mine als
A icle
Impac o Fly Ash as a Raw Ma e ial on he P ope ies
o Re ac o y Fo s e i e–Spinel Ce amics
Ma in Nguyen * and Radomí Sokoláˇ
Facul y o Ci il Enginee ing, Ins i u e o Technology o Building Ma e ials and Componen s,
B no Uni e si y o Technology, Ve eˇ í331/95, 602 00 B no, Czech Republic; sokola [email p o ec ed].cz
*Co espondence: [email p o ec ed].cz
Recei ed: 17 Augus 2020; Accep ed: 21 Sep embe 2020; Published: 22 Sep embe 2020


Abs ac :
This a icle examines he p ocess o he syn hesis o o s e i e–spinel
(2MgO
·
SiO
2
/MgO
·
Al
2
O
3
) e ac o y ce amics om ly ash and alumina as sou ces o aluminum
oxide. Raw ma e ials we e milled, mixed in di e en a ios and sin e ed a 1500
◦
C o 2 h.
Sin e ed samples we e cha ac e ized by XRD, he mal analyses and SEM. Po osi y, wa e abso p ion,
bulk densi y, e ac o iness, e ac o iness unde load and he mal shock esis ance we e also
in es iga ed. The impac o ly ash as a aw ma e ial was in es iga ed in acco dance wi h he esul ing
p ope ies and mic os uc u e o samples wi h ly ash and alumina as he aw ma e ials. Due o he
posi i e e ec o lux oxides (i on oxides and alkalis) on sin e ing, he mulli e con ained in ly ash
comple ely decomposed in o silica and alumina, which, oge he wi h magnesium oxide, o med
spinel. This led o imp o ed mic os uc u al and mechanical p ope ies and he mal shock esis ance.
In pa icula , mix u es wi h 10 w .% and 20 w .% o ly ash had he mos p omising esul s compa ed
o alumina mix u es. Bo h modulus o up u e and he mal shock esis ance we e imp o ed, while
he impac on e ac o y p ope ies was minimal. The no el y o his esea ch lies in he ecycling
o ly ash, a by-p oduc om coal-bu ning powe plan s, in o a aw ma e ial o he p oduc ion o
o s e i e–spinel e ac o y ce amics.
Keywo ds: sin e ing; spinel; o s e i e; e ac o y ce amics; ly ash; alumina
1. In oduc ion
Re ac o y o s e i e ce amics a e an impo an ype o ma e ial and a e used in he me allu gical
and cemen indus ies as he lining o me allu gical u naces and o a y kilns due o hei high mel ing
poin o 1890
◦
C [
1
–
3
]. Fo s e i e is also used in elec o echnical enginee ing o ce amic–me al join s
because o i s high he mal expansion coe icien , which is simila o he coe icien o me als [
4
,
5
].
To da e, a ious s udies ha e been conduc ed on he use o o s e i e as a c ys al in ing ca i y
lase s [
6
], as a p omising bioma e ial o bone ansplan s [
7
,
8
] and as a ma e ial o composi e
nanoma e ials [9–11].
As a esul o i s high he mal expansion coe icien , o s e i e has a low he mal shock esis ance.
This can be imp o ed by he addi ion o magnesium–alumina spinel (MA-spinel; commonly abb e ia ed
as spinel), which imp o es he mic os uc u e, he mechanical p ope ies and he he mal shock
esis ance [
12
,
13
]. Spinel can be syn hesized om aluminum and magnesium oxides. Gene ally,
spinel is syn hesized om alumina o bauxi e and magnesium oxide [
9
,
14
,
15
]. Howe e , ly ash, which
is ob ained as a seconda y p oduc om coal-bu ning powe plan s, can be used as an inexpensi e
sou ce o aluminum and silicon oxides, which in u n can be used as aw ma e ials o sin e ing
e ac o y ce amics [
16
–
21
]. Despi e he u iliza ion o ly ash in he syn hesis o aluminosilica e
e ac o ies, li le esea ch has been ca ied ou on he use o ly ash in he syn hesis o o he e ac o ies
con aining silicon and/o aluminum oxides, such as o s e i e–spinel ce amics. The mulli e-con aining
Mine als 2020,10, 835; doi:10.3390/min10090835 www.mdpi.com/jou nal/mine als
Mine als 2020,10, 835 2 o 12
ly ash decomposes in o silica and alumina in he p esence o lux oxides, and hese, oge he wi h
magnesium oxide, o m spinel, as p o en by bo h he li e a u e [
17
–
19
] and p e ious esea ch [
22
,
23
].
