Document Actions

ijege-16_01-nmiri-et-alii.pdf

background image
5
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
DOI: 10.4408/IJEGE.2016-01.O-01
A
hmed
NmIRI
(*)
,
(**)
, O
umAyA
YAZOGHLI-MARZOUK
(***)
, m
yRIAm
DUC
(****)
,
N
OuReddINe
HAMDI
(*)
& e
zzeddINe
SRASRA
(*)
(*)
Centre National des Recherches en Sciences des Matériaux - Laboratoire de Physico Chimie des Matériaux Minéraux et leurs applications
Technopôle Borj Cedria, BP 73 - 8027 Soliman, Tunisia
(**)
Faculté des Sciences de Tunis - Campus Universitaire - 2092 El Manar, Tunis, Tunisia
(***)
CEREMA - Direction Départementale Centre-Est - Département Laboratoire Autun - BP 141 Boulevard Bernard Giberstein - 71404 Autun, France
(****)
Université Paris Est, IFSTTAR, Département Géotechnique, Environnement, Risques Naturels et Sciences de la Terre, 14-20 boulevard Newton -
77447 Marne-la-Vallée cedex 2, Champs-sur-Marne, France. Corresponding author: myriam.duc@ifsttar.fr
TEMPERATURE EFFECT ON MECHANICAL AND PHYSICAL PROPRIETIES OF
Na OR K ALKALINE SILICATE ACTIVATED METAKAOLIN-BASED GEOPOLYMERS
EXTENDED ABSTRACT
Il cemento risulta un materiale edile di alta qualità, usato in tutto il mondo in molti progetti di costruzione. La produzione globale
di cemento è stata stimata nel 2013 attorno a 4 miliardi di tonnellate all’anno. Tale valore è destinato a crescere tra il 2030 e il 2050
fino ai 5 miliardi di tonnallate; un esempio è la Tunisia dove, nel 2013, la produzione crescente raggiunge i 5.88 milioni di tonnellate
all’anno. I processi di fabbricazione del cemento attuale comportano effetti dannosi sull’atmosfera, causando un riscaldamento globale
dovuto all’emissione di grandi quantità di gas ad effetto serra. Il cemento geopolimerizzato, caratterizzato da proprietà meccaniche,
ignifughe e isolanti, puo’ essere utilizzato nell’industria edilizia come cemento ecologico e resistente al fuoco. Questo lavoro tratta della
sintetizzazione di geopolimeri con soluzioni matacaoline (Mk) e alcaline (K
2
SiO
3
or Na
2
SiO
3
miste a KOH o NaOH rispettivamente).
Le prestazioni tecniche dei geopolimeri di tipo (Na-) Mk e (K-) Mk sono state comparate esponendoli prima a trattamenti ad alte tem-
perature. Test di laboratorio sono stati condottti per seguire le loro proprietà fisico-meccaniche e i cambiamenti microstrutturali dopo
l’esposizione ad alte temperature per due ore (200, 400 o 600 °C). Le proprietà meccaniche dei campioni sono state studiate consideran-
do la resistenza alla compressione che puo’ essere vista come un indicatore della qualità del processo di geopolimerizzazione stesso. La
resistenza alla compression dipende dal grado di dissoluzione dei materiali alluminosilicati (come la metacaolina) in soluzioni alcaline.
Particelle non dissolte rimangono legate alla matrice, e impurità presenti nei materiali grezzi possono subire reazioni collaterali che
possono influenzare la cinetica della geopolimerizzazione e quindi la resistenza meccanica del geopolimero ottenuto. La misura della
resistenza alla compressione è stata eseguita su campioni ciclindrici preparati in maniera da avere un rapporto diametro/altezza uguale a
1/2. I campioni sono stati anche caratterizzati con diffrazione ai raggi X (XRD) e via spettrometria a infrarossi (FT-IR); per confermare
l’avvenuto processo di formazione del geopolimero e per determinare le differenze mineralogiche fra i polimeri anche scansioni al mi-
croscopio elettronico (SEM) e porosimetria all’intrusione di mercurio (MIP) sono state eseguite prima e dopo il trattamento termico. I
risultati hanno mostrato che i geopolimeri di tipo Na-Mk esibiscono resistenze alla compression maggiori di quelle dei polimeri di tipo
K-Mk dopo la polimerizzazione a temperature ambiente. L’alta resistenza del polimero Na-Mk sembra essere associata alla mineralogia
(con presenza di faujasite e alluminosilicate di sodio) combinata ad una bassa porosità, a pori di piccole dimensioni (osservati con im-
magini SEM e come dedotto dalla curva di distribuzione ottenuta con MIP). I geopolimeri di tipo Na-Mk sono caratterizzati da un più
alto grado di condensazione rispetto al polimero di tipo K-Mk, grazie ad una maggiore dissoluzione del precursore alluminosilicato in
presenza di cationi Na
+
rispetto ai cationi K
+
. Cio’ spiega la presenza di metacaolina inerte nei geopilimeri K-Mk, identificata via analisi
termiche e con infrarossi. Questa fase è anche responsabile della diminuzione osservata della resistenza meccanica nei polimeri di tipo
K-Mk. Inoltre, il maggior linear shrinkage cosi’ come una perdita significativa di ignizione (essenzialmente acqua fisicamente assorbita
e rilasciata a temperature <400°) osservati per i geopolimeri di tipo Na-Mk rispetto ai polimeri di tipo K-Mk, sembra comunque non
governare il comportamento meccanico del polimero; nessuna microfrattura (potenzialmente osservabile via linear shrinkage) è stata
osservata in microscopio a piccolo scala. I risultati ottenuti dopo la calcinazione messi a confronto con quelli ottenuti a temperature am-
biente dimostrano che la porosità e la densità dei geopolimeri crescono lievemente o rimangono circa costanti cosi’ come la resistenza
a compressione. Tuttavia, la prestazione meccanica diminuisce a 600° (a causa di un aumento della taglia dei pori per effetti termici)
sui polimeri di tipo K-Mk. Questo studio conclude che i geopolimeri di tipo Na-Mk (se comparati a quelli di tipo K-Mk) risultano i più
appropriati per la produzione di cementi geopolimerizzati resistenti alle alte temperature. Questo risultato sembra essere in opposizione
a quanto presentato in letteratura sui geopolimeri basati su ceneri volatili e fra i quali i geopolimeri di tipo K- presentano generalmente
maggiori resistenze al fuoco. Questi tipi di polimeri hanno una grande quantità di piccoli pori che facilitano l’uscita dell’umidità quan-
do il materiale viene riscaldato. Questa rete di pori permette la minimizzazione dei danni alla matrice del geopolimero. L’aumento di
resistenza meccanica nei geopolimeri contenenti ceneri volatili viene parzialmente attribuito alle reazioni di sinterizzazione delle parti-
celle di cenere. Nel caso dei geopolimeri di tipo Mk, la piccola quantità di pori nei geopolimeri di tipo Na-Mk sembra non abbassare la
resistenza termica del materiale rispetto a quella del geopolimero K-Mk.
