Research Article Thermodynamic Stability of Ettringite...

Research ArticleThermodynamic Stability of Ettringite Formed byHydration of Yersquoelimite Clinker

Marcela Fridrichovaacute Karel Dvolaacutek Dominik GazdiI Jana Mokraacute and Karel Kuliacutesek

Brno University of Technology Veverı 33195 602 00 Brno Czech Republic

Correspondence should be addressed to Karel Dvorak dvorakkfcevutbrcz

Received 31 May 2016 Revised 29 July 2016 Accepted 16 August 2016

Academic Editor Kestutis Baltakys

Copyright copy 2016 Marcela Fridrichova et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

In order to save limited natural resources by utilising industrial by-products this paper focuses on an entirely new application offluidized bed combustion fly ash (FBCFA) into Portland composite cements It is not currently used because undesirable ettringite3CaOsdotAl

2O3sdot3CaSO

4sdot32H2O is formed during the hydration of FBCFA Although the stability of ettringite has been the subject of

much research the solution is not yet fully clear Ettringite is generally considered to be stable up to a temperature of 110∘C howeversome investigators claimed that ettringite may already decompose at even ambient temperatures To prove these statementsettringite was prepared by the hydration of yersquoelimite 3CaOsdot3Al

2O3sdotCaSO

4 and the system stored at laboratory temperature in two

environments in laboratory settings and in an environment of saturated water vapourThemineralogical composition of ettringitewas long term (up to 160 days of hydration) and was analysed by X-ray diffraction (XRD) and differential thermal analysis (DTA)The hydration of yersquoelimite is a relatively complex process Only approximately 30 of ettringite was formed under laboratoryconditions that appeared to gradually convert into metaettringite Within an environment of saturated water vapour we observedthe conversion of ettringite into monosulfate Original yersquoelimite was indicated as the dominant phase of both storages

1 Introduction

The production of building materials currently focuses onstrength values as well as lifetime and durability Related tothis the formation of AFt phases is most often mentioned asa display of sulfate attacks upon cementitious materials TheldquoArdquo from AFt phases means Al

2O3 the ldquoFrdquo represents Fe

2O3

and ldquotrdquo is an abbreviation of the numeral ldquothreerdquo in Latin AFtphases are described by the formula

[Ca3(Y) (OH)

6sdot 12H2O] sdot X

119911sdot 119909H2O (1)

where Y represents cation most often Al3+ Fe3+ or Cr3+ Xrepresents CO

3

2minus SO4

2minus and less often SeO4

2minus or CrO4

2minusand 119909 le 2 119911 le 2

The term ldquoAFtrdquo responds to 3 molecules of CX and theformula can generally be written as

C3(A F) sdot 3CX sdot 119910H

2O (2)

where C represents CaO A represents Al2O3 F represents

Fe2O3 and 119910 = 119909 + 30

Ettringite (3CaOsdotAl2O3sdot3CaSO

4sdot32H2O) is the most

important representative of this group It is a high watermineral that is primarily formed at an early period of thehydration of Portland cement Its structure is hexagonal withneedle-like crystals [1]

Although the decomposition of ettringite has been thesubject of many studies neither accurate temperatures ofdecomposition nor mechanisms and kinetics of this processwere clarified Moreover there is some discrepancy amongthe conclusions of various authors

Taylor et al [2] claim that ettringite is internally unstableat temperatures greater than 70∘C This value is shiftedupward to 90∘Cwhen a sufficient amount of sulfate is present

Zhou and Glasser [3] agree with the generally validopinion that ettringite is unstable at higher temperaturesFor instance it is less stable than hydrogarnet (3CaOsdotAl

2O3sdot

6H2O) which decomposes at a temperature of 325∘C Zhou

and Glasser further claim that metaettringite the decom-posed product of ettringite is typically formed at temperatureranges between 50 and 100∘C At temperatures of 110ndash114∘C

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2016 Article ID 9280131 7 pageshttpdxdoiorg10115520169280131

2 Advances in Materials Science and Engineering

ettringite decomposes into calcium sulfate hemihydratewater and an amorphous compound that is probably AFmphase

When the ettringite is exposed to very low values ofwater vapour pressures (6mm) metaettringite is the primaryproduct of decomposition but it spontaneously and graduallytransforms into AFm phase

Satava and Veprek [4] found a spontaneous decom-position of ettringite at a temperature of 125∘C Certainsimilar results were also found by Ogawa and Roy [5] underhydrothermal conditions at temperatures over 100∘C Hallet al [6] claimed that ettringite begins to decompose at awater vapour pressure of 163 bar (approximately 163 kPa) ata temperature of 114 plusmn 1∘C The study of Abo-el-enein etal [7] determines weight losses in ettringite at temperaturelower than 100∘C Taylor [1] determines the temperatureof decomposition to be 110∘C Artificial ettringite becomesnearly amorphous due to either higher temperature orvacuum In an environment with ambient relative humidityettringite already begins to lose water molecules at 50∘CThe endotherm of ettringite from DTA analysis is located attemperatures ranging between 110 and 150∘CWhen ettringiteoriginates from cement paste the typical peak can be foundat temperatures between 125 and 130∘C

Pourchez et al [8] claim that ettringite may be ther-mally unstable at temperatures lower than 120∘C dependingon the pressure of water vapour Decomposing ettringiteformsmetaettringite containing 10ndash13 molecules of water itsamount depending upon time

Zhou et al [9] also mention temperatures of decompo-sition in a range between 114 and 116∘C at water vapourpressures greater than 760 Torr (approximately 1333 Pa)In the presence of water ettringite decomposes forminghemihydrate CaSO

4sdot05H2O water and a compound that

may be monosulfate depending on its CaOAl2O3SO3ratio

The stability of ettringite depends among other factorson pH value According to Santhanam et al [10] ettringite isunstable in a low-lime environment when pH value decreasesmore than 115ndash120 Ettringite decomposes into gypsumbelow this pH value Shimada and Young [11] extend thestable range of ettringite pH value to 90 to 134

Warren andReardon [12] studied the stability of ettringitewith respect to its solubility They presume that ettringitedissociates according to the following reaction

Ca6Al2O6(SO4)

3sdot 32H2O

997888rarr 6Ca2+ + 2Al (OH)4

minus+ 3SO

4

2minus+ 4OHminus

+ 26H2O

(3)

Zhou and Glasser [3] as well as Hargis et al [13 14]claim that yersquoelimite hydrates to give off monosulfoaluminateand aluminium hydroxide as can be seen in the followingreaction

3CaO sdot 3Al2O3sdot CaSO

4+ 18H

2O

997888rarr 3CaO sdot Al2O3sdot CaSO

4sdot 12H2O + 4Al (OH)

3

(4)

When yersquoelimite is accompanied by calcium and sulfateions and then reacts with water ettringite is formed see thefollowing reaction


4+ 8CaSO

4+ 6Ca (OH)

2

+ 90H2O

997888rarr 3 (3CaO sdot Al2O3sdot 3CaSO

4sdot 32H2O)

(5)

When gypsum is depleted formations of monosulfatebecome dominant Fridrichova et al [15] probably performedthe latest experiment that synthesised ettringite by a pro-cedure involving the direct hydration of mineral yersquoelimite(3CaOsdot3Al

2O3sdotCaSO

4) for 28 days at laboratory tempera-

tures However the long-term stability of this ettringite hasnot yet been fully described Based on the above-mentionedfacts because of its low temperature of decomposition andhigh content of relatively free bonded water we can presumethat ettringite will gradually decompose due to the evapora-tion of free bonded water under dry conditions and be morestable in conditions of higher humidity

The aim of this paper is to describe the long-term stabilityof ettringite formed by yersquoelimite clinker hydration underconditions of varying humidity

2 Materials and Methods

In order to observe the long-term thermodynamic stabilityof ettringite it was prepared by the hydration of yersquoelimiteclinker 3CaOsdot3Al

2O3sdotCaSO

4 Samples of hydrated yersquoelimite

clinker were next exposed to two varied conditionsThe raw meal for yersquoelimite clinker preparation con-

sisted of 3 compounds limestone CaCO3with a purity of

99 corundum Al2O3with a purity of 100 and gypsum

CaSO4sdot2H2O with a purity of 986 Their ratios were

calculated according to the stoichiometric ratio of yersquoelimiteThe raw meal then contained 3844 limestone 3921corundum and 2235 gypsumThe three components werehomogenized andmilled under wet conditions in a PlanetaryMonoMill PULVERISETTE 6The volume of the capsule was500mL and the size of milling bodies was 20mm Isopropylalcohol was chosen as the inert liquid for milling andhomogenization Milling and homogenization were carriedout gradually in 200 g doses at 500 rpm for 15min Theindividual doses were next blended using an electric blenderThe resulting mixture was then dried in a laboratory drier ata temperature of 50∘C for 24 h 3 kg of rawmeal was preparedin total

The raw meal was then placed into 30mL platinumcrucibles and compacted by stomping A 4mm diametercavity was made in the centre of the material to enable theeasy release of gas The firing process was performed in aclassic 1700 kiln at a temperature of 1200∘C soaking for3 h and a heating rate of 10∘Cmin The firing process wassubsequently terminated by quenching using pressurized air

The resulting clinker was then ground in a planetarymill under dry conditions at 300 rpm allowing no particleslarger than 0063mm at completion Prior to that the samplefor the powder diffraction was refined using a McCrone

Advances in Materials Science and Engineering 3

Micronising Mill for 150 s The mineralogical composition ofthe yersquoelimite clinker was examined by XRD analysis using aPanalytical Empyrean diffractometer (copper anode 120582K

1205721=

154056 A generated at 45mA and 40 kV) The measurementstep size was 0013∘ 2120579 with integration at the rate of 150 sper step The high score+ analysis was used for XRD patternassessments

The yersquoelimite clinker was hydrated becoming paste Thewaterclinker ratio of 055 was chosen to prevent hydrationpauses due to lack of water 10 times 10 times 30mm prisms weremanufactured in order to easily manipulate the hydratedpaste During the initial 24 hours samples were stored underdamp conditions RH gt 90 and a temperature of 21∘CFollowing that the samples were demoulded and were storedunder two conditions

(i) laboratory conditions temperature of 21∘C and RH of40

(ii) an environment of saturated water vapour at a tem-perature of 21∘C

In order to observe the process of hydration threesamples for XRD analysis as well as for DTA were taken fromboth conditions twice a week up to a hydration age of 160days For all analyses hydrated yersquoelimite samples were firstcrushed in an agate bowl and hydration was stopped by triplewashing in isopropyl alcohol The material was subsequentlyrefined using a McCrone Micronising Mill for 150 s (also inisopropyl alcohol) Following those procedures the sampleswere gently dried at 30∘C in a laboratory drier and analysedDTA were performed using a Mettler Toledo TGDSC1 witha temperature range up to 1000∘C Approximately 50ndash70mgof the sample was analysed Average values of the threeDTA measurements were used for the quantification of theettringite and the calcium monosulfoaluminate hydrate

For the XRD analysis the same settings were used as inthe previous case

Apart from phase composition the pH value of thesamples was observed once a week One prism of eachsample was weighed and ground in an agate bowl Next thepowder was mixed with an equal weight of distilled waterThis mixture was filtered after 24 hours and the pH of thesolution was measured using an electronic pH-meter MettlerToledo Easy 21 The pH of the hydrated pastes under bothstorage conditions was between 95 and 10 For this reasonthe influence of the pH value on the stability of ettringite wasnot taken into account

3 Results and Discussion

31 Phase Composition of Yersquoelimite Clinker The results ofthe phase andmineralogical composition of yersquoelimite clinkerimmediately after firing can be seen in Figure 1

Yersquoelimite (Y) 3CaOsdot3Al2O3sdotCaSO

4(d-spacing = 3758

2169 2657gt) was identified as virtually the only mineralBesides that residua from precursor minerals of raw mate-rials were slightly present namely anhydrite II (A) CaSO

4

(d-spacing = 3496 2848 2209gt) and corundum (Co) 120572-Al2O3(d-spacing = 1390 2550 2110gt)

A anhydrite P portlandite Co corundum

YY

Y

Y

Y

YYYY

YY

Y

YY

Y

CoP

PA

A

A A Co

12 16 20 24 28 32 36 40 44 4882120579(

∘)

Y yersquoelimite

Figure 1 XRD pattern of yersquoelimite clinker

E ettringite A anhydrite P portlandite

AY

Y

E E E EE Y + EY+ E + P

100 120 140 160 180 200 220 240 260 280 300802120579(

∘)

Y yersquoelimite

1 day

7 days19 days29 days57 days92 days145 days

154 days

Figure 2 XRD patterns of hydrated yersquoelimite clinker exposed tolaboratory settings age 1ndash159 days of hydration

Very weak diffraction lines corresponded to portlandite(P) Ca(OH)

2(d-spacing = 2627 4922gt) This phase was

present in the XRD pattern due to the hydration of a verysmall amount of free lime by air humidity during XRDanalysis when the sample was stored in the sample changermagazine

Thus it can be claimed that raw meal components werenearly completely synthesised for yersquoelimite during firing

32 Phase Composition of Hydrated Yersquoelimite Clinker Sam-ples of hydrated yersquoelimite clinker from laboratory conditionsand environment of saturated water vapour were regularlyanalysed by XRD analysis and DTA analysis

321 Phase Analysis Figures 2 and 3 show XRD patterns ofhydrated yersquoelimite from both storage conditions In order toobtain a clear overview only patterns of hydrated yersquoelimiteaged 1 7 19 29 57 92 145 and 159 days are presented

Yersquoelimite (Y) and ettringite (E) (d-spacing = 97205610gt) were identified in XRD patterns of the samplesexposed to laboratory conditions

Except for those minerals mentioned above calciummonosulfoaluminate hydrate (M

1) 3CaOsdotAl

2O3sdotCaSO

4sdot

12H2O (d-spacing = 8932 4466gt) calcium monosulfoalu-


E ettringiteG Gibbsite A anhydrite P portlandite

AY

Y

E

G GG

E E E

E

E E

100 120 140 160 180 200 220 240 260 280 300802120579(

∘)

M1M2

M1

M1M2HM1M2H

E + M1

E + M1G

E + Y

Y+ EE + Y + P

M1 + M2G

1 day


154 days

Y yersquoelimiteM1 monosulfate-12 H2OM2 monosulfate-11 H2O

H C4AHX

Figure 3 XRD patterns of hydrated yersquoelimite clinker exposedto environment of saturated water vapour aged 1ndash159 days ofhydration

minate hydrate (M2) 3CaOsdotAl

2O3sdotCaSO

4sdot11H2O (d-spacing

= 8548 4273gt) C-A-H phase (H) (d-spacing = 82002860) and possibly Gibbsite (G) Al(OH)

3(d-spacing =

4850 4350) were identified in XRD patterns in the case ofthe samples exposed to an environment of saturated watervapour Residua of minerals from the original raw materialswere rarely found either However these negligible contentsdid not affect the observed processes

In early stages of hydration under laboratory conditionshydrated samples showed a high intensity of the diffractionlines of the original yersquoelimite and a comparatively lowintensity of resulting ettringite No growth of ettringitediffraction lines was noticed during early stage hydrationunder laboratory conditions

Conversely its intensity apparently decreases under long-term observation We claim that this effect was associatedwith the drying of ettringite that is the loss of relativelyfree bonded water molecules from its structural cavitiesAs no other crystalline phases or products of hydrationwere detected during yersquoelimite hydration under laboratorysettings it confirms the opinion according to [3] that the lossofmolecular water results in the transformation of crystallineettringite into an amorphous substance designated in litera-ture as a metaettringite [9]

In the environment of saturatedwater vapour the gradualincrease of intensity of ettringite diffraction and correspond-ing decrease of yersquoelimite diffraction were observed up toapproximately 60 days of hydration Amarked release of basaldiffraction of monosulfate (M

1) initially with a very diffuse

character was observed between 15 and 20 days of hydrationas can be seen in Figure 3

Over the course of time the intensity and sharpnessof this diffraction rose and other types of diffraction ofmonosulfate became apparent With increasing intensities ofmonosulfate the intensity of diffraction of ettringite virtuallystopped This means ettringite is not the only hydrationproduct of yersquoelimite Monosulfate is also formed which hasthe same stoichiometric ratio of CaOSO

3as that of yersquoelimite

40 90 140 190 240 290 340 390 440 490 540

Weight loss89186581271368751653754251955418271818180217781913

EE

Temperature 40ndash200∘C



t (∘C)

minus250

minus200

minus150

minus100

minus050

DTG

(m

in)

50 days

15 days159 days

1day5 days

Figure 4 DTA of hydrated yersquoelimite clinker exposed to laboratoryconditions (E = ettringite)

Weight loss

8911011985

11031134

516658793984

1067

18272078227226292774

minus320

minus270

minus220

minus170

minus120

minus070

minus020

DTG

(m

in)

90 140 190 240 290 340 390 440 490 54040

t (∘C)




1 day5 days15 days

50 days159 days

E

(A)

E +

+

M

Figure 5 DTA analysis of hydrated yersquoelimite clinker exposed toenvironment of saturated water vapour (E is ettringite M is calciummonosulfoaluminate hydrate and A is calcium aluminate hydratephases)

Thus it can be assumed that monosulfate will prevail in thesystemover long-term storage in an environment of saturatedwater vapour For that reason ettringite will also graduallytransform into it

Over approximately 60 days very diffuse diffractionlines of Gibbsite were identified and later after 150 days ofhydration also C

4AHX as a representative of C-A-H phases

was found All these findings correspond to [3 13 14]According to [10 11] gypsum was not identified in anycases Nevertheless this fact should be verified in follow-upresearch

322 DTA and the Quantification of Hydration ProductsFigures 4 and 5 present DTA analysis curves of hydratedyersquoelimite from the laboratory settings and environment ofsaturated water vapour In order to obtain a clear overviewand better resolution only curves of hydrated yersquoelimite at


ages 1 5 15 50 and 159 days and temperatures up to 600∘Care presented in the graphs

The first endothermic effect in the temperature ranges of40ndash200∘C corresponds to the loss of molecular water fromettringite The second effect at 240ndash390∘C represents thelosses of water arising from dehydration of calcium mono-sulfoaluminate hydrate assuming the monosulfate has beenpresented in the sample [1] However the same second effectis also presented in the typical DTA of a pure ettringite curve[1] In this case this effect may correspond to a loss of the restof molecular water and hydroxyl water from the ettringite

Total loss of annealing at temperatures of 40 to 1000∘Cwas always about 4 to 65 higher than the loss of annealingin the observed temperature range from 40 to 390∘C In thecomplete DTA curves the endothermic effect of about 750∘to 790∘C occurred in addition to the endothermic effects inthe range of 40 to 390∘C The data is presented in Figures 4and 5

This effect corresponded to the decomposition of calciumcarbonate The carbonate formed in the samples due to thehydration and subsequent carbonation of free lime fromclinkerHowever the content of this phasewas not substantialenough for the study of the issue of quantifying the contentof ettringite and monosulfate

Upon storage in a laboratory environment the loss ofignition ratios between the first and second endothermiceffect decreased with the period of hydration Becauseaccording to XRD analysis results monosulfate is not presentin the samples (or its amount is below the methodrsquos detectionthreshold) this phenomenon may be explained as a conse-quence of the gradual evaporation of molecular water relatedto the transformation of ettringite into metaettringite [8] Onthe contrary upon storage in an environment saturated withwater vapours this effect cannot occur Thus in this casethe ratio of loss of annealing between the first and secondendotherm for the hydration time period of one day was usedto quantify monosulfate in the presence of ettringite whereonly ettringite was actually present in the sample

Thus the established ratio of the overall loss by annealingunder the second endoeffect was subtracted from the overallannealing loss that corresponds to the decomposition ofettringite in samples stored in an environment saturated withwater vapours The remaining loss by annealing under thesecond endoeffect was then attributed to the decompositionof monosulfate Upon long-term hydration in this environ-ment there was also an initial development of the C-A-H phases of the C

4AHX type The eventual development

of Gibbsite was observed from approximately day 150 onThe presence of these phases was disregarded during thequantification of these monosulfates

(1) Laboratory Conditions-Quantification In consideration ofthe absence of other hydration products it was possible toquantify the amount of ettringite that was produced dueto hydration of yersquoelimite clinker stored under laboratoryconditions see Figure 6

Amounts of the ettringite decreased slightly but gradu-ally when stored under laboratory conditionsThis statementcorresponds very well to the results of XRD analysis In order

8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159

Age of hydration (days)

Amount of ettringite ()

0

5

10

15

20

25

30

Am

ount

of e

ttrin

gite

()

Figure 6 Average amount of ettringite in hydrated yersquoelimite clinkerexposed to laboratory conditions

8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159


Molecularhydroxyl water (mdash)

002040608

112141618

Mol

ecul

arh

ydro

xyl w

ater

(mdash)

Figure 7 Molecular water to hydroxyl water ratio of hydratedyersquoelimite clinker exposed to laboratory conditions

to verify the gradual loss of molecular water hypothesisthe loss of water ratios between the first (molecular water)and the second (hydroxyl water or possibly the remainderof molecular water) endotherm was calculated for all thehydration ages see Figure 7

It follows that molecular water to hydroxyl water ratiogradually decreased under these laboratory conditions Thisfinding is consistent with the hypothesis [8] that there isa slight but progressive loss of molecular water located inthe structural cavities of ettringite during hydration duringtheir exposure under laboratory conditions Due to thisphenomenon the conversion of ettringite into amorphousmetaettringite occurs

(2) Environment of Saturated Water Vapour-QuantificationContrary to the yersquoelimite clinker hydration behaviour inlaboratory conditions the hydration of yersquoelimite in an envi-ronment of saturated water vapour performed differentlyBased on the results from DTA it is obvious that the amountof ettringite gradually increased up to 50 days of hydrationMonosulfate was simultaneously formed and its contentachieved approximately 10 of the whole volume at the end


05

1015202530354045

1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159A

mou

nt o

f ettr

ingi

te m

onos

ulfa

te (

)


Amount of ettringite ()Amount of monosulfate ()

Figure 8 Average amount of ettringite andmonosulfate in hydratedyersquoelimite clinker exposed to environment of saturated water vapour

1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159



08

1

12

14

16

18

2

22

Mol

ecul

arh

ydro

xyl w

ater

(mdash)

Figure 9 Molecular water to hydroxyl water ratio of hydratedyersquoelimite clinker exposed to environment of saturated water vapour

of the observed hydration periodThis content accounted forabout one-quarter of the amount of ettringite see Figure 8

The molecular water to hydroxyl water ratio was alsoreduced as well as in the case of hydration under laboratoryconditions This was due to the formation of monosulfate inthis case as can be seen in Figure 9 The results of thermalanalysis agreed very well with those of XRD analysis

4 Conclusion

The stability of ettringite that was formed by the hydrationof yersquoelimite clinker was in the study of this paper Twoconditions with different relative humidity were chosen forthis experiment

Under laboratory conditions the transformation of yersquoe-limite into ettