemical composition of the original SSA and their size fractions %

SiO2 Al2O3 Fe2O3 CaO MgO SO3 P2O5 Moisture LOI I.R.

Sewage sludge ashes SSA

SSAC

SSAM

SSAF

20.8 14.9 7.4 31.3 2.6 12.4 6.7 0.5 5.1 16.1

30.1 11.9 7.1 25.9 2.2 10.7 6.3 0.5 3.6 28.0

16.2 14.7 8.2 32.9 2.4 13.1 7.6 0.4 5.2 11.7

15.5 14.0 8.3 32.1 2.6 13.7 6.9 0.4 5.2 11.3

SSA, Sewage sludge ash Adapted from Monzo´, J., Paya´, J., Borrachero, M.V., Co´rcoles A., 1996. Use of sewage sludge ash (SSA) cement admixtures in mortars. Cem. Concr. Res. 26 (9), 1389
1398. Available from: https://doi.org/10.1016/0008-8846(96) 00119-6.

conductivity of the aqueous ash suspension (9.57 mS/cm) revealed that the ash had a high soluble salt content (Cl2 5 7.8 mg/g, SO422 5 37.2 mg/g) (Garcı´a-Alcocel et al., 2006). In Fig. 5.3, SEM micrographs of SSA are shown, revealing irregularly shaped particles with a rough surface. More recently, Pe´rez-Carrio´n et al. (2014) used the SSA from Pinedo (Spain) as a substitute for cement in the manufacture of precast concrete blocks. In this case,

Sewage sludge ash

127

Figure 5.3 SEM micrographs of SSA particles (Monzo´ et al., 2003). SSA, Sewage sludge ash. Table 5.4 Chemical composition of sewage sludge ash from Pinedo (Spain) Oxide (%)

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O TiO2 P2O5 Cl2 SO3 Others LOI

Reference Pe´rez-Carrio´n et al. (2014)

Baeza et al. (2014a)

19.2 8.9 10.0 30.6 2.7 0.8 1.4 1.0 12.3 -11.1
5.1

16.98 9.48 8.38 29.73 3.17 0.92 1.26 0.90 14.01 0.15 8.80 0.56 4.55

the characterisation of the ash was somewhat different than in previous studies (Table 5.4). In the X-ray pattern, various crystalline compounds [quartz SiO2, anhydrite CaSO4, calcite CaCO3, forsterite Mg2SiO4, Magnetite, Fe3O4, lime CaO and whitlockite, β-Ca3(PO4)2] were identified, more so than an amorphous material. Ash particles had a specific Blaine surface of 3000 cm2/g. Moreover, Baeza et al. (2014a) studied binary and ternary combinations of SSA with marble dust (MD), FA and RHA as a replacement for PC pastes. The chemical composition of the SSA was collected in Table 5.4. For this work, the ash used had a very small mean particle diameter (16.67 μm). Pan et al. (2003) investigated the effect of SSA fineness on mortar properties. This ash was taken from a wastewater treatment plant in Taipei City (Taiwan) and it was obtained from a typical sewage treatment plant using a primary process. The

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New Trends in Eco-efficient and Recycled Concrete

sludge was incinerated at 700 C for 3 h by a modular incinerator. The chemical composition is shown in Table 5.5. Note the high percentage of silica and the near absence of CaO from this sample in relation to others, probably due to only a primary treatment being used. In the study on the influence of fineness (Pan et al., 2003), the SSA was ground in a ball mill under several conditions of grinding time, amount of ash and jar rotation speed. It was confirmed that with a grinding time in the range of 10
360 min, the variation of specific gravity and Brunauer
Emmett
Teller (BET) specific surface area were negligible (specific gravity 2.68
2.70 g/cm3 and BET area 11.59
12.48 m2/g). However, between 10 and 60 min of grinding, the SSA increased its Blaine fineness (from 496 to 975 m2/kg). An increase of grinding time did not improve fineness. The mineralogical composition was not modified by grinding as the main crystalline phases that were detected were two polymorphs of silicon dioxide: quartz and manganite. In addition, heavy metal analysis was also carried out. The results are summarised in Table 5.6. Lin et al. (2008a,b) used mixtures of nanosilica and SSA for the improvement of the pozzolanicity of FA in PC mortars. The sludge was obtained from a municipal wastewater treatment plant in Taiwan. These samples were initially oven dried at 105 C, incinerated in an electronic furnace at 800 C, ground into fine particles and then passed through a #200 sieve. The chemical compositions are shown in Tables 5.5 and 5.6. Dyer et al. (2011) studied the hydration chemistry of SSAs used as a cement component using four SSAs from the United Kingdom. The chemical composition of these samples is listed in Table 5.5. Also, the X-ray difractometry (XRD) analysis indicated that phosphorous was present as the crystalline phase whitlockite. Other crystalline phases detected were silica polymorphs (quartz, trydimite and cristobalite) and albite (Table 5.7). The authors estimated the composition of the amorphous fraction by subtracting the composition of each crystalline phase from the chemical composition of the material as a whole (Table 5.7). There were important differences among the studied samples. Cyr et al. (2007) characterised a French SSA and assessed its use in cement-based materials. This SSA came from a fluidised bed combustor operating at 850 C. The authors published a complete chemical and mineralogical analysis, and they compared the results with other previously reported SSA (see Other SSA in Tables 5.8 and 5.9). As seen in Tables 5.8 and 5.9, the authors concluded that the SSA was a material made of several crystalline minerals (  60%) and a vitreous phase (  40%). Its main oxides were CaO, SiO2, P2O5 and Al2O3. SSA usually contains a significant amount of sulphates, crystallised in the form of anhydrite. A trace element analysis showed the presence of heavy metals, such as Zn, Cr and Cu. Also, they commented about the high variability in the chemical composition between the different types of SSA. With respect to the mineralogical composition of the SSA (Table 3.8), all samples contained quartz. Some SSA contained calcium phosphates, haematites, anhydrite and some feldspars, such as orthoclase. The physical characteristics of SSA showed a particle size distribution between 1 and 100 μm, with a mean diameter around 26 μm, a Blaine fineness of 640 m2/g and a BET of 19000 m2/g. The authors stated that compared to other SSA samples, it had particles

Table 5.5 Chemical composition of sewage sludge ash Reference

SiO2

Al2O3

Fe2O3

CaO

MgO

Na2O

K2O

P2O5

SO3

Cl2

Pan et al. (2003) Lin et al. (2008a) Dyer et al. (2011) SSA-1 Dyer et al. (2011) SSA-2 Dyer et al. (2011) SSA-3 Dyer et al. (2011) SSA-4

50.6 64.6 29.4 35.2 26.4 33.0

12.8 23.1 10.9 12.4 14.1 24.0

7.21 1.31 13.0 7.3 11.3 6.7

1.93 8.6 14.3 12.4 9.6 5.7

1.48 1.1 1.4 2.2 1.4 1.0

0.32
0.2 0.2 0.4 0.0

1.70
1.2 1.8 1.6 1.3

1.67
13.5 12.7 15.7 14.0

2.38
1.3 0.5 0.6 0.2



0.14 0.04 0.01 0.0

SSA, Sewage sludge ash.

