Efecto de la temperatura, composición química y agregados de los morteros basados en geopolimeros con bajo grado de metakaolin

1. Geopolymer mortars based on a low grade metakaolin: Effects of the
chemical composition, temperature and aggregate:binder ratio
Raúl Arellano-Aguilar a
, Oswaldo Burciaga-Díaz a
, Alexander Gorokhovsky b
, José Iván Escalante-García a,⇑
a
Cinvestav Saltillo, Ceramics Engineering Group, Av. Industria Metalúrgica No. 1062, Parque Industrial, Ramos Arizpe, Coahuila, MX C.P. 25900, Saltillo, Coahuila, Mexico
b
Department of Chemistry, Saratov State Technical University, 77 Politekhnicheskaya Str., Saratov 410054, Russia
h i g h l i g h t s
Low grade metakaolin is effective for green binders of low demand of alkalis and high strength.
The strength was studied as a function of the chemical composition and temperature.
Higher aggregate:binder ratio reduced the strength, but values 30 MPa were obtained.
Consumptions of 280 kg/m3
of low grade metakaolin resulted in 30 MPa at 28 days.
Curing at 75 °C for 24 h resulted in more than 85% of the 28 day strength at 20 °C.
a r t i c l e i n f o
Article history:
Received 28 May 2013
Received in revised form 5 September 2013
Accepted 14 October 2013
Keywords:
Geopolymer
Mortar
Compressive strength
Microstructure
Low grade metakaolin
a b s t r a c t
A low grade mineral composed of 50% kaolinite and 50% quartz was activated with solutions of Na2-
OÁrSiO2ÁxH2O. Pastes and mortars with aggregate:binder ratios of 3:1, 5:1 and 7:1 were investigated
varying the molar ratios SiO2/Al2O (2.7–3.3), Na2O/Al2O3 (0.7–1.0) and H2O/Na2O (11–13). The results
showed that the compressive strength was reduced by increasing the aggregate:binder ratio, nonetheless,
mortars with a ratio 7:1 achieved 30 MPa at 28 days. The curing of mortars at 75° for 24 h favored a rapid
development of strength at 1 day, but without a significant increase afterwards at 20 °C. The microstruc-
tures of samples cured at 20 °C were dense, while those exposed at 75 °C showed porosity, as well as
unreacted particles of quartz, limestone aggregates and metakaolin in a matrix of geopolymeric gel.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Geopolymers are alternative cementitious materials produced
by means of a chemical process in which amorphous aluminosili-
cate materials precursors of natural or industrially processed origin
may be transformed into binders under highly alkaline conditions,
yielding compact microstructures that assemble: disordered alkali
aluminosilicate gel phases, unreacted solid precursor particles and
pores ranging from below 10 nm up to about 10 lm [1–3]. The
structural description of the tridimensional disordered network
of the gel formed is described elsewhere [3–9].
Among the most used raw materials to prepare alkali-activated
aluminosilicate binders, are the coal ash from power stations [10–
13] and kaolinitic mineral clays [14,15]. The latter are commonly of
high purity as reported by many authors [16,17]. The use of
binders based on kaolinitic minerals is environmentally advanta-
geous relative to ordinary Portland cement (PC), as less energy is
required during the thermal treatment of kaolinite at about
750 °C to obtain metakaolin (MK), and there are no CO2 emissions
from calcination of raw materials. Nonetheless, it is also necessary
to consider the CO2 emissions associated to the activating agents,
for which the reported data may present a wide variation [18]. A
recent study [19] concluded that for concretes of similar strength,
the CO2 footprint of a PFA based geopolymer concrete was approx-
imately 9% less than that based on PC. On the other hand, other
studies indicated that the CO2 emissions in concretes based on
PFA geopolymers were 45% lower than those of PC [20]. Moreover,
other authors concluded that geopolymeric binders may result in
improvements of 44–64% in greenhouse emissions over Portland
cement [18].
Despite the widespread availability of kaolinitic minerals, they
have not been completely regarded as a chief alternative to
produce new geopolymer-based construction products for
large-scale applications because of their relative high cost and dif-
ficulties caused by the high surface area of their particles, such as
0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.conbuildmat.2013.10.023
⇑ Corresponding author.
E-mail address: ivan.escalante@cinvestav.edu.mx (J.I. Escalante-García).
Construction and Building Materials 50 (2014) 642–648
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat

2. segregation, drying shrinkage, cracking and incomplete reactions;
all of these derived mostly from the high water demand required
to prepare flowable pastes [21,22]. Nonetheless, this drawback
could be surmounted if low purity kaolins with coarse particle size
are used [23] which is also the case of this investigation.
The absence of a detailed scientific understanding about some
factors affecting the short and long-term properties of MK-based
geopolymers, has also limited their commercial adoption and
standardization, mainly because the exploitation properties are
sensitive to the specific characteristics of the kaolin as the chemical
composition, particle size, and crystallinity. Such parameters vary
depending on the ore, so the specific response to the chemical acti-
vation varies substantially from source to source. Even though
some studies [7,21,24–26] have reported on the influence of the
chemical composition of the mixtures, curing temperature, water
content and the nature of the alkaline-activators on the properties
of MK-based geopolymers, little attention has been granted to the
factors affecting the properties of alkali-activated geopolymeric
pastes based on low purity MK minerals [23] which are naturally
abundant and cheaper than the high purity MK [27,28].
