re

ce

600

stry

nto

dou

gra

s w

es, 7

d t

aZ5.7507(3) A and cZ20.164(2). A structural model is proposed with CrO4 species close to the seventh coordination position of Ca atoms. Insitu HTXRD measurements in the temperature range 25350 8C reveal different thermal behaviors depending on the oxoanion. At low

an anionic clay mineral belonging to the layered double calumite was first examined. Experiments were performed at

Journal of Physics and Chemistry oE-mail addresses: Fabrice.leroux@univ-bpclermont.fr (F. Leroux), Chris-

tine.taviot-gueho@univ-bpclermont.fr (C. Taviot-Gueho).temperature, a dehydrationhydration process partly reversible was observed for Ca2AlV2O7 and Ca2AlCrO4 phases, between 100200 and

50120 8C, respectively. At higher temperature, strong interlayer contractions were observed due to a pillaring process: Dd/dZK13% forCa2AlV2O7 at 200 8C, Dd/dZK32% for Ca2AlCrO4 at 250 8C and Dd/dZK35% for Ca2AlSiO4 at 135 8C.q 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Multilayers; C. X-ray diffraction; D. Microstructure

1. Introduction

Interest in hazardous waste stabilization processes based on

the formation of ettringite and hydrocalumite has grown

steadily in the last few years [1]. Ettringite and hydrocalumite

are major hydration products of ordinary Portland cements.

Both of them are also commonly identified as secondary

precipitates during the hydration of fly ash and spent oil shale

or can be induced to precipitate in situ from the solid waste

materials [2]. It has been documented that extensive solid-

solutions between different anions can form in both hydro-

calumite and ettringite and that a conversion can occur between

these two phases [3].

This study focused on the anion uptake by hydrocalumite

which is larger than that by ettringite. Hydrocalumite is

the naturally occurring hydrotalcite represented by the

general formula MII1KxMIIIx OH2xCAmKx=m$nH2O (abbreviatednotation MIIRM

IIIKA with RZ(1Kx)/x) where MII and MIII

represent metallic cations and A the interlayer anion. The

layered structure of LDH is built by the periodical stacking

of positively charged (M2C, M3C)(OH)6 octahedral layers

related to brucite and negatively charged interlayers

consisting of anions and water molecules [4]. For hydro-

calumite, Ca2C and M3C ions (Al3C, Fe3C, Ga3C and Sc3C)

are ordered within the hydroxide layers and the interlayer

water molecules are coordinated to Ca atoms, creating 7-fold

coordinated Ca sites [5]. These layered Ca-aluminate

hydrates are also know collectively as AFm phases in

cement science litterature [1].

The incorporation of Cr and V oxoanions into hydro-Hydrocalumite-type materials: 1. Inte

Rachid Segni a, Laetitia Vieille b, Fabri

a Laboratoire des Materiaux Inorganiques, CNRS-UMR no.b University of Bristol, School of Chemi

Abstract

This study investigated the incorporation of Cr, V and Si oxoanions i

hydration of Portland cement and the use of these materials in hazar

examined in terms of the structural characteristics by means of thermo

in situ high temperature X-ray diffraction (HTXRD). The synthese

interlamellar distances observed for Ca2AlV2O7 and Ca2AlSiO4 phas

the hydroxide layers. The good crystallinity of Ca2AlCrO4 allowest in hazardous waste immobilization

Leroux a,*, Christine Taviot-Gueho a,*

2, Universite Blaise Pascal, 63177 Aubie`re cedex, France

, Cantocks close, Bristol BS8 1TS, UK

hydrocalumite. Hydrocalumite together with ettringite form during the

s waste immobilization is of current concern. The anion uptake was

vimetric analysis, infrared spectroscopy, X-ray diffraction (XRD) and

ere carried out by coprecipitation or exchange reaction. The short

.90 and 10.93 A, respectively, are explained by a grafting process onto

he refinement of the cell parameters in the P-3 space group with

f Solids 67 (2006) 10371042

www.elsevier.com/locate/jpcsoxoanions or under exchange conditions by interlayer anionic

substitution of nitrate or chlorideanions. In addition, the

incorporation of silicate anions in hydrocalumite was reported.