Table 5.6 Heavy metal contents in sewage sludge ash (g/kg) Reference

Ag

As

Cr

Cu

Mn

Ni

Pb

Zn

Cd

Pan et al. (2003) Lin et al. (2008a) Lin et al. (2008b)

0.049



0.023
0.084

0.564 0.01 0.034

1.09 11.9 8.725

0.44



0.72 0.06 0.033

0.18 0.02 0.027

2.62 2.07 1.7


0.03 0.02

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New Trends in Eco-efficient and Recycled Concrete

Table 5.7 Mineralogical composition of the SSA samples and estimated composition of the amorphous phase Mineral phase

SSA1

SSA2

SSA3

SSA4

Anhydrite, CaSO4 Quartz, SiO2 Haematite Fe2O3 Ettringite, Ca6Al2(SO4)3(OH)12.26H2O Silicon carbide, SiC Albite, NaAlSi3O8 Iron whitlockite, Ca9Fe(PO4)7 Cristobalite, SiO2 Tridymite, SiO2

x xx x X x x xx



x xxx x
x x xx
x

x xxx x
x x xx x x


xxx x
x
xx x x

20 25 15 22 10

24 42 8 17 3

Chemical compound in amorphous phase SiO2 Al2O3 Fe2O3 P2O5 CaO

33 15 15 16 15

37 19 5 15 14

x
traces; xx
minority compounds; xxx
majority compounds. SSA, Sewage sludge ash. Adapted from Dyer, T.D., Halliday, J.E., Dhir, R.K., 2011. Hydration chemistry of sewage sludge ash used as a cement component. J. Mater. Civil Eng. 23 (5), 648
655. Available from: https://doi.org/10.1061/(ASCE)MT.19435533.0000221.

finer than the mean values for the diameter and specific surface area found in the literature [44 μm and a 450 (Blaine) or 15,000 (BET) m2/g]. Donatello et al. (2010a,b) studied the effects of milling and acid washing on the pozzolanic activity of SSA from the United Kingdom. The acid washing was made to extract the phosphates from the ash. The chemical composition and physical parameters are listed in Table 5.10. In Fig. 5.4, the X-ray diffractogram of SSA before and after acid washing is shown. It is observed that acid washing caused the complete dissolution of Ca and P from whitlockite and precipitation of CaSO4. Naamane et al. (2016) carried out studies about the influence of the incineration temperature (300
800 C) of sludge on the characteristics of the ash, and the influence of these ashes obtained at different temperatures on cement-based materials. The SSAs were characterised from chemical, physical and mineralogical points of view. SS used in this investigation came from an industrial wastewater treatment plant located in Fez (Morocco). The sludge samples were dried at 40 C, homogenised, ground and sieved to a particle size smaller than 1 mm. Then, the dried SS was calcined at different temperatures (300, 400, 500, 600, 700 and 800 C) during 50 min in an electrical muffle. Then, the sludge ash was cooled down to an ambient temperature. The chemical composition of the ashes obtained at different temperatures is shown in Table 5.11.

Table 5.8 Chemical composition and trace analysis of SSA compared to previously studied SSA (other SSA ) Oxide (%)

SiO2

Al2O3

Fe2O3

CaO

P2O5

SO3

Na2O

K2O

TiO2

MgO

MnO

LOI

SSA (Cyr et al., 2007)

34.2

12.6

4.7

20.6

14.8

2.8

1.0

1,7

0.9

1.9

0.06

5.5

Other SSA

36.1 14.4 65

14.2 4.4 34.2

9.2 2.1 30.0

14.8 1.1 40.1

11.6 0.3 26.7

2.8 0.01 12.4

0.9 0.01 6.8

1.3 0.1 3.1

1.1 0.3 1.9

2.4 0.02 23.4

0.3 0.03 0.9

6.1 0.2 41.8

As

Ba

Cd

Co

Cr

Cu

Ni

Pb

Sn

Sr

V

Zn

23

1430

14

669

2636

2483

621

720

283

623

63

7103

87 0.4 726

4142 90 14600

20 4 94

39 19 78

452 16 2100

1962 200 5420

671 79 2000

600 93 2055

400 183 617

539 539 539

35 14 66

3512 1084 10,000

Mean Min Max

Element (mg/kg) SSA Other SSA

Mean Min Max

SSA, Sewage sludge ash. Adapted from Cyr, M., Coutand, M., Clastres, P., 2007. Technological and environmental behavior of sewage sludge ash (SSA) in cement-based Materials. Cem. Concr. Res. 37, 1278
1289, Available from: https://doi.org/10.1016/j.cemconres.2007.04.003.

