Vacuum ultraviolet photolysis of ethane: molecular detachment of hydrogen

Cement and Concrete Research 52 (2013) 140–148

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Ettringite decomposition in the presence of barium carbonate P.M. Carmona-Quiroga ⁎, M.T. Blanco-Varela Instituto de Ciencias de la Construcción Eduardo Torroja (IETcc-CSIC), C/Serrano Galvache 4, 28033 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 6 July 2012 Accepted 28 May 2013 Available online xxxx Keywords: Ettringite (D) Barium carbonate Sulfate-resistant cements (D) Thermal analysis (B) Thermodynamic calculations (B)

a b s t r a c t The interaction between cement paste and sulfate-rich solutions or soils induces gypsum, ettringite or thaumasite precipitation. These expansive processes may be mitigated by using BaCO3 (witherite) as a setting regulator. The present study explored ettringite decomposition in the presence of witherite at different temperatures (25, 40 and 65 °C) and reaction times (up to 90 days) to further the understanding of this process as grounds for developing new sulfate-resistant cements (SR-PC). According to the XRD, FTIR and DSC-TG findings, sulfoaluminate decomposition and barite formation begin in the first 24 h of the reaction, even at ambient temperature (25 °C), and progress rapidly for the first 30 days. The reaction follows a different pathway at 65 °C than at 25 and 40 °C due to ettringite instability at high temperatures. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The three main types of (chemical) attack summarized by Collepardi [1] are:

In the first few hours of OPC hydration, C3A, gypsum and water react to yield ettringite (Eq. (1)), whose formation, attendant upon high crystallisation pressures, causes no damage in the cement while still in its plastic state [1]. In fact, so-called early ettringite formation (EEF) even contributes to material strength. Depletion of the dissolved sulfate during C3A and C4AF hydration destabilises ettringite, yielding calcium monosulfoaluminate hydrate [2] as shown in Eq. (2): C3 A þ 3CaSO4 ⋅2H2 O þ 26H2 O→C3 A⋅3CS⋅32H2 O

ð1Þ

2C3 A þ C3 A⋅3CS⋅32H2 O þ 4H2 O→3C3 A⋅CS⋅12H2 O:

ð2Þ

In concretes cured at over 65–70 °C, monosulfoaluminate becomes increasingly stable at the expense of early ettringite [3]. If these materials are subsequently kept in water or under moist conditions at ambient temperature, so-called delayed ettringite (DEF) appears. As this may generate cracking in hardened concrete, DEF is viewed as one of the mechanisms governing internal sulfate attack [4,5]. Traditional sulfate attack, by contrast, arises in the presence of external sulfates (interaction between cement paste and sulfate-rich water or soil). Chemical sulfate attack, related essentially to gypsum or ettringite formation [1], is distinguished by some authors [6] from physical attack, due to salt crystallisation in the pores of the material.

⁎ Corresponding author. Tel.: +34 913020440; fax: +34 913026047. E-mail address: [email protected] (P.M. Carmona-Quiroga). 0008-8846/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconres.2013.05.021

a) sulfate attack on CH and C\S\H to form gypsum; and in the presence of MgSO4, to form gypsum, brucite and silica gel; b) sulfate attack on C\A\H and monosulfate to form ettringite and c) sulfate attack on C\S\H and CH in the presence of carbonate ions to form thaumasite. Three factors are requisite to such attacks: high concrete permeability, a sulfate-rich environment and the presence of water for ion ingress [1]. With these factors in mind, a number of strategies have been implemented to prevent sulfate-induced expansive processes, including the use of sulfate-resistant cement [7] or impermeable concrete [1]. These solutions are not always satisfactory, however [8]. Another way to mitigate sulfate attack would be to eliminate or immobilize the sulfate ions involved in the respective reactions. This would entail the addition to cement or clinker of a compound containing an element such as Ba in its composition able to form an insoluble salt with the external sulfates [9,10]. In cement industry Ba compounds have not been used yet to this end, but in brick manufacture [11] or in minery [12], the use of BaCO3 as an immobilizing agent of soluble salts is a normal practice. Several authors have explored in some extent the role of BaCO3 in cementitious systems. As Utton [13] noted, BaCO3 (present in nuclear waste slurries) is not inert inside cementitious matrices. Moreover, Ciliberto [14] reported that thaumasite and ettringite can evolve into more stable phases (barite, witherite and scarbroite) in the presence of Bacontaining solutions such as Ba(NO3) and Ba(OH)28H2O. According to Carmona-Quiroga et al. [10], the ettringite in the CaO–BaO–Al2O3– CaSO4–CaCO3–H2O system decomposes when the Ba concentration in the solution is over 0.1176 mmol/kg and in OPC blends with 10 and 20 wt.% of BaCO3 its precipitation is inhibited with up to approximately 5.3 and 19.4 g of Na2SO4, respectively [15].

P.M. Carmona-Quiroga, M.T. Blanco-Varela / Cement and Concrete Research 52 (2013) 140–148

The ettringite formed was filtered, rinsed with acetone and dried at ambient temperature in a desiccator. Mixes (0.7359 g) of ettringite and barium carbonate (Probus) were prepared with a molar ratio of 1:3 in 100 ml of decarbonated water to study ettringite decomposition in the presence of the carbonate. These mixes were stored in closed containers at three temperatures (25, 40 and 65 °C) for different test times (from 6 h to 90 days) (Table 1). The samples were filtered, treated first with acetone and then with ethanol to detain the reactions, and vacuum-dried to a constant weight. Both the synthetic ettringite and the ettringite–witherite reaction products were characterised with XRD, FTIR and DSC-TG. The XRD trials were conducted on a 2.2-kW Bruker D8 Advance diffractometer under the following instrumental conditions: copper anode X-ray tube (CuKα1 radiation: 1.5406 Å and CuKα2 radiation: 1.5444 Å); operating parameters, 40 kV and 30 mA; fixed divergence slit, 0.5°; Lynxeye Super Speed Detector fitted with a 3-mm anti-scatter slit, a 2.5° secondary Soller slit, and a (0.5%) Ni K-beta filter and no monochromator. For Rietveld refinement of XRD pattern, the conditions were 2θ recording range 5–70°; step size, 0.02°; total recording time per diffractogram, approximately 1 h. Rietveld refinement was performed using GSAS software [17]. The parameters refined included background coefficients, cell parameters, zero shift error, peak shape parameters and phase fractions. Atomic parameters (position, thermal displacement and site occupation) were not refined.

Table 1 Selected temperatures (°C) and reaction times for ettringite and Ba carbonate suspensions (molar ratio 1:3).

25 °C 40 °C 65 °C

6h

15 h

24 h

72 h

7 days

30 days

60 days

90 days

– – X

– – X

X X X

– – X

X X X

X X X

X X X

X X X

141

Ettringite stability therefore depends not only on sulfate content and temperature in the system, but also on the chemical composition of the solution in contact with the salt [3]. This study focuses on the latter two questions and explores ettringite decomposition in the presence of BaCO3 (witherite) at different temperatures (25, 40 and 65 °C) and reaction times to further the understanding of this process as grounds for developing new sulfate-resistant cements (SR-PC).

2. Experimental One of the methods proposed by Struble [16] was used to synthesise ettringite. A suspension of Al2(SO4)3·18H2O (Panreac) and CaO (obtained by decarbonating CaCO3 instead of Ca(OH)2 as per the original procedure) was prepared with a molar ratio of 1:6 in one litre of water, and stirred at ambient temperature in a nitrogen atmosphere for 48 h.

Table 2 Attribution of infrared spectrum bands and the signals on DSC-TG curves for synthetic ettringite and witherite (*impurities; (e) endothermal).

Witherite Present study

3432

Assignment

O-H stretching

Synthetic ettringite Present study

[21]

[22]

[14]

Assignment

3637

3643

3635

3633

O-H stretching

3434

3433

3420

3425

O-H stretching

1655

H-O-H bending

2190 1666 1628

H-O-H bending

1676

1647

1675 1640

H-O-H bending

1491 C-O stretching

1115

S-O stretching*

1059

C-O stretching

989

857

CO3 bending

854

FTIR

1448

1428

C-O stretching* 1429

1422

C-O stretching*

S-O stretching

1198 1114

S-O stretching 1120

1120

1116 991

S-O stretching

863

855

867

Al-O-H bending SO42- bending (v4) [23], Ca-OH bending [14, 23], Al-O-H bending

694

CO3 bending

615

626

592

SO4 bending

561

544

Al-O-H bending

426

420

SO42- bending (v2)

Present study

Assignment [24, 25]

Present study

DSC

DTG

97 1133

Decomposition

240

[26]

610

[27]

[28]

[21]

[29]

70

Assignment Ettr. dehydration Ettr. dehydration (or alumina gel

~225

[29])

818 (e)

Rhomboid to hexagonal

102 (e)

110 (e)

120 (e) (DTA)

977 (e)

Hexagonal to cubic

200 shoulder (e)

160 (e)

Ettringite dehydration

Ettringite dehydration

1166 (e)

Decomposition

242 (e)

250 (e)

Ettr. dehydration (or alumina gel

EGA

[29]) ~140 ~240

Ettringite dehydration

142

P.M. Carmona-Quiroga, M.T. Blanco-Varela / Cement and Concrete Research 52 (2013) 140–148

KBr pellets were analysed on a Nicolet 6700 infrared spectrophotometer with the following settings: range, 4000–400 cm−1; scans, 10; spectral resolution, 4 cm−1. Thermogravimetric and heat flow analysis was conducted on a Q600 TA SDT analyser with platinum crucibles (reference: empty crucibles). The conditions were: temperature, up to 1200 °C (1050 °C for synthetic ettringite); atmosphere, air; flow speed, 4 °C/min. The reaction products obtained at the three temperatures in the presence and absence of CO2 were quantified using the GEMS geochemical code [18]. This thermodynamic modelling software which includes built-in thermodynamic databases (general [19] and cement-specific, cemdata2007 [20]), computes equilibrium phase assemblage and speciation of the defined systems by Gibbs free energy minimization (GEM). The bulk chemical composition of the system examined was the same as in the experimental study: a mix of ettringite and barium carbonate with a molar ratio of 1:3 in 100 ml of water at 25, 40 and 65 °C in the presence of CO2 (that guarantees a complete carbonation) or in its absence (just 10 g CO2 free air).

interval, with an endothermal peak at 102 °C; a further 4.3 molecules of water were lost in the second interval, with a peak at 242 °C, and an additional 3.7 molecules through the end of the trial. These findings corroborated the results reported in the literature [26–28] to the effect that ettringite dehydration takes place by stages and the water molecules eliminated in each stage are characterised by different bond energies. Consequently, both the first (102 °C) and second (242 °C) endothermal signals as well as the subsequent ongoing water loss were generated by ettringite dehydration. The XRD study showed that the witherite used was also very pure. Impurities were only detected on the FTIR spectrum in the form of very low intensity bands attributable to the vibrations generated by the S\O bonds in sulfates. The DSC for the sample (Fig. 1, Table 2) exhibited two initial endothermal signals at 818 and 977 °C, induced by witherite polymorphic transformations, the first from the rhomboid to the hexagonal and the second from the hexagonal to the cubic system. A third signal, at 1166 °C, reflected witherite decomposition.

3. Results and discussion

3.2. XRD study of the reaction

3.1. Characterisation of synthetic ettringite and witherite

Further to the XRD findings (Fig. 2), at both 25 and 40 °C, ettringite began to decompose in the presence of witherite 24 h into the trial, forming calcite and barite (a very stable salt; log K = −9.97, BaSO4 = Ba2+ + SO42− [30]) and at 40 °C also calcium monocarboaluminate hydrate which co-existed with the reagents. Ettringite and witherite decomposition and the rise in the intensity of the reflections of their reaction products progressed more quickly at 40 than at 25 °C (Fig. 2): reflections generated by monocarboaluminate that were present in the 24-hour XRD pattern for the former first appeared on the 7-day diffractogram for the latter. Moreover, while neither ettringite nor witherite was fully consumed in the 3-month period, their end-of-trial content was lower and the proportion of calcite and monocarboaluminate higher in the material stored at 40 °C. At 40 °C, traces of gibbsite were observed in the 60-day sample. In addition, new reflections appeared in the 90-day specimen, the most intense at 2θ = 10.98° and the second most intense at 22.05°, possibly indicating the formation of a calcium aluminate hydrate (Fig. 2).

According to Rietveld refinement phase quantification, the synthetic ettringite was 98.66% pure, with the remaining 1.34% consisting of gypsum and gibbsite (wRp = 8.60%). The FTIR spectrum exhibited the bands characteristic of ettringite (Table 2) [14,21–23], in which the only impurities identified, in the form of carbonate C\O bond ν3 vibration bands, were interpreted to be the result of slight sample weathering. Fig. 1 shows the TG, DTG and DSC curves for the witherite and synthetic ettringite, while Table 2 compares the thermal analysis signals observed in this study and their attribution to data from the literature [24–29]. The purity of the ettringite sample may also be assessed from TG findings (Fig. 1, Table 2). The loss of a total of 30.8 molecules of water at temperatures of up to 1050 °C indicated that the ettringite was 96.51% pure. A loss of 22.8 molecules was recorded in the first

105

5

synthetic ettringite

6.209% 101.80°C

5.337% witherite 0.8572%

[ –– –– – ] Heat Flow (W/g)

[ ––––– · ] Weight (%)

32.74%

[ ––––––– ] Deriv. Weight (%/°C)

44.29%

242.19°C

18.47% 976.73°C 818.00°C 1165.79°C 10

-5 0

Exo Up

200

400

600

800

Temperature (°C)

1000

1200

Universal V4.5A TA Instruments

Fig. 1. TG, DTG and DSC curves for ettringite (above) and witherite (below).

P.M. Carmona-Quiroga, M.T. Blanco-Varela / Cement and Concrete Research 52 (2013) 140–148

At 25 and 40 °C, therefore, the reaction would progress as described in Eq. (3):

MC E

MC B B B C B B B BB

E

143

BB B

90 d

Ca6 Al2 ðSO4 Þ3 ðOHÞ12 ⋅26H2 O þ 3BaCO3 →3BaSO4 þ ð1−xÞCa4 Al2 ðCO3 ÞðOHÞ12 ⋅5H2 O þ ð2 þ xÞCaCO3 þ 2xAlðOHÞ3 þ 21H2 O: ð3Þ

60 d W 30 d E

7d

24 h 10

20

30

40

50

60

2θ

MC MC

E

*

E

B BC B BB BB B B *

BB B B

90 d

60 d W 30 d E

EE

7d

Due to ettringite instability at 65 °C [3], the decomposition pathway at that temperature differs from the above (Eq. 3). The 6-hour XRD pattern showed reflections generated not by ettringite but by calcium monosulfoaluminate hydrate, as well as signals for calcite, barite and witherite. Traces of bayerite were detected at early ages, and calcium hemicarboaluminate in the 15-hour sample only. Contrary to what was observed for the 25 and 40 °C samples, the presence of calcium monocarboaluminate hydrate was not significant at short reaction times because the formation of calcium monosulfoaluminate hydrate rather than ettringite reduced the Al available for its formation [31]. After 30 days, with the increase in calcite content, monosulfoaluminate was observed to turn into monocarboaluminate. At that age practically all the witherite had been consumed, while not all the monosulfoaluminate had therefore been converted into barite. Moreover, a small amount of a calcium aluminate hydrate-like phase began to form, a development likewise observed after 3 months in the material stored at 40 °C. In the 90-day sample, a minor fraction of calcium monosulfoaluminate hydrate remained intact, like the ettringite in the material stored at 25 and 40 °C, and traces of gibbsite were also recorded. According to Hsu [32], slow precipitation favours gibbsite formation, here detected after 3 months, while rapid precipitation favours the appearance of bayerite, detected here in the early (6- to 24-) hour samples. At 65 °C, then, the reaction between ettringite and witherite proceeds as described in Eqs. (4) and (5): Ca6 Al2 ðSO4 Þ3 ðOHÞ12 ⋅26H2 O

24 h 10

20

30

40

50

60

2θ

Ca4 Al2 O6 ðSO4 Þ⋅12H2 O þ BaCO3 →Ca4 Al2 ðCO3 ÞðOHÞ12 ⋅5H2 O

ð5Þ

3.3. FTIR study of the reaction

MC B BC MS B B BB B B *

BB

90 d 60 d 30 d

W

7d 72 h

HC

24 h 15 h 6h

10

ð4Þ

þBaSO4 þ H2 O:

MC MS *

þ 3BaCO3 →ð1−xÞCaO⋅Al2 O3 ⋅CaSO4 ⋅12H2 O þ ð2 þ xÞ BaSO4 þ ð2 þ xÞCaCO3 þ ð1−xÞBaCO3 þ ð2xÞAlðOHÞ3 þ 17H2 O

20

30

40

50

60

2θ Fig. 2. 24-h (6-h for 65 °C trials) to 90-day diffractograms for the ettringite/witherite reaction products after storage at 25, 40 and 65 °C. E = ettringite; W = witherite; C = calcite; B = barite; MC = calcium monocarboaluminate hydrate; HC = calcium hemicarboaluminate hydrate; MS = calcium monosulfoaluminate hydrate; * = calcium aluminate hydrate, not clearly identified.

The infrared spectroscopic study of ettringite decomposition at the three temperatures analysed (Figs. 3 and 4) revealed that the witherite was consumed over time. The bending bands at 857 and 692 cm−1 generated by its CO2− 3 groups (Table 2) (nearly) disappeared from the spectra for the samples after the first month (although the presence of a small amount of these carbonate continued to be detected by XRD in the 25 and 40 °C samples). At the same time, the calcite (intensification of the bands at 875 and 712 cm−1) and barite (S\O stretching bands at 1173–1176 bending bands at 636 cm−1) [33] conand 1079–1082 cm−1 and SO2− 4 tents grew, although after 30 days the reaction apparently reached a standstill at all three temperatures, for no significant changes were observed in the subsequent spectra. Temperature quickened the ettringite decomposition rate, as the intensity of its characteristic stretching bands, S\O at 1116 cm−1 and O\H at 3630 cm−1, waned throughout the trial at shorter reaction times in the 40 than in the 25 °C samples (Fig. 3, Table 2). Moreover, the three ν3 (S\O) vibration signals of barite in the 1082–1176 cm−1 range [33–35] appeared earlier (after 7 days) at 40 °C, although the S\O stretching vibration band of the ettringite at 1116 cm−1 appeared to overlap with the central band of the vibrational mode ν3 (1127–1111 cm−1 [34]; 1136 cm−1 [33]), given the relative intensity and position of the resulting band (1119 cm−1). At 65 °C (Fig. 4), the presence of barite was recorded from the beginning of the reaction, deduced primarily from the existence of the

144

P.M. Carmona-Quiroga, M.T. Blanco-Varela / Cement and Concrete Research 52 (2013) 140–148

24 h 3630

24 h

693

857

1116

7d

30 d

1120

693

857

1116

3630

7d

30 d

60 d

60 d

4000 3500

1500

1250

1000

750

500

wavenumber (cm-1)

4000 3500

1500

1250

1000

712 636 610

875

1174 1125 1079 983

1435

1636

90 d 3676 3621 3547 3442

713 636 610

875

1173 1121 1081 984

1437

3620 3547 3438

1628

90 d

750

500

wavenumber (cm-1)

Fig. 3. Infrared spectra for the ettringite–witherite reaction products after storage at 25 °C (left) and 40 °C (right) for 24 h and 7, 30, 60 and 90 days (from top to bottom).

three ν3 (S\O) intense bands between 1176 and 1078 cm−1. Higher temperatures and longer reaction times were associated with a shift to a higher wavenumber by the central band recorded in this region (Figs. 3 and 4). That shift was attributed to the progressive decline in interference from the S\O vibration bands generated by ettringite (Table 2) at 25 and 40 °C and by monosulfoaluminate at 65 °C (Table 3 [22]). Calcium carboaluminate formation was first identified on the 30-day spectra for the 40 and 65 °C samples, based primarily on the joint appearance of O\H stretching bands at 3676 and 3620 cm−1 [36,37]

6h

15 h 24 h 72 h

693

857

7d

30 d 60 d

4000

3500

1750

1500

1250

1000

712 636 610

875

1176 1132 1078 983

1434

3676 3547 3470

1634

90 d

750

500

wavenumber (cm-1) Fig. 4. Infrared spectra for the ettringite–witherite reaction products after storage at 65 °C for 6, 15, 24, and 72 h, 7 days, and 1, 2 and 3 months (from top to bottom).