In a recent study [29] compressive strengths of 44–58 MPa at
75 days of curing were reported for high purity MK-based geopoly-
mer mortars with aggregate:binder ratio of 0.5; while Steinerova
[30] observed that for mortars with quartz sand content of 75–
78 wt.% (equivalent aggregate:binder ratios of 3.0:1 to 3.5:1), the
maximum compressive strengths were of 60–70 MPa, using a high
purity MK geopolymer binder elaborated with chemical composi-
tion Si/Al = 1.4, Na/Al = 1 and H2O/Na = 7.14. Other reports
[31,32] have also described mortars based on MK geopolymers
with aggregate:binder ratios of 2:1 and 2.2:1. We did not find
reports of mortars using ratios greater than 3.5:1 as in the case
of this investigation, which also reports about the influence of
some factors affecting the chemical activation process, as well as
the effect of the aggregate:binder ratio on the microstructural
features and the mechanical behavior of geopolymeric mortar
systems based on a low grade kaolin mineral. A fundamental
understanding of aluminosilicate chemistry and the effect of such
variables are important for producing low cost green cements
and construction materials with tailored properties.
3. Experimental procedure
2.1. Materials
Geopolymer pastes were synthesized using a low grade kaolin mineral from
central Mexico, with a specific gravity of 2.60 g/cm3
and a chemical composition
determined as the average of 30 spot analyses by energy dispersive spectroscopy
(EDS) as shown in Table 1. The particle size range determined by laser diffraction
was between 1.8 and 100 lm, with a d50 of 13.27 lm and 10% of particles finer than
2.27 lm. The kaolin was heat treated at 750 °C in batches of 6 kg for 6 h in air to
obtain reactive MK.
Fig. 1 shows the mineralogical analysis of the raw materials (kaolin and MK) by
X-ray diffraction (XRD). The results indicate that the major crystalline phases are
quartz (SiO2; Powder Diffraction File (PDF) card # 00-046-1045) as the main
impurity in the MK, as well as some traces of cristobalite. The characteristic peaks
of kaolinite disappeared after the thermal treatment, indicating the induction of a
high degree of structural disorder in the fraction of kaolinite.
For the preparation of the chemical formulations it was assumed that all of the
Al2O3 was present in the fraction of MK, so the mineral was estimated to contain
$50% of MK and 50% of quartz (SiO2). On this basis a ‘‘reactive’’ fraction was consid-
ered, which excluded the quartz, and its chemical composition was defined and
used as the base for the preparation of all the assessed formulations.
The alkaline activators were blends of commercial soluble sodium silicate
(SiO2 = 29.5 wt.%, Na2O = 14.7 wt.% and H2O = 55.8 wt.%; modulus Ms = 2) and so-
dium hydroxide pellets. Purified water was used to dissolve the sodium hydroxide
pellets.
A commercially available crushed limestone sand No. 4, with particle size lower
than 5 mm, fineness modulus of 2.9 and specific gravity of 2.56 g/cm3
was used to
prepare mortars. The physical appearance was pale gray and the mineralogy con-
sisted predominantly of CaCO3.
2.2. Sample synthesis and tests conducted
Based on the chemical composition of the MK (the reactive fraction), geopoly-
meric pastes and mortars with an aggregate:binder weight ratio of 3:1 were elab-
orated, the details are given in Table 2. Mortars with aggregate:binder ratios of
5:1 and 7:1 were also prepared for some formulations. The alkaline solutions were
elaborated in the appropriate proportions and then mixed with the MK powders for
3 min in a planetary mixer to obtain overall (activator + MK) molar ratios of SiO2/
Al2O3 (S/A) of: 2.7, 3.0 and 3.3; Na2O/Al2O3 (N/A) of: 0.7, 0.85 and 1.0; H2O/Na2O
(H/N) of: 11 and 13. The sand, previously saturated with 4.0 wt% of water was then
added to the geopolymeric pastes and mixed for about 3 min. The mortars were cast
into cubic molds of 50 mm per side and vibrated for 2 min. The samples were cov-
ered with plastic film and cured at 20 °C in isothermal chambers up to 28 days; in
order to evaluate the effect of the curing temperature, some selected compositions
were also initially cured at 75 °C for 24 h with a subsequent curing at 20 °C for
28 days.
For each formulation, the compressive strength (CS) was determined using an
automatic hydraulic press according to the standard procedure ASTM C109 [33]
after 1, 3, 7, 14 and 28 days; the data were reported as the average from four tested
cubes randomly taken from the batch. After the mechanical testing, samples of
Table 1
Chemical composition of the MK from 30 EDS spot analysis.
Oxide (wt.%) SiO2 Al2O3 Fe2O3 TiO2 K2O
MK 75.41 22.91 0.64 0.52 0.52
Fig. 1. X-ray diffraction patterns of kaolin and metakaolin treated at 750 °C for 6 h.
Table 2
Compositions of binders prepared to study the effect of the chemical composition on
the strength development of geopolymer mortars.