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Table 5.9 Mineralogical composition of SSA compared to those found in the literature (Other SSA ) SSA (Cyr et al., 2007) Other SSA

(Q), (G), (W), micas, and amorphous material Monzo´ et al. (1999b) Roland (2002) Dyer et al. (2001) Fontes et al. (2004) Cheeseman and Virdi (2005) Anderson et al. (2002)

(Q), (C) (Ma), (A), calcium phosphates (HAP, CPH) (Q), (C) (H), (W), (Q), (H) (P), (Mu), and amorphous material (Q), (O), (Mu), and amorphous material (Q), (H) (W) (Q), (C), (H), (A) and amorphous material

(A) Anhydrite -γ-CaSO4; (C) Calcite CaCO3; (CPH) calcium phosphate hydrateCa3(PO4)2. xH2O; (G) Gypsum- CaSO4  2H2O; (H) Haematite-Fe2O3; (HAP) Hydroxylapatite-Ca5(PO4)3(OH); (Mu) Mica Moscovite-KAl2(Si3Al)O10(OH)2¸(O) Ortoclase-KAlSi3O8; (Q) Quartz-SiO2; (W) Whitlockite-β-Ca3(PO4)2. SSA, Sewage sludge ash Adapted from Cyr, M., Coutand, M., Clastres, P., 2007. Technological and environmental behavior of sewage sludge ash (SSA) in cement-based Materials. Cem. Concr. Res. 37, 1278
1289, Available from: https://doi.org/10.1016/j. cemconres.2007.04.003.

Table 5.10 Chemical composition and physical properties of SSA Chemical composition (%) SiO2 Al2O3 Fe2O3 CaO MgO SO3 P2O5 K2O Na2O TiO2 LOI

35.8 11.2 16.9 12.9 1.9 3.1 11.9 1.5 0.2 1.0 0.8

Physical properties pH Specific gravity (g/cm3) Loose bulk density (kg/cm3) BET surface area (m2/g) Mean particle size (μm) d10 d50 d90

7.2 2.43 700 6.4 140.8 13.5 106.8 314.4

SSA, Sewage sludge ash. Adapted from Donatello, S., Freeman-Pask, A., Tyrer, M., Cheeseman, C.R., 2010a. Effect of milling and acid washing on the pozzolanic activity of incinerator sewage sludge ash. Cem. Concr. Compos. 32, 54
61. Available from doi:10.1016/j.cemconcomp.2009.09.002.; Donatello, S., Tyrer, M., Cheeseman, C.R., 2010b. Comparison of test methods to assess pozzolanic activity. Cem. Concr. Compos. 32, 121
127. Available from: https://doi.org/ 10.1016/j.cemconcomp.2009.10.008.

Sewage sludge ash

133

Figure 5.4 XRD patterns for SSA before (bottom) and after (top) acid wash with sulphuric acid (Donatello et al., 2010a). SSA, Sewage sludge ash. Table 5.11 Chemical composition (traces, ICP-AES) of sewage sludge ash calcined in the range of 300
800 C Temperature of calcination ( C)

Element (mg/L)

Ca Fe Mg Mn Na P K Cd Co Cr Ba Cu Ni Pb V Zn

300

400

500

600

700

800

342.64 33.82 37.74 0.18 40.64 43.91 16.82 0.07 0.026 0.38 0.31 1.27 0.27 1.07 0.05 3.55

394.20 33.87 46.34 0.20 41.76 43.95 17.03 0.07 0.018 0.39 0.33 1.28 0.30 1.11 0.07 3.76

470.46 33.98 55.54 0.20 61.44 43.26 17.25 0.08 0.013 0.42 0.36 1.33 0.29 1.13 0.09 3.79

565.18 34.23 57.38 0.22 61.36 44.28 15.04 0.07 0.015 0.41 0.38 1.09 0.42 1.09 0.08 3.83

687.24 34.16 69.42 0.33 79.23 55.82 18.96 0.04 0.021 0.79 0.53 1.04 0.67 1.04 0.10 3.82

712.68 34.08 78.82 0.29 37.78 69.78 14.55 0.02 0.019 0.74 0.62 0.79 0.83 0.79 0.04 4.02

Adapted from Naamane, S., Rais, Z., Taleb, M., 2016. The effectiveness of the incineration of sewage sludge on the evolution of physicochemical and mechanical properties of Portland cement. Constr. Build. Mater. 112, 783
789. Available from: https://doi.org/10.1016/j.conbuildmat.2016.02.121.

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The chemical composition revealed a high amount of Ca, Mg, Na, P and K. Usually, increasing the calcination temperature increases the concentration of these elements. Other elements were present as minor quantities, and some elements, such as Pb and Cd, showed up as highly volatile elements. Thus, the concentration of these last elements decreased from 300 to 800 C. In this paper, a SEM study was carried out to characterise the ashes. A large amount of organic matter was observed in the micrographs of the ash at 300 C, whereas that matter disappeared in the ashes obtained at 500 C. Generally, all the samples showed the presence of some crystalline aggregates and amorphous phases with an irregular morphology. The XRD showed the presence of quartz and calcite as the main minerals. Also, dolomite, anhydrite and phosphorous compounds were identified. In the ash calcined at 800 C, free-lime due the calcite decomposition was detected and complex additional phases were formed that had a chemical structure close to wilkeite. Recently, Kappel et al. (2017) studied the colour, compressive strength and workability of mortar when cement was partly replaced by iron-rich SSA that was ground into six different fractions. SSA was collected at a wastewater treatment plant in Copenhagen (Denmark) and incinerated in a fluidised bed combustor at 850 C. In the treatment plant, phosphorous was removed by chemical precipitation with iron salts. Thus, the SSA had a red iron-oxide colour. The SSA was a mixture of at least 95% of the ash captured in the electrostatic filter and 5% of residues from the filter bag. The pH of the SSA was 9.9 (very alkaline) and it had a water-soluble fraction of 1.5%. The chemical composition and trace metals are shown in Table 5.12. The distribution of the major oxides for the SSA were: CaO . P2O5 . SiO2 . Fe2O3 . Al2O3. In comparison to mean values reported by Cyr et al. (23), the SSA of this study had a high content of P2O5 and relatively low content of SiO2 and Al2O3. The content of P2O5 was 20% and was at the same level as the CaO (23.8%) and Fe2O3 (17.5%).

Table 5.12 Characterisation of SSA

Major oxides (%) SiO2 17.1

Al2O3 5.1

Fe2O3 15.7

CaO 23.8

MgO 2.32

Na2O 1.15

K2O 1.57

P2O5 20.2

SO3 2.02

TiO2 0.83

Cl2 0.01

MnO 0.09

Trace elements (mg/kg) Ni 57.5 6 1.53

Cr 38.7 6 0.76

Cu 688 6 17.3

Zn

Pb

LOI (%)

1930 6 26.8

144 6 2.00

1.35 6 0.04

SSA, Sewage sludge ash. Adapted from Kappel, A., Ottosen, L.M., Kirkelund, G.M., 2017. Colour, compressive strength and workability of mortars with an iron rich sewage sludge ash. Constr. Build. Mater. 157, 1199
1205. Available from: https://doi.org/10.1016/j. conbuildmat.2017.09.157.