background image
A. NMIRI, O. YAZOGHLI-MARZOUK, M. DUC, N. HAMDI & E. SRASRA
6
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
ABSTRACT
In this work geopolymeric materials were synthesized in or-
der to reach the best thermal characteristics allowing the use of
them as flame retardant natural stone. The mechanical strength
(notably after high temperature exposure) as well as the micro-
structure were investigated on hardened specimens of geopoly-
mer manufactured at ambient temperature with metakaolin and
a mixture of alkali hydroxide solution (MOH) and alkali silicate
solution (M
2
SiO
3
) with M = K or Na. Geopolymers were subject-
ed to thermogravimetric analysis, scanning electron microscopy
and mercury intrusion porosimetry tests and linear shrinkage,
loss on ignition and compressive strength measurements. Results
showed that Na-Mk geopolymer exhibits higher compressive
strength at ambient temperature after the curing, du to a minor
value of porosity compared to the value for the K-Mk geopoly-
mer. Na-Mk geopolymer gave a geopolymer with a high degree
of condensation compared to K-Mk geopolymer because the dis-
solution of aluminosilicate precursor is still greater in presence
of Na
+
cations than in presence of K
+
cations. The strength of the
Na or K-based geopolymer decreased after exposure to elevated
temperatures (until 600°C). The conclusion of the study is that
Na-Mk geopolymer is the most appropriate for using in the pro-
duction of thermal resistant geopolymer cement than K-Mk based
geopolymer.
K
eywords
: metakaolin, alkaline solution, geopolymer
INTRODUCTION
Portland cement concretes (PCC), subjected to high tempera-
tures such as 800°C or 1000°C, are damaged by dehydration phe-
nomena. CSH hydrates (and some other crystalline compounds)
are deteriorated and that causes a significant decreasing of the
compressive strength down to about 70% (S
INgh
et alii, 2015).
Thermal treatment or fire involves strong temperature gradients
which are at the origin of catastrophic concrete spalling (N
evIlle
,
2011). Geopolymers characterized by a low shrinkage during the
setting, a low permeability, as well as a good durability under
acid attack offer a good alternative to PCC. Geopolymer cement
is incombustible and contains no volatile chemical compounds,
which are at the origin of harmful fumes. It doesn’t release toxic
compounds at high temperatures because of its composition that
consists mainly of silica mixed at ambient conditions with alka-
line solutions.
The effect of thermal treatment (or fire conditions) on me-
chanical and physical proprieties of geopolymer depends on sev-
eral parameters conducting to a more or less important decrease
of the mechanical strength. Each parameter impacting on the
metakaolin-based behavior at ambient temperature may impact
also after thermal treatment. Parameters such as the aluminosili-
cate sources (metakaolin, fly ash, blast furnace slag…), the Si/Al
ratio, the solid/liquid ratio (S/L) (or the water content) and the
chemical composition of the geopolymer (the concentration and
Na or K metal cations in alkaline solutions used as activator) were
highlighted in the follow for their effect on its thermal properties
and linear shrinkage curves (m
Ohd
S
AlAhuddIN
et alii, 2015).
(1) The aluminosilicate source: the metakaolin-based ge-
opolymers (Mk geopolymers) are the most widespread geopol-
ymer. Metakaolin is obtained after kaolin calcination at tem-
peratures above 550°C. Kaolin contains in majority kaolinite
(Al
2
Si
2
O
5
(OH)
4
) that is widely used in ceramic for its low ex-
pansion/shrinkage under temperature. The mechanical strength
of Mk geopolymers usually depends on the rate of kaolinite
amorphization varying with the temperature and the duration
of calcination. Above 550°C, K
ONg
et alii (2008) showed that
the calcinating temperature between 650 and 800°C of kaolin
have a little influence on the performance of Mk geopolymers
at elevated temperature. If geopolymers containing metakaolin
showed good mechanical properties at room temperature, these
binders are prone to a large degree of drying shrinkage and crack-
ing conducting to the degradation of their mechanical strength
when exposed to high temperatures (B
eRNAl
et alii, 2011). Fly
ash generally proved to be much more effective to produce ther-
mal resistant geopolymer.
(2) The SiO
2
/Al
2
O
3
ratio (as well as the Na
2
O/Al
2
O
3
) may
change with the type of raw material used as source of alumino-
silicate (v
AN
J
AARSveld
et alii, 2003) and then impact on geopol-
ymers properties (K
OmNItSAS
& z
AhARAKI
, 2007). The physical
properties of Mk geopolymer are improved when SiO
2
are added
to the mixture (d
AvIdOvItS
, 2013). d
e
S
IlvA
& S
AgOe
-C
ReNtSIl
(2008) demonstrated that the setting time of Mk-geopolymer
(based on a calcined commercial kaolin at 750°C) is mainly con-
trolled by Al
2
O
3
content and it increases with increasing SiO
2
/
Al
2
O
3
ratio of initial mixture (when Al
2
O
3
decreases). At the same
time, the increase of SiO
2
/Al
2
O
3
molar ratios up to 3.4-3.8 leads
to high compressive strength development. The increase in Al
2
O
3
(low SiO
2
/Al
2
O
3
) leads to products of low strengths, accompanied
by microstructural change in Na-Al-Si matrix. The best resist-
ances (at ambient temperature) are obtained when the molar ratio
SiO
2
/Al
2
O
3
is between 3 and 3.8 and the molar ratio Na
2
O/Al
2
O
3
is about 1 (S
IlvA
et alii, 2007; B
eRNAl
et alii, 2011). K
ONg
& S
AN
-
JAyAN
(2008) stated also that alkaline solution selection and con-
centration ratio are critical parameters necessary to optimize the
performance of Mk geopolymer (at ambient or at elevated tem-
perature). SiO
2
/Al
2
O
3
ratio has a significant influence on elevated
temperature exposure deterioration and lesser strength loss due
to elevated temperature exposures were observed in geopolymer
with high SiO
2
/Al
2
O
3
ratios (>3).
(3) The solid-liquid ratio (S/L): water in alkaline solution
is the third component of the mixture that impacts the geopoly-
mer synthesis. During geopolymerization, water acts as a way
background image
TEMPERATURE EFFECT ON MECHANICAL AND PHYSICAL PROPRIETIES OF
Na OR K ALKALINE SILICATE ACTIVATED METAKAOLIN-BASED GEOPOLYMERS
7
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
of ion transport. Furthermore, water is essential for good fluid-
ity of fresh geopolymer cement paste (y
uNSheNg
et alii, 2010).