Composition name Molar ratios Water/binder
SiO2/Al2O3 Na2O/Al2O3 H2O/Na2O
A1 2.7 0.7 11 0.31
A2 13 0.37
A3 0.85 11 0.38
A4 13 0.44
A5 1.0 11 0.45
A6 13 0.52
B1 3.0 0.7 11 0.31
B2 13 0.37
B3 0.85 11 0.38
B4 13 0.44
B5 1.0 11 0.45
B6 13 0.52
C1 3.3 0.7 11 0.31
C2 13 0.37
C3 0.85 11 0.38
C4 13 0.44
C5 1.0 11 0.45
C6 13 0.52
R. Arellano-Aguilar et al. / Construction and Building Materials 50 (2014) 642–648 643

3. fractured pastes and mortars were chosen, crushed by hand with a metallic mortar
and pestle and then immersed in acetone for 2 days to eliminate the remaining
water, and then dried in a vacuum oven at 40 °C for 48 h to stop the reactions. Sam-
ples were ground in a planetary mill (PM 400/2; Restch, Newton, PA), using agate
media, to pass the #100 sieve. The ground powders were characterized by X-ray dif-
fraction (XRD, Philips D-Expert, Netherlands) in a range of 7–60° (2h) with a step
size of 0.03° and count time of 2 s/step, using Cu Ka (1.542 Å) radiation.
For scanning electron microscopy (SEM) analysis, samples were mounted in
epoxy resin, then ground and polished using a series of silicon carbide sand paper
of numbers from 320 to 800 and diamond pastes down to 1/4 lm. The samples
were carbon coated to make them conductive under the microscope (ESEM Philips
XL30, Eindhoven, the Netherlands) equipped with energy-dispersive spectroscopy
(EDS). Representative backscattered electron images of microstructures were taken
at 500 magnifications in high vacuum mode using an accelerating voltage of 20 kV,
and a working distance of 9.8–10 mm. The EDS spot analyses were collected
throughout the microstructures at 20 kV with a time of analysis of 50 s.
3. Results and discussion
3.1. Compressive strength (CS)
This section describes the effect of the chemical composition on
the CS development for mortars cured at 20 °C up to 28 days.
Fig. 2(a) shows the influence of the molar ratios S/A and N/A on
the strength development for geopolymer mortars with
aggregate:binder ratio = 3:1 and H/N = 11. After 1 day, none of
the mortars developed CS over 2 MPa, nevertheless after 7 days
of curing, all compositions surpassed the 10 MPa; the curing time
favored the progress of the reactions of geopolymerization of the
binder and hence the CS development of the mortars. It was note-
worthy, that increasing the S/A and N/A ratios, the CS was favored,
suggesting a strong influence of the chemical composition of the
activating agent [23,34,35].
After 28 days the addition of soluble silicate species, observed
by the increase of the S/A ratio, favored the gain of CS for mortars
with N/A = 0.85 and 1.0. The latter showed the highest CS of 45, 55
and 58.3 MPa for S/A = 2.7, 3.0 and 3.3, respectively. Previous stud-
ies [34,36] have reported that the densification of the geopolymer
gel increases with the S/A ratio, so the aggregate particles were
effectively bonded by a dense geopolymeric gel; also, by increasing
the S/A ratio, it is probable that more Si–O–Si bonds were formed,
which are stronger than the Si–O–Al and Al–O–Al bonds [37–39].
On the other hand, for the mortars with N/A = 0.7 the effect of
the S/A ratio showed a bell-type behavior, in which the higher CS
of $45 MPa was for the mean value of S/A = 3.0.
Regarding the effect of the N/A ratio, for S/A = 2.7, the strength
increased with higher N/A ratios, and a similar effect was noted for
samples with S/A = 3.0 and 3.3; however, for the latter the increase
of the N/A ratio from 0.85 to 1.0, did not improve substantially the
CS. The results suggest that for H/N = 11 and S/A P 3.0, an attempt
to enhance the CS using a stronger alkaline solution, by increasing
the N/A ratio beyond 0.85 through the addition of NaOH, seems lit-
tle practical and will increase the costs and environmental impact
of the mortars. In fact, a higher alkalinity over a certain limit has
deleterious effects, because the excess of alkali hinders the conden-
sation of geopolymeric gel, resulting in problems of efflorescence
and brittleness due to the excess of free alkalis [23,40,41].
Fig. 2(b) shows that the patterns of CS vs time for mortars with
H/N = 13 were fairly similar to those described for H/N = 11, in that
the highest values of CS at 14 and 28 days, of 44.8 and 51.2 MPa
respectively, were developed for mortars with ratios S/A = 3.3
and N/A = 1.0. The effect of increasing the CS with the N/A was
noticeable only for S/A = 3.3, but not very clear for mortars with ra-
tios S/A = 2.7 and 3.0; for the latter, similar CS were noted for the
use of N/A = 0.7 and 1.0, and the highest CS were for samples with
N/A = 0.85. It was noticeable that the CS decreased as the H/N
molar ratio increased, indicating a strong influence of the water
content in the initial mixture of the binder. Although a sufficient
amount of water promotes a better workability and mobilization
of the alkaline ions in the fresh pastes, the excess of water may
result in the leaching of alkalis, incomplete reactions, extensive
shrinkage cracking and high porosity in the hardened microstruc-
tures [14,21,42]. The latter means that any water that does not
participate in the reactions finally represents voids, porosity or de-
fects which affect negatively the CS of the formulated mortars.
The effect of the curing temperature was investigated for some
selected mortars with aggregate:binder ratio of 3:1, H/N = 11, N/
A = 0.85 and S/A = 3.0 and 3.3 (compositions B3 and C3, respec-
tively), the comparative results are presented in Fig. 3. After
1 day of curing, the higher temperature (75 °C) accelerated notably
the chemical reactions of the fresh mixtures inducing a rapid
development of CS at early ages of curing relative to mortars cured
at 20 °C. The CS of mortars B3 and C3 were similar at $48 MPa,
while those for samples cured at 20 °C were lower than 2.5 MPa
due to the retarded setting of the geopolymer binder. At 14 days
the samples cured at 20 °C reached more than 27 MPa, whereas
those cured at 75 °C only showed minor variations relative to the
CS achieved at 1 day.
Fig. 2. Effect of the molar ratios SiO2/Al2O3 and Na2O/Al2O3 on the compressive strength of mortars with aggregate:binder ratio of 3:1: (a) with ratio H2O/Na2O = 11 and (b)
ratio H2O/Na2O = 13.
644 R. Arellano-Aguilar et al. / Construction and Building Materials 50 (2014) 642–648

4. After 28 days, the CS of mortars cured at 20 °C increased notice-
ably reaching between 53 and 57 MPa, while that the values of CS
of samples exposed at 75 °C were of 48–51 MPa. This indicates that
although the curing at high temperature accelerates the reactions
during the first day, the polymerization processes does not show
a significant progress at later ages of curing and the CS remains
with small variations. It is also important to note that the curing
at high temperature (75 °C) also favors the condensation of less
dense microstructures with lower strengths [43] than those
showed by samples cured at 20 °C. Nonetheless, the attained CS
of the studied mortars in both regimes of curing is of interest con-
sidering that the starting mineral contains a very low proportion of
reactive phase ($50% kaolinite).
Fig. 4 shows results of CS versus time as a function of the aggre-
gate:binder ratio of 3:1, 5:1 and 7:1 for the formulations B3 and C3
cured at 20 °C; the results of neat pastes (without aggregates) of B3
and C3 are also included for reference. Although at 1 day the neat
pastes and mortars did not develop strength (regardless of the ra-
tios S/A, N/A and aggregate content), the CS increased over time.
After 14 days the mortars B3 and C3 with aggregate:binder ratio
of 5:1, reached 30 and 25.5 MPa, respectively; moreover, both mor-
tars surpassed the 35 MPa at 28 days and showed a trend towards
higher strengths over time. On the other hand, mortars B3 and C3
with 7:1 developed about 30 MPa after 28 days, and also showed a
favorable trend to increase the CS vs time.
The CS of the geopolymeric mortars decreased with increasing
the aggregate:binder ratio. It has been reported that the addition
of aggregate or granular fillers in some cases may strengthen the
geopolymer matrix [44]; however, in most cases fine-grained fill-
ers typically reduce the mechanical properties of the binders, in
agreement with our results. Nonetheless, the CS 30 MPa after
28 days may be considered as promising for extrapolation in con-
cretes for structural purposes, alternatively, the amount of aggre-
gate could be increased for applications in masonry materials
[45]. This is important because the reduction in the amount of bin-
der (increasing that of aggregate) could be reflected in lower costs,
so the proper amount of aggregate is to be used depending on the
required application. The consumptions of MK for the mortars with
ratios 3:1, 5:1 and 7:1 were of about 500, 360 and 280 kg/m3
of
mortar, respectively; however, it must be borne in mind that the
actual metakaolinite consumption is 50% of such figures (250,
180 and 140 kg/m3
), which accounts for a lower demand of alka-
line agents when compared to a high purity MK. Although it is
noted in the literature that MK geopolymers require high amounts
of sodium silicate and sodium hydroxide, resulting in high CO2
emissions [20], the use of low grade metakaolin is interesting be-
cause a low demand of alkaline agents also would allow the reduc-
tion of the CO2 emissions.
The CO2-e associated to the amount of binder B3 required to
produce one m3
of a mortar of an aggregate: binder ratio of 5:1
and 40 MPa at 28 days (mortar identified as B35:1), can be esti-
mated using data of CO2 emissions from the literature, and com-
pared to concretes based on PC of similar compressive strength.
Concretes made with PC of 28 day strengths of 36–40 MPa, simi-
lar to that of mortar B35:1, have been reported to consume 328–
356 kg/m3
of cement [18,20]. Table 3 shows, that by using data of
CO2-e reported by Turner and Collins [19], one m3
of mortar
B35:1 produces 383.1 kg of CO2-e/m3
, while for the concrete of
PC the emissions are of 280.4 kg of CO2-e/m3
; with this data
set, the emissions of mortar B35:1 are 26.8% higher than the PC
concrete. In contrast, when the data set reported by McLellan
et al. [18] is considered, the fabrication of one m3
of mortar
B35:1 results in emissions of 226.7 kg of CO2-e/m3
, while a PC
concrete results in 342 CO2-e/m3
; thus the geopolymeric mortar
is environmentally more advantageous than the PC concrete with
a reduction of 33.71% less CO2-e/m3
. These results suggests that
caution must be exerted when evaluating the environmental im-
pact of geopolymers and comparing it to other binders, as many
factors related to the energy and CO2 emissions during the pro-
duction of the raw materials must be considered, which accord-
ing to McLellan et al. [18] may present a wide variation;
moreover, the mineralogy of the metakaolin is also to be taken
into account.
