Evolution of the pozzolanic activity of a thermally treated zeolite

Raquel Vigil de la Villa • Raul Fernandez •

Olga Rodrıguez • Rosario Garcıa •

Ernesto Villar-Cocina • Moises Frıas

Received: 1 October 2012 / Accepted: 13 December 2012 / Published online: 27 December 2012

� Springer Science+Business Media New York 2012

Abstract Zeolites generally show pozzolanic activity due

to their structural characteristics. The utilisation of po-

zzolans as additions to cements results in added technical

advantages of the construction materials. In this study, the

pozzolanic activity of a thermally treated natural morde-

nite-type zeolite from the Palmarito open-air deposit

(Cuba) was evaluated with respect to a non-treated zeolite.

Initially, a thermal treatment of the zeolite was performed

at different temperatures within the range 300–1000 �C for

5 h in order to evaluate the better temperature of treatment.

Afterwards, the pozzolanic activity was determined for

each temperature after 7 days of reaction with a saturated

Ca(OH)2 solution that simulates the release of lime from

ordinary Portland cement during the hydration reaction.

The higher pozzolanic activity was achieved with the

thermal treatment performed at 300 �C. Therefore, a fur-

ther study studied the evolution of the mineralogical phases

produced during pozzolanic reaction up to 90 days, carried

out with the zeolite treated at that temperature. Consump-

tion of Ca2? in solution and formation of C–S–H-like

phases with low Ca/Si ratio were experimentally observed

as the main pozzolanic products. The thermodynamic study

confirms high reactivity of the zeolite at short-term and

chemical stability of the reaction products after 28 days.

The zeolite thermally treated at 300 �C confirmed an

increase in the pozzolanic activity with respect to the non-

treated zeolite.

Introduction

Natural, zeolite-rich tuffs are presently used in the con-

struction industry as pozzolanic materials [1–3]. A poz-

zolanic material is generally defined as a natural or

artificial material mainly consisting of SiO2 and Al2O3,

lacking in cementitious properties and hydraulic activity,

but able to combine with portlandite [Ca(OH)2] or lime-

releasing materials (e.g. Portland cement) in the presence

of water, at atmospheric temperature, to form reaction

products with hydraulic binding character, calcium silicates

hydrates and calcium aluminates hydrates-like phases

(C–S–H and C–A–H) [4, 5]. Pozzolans are commonly used

as additions to Portland cement and concrete manufactured

to improve the durability characteristics [6–9], reduce the

amount of clinker in the blended matrix and decrease the

energy and environmental costs [10, 11]. In addition, the

heat evolution during cement hydration is decreased

[12–14].

Synthetic and natural zeolite materials, when properly

prepared, achieve higher pozzolanic activity than other well-

known pozzolans, such as silica fume, fly ash, metakaolin-

based activated product or blast furnace slag [15–18].

Several authors have studied the pozzolanic activity of

thermally treated zeolites. Upon heating, zeolites undergo a

series of chemical and structural changes (loss of water,

R. Vigil de la Villa � R. Fernandez (&) � R. Garcıa

Department of Geology and Geochemistry (Associated Unit

CSIC-UAM), Autonomous University of Madrid, Madrid, Spain

e-mail: raul.fernandez@uam.es

R. Fernandez � O. Rodrıguez � M. Frıas

Eduardo Torroja Institute for Construction Science (CSIC),

Madrid, Spain

O. Rodrıguez

National Center for Metallurgical Research (CSIC),

Madrid, Spain

E. Villar-Cocina

Department of Physics, Central University of Las Villas,

Santa Clara, Villa Clara, Cuba

123

J Mater Sci (2013) 48:3213–3224

DOI 10.1007/s10853-012-7101-z

modification of cage dimensions etc.) until they are largely

converted to an X-ray amorphous material [19]. It is widely

accepted that the thermal behaviour of the zeolite frame-

work is related to the Al concentration and that the thermal

stability increases as the Si/Al ratio increases. The expla-

nation is straightforward, as more energy is required to

break the Si–O bond compared to the Al–O bond [20]. The

heating of the zeolite samples normally decreases the early

strength but improves the resistance at long term [21, 22].

In general, the activation of zeolites destabilises the crys-

talline structure to form a less stable and more reactive

aluminosilicate framework; however, it has been evidenced

that the effect of heating on natural zeolites may produce

negligible improvement of the pozzolanic activity or even

reduce it [23].

Because of the several factors controlling the achieve-

ment of higher pozzolanic activity on zeolites and the wide

range of compositions and structures, there is no consensus

on the adequate temperature and dwell time in furnace.

Several natural zeolites have been heated to tempera-

tures in the range 350–1000 �C, for times in the range

2–15 h, in order to study the pozzolanic reaction and

compressive strength of cement–pozzolan mortars [19–23].

Perraki et al. [19] observed higher pozzolanic for an acti-

vated natural zeolite activity after treatment at 400 �C, the

lower temperature selected within the range proposed in

that study. The thermal treatment of the zeolite at 400 �C

improved its pozzolanic reactivity and accelerated the

reaction with Ca(OH)2, compared with a non-treated

sample, but thermal activation at temperatures over 400 �C

had a negative effect. Habert et al. [21] found that when the

zeolites are destabilised, the pozzolanic activity is

improved. They found that the higher pozzolanic activity

was achieved when the zeolites structure had partially

collapsed but had not already recrystallised. The mineral-

ogical composition was a key factor controlling the opti-

mum temperature to treat each sample (350 �C for a natural

phillipsite and 500 �C for a heulandite-rich zeolite). They

proposed the term metazeolite to describe the reactive

phase created by thermal activation of the zeolite before its

recrystallisation into a weakly reactive phase.