Magnesium–alumina spinel is used wi h magnesia as a lining in cemen and lime kilns because o i s
high mel ing poin o 2135
◦
C, low he mal expansion coe icien in compa ison wi h o s e i e and
good he mal shock, chemical and co osion esis ance [9,12,13].
The main objec i e o his wo k was o de e mine he impac o ly ash as a aw ma e ial on
he syn hesis o e ac o y o s e i e–spinel ce amics. The no el y o his wo k lies in he ecycling
o ly ash, a was e p oduc o coal-bu ning powe plan s, in o a aw ma e ial o he p oduc ion o
e ac o y o s e i e–spinel ce amics, while compa ing he esul ing p ope ies wi h he syn hesis o his
e ac o y ce amic om alumina as a aw ma e ial. Physico-mechanical p ope ies, phase composi ion,
mic os uc u e, he mal analyses and e ac o y and mechanical p ope ies we e in es iga ed.
2. Ma e ials and Me hods
2.1. Raw Ma e ials
Raw ma e ials we e ob ained om mul iple sou ces. Oli ine om he No wegian company A/S
Oli in, calcined caus ic magnesi e (CCM 85) om SMZ Jelša a (Slo akia), coal ly ash class F (acco ding
o s anda d ASTM C618; d
50
=14
µ
m) om powe plan Mˇeln
í
k (Czech Republic), eac i e alumina
CTC 22 (d
50
=1.9
µ
m) om Alma is (Ge many) and kaolin Sedlec Ia (d
50
=1.3
µ
m) om Sedleck
ý
kaolin (Czech Republic). Table 1p esen s he chemical composi ion o he used ma e ials, which was
de e mined by chemical composi ion analysis and X- ay luo escence (XRF).
Table 1. The chemical composi ion o used aw ma e ials.
Raw Ma e ials MgO
[%]
SiO2
[%]
Al2O3
[%]
CaO
[%]
Fe2O3
[%]
K2O+
Na2O
[%]
LOI *
[%]
CCM 85 85.0 0.5 0.8 5.2 7.30 0.20 1.0
Oli ine 24.1 64.7 1.0 0.7 8.80 0.50 1.0
Fly ash 1.4 57.3 29.3 2.2 5.10 1.70 1.2
Alumina 0.0 0.0 99.7 0.0 0.03 0.12 0.1
Kaolin 0.5 46.8 36.6 0.7 0.85 1.20 13.2
* Loss on igni ion.
A o al o eigh di e en mix u es we e designed and di ided in o wo se s, depending on he
sou ce o aluminum oxide (Al
2
O
3
) o he syn hesis o spinel. The i s se o ou mix u es had ly
ash (FA10-FA40) as he sou ce o Al
2
O
3
, and he second se o ou mix u es had eac i e alumina
(RA10-RA40) as he sou ce o Al
2
O
3
. The quan i y o ly ash in he mix u es chosen anged om
10–40 w .% wi h 10 w .% inc emen s and he quan i y o alumina was calcula ed o ma ch he exac
con en o Al
2
O
3
p esen in he ly ash mix u es. The designed composi ions o all he mix u es used o
ob ain s oichiome ic o s e i e a e p esen ed in Table 2.
Table 2. Designed composi ions o all aw ma e ial mix u es.
Mix u es CCM 85
[w .%]
Oli ine
[w .%]
Fly Ash
Mˇelník [w .%]
Alumina CTC
22 [w .%]
Kaolin Sedlec
Ia [w .%]
FA10 43.2 41.8 10.0 - 5.0
RA10 41.0 51.1 - 2.9 5.0
FA20 44.1 30.9 20.0 - 5.0
RA20 39.8 49.4 - 5.8 5.0
FA30 45.0 20.0 30.0 - 5.0
RA30 38.6 47.8 - 8.6 5.0
FA40 45.9 9.1 40.0 - 5.0
RA40 37.4 46.1 - 11.5 5.0
Mine als 2020,10, 835 3 o 12
The e na y phase diag am o he MgO-Al
2
O
3
-SiO
2
sys em is p esen ed in Figu e 1wi h all
designed mix u es plo ed on he line be ween o s e i e and spinel.
Figu e 1. Te na y phase diag am o MgO-Al2O3-SiO2sys em wi h plo ed mix u es.
Figu e 2A–C ep esen he scanning elec on mic oscope (SEM, Tescan Mi a 3, Tescan O say
Holding a.s., B no, Czech Republic) mic opho og aphs o un ea ed oli ine, alumina and ly ash. The e
is an appa en ib ous mic os uc u e o un ea ed oli ine. Alumina was inely g ound o he desi ed
pa icle size, whe e d
50
=1.9
µ
m. Fly ash has i s ypical sphe ical pa icles wi h a diame e be ween
0.4–90 µm (d50 =14 µm).