Sewage sludge ash

5.4

135

Reactivity of sewage sludge ashes in Portland cement mixtures

The data referring to the reactivity of SSA have been compiled for this area, taking into account the different methods used from thermogravimetric analysis (TGA) to Frattini tests. In general, in all of the reported articles, the pozzolanic character of SSA was demonstrated. Cyr et al. (2007) studied the reactivity of SSA in pastes with lime. The authors made pastes with a ratio of 4:1:1 (SSA, lime and water respectively). They concluded that the principal phases formed during the pozzolanic reaction were ettringite, hydrated calcium aluminates and hydrated calcium aluminosilicates. After three weeks, all the calcium hydroxide had reacted and the presence of hydrated silicate phase was not detected. Donatello et al. (2010a; 2010b) studied the pozzolanic activity and published two articles on their findings. In the first paper, they studied the pozzolanic activity using the Frattini test. Metakaolin (MK), quartz sand and FA were used for comparative purposes. SSA, sand and FA were milled and all materials were washed with sulphuric acid in order to study the influence of this process in the pozzolanic activity. The Frattini test focused on the study of the solubilisation of Ca(OH)2 in a mixture of cement (80%) and pozzolan (20%) with water. The samples were left for 8 or 15 days in a bath at 40 C, after which the samples were filtered. The filtrate was analysed to determine concentrations of OH2 and Ca21 ions. The results were plotted on the solubility chart of Ca21/OH2 system and the points below the line indicate a positive pozzolanic activity. The results obtained are represented in Fig. 5.5. As can be seen in the figure, the grinding process improved the pozzolanic activity of SSA. The process of washing the samples with acid worsened the behaviour of all the samples. 20 18

[CaO] mmol

16

MK acid

Sand acid

14 Sand mil

12

ISSA acid

Sand unmil

10 8

ISSA unmil

ISSA mil

6

PFA acid

PFA unmil

4 2

Lime solubility curve

PFA mil MK unmil

0 35

45

55

65

75

85

[OH] mmol

Figure 5.5 Frattini test results (8 days) for unmilled, milled and sulphuric acid washed materials (sand, MK, PFA, ISSA) (Donatello et al., 2010a). MK, Metakaolin; PFA, Pulverised fuel ash; ISSA, incinerated sewage sludge.

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New Trends in Eco-efficient and Recycled Concrete

In the second article published, the authors compared the results obtained by different pozzolans and diverse methods [strength activity index (SAI), Frattini test and the saturated lime test] (Donatello et al., 2010b). The saturated lime method is similar to the Frattini test, but in this test, the pozzolan is mixed with a saturated lime solution. The amount of lime fixed is measured with the determination of the residual dissolved calcium. The authors concluded that the SAI and Frattini test are standardised methods with good correlation between them. The saturated lime test did not have a good correlation with the other methods. The reactivity of SSA is less than MK, pulverised FA and SF. Dyer et al. (2011) studied pastes with SSA with XRD and showed the presence of a poorly crystalline component with phosphorus. The compound was hydroxyapatite (Ca5(PO4)3OH) and it was observed in the diffraction analysis together with the AFm phases, portlandite and CSH gel. The presence of hydroxyapatite depends on the source used. It is possible that the presence of this compound could be used in waste solidification applications. In the same paper, the authors concluded that the additional aluminium and iron in the ashes formed a greater quantity of AFm phases. This statement was corroborated by the presence of a secondary peak in the curves obtained by the isothermal conduction calorimetry (ICC) analysis. The characterisation of pastes by TGA was employed by various authors. Yen et al. (2012) studied the properties of binary and ternary mixtures. The replacement of cement was 50% and 70% and the pozzolans employed were FA, GBS and SSA. The peak of portlandite was less in the pastes with pozzolan. In the pastes with SSA, a large amount of AFm phases were formed. Baeza et al. (2014a) studied the percentage of fixed lime by TGA in pastes with a 10% of substitution of cement by FA, RHA and SSA. They corroborated that the most reactive pozzolan was RHA, and that at 28 days of curing, the percentage of fixed lime was greater in the paste with SSA than in the paste with FA. Wang et al. (2017) studied the influence of co-combustion of SS and rice husk on the hydration properties. The ashes were burnt in two proportions. The ash defined as H had 20% SS and 80% rice husk. The other proportion was 30% and 70%, respectively, and the ash was referred to as W. The heat flow curves show that the presence of the ashes retarded the cement hydration with an induction period higher than that for the control paste. However, in the final hours of the experiment, an exothermic effect was observed as a consequence of the pozzolanic reaction. The total hydration peak showed that the paste with the highest heat was the paste with H. By means of the data obtained in the ICC experiment, the authors proposed a hydration kinetic model. They concluded that the inclusion of ashes inhibited the hydration process at the early ages, but the quantity of the hydrates was higher due to the pozzolanic reaction. Chen and Poon (2017a) studied the reactivity of SSA and FA using the Frattini test, ICC and XRD analysis. In the case of SSA, the authors used two ashes, one with a small mean particle diameter (the FSSA had a mean diameter of 6 μm). All the ashes had pozzolanic behaviour from Frattini test results. In all the pastes studied through XRD, the major components were portlandite, ettringite, alite and belite. In pastes that contained SSA or FSSA, a new crystalline compound, brushite

Sewage sludge ash

137

(CaHPO4  2H2O), was observed, which was generated from the reaction between iron phosphate from the SSA and the Portlandite generated in the cement hydration. The ICC test demonstrated that the evolved heat of the pastes containing SSA or FSSA were higher than the control mix. In this case, the authors said that the nucleation effect was the reason for this phenomenon.