In a prior study [24], the pozzolanic reaction of a natural

zeolite-rich mordenite from the Palmarito deposit (Cuba)

was studied as a function of time. The most important

mineralogical and chemical changes observed were the

precipitation of calcium silicates hydrates (C–S–H phases)

with low Ca/Si ratio on the zeolite aggregates and the

consumption of aqueous Ca2? through the pozzolanic

reaction. However, the transformation of the original

mordenite to clinoptilolite was also detected.

In order to assess the potential benefit of heat treatment,

the pozzolanic reaction of the same starting material has

been determined in this study at 7 days after heating to

300, 400, 500, 600, 700, 800, 900 and 1000 �C. The

maximum consumption of Ca2? in the pozzolanic reaction

occurred in the material heat treated at 300 �C; therefore,

this temperature was chosen as the optimum to repeat the

experiment as in [24] and to study the pozzolanic reaction

as a function of time, up to 90 days, to assess the benefit of

using heat-treated rather than as mined zeolitic material in

concrete.

The present study complements the research performed

previously in [24] and compares the evolution of the poz-

zolanic activity of the as mined zeolite against the opti-

mised thermally treated sample at 300 �C. As a novelty

with respect to previously published studies, analysis of the

aqueous solutions have been performed and used for a

thermodynamic study that permits to evaluate the miner-

alogical stability of the new-formed minerals and their

evolution with time. This is important to predict the

chemical stability of the low crystalline phases typical of

cement systems.

Materials and methods

Thermally treated natural samples from the Palmarito

zeolite deposit were studied along the reaction with port-

landite [Ca(OH)2] in order to determine their pozzolanic

activity as a function of time. The zeolite deposit of

Palmarito is located within the province of Santiago de

Cuba, 50 km N of Santiago de Cuba city and 2 km SE of

Palmarito de Cauto town. The deposit stratigraphy is

constituted by several well-defined sedimentary sequences,

dominated by conglomeratic varieties in the base and

vitroclastic zeolitized tuffs on the top. The zeolite deposit

is related to back-arc sequences from Palaeocene–Eocene

volcanic island arcs. Due to the transformation of volcanic

shards of medium-acid composition, clinoptilolite origi-

nates substituting volcanic shards, and in a latter hydro-

thermal process mordenite substitutes for clinoptilolite.

The rocks, of green colour, are found in a well-stratified

discontinuous zone in layers of 10–15 cm thickness [25].

The crystalline components in the bulk sample were

quantified by X-ray diffraction (XRD) using the procedure

proposed by Schultz [26]. The integrated intensities (areas

under the XRD peaks) were adjusted to Gaussian functions

with subtraction of the base line, using the software

DRXWin� [27]. The comparison of the integrated inten-

sities of the diffraction peaks determines the weight frac-

tions of constituents. The XRD analysis is therefore

considered a semi-quantitative method that serves as a

proxy to identify the minerals evolution. The rock sample

from Palmarito is predominantly constituted by mordenite

(79 wt%), K-feldspar and plagioclases (21 wt%). The

observations by SEM showed aggregates of mordenite with

3214 J Mater Sci (2013) 48:3213–3224

123

poorly defined geometry in addition to other matrix min-

erals rich in iron and K-feldspars and plagioclases with

variable concentration in K, Na and Ca [24].

One gram of the thermally treated zeolites was sieved to

obtain a homogeneous sample and placed into a 75 ml

polyethylene flask with a saturated portlandite [Ca(OH)2]

solution that simulates the release of lime produced by an

OPC during the hydration reaction of a cement matrix. The

container was hermetically sealed, and the thermally trea-

ted zeolite–portlandite mixture was wet-cured at 40 �C for

7 days. It was proved that the higher pozzolanic activity

was achieved after treatment at 300 �C; therefore, the

pozzolanic activity was evaluated at this temperature by the

method described above for independent periods of 1, 7, 28

and 90 days.

The solid commercial Ca(OH)2 was calcined at 900 �C

for 2 h in order to decarbonate the chemical reagent, and

remove volatile impurities, mainly CO2. Carbonation of the

samples is a side effect that inhibits the pozzolanic reaction

because Ca(aq)2? is consumed to form CaCO3 instead of reac-

tion with the reactive aluminosilicate framework. Partial

carbonation is, however, expected during manipulation of

samples once the pozzolanic reaction has ceased due to

interaction with the atmospheric CO2(g). The Ca2? concen-

tration of a solution at portlandite saturation and 40 �C is

17.7 mmol/l. All experiments were performed in duplicate.

At the end of the reaction time, the hydrated solid

sample was filtered, washed with ethanol and desiccated

for 24 h in order to stop the hydration reaction. Then, the

mineralogy was studied by XRD and scanning electron

microscopy (SEM) to identify the products of the pozzo-

lanic reaction.

X-ray powder diffraction patterns were obtained using a

Siemens D-5000 diffractometer with a Cu anode, operating

at 30 mA and 40 kV, using divergence and reception slits

of 2 and 0.6 mm respectively, and measuring in 0.04 2hgoniometric steps for 3 s. The SEM equipment was a FEI

INSPECT microscope with an energy dispersive X-ray

analyser (EDX).