Figu e 2.
SEM mic opho og aphs o selec ed aw ma e ials: (
A
) oli ine, (
B
) alumina and (
C
) ly ash;
magni ica ion =2000×.
Mine als 2020,10, 835 4 o 12
The mine alogical composi ion o he aw ma e ials is p esen ed in Figu e 3. CCM 85 was p ima ily
composed o pe iclase wi h ace quan i ies o i on compounds. The majo c ys al phase in oli ine was
o s e i e (2MgO
·
SiO
2
) wi h mino phases o ayali e (2FeO
·
SiO
2
), se pen ini e (3MgO
·
2SiO
2·
2H
2
O)
and qua z (SiO
2
). The Class-F ly ash was p ima ily composed o mulli e (3Al
2
O
3·
2SiO
2
) and qua z
(SiO
2
). Bo h oli ine and ly ash had backg ound cu a u e, indica ing he p esence o an amo phous
glass phase. Reac i e alumina was composed o almos pu e co undum (Al
2
O
3
), and kaolin was
p ima ily kaolini e (Al2O3·2SiO2·2H2O) wi h aces o bio i e (K(Mg,Fe)3AlSi3O10(F,OH)2).
Figu e 3. X- ay di ac ion analysis o he used aw ma e ials.
2.2. Expe imen
All mix u es we e p epa ed by milling oli ine and CCM o a pa icle size ha anged be ween 1 and
80
µ
m (whe e d
50
=10–20
µ
m) and hen mixing wi h o he ine aw ma e ials. Pa icle size dis ibu ion
was de e mined by lase g anulome y (Mal e n Mas e size , Mal e n Panaly ical, Mal e n, Uni ed
Kingdom). Mix u es we e hen homogenized in a o a y mechanical homogenize o 24 h, hen mixed
wi h wa e o gain he op imal plas ici y using P e e ko n appa a us (s anda d ˇ
CSN 72 1074). Samples
we e hen molded in o b ass molds om plas ic pas e. The app oxima e dimensions o he es samples
we e 20
×
25
×
100 mm
3
p isms, cylinde s wi h a diame e o 50 mm and a heigh o 50 mm o
e ac o iness unde load and 230
×
64
×
54 mm
3
p isms (dimensions o hal o a s anda d b ick) o
he mal shock esis ance. The py ome ic cones used o he e ac o iness expe imen s (s anda d EN
993-13:1995) we e also p epa ed om he same mix u es in a se o h ee o each mix u e. All es
samples we e hen d ied in a labo a o y d ye a 105
◦
C. A e d ying, es samples we e i ed in
a labo a o y u nace wi h an ai a mosphe e a 1500
◦
C, which used a hea ing a e o 4 K/min and
a soaking ime o 2 h a maximum empe a u e.
Tes samples we e hen subjec ed o se e al expe imen al p ocedu es ha assessed he
appa en po osi y, wa e abso p ion and bulk densi y, which we e de e mined by a acuum wa e
abso p ion me hod wi h subsequen hyd os a ic weighing (s anda d EN 993-1:1995); he mal analyses:
he mog a ime y (TG), de i a i e he mog a ime y (DTG; Me le Toledo TGA/DSC1, s anda d
ˇ
CSN 72 1083); and change in dimension du ing i ing (s anda d EN 993-10:1997). Re ac o iness o
py ome ic cones (s anda d EN 993-12:1997) was pe o med wi h a se o h ee iden ical py ome ic
cones om each mix u e in a labo a o y u nace wi h an obse a ion po wi h a came a which enabled
eal- ime obse a ion in he u nace and allowed o he cap u e o he exac momen o he bending o
py ome ic cones. Re ac o iness unde load (s anda d ISO 1893:2007) was pe o med on cylind ical
samples acco ding o he s anda d and he empe a u e a 0.5% de o ma ion (T0.5) was measu ed.
Mine als 2020,10, 835 5 o 12
De e mina ion o he mal shock esis ance was ca ied ou acco ding o s anda d EN 993-11:2007
me hod B, which de ines pa ame e “ esidual modulus o up u e (MOR)”, his pa ame e enables
a quan i a i e app oach o measu ing he mal shock esis ance. I is a a io be ween he modulus o
up u e (MOR) o cycled samples and he MOR o samples a a no mal labo a o y empe a u e o
25
◦
C. Cycling o samples was done acco ding o he s anda d EN 993-11:2007. The mal dila ome ic
analysis (s anda d EN 993-19:2004); modulus o up u e (MOR; Tes ome ic M350-20CT, s anda d EN
993-6:1995); and X- ay di ac ion analysis (XRD; Panaly ical Empy ean, PANaly ical B.V., Almelo,
Ne he lands) ha used CuK
α
as a adia ion sou ce, an accele a ing ol age o 45 kV and a beam cu en
o 40 mA and SEM wi h an EDX p obe (Tescan Mi a 3, Tescan O say Holding a.s., B no, Czech Republic)
we e used o de e mine mo phology and elemen al analysis o he c ys al s uc u e.
