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Waste and Biomass Valorization ISSN 1877-2641 Waste Biomass ValorDOI 10.1007/s12649-014-9299-2

Incinerator Bottom Ash and Ladle Slag forGeopolymers Preparation

Isabella Lancellotti, Chiara Ponzoni,Maria Chiara Bignozzi, Luisa Barbieri &Cristina Leonelli

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ORIGINAL PAPER

Incinerator Bottom Ash and Ladle Slag for GeopolymersPreparation

Isabella Lancellotti • Chiara Ponzoni •

Maria Chiara Bignozzi • Luisa Barbieri •

Cristina Leonelli

Received: 25 July 2013 / Accepted: 18 February 2014

� Springer Science+Business Media Dordrecht 2014

Abstract Ladle slag (LS) and urban incineration bottom

ash (BA) are two types of non hazardous inorganic wastes

which do not contain significant amounts of pollutants such

as heavy metals and organics. Nowadays they are neces-

sarily disposed of with little attention placed for re-use or

recycling. Considering their chemical composition, rich in

silica and alumina with substantial levels of lime and iron

oxides, these residues can be suitable for generating new

alkali activated materials. A safe reuse of these residues in

high percentages (60–70 wt%) is presented in this study for

mortar production. The final room-temperature consoli-

dated materials, also known as geopolymers, have been

characterized in terms of thermogravimetric analysis,

morphology, porosity, and crystalline phases evolution.

When incinerator BA is used, the morphology of the

resulting geopolymer is very close to that of pure

metakaolinic pastes, whereas for LS based geopolymers

calcium presence promotes the formation of calcium–alu-

minate–silicate–hydrate phase. This investigation also

demonstrated that the content of reactive fraction of BA is

of primary importance to assess its possible use in alkaline

activation process.

Keywords Alkali activation � Geopolymers � Ladle slag �Incinerator bottom ash

Introduction

With the aim of transforming inorganic industrial waste to

valuable materials, two categories of aluminosilicates have

been here activated via alkali solutions and consolidated at

room temperature as cement-like materials. The first type

of waste is a residue of steel industry. Steel slag comes

from the ladle slag (LS) where the refining process of steel

produced by scrap melting in arc electric furnace or by the

conversion of iron to steel in a basic oxygen furnace (BOF)

occurs [1, 2]. In the steel making process, slags are used to

prevent oxidation of the steel through contact with air, to

limit heat losses through radiation and to remove impurities

from the molten steel [3]. Hence slag is an unavoidable by-

product of steelmaking process and its numerous functions

are very well known.

Ladle slag accounts for about 1/3 of the total amount of

slag usually produced in electric arc furnaces and its

European production can be estimated about 4 millions

t/year [4]. The amount of LS and, more in general, of steel

slag is going to increase in the near future as arc-electric

furnaces represent a more efficient and sustainable process

for steel production compared to blast furnaces [5]. Now-

adays LS is generally treated as non hazardous waste and

dumped in landfills and very few attempts are present to

evidence their commercial value when properly prepared

and processed [6]. Beside aluminosilicate, LS contains

calcium–silicate, magnesium–silicate and calcium–alumi-

nate compounds and minor amounts of other metallic

oxides, mainly iron. For its chemical composition LS was

investigated as cement constituent in mortar and concrete

preparation [2, 7] and geopolymer matrix for fiber rein-

forced composites materials [8].

The second type of waste material, very close to LS in

terms of chemical composition and percentage of

I. Lancellotti (&) � C. Ponzoni � L. Barbieri � C. Leonelli

Department of Engineering ‘‘Enzo Ferrari’’, University

of Modena and Reggio Emilia, 41125 Modena, Italy

e-mail: isabella.lancellotti@unimore.it

M. C. Bignozzi

Department of Civil, Chemical, Environmental, and Materials

Engineering, University of Bologna, 40131 Bologna, Italy

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DOI 10.1007/s12649-014-9299-2