Immediately after solid–liquid separation, a fraction of

the aqueous solution was used to measure pH. Major cations

and anions in solution were measured by ICP-MS with an

Elan 6000 Perkin-Elmer Sciex analyser. The amount of

Ca(OH)2 consumed in the reaction was determined by

titration of aqueous Ca2? and OH- with ethylenediamine-

tetraacetic acid (EDTA) and diluted HCl, respectively. The

measurements obtained for Ca2? at 25 �C were normalised

with respect to a reference blank solution saturated in port-

landite at 40 �C using the geochemical code PHREEQC

[28].

In addition, analyses by Fourier transform infrared

spectroscopy (FTIR), and thermal analyses: thermogravi-

metry (TG) and differential thermal analysis (DTA) were

performed in order to confirm the mineralogical changes

observed by XRD and SEM-EDX. The derivative ther-

mogravimetry (DTG), which is the mathematical derivative

of the TG, was also used. The IR spectral analyses were

performed using a spectrometer ThermoScientific NICO-

LET 6700 and the TG/DTG-DTA analyses using a STA-

TON, STA 781 instrument, working at temperatures from

25 to 1050 �C, at a heating rate of 10 �C/min.

To study the state of equilibrium of the system, the

activities of aqueous species were calculated by entering,

in PHREEQC, the concentrations of Ca, Na, K, Mg, Al and

pH measured in solution. Aqueous Si, the dominant spe-

cies, at high pH, of which is HSiO3-, was not determined

by ICP-MS because the range of error for this species is too

large in the equipment used. Instead of the analytical

quantification, aqueous Si was equilibrated in PHREEQC

with quartz to fix a constant value (SiO2(aq) = 170 mmol/l).

This is coherent with calculations performed in previous

studies for zeolites dissolution reactions [24, 29]. The

electrochemical charge in solution was balanced with pH.

The saturation indices (SI) of minerals able to dissolve/

precipitate in the system were calculated with PHREEQC

to predict the stability of reaction products as a function of

time. The thermodynamic database used in PHREEQC was

the Lawrence Livermore National Laboratory (LLNL)

database which is a reformatted version of the EQ3/EQ6

database [30]. Crystalline C–S–H phases (with Ca/Si ratios

0.8, 1.2 and 1.6) and C–A–S phases (C3AH6 and C4AH13),

not included in LLNL database, were added from the

database THERMODDEM [31]. The dissolution reactions

of minerals, acquired from THERMODDEM, were prop-

erly adapted to fit the same aqueous species as in LLNL

(SiO2 instead of H4SiO4).

Results of the thermal treatment

XRD

From 300 to 800 �C, the crystalline phases were mainly

composed of mordenite and Na- and K-feldspar, as in the

raw sample [24]. At 900 �C, the structure of the zeolite was

destroyed and broad reflections were observed, indicating

the presence of an amorphous material. Characteristic

reflections of K-feldspars were still present at 900 and

1000 �C, although showing low intensity. At 1000 �C, the

reflections at 4.15 and 2.53 A indicated the formation of a

carnegieite-like phase (carnegieite is a crystalline phase of

aluminium silicate with ideal formula NaAlSiO4). The

same phenomenon observed in this study at 900 and

1000 �C (conversion to amorphous material followed by

formation of crystalline material with sharp diffraction

peaks) has been observed on the thermal treatment of NaX

J Mater Sci (2013) 48:3213–3224 3215

123

zeolite [32]. The transition from the initial silicates

(mordenite and K-feldspars) to the carnegieite-like phase

requires the breaking up and reforming of the silica

framework; therefore, the original minerals of the natural

sample were mostly destroyed. The carnegieite-like phase

was extensively observed by SEM as spheres uniformly

packed (Fig. 1).

Thermal analysis

Although thermal analyses are not conclusive tools by

themselves for identifying minerals, they allow clear dis-

tinctions to be made between the zeolite thermally treated

at different temperatures. Three thermal effects could be

highlighted: loss of zeolitic water, collapse of the zeolite

structure, and the progressive amorphisation of the zeolite

structure caused by the thermal treatment (Table 1).

In the samples thermally treated in the range

300–800 �C, superficial dehydration and loss of the initial

water content was achieved in the range of temperatures

60–200 �C. The TG analyses showed that between 4 and

6 wt% was lost within this range of temperature, corre-

sponding to the initial humidity of the samples and the

zeolite dehydration steps. This water content in the samples

was achieved after the thermal treatment, and the degree of

humidity is independent of the temperature applied.

Because much of the water in structural cavities is asso-

ciated with extra-framework cations, the nature of zeolite

dehydration (and the total amount of water) is strongly

dependent on the extra-framework cations, in particular

their hydration energy. In general, studies on zeolites

indicate that most zeolites undergo dehydration-induced

reductions in volume that collapse when water molecules

are removed. Specific types of water do not exist in zeo-

lites; instead, the minerals dehydrate in a pseudo-continu-

ous manner, and the water–cation interactions are dynamic

during dehydration [33].

From 200 to 800 �C, the loss of mass observed in the

TG analyses was attributed to dehydroxylation steps of the

zeolite structure. A uniform decrease of mass was deter-

mined with the increasing temperature of treatment.

Therefore, this should be related to the structure achieved

after heating.

The samples heated at 900 and 1000 �C did not show

any loss of mass by TG because the structures were already

destroyed. The new-formed phases were not even able to

hydrate in atmospheric conditions.