3. Resul s and Discussion
The main aim o his pape was o in es iga e he impac o ly ash as he aw ma e ial used
du ing p oduc ion on he esul ing p ope ies o e ac o y o s e i e–spinel ce amics. A compa ison o
mix u es ha used ly ash wi h mix u es ha used alumina wi h equal p opo ions o aluminum oxide
was in es iga ed.
3.1. Mine alogical Composi ion and Mic os uc u e
The X- ay di ac ion (XRD) analysis o samples o all he designed mix u es wi h an equal
p opo ion o Al
2
O
3
con en is p esen ed in Figu e 4. All mix u es con ained o s e i e (2MgO
·
SiO
2
),
spinel (MgO
·
Al
2
O
3
), pe iclase (MgO) and mon icelli e (CaO
·
MgO
·
SiO
2
) mine als. The cu ed
backg ound o he XRD di ac og am also indica ed he p esence o an amo phous glass phase,
and he backg ound noise indica ed he p esence o i on oxides (Fe
2
O
3
—hema i e, Fe
3
O
4
—magne i e)
due o he use o CuKαas a adia ion sou ce.
Figu e 4.
Compa ison o he X- ay di ac ion analysis o all designed samples wi h ly ash (FA10-FA40)
o alumina (RA10-RA40).

Mine als 2020,10, 835 6 o 12
As can be seen in Figu e 3, XRD analysis con i med he p esence o mulli e in ly ash. Howe e ,
no aces o mulli e we e ound in he XRD o i ed samples which used ly ash (Figu e 4). A he
same ime, XRD analysis con i med he p esence o co undum in alumina (Figu e 3), bu no aces
o co undum we e ound in he XRD o he i ed samples which used alumina. I can he e o e be
concluded ha all mulli e om ly ash, and all co undum om alumina, ans o med in o spinel.
Simila ly, as can be seen om Figu e 3, oli ine con ained mino quan i ies o ayali e (2FeO
·
SiO
2
),
which decomposed a 1205
◦
C in o i on(II) oxide and silica. I on(II) oxide hen oxidized in o
hema i e (Fe
2
O
3
) and magne i e (Fe
3
O
4
), and silica eac ed wi h magnesium oxide in o o s e i e [
24
].
The decomposi ion o ayali e can also be obse ed in he he mal analyses (Figu es 6 and 7) as
an endo he mic peak abo e 1200
◦
C. The p esence o i on oxides was indica ed in he XRD pa e ns
(Figu e 3) as backg ound noise, which was mo e p ominen in samples using ly ash. T ace quan i ies
o mon icelli e (CaO
·
MgO
·
SiO
2
) we e also p esen in all samples due o he calcium oxide con en ,
mainly in CCM (5.2%) and ly ash (2.2%). The p esence o un eac ed magnesium oxide (pe iclase) can
be explained by he ac ha pa o he silicon dioxide ans o med due o he enhanced sin e ing e ec
o lux oxides in o an amo phous glass phase, which is indica ed in Figu e 4as a cu ed backg ound in
he XRD di ac og am.
Mo phology and mic os uc u e we e examined by SEM wi h ene gy-dispe si e X- ay
spec oscopy (EDX), mic opho og aphs a e p esen ed in Figu e 5A–D. Spinel c ys als o med in
pa ches in bo h ly ash and alumina mix u es wi h a diame e ange o 2–4
µ
m. Spinel c ys als we e
loca ed on he edges o he la ge o s e i e ma ix o connec ed o s e i e pa ches oge he . Spinel
c ys als ha o med om he alumina mix u es we e uni o m wi h a smoo h su ace and no c acks,
whe eas spinel c ys als ha o med om ly ash mix u es we e c acked wi h inden ed and i egula
edges. This can be explained by he ac ha spinel c ys als om alumina mix u es o med di ec ly
om magnesium oxide and eac i e aluminum oxide, whe eas spinel c ys als om ly ash mix u es
o med indi ec ly om mulli e decomposi ion in o aluminum oxide and silicon dioxide and subsequen
eac ion be ween aluminum oxide wi h magnesium oxide.