On the other hand, the CS of concretes formulated using lime-
stone aggregates with a cement consumption of 230 and 330 kg/
m3
, similar as the mortars of this investigation, attained the target
28 days CS of 17 and 25 MPa, respectively [46]. The CS of such PC
concretes may be enhanced by reducing the water/cement ratio
by using superplasticizers; whereas additives were not required
for the geopolymeric mortars investigated, and the ratio water/bin-
der of about 0.38 allowed an excellent workability and strength.
Moreover, the 28 days CS of concretes based on activated blast fur-
nace slag, with consumptions of 230 and 330 kg/m3
, were of 28
and 35 MPa respectively [46], which are similar to those reported
in this study for the mortars 7:1 and 5:1. This evidences the poten-
tial advantages of the use of low grade MK to produce concretes
with suitable properties similar to those showed by alkali-acti-
vated blast furnace slag cements, which have been used for dec-
ades [47,48].
Fig. 3. Effect of the curing temperature at 20° and 75 °C on the compressive
strength of geopolymeric mortars with aggregate:binder ratio of 3:1.
Fig. 4. Compressive strength versus time of geopolymeric pastes and mortars with
aggregate:binder ratios of 3:1, 5:1 and 7:1.
R. Arellano-Aguilar et al. / Construction and Building Materials 50 (2014) 642–648 645

5. The high CS observed is also attributable to the inherent
strength of the neat binders and their effective bond with the lime-
stone aggregates. After 28 days, both pastes (B3 and C3) reached
values of CS of $84 MPa, which were higher than other reported
using MK of high purity. Rowles and O’Connor [17] reported values
of CS of 64 MPa for geopolymer pastes cured at 75 °C 24 h and
7 days at 20 °C using a high purity kaolin (containing $93 wt.% of
kaolinite as reactive phase) and S/A = 5.0 and N/A = 1.29 (equiva-
lent atomic ratios Si/Al = 2.5 and Na/Al = 1.29); while Burciaga-
Diaz et al. [23] reported CS between 60 and 75 MPa for geopolymer
pastes using a clay mineral with 70% of kaolinite (reactive phase)
and molar ratios of S/A = 2.96–3.29 and N/A = 0.93.
The aforementioned evidences that the use of a kaolin mineral
of low purity (50% kaolinite) is also suitable to obtain geopolymer
binders of high CS (85 MPa) and low costs associated to cheaper
raw materials and lower demand of alkaline agents.
3.2. X-ray diffraction (XRD)
Fig. 5 shows the XRD patterns of MK, geopolymer pastes and
mortars 3:1 (B3 and C3) with ratios S/A = 3.0 and 3.3 and N/
A = 0.85 after 28 days of curing at 20 °C and 75 °C. The patterns
of the pastes show sharp peaks of crystalline phases of quartz
and cristobalite from the parent MK, this indicates that the crystal-
line phases were not involved in the geopolymerization process,
but were rather present as inactive fillers. Only the amorphous
fraction was reactive and participated in the geopolymerization
reactions.
After 28 days, a broad hump among 20°–35° 2h was observed,
regarded as an indication of the characteristic formation of amor-
phous aluminosilicates or geopolymeric gel which is responsible
for the strength. In agreement with other studies, the geopolymer-
ization process shifted the location of the amorphous hump of the
initial MK towards higher angles in the XRD patterns [4,7,22]. The
curing at elevated temperature of binders B3 and C3 did not signif-
icantly modified the crystalline fraction of the reacted pastes, and a
similar wide diffuse halo at about 20°–35° 2h was observed, indi-
cating that regardless curing temperature all geopolymeric binders
are mainly X-ray amorphous, lacking of a long range order in the
atomic structure. On the other hand, the patterns of both mortars
also showed sharp peaks of quartz and CaCO3 from the aggregate,
the amorphous hump previously observed in neat pastes, was less
clear as the CaCO3 predominates in the composition of the pow-
dered samples.
3.3. Microstructural characterization
Fig. 6 presents backscattered electron images of microstruc-
tures from various geopolymer mortars 3:1 cured at both regimes
previously described. EDS spot analyses were taken on the matrix
of geopolymeric products. In general, the mortars exhibited heter-
ogeneous structures at microscale level. Particles of unreacted
crystalline quartz from the MK are readily identified by their irreg-
ular angular shape and smooth surface. CaCO3 particles from the
aggregate, appear with an irregular morphology and porous sur-
faces, with a brighter tone compared to quartz, and unreacted
MK particles that remained after the incomplete dissolution under
the alkaline attack. Such particles were bonded by a dense matrix
of reaction products (MP) that showed the darkest gray tone. The
darkest tone resulted because the water also takes part in the dis-
solution and polycondensation processes (during the geopolymer
synthesis), and a certain proportion of nonevaporable water re-
mains in the final structure [42] reducing the average atomic num-
ber of the MP due to the presence of hydrogen, lowering the
backscatter coefficient and thus the brightness of the phase.