Results of the pozzolanic reaction after 7 days

XRD

The X-ray diffractograms on the thermally treated samples

after 7 days of pozzolanic reaction show differences

compared to the samples before the pozzolanic reaction

(Fig. 2). The quantification of XRD reflections on the

thermally treated solid samples that reacted for 7 days at

40 �C with the saturated Ca(OH)2 solution permitted the

identification of the mineralogical transformations

involved in the reaction as a function of temperature

(Table 2). The limitations on this quantification are

assumed in advance; therefore, the only objective is to

compare the mineralogical transformations as a function of

the thermal treatment applied. Mordenite was quantified in

the (110) reflection, at 13.58 A, K-feldspars at 3.83 A, the

carnegieite-like phase at 4.25 A, the isomorphic series

heulandite–clinoptilolite at 7.91 A, the hydrotalcite-like

phase at 7.56 A, calcite at 3.03 A and C–S–H phases at

3.08 A. In general, C–S–H gels are the main product in the

pozzolanic reaction of raw zeolitic rocks [34], and have

been already observed after 7 days of reaction [35]. C–S–H

phases were quantified by XDR in this study, but the broad

Fig. 1 SEM photograph of the sample treated at 1000 �C. The sphere

aggregates are identified as carnegieite-like phases

Table 1 Loss of mass (in wt%) obtained by TG analyses in samples thermally treated in the range 300–1000 �C

T (�C) Raw 300 �C 400 �C 500 �C 600 �C 700 �C 800 �C 900 and 1000 �C

60–200 5.79 5.97 4.23 5.13 3.90 5.21 4.60 0.00

200–800 6.47 4.04 4.06 2.95 2.34 1.92 1.37 0.00

800–1000 0.01 0.08 0.04 0.02 0.03 0.04 0.03 0.00

3216 J Mater Sci (2013) 48:3213–3224

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reflections indicate their low crystallinity. At 300 �C, a

shift to lower d-spacing in the (200) reflection of mordenite

(9.06 A) and splitting into the range 8.95–9.03 A were

characteristic of the coexistence of the isomorphic series

heulandite–clinoptilolite (Fig. 3), as in the original raw

sample [24]. However, the coexistence of the isomorphic

series heulandite–clinoptilolite was lost at higher temper-

atures. The (200) reflection of mordenite became wider in

the range 9.07–9.05 A indicating an increasing effect of

amorphisation. Calcite is observed in all the diffractograms

from 300 to 1000 �C. This mineral does not form as a

consequence of the pozzolanic reaction since carbonates

were not present in the raw zeolite or in the thermally

treated samples, and were excluded in the pozzolanic

reaction. Then, it is assumed that calcite forms as a

consequence of the manipulation of samples after the

pozzolanic reaction. This reaction cannot be prevented but

influences in the calculation of Ca2? consumed in the

pozzolanic reaction must be corrected thereafter.

A hydrotalcite-like structure only formed at 1000 �C. This

phase was observed in a similar study as a reaction product

ofthe pozzolan/lime/water system, using paper sludge as

pozzolan thermally activated at 700 �C for 2 h [36].

SEM

The sample treated at 300 �C after pozzolanic reaction for

7 days showed web-like porous surfaces of C–S–H phases

covering mordenite and feldspar aggregates (Fig. 4a). The C–

S–H phases are the main product of the reaction. The external

morphology of aggregates was very similar to those obtained

for the raw sample without thermal treatment after pozzolanic

Fig. 2 XRD on disoriented powder of the natural zeolite from

Palmarito thermally treated from 300 to 1000 �C, after reaction with a

Ca(OH)2 saturated solution at 40 �C for 7 days. Ca calcite, Cgcarnegieite-like phase, CSH C–S–H, Fel K-feldspar, HT hydrotalcite-

like phase, Mor mordenite

Table 2 Mineralogical composition (wt%) quantified by XRD of the thermally treated zeolite in the range 300–1000 �C after 7 days of reaction

with the saturated Ca(OH)2 solution

T (�C) Mordenite Feldspars HEU–CLI C–S–H Calcite Carnegieite HT

Initial 56 20 9 15 – – –

300 58 17 6 11 8 – –

400 60 17 – 11 12 – –

500 55 20 – 10 15 – –

600 60 15 – 10 15 – –

700 56 15 – 9 20 – –

800 52 12 – 8 28 – –

900 – 8 – 6 – 86 –

1000 – 4 – 4 – 89 4

HEU heulandite, CLI clinoptilolite, HT hydrotalcite-like phase

Fig. 3 Details of random-powder XRD patterns showing the forma-

tion of a secondary clinoptilolite-type zeolite in the raw sample and

after thermal treatment at 300 �C (values given in d-spacing)

J Mater Sci (2013) 48:3213–3224 3217

123

reaction for 7 days in Fig. 4b [24]. The aspect of the C–S–H

phases presented a very disordered layer microstructure. The

pozzolanic reaction proceeds by rupture of Si–O and Al–O

bonds on the surface of the pozzolan (the thermally treated

natural zeolite) by attack of hydroxyl anions (OH-) and

reaction of the silicates and aluminates anions with Ca2? in

solution, to form C–S–H phases. The morphology of the C–S–

H phases covering mordenite and feldspar aggregates slightly

varied with the increasing temperature of treatment up to

800 �C, but the porous morphology decreased. At 800 �C,

incipient growth of C–S–H phases is observed only on very

irregular surfaces and cracked edges of mordenite and feld-

spar aggregates (Fig. 4b).

The formation of C–S–H occurs on the surface of the

aggregates that serve as substratum for the nucleation.