Moreover, the compressive strength of Mk geopolymer may vary
when similar S/L ratios are imposed but the amount of sodium
silicate is changed (g
AO
et alii, 2013). The compressive strength
is high when the S/L ratio is 1.03 and the amount of sodium sili-
cate is close to 1% of the total solid weight. A S/L ratio higher
than 1.03 generates an improvement of the compressive strength
because of the good contact between the activating solution and
the reactive raw materials but for the highest S/L ratio (close to
1.19), the dissolution of aluminosilicate raw material is limited
and the geopolymer paste hardens quickly while some particles
of raw materials remain unreacted. The uncompleted dissolution
produces a low level of polycondensation generating the decrease
of the compressive strength of geopolymer. When the ratio S/L
is low (around 0.97), the contact between activating solution and
raw material is limited due to the large volume of liquid which
weakens the compressive strength of geopolymer. The compres-
sive strength follows the same trend before or after thermal treat-
ment (K
ONg
& S
ANJAyAN
, 2008). The replacement of metakaolin
by fly ash allows to have a high solid/liquid ratio in the starting
mixture and then less water has to diffuse and evaporate when
the temperature rises during fire. Less moisture by adding more
fly ash in geopolymer improves the fire resistance (K
ONg
et alii,
2007; z
hANg
et alii, 2014). This property is in relation with the
water demand of the reacting mixes (P
ROvIS
et alii, 2010) and Mk
is usually characterized by a high water demand.
(4) The nature of the alkali metal cation in alkaline solu-
tion usually a mixture of M
2
SiO
3
and MOH with M = Na or K
plays also a role. In presence of sodium, the Na
2
SiO
3
/NaOH alkali
activation of metakaolin is known to give material with better
mechanical properties compared to the activated geopolymers
with only NaOH (K
OmNItSAS
& z
AhARAKI
, 2007). S
INdhuNAtA
et
alii (2006) studied also the effect of the silicate ratio of the ac-
tivating solutions (SiO
2
/M
2
O, with M = Na or K) as well as the
effect of curing temperature on fly-ash-based geopolymer. Very
high silicate ratios (SiO
2
/M
2
O) are believed to slow the reactions
and the pore structure of K-based geopolymer is more suscepti-
ble to change in temperature than the one associated to Na-based
geopolymer (an increase in the total pore volume and the sur-
face area when the cure temperature increases was observed).
It could explain C
heNg
& C
hIu
(2003)‘s results showing that
KOH gives better results in terms of fire resistance as confirmed
also by (K
ONg
et alii, 2008). C
heNg
& C
hIu
(2003) showed that
when a geopolymer panel of 10 mm of thickness is exposed to
a flame of 1100°C, the temperature on the opposite face of the
panel decreases depending on the initial concentration of KOH
(the temperature reached 250°C-300°C after 15 minutes). J
AARS
-
veld
& d
eveNteR
(1999) showed also on fly ash-based geopoly-
mer that alkali metal cations play a determining role in the raw
material dissolution, and in the nucleation process which leads
to a 3-dimensional structure. The size of the cations affects the
crystal morphology: K
+
produces a geopolymer with a high de-
gree of condensation compared to Na
+
in the same conditions
whilst the dissolution of alumina and silica oligomers is greater
in presence of Na
+
cations than in the presence of K
+
cations. The
densification and the weight loss of Na-geopolymer for 1.15≤ Si/
Al ≤2.15 (2.3≤ SiO
2
/Al
2
O
3
≤4.3) were observed by d
uxSON
et
alii (2007) when hardened paste is heated at 1000°C. Despite the
great differences in shrinkage and densification with temperature,
the weight losses of Na-geopolymer for different Si/Al ratios are
similar. These differences are to be linked to the geopolymer gel
structure (
vAN
J
AARSveld
et alii, 1997).
As showed for the nature of alkali metal cation in geopoly-
mer, a better understanding of geopolymer microstructure can
explain its thermal behavior. v
IllAquIRáN
-
CAICedO
et alii (2015)
showed clearly that the thermal properties of geopolymer (spe-
cific heat diffusivity and thermal conductivity) are connected
tightly to the pore volume, the water content and the microstruc-
ture of geopolymer which are parameters influencing the heat
flow within the material. Geopolymer concretes (with fly ash or
fiber for reinforcement) clearly show a better resistance against
the spalling effect because of the presence of temperature gradi-
ents much lower between the surface and the heart of the material
(compared to what happens in Portland concrete). This character-
istic limits the cracking surface and conduct to a decreasing of the
spalling phenomena (S
ARKeR
et alii, 2014). Moreover, d
uxSON
et
alii (2007) explain that geopolymers are highly resistant to fire
because of the existence of nano-pores allowing the physically
and chemically bounded water to migrate and evaporate without
damaging the alumino-silicate geopolymer network. S
INgh
et alii
(2015) and m
Ohd
S
AlAhuddIN
et alii (2015) described also the
steps occurring in fly ash-based geopolymers when exposed to
fire: it occurs dehydration of free water with minimal shrinkage,
collapse of pores with constant shrinkage accompanied by de-
hydroxylation (OH departure from the structure), a step of den-
sification (550-650°C) followed by crystallization and sintering
of unreacted particles (600-800°C) which probably contributes to
the strengthening of the geopolymer subjected to high tempera-
tures. The densification is due to geopolymerization and to sinter-
ing of fly ash particles under temperature (S
ARKeR
et alii, 2014;
K
ONg
et alii, 2007, 2008). The geopolymerization exclusively
involves the amorphous parts of ashes (F
eRNANdez
& S
CRIveNeR
,
2011) and other crystallized particles undergo sintering and den-
sification during fire exposure which significantly increases the
strength at high temperatures. In the case of Mk geopolymer, the
same phenomenon may happen if much of metakaolinite parti-
cle is not involved in geopolymerization reactions (S
ChmüCKeR
&
m
AC
K
eNzIe
, 2005). In this case, just a fraction of the raw material
is dissolved, producing free SiO
2
and Al
2
O
3
amounts. If all the
background image
A. NMIRI, O. YAZOGHLI-MARZOUK, M. DUC, N. HAMDI & E. SRASRA
8
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
amorphous Mk material reacts by geopolymerization, no den-
sification is then possible during a fire exposure (z
hANg
et alii,
2014).
The purpose of this paper is to synthesize geopolymer materi-
als from mixtures of metakaolin and alkaline solution in order to
reach the best thermal characteristics to use the material as flame
retardant natural stone. (Na-) and (K-) Mk geopolymers are com-
pared and their performance when exposed to high temperatures
are determined by following their physico-mechanical properties
and their microstructural change after spending two hours at high
temperature (200, 400 and 600°C).