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amorphous phase and soluble salts, is bottom ash (BA)

deriving from urban waste incineration. At present the

incineration process represents an efficient alternative to

dumping and it is safely conducted till full oxidation of all

the combustible materials. Thermal treatment aims at a

volume reduction of waste; at the destruction, capture, and

concentration of hazardous substances; and evidently at the

recovery of energy [9]. One of the most important disad-

vantages of the incineration technology is the production of

significant amounts of solid residues: fly ashes (5 % of

initial weight of wastes) and bottom ashes (up to 30 % of

initial weight of waste) [10]. The BA, used in this paper, is

classified as non hazardous waste, differently from fly ash,

and it represents a residue with interesting chemical com-

position capable of matching with cement and ceramic

formulations requirements [11]. The similarities in terms of

physical characteristics and chemical composition between

granular construction materials and incinerator BA make

this residue virtually suitable for recycling as natural

aggregates replacement. In some countries, such as Japan,

German, Denmark and the Netherlands, incinerator BA is

used extensively in roads, aggregates, asphalt concrete, soil

amendment, soundproofing for walls, etc. The recycling

rate is typically 70–90 % and can be even up to 100 %

[12]. Notwithstanding this scenario, increasing regulatory

focus on leaching properties of bottom ashes has increased

the demand for dedicated techniques for upgrading of ashes

prior to utilization. Within the last decade, many efforts are

made for minimizing leaching of salts and heavy metals

such as Cu, Cr, As, Ni, Cd, and Pb in full-scale applica-

tions, especially using washing processes. No single pro-

cess has been found to ensure compliance with limit values

on leaching; however, extended curing as well as washing

could in most cases decrease leaching significantly.

Remaining problems were primarily associated with Cu, Cr

and sulfate leaching. [13]. In Italy only recently specific

plants for incinerator BA treatment are developing. In

particular these treatments consists of ageing, grinding and

Fe and Al separation by means of magnetic and eddy

current systems. The separated metals are about 8 % of the

total BA and are sent to recycling.

Thus both LSs and urban waste incineration ash repre-

sent a challenging problem of waste management due to

the presence of soluble salts and high aluminium-contain-

ing phases making their use not particularly appreciated in

cement industry. On the other hand their calcium content is

too high for completely substituting metakaolin (MK) in

structural geopolymer matrix [14, 15]. Nevertheless, both

wastes possess an important amount of vitreous fraction

making them promptly attacked by alkali solution. LSs

have already been activated with sodium silicate but a

complete investigation on the NaOH activation has never

been carried out previously by other authors [10]. Further,

only few papers [16] are present in literature, except our

work [17], which report an investigation about the use of

high amount of bottom ashes in geopolymer formulations.

On the basis of previous researches, this work aim at

demonstrating the possibility to use alkali activation for the

transformation of inorganic industrial and urban waste to

valuable materials. We report recent results in the micro-

structural, mineralogical, thermogravimetric and spectro-

scopic characterization of the newly proposed

formulations. These results represent a step forward in the

understanding the consolidation process of MK based

geopolymers with high content of LS or BA proposed for

the industry of building materials.

Materials and Methods

Ladle slag, incineration BA and MK’s chemical charac-

terization (X-ray fluorescence, Philips PW 2004), soluble

fraction in water (solid/liquid = 1/4, for 2 h) and loss of

ignition (LOI) (at 1,100 �C for 2 h) were performed and

reported in Table 1 [14, 17, 18].

Incineration BA was ground for 30 min in a ball mill

and sieved below 75 lm to reach grain size similar to that

of MK powder. LS was supplied as fine sand. The fraction

greater than 1 mm was sieved in order to obtain a grain size

distribution close to MK powder.

Three oxide ratios (CaO/SiO2; SiO2/Al2O3; Na2O/

Al2O3) are reported in Table 1 as they are indicators of

Table 1 Chemical composition and other physical data of LS and

BA added to MK

(wt%) Ladle slag Bottom ash Metakaolin

CaO/SiO2 3.32 0.35 _

SiO2/Al2O3 1.48 5.30 1.29

Na2O/Al2O3 _ 0.61 _

SiO2 16.4 49.45 53.6

Al2O3 11.1 9.33 41.7

CaO 54.5 17.49

MgO 4.0 2.88

Na2O Traces 5.70

K2O 0.04–0.2 1.41 2.5

Fe2O3 2.0–3.0 4.81 1.0

MnO 0.05–0.5 0.16 _

TiO2 0.04–0.2 0.81 0.5

Soluble fractiona 1.5 1.3 _

LOIb 9.81 4.77 _

Grain size \1 mm \75 lm \75 lm

a Determined as solubility in water (solid/liquid = 1/4) for 2 h for

five repeated treatmentsb Loss of Ignition at 1,100 �C, for 2 h

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good quality for structural building materials. In particular,

for LS a high CaO/SiO2 is observed according to its large

content ([50 %) of calcium oxide. On the contrary, a

higher SiO2/Al2O3 ratio is present for BA compared with

MK ratio (1.29), thus indicating that an extra alumina-rich

source should be used as precursor to reach geopolymer

formulations with high performances.