These results should provide insights into the behavior of

chromium and vanadium in cement, as well as the ability of

hydrocalumite to bind silica.0022-3697/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jpcs.2006.01.081

* Corresponding authors.precipitating hydrocalumite from a solution containing

hydroxide (LDH) family. Most of LDH resemble conditions relevant to cement chemistry, either by directly

2. Experimental

2.1. Synthesis

The synthesis of LDHs is usually achieved using the

coprecipitation method [6]. We have used this method for the

method. Ca2AlCrO4exch phase was also prepared using this

R. Segni et al. / Journal of Physics and Ch1038method. Practically, 0.10 g of Ca2AlCl or Ca2AlNO3powders and 0.217 g of NaVO3 or 1.38 g of K2CrO4 or

3.80 g of Na2SiO3, were added to 50 mL of deionized water.

The mixture was stirred for 24 h under nitrogen atmosphere at

room temperature for Ca2AlV2O7exch and Ca2AlCrO4exch

and under refluxing conditions at 80 8C for Ca2AlSiO4exch.The pH was fixed at a value of 11.5 with 2M NaOH,

compatible with both Ca2Al- host and anions stability.

Elemental analyses were performed at the Vernaison Analysis

Center of CNRS using inductive conduction plasma coupled to

atomic emission spectroscopy (ICP/AES).The chemical com-

positions are reported in Table 1.

2.2. Instrumentation

Powder X-ray diffraction patterns were recorded on XPert

Pro Philips diffractometer using Cu Ka radiation and adiffracted beam graphite monochromator. For the structure

investigation, data were recorded in steps of 0.0128 with acount time of 30 s at each point. The program FULLPROF was

used for the Rietveld refinement [7]. The intensities of the

reflections were calculated using the analytical pseudo-Voigt

peak-shape function; the background was defined by linear

interpolation between points in the pattern. A high-temperature

chamber (Anton Paar HTK-16) was installed, enabling in situ

high-temperature X-ray diffraction (HTXRD) measurements

under a controlled atmosphere. Measurements were carried out

Table 1

Chemical Analyses of Ca2Al-oxoanions phases.

Sample Ca/Al Al/AmK n.H2O

Ca2AlV2O7exch 2.08 0.24 2.94

Ca2AlCrO4exch 2.31 0.54 3.53preparation of Ca2AlCl and Ca2AlNO3 precursors as well as

Ca2AlCrO4cop. Practically, a mixed solution of 0.66 M

CaCl2$2H2O, and 0.33 M AlCl3$6H2O was added dropwiseto 250 mL of water (precursor compounds) or 250 mL of a

solution of K2CrO4 0.30M (Ca2AlCrO4cop). The pH was kept

constant at 11.5 by the simultaneous addition of 2 M NaOH.

The syntheses were carried out at room temperature under

vigorous stirring and nitrogen atmosphere. After the complete

addition of the metallic salts, the precipitates obtained were

aged in the mother solution for 24 h at 65 8C, then centrifuged,washed twice with water and finally dried under vacuum at

room temperature.

The synthesis of the other phases intercalated with vanadate

and silicate species was carried out with the anion exchangeCa2AlCrO4cop 1.97 0.66 2.97

Ca2AlSiO4exch 1.95 0.27 2.26under air atmosphere in the temperature range 25350 8C, after10 min equilibration at each temperature, and the heat/cooling

rate was 5 8C/min. Repetitive treatments consisting ofcalcinations at increased temperatures and subsequent coolings

to room temperature were performed, leading to two different

sets of data, those obtained in situ at high temperature and those

recorded ex situ after the sample was cooled down to room

temperature. Typical measurement conditions were in the 2q

range 2708, step size 0.088 (2q), and step counting time 4 s.Thermogravimetric analyses (TGA) were performed on a

Setaram TGA 92 instrument with a linear heating of 5 8C/minunder air atmosphere. IR spectra were recorded from 200 mg

KBr pellets containing 2 mg of sample on a 16PC Perkin-

Elmer FTIR spectrometer.

3. Results and discussion

3.1. Incorporation of V, Cr and Si oxoanions.

The aqueous chemistry of chromate, vanadate and silicate

anions has been thoroughly examined. The distribution of the

anions depends on the concentration and the pH. V2O4K7 are the

major species at pHO9 and acidification leads to the formationof V4O

4K12 in the pH range 68 and of V10O

6K28 in the pH range

46. At pHO6, CrO2K4 are the major anionic species whileCr2O

2K7 are stabilized at lower pH values. Eventually, the

prepared silicate solution must contain both HSiOK3 and SiO2K3

species.

One can see that the Ca/Al molar ratios are well maintained

after the intercalation of the oxoanions (Table 1). The Al/AmK

ratios are also those expected for free intercalated oxoanions

assuming that AmK are CrO2K4 , V2O4K7 and SiO

2K3 . The water

contents are very similar, as well. It is worth noting that the

incorporation of all these oxyanions was attempted both using

the coprecipitation method and through anion exchange. Yet,

only the chromate phase was prepared successfully by the two

methods while the silicate and vanadate phases were obtained

only via exchange by nitrate anions and chloride anions

respectively. In addition, a moderate heating was required for

the formation of the silicate phase.

The powder X-ray diffraction patterns of the precursor

phases (Fig. 1a) are characteristic of well crystallized products

with a bidimensionnal structure of symmetry R-3 for Ca2AlCl

[5f] and P-3c1 for Ca2AlNO3 [5b]. As it is often the case, the

exchange process leads to a lowering of the crystallinity. For

Ca2AlV2O7exch and Ca2AlSiO4exch, the number of diffrac-

tion lines is very limited and it is only possible to give

approximated values of the intermetallic and interlayer

distances from, respectively, the (110) and the first observed

(001) diffraction lines. For Ca2AlCrO4, the X-ray diffraction

patterns are very similar to each other indicating that the same

intercalated phase was obtained by the two synthesis methods.

The good crystallinity of Ca2AlCrO4cop allowed the

indexation of the diffraction lines which was carried out in

the P-3 space group (Fig. 1b). This space group arises from a

emistry of Solids 67 (2006) 10371042peculiar ordering of chromate entities in the interlayer domain

as discussed below. The refined parameters of the hexagonal

Z15.8, c2Z6.06). The structural informations are thus

ases. (b) Cell parameters refinement for Ca2AlCrO4cop in P-3 space group:

ence profile.

Chemistry of Solids 67 (2006) 10371042 1039unit cell are aZ5.7507(3), cZ20.164(2) A. In all cases, theinterlamellar distances indicate the presence of intercalated

anions. Indeed, these oxoanions have already been intercalated

in various hydrotalcite-like LDH and the following discussion

compares their incorporation into hydrocalumite with that in

hydrotalcite.

The short interlamellar distance observed for Ca2AlV2O7exch is not compatible with free intercalated oxoanions at

hydrogen bond distances from hydroxyl planes. The d-spacing

value obtained 7.90 A is slightly higher than that reported by

Depe`ge et al. for polyoxovanadates anions intercalated in

Cu2Cr-LDH at pH 10.0: 7.62 A [8]. Like these authors, we

assume a grafting of V2O4K7 species onto the hydroxide layers.

Such a process has been reported for sulphate and chromate

anions in LDH and can occur either spontaneously during

ageing or storing, or under moderate thermal treatment

(100150 8C) [9]. In the infrared spectrum (Results not

shown) of this phase, the disappearance of the two typicalK1

Fig. 1. (a) X-ray diffraction patterns of freshly prepared Ca2Al-oxoanion ph

experimental data (cross), calculated (line), Bragg reflections (ticks) and differ

R. Segni et al. / Journal of Physics andbands of the hydroxide layers (nCaO at 420 cm and dOAIO at530 cmK1) and the shift of the VO4 stretching band (nOVO at950 cmK1 in NaVO3 salt) towards high energies (820 cm

K1),

support the grafting phenomenon); a small band at 614 cmK1

(nVOV) can be attributed to corner-sharing VO4 tetrahedra.As already said, the CrO4 phases obtained both by the

coprecipitation method and through anion exchange display the

same interlamellar distance i.e. 10.02 A. This value is higher

than that reported in hydrotalcite system, ranging from 8.08 to

8.42 A for freshly prepared Cu2Cr-LDH [9]. The capability of

chromate-containing LDH to intercalate variable amount of

water molecules has been demonstrated and strong variations

of the interlamellar distance have been observed relative to the

storage conditions. Considering previous structural data

reported by M. Francois et al. [5g], we attempted to perform

a structural refinement of the X-ray powder diffraction diagram

of Ca2AlCrO4cop material. Yet, due to a statistic and dynamic

disorder of the chromate species and the water molecules, the

minimization of the difference was obtained with rather high

conventional Rietveld factors (RpZ15.7, RwpZ21.1, RBraggapproximate but a structural model can be proposed. The

structure projected along the [100] direction is shown Fig. 2.