MATERIALS AND METHODS
Materials
Natural kaolin from a quarry in Tabarka region (in Tunisia)
was used as the primary aluminosilicate source for geopolymeri-
zation (its chemical composition is given in Tab. 1). After a crush-
ing in a Retsch RMO mortar lab grinder (with agate mortar) until
the full passage through 106 µm sieve, the kaolin was calcined at
700°C for 2 hours (previous tests evidenced that this temperature
allows to reach the best mechanical properties of geopolymer).
Sodium hydroxide powder (NaOH, 99%) and potassium hy-
droxide powder (KOH, 99%), both from Sigma-Aldrich were
used as well as sodium silicate solution Na
2
SiO
3
(SiO
2
/Na
2
O = 2)
supplied by Fisher Chemicals. Potassium silicate solution (SiO
2
/
K
2
O = 2.5) was supplied by Alfa Aesar. The alkali metal hydrox-
ide MOH solutions (M: K or Na) were obtained by dissolving the
dry powders in distilled water. After the cooling down to ambi-
ent temperature (around 25°C), solutions at 10 mol/L were kept
safe from air. Afterwards, NaOH and KOH solutions were mixed
with the alkali silicate Na
2
SiO
3
and K
2
SiO
3
solutions respectively.
Mixture, using a mechanical mixer, took place just 24 h before
the geopolymer manufacture to ensure that the activator compo-
nent was mixed uniformly. The chemical composition of activa-
tors used to make geopolymer cement is listed in Tab. 2.
Preparation of geopolymers
Two types of geopolymer were synthesized and studied:
Geopolymer G1 (K-geopolymer obtained from metakaolin
It was obtained making the following mixture:
Metakaolin + KOH (10M; d=1.46 d=1.39) + K
2
SiO
3
(0,55M;
d=1.07) with mass ratio K
2
SiO
3
(solution)/KOH (solution)
(g/g) = 0.14
Geopolymer G2 (Na-geopolymer obtained from metakaolin)
It was obtained making the following mixture:
Metakaolin +
NaOH (10M; d=1.34 d=1.330) + Na
2
SiO
3
(0,55M; d=1.04) with
mass ratio Na
2
SiO
3
(solution)/NaOH (solution) (g/g) = 0.14
Each solutions of alkali metal hydroxide mixed with alkali
silicate was added on metakaolin powder and further manually
mixed for 3 to 5 minutes. The quantities of each components
added follow the weight ratios given in Tab. 3. After mixing, the
fresh geopolymer paste is rapidly poured into cylindrical PVC
molds with a 1:2 diameter-to-length ratio (l
AtellA
et alii, 2008;
vAN
J
AARSveld
&
vAN
d
eveNteR
, 1999; B
eRNAl
et alii, 2011). All
samples were vibrated for 2 min on a vibration table to remove
trapped air bubbles.
Methods of solid characterization
The temperature effects on the physical properties of geopol-
ymer were tested using linear shrinkage and loss on ignition as
well as compressive strength testing. Linear shrinkage and loss
on ignition were measured on hardened geopolymer specimens
after heat treatment during 2 hours at 200, 400 or 600°C in a
programmable furnace (Nabertherm B180). A thermal increase
of 2°C/min was imposed to reach each final temperature in order
to avoid the cracking of material (
vAN
J
AARSveld
et alii, 2002).
The linear shrinkage was determined using a calliper by
measuring the variations in length of the samples before and after
heating, both after previous 28 days of curing at ambient tem-
perature. Linear shrinkage (Ls) was calculated for each specimen
according to equation 1:
Eq. 1
where, L
0
and L are the lengths of specimen before and after cal-
cination, respectively.
Tab. 1 - Chemical composition of kaolin and metakaolin (LOI: Loss on ignition at 1100°C)
Tab. 2 - Chemical composition of alkaline solutions used as activators
Tab. 3 - Weight and molar ratios calculated for G1 and G2 geopolymer mixtures. L/S is the liquid upon solid ratio
background image
TEMPERATURE EFFECT ON MECHANICAL AND PHYSICAL PROPRIETIES OF
Na OR K ALKALINE SILICATE ACTIVATED METAKAOLIN-BASED GEOPOLYMERS
9
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
Loss on ignition (Li) was calculated for each specimen ac-
cording to equation 2, where m
1
and m
2
are the masses of the
specimen before and after calcination, respectively (both after
previous 28 days of curing at ambient temperature).
Eq. 2
Compressive strength testing was performed using cylinders
with a 1:2 diameter-to-length ratio (v
AN
J
AARSveld
& v
AN
d
eveNteR
, 1999). Three cylinders of each geopolymer sample
were tested to calculate average experimental values. All the
samples were tested after 28 days of curing under ambient
laboratory atmosphere. A Zwick/Roell compressive strength
testing apparatus was used, with a constant and controlled
displacement rate (5 mm per min) for all the tests (A
l
B
AKRI
et
alii, 2013). Compressive strength was measured at 20°C on the
200, 400 or 600°C heated geopolymers.
The structural characteristics of geopolymers (morphology
and particles arrangement) have been studied and compared on
fresh fractured surface before and after calcination from both G1
and G2 geopolymer specimens using environmental scanning
electron microscopy (FEI Quanta 400).
Thermal analysis (DTA–TGA) using a STA-409 E apparatus
from NETZSCH was performed under air sweeping on 180 mg of
ground geopolymers (at 80 µm). It allows to identify the chemi-
cal reactions or the phase changes that may occur in the sample
under heating. Indeed, the powder was placed in a platinum cru-
cible and was heated at a temperature ranging from 20 to 1250°C
with a heating rate of 10°C/min. In DTA, alumina is used as a
reference (inert substance) that shows no thermal change in the
explored temperature interval.
The functional groups of ground synthesized geopolymers
were identified by FT infrared spectroscopy (from 4000 to 450
cm
-1
) using a Perkin Elmer spectrometer 180. Mineralogical
qualitative analyses were realized by X-ray diffraction (XRD)
using a D8 Advance from Bruker with a cobalt anode (λ Cokα1
= 1.79 Å). The XRD patterns were measured from 5° to 80° 2θ
at a scan rate of 0.01°2θ/1s. The porosity was carried out on
freeze-dried samples using a mercury porosimeter Autopore
IV from Micrometritics. The freeze drying is a technique that
remove most of the water contained in the specimen by water
sublimation at low-temperature and low-pressure and that
prevents microstructure change during usual drying.
EXPERIMENTAL RESULTS
STRUCTURAL AND MINERALOGY OF SYNTHESISED
GEOPOLYMERS
X-
ray
diffraction
(Xrd)
XRD patterns of raw kaolin (Kb), metakaolin (Mk) (calcined
kaolinite Kb) and geopolymers G1 and G2 are shown in Fig. 1.
XRD patterns were performed on geopolymers fragments after
mechanical measurements applied after 28 days of curing at am-
bient temperature.