As previously reported [14], geopolymers made by

100 % of LS do not exhibit good mechanical properties

while the addition of MK as minor component for both the

slag has been optimized in two separate studies [13, 16]. It

was previously demonstrated that the minimum quantity of

30 wt% was suitable for both wastes. For LS based geo-

polymers the addition of 40 wt% of MK was also taken

into consideration. Different amount of NaOH (8 M, Carlo

Erba, Italy) and sodium silicate (SiO2/Na2O = 3, Ingessil,

Verona, Italy) solutions were added to the different mixes

as reported in Table 2. The investigated waste was alter-

natively mixed with MK and finally to the alkali solutions

with solid/liquid [g/g] ratio of 1.7 and 2.5 for LS and BA

based samples, respectively. A lower content of alkali

solutions was used for BA based samples comparing to LS

based geopolymers as BA is very rich of sodium oxide.

However, water addition was required to reach a good

workability when BA was used as precursor.

After 10 min mixing, geopolymers were cast in pris-

matic moulds with dimension of 3 cm 9 2 cm. Three

specimens for each mix were prepared.

In order to evaluate the reactive fraction of BA and

better tailor the formulation, an alkaline attack with NaOH

8 M was performed according to the test reported in lit-

erature [19]: 1 g of BA was placed in 100 ml of 8 M NaOH

solution and stirred constantly for 5 h in a flask bathed at

80 ± 2 �C. The analysis of liquid fraction after the test

performed by ICP-AES (Philips Varian, Liberty 200, using

k = 251,611 nm for Si and 396,152 nm for Al) shows the

amounts of Si and Al dissolved in an environment similar

to that of alkaline activation conditions. The dissolved

cations were determined to be: Si = 1.53 and

Al = 0.5 wt% for a ratio of Si/Al = 3.06. On this basis

new geopolymers were prepared and reformulated, here-

after indicated as MK30_BA70R, as reported in Table 2.

The reformulation was performed considering as starting Si

and Al content the amount dissolved in the alkaline

solution, i.e. the effective reactive fraction. This reformu-

lation, therefore, required a different dosage of NaOH and

Na-silicate as reported in Table 2.

Mineralogical and microstructural characterizations

were performed on both LS and BA based geopolymers

after curing for 24 h in sealed mould plus 30 days

unsealed. Curing procedure was carried out at room tem-

perature and relative humidity (R.H.) equal to 60–70 %.

Morphology studies were conducted on freshly fractured

samples by ESEM (ESEM, Quanta200, FEI, NL) equipped

with EDS in order to evidence phase distribution and

degree of reactivity of LS and BA in the geopolymeric

amorphous matrix. The mineralogical analysis of the two

geopolymers was carried out by a powder diffractometer

(PW3830, Philips, NL) with Ni-filtered Cu Ka radiation in

the 5–70 2h range on powdered samples (30 lm particle

size). Pore size distribution measurements were carried out

by a mercury intrusion porosimeter (MIP, Carlo Erba 2000,

Italy) equipped with a macropore unit (Model 120, Fison

Instruments, UK) on samples previously dried at 80 �C.

Samples for porosimetry were cut by diamond saw to

approximately 1 cm3, dried under vacuum and kept under a

P2O5 atmosphere in a vacuum dry box until testing.

Thermogravimetric characterizations of the geopoly-

mers were performed by means of TG/DTA equipment

(Netzsch 429, Germany) on 30 mg of powdered samples

with an heating rate of 20 �C/min up to 1,400 �C.

FT-IR spectra for samples containing incinerator BA were

recorded (Avatar 330 FT-IR Thermo Nicolet, USA); 32

scans between 2,000 and 650 cm-1 were averaged for each

spectrum at intervals of 1 cm-1. The FT-IR spectra have

been collected on geopolymers without (MK30_BA70) and

with the reformulation (MK30_BA70R), taking into account

the dissolved fraction of BA, in order to assess the differ-

ences in structure.