Mine als 2020,10, 835 7 o 12
Figu e 5.
SEM images o mix u es (
A
,
B
) FA20 and (
C
,
D
) RA20 wi h magni ica ions o 2000
×
(le ) and
20,000×( igh ) wi h EDX p obe esul s.
3.2. The mal and The mo-Mechanical Analyses
The mal analyses (Figu es 6and 7) o bo h ly ash and alumina mix u es we e almos iden ical
un il empe a u es eached 1200
◦
C and abo e. Endo he mic peaks on he DTG cu e be ween 100
and 200
◦
C co espond o he loss o physically bound wa e , while peaks be ween 350–380
◦
C and
430–480 ◦C co espond o he dehyd oxyla ion o se pen ini e (Equa ion (1)) [24].
3MgO·2SiO2·2H2O→3MgO·2SiO2+2H2O. (1)
Figu e 6. The mal analyses o mix u es wi h ly ash (FA10-FA40).
Mine als 2020,10, 835 8 o 12
Figu e 7. The mal analyses o mix u es wi h eac i e alumina (RA10-RA40).
Peaks a ound 530
◦
C co espond o a modi ica ion change om
α
-qua z o
β
-qua z, and peaks
a ound 720 ◦C co espond o he dehyd oxyla ion o kaolini e o o m me akaolin (Equa ion (2)):
Al2O3·2SiO2·2H2O→Al2O3·2SiO2+2H2O. (2)
Endo he mic peaks in ly ash mix u es (Figu e 6) a 1260
◦
C and 1400
◦
C co espond o ayali e
(Equa ion (3)) and mulli e (Equa ion (4)) decomposi ion:
2FeO·SiO2→2FeO +2SiO2, (3)
3Al2O3·2SiO2→3Al2O3+2SiO2, (4)
wi h subsequen spinel (Equa ion (5)) and o s e i e (Equa ion (6)) o ma ion om he decomposi ion
p oduc s, along wi h he c ea ion o he liquid glass phase [17,21]:
MgO +Al2O3→MgO·Al2O3, (5)
2MgO +SiO2→2MgO·SiO2. (6)
B oad endo he mic peaks in alumina mix u es (Figu e 7) a 1230–1290
◦
C also co espond o
ayali e decomposi ion, and he peaks a 1450
◦
C co espond o spinel c ea ion om magnesium and
aluminum oxides [17,21,24].
The mal dila ome ic analysis o d ied un i ed samples is p esen ed in Figu e 8. Fo be e cla i y
and demons a ion, only ou samples we e selec ed: FA10 and RA10, which espec i ely had he
lowes quan i ies o ly ash and alumina; and FA40 and RA40, which espec i ely had he highes
quan i ies o ly ash and alumina. No iceable di e ences be ween mix u es wi h ly ash and alumina
we e obse ed abo e 1000
◦
C. Samples wi h ly ash had ou di e en s ages. In he i s s age,
he e was a linea expansion o aw ma e ials ha occu ed up o 1000
◦
C, which was hen ollowed by
a sh inkage s age in he empe a u e in e al o 1000–1250
◦
C. The hi d s age obse ed an expansion
in he empe a u e in e al 1250–1350
◦
C, which was caused by mulli e c ys al g ow h and eac ion
wi h magnesium oxide o o m spinel, leading o he c ea ion o he liquid glass phase. This beha io
was also epo ed in o he wo ks [
17
,
21
] and p e ious esea ch [
22
,
23
]. The ou h s age occu ed a
empe a u es abo e 1350
◦
C and caused e-sh inkage due o he sin e ing p omo ion ac ion o he
liquid glass phase [17,21].
Mine als 2020,10, 835 9 o 12
Figu e 8. Selec ed esul s o he he mal dila ome ic analysis.
Resul s o e ac o iness unde load (Figu es 9and 10) showed ha in mix u es wi h inc easing
amoun s o ly ash, he he mal expansion is sligh ly highe due o he highe con en o he amo phous
glass phase. Wi h inc easing amoun s o ly ash in he mix u e, he empe a u e a 0.5% de o ma ion
dec eased due o he e ec o lux oxides and he amo phous glass phase. Di e en ial cu es o
mix u es wi h alumina we e almos iden ical, including empe a u es a 0.5% de o ma ion. The highe
he mal expansion in alumina mix u es han in ly ash mix u es can be explained by he highe amoun
o un eac ed pe iclase (MgO) which has a highe coe icien o he mal expansion han o s e i e
o spinel.
Figu e 9. Resul s o e ac o iness unde load o mix u es wi h ly ash.