Therefore, the chemical analyses by EDX on the C–S–H

phases did not yield acceptable compositions since the

electron beam damaged the very thin C–S–H layers pene-

trating into the zeolite and giving a mixed composition. The

chemical compositions obtained by EDX revealed a

decreasing Ca content with the increasing temperature, while

the silica content increased shortly from 72 to 78 wt%

(Table 3). These results indicated that higher content of

Ca2? reacted in the pozzolanic reaction at lower

temperatures, as supported by the determination of the

pozzolanic activity (see below) and also the increasing Ca/Si

ratio with the decreasing temperature of treatment. The

mixed compositions obtained by the EDX analyses on

external surfaces did not permit to calculate the atomic Ca/Si

ratios of the C–S–H phases; however, an approximation was

performed assuming an ideal composition for the mordenite:

K0.9Na2.1Ca1.5Al6Si30O72:22H2O [29], occluded under the

C–S–H recovering. The selected composition corresponds to

a mordenite originated by diagenetic alteration in volcanic

tuffs (the same origin as in Palmarito). Assuming that Al did

not react to form C–A–H phases and remained constant in the

mordenite, and considering that the ideal composition

maintained the Si/Al and Ca/Al ratios at 5 and 0.83,

respectively, the excesses in Si and Ca were accounted as

part of the new-formed C–S–H phases. Then, the Ca/Si ratios

were calculated as 1.2 and 0.9 for the samples treated at 300

and 400 �C, respectively. At higher temperatures, the pres-

ent assumption lost validity because the thermal treatments

are able to vary the framework Si/Al ratio in mordenite. In

any case, taking into account this calculation and the aspect

of the C–S–H phases observed by SEM, a decreasing Ca/Si

ratio with the increasing temperature of treatment could be

inferred.

Fig. 4 Aspect of reaction products identified by SEM after 7 days of

pozzolanic reaction a nucleation of C–S–H phases recovering

aggregates of mordenite in the sample treated at 300 �C, and

b mordenite1 and feldspar2 aggregates in the sample treated at

800 �C with incipient C–S–H phases3 nucleating on irregular surfaces

Table 3 Chemical compositions in oxides (wt%) obtained by EDX analyses on samples thermally treated in the range 300–800 �C after 7 days

of pozzolanic reaction

Oxide 300 �C 400 �C 500 �C 600 �C 700 �C 800 �C

Na2O 0.8 ± 1.0 1.3 ± 0.8 1.5 ± 0.4 2.8 ± 0.8 1.4 ± 0.9 2.7 ± 0.7

Al2O3 11.6 ± 1.0 12.0 ± 0.7 11.3 ± 0.3 12.8 ± 0.8 13.2 ± 1.0 12.7 ± 1.3

SiO2 72.0 ± 1.3 72.5 ± 1.0 76.6 ± 1.1 76.0 ± 1.3 77.2 ± 1.1 78.3 ± 1.4

K2O 0.7 ± 0.5 1.5 ± 0.4 1.0 ± 0.4 0.9 ± 0.1 1.5 ± 0.9 1.6 ± 0.8

CaO 14.9 ± 0.4 12.7 ± 0.8 9.6 ± 0.8 7.5 ± 0.5 6.6 ± 0.4 4.8 ± 0.6

3218 J Mater Sci (2013) 48:3213–3224

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Consumption of Ca2? in solution

The reaction between the thermally treated zeolites and the

Ca(OH)2 at 40 �C after 7 days of reaction showed that the

higher pozzolanic activity was achieved for the sample

treated at 300 �C (16.1 out of 17.7 mmol/l of portlandite

solution was consumed, ideally to form C–S–H and other

cement phases). The pozzolanic activity slightly decreased

with the increasing temperature of treatment up to 900 �C,

but at 1000 �C, the lack of mordenite and K-feldspars and

formation of a carnegieite-like phase which is chemically

inert and alkali resistant, reduced considerably the searched

reactivity of the solid (Fig. 5). A correction of the pozzo-

lanic activity was calculated assuming that the calcite

quantified by XRD corresponded to Ca2? adsorbed on the

surface of mordenite aggregates that did not react in the

pozzolanic reaction. Once the reaction was terminated, and

the solid and aqueous phases were separated, this Ca2?

carbonated easily during manipulation of the solid samples

by interaction with atmospheric CO2(g). The EDX analyses

confirmed high concentration of calcium carbonates on the

surface of aggregates, indicating that calcite did not pre-

cipitate as a secondary phase in the pozzolanic reaction but

as a residue of the non-reactive Ca2? that formed small

crystals with very poor morphology. The mass of calcite

quantified by XRD was transformed into concentration of

Ca2? required to form this mineral and then discounted

from the calculated Ca2? consumed in the pozzolanic

reaction. This assumption reduced moderately the pozzo-

lanic activity of samples treated up to 800 �C. At 900 �C,

calcite could not be quantified, and the correction at this

temperature was not performed, but the low pozzolanic

activity achieved compared to that obtained at lower tem-

peratures discarded any interest to study the reaction at

900 �C or higher temperatures in further detail.

Vigil de la Villa et al. [24] postulated that part of the

Ca2? consumed in solution, normally attributed to the

pozzolanic reaction, could be spent by an exchange reac-

tion between the original zeolite and a secondary zeolite or

by a dissolution/precipitation reaction, also to form a sec-

ondary zeolite, because for some zeolites (e.g. clinoptilo-

lite) the Ca-dominant type is the most thermodynamically

stable variety [37]. In agreement with this argument, the

calcite present in samples after the pozzolanic reaction,

detected in the range 300–800 �C, could have formed if

hydrated Ca2? was weakly adsorbed either on exchange

sites or at the external surface of the zeolite and would

react easily with the atmospheric CO2(g). This spontaneous

reaction would create a large number of nanocrystals of

calcite, not observable by SEM. The fact that calcite was

not detected at 900 �C after the pozzolanic reaction would

confirm that once the initial zeolite is destroyed, the weak

adsorption of Ca2? in (or on) the zeolite, and therefore the

carbonation, can no longer occur.

These results were compared to previous determinations

obtained for the non-treated raw zeolite using the same

methodology after 7 days of reaction [24]. The determi-

nation of the pozzolanic activity, even with the correction

of consumed Ca2? (not performed for the raw sample),

showed that heating at 300 �C for 5 h is the optimal con-

dition to improve the pozzolanic activity.

Results of the pozzolanic reaction for the sample

treated at 300 �C

XRD

Mordenite and K-feldspars were the sole mineral phases

identified by XRD after the thermal treatment at 300 �C.

Although the thermal effect produced structural changes on

the minerals, they were not evidenced by XRD, and the

quantification of the sample heated at 300 �C provided the

same mineral distribution as that in the raw sample [24].

The pozzolanic reaction evolved as a function of time

consuming mordenite and generating C–S–H phases as the

main product of the reaction (Table 4). A zeolite-type

phase within the isomorphic series heulandite–clinoptilo-

lite was detected and quantified after only one day of

reaction; however, this phase did not evolve significantly as

a function of time. Calcite was also quantified; however, as

previously reported, this mineral phase did not occur dur-

ing the pozzolanic reaction.

Fig. 5 Consumption of Ca2? in the pozzolanic reaction of the

zeolites thermally treated in the range 300–1000 �C after 7 days and

comparison of results with the non-treated raw zeolite. White areas

show the Ca2? concentration assumed in the precipitation of calcite,

not involved in the pozzolanic reaction

J Mater Sci (2013) 48:3213–3224 3219

123

SEM

After 1 day of reaction, the mordenite aggregates presented

microcrystalline porous lattices and C–S–H phases nucle-

ating on the cracked surfaces (Fig. 6a). The SEM images

show a porous network with cavities. These cavities are not

produced by the dissolution of mordenite; they are the

result of aggregation of the initial small and poorly crys-

talline mordenite particles interconnected by the incipient

growth of C–S–H phases. The reaction time favoured the

crystallisation of C–S–H phases on the surface of aggre-

gates, but after 28 days, the morphology and composition

persisted without any significant change up to 90 days

(Fig. 6b, c), indicating that the reaction ran to near com-

pletion and stopped due to surface layer of C–S–H block-

ing access to zeolite reactant.

Consumption of Ca2? in solution

In addition to the standard times selected (1, 7, 28 and

90 days), the pozzolanic activity was also determined

after 2 and 6 h and 3 days in order to establish a detailed

path of reaction at short-term. It was observed that the

reaction occurred rapidly and consumed most of the

aqueous Ca2? available within the first week (Fig. 7).

This is related with the structure of the zeolite after the

thermal treatment. Due to the low crystallinity of the raw

zeolite, the thermal treatment at 300 �C caused limited

processes of dissolution and re-crystallisation on the sur-

face of mordenite aggregates. The same correction per-

formed for the samples thermally treated in the range of

temperatures 300–1000 �C that reacted for 7 days with

the saturated Ca(OH)2 solution was accomplished at

300 �C after 1, 7, 28 and 90 days, deducting the Ca2?

consumed to form calcite to the Ca2? determined in

solution. After 2 and 6 h and 3 days, the solid samples

were not analysed, and therefore the correction could not

be performed. The correction decreased shortly the con-

centration of Ca2? consumed in the pozzolanic reaction.

The pozzolanic activity achieved with the zeolite treated

at 300 �C was considerably high after 7 days and

increased briefly after 28 days. Afterwards, it remained

almost constant, indicating the completion of the pozzo-

lanic reaction observed by XRD and SEM.

Table 4 Mineralogical composition (wt%) in the initial sample

thermally treated at 300 �C before the pozzolanic reaction and after 1,

7, 28 and 90 days

Time Mordenite Feldspars HEU–CLI Calcite C–S–H

Initial 79 21 – – –

1 day 66 13 6 8 7

7 days 58 17 6 8 11

28 days 51 19 5 6 19

90 days 49 21 5 6 19

Fig. 6 Aspect of reaction products identified by SEM in the sample treated at 300 �C a after 1 day; b after 28 days, and c after 90 days of

pozzolanic reaction

Fig. 7 Consumption of Ca2? in the pozzolanic reaction of the zeolite

treated at 300 �C as a function of time. The correction of results

(white squares) assumed the precipitation of calcite as a side effect

3220 J Mater Sci (2013) 48:3213–3224

123

Fourier-transform infrared spectroscopy

The analyses by FTIR showed wide and poorly defined

bands for the initial zeolite heated at 300 �C before the

pozzolanic reaction, indicating that the material is rather

amorphous. Not many differences were observed by FTIR

in the pozzolanic reaction as a function of time, but in

general, the IR bands became wider (Fig. 8).

The bands at 1429 and 875 cm-1 are normally assigned to

carbonates. The band at 875 cm-1 was not observed in the

initial sample, but it was present in all the IR spectra of

samples after the pozzolanic reaction attributed to the car-

bonation of samples during manipulation once the pozzola-

nic reaction finished.

The IR band at 1047 cm-1 in the initial sample progres-

sively shifted as a function of the reaction time to 1034 cm-1

after 90 days of reaction. This band was attributed to

asymmetrical stretching Si–O(Si) and Si–O(Al) vibrations in

external tetrahedral sites, and the shift indicates that alu-

minium species are inserted into the framework of zeolite,

which is consistent with the formation of the heulandite–

clinoptilolite isomorphic series from mordenite. This effect

was also observed in the pozzolanic reaction of the zeolite

without thermal treatment [24]. In addition to the shift, the

decrease in the intensity of this IR band indicated the con-

sumption of mordenite in pozzolanic reaction.

The band at 671 cm-1, and the shoulder at 970 cm-1

found in the spectrum after 90 days of pozzolanic reaction

is assigned to C–S–H phases and is characteristic of C–S–

H(I)-type phases [38].

Thermal analysis

The thermal analyses (DTG and DTA) were used to

identify structural changes in the initial zeolite compared

with the samples that reacted in the pozzolanic reaction.

Despite the low crystallinity of the thermally treated

samples before and after the pozzolanic reaction confirmed

by low intensity peaks in DTA and location in non-fixed

positions (i.e. the peaks were found at certain temperatures

within range shift from one sample to another), four dif-

ferent phenomena were identified by DTG (Fig. 9) and

DTA (not shown):

In the initial zeolite, a single broad endothermic peak is

observed with a maximum localised at 117 �C, which is

attributed to zeolitic water. During the pozzolanic reaction,

this peak was modified substantially. Two endothermic

peaks appear after only 1 day of reaction, localised at 60

and 117 �C, which are assigned respectively to hygro-

scopic water in low crystalline C–S–H phases and the

zeolitic water mentioned above.

An endothermic weak peak at 450 �C, observed in the

thermogram, was attributed to structural changes in the

poorly crystalline zeolite [19–21].

The endothermic peaks localised at 650 and 800 �C are

assigned to isomorphic changes of the secondary zeolite

formed within the isomorphic series heulandite–clinoptil-

olite that involve a change in the structure and decarbon-

ation of calcite, respectively. Both endothermic processes

were also observed during the pozzolanic reaction using a

non-thermally treated zeolite sample [24].

Specific surface area

The specific BET surface area of the solid samples treated

at 300 �C increased as the pozzolanic reaction evolved

(Table 5). The increase in BET surface area evolved as a

function of time in agreement with the consumption of

Ca2? in solution, and it is mainly attributed to the

Fig. 8 FTIR spectra of the initial sample (before the thermal treatment

at 300 �C) and after 1, 7, 28 and 90 days of pozzolanic reaction

Fig. 9 DTG curves of the initial sample (before the thermal treatment

at 300 �C) and after 1, 7, 28 and 90 days of pozzolanic reaction

J Mater Sci (2013) 48:3213–3224 3221

123

precipitation of C–S–H-like phases on the mordenite

aggregates, as observed by SEM.

Thermodynamic stability

Thermodynamic modelling is a valuable tool to predict the

stability of minerals involved in the pozzolanic reaction

and reaction products that could potentially form (i.e.

C–A–H phases).

The SI shows the tendency of equilibrium for the min-

erals of interest to precipitate or dissolve under the

experimental conditions [39]. The SI for mordenite, C–S–H

and C–A–H phases were calculated from the activities of

species in solution and the equilibrium constants of the

dissolution reactions for the minerals evaluated (Fig. 10).

A positive SI indicates that the aqueous solution is over-

saturated with respect to a mineral and, therefore, that the

mineral is favoured to precipitate. A negative value of the

SI indicates that the aqueous solution is undersaturated

with respect to a mineral and, therefore, that the mineral is

favoured to dissolve. The SI for mordenite increased

shortly for the first day of reaction, reached a maximum

after 3 days of reaction and decreased after that to values

near to zero after 28 and 90 days. This indicates that

mordenite was close to equilibrium conditions at any time

with some tendency to dissolve after 3 days. The C–S–H

phases with higher Ca/Si ratio were predicted to be more

stable at short term (before 3 days), which is consistent

with the nucleation of Ca2? on the surface of the mordenite

aggregates. After that time, C–S–H phases with lower Ca/

Si ratio were predicted to be more stable. The tobermorite-

type C–S–H mineral with Ca/Si = 0.8 is the only C–S–H

phase stable (SI [ 0) after approximately 12 days of

reaction up to 90 days. This phase was confirmed to form

by heterogeneous nucleation on the solid–liquid interface

in the pozzolanic reaction of a zeolitic tuff [40]; however,

other studies [5, 41] showed that a shift in the main C–S–H

position of the XRD reflection from 3.06 to 3.07 A

d-values (3.08 A in the present study) indicated a varying

Ca/Si range in the composition and changes in structural

parameters in comparison to an ideal synthesised toberm-

orite-type C–S–H phase.

None of the experimental evidence indicated that C–A–H

phases could form in the studied pozzolanic reaction.

Moreover, the thermodynamic model supported that these

phases were far too unsaturated to precipitate under the

experimental conditions. Low and stable concentrations of

Al(aq)3? ( the dominant species, at high pH, of which is

AlO2-) were determined in solution without any observed

tendency as a function of time (in the range 10-4–10-5 M).

This Al(aq)3? , originated by the dissolution of the activated

zeolite in the alkaline medium, could partially be incor-

porated into the C–S–H phases (to form C–A–S–H phases)

but not C–A–H.

Discussion

The thermal treatment of the natural zeolite from Palmarito

at 300 �C confirmed an increase in the pozzolanic activity

with respect to the non-treated zeolite because the zeolite

destabilised its structure and converted to a more reactive

phase (metazeolite), at a temperature below the structural

collapse. However, the thermal treatment at higher tem-

peratures than 300 �C had a negative effect, in agreement

with [19, 21], because the zeolite converted to a weaker

reactive phase. Liebig and Althaus [23] had already

showed that the initial mineral composition of a natural

zeolite controls the response to the thermal treatment, and

that the heating of the zeolite can even reduce the

Table 5 Specific BET surface area (m2/g) as a function of the

reaction time for the sample thermally treated at 300 �C

Time BET surface (m2/g)

Initial 15.5 ± 0.1

6 h 17.1 ± 0.1

1 day 22.6 ± 0.1

7 days 27.5 ± 0.2

28 days 35.4 ± 0.2

90 days 39.0 ± 0.3

Fig. 10 Saturation indices of evaluated minerals as a function of time

(in logarithmic scale)

3222 J Mater Sci (2013) 48:3213–3224

123

compressive strength of cement–pozzolan mortars. This

justifies the evaluation accomplished on the effect of the

thermal treatment at different temperatures in the range

300–1000 �C because the performance improvement in the

pozzolanic activity is specific of the starting material and

cannot be directly derived from other studies carried out on

different materials.

The thermal treatment necessary to destabilise a zeolite

is usually related to its framework topology and the Si/Al

ratio. Vigil et al. [24] predicted that the required temper-

ature to start decomposing the silicoaluminate network of

the mordenite sample used in the present study (Si/Al C 5)

should be higher than 400 �C, since [19] reported the better

pozzolanic activity for a heulandite-type zeolite (Si/

Al \ 4) after thermal treatment at that temperature. Sur-

prisingly, the pozzolanic activity of the natural zeolite

thermally treated at 300 �C required lower temperature of

treatment, which can be considered as an energy

improvement.

Partial carbonation of the zeolite samples was detected

after the pozzolanic reaction. The Ca2? consumed to form

CaCO3 was calculated and discounted from the pozzolanic

activity because the carbonation was a secondary effect

attributed to weakly adsorbed Ca on external or

exchangeable sites in the zeolite that reacted with atmo-

spheric CO2 during the manipulation of the samples.

C–S–H phases formed by means of the pozzolanic

reaction between the thermally treated zeolite and the

Ca(OH)2 solution simulating the release of lime. These

phases are considered responsible for the mechanical

strength of hydrated Portland cements. In addition, their

structure controls important physical properties of cement

pastes, such as shrinkage, creep, porosity, permeability and

elasticity. The formation of low-density C–S–H-like phases

at early stages evolved to denser porous structures at later

stages as a function of the pozzolanic reaction rate influ-

encing the increase in the specific surface area.

In agreement with the experimental observations, the

thermodynamic study predicted that a tobermorite-type

C–S–H phase (Ca/Si = 0.8) was the most stable phase at

long-term, while C–A–H phases were too unsaturated to

form in the system.

Materials such as fly ashes and silica fumes are also

frequently examined in the scientific literature in similar

fashion as raw or calcined natural pozzolans, e.g. [42], but

the use of any of these materials as admixtures for concrete

production must be evaluated according to the local

availability, costs and energy saving reasons. Natural

zeolites are abundant materials, already broadly consumed

in the construction industry in countries like Cuba [1] or

China [43]. Scientific studies as the present one should be

of use to promote better performance of the materials with

added technical improvements.

The chemical and mineralogical results obtained in the

present study are promising to consider the zeolite treated

at 300 �C a good pozzolan. However, in a further study, the

mechanical behaviour of both materials, the raw and the

thermally treated zeolite at 300 �C, should be studied in

their performance as addition to a cement mortar in order

to confirm or reject the advantages of the thermal

treatment.

Conclusions

The natural mordenite-type zeolite from Palmarito (Cuba)

demonstrated good pozzolanic properties once thermally

treated at 300 �C for 5 h. The thermal treatment at 300 �C

did not modify significantly the mineralogy of the zeolite but

activated the zeolite surface creating a more reactive mate-

rial, and increasing slightly its pozzolanic activity. Thermal

treatment at higher temperatures (400–1000 �C) is not rec-

ommended because the pozzolanic activity decreases. With

the increasing temperatures of treatment, the dissolution/re-

crystallisation reactions resulted in larger compaction of

aggregates and formation of lower size but well-defined

crystals that partially inhibited the pozzolanic reaction.

In the zeolite treated at 300 �C, C–S–H-like phases

formed on the aggregates of mordenite according to the

pozzolanic reaction. The reaction proceeded rapidly during

the first week of reaction indicating the completion of the

pozzolanic reaction due to C–S–H blocking access to

zeolite reactant after 28 days. The thermal treatment of the

zeolite at 300 �C shortly increased the pozzolanic activity

compared to the as mined non-activated sample.

The mechanical performance of the thermally treated

zeolite in a cement mortar with different ratios of zeolites

replacing cement (ASTM test) should be studied to con-

clusively evidence the pozzolanic activity of the thermally

treated zeolite. In addition, the proposed further study

should clarify the advantage of thermal treatment of the

zeolite in comparison with the non-treated material.

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