Each diffractograms show the presence of characteristic
peaks of quartz (d=3.34 Å close to 31°2θ), anatase (TiO
2
) usually
associated to kaolin and illitic clay (d=10Å near 10°2θ). These
minerals were present initially in raw kaolin before heating and
are not modified by calcination at 700°C. They are not involved
in the synthesis of geopolymer (B
uRCIAgA
-d
IAz
et alii, 2012).
Compared to Kb pattern, XRD pattern of metakaolin shows the
disappearance of peaks of kaolinite phase after calcination (the
main peak is positioned at d= 7Å around 14°2θ) and the presence
of a larger peak with weak amplitude between 20° and 33°2θ. It
reveals the amorphous character of metakaolin. This wide peak
shifts to 28°- 40° 2θ range on G1 and G2 XRD patterns which
is characteristic of geopolymer systems (V
AN
J
AARSveld
& v
AN
d
eveNteR
, 1999).
In the case of Na-geopolymer G2 contrary to K-geopolymer
G1, it appears new peaks representative of sodium aluminosili-
cate (d= 6.35 Å and d= 3.65 Å close to 16.23°2θ and 28.4°2θ
respectively) and faujasite (Na
2
Al
2
Si2.4O
8.8
.6.7 H
2
O) (d=14.3
Å near 7.17°2θ and d=8.84 Å near 11.67°2θ.) (z
IBOuChe
et alii,
2009).
FTIR analysis
FTIR curves obtained on metakaolin (Mk) and geopolymers
G1 and G2 are shown in Fig. 2. The absorption broadband at
about 3440 cm
-
1 and 1640 cm
-1
are the stretching and bending
vibration frequencies of OH groups associated to water, respec-
tively (z
heNg
et alii, 2009).
The absorption peak at about 2350 cm
-1
on Mk spectrum is
due to adsorptive CO
2
vibration on clay particle surface. Two
bands located at 1093 cm
-1
and 994 cm
-1
are characteristic of Si-O
and Si-O-Si stretching vibrations respectively (d
uxSON
, 2006).
Compared to Mk spectrum, IR spectra collected on G1 and G2
Fig. 1 - X-ray diffraction patterns on raw kaolin Kb, metakaolin Mk,
K-geopolymer G1 and Na-geopolymer G2
background image
A. NMIRI, O. YAZOGHLI-MARZOUK, M. DUC, N. HAMDI & E. SRASRA
10
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
geopolymers show the appearance of a new absorption band at
about 1437 cm
-1
which is assigned to the C-O stretching vibration
of carbonate (experiments are done under air). In addition, the
vibration frequency at 798 cm
-1
on metakaolin spectrum, linked to
Al-O stretching vibration, was reduced after geopolymerization.
It corresponds to the change of aluminum coordination from VI
to IV, the coordination number IV characterizing the geopolymer
structure (y
uNSheNg
et alii, 2010). This band is still present on
spectrum associated to geopolymer G1, indicating that the ge-
opolymer structure contains more unreacted metakaolin than the
geopolymer G2. The absorption bands at about 797 and 724 cm
-1
on both G1 and G2 spectra are respectively attributed to asym-
metric and symmetric vibrations of Si-O and Si-Al-O-Si bonds
that provide the cohesion between AlO
4
and SiO
4
tetrahedrons
in geopolymeric structure (R
AttANASAK
& C
hINdAPRASIRt
, 2009).
Finally, the band at approximately 470 cm
-1
is attributed to O-Si-
O bending mode.
DTA–TGA thermal analysis and loss on ignition
Endothermic reactions corresponding to successive release of
physically absorbed, interlayer and crystalline water are identified
by thermal analyses. Exothermic reactions (identified by peaks
pointing up on DTA curve as at 977°C on Fig. 3) may correspond
to new compounds formation or organic matter calcinations.
Thermal behavior of kaolin Kb is shown in Fig. 3. The total
weight loss is 6.06% at 1150°C. The first loss (0.86%) between
room temperature and 320°C corresponds to the release of mois-
ture (physically and partially chemically adsorbed water). In DTA
curve the broad endothermic peak at around 565°C (with intense
loss weight on TGA curve corresponding to 5.10 %) is attributed
to dehydroxylation of kaolinite and metakaolin formation (S
h
-
vARzmAN
et alii, 2003) according to the following reaction given
by K
AKAlI
et alii (2001):
Al
2
Si
2
O
5
(OH)
4
→ Al
2
Si
2
O
5
(OH)xO2-x + (2-x/2)H
2
O Eq. 3
with values much lower than x (x depends on the time and tem-
perature of calcination).
In our case, the heating at 700°C during two hours allows to
make disappear almost all OH from kaolinite but the OH from
illite remains. Such OH remains until 900°C but above 800°C,
metakaolin may be transformed into crystallized form. The small
exothermic peak on DTA curve at 977.7°C, with negligible loss of
weight on the TGA, indicates such crystallization into crystalline
aluminosilicate such as spinelle (Al
4
Si
3
O
12
). The exothermic peak
at 1203.1°C corresponds to transformation of spinelle to another
crystalline form such as mullite (Al
6
Si
2
O
13
) (S
hvARzmAN
et alii,
2003; K
AKAlI
et alii, 2001). These new crystallized phases de-
crease the reactivity of metakaolin and then the mechanical prop-
erties of synthesized geopolymer. The TGA and DTA curves of
metakaolin is not given as they correspond to the state of kaolin
after heating at 700°C (the total weight loss of metakaolin is al-
most zero with just the presence of the DTA peak at 977°C). The
temperature resistance of geopolymers is partially explained by
thermal analysis. The TGA curve of K-geopolymer G1 on Fig. 4a
shows several losses which gives a total weight loss of 12%. The
first weight loss of 9.7% (that represents almost 81% of the total
loss) characterized by an endothermic DTA peak at 157.1°C cor-
responds to evaporation of physically absorbed water.