5.5

Effect of sewage sludge ash on the rheology of concrete

The principal characteristics of rheology studied on the role of SSA in cement mixtures are focused on the workability and the setting time parameters. The results obtained in several of these investigations often do not agree, and this makes sense, since the origins and compositions of the ashes are very variable. Tay (1987) used SSA passing through a 150 μm sieve and established that the initial setting time was similar to the control system with PC, but the final setting time was less. The workability of the mixtures was superior, or similar, to the control for the proportions with 15% and 20% of SSA substituted for cement. These results obtained by Tay in 1987 differed, for example, from those obtained by Monzo´ et al. (1996). In the last research, the SSA was used in three different fractions: SSA (ash as-received, original ash), SSAC (ash retained in the 80 μm sieve) and SSAM (ash retained in the 40 μm sieve). The percentage of substitution was 15%, and in this case, the workability was inferior to the control mortar for all ashes. The authors concluded that the irregular morphology and the water absorption on the surface of the particles were responsible for this loss of workability. In the other article, the same authors studied the influence of different percentages of substitution, water/cement ratios and the use of superplasticiser (Monzo´ et al., 2003). The increase in the substitution produced a decrease in workability, but the use of a superplasticiser improved the fluidity of the mortars. The finest SSA had the worst workability. One alternative to improve the workability of mortar with SSA is the use of FA as a second replacement material. Paya´ et al. (2002) demonstrated the good performance of FA in mortars with RHA, SSA, SF and fluid catalytic cracking residue. Pan et al. (2003) used milled SSA and established that a 20% substitution enhanced the workability of the mixture. They reported that the mechanical grinding of particles promoted the morphology change in the particles from an irregular to a spherical form. It is likely that the use of the original ash without milling was the cause of the poor workability due to the morphology and texture of the particles. The initial setting time was larger, but the final setting time was less than the control. The reason for this trend was attributed to the greater quantity of aluminium in the SSA. Garce´s et al. (2008) studied the effect of substitution of cement by SSA in percentages between 0% and 30%. Only the samples that had more than a 10%

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New Trends in Eco-efficient and Recycled Concrete

Figure 5.6 Workability behaviour (FTS values) for SSA-CEM I 42.5 R blended cements containing 0%
40% of SSA replacement. SSA, Sewage sludge ash; FTS, flow table spread. From Garce´s, P., Pe´rez Carrio´n, M., Garcı´a-Alcocel, E., Paya´, J., Monzo´, J., Borrachero, M.V., 2008. Mechanical and physical properties of cement blended with sewage sludge ash. Waste Manage. 28 2495
2502. Available from: https://doi.org/10.1016/j.wasman.2008.02.019.

substitution met established standards. As can be seen in Fig. 5.6, the loss of workability was very important for substitution percentages greater than 10%. The porous and rough nature of SSA can be the cause of this workability loss. This conclusion was corroborated by Chen and Poon (2017a) as their results indicated that milled ash had better performance than the original in terms of workability. The same conclusion was reached by Kappel et al. (2017). The authors confirmed that pre-treatment with milling improved the performance of mortars with SSA. Li et al. (2017) suggested that the loss of workability was due to increased water absorption, which may be an advantage to obtain mortars with lower segregation. Stirmer et al. (2016) studied the influence of the calcination temperature on the fresh properties of mortars with SSA. They concluded that a greater calcination temperature produced mortars with less air content and better workability. For example, the ashes calcined at 1000 C had a lesser absorption of water than the ashes calcined at 800 C. However, the high temperature treatment reduced the pozzolanic properties. Vouk et al. (2017a) observed that the air content in concrete was increased with the use of SSA, especially for ashes with added lime. It was also noted that a higher temperature treatment at 1000 C reduced the air content because it increased the specific density of the ashes.

5.6

Mechanical and durability properties of concrete containing sewage sludge ash

Many studies indicated that SSA can be used in mortars or concrete preparations because of its reactivity and size distribution.

Sewage sludge ash

139

Tay (1987) studied the use of SSA as a filler in concrete. Dewatered sludge samples were collected weekly for 12 months, and burnt in a furnace at above 550 C to remove organic matter. In order to study the feasibility of the use of ash as a filler, it was pulverised and passed through a 150 μm sieve. Then 5%
20% (by weight) of PC was replaced by SSA. The results showed a decrease of compressive strength as the percentage of sludge increased. After 28 days of curing, a reduction in strength of 5% and 11% for 5% and 10% replacements, respectively, was observed. Additionally, the shrinkage strain was reduced by 10% with respect to the control sample. The author concluded that SSA could be used as a partial replacement of cement for concrete mixing. Monzo´ et al. (1996, 1999a) studied the mechanical behaviour of PC mortars adding SSA and curing at 40 C. In the first paper (Monzo´ et al., 1996), 15% (by weight) of PC was replaced by SSA and three size fractions (coarse, medium and fine) were obtained by sieving. In general, all mortars containing SSA exceeded the control mortar, and an increase of compressive strength with fineness was observed (Fig. 5.7). The pozzolanic behaviour for the 3
28 day curing period was then confirmed when the curing temperature was slightly raised. In their second paper (Monzo´ et al., 1999a), the assessment of the sulphate content of SSA was carried out. This parameter was very high in SSA (12.4%) and could influence the volume stability of the concrete. The behaviour of mortars containing SSA (15% and 30% replacements) and several PCs with different tricalcium aluminate contents was presented. Results showed that this SSA was compatible with PC with high C3A content, and a decrease in the mechanical properties were not observed after the 28-day curing time at 40 C. This meant that the sulphate in the SSA was not reactive, and no expansive products (e.g., secondary ettringite) were formed during the curing process. The expansion was measured (Garce´s et al., 2008) for SSA-cement

Figure 5.7 Compressive strength development of mortar cured at 40 C for 3
28 days. Control mortar (only cement) was compared with 15% replaced mortars: SSA (as-received), SSAC (coarse fraction) and SSAM (medium fraction) (Monzo´ et al., 1996). SSA, Sewage sludge ash.