Results

The as-received BA presents a chemical composition

(Table 1) with insufficient Al content with respect to sili-

con (ratio Si/Al = 5.3 in Table 1), therefore the ash has

been admixed with MK to ensure the proper Si/Al ratio for

Table 2 Formulation of geopolymers referred to 100 g of solid and final Si/Al and Na/Al mass ratio

MK (g) LS (g) BA (g) NaOH 8 M (ml) Na silicate (ml) H2O (ml) Si/Al Na/Al

MK40-LS60 40 60 – 30 30 – 1.51 0.71

MK30_LS70 30 70 – 30 30 – 1.58 0.82

MK30_BA70 30 – 70 14 20 3 3.26 1.09

MK30_BA70R 30 – 70 10 24 3 2.06 1.24

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geopolymerization of structural materials. Many authors

found Si/Al ranging between 1.5 and 2.5 as most suitable

value for two parameters: material leachability [20], as a

consequence of high degree of 3D-polymerization in the

structure; and mechanical properties [21]. Furthermore,

both a soluble fraction, related to chlorides and sulphates,

and loss of ignition, imputable to inorganic and organic

carbon are present. These phases have also been confirmed

by XRD analysis (see below). In a previous paper [17], the

leaching of Ba, Zn, Pb, Cu, Cr and Ni was analysed to

evaluate the hazardousness of the BA. All the values were

below the regulation limits for disposing on landfill for not

dangerous wastes. For this reason geopolymers containing

this BA are not further subjected to leaching test.

SEM images of as-received LS and BA (Fig. 1a, c)

present the starting raw materials used for the preparation

of the specimens. Particles size distribution is particularly

wide in both the samples promising a good packing ability

of the two powders. LS presents irregularly shaped grains

(Fig. 1a) as well as more spherical particles according to

the high temperature environment in which they were

generated.

Fig. 1 SEM images of

untreated LS and BA (a) and

(c) and geopolymers

MK30_LS70 with

corresponding Si/Al and Si/Ca

ratios by EDS analysis (b),

MK30_BA70 cured for 30 days

(d), and MK30_BA70R cured

for 15 days with corresponding

Si/Al ratio by EDS analysis (e)

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Freshly fractured surfaces of geopolymers containing

70 wt% of LS or BA are reported in Fig. 1b, d, e,

respectively. In LS based geopolymer, un-reacted slag

particles are tightly connected to the matrix, however two

gel morphologies are clearly distinguished. The EDS ana-

lysis has highlighted the presence of two Ca–Na–alumi-

nosilicate (C–N–A–S–H) gels with different chemical

compositions: one gel is richer in Si, Al and Na and poorer

in Ca, compared to the other gel. In particular, calculating

the Si/Al and Si/Ca weight ratio, it has been determined

that the Si/Al ratio is quite similar for both gels (1.5 and

1.6), whereas the Si/Ca ratio changes from 0.6 to 1.3. These

findings agree with EDS analysis previously reported [14]

for alkali activated mixes containing larger content of MK.

Indeed, the dissolution of LS particles promotes the ion

exchange between sodium and calcium, leading to gel

richer in calcium with the increase of LS in the

formulation.

Comparing the two morphologies of BA and corre-

sponding geopolymer reported in Fig. 1c, d, it can be seen

that after 30 days curing the geopolymeric matrix appears

denser for the formation of gel, but particles of 30–50 lm

are still evident. This observation demonstrates that

incinerator ash is reactive in alkaline environment and can

be transformed in geopolymers, but the optimization of the

formulation can help to obtain complete reactivity. Fig-

ure 1e shows the microstructure of geopolymer containing

BA after reformulation cured for only 15 days, ash parti-

cles are still evident but a larger amount of geopolymeric

gel is formed, confirming that a Si/Al ratio near to 2 leads

to more extended reactivity. Furthermore, from EDS ana-

lysis it can be observed that the values of the Si/Al ratios

analysed in two areas are near to the theoretical value of

2.06 (reported in Table 2), confirming the high reactivity of

BA. For sake of comparison, in the sample before refor-

mulation the theoretical value of Si/Al, considering the

overall chemical composition only, was 3.26, but from

EDS analysis only an average value of 1.5 was found,

confirming the lower reactivity of the ash in this condition.

Moreover, higher amount of calcium (about 3 wt%) is

dissolved in the gel versus 1.3 wt% recorded for the initial

formulated geopolymers; with respect to the value

observed for LS the content of Ca in the gel remains lower.

Ladle slag exhibits a complex structure (Fig. 2) where

an amorphous matrix coexists with crystalline phases, such

as olivine (c-C2S Ca2SiO4 ICDD #180-941), gehlenite

(Ca2(Al(AlSi)O7) ICDD #174-1607), mayenite, syn

(Ca12Al14O33 ICDD #170-2144) and iron–magnesium–

calcium–silicate as previously described [14]. Traces of:

(1) periclase (MgO, ICDD #45-0946), from the ladle

refractory lining; (2) oldhamite (CaS, ICDD #38-1420),

from the desulfuration process; and (3), b-C2S (Ca2SiO4),

Ca(OH)2, calcium and magnesium carbonates have also

been identified [18]. The large extent of crystalline phases

makes difficult the quantitative determination of the

amorphous phases, as reported in the literature [18–22].