Then, there is a small loss of 2.22% at 800°C that corre-
sponds to the release of crystalline water from illite and other
components. The exothermic peak at 1020.9°C may correspond
to a recrystallization of residual metakaolin to spinelle (K
AKAlI
et alii, 2001). The exothermic peak at 1226.2°C corresponds to
the mullite appearance from spinelle formed from unpolymerised
metakaolin (S
ChmüCKeR
& m
AC
K
eNzIe
, 2005). Thermal analysis
of Na-geopolymer G2 on Fig. 4(b) shows a higher total weight
loss of 14.99%. The first weight loss (until 400°C) of 12.86%
characterized by a DTA peak at 139.6°C corresponds as previ-
ously to evaporated superficial moisture (the water content is
higher than 9.7% lost by G1 in similar condition). Then, there
is a loss of remaining water (2.12% close to 2.22% lost by G1)
until 635°C. The exothermic peak at 864°C may be attributed to
illite deshydroxylation (although this phenomenon is not visible
on geopolymer G1). G2 curve does not show an exothermic peak
at temperature above 1200°C, which may be explained by the
polymerization of all aluminosilicate when NaOH and Na
2
SiO
3
are used as activator. Such observation validates the fact that the
dissolution of aluminum and silicon precursor is greater in the
presence of Na+ cations that in the presence of K+ cations. It
contributes to explain why Na-geopolymer G2 is steadier than
K-geopolymer G1. Finally, both G1 and G2 geopolymers appear
stable under heating: the main loss of mass comes from superfi-
cially adsorbed water and no exo- or endo-thermal reaction (ex-
cept in presence of side mineral phases in raw material) seems to
Fig. 2 - FTIR spectra of metakaolin Mk, K-geopolymer G1 and Na-
geopolymer G2
background image
TEMPERATURE EFFECT ON MECHANICAL AND PHYSICAL PROPRIETIES OF
Na OR K ALKALINE SILICATE ACTIVATED METAKAOLIN-BASED GEOPOLYMERS
11
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
occur between 400 and 1000°C.
In parallel to thermal analysis, the loss on ignition of geopol-
ymers (a macroscopic and simple measure compared to TGA-
DTA) was measured as a function of temperature. Measures
shown in Fig. 5 are in accordance with previous thermogravi-
metric data on Fig. 4. We observe that both geopolymers G1 and
G2 reached a maximum loss at 600°C: 11.42% in G1 and 13.42%
in G2. The higher the temperature of calcination is, the greatest
the loss on ignition appears (with systematic larger amount of
released water for Na-geopolymer G2).
TEMPERATURE EFFECT ON THE PHYSICAL
PROPERTIES OF GEOPOLYMER
Compressive strength
The compressive strength measured on geopolymer speci-
mens G1 and G2 after calcination at various temperatures are
shown in Fig. 6 (average values calculated from 3 replicates on
each geopolymer tested for each temperature). A slight increase
of the compressive strength is observed at temperatures ranging
between 25 and 400°C in both geopolymer formulations due to
acceleration of the geopolymerization reaction which is not yet
completed at 25°C after the 28 days of curing. A calcination of G1
and G2 at higher temperature leads to a decrease in mechanical
properties with a compressive strength that drops to 12.74 MPa
in G1 and 18.47 MPa in G2. It is correlated to the vaporization
of the remaining water linked to the presence of illite (and other
components) not affected by geopolymerization as shown on
XRD patterns, and the creation of micro cracks in specimens. If
geopolymers G1 and G2 behave similarly with the temperature
of calcination, they are clearly differentiated on Fig. 6. The new
crystalline phases detected by XRD on Fig. 1 on geopolymer G2
(compared to G1) are probably at the origin of the improved me-
chanical properties of G2 (compressive strength for G1: 14 MPa
and G2: 20.64 MPa without calcination and 12.74 MPa for G1
and 20.64 MPa for G2 after calcination at 600°C).
Linear shrinkage
The slight increase in resistance after calcination on Fig. 6
is accompanied by a slight increase of weight loss on Fig. 5 due
to evaporation of superficial water adsorbed on external surface
and of gravity and capillary water in pores. Such phenomenon
causes also partial shrinkage of porous material (after release of
water) resulting in a linear shrinkage presented on Fig. 7. Linear
shrinkage may come also from the development of microcracks
under heating. Results show that the linear shrinkage increases
with the temperature of calcination. The lower linear shrinkage
Fig. 3 - Thermal analysis on kaolin Kb (TGA curve in red, DTG curve
derived from TGA in green and DTA curve in blue)
Fig. 4 - Thermal analysis on geopolymers G1 (a) and G2 (b) (TGA
curve in red, DTG curve derived from TGA in green and DTA
curve in blue)
Fig. 5 - Loss on ignition of G1 and G2 heated at different temperatures
background image
A. NMIRI, O. YAZOGHLI-MARZOUK, M. DUC, N. HAMDI & E. SRASRA
12
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
values (2.18% and 2.93%) are obtained respectively for G1 and
G2 treated at 200°C. The greatest values (3.46% and 4.27%) cor-
respond to G1 and G2 treated at 600°C.
Furthermore correlated to the loss on ignition, K-geopolymer
G1 has the lowest linear shrinkage compared to Na-geopolymer
G2. Surprisingly, G1 has the lowest mechanical strength while
G2 has the greatest (the geopolymer with strong water release and
with the highest linear shrinkage in favor of microcracks, is ex-
pected to be the weakest mechanically). Such behavior should be
linked to the mineralogical difference observed on G1 and G2 ge-
opolymers. The presence of faujasite and sodium aluminosilicate
probably strengthen the Na-geopolymer G2 (more rigid structure)
but such structure may shrink with heating without or with a few
microcracks.
MIP analysis
The mercury intrusion porosimetry (MIP) on hardened ge-
opolymers paste after 28 days of curing and heating at different
temperature gives complementary information on the presence of
microcracks and on pore distribution. As the pore diameter is a
function of the mercury pressure, the volume of mercury entering
into the sample (per unit mass of the material - mL/g) gives the
pore volume for each pore size and the total porosity (%) may be
calculated. Results are given in Tab. 4.
Before heating, Na-geopolymer G2 is less porous than K-
geopolymer G1with a porosity equal to 24.3% and 30.1% respec-
tively. Such observation is consistent with the higher mechani-
cal resistance of geopolymer G2. When heat treatment is applied
with increasing temperature of calcination, porosity remains quite
constant for G1, but it increases in geopolymer G2 which reaches
33.2% at 400°C and then decreases at 600°C. The microcracks or
porosity seems to increase with temperature for Na-geopolymer
G2 (while it remains stable until 400°C for K-geopolymer G1)
and then falls at 600°C for both geopolymers. However, G2 is
more resistant than G1. The apparent density of both geopolymers
follows the same trend: it increases with calcination temperature
and then decreases after a 600°C calcination. It’s correlated to the
decrease of the compressive strength at 600°C for both geopoly-
mers and to the variation of pore size under the thermal effect.