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New Trends in Eco-efficient and Recycled Concrete

mixtures and compared to gypsum-cement mixtures with the same sulphate content, and significant differences were found in their behaviour. For example, gypsumcement mixtures showed expansion higher than 1 mm/m after 180 days, while the SSA-cement mixture had much lower expansion (0.190 mm/m). Garce´s et al. (2008) evaluated the suitability of SSA for blending with different commercially available PCs. Durability aspects were tested (porosity and shrinkage expansion) on mortars containing SSA. The results showed that total porosity increased in all mortars containing SSA. In general, total porosity increased with the PC replacement by SSA (Fig. 5.8). This fact suggests that water demand is a crucial factor when the percentage of replacement is high, and consequently the compaction level is low. No significant differences were observed between CEM IIB-M(V-LL) 42.5R (a commercial cement with FA and ground limestone) and CEM I 52.5R. This similar behaviour confirmed the advantage of using PC with mineral additions plus SSA instead of PC without mineral additions, because blended cements are cheaper and more environmentally friendly. Paya´ et al. (2002) studied the influence of binary binder admixture [SSA-coal FA (CFA)] on the development of mortar strength. The results showed that the presence of FA, in association with SSA and PC, became important for long curing times (90 days), where the compressive and flexural strengths of the mortars were similar, or higher, that the control one. The influence of SSA fineness on mortar strength was studied by Pan et al. (2003). Mortars replaced 20% by weight of PC by SSA were prepared. SSA was ground in order to increase the fineness. SSA ground for 10 min showed 496 m2/kg Blaine fineness, and for 360 min it increased to 872 m2/kg. The corresponding 20% SSA-replaced mortars for each fineness yielded, at 28 days, 18.4 and 29.5 MPa, respectively. These results show that the SSA pozzolanic activity and compressive

Figure 5.8 Total porosity of SSA containing mortars (0%
30% replacement of different commercial cements) (Garce´s et al., 2008). SSA, Sewage sludge ash

Sewage sludge ash

141

strength of the mortars increased with the SSA fineness. Donatello et al. (2010a) also demonstrated that the milling is an appropriate treatment for activating the reactivity of ashes, and a significant increase in the SAI was achieved. For example, 0.8 was the SAI value for 20% SSA replaced mortars after 28 days, while the SAI value was raised to 0.96 for mortar with milled SSA. They also reported that acid washing (H2SO4) had a negative effect in the pozzolanic behaviour of SSA in terms of compressive strength development. Kappel et al. (2017) found a similar enhancement in strength with the milling of the SSA when a 20% replacement in mortar was tested. The higher SSA fineness led to a reduction in the ASR process with respect to the control mortar (Chen and Poon, 2017b). After 14 days of storing the control mortar in 1 M NaOH solution, it developed an expansion close to 0.25%. However, when ground SSA was used for 10% replacement, the expansion was reduced to 0.06%. The replacement of 20% of cement by SSA (as-received or ground) led to a decrease in the ASR expansion after 14 days (less than 0.01%). On the other hand, due to water absorption ability of SSA particles, the shrinkage rate for SSA containing mortar was significantly greater than that observed for the control mortar. The reactivity of SSA also was studied with regards to calcination temperature. Naamane et al. (2016) compared the development of strength for mortars containing SSA in the 0%
30% range, and submitted the SSA to different calcination temperatures (300
800 C). Fig. 5.9 shows the developed strengths after 90 days of curing. Calcination temperatures in the 300
500 C range did not improve the strength development, whereas the 700
800 C range of treatment temperatures yielded the best results. Yang et al. (2014) also found that the calcination of SS at 800 C for 2.5 h was the best option in terms in compressive strength. Also, a complementary study in terms of lime addition in the obtaining of SSA was carried out (Vouk et al., 2017b). Different SSA samples were obtained from: (1) calcination of SS; (2) calcination of SS previously stabilised with lime; and

Figure 5.9 Influence of SSA calcination temperature. Compressive strength values for mortars with 0%
30% SSA content after 90 days of curing (Naamane et al., 2016). SSA, Sewage sludge ash.

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New Trends in Eco-efficient and Recycled Concrete

(3) calcination of SS with the addition of lime. There were minor differences among the types of SSA obtained at 800 C, although SSA without any lime added (as stabilising agent or as addition in the calcination) presented the best mechanical results when used for 10 and 20 replacement percentages for cement mortars. Mixtures of PC-SSA in the ratios of 20:80, 30:70 and 40:60 (by weight) were used to prepare foamed lightweight materials (Wang and Chiou, 2004) using aluminium powder (0.9%
1.3% referred to as total solid) or mixed scrap metal waste powder as foaming agents. The results show that compressive strength of aluminium powder specimens was lower than those that used the mixed scrap metal waste as a foaming agent. This tendency was confirmed by the total pore volume data. The presence of SSA in the mixtures increased the proportion of pores smaller than 10 μm, probably due to pozzolanic reaction products and the morphology of the ashes. Garcı´a-Alcocel et al. (2006) demonstrated that the pozzolanic reactivity of SSA was moderated and studied the corrosion of embedded rebars in PC mortars containing 0%
60% of SSA replacement. They found that metallic corrosion for mortars containing 10% SSA had similar behaviour than plain cement mortar in 100% RH and under seawater. The washing of SSA with distilled water was an interesting procedure for enhancing mortar properties from the corrosion point of view. Cyr et al. (2007) studied SSA replaced mortars by 25% and 50% by weight of PC. Compressive and flexural strength development for these mortars are shown in Fig. 5.10. It can be seen that the strength evolution for the short curing time was much lower for SSA mortars, especially for the 50% replaced case. However, after 28 days, a significant contribution of SSA was highlighted. Calorimetric studies showed that heat flow for plain cement mortar started after 3 h of mixing. However, for 25% and 50% SSA samples, heat flow started after 4.5 and 6.5 h, respectively. This delay was attributed to both a dilution effect and the presence of several chemical elements (P, Zn) in the ash. Blending different mineral admixtures [such as FA and blast furnace slag (BFS)], Yen et al (2012) obtained interesting results by replacing 50% of PC. Binary SSA 1 FA and SSA 1 BFS and ternary SSA 1 FA 1 BFS were studied. The authors reported that the best mixtures were those that contained the binary system 25% SSA 1 75% BFS and the ternary system 25% SSA 1 50% FA 1 25% BFS, reaching relative compressive strengths, with respect to the control mortar, of 93.7% and 92.9% at 56 days of curing. In the same way, Baeza et al. (2014a) tested binary SSA blends with FA, MD and RHA. They found that after 90 days of curing, 84.7% of the reference mortar was achieved for the 10% SSA 1 10% FA mortar and 91.3% for the 10% SSA 1 10%RHA one. However, MD did not contribute to good strength development (75.8% for 10% SSA 1 10% MD with respect to the reference) and it was lower than that found for 20% SSA (78.5%). The ternary mixture 10% SSA 1 10% FA 1 10% RHa yielded the best result, with 109.4% respect to the reference, which was much higher than that found for 30% SSA (59.3%). SSA has been also tested as a sand replacement in the fabrication of dry consistency concrete blocks (Baeza et al., 2014b). A similar strength to the control block was obtained, and 10% of sand replacement by SSA yielded blocks with the best performance in terms of absorption, density and capillary suction. The

Sewage sludge ash

143

Compressive strength (MPa)

70

Reference 25% SSA 50% SSA

60 50

54.8

55.7

45.0

41.3

32.7

31.1

28.7

19.4

19.1

20

66.6 61.2

50.7

40 30

62.0 57.0

12.5

10 0 1

Reference 25% SSA 50% SSA 6.4

9 Flexural strength (MPa)

8 7 6 5

2 7 28 Hydration time (days) 7.8 7.2

7.5 7.1 6.7

6.2 5.3

5.2 4.9 4.3

7.06.6

84

4.0

4 3

1.9

2 1 0 1

2 7 28 Hydration time (days)

84

Figure 5.10 Flexural and compressive strength development for control mortar (reference) and mortars containing SSA (25% and 50% by weight) for 1
84 day time period (Cyr et al., 2007). SSA, Sewage sludge ash.

enrichment of the mix with the fine SSA particles likely enhanced the development of the block performance. Jamshidi et al. (2013) found that replacement of sand by SSA in 5%
10% by weight of cement yielded similar compressive strength than reference concrete after 90 days of curing. In contrast, Bhatty and Reid (1989) found that replacement of sand by SSA reduced the strength significantly. Reactivity of SSA was compared to the FA case (Piasta and Lukawska, 2016) and it was concluded that SSA presented lower pozzolanic behaviour in terms of strength gain. Thus, pozzolanic activity of SSA can be considered as moderated. Pavlı´k et al. (2016) also found a low reactivity of SSA in the first 28 days of curing, attributing this to a simple filler effect that contributed to cement paste strength. The authors analysed the porosity and mechanical behaviour of cement pastes containing SSA in the 0%
60% replacement range. The total porosity increased with the replacement percentage (Fig. 5.11A) and they found a relationship between porosity, compressive and bending strength (Fig. 5.11B).

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New Trends in Eco-efficient and Recycled Concrete

Figure 5.11 SSA cement pastes (0%
60% replacement): (A) cumulative curve for pore size distribution, and (B) relationship between porosity and mechanical properties (Pavlı´k et al., 2016). SSA, Sewage sludge ash.

In contrast, Chen and Poon (2017a) found a good reactivity of SSA and ground SSA in early-stage cured mortars, and that it was superior to that for FA. They assumed that several factors affected the behaviour: (1) the absorption of water in SSA particles, which reduced the effective w/c ratio; (2) the slow release of water from SSA particles, which allowed them to complete the hydration of the cement particles; and (3) the angular shape of SSA particles, which caused an interlocked structure among the gelatinous and unreacted particles.

Sewage sludge ash

5.7

145

Alternative binders containing sewage sludge ashes

There are few articles in which the use of SSA as a precursor in geopolymer mortars is described. In the first investigation (Yamaguchi and Ikeda, 2010), the waste used was in the slag form, not in the ash form. SSS (SS slag) with CFA were employed to fabricate geopolymeric materials cured at 80 C (Yamaguchi and Ikeda, 2010). They concluded that the optimum percentage to obtain materials with the best mechanical strength is 75% of CFA and 25% SSS. Additionally, they demonstrated that 100-SSA geopolymer systems developed strength slowly, and solidification was very sluggish. Istuque et al. (2016) studied the behaviour of incorporating SSA in MK-based geopolymeric mortars. One of the principal problems of geopolymeric materials based in MK is that at high curing temperature reduces the compressive strength with time. The formation of zeolites can be the source of the strength reduction. The authors proposed replacement of part of MK by SSA to improve this reduction. The percentages of replacement were 10% and 20% by mass. The curing conditions employed were 25 and 65 C, and the compressive strength was measured after 1, 3 and 7 days of curing. As seen in Fig. 5.12, the mortars cured at 65 C presented a reduction in compressive strength with curing time, while at room temperature the

(A) Curing temperature: 65°C

28 .8 27 ± 2 .9 . ± 4 2. 5

5 8

± 5

± .9

1 ± .2 17

.7

±

1.

± .3

15

17

1. 0 ± .0

15

3

0.

9

2.

0

21 .

2 2. ± 6 16 .

5 0. ± .5

16

1.

8 ±

± .3 20

6 1. ± 0 19 .

20

1.

2.

.4 24

25

3

25 .

7

±

0.

5

26 .4

2.

±

1

2.

3

2. ± 4

26 .

30

21

1. ± .0

MKB-0 MKB-10 MKB-20

1. ± .8 29

Compressive strength (MPa)

35

37

40

(B) Curing temperature: 25°C MKB-0 MKB-10 MKB-20

8

45

15 10 5 0 1 day

3 day

7 day

1 day

3 day

7 day

Curing time

Figure 5.12 Compressive strength of metakaolin-based geopolymer mortars with different replacement by SSA (0%, 10% and 20%) cured at: (A) 65 C (thermal bath, B series), and (B) 25 C (room temperature, R series) (Istuque et al., 2016). SSA, Sewage sludge ash.

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New Trends in Eco-efficient and Recycled Concrete

strength increased. The addition of SSA to the mortars showed advantages in both curing conditions. At 65 C, the compressive strength loss of mortars with SSA was slightly less than in mortars with only MK. At room temperature, the mortar with 10% SSA replacement and the control with 100% MK presented a similar compressive strength after 7 days. Chakraborty et al. (2017) studied the improvement of adding quick lime (QL) and blast furnace slag (BFS) to systems with SSA. They compared the performance of mortars of SSA mixed with only water and mortars mixed with NaOH. They studied different concentrations of NaOH and concluded that the best was 10 M. With this concentration of NaOH, they studied the influence of substituting SSA by QL and BFS. The maximum compressive strength was achieved with the mortar that contained 70% SSA, 20% QL and 10% BFS. The addition of QL and BFS promoted the formation of more hydrated products, which developed a more compact matrix. Finally, Tashima et al. (2017) published an investigation using SSA and ground granulated blast furnace slag (GGBS). They activated the mixtures with only sodium hydroxide, which were cured at room temperature. The mortar with 20% of SSA and 6 mol/kg of NaOH achieved 31 MPa of compressive strength after 90 days of curing. This result is very important because it demonstrated the existence of good mortars without using water by curing at room temperature. The main product formed was a cementitious gel type, C-A-S-H. Other alternative binders are the mixture of soil with cement and Chen and Lin (2009) studied the stabilisation of a soft soil (with clay and silt) with a mixture 4:1 of SSA:cement. They substituted 16% of soil with the mixture of SSA/cement, because the addition of this mixture promoted better soil performance. The characteristics of the resulting soil were better than those demanded by legislation. Along the same lines as previous article, Lin et al. (2016), studied the same SSA/cement system and soil, but with the incorporation of nanosilica. The hydration products also increased with the addition of nanosilica, and therefore the unconfined compressive strength (UCS) was 30
47 kPa higher than the soil without this component. Gu¨llu¨ and Fedakar (2017) proposed the stabilisation of soils with SSA and polypropylene fibre (PF). They employed ANOVA analysis to obtain the best proportion of materials. The addition of 19.5% of SSA, 0.57% of PF and 12 curing days resulted in a mixture with around 1900 kPa of UCS.

5.8

Other applications of sewage sludge ashes in construction materials

The direct use of SSA in the preparation of cement includes blending it with Portland clinker replacing part of the commercial cement by SSA, and using alternative binding materials. Some reports have also been done regarding the indirect

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use of SSA in building materials, such as with raw materials in the fabrication of clinker and ceramics. Lin and Lin (2004) and Lin et al. (2005) developed three types of hydraulic cements incorporating SSA and slag from steelworks (ferrate) as a partial replacement of clay, silica, alumina and iron oxide in raw materials for PC production. A pilot-scale study was conducted, heating it up to 1400 C for 6 h in a clinkerisation process using a simulated incineration and smelter. Results showed that eco-cement A (6.47% SSA) was most similar in compressive strength to PC, while eco-cement B (4.86% SSA) showed early strength development. Tay et al. (1991) studied the use of dewatered sludge clay mixtures after incineration as a lightweight coarse aggregate. The properties of the obtained aggregates, as well as concrete, were also studied. The strength of the concrete with the lightweight aggregate increased along with the percentage of clay after 28 days of curing. Compressive strength results agreed with BS 8110 for structural concrete. A bench-scale was developed to determine whether SSA could be used in brick making clay mixtures in order to reduce SSA that is landfilled (Trauner, 1993). The study concluded that bricks with even 30% by weight SSA addition agreed with ASTM standard specifications regarding compressive strength. Chen and Chiou (2006) also used SSA as a main material to produce foamed lightweight materials, investigating how sintering temperature affected the macro/ micro properties. Firing tests were conducted at different temperatures, and it was shown that the SSA/cement foamed samples (20/80, 30/70 and 40/60 ratios were tested) presented better performance than pure cement foamed paste (Fig. 5.13). Large volume shrinkage (25%) was measured with increasing firing temperature from 20 to 1093 C. However, because the sintering effect was favoured by SSA, the 20/80 and 30/70 samples showed an increase in compressive strength from 600 C (8.5
9.0 MPa) to 1093 C (11.0
13.5 MPa).

Figure 5.13 Appearance changes for foamed pastes after firing at different temperatures: (A) pure cement paste (W/S 5 0.35), and (B) C/SSA 5 30:70 (W/S 5 0.6) (Chen and Chiou, 2006). SSA, Sewage sludge ash.

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New Trends in Eco-efficient and Recycled Concrete

Conclusions

Chemical, physical and mineralogical properties of SSAs are highly variable because of the different sources of wastewater, the nature of the soil and the type of treatment in the WWTP, among other factors. In general, silicon dioxide is usually the major component, although depending on the SS, calcium carbonate, iron oxides, phosphorous and sulphur-derived compounds are also present. This variability must be taken into account, because it could result in significant differences in the behaviour of cement-based SSA containing mortars and concrete. SSA has interesting pozzolanic reactivity, which has been tested by several techniques: thermogravimetry, Frattini test and compressive strength development, among others. In some cases, SSA delays the hydration rate of PC. However, this behaviour apparently does not affect the final strength obtained. It is important to highlight that this pozzolanic reactivity, because it opens up the possibility of using SSA containing mortars and concrete. Another important fact that must be taken into account in the use of SSA in cement mixtures is the effect on the rheology of the systems. Usually, a significant decrease in workability was observed in fresh concrete or mortar, due to the irregular morphology, porosity and roughness of SSA particles. Several treatments, such as milling or high temperature calcination, have been proposed in order to minimise this negative effect. The pozzolanic role of SSA in cement mixtures has been widely explored. The reactivity of the ash in terms of strength contribution has been considered as a moderate rate. In some cases, increasing reactivity has been achieved by controlling the calcination temperature of the SS (usually 800 C was the best condition), or by grinding. Durability aspects were less assessed, and research was focused on parameters such as porosity, absorption, ASR expansion and drying shrinkage. In the last decade, there has been an interest in the use of SSA in geopolymer binders. Reported results demonstrated that in most cases, blending SSA with other reactive mineral additions (MK, BFS) led to the production of stable systems with sufficient mechanical properties. SSA was also successfully used in the fabrication of clinker and ceramic bricks. The global view of the treatment of wastewater can be seen as a good example of the zero-waste concept and circular economy. The removal of contaminants from the wastewater provides an opportunity to reuse the water and/or to avoid damage to the environment, mainly rivers, lakes and the ocean. The by-product from the water treatment, namely the SS, can be partially reused, mainly in the agriculture sector. Finally, the discarded SS can be transformed into ash by calcination, in many cases taking advantage of its organic matter content. The resulting ash, namely SSA, can be reused in many fields of the construction sector: ceramics, clinker and cement/mortar/concrete. Certainly, the reuse of this ash in the preparation of mortars and concrete is a very promising option. Properties of the manufactured elements based on the cement
ash system showed that, in many cases, the quality of the final product was appropriate. In this way, the circle is closed and no material is lost through the different processes.

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