The microstructures confirmed that the crystalline phases are
generally nonreactive and these are present as inactive fillers.
Table 3
CO2-e emissions for the binder B3 used to prepare one m3
of mortar with aggregate:binder ratio of 5:1, and compared to a concrete with PC of similar strength. Data associated to
the aggregates are not considered.
Material Binder composition
(kg/m3
)
Emissions (kg of CO2-e/kg)
using data from Turner
and Collins [19]
Calculated
(kg of CO2-e)
Emissions (kg of CO2-e/kg)
using data from
McLellan et al. [18]
Calculated
(kg of CO2-e)
Metakaolin 360 0.245a
88.2 0.245 88.2
Sodium silicate 165 1.514 249.8 0.38 62.7
Sodium hydroxide (from this work) 24 1.915 45.9 3.16 75.8
MK-geopolymer binder 383.1 226.7
Total emissions
Concrete of Portland cement 342b
0.82 280.4 1.0 342
Data from [19,20]
a
Data not offered by [19] and taken from [18].
b
Average of 328 and 356 kg/m3 from Turner and Collins [19] and Habert et al. [20].
Fig. 5. XRD patterns of MK, geopolymer pastes and mortars B3 and C3 after 28 days
of curing at 20 °C and 75 °C.
646 R. Arellano-Aguilar et al. / Construction and Building Materials 50 (2014) 642–648

6. The EDS results, together with those from XRD, showed that the
main binding phase is constituted by an amorphous gel composed
of Si, Al, Na, O and Ca commonly reported as N–A–S–H gel [22,49].
The presence of Ca in the MP is attributable to the intermixed lime-
stone fine particles into the geopolymeric gel, or to the formation
of Ca-substituted aluminosilicate as the aluminosilicate gel has
also the capacity to incorporate Ca when a calcium source is avail-
able [31].
At 20 °C the microstructure of the mortar B3 was dense and
showed a compact interface between the aggregate and the geo-
polymer binder without cracks or pores, suggesting a very strong
bond with the aggregate which enhanced the CS. Similar features
were also observed for the mortar C3 at 20 °C, and although
some cracks were noted around the interface of some quartz
particles and through the aggregate particles, the CS was not
affected.
On the other hand, the microstructures of mortars cured at
75 °C, showed a larger proportion of unreacted particles relative
to the mortars cured at 20 °C, and the formation of finely distrib-
uted pores of approximately 10 lm, which probably resulted from
the rapid evaporation of water and the fast binder densification as
an effect of increasing the curing temperature. This is in agreement
with Muñiz-Villareal et al. [50], who concluded that the CS of MK-
based geopolymers is strongly dependent on the size and percent-
age of porosity, which are influenced by the curing temperature. In
a similar study, Rovnaník [43] analyzed the effect of the curing
temperature on the structural development of MK-based geopoly-
mers and reported a tendency to increase the pore size and cumu-
lative pore volume with rising the temperature, which in turn
reduced the mechanical properties. Additionally, a higher porosity
promote higher permeability for the penetration of harmful spe-
cies such as chlorides into the structure, so the curing at 20 °C
seems to be more suitable to avoid the porosity and promote the
durability of the geopolymers [39].
4. Conclusions
1. Calcined kaolinitic minerals with relatively low content of
reactive phase (50% MK–50% quartz) can be used to pro-
duce geopolymeric pastes of compressive strengths above
85 MPa after 28 days.
2. The compressive strength of mortars increases with the
ratios S/A and N/A, but decreases as the ratio H/N aug-
ments. The optimal ratios that yielded the greatest strength
were S/A = 3.0–3.3 and N/A = 0.85–1.0.
3. Curing at 75 °C for 24 h was favorable for a rapid strength
gain at early ages and at later ages the CS remained rela-
tively stable. Curing at 20 °C showed slower strength devel-
opment than samples cured at 75 °C, but higher values of
CS after 28 days of curing.
4. When designing a MK-based geopolymer mortar, the deter-
mination of the optimum balance between the aggregate
and binder content as well as the composition of the binder
is important in order to produce geopolymers of suitable
properties.
5. Although the incorporation of limestone sand reduces the
mechanical strength of geopolymeric mortars, green
construction materials can be formulated with aggre-
gate:binder ratios of up to 7:1 with CS higher than
30 MPa at 28 days of curing.
Fig. 6. Microstructures and chemical composition of mortars B3 and C3 with aggregate.binder ratio of 3:1 at 28 days of curing at 20 °C and 75 °C.
R. Arellano-Aguilar et al. / Construction and Building Materials 50 (2014) 642–648 647

7. 6. The matrix of reaction products of mortars showed the con-
densation of compact and dense microstructures, nonethe-
less increasing the curing temperature from 20 °C to 75 °C
accelerated the reactions and the rapid evaporation of
water resulted in the formation of micropores.