Parallel to the quantity of pore, the mercury intrusion curves
on Fig. 8 allow to observe the pore family and their evolution
under heating. Results show clearly that G1 and G2 pore fam-
ily characteristics are different. Before heat treatment, they are
characteristic of unimodal and bimodal systems for G2 and G1
respectively. The average pore diameters range from 50 to 200
nm in G2 and from 150 to 500 nm in G1 (the two pore families
in G1 are close, centered on 400 nm for the main pores and to
350 for the second ones). The literature indicated that pores larger
than 200 nm in geopolymer pastes were probably associated with
Fig. 6 - Compressive strength of G1 and G2 geopolymers heated at dif-
Compressive strength of G1 and G2 geopolymers heated at dif-
ferent temperatures
Fig. 7 - Linear shrinkage of G1 and G2 heated at different tempera-
Linear shrinkage of G1 and G2 heated at different tempera-
tures
Tab. 4 - Mercury intrusion porosimetry of geopolymers before and after heating
background image
TEMPERATURE EFFECT ON MECHANICAL AND PHYSICAL PROPRIETIES OF
Na OR K ALKALINE SILICATE ACTIVATED METAKAOLIN-BASED GEOPOLYMERS
13
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
interfacial spaces between partially reacted or unreacted raw ma-
terial and the geopolymer gel (g
AO
et alii, 2013). The presence
of such pores in K-geopolymer G1, indicating the presence of
unreacted metakaolin, agrees with previous TGA-DTA and FTIR
results.
Na-geopolymer G2 has not only quantitatively a lower po-
rosity than G1, but also smaller pores, which contributes to its
highest mechanical resistance. After a 2 hours heat treatment at
200, 400 and 600°C, mercury intrusion curves on K-geopolymer
G2 demonstrated the pore family changes with the appearance
of a bimodal system (the pore sizes of the two families are posi-
tioned around 160 nm and 300 nm). For G1, after the decrease of
the mean size of the two pore families (close to each other) until
400°C, the calcination at 600°C induces the increase of pores size
with clearly two families with average diameters about 300 nm
and 450 nm.
Environmental scanning electron microscopy observations
The environmental SEM images of geopolymers G1 and G2
(Fig. 9 and Fig. 10) show matrices containing particles with a
certain porosity which directly influences the resistance of ge-
opolymer.
Before heating, G1 and G2 present various morphology and
particle size even if raw material used to synthesized them are
similar., The fresh-fractured surface observed on geopolymer G2
shows globally a denser matrix (Fig. 10d) with very fine parti-
cles. Local area show also the presence of gathered hexagonal
platelets (Fig. 10c) that could be identified as unreacted metakao-
linite particles, even if such particles are expected in geopolymer
G1 as demonstrated by previous FTIR and TGA-DTA analysis.
Geopolymer G1 shows a more heterogeneous matrix with large
amount of particles with lumpy texture. The more compact struc-
ture observed on polymer G2 contribute to explain the increase
in compressive strength, but no significant porosity differences
in both geopolymers had been recorded because observations are
somewhat subjective and remain local.
CONCLUSION
As a conclusion, Na-geopolymer (G2) obtained from
metakaolin is mechanically more resistant than geopolymer
K-geopolymer (G1) obtained also from metakaolin. The
high resistance of geopolymer G2 seems to be linked to the
mineralogy (with the presence of faujasite and sodium alumino-
silicate) combined to a lower porosity with small-sized pores
(demonstrated by SEM images and MIP analysis). Metakaolin
and Na alkaline silicate give a geopolymer with high degree of
condensation relative to metakaolin and K alkaline silicate. The
probable presence of unreacted metakaolinite in K–geopolymer
G1 identified by thermal or infrared analyses contributes also to
decrease the mechanical strength of G1 (it agrees with literature
that mentions that sodic activators compared to potassic promote
the dissolution of aluminosilicate precursor). Furthermore, the
Fig. 8 - MIP curves from G1 and G2 geopolymers heated at different
temperatures
Fig. 9 - Environmental SEM images of fresh fractured geopolymers (a)
G1 and (b) G1 after heating at 600°C
background image
A. NMIRI, O. YAZOGHLI-MARZOUK, M. DUC, N. HAMDI & E. SRASRA
14
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
greater linear shrinkage of Na-geopolymer G2 (compared to G1)
as well as its significant loss on ignition (essentially physically
adsorbed water released at temperature < 400°C) seems not to
govern the mechanical behavior of geopolymer. No microcracks
that could appear with high linear shrinkage were observed
by microscopy at small scale. After calcination, the porosity
and density of geopolymer slightly increases or remains quite
constant as well as the compressive strength, but at 600°C, they
decreased (due to the increase of pore size under the thermal
effect for G1). To conclude, the Na-metakaolin based geopolymer
compared to K-metakaolin based one is the most appropriate to
produce heat resistant geopolymer cement. This material, with
a light color after calcination, could be used in the construction
industry as new ecological cement (for flooring, artificial stone
wall, pedestrian roads…) or applied instead of asbestos as thermal
insulator in electric oven.
REFERENCES
A
l
B
AKRI
A.m.m., K
AmARudIN
h., B
NhuSSAIN
m., l
IyANA
J. & R
uzAIdI
g
hAzAlI
C.m. (2013) - Nano geopolymer for sustainable concrete
using fly ash synthesized by high energy ball milling. Appl. Mech.
Mater., 313-314: 169-173.
B
eRNAl
S.A., R
OdR
í
guez
e.d., m
eJíA
d
e
g
utIéRRez
R., g
ORdIllO
m &
P
ROvIS
J.L. (2011) - Mechanical and thermal characterisation of
geopolymers based on silicate-activated metakaolin/slag blends. J.
Mater. Sci, 46(16): 5477-5486.
B
uRCIAgA
-d
IAz
O., e
SCAlANte
-g
ARCIA
J.I. & g
OROKhOvSKy
A. (2012)
- Geopolymers based on a coarse low-purity kaolin mineral:
mechanical strength as a function of the chemical composition and
temperature.
Cem. Concr. Compos., 34(1) : 18-24.
C
heNg
t.W. & C
hIu
J.P. (2003) - Fire-resistant geopolymer produced by
granulated blast furnace slag. Miner. Eng. 16(3): 205-210.
d
AvIdOvItS
J. (2013, Eds) - Geopolymer cement 1–11, Saint-Quentin,
France.
d
e
S
IlvA
P. & S
AgOe
-C
ReNtSIl
K. (2008) - The effect of Al
2
O
3
and SiO
2
on
setting and hardening of Na
2
O-Al
2
O
3
-SiO
2
-H
2
O geopolymer systems.
J. Australia Ceram. Soc., 44(1): 39-46.
d
uxSON
P. (2006) - The structure and thermal evolution of metakaolin
geopolymers. Ph.D. thesis, Department of Chemical & Biomolecular
Engineering, The University of Melbourne.
d
uxSON
P., l
uKey
g.C. & v
AN
d
eveNteR
J.S.J. (2007) - Physical evolution
of Na-geopolymer derived from metakaolin up to 1000°C. J. Mater.
Sci., 42: 3044-3054.
F
eRNANdez
R. & S
CRIveNeR
K. (2011) - Des argiles calcinées comme
substitut au ciment. Bull. Tech. la Suisse Rom., 137: 11-14.
g
AO
K., l
IN
K., W
ANg
d., S
hIu
h. & h
WANg
C. (2013) - Effects of nano-
SiO
2
on setting time and compressive strength of alkali-activated
metakaolin-based geopolymer. The Open Civil Engineering Journal,
7: 84-92.
K
AKAlI
g., P
eRRAKI
t., t
SIvIlIS
S. & B
AdOgIANNIS
e. (2001) - Thermal
Fig. 10 - Environmental SEM images of fresh fractured geopolymers (c)-
(d) G2 and (e) G2 after heating at 600°C
background image
TEMPERATURE EFFECT ON MECHANICAL AND PHYSICAL PROPRIETIES OF
Na OR K ALKALINE SILICATE ACTIVATED METAKAOLIN-BASED GEOPOLYMERS
15
Italian Journal of Engineering Geology and Environment, 1 (2016)
© Sapienza Università Editrice
www.ijege.uniroma1.it
treatment of kaolin: the effect of mineralogy on the pozzolanic activity. Appl. Clay Sci., 20: 73-80.
K
OmNItSAS
K. & z
AhARAKI
d. (2007) - Geopolymerisation: a review and prospects for the minerals industry. Miner. Eng., 20(14): 1261-1277.
K
ONg
d.l.y. & S
ANJAyAN
J.G. (2008) - Damage behavior of geopolymer composites exposed to elevated temperatures. Cem. Concr. Compos., 30(10):
986-991.
K
ONg
D.L.Y., S
ANJAyAN
J.G. & S
AgOe
-C
ReNtSIl
K. (2008) - Factors affecting the performance of metakaolin geopolymers exposed to elevated temperatures.
J. Mater. Sci., 43(3): 824-831.
K
ONg
D.L.Y., S
ANJAyAN
J.G. & S
AgOe
-C
ReNtSIl
K. (2007) - Comparative performance of geopolymers made with metakaolin and fly ash after exposure to
elevated temperatures. Cem. Concr. Res., 37(12): 1583-1589.
l
AtellA
B.A., P
eReRA
d.S., d
uRCe
d., m
ehRteNS
e.g. & d
AvIS
J. (2008) - Mechanical properties of metakaolin-based geopolymers with molar ratios of Si/
Al ≈ 2 and Na/Al ≈ 1. J. Mater. Sci., 43(8): 2693-2699.
m
Ohd
S
AlAhuddIN
m.B., N
ORKhAIRuNNISA
m. & m
uStAPhA
F. (2015) - A review on thermophysical evaluation of alkali-activated geopolymers. Ceram. Int.,
41(3): 4273-4281.
N
evIlle
A. (2011, Eds) - Properties of concrete. 597-602.
P
ROvIS
J.l., d
uxSON
P. & v
AN
d
eveNteR
J.S.J. (2010) - The role of particle technology in developing sustainable construction materials. Adv. Powder
Technol., 21(1): 2-7.
R
AttANASAK
u. & C
hINdAPRASIRt
P. (2009) - Influence of NaOH solution on the synthesis of fly ash geopolymer. Miner. Eng., 22(12): 1073-1078.
S
ARKeR
P.K., K
elly
S. & y
AO
z. (2014) - Effect of fire exposure on cracking, spalling and residual strength of fly ash geopolymer concrete. Mater. Des., 63:
584-592.
S
ChmüCKeR
M. & m
ACKeNzIe
K.J.D. (2005) - Microstructure of sodium polysialate siloxo geopolymer. Ceram. Int. 31(3): 433-437.
S
hvARzmAN
A., K
OvleR
K., g
RAdeR
g.S. & S
hteR
g.e. (2003) - The effect of dehydroxylation / amorphization degree on pozzolanic activity of kaolinite.
Cem. Concr. Res., 33: 405-416.
S
IlvA
P. d
e
, S
AgOe
-C
ReNtSIl
K. & S
IRIvIvAtNANON
v. (2007) - Kinetics of geopolymerization: role of Al
2
O
3
and SiO
2
. Cem. Concr. Res., 37(4): 512-518.
S
INdhuNAtA
, v
AN
d
eveNteR
J.S.J., l
uKey
g.C. & x
u
H. (2006) - Effect of curing temperature and silicate concentration on fly-ash-based geopolymerization.
Ind. Eng. Chem. Res., 45(10): 3559-3568.
S
INgh
B., I
ShWARyA
g., g
uPtA
m. & B
hAttAChARyyA
S.K. (2015) - Geopolymer concrete: a review of some recent developments. Constr. Build. Mater., 85:
78-90.
v
AN
J
AARSveld
J.G.S. & v
AN
d
eveNteR
J.S.J. (1999) - Effect of the alkali metal activator on the properties of fly ash based geopolymers. Ind. Eng. Chem.
Res. 38 (10): 3932-3941.
v
AN
J
AARSveld
J.G., v
AN
d
eveNteR
J.S. & l
uKey
G. (2002) - The effect of composition and temperature on the properties of fly ash- and kaolinite-based
geopolymers. Chem. Eng. J., 89(1-3): 63-73.
v
AN
J
AARSveld
J.G.S., v
AN
d
eveNteR
J.S.J. & l
OReNzeN
L. (1997) - Potential use of geopolymeric materials to immobilize toxic metals: Part I. Theory and
applications. Miner. Eng., 10(7): 659-669.
v
AN
J
AARSveld
J.G.S., v
AN
d
eveNteR
J.S.J. & l
uKey
G.C. (2003) - The characterisation of source materials in fly ash-based geopolymers. Mater. Lett.,
57(7): 1272-1280.
v
IllAquIRáN
-C
AICedO
m.A., m
eJíA
d
e
g
utIéRRez
R., S
uleKAR
S., d
AvIS
C. & N
INO
J.C. (2015) - Thermal properties of novel binary geopolymers based on
metakaolin and alternative silica sources. Appl. Clay Sci., 118: 276-282.
y
uNSheNg
z., W
eI
S. & z
ONgJIN
l. (2010) - Composition design and microstructural characterization of calcined kaolin-based geopolymer cement. Appl.
Clay Sci., 47(3-4): 271-275.
z
hANg
h.y., K
OduR
v., q
I
S.l., C
AO
l. & W
u
B. (2014) - Development of metakaolin-fly ash based geopolymers for fire resistance applications. Constr.
Build. Mater., 55: 38-45.
z
heNg
g., C
uI
x., z
hANg
W. & t
ONg
z. (2009) - Preparation of geopolymer precursors by sol–gel method and their characterization. J. Mater. Sci. 44(15):
3991-3996.
z
IBOuChe
F., K
eRdJOudJ
h., d
e
l
ACAIlleRIe
J.-B., d’e
SPINOSe
& v
AN
d
Amme
h. (2009) - Geopolymers from Algerian metakaolin. Influence of secondary
minerals. Appl. Clay Sci., 43(3-4): 453-458.
Received February 2016 - Accepted April 2016
Statistics