This representation has been ordered for clarity. It can be

described as the stacking of two kinds of layers, one negatively

charged [Ca2Al(OH)6$CrO4$H20]K and the other positively

charged [Ca2Al(OH)6$ 2H20]C plus four interlayer free water

molecules. Such a stacking sequence is likely to be induced by

the size of chromate anions. The two adjacent [Ca2Al(OH)6]2C

main layers are separated by a distance of 10.082 A. The

seventh coordination position of the Ca atoms is assumed

statistically either by a water molecule (OW1) or one of the

oxygen atoms of the chromate anions (OCr1). Because of the

great disorder, OW1 and OCr1 could not be localized

separately. The Ca1 OCr1/OW1 distance obtained 2.96(3) A

indicates that Ca atoms are nearly linked to chromate anions.

To check this assumption, we studied the reversibility of theFig. 2. Structural model for [Ca2Al(OH)6][0.5CrO4, 3.5H2O]. The structure is

represented in an ordered way with the seventh coordination position of the Ca

atoms occupied statistically either by a water molecule or a chromate oxygen

atom.

exchange reaction by suspended Ca2AlCrO4cop sample in a

solution containing a large excess of chloride anions. After

24 hours of mixing, the formation of Ca2AlCl was observed

but a small amount of the Ca2AlCrO4 phase was still present

indicating strong interactions between CrO2K4 and the hydrox-

ide layers, yet the question of the existence of a grafting

remains since part of the solid has been exchanged.

Noteworthy here is the easiness of grafting at low temperature

in hydrocalumite system and the reversibility of this process as

already reported with intercalated oxygen-containing anions

such as nitrate and carbonate anions [5bd]. The interlamellar

distance observed for Ca2AlSiO4exch 10.93 A is much shorter

than that reported in hydrotalcite phases: 12.45 A for ZnRAl

LDH, and 12.012.2 A for Mg3Al-LDH and 11.9 for Zn2Cr

LDH [10]. For these materials, a polymerization of SiO4 units

in the interlamellar domain as inverted tetrahedral layers has

been evidenced by 29Si NMR [10c]. A strong interlayer spacing

contraction from 11.90 A at 25 8C to 7.65 A at 140 8C was also

reported arising from a condensation of the silanol groups and

the metallic hydroxyls. In the case of Ca2AlSiO4exch, as the

exchange was carried out at 80 8C, we may assume a partial

grafting process. The infrared spectrum shows a band at

1150 cmK1 dSiOSi characteristic of corner-sharing SiO4

tetrahedra, yet the bands ascribed to the hydroxide layers are

still visible but strongly attenuated.

3.2. Thermal behavior

In the present study, we also aim at clarifying the thermal

behavior of Ca2Al-oxoanions phases in the temperature

interval 25350 8C. Of particular interest here is the use ofin situ high temperature powder X-ray diffraction (HTXRD)

technique as most of the thermal behavior investigations

published were carried out ex situ after cooling the sample at

room temperature. Recently, by means of HTXRD measure-

ments, we have shown the reversibility of the hydration

dehydration process for the Friedels salt Ca2Al(OH)6Cl.2H2O

in the temperature range 80100 8C [11]. Besides, thestructural determination of the dehydrated phase at 116 8Creveals a quasi-pillared layer structure with chloride anions

situated midway in the interlamellar space at only 2.904(3) A

from Ca atoms of adjacent hydroxide layers. Fig. 3 gives a

general view of the X-ray diffraction patterns of Ca2Al-

oxoanion phases between 25 and 350 8C recorded by means ofin situ HTXRD in air. Repetitive treatments consisting of

calcinations at increased temperatures and subsequent coolings

R. Segni et al. / Journal of Physics and Chemistry of Solids 67 (2006) 103710421040Fig. 3. High-temperature in situ (black line) powder X-ray diffraction patterns o