(Figs. 3 and 4 and Table 3). At 25 °C, its 30-day presence was inferred from the vibration signals at 3621 (O\H stretching) and 1401 cm−1 (CO2− group ν3 vibrations (shoulder)) [36–38] (Fig. 3, Table 3). 3 3.3. DSC-TG study of the reaction After the first 24 h, the DSC curves for the 25 and 40 °C samples (Fig. 5: 25 °C curves not shown for reasons of similarity) contained endothermal signals at approximately 100 and 240 °C, attributed to synthetic ettringite dehydration; at 805 °C, to witherite polymorphic transition from the rhomboid to the hexagonal system; at 680 °C, to calcite decarbonation (Table 2); and at 1158 °C, to barite polymorphic transition from the rhombohedral to the monoclinic system (Table 3) [39]. The intensity of the last two signals mentioned grew with reaction time (Fig. 5) and the one from calcite also shifted to a higher temperature denoting an increase in its degree of crystallinity (the decarbonation temperature was higher in the 40 than the 25 °C material at the same reaction time). The weak endothermal signal of witherite declined steadily and finally disappeared on the 30-day curves for samples stored at both 25 and 40 °C (Fig. 5), a finding consistent with the FTIR observations. The intensity of the most intense endothermal signal, at 103 °C, generated by ettringite dehydration (Table 2), waned with reaction time, corroborating the progressive decline in its content detected by XRD. Its simultaneous shift to lower temperatures (to approximately 80 °C) denoted a loss of crystallinity, higher at 40 °C than at 25 °C. This low temperature (b90 °C) signal might also be attributed to the presence of free water, although water loss in ettringite is also common at temperatures of under 100 °C [40,41]. The intensity of the endothermal signal at 240 °C, by contrast, not only failed to decline as the reaction advanced (Fig. 5), but rose due to the loss of bound water in the Al(OH)3 groups. One of the stages of calcium monocarboaluminate dehydration [37] might also contribute slightly to these signal increase. Beginning on the 7th day in both the 25 and 40 °C material, an endothermal signal (shoulder) attributed to this phase could be distinguished at around 130 °C. By the 30th day it had developed fully, along with another peak at 105 °C (Fig. 5). These two signals represented the first two stages of calcium monocarboaluminate dehydration (Table 3) [31,37,42]. The third appeared at approximately 240 °C, as mentioned above. The decarboxylation signal for calcium monocarboaluminate at around 865 °C [37], which is less intense than the aforementioned ones, was not detected on the respective DSC curves.

P.M. Carmona-Quiroga, M.T. Blanco-Varela / Cement and Concrete Research 52 (2013) 140–148

145

Table 3 Calcium monocarboaluminate and monosulfoaluminate bands on infrared spectra and signals on DTG and DSC curves.

Monosulfoaluminate

Monocarboaluminate

[22]

Assignment

Present study

[36]

[37]

[38]

Assignment

3675

O-H stretching

3676

3680

3676

3676

O-H stretching

3621

3622

3624

3624

O-H stretching

3546

3550

3543

3555

O-H stretching ( v3 H2O)

3363

3371

O-H stretching (v1H2O)

1641

1645/1636

1647

H-O-H bending (H2O)

1439

1420

1417

1426

C-O stretching (assym. v3)

1401

1369

1365

1365

C-O stretching (assym. v3)

1067

1066

C-O stretching (sym. v1)

954

957

AlO6

812

AlO6

3540

O-H stretching

3523 3500−3100

O-H stretching

3369 3130 3007

H-O-H bending

FTIR

1600

1170

S-O stretching

1100

S-O stretching 953

850

Al-O-H bending 750

This work

[43]

[44]

Assignment

Present study

671

668

674

AlO6

547

539

539

AlO6

424

424

427

AlO6

[42]

[37]

[31]

Assignment

DTG

100−107

120

130−135

150

151−155

~240

240

(241) 262

150

~260 ~650

DSC/DTA

865 ~ 150

165−170

Decarbonation Decarboxylation

~200 (230)

213 (small) ~280 (doublet)

Dehydration

Dehydration 300

According to the thermogravimetric findings in Table 4, calcite content, which was somewhat higher (nearly 25%) in the samples stored at 40 than at 25 °C, remained flat after 30 days. These results, together with the weight loss stabilisation values observed for the other temperature ranges on the thermograms, are consistent with the infrared spectroscopic findings whereby at both 25 and 40 °C, ettringite decomposition reactions came to an end after 30 days. The weight losses at temperatures of under 180 °C (Table 4), attributed essentially to ettringite dehydration (major contributor), were greater at 25 than at 40 °C material, inferring that this phase decomposed more quickly at 40 °C. At 65 °C (Fig. 6), the witherite polymorphic transformation signal on the DSC curve waned into non-existence after 30 days, while the

(increasingly more crystalline) calcite decomposition signal continued to grow. Further to the gravimetric findings (Table 4), the highest percentages of calcite were obtained at this high temperature (27% in the 90-day samples) and lower hydrates losses were recorded (observed up to a temperature of 280 °C). The presence of calcium monosulfoaluminate hydrate, which is stable at this temperature, instead of ettringite, was deduced from the endothermal signals present at approximately 150, 213 (very small signal) and 280 °C (double band) on all except the 90-day DSC curves (Fig. 6, Table 3) [43,44]. Lastly, the appearance of two endothermal signals after 30 days at approximately 100 and 130 °C (double signal) (Fig. 6, Table 3) denoted monocarboaluminate precipitation at the expense of calcium monosulfoaluminate.

146

P.M. Carmona-Quiroga, M.T. Blanco-Varela / Cement and Concrete Research 52 (2013) 140–148

0 90 d 60 d

30 d

Heat Flow (W/g)

MC MC MC

7d 24 h

Calcite unhydrated Barite Witherite Alumina gel

Ettringite

40°C

-6 0

100

200

300

400

500

Exo Up

600

700

800

900 1000 1100 1200

Temperature (°C)

Universal V4.5A TA Instrl

97.60°C

40°C

Deriv. Weight (%/°C)

0.8

84.72°C

0.12% min/°C

238.58°C

unhydrated

0.20% min/°C

24 h

0.22% min/°C

73.50°C 104.52°C 133.19°C 0.26% min/°C

30 d

0.37% min/°C

60 d

76.37°C

106.86°C 134.75°C 0.36% min/°C

7d

240.56°C 90 d

-0.2 25

75

125

175

225

Temperature (°C)

275

Universal V4.5A TA Instrl

Fig. 5. DSC (above) and DTG (below) curves for ettringite decomposition in the presence of barium carbonate at different ages and 40 °C.

Table 4 Weight loss (up to 1200 °C) and calcite content in ettringite–witherite suspensions stored at different temperatures (25, 40 and 65 °C) and reaction times (24 h to 3 months). T Reaction Weight loss (%) (°C) time Total Up to Approx. From approx. 180–280 °C approx. 180 °C 280 to 535–580 °C 25

40

65

24 h 7 days 30 days 60 days 90 days 24 h 7 days 30 days 60 days 90 days 6h 15 h 24 h 72 h 7 days 30 days 60 days 90 days

34.5 30.7 24.5 22.8 23.2 31.8 26.9 23.1 22.7 23.8 23.5 22.8 22.3 21.3 22.6 20.5 21.3 19.5

19.8 15.3 8.3 7.8 7.3 16.7 11.0 5.5 4.9 6.3 4.7 5.4 4.4 4.4 5.8 2.9 3.5 2.2

3.7 3.4 3.1 3.1 3.1 3.5 2.8 3.1 3.7 3.7 2.8 3.5 2.7 2.9 2.4 2.2 2.2 2.4

2.4 2.3 2.3 2.0 2.3 2.4 2.4 2.1 2.0 2.5 2.4 3.2 2.5 2.8 3.1 3.0 3.2 2.2

From Up to 535–580 to 1200 °C 730–770 °C (% calcite) 3.2 (7.2) 5.1 (11.7) 9.0 (20.3) 8.4 (19.1) 8.5 (19.3) 4.6 (10.6) 7.5 (17.0) 10.7 (24.3) 10.8 (24.5) 9.9 (22.5) 4.3 (9.9) 5.6 (12.8) 5.9 (13.4) 7.5 (17.0) 8.6 (19.6) 11.4 (25.9) 11.6 (26.4) 11.7 (26.5)

5.4 4.7 1.8 1.7 2.1 4.5 3.3 1.6 1.4 1.5 9.3 5.1 6.8 3.8 2.7 1.0 0.9 1.0

3.4. Thermodynamic modelling According to thermodynamic modelling predictions, BaSO4 is the majority product of the ettringite–witherite reaction at all three temperatures (1.19 mmoles, from 52.02 to 54.47%; Table 4). Only a small fraction of sulfates (0.4% of ettringite) failed to evolve into this stable phase at 25 °C. The calcium monocarboaluminate hydrate content, the second most abundant reaction product, declined gradually with rising temperature (from 27.01% to 23.49%), inducing gibbsite precipitation as a minority phase (≤3.4%), while calcite concentration grew (from 17.6 to 19.3%) (Table 5). The fact that experimental thermogravimetry detected more calcite and less monocarboaluminate than predicted by thermodynamic modelling (Table 6) is an indication of samples weathering and reaction with atmospheric CO2. This partial conversion of monocarboaluminate into calcite might explain the minority formation of the calcium aluminate hydrate-like species detected with XRD. Final equilibrium of the reaction in an uncontrolled atmosphere (with CO2) would therefore be reached with maximum calcite (40.73%) and gibbsite (10.83%) precipitation in absence of calcium monocarboaluminate hydrate and presence of barite (Table 5).

P.M. Carmona-Quiroga, M.T. Blanco-Varela / Cement and Concrete Research 52 (2013) 140–148

147

0 90 d 60 d 30 d

Heat Flow (W/g)

7d 72 h

MC 24 h 15 h

MS MS MS

Calcite Witherite

6h

Barite

Alumina gel 65°C

-5 0

100

200

300

400

Exo Up

500

600

700

800

900 1000 1100 1200

Temperature (°C)

Universal V4.5A TA Instrl

0.18

65°C 73.75°C

Deriv. Weight (%/°C)

0.13

234.94°C

88.87°C 126.34°C 139.91°C 196.26°C

277.76°C 6h 15 h

0.08 24 h 72 h 30 d

0.03

60 d 90 d

-0.02 25

100

175

250

325

Temperature (°C)

400

Universal V4.5A TA Instrl

Fig. 6. DSC (above) and DTG (below) curves for ettringite decomposition in the presence of barium carbonate at different ages and 65 °C.

4. Conclusions

Since at 65 °C the stable phase is Ca monosulfoaluminate hydrate, an intermediate stage is involved in the reaction pathway:

This study shows that barium carbonate can decompose ettringite and stabilise sulfates in the form of barite. While decomposition advances more swiftly at higher temperatures, it begins during the first 24 h even at ambient temperature (25 °C) and reaches completion in 30 days. At 25 and 40 °C, ettringite decomposes as shown in the following equation:

Ca6 Al2 ðSO4 Þ3 ðOHÞ12 ⋅26H2 O þ 3BaCO3 →ð1−xÞ CaO⋅Al2 O3 ⋅CaSO4 ⋅12H2 O þ ð2 þ xÞ BaSO4 þ ð2 þ xÞ CaCO3 þ ð1−xÞ BaCO3 þ ð2xÞ AlðOHÞ3 þ 17H2 O: Ca4 Al2 O6 ðSO4 Þ⋅12H2 O þ BaCO3 →Ca4 Al2 ðCO3 ÞðOHÞ12 ⋅5H2 O

Ca6 Al2 ðSO4 Þ3 ðOHÞ12 ⋅26H2 O þ 3BaCO3 →3BaSO4 þ ð1−xÞCa4 Al2 ðCO3 ÞðOHÞ12 ⋅5H2 O þ ð2 þ xÞCaCO3 þ 2xAlðOHÞ3

þBaSO4 þ H2 O:

þ 21H2 O:

Table 5 Amount (mmoles and wt.%) of stable hydrates at three reaction temperatures (25, 40 and 65 °C) in the presence and absence of CO2. T (°C)

No CO2

CO2

25 40 65 25/40/65

Table 6 CO2 weight loss of calcite and calcium monocarboaluminate hydrate: comparison of experimental thermogravimetric findings and thermodynamic modelling calculations. CO2 weight loss (%)

mmol (wt.%)

Calcite

Ettringite

C4A C H11

Barite

Calcite

Gibbsite

1.80E−04 – – –

0.25 0.24 0.21 –

1.19 1.19 1.19 1.19

0.94 0.95 0.98 2.33

0.23 0.23 0.18 0.80

Monocarboaluminate

T (°C)

Calculated (GEMS)

Measured at 90 days (TG)

Calculated (GEMS)

Measured at 90 days (TG)

25 40 65

7.72 7.97 8.46

8.51 9.91 11.68

2.09 1.99 1.82

1.67 1.10 0.69

148

P.M. Carmona-Quiroga, M.T. Blanco-Varela / Cement and Concrete Research 52 (2013) 140–148

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