Nevertheless, LS are alkali activated by NaOH solution

when mixed with MK in proper amounts. Indeed, MK

brings reactive silica and alumina in the mix thus allowing

the geopolymerization process, which is very difficult

when only LS is used as its oxides are mainly in crystalline

form.

Figure 2 reports also MK30_LS70 XRD pattern, char-

acterized by an amorphous structure (broad diffraction

hump from 25� to 37� 2h) with peaks corresponding to

calcite, mayenite, cristobalite and periclase. Although dif-

ferent types of slag, reacting with water, are able to form

hydrate products similar to C–S–H [23, 24], LS exhibits

Fig. 2 XRD patterns of as-

received LS and geopolymer

MK30_LS70 after 30 days of

curing (O olivine, P periclase,

M mayenite, C calcite, Q quartz,

cr cristobalite, G gehlenite)

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binding property mainly in the presence of an alkaline

activator and LS reactivity increases with the decrease of

the particle size [25]. The chemical nature of thephases

formed in LS/MK geopolymers has been reported else-

where [14], but we can mention that it was demonstrated

from XRD patterns and EDS analysis that C–A–S–H and

3D aluminosilicate (N–A–S–H) network can coexists in

different extent. Ion exchange between sodium and calcium

also modifies the structure typical of metakaolinitic geo-

polymers. Recent studies carried out on synthetic sodium

aluminosilicate hydrate gel [26] show the existence of a

C–A–S–H gel more stable than N–A–S–H at high pH.

Figure 3a reports the XRD pattern of the MK, the as-

received BA and its corresponding geopolymer. The wide

amorphous hump of BA sample (more significant with

respect to LS) indicates its nature being both amorphous

and crystalline.

The crystalline phases present in the starting BA, aquartz (a-SiO2, JCPDF file 33-1161), calcite (CaCO3,

ICCD #5-586), anhydrite (CaSO4, ICCD #37-1496), gehl-

enite (Ca2Al(Al,Si)O7, ICCD #35-755) and plagioclase

((Na,Ca)(Si,Al)4O8), are still present in the consolidated

product (except for calcium sulfate, due to its solubility)

together with muscovite, present in MK.

Guo et al. [27] found as newly phase formed in geo-

polymers containing waste materials containing calcium,

gismondine (CaAl2Si2O8. 4H2O, ICCD #20-452), corre-

sponding to calcium aluminum silicate hydrate. In the

sample investigated in this paper the presence of gismon-

dine is difficult to identify with certainty because all its

peaks overlap with the peaks of quartz. The formation of

gismondine could be due to the main reactivity of calcium

present in the amorphous fraction in the ash rather than

calcium in crystalline phases. The higher reactivity of

amorphous phase is evidenced in Fig. 3b where is reported

the comparison of XRD patterns of ash and ash treated in

NaOH. It appears evident the significant decrease of the

amorphous hump after the treatment with NaOH 8 M

confirming its higher reactivity in the geopolymers

environment.

Pore size distributions, reported in Fig. 4, were strongly

influenced by the waste based precursors used for geopo-

lymerization. Indeed, LS and BA geopolymers exhibit a

very different behavior: the former shows the lowest total

intruded volume with the most part of pore sizes in the

range of 0.010–0.10 lm, whereas the latter has large pores

and elevate intruded volume, thus indicating a large open

porosiy.

Pore size distributions were strongly influenced by feed

composition. For LS based geopolymers, by increasing the

content of LS, porosity curves move towards pores with

large dimensions. As LS promotes the formation of cal-

cium rich gel, it can be hypothesized that such a gel is

responsible for porosity increase. Indeed, both LS60-MK40

as well as LS70-MK30 exhibits two types of gel, as

reported [14].

Fig. 3 XRD patterns of a MK, BA and geopolymer MK30_BA70

after 30 days of curing and b of BA before and after treatment in

NaOH 8 M

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BA based geopolymers exhibit a different behavior as

function of the different amounts of sodium hydroxide and

sodium silicate solutions used in the mix. The absence of

C–A–S–H phase generates a more porous structure, which,

in its turn, is strictly influenced by the amount of Al and Na

available for geopolymerization process. In fact BA70-

MK30 presents a porosity mainly concentrated in the range

of 0.010–0.21 lm, whereas the reformulation of the alka-

line solutions in BA70-MK30R lead to a geopolymer with

a bimodal pores distributions in the range of 0.010–0.10

and 10–35 lm.