Acknowledgement
Thanks to Conacyt México for the scholarship of Arellano-
Aguilar and for funding of the Projects 182424 and 53563-Y.
References
[1] Provis JL, Myers RJ, White CE, Rose V, van Deventer JSJ. X-ray microtomography
shows pore structure and tortuosity in alkali-activated binders. Cem Concr Res
2012;42:855–64.
[2] Maitland CF, Buckley CE, O´ Connor BH, Butler PD, Hart D. Characterization of
the pore structure of metakaolin-derived geopolymers by neutron scattering
and electron microscopy. J Appl Cryst 2011;44:697–707.
[3] Pacheco-Torgal F, Castro-Gomes J, Jalali S. Alkali-activated binders: a review
Part1. Historical background, terminology, reaction mechanisms and
hydration products. Constr Build Mater 2008;22:1305–14.
[4] Barbosa VFF, MacKenzie KJD, Thaumaturgo C. Synthesis and characterization
of materials based on inorganic polymers of alumina and silica: sodium
polysialate polymers. Int J Inorg Mater 2000;2:309–17.
[5] Lee WKW, van Deventer JSJ. Use of infrared spectroscopy to study
geopolymerization of heterogeneous amorphous aluminosilicates. Langmuir
2003;19:8726–34.
[6] Singh PS, Trigg M, Burgar I, Bastow T. Geopolymer formation processes at room
temperature studied by 29
Si and 27
Al MAS-NMR. Mater Sci Eng A
2005;396:392–402.
[7] Rahier H, Simons W, Van Mele B, Biesemans M. Low-temperature synthesized
aluminosilicate glasses. J Mater Sci 1997;32:2237–47.
[8] Palomo A, Glasser FP. Chemically bonded cementitious materials based on
metakaolin. British Ceram Trans J 1992;91:107–12.
[9] Provis JL, Lukey GG, van Deventer JSJ. Do geopolymers actually contain
nanocrystalline zeolites? A reexamination of existing results. Chem Mater
2005;17:3075–85.
[10] Palomo A, Grutzek MW, Blanco MT. Alkali-activated fly ashes cement for the
future. Cem Concr Res 1999;19:1323–9.
[11] van Jaarsveld JGS, van Deventer JSJ, Lukey GC. The characterization of source
materials in fly ash based geopolymers. Mater Lett 2003;57:1272–80.
[12] Yang T, Yao X, Zhang Z, Wang H. Mechanical property and structure of alkali-
activated fly ash and slag blends. J Sust Cem-Based, Mater 2012;1(4):167–78.
[13] Montes C, Zang D, Allouche EN. Rheological behavior of fly ash-based
geopolymers with the addition of superplasticizers. J Sust Cem-Based Mater
2012;1(4):179–85.
[14] Rashad AM. Alkali-activated metakaolin: a short guide for civil engineer – an
overview. Constr Build Mater 2013;41:751–65.
[15] Wang H, Li H, Yan F. Synthesis and mechanical properties of metakaolinite-
based geopolymer. Colloid Surf A 2005;268:1–6.
[16] Davidovits J. Geopolymers: inorganic polymeric new materials. J Thermal Anal
1991;37:1633–56.
[17] Rowles M, O’Connor B. Chemical optimization of the compressive strength of
aluminosilicate geopolymers synthesized by sodium silicate activation of
metakaolinite. J Mater Chem 2003;13:1161–5.
[18] McLellan BC, Williams RP, Lay J, van Riessen A, Corder G. D. Costs and carbon
emissions for geopolymer pastes in comparison to ordinary Portland cement.
J Cleaner Prod 2011;19:1080–90.
[19] Turner LK, Collins FG. Carbon dioxide equivalent (CO2-e) emissions: a
comparison between geopolymer and OPC cement concrete. Constr Build
Mater 2013;43:125–30.
[20] Habert G, d’Espinose de Lacaillerie JB, Roussel N. An environmental evaluation
of geopolymer based concrete production: reviewing current research trends.
J Cleaner Prod 2011;19:1229–38.
[21] Kuenzel C, Vandeperre LJ, Donatello S, Boccaccini AR, Cheeseman C. Ambient
temperature drying shrinkage and cracking in metakaolin-based geopolymers.
J Am Ceram Soc 2012;95(10):3270–7.
[22] Provis J, van Deventer J. Geopolymers: structure, processing properties and
industrial applications. Sawston, Cambridge (UK): Woodhead Publishing Ltd.;
2009.
[23] Burciaga-Diaz O, Escalante-García JI, Gorokhovsky A. Geopolymers based on a
coarse low-purity kaolin mineral: mechanical strength as a function of the
chemical composition and temperature. Cem Concr Comp 2012;34:18–24.
[24] Luz-Granizo M, Blanco-Varela MT, Martínez-Ramírez S. Alkali activation of
metakaolins: parameter affecting mechanical, structural and microstructural
properties. J Mater Sci 2007;42:2934–43.
[25] Jaarsveld JGS, Deventer JSJ, Lukey GC. The effect of composition and
temperature on the properties of fly ash and kaolinite based geopolymers.
Chem Eng J 2002;89:63–73.
[26] Duxson P, Fernández-Jiménez A, Provis JL, Lukey GC, Palomo A, van Deventer
JSJ. Geopolymer technology: the current state of the art. J Mater Sci
2007;42:2917–33.
[27] Galan Huertos E. Kaolin in Spain, features, identification and ceramic tests. Soc
Esp Ceram V Madrid 1974:229 (in Spanish).
[28] Nkoumbou C, Njoya A, Njoya D, Grosbois C, Njopwouo D, Yvon J, et al. Kaolin
from mayouom (western Cameroon): industrial suitability evaluation. Appl
Clay Sci 2009;43:118–24.
[29] Kuenzel C, Grover LM, Vandeperre L, Boccaccini AR, Cheeseman CR. Production
of nepheline/quartz ceramics from geopolymer mortars. J Eur Ceram Soc
2013;33(2):251–8.
[30] Steinerova M. Mechanical properties of geopolymer mortars in relation to
their porous structure. Ceram-Silik 2011;55(44):362–72.
[31] Lee WKW, van Deventer JSJ. The interface between natural siliceous
aggregates and geopolymers. Cem Concr Res 2004;34:195–206.
[32] Pacheco-Torgal F, Moura D, Ding Y, Jalali S. Composition, strength and
workability of alkali-activated metakaolin based mortars. Constr Build Mater
2011;25:3732–45.
[33] ASTM C109. Standard test method for compressive strength of hydraulic
cement mortars. Am Soc Test Mater; 2011.
[34] Duxson P, Provis JL, Lukey GC, Mallicoat SW, Kriven WM, van Deventer JSJ.
Understanding the relationship between geopolymer composition,
microstructure and mechanical properties. Colloids Surf, A 2005;269:47–58.
[35] Zhang Y, Sung W, Li Z. Composition design and microstructural
characterization of calcined kaolin-based geopolymer cement. ACI Mater J
2008;105-M18:156–64.
[36] Lizcano M, Kim HS, Basu S, Radovic M. Mechanical properties of sodium and
potassium activated metakaolin-based geopolymers. J Mater Sci
2007;47:2607–16.
[37] de Jong BHWS, Brown GE. Polymerization of silicate and aluminate tetrahedra
in glasses, melts, and aqueous solutions – I. Electronic structure of
H6Si2O7; H6AlSiO
À1
7 , and H6Al2OÀ2
7 . Geochim Cosmochim Acta
1980;44(3):491–511.
[38] Kong DLY, Sanjayan JG, Sagoe-Crentsil K. Factors affecting the performance of
metakaolin geopolymers exposed to elevated temperatures. J Mater Sci
2008;43:824–31.
[39] Burciaga-Diaz O, Escalante-García JI. Strength and durability in acid media of
alkali silicate-activated metakaolin geopolymers. J Am Ceram Soc
2012;95(7):2307–13.
[40] Duxson P, Lukey GC, Separovic F, van Deventer JSJ. Effect of alkali cations on
aluminum incorporation in geopolymeric gel. Ind Eng Chem Res
2005;44:832–9.
[41] Khale D, Chaudhary R. Mechanism of geopolymerization and factors
influencing its development: a review. J Mater Sci 2007;42:729–46.
[42] Zuhua Z, Xiao Y, Huajun Z, Yue C. Role of water in the synthesis of calcined
kaolin-based geopolymer. Appl Clay Sci 2009;43(2):218–23.
[43] Rovnaník P. Effect of curing temperature on the development of hard structure
of metakaolin-based geopolymer. Constr Build Mater 2010;24:1176–83.
[44] Joseph B, Mathew G. Influence of aggregate content on the behavior of fly ash
based geopolymer concrete. Sci Iran A 2012;19:1188–94.
[45] BS 6073-1-1981. British standard, precast concrete masonry units: Part 1:
Specifications for precast concrete masonry units. British Standards Inst; 1981.
[46] Escalante-Garcia JI, Espinoza-Perez LJ, Gorokhovsky A, Gomez-Zamorano LY.
Coarse blast furnace slag as a cementitious material, comparative study as a
partial replacement of Portland cement and as an alkali activated cement.
Constr Build Mater 2009;23:2511–7.
[47] Burciaga-Díaz O, Díaz-Guillén MR, Fuentes AF, Escalante-Garcia JI. Mortars of
alkali-activated blast furnace slag with high aggregate:binder ratios. Constr
Build Mater 2013;44:607–14.
[48] Shi C, Krivenko PV, Roy D. Alkali-activated cements and concretes. London and
New York: Taylor and Francis; 2006. ISBN I3 978-0-415-70004-7.
[49] Li Chao, Sun Henghu, Li Longtu. A review: the comparison between alkali-
activated slag (Si+Ca) and metakaolin (Si+Al) cements. Cem Concr Res
2010;40:1341–9.
[50] Muñiz-Villareal MS, Manzano-Ramírez A, Sampieri-Bulbarela S, Gasca-Tirado
JR, Reyes JL, Rubio-Ávalos JC, et al. The effect of the temperature on the
geopolymerization process of a metakaolin-based geopolymer. Mater Lett
2011;65:995–8.
648 R. Arellano-Aguilar et al. / Construction and Building Materials 50 (2014) 642–648