Ca2AlSiO4exch at different temperatures compared with those recorded after coof (a) Ca2AlCrO4cop, (b) Ca2AlCrO4exch, (c) Ca2AlV2O7exch and (d)

ling to room temperature (gray line).

to room temperature were performed. The thermal behavior

was also examined by thermogravimetric analysis and to

combine XRD and TGA results, the same thermal treatment

was applied. Changes in the diffraction patterns indicate three

steps in the thermal decomposition process over the tempera-

ture ranges 25%T%350, 350%T%650 and 650%T%950 8Cwhich are commonly ascribed to the following three main

processes: dehydration, dehydroxylation and anion decompo-

sition or recombination. The evolution of the interlamellar

distances between 25 and 300 8C for both series of

measurements, i.e. in situ and ex situ measurements, is

depicted in Fig. 4. This evolution differs between compounds.

For Ca2AlV2O7exch, almost no change in the interlayer

distance is observed up to 100 8C confirming that the grafting

the of V2O7 units onto hydroxide layers has already occurred

during the preparation. Consistent with XRD results, the

material starts losing weight only at 100 8C on the TGA curve.

Above 100 8C, the interlamellar distance starts decreasing from

7.90 A to c.a. 6.85 A at 200 8C (Dd/dZK13%) however, thesample was found to partly recover the original distance on

cooling. This reversible contraction is ascribed to the reversible

elimination of the interlayer water molecules. The air moisture

is sufficient to induce such reversibility. Above 200 8C, the

reversibility is lost and at 350 8C the sample is converted to an

amorphous phase.

HTXRD and TGA measurements on Ca2AlCrO4samples first show the effect of the crystallinity and the

structural order on the quality and the interpretation of such

experimental data. Indeed, while Ca2AlCrO4cop unequi-

vocally exhibits a reversible interlayer contraction in the

temperature range 50120 8C due to the reversible elimin-

ation of the interlayer water, the process appears less

collective in Ca2AlCrO4exch sample of lower crystallinity

and reversibility is completely lost at 70 8C. Yet, in situ

measurements indicate a decrease of the interlayer distance

between 25120 8C of a comparable amplitude in both

cases, from 10.08 to c.a. 8.1 A (Dd/dZK20%). Above60 8C, both the dehydrated and hydrated phases are present

on cooling in the two samples. Comparatively, for Ca2AlCl

Friedels salt, the coexistence of the two phases was

observed only above 260 8C [11]. Although the rehydration

rate depends on the humidity level and other experimental

conditions, this result again indicates strong interactions

between the chromate anions and the hydroxide layers,

agreeing with the structural model. The short interlayer

distance of 8.1 A probably is the lower limit under which a

pillaring process may occur, with adjacent hydroxide layers

bridged by chromate species. Indeed, above 120 8C

reversibility in Ca2AlCrO4cop is lost and the interlayer

distance decreases rapidly from 8.1 A at 120 8C down to

R. Segni et al. / Journal of Physics and Chemistry of Solids 67 (2006) 10371042 1041Fig. 4. Variation of basal spacing d for (a) Ca2AlCrO4cop, (b) Ca2AlCrO4exch

measured in situ (open circles) and after cooling to room temperature (filled circle, (c) Ca2AlV2O7exch and (d) Ca2AlSiO4exch as a function of temperature

s). The corresponding TG curves are given in inset.

c.a. 6.8 A at 250 8C (Dd/dZK16%).For Ca2AlSiO4exch,the behavior versus temperature is totally different. A strong

interlayer contraction from 10.93 A to c.a. 7.1 A was

observed at 135 8C (Dd/dZK35%), with no recovering ofthe original distance upon cooling. Above 135 8C, the

reflections are strongly reduced in intensity and almost

disappeared at 165 8C. This irreversible contraction of theinterlamellar space must arise from a condensation process

of silanol groups onto the hydroxide layers as demonstrated

elsewhere [10]. Consistent with this interpretation, the

TGA curve indicates that dehydroxylation and anion

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hydrocalumite was investigated. A partial grafting process

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prepared samples. In situ HTXRD experiments reveal different

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Hydrocalumite-type materials: 1. Interest in hazardous waste immobilizationIntroductionExperimentalSynthesisInstrumentation

Results and discussionIncorporation of V, Cr and Si oxoanions.Thermal behavior

ConclusionReferences