Figure 5 shows the differential thermogravimetric

(DTA/TG) curves of BA based samples. Both geopolymers

show mass loss as a function of temperature increase. An

average mass reduction of 12 %, occurred over the tem-

perature range of 100–1,400 �C, was recorded for geo-

polymers as observed by other authors [28, 29]. This loss is

significantly lower than that of pure MK geopolymer (30 %

mass reduction). The main weight loss is registered at

temperatures below 250 �C and it is imputable to water, the

exposure, in fact, to high temperatures leads to changes in

chemical structure and the dehydration of free and chem-

ically-bound water. As the external temperature increases,

moisture within the specimen rapidly migrates towards the

surface of the specimen and escape. The mass loss at about

250–600 �C is attributed to the removal of more tightly

bound water and probably also reflects the decomposition

of hydroxides formed in alkaline environment from the

soluble salts of metals. At 700 �C, weight decrease,

imputable to decomposition of calcium carbonate, is evi-

dent and is related to the presence of CaCO3 both in

incinerator ash as well as in geopolymers as confirmed by

XRD pattern showed in Fig. 3.

Samples MK30_BA70 and MK30_BA70R, before and

after reformulation, do not evidence significant differences

in the thermal properties. Small peaks in the range between

650 and 800 �C for the sample before reformulation shows

the higher presence of carbonates both residual from BA

and formed by the reaction with atmospheric carbon

dioxide. These results are confirmed by FT-IR spectros-

copy (see below).

The differential thermogravimetric (DTA/TG) curves

for LS70-MK30 and LS60-MK40 (Fig. 6) show higher

weight loss than BA based geopolymers. As XRD and

porosity data suggest, the total weight loss is influenced by

the LS content in the feed: indeed increasing the LS

amount, the weight % loss moves from 15 % for LS60-

MK40 to 20 % for LS70-MK30. As previously described,

the main contribution to the weight loss (up to 650 �C) is

due to dehydration of free and chemically bonded water.

For these materials the dehydration of C–A–S–H phase

must also be taken into account together with the decom-

position of Ca(OH)2 occurring in the range of 350–450 �C

(this peak is particularly evident for LS70-MK30 geo-

polymer which is rich in Ca thanks to the high content of

LS). The weight loss detected between 700 and 750 �C is

due to calcium carbonate, present in traces in the starting

LS.

The comparison between geopolymers MK30_BA70

without and with reformulation is reported in FT-IR spectra

(Fig. 7). For both the samples a common feature is

observed:

• strong Si–O–T (T = Si, Al) asymmetrical stretching

peak at about 1,000 cm-1; by comparing this signal to

which of the mixture of raw materials (MK and BA)

before alkali activation (1,035 cm-1), reported in a

previous paper [17] appears evident the shift towards

lower values, implying a chemical change in the matrix,

indicating the degree of geopolymerization associated

to the inclusion of tetrahedrally-coordinated Al in the

Si–O–Si skeletal structure as observed also by other

Fig. 4 Pore size distributions of the investigated geopolymers

Fig. 5 DTA/TG curves of geopolymers MK30_BA70 before and

after reformulation

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authors [16, 30]. No signals near 900 cm-1, related to

Al(VI)–OH stretching vibrations, are visible, as

observed for examples by the authors in MK geopoly-

mers [31], notwithstanding traces of residual muscovite

are identified by XRD results in MK30_BA70. The

presence of Al(VI) signal could be related to low

geopolymerization degree in the samples.

• band at 1,460 cm-1, typical of carbonate species,

probably sodium carbonate, derived from the reaction

of free Na? ions with atmospheric carbon dioxide. In

sample with reformulation this band is less intense

probably due to the lower amount of Na2CO3 formed

by unreacted NaOH as consequence of the best

tailoring of mixture. This band was also observed by

the authors in MK geopolymers moulded and cured in

similar conditions (room temperature and polymeric

mould) [31].

• low intensity band at 875 cm-1, already attributed to

CaCO3 by other authors [32], therefore this signal can

be related to the presence of carbonates in geopolymer

as a consequence of their presence in BA.

Conclusions

The valorisation of LS and BA derived from the inciner-

ation of bottom ashes has been investigated via alkali

activation using a concentrated NaOH and sodium silicate/

water glass solutions.

The consolidated materials, aslo known as geopolymers,

contain high percentages (60 and 70 wt%) of LS and BA

derived from the incineration of urban wastes. Their mor-

phology is very close to that of pure metakaolinic pastes,

i.e. structural 3D geopolymers, although calcium presence

in the precursors tend to promote the formation of calcium–

aluminate–silicate–hydrate phase. There is a complex

evolution of the amorphous as well as the crystalline por-

tion of the wastes.

According to the literature [22], the presence of calcium

coming from LS and BA can promote an ion exchange

process with sodium, replacing it in the N–A–S–H gel and

consequently forming C–A–S–H. The structure of the C–

A–S–H gels has not been investigated in this paper, how-

ever it has been reported that the 3D structure typical of N–

A–S–H is usually preserved at these pH values [13].

This investigation has also demonstrated that the

determination of reactive fraction of BA is of primary

importance when assessing the possible use of this raw

material in the alkaline activation process. In particular, the

reactive Si/Al ratio is an important parameter to take into

account for a proper geopolymers formulation [18]. For

BA, significant differences have been observed in the Si/Al

ratios and microstructure with or without reformulation by

considering reactive Si/Al due to the presence of both

amorphous and crystalline fractions with a different degree

of reactivity.

Acknowledgments Authors are grateful to dr. Mirko Braga and dr.

Pasquale Pansini, from R.S.A. Laboratory, INGESSIL S.r.l., Monto-

rio (Verona, Italy) for supplying sodium silicate solutions.

References

1. Crompton, P.: The diffusion of new steelmaking technology.

Resour. Policy 27(2), 87–95 (2001)

2. Shi, C.: Characteristics and cementitious properties of ladle slag

fines from steel production. Cem. Concr. Res. 32, 459–462 (2002)

3. Engh, T.A., Simensen, C.J., Wijk, O.: Principles of Metal

Refining. Oxford University press, Oxford (1992)

4. Kourounis, S., Tsivilis, S., Tsakiridis, P.E., Papadimitriou, G.D.,

Tsibouki, Z.: Properties and hydration of blended cements with

steelmaking slag. Cem. Concr. Res. 37(6), 815–822 (2007)

Fig. 6 DTA/TG curves of geopolymers LS70-MK30 and LS60-

MK40

Fig. 7 FT_IR spectra of MK30_BA70 before and after reformulation

Waste Biomass Valor

123

Author's personal copy

5. Kirschen, M., Badr, K., Pfeifer, H.: Influence of direct reduced

iron on the energy balance of the electric arc furnace in steel

industry. Energy 36(10), 6146–6155 (2011)

6. Danilov, E.V.: Modern technology for recycling steelmaking

slags. Metallurgist 47(5–6), 232–234 (2003)

7. Bignozzi, M.C., Sandrolini, F., Andreola, F., Barbieri, L.,

Lancellotti, I.: recycling electric arc furnace slag as unconven-

tional component for building materials. In: Proceeding of 2nd

International Conference on Sustainable Construction Materials

and Technologies, 28–30th June 2010, Ancona (Italy), vol. 1,

pp. 557–567 (2010)

8. Natali, A., Manzi, S., Bignozzi, M.C.: Novel fiber-reinforced

composite materials based on sustainable geopolymer matrix.

Procedia Eng. 21, 1124–1131 (2011)

9. Van Caneghem, J., Brems, A., Lievens, P., Block, C., Billen, P.,

Vermeulen, I., Dewil, R., Baeyens, J., Vandecasteele, C.: Fluid-

ized bed waste incinerators: design, operational and environ-

mental issues. Prog. Energy Combust. Sci. 38(4), 551–582 (2012)

10. Lapa, N., Barbosa, R., Morais, J., Mendes, B., Mehu, J., Santos

Oliveira, J.F.: Ecotoxicological assessment of leachates from

MSWI bottom ashes. Waste Manag 22, 583–593 (2002)

11. Lancellotti, I., Kamseu, E., Michelazzi, M., Barbieri, L., Corradi,

A., Leonelli, C.: Chemical stability of geopolymers containing

municipal solid waste incinerator fly ash. Waste Manag 30,

673–679 (2010)

12. Lin, C., Weng, M., Chang, C.: Effect of incinerator bottom-ash

composition on the mechanical behavior of backfill material.

J. Environ. Manag. 113, 377–382 (2012)

13. Astrup, T.: Pretreatment and utilization of waste incineration

bottom ashes: Danish experiences. Waste Manag 27, 1452–1457

(2007)

14. Bignozzi, M.C., Manzi, S., Lancellotti, I., Kamseu, E., Barbieri,

L., Leonelli, C.: Mix-design and characterization of alkali acti-

vated materials based on metakaolin and ladle slag. Appl. Clay

Sci. 73, 78–85 (2013)

15. Bernal, S.A., Rodrıguez, E.D., de Gutierrez, R.M., Provis, J.L.,

Delvasto, S.: Activation of metakaolin/slag blends using alkaline

solutions based of chemically modified silica fume and rice husk

ash. Waste Biomass Valorization 3(1), 99–108 (2012)

16. Onori, R., Will, J., Hoppe, A., Polettini, A., Pomi, R., Boccaccini,

A.R.: Bottom ash-based geopolymer materials: mechanical and

environmental properties. In: Ceramic Engineering and Science

Proceedings, vol. 32, pp. 71–82. Wiley, NJ, USA (2011)

17. Lancellotti, I., Ponzoni, C., Barbieri, L., Leonelli, C.: Alkali

activation processes for incinerator residues management. Waste

Manag 33, 1740–1749 (2013)

18. Natali Murri, A., Rickard, W.D.A., Bignozzi, M.C., van Riessen,

A.: High temperature behaviour of ambient cured alkali-activated

materials based on ladle slag. Cem. Concr. Res. 43, 51–61 (2013)

19. Ruiz-Santaquiteria, C., Fernandez-Jimenez, A., Palomo, A.:

Quantitative determination of reactive SiO2 and Al2O3 in alu-

minosilicate materials. In: Proceedings of XIII International

Congress on the Chemistry of Cement, Madrid, Spain (2011)

20. Aly, Z., Vance, E.R., Perera, D.S., Hanna, J.V., Griffith, C.S.,

Davis, J., Durce, D.: Aqueous leachability of metakaolin-based

geopolymers with molar ratios of Si/Al = 1.5–4. J. Nucl. Mater.

378, 172–179 (2008)

21. Duxson, P., Mallicoat, S.W., Lukey, G.C., Kriven, W.M., van

Deventer, J.S.J.: The effect of alkali and Si/Al ratio on the

development of mechanical properties of metakaolin-based geo-

polymers. Coll. Surf. A 292, 8–20 (2007)

22. Bignozzi, M.C., Sandrolini, F., Barbieri, L., Lancellotti, I.: New

geopolymers based on electric arc furnace slag. Adv. Sci. Tech-

nol. 69, 117–122 (2010)

23. Pal, S.C., Mukherjee, A., Pathak, S.R.: Investigation of hydraulic

activity of ground granulated blast furnace slag in concrete. Cem.

Concr. Res. 33, 1481–1486 (2003)

24. Srinivasa Reddy, A., Pradhan, R.K., Chandra, S.: Utilization of

basic oxygen furnace (BOF) slag in the production of a hydraulic

cement binder. Int. J. Miner. Process. 79(2), 98–105 (2006)

25. Shi, C.: Characteristics and cementitious properties of ladle slag

fines from steel production. Cem. Concr. Res. 32(3), 459–462

(2002)

26. Garcia-Lodeiro, I., Palomo, A., Fernandez-Jimenez, A., Mac-

phee, D.E.: Compatibility studies between N-A–S–H and C–A–

S–H gels. Study in the ternary diagram Na2O–CaO–Al2O3–

SiO2–H2O. Cem. Concr. Res. 41(9), 923–931 (2011)

27. Guo, X., Shi, H., Dick, A.: Compressive strength and micro-

structural characteristics of class C fly ash geopolymer. Orig.

Cem. Concr. Compos. 32(2), 142–147 (2010)

28. Xu, H., Li, Q., Shen, L., Wang, W., Zhai, J.: Synthesis of ther-

mostable geopolymer from circulating fluidized bed combustion

(CFBC) bottom ashes. J. Hazard. Mater. 175(1–3), 198–204

(2010)

29. Subaer, A., van Riessen, A.: Thermo-mechanical and micro-

structural characterization of sodium-poly(sialate-siloxo) (Na-

PSS) geopolymers. J. Mater. Sci. 42, 3117–3123 (2007)

30. Catauro, M., De Gaetano, F., Marotta, A.: Devitrification

behaviour of CaO rich calcium silicate glasses. Phys. Chem.

Glasses 44(1), 39–41 (2003)

31. Lancellotti, I., Catauro, M., Ponzoni, C., Bollino, F., Leonelli, C.:

Inorganic polymers from alkali activation of metakaolin: effect of

setting and curing on structure. J. Solid State Chem. 200,

341–348 (2013)

32. Zhang, J., Provis, J.L., Feng, D., van Deventer, J.S.J.: Geopoly-

mers for immobilization of Cr6?, Cd2?, and Pb2?. J. Hazard.

Mater. 157(2–3), 587–598 (2008)

Waste Biomass Valor

123

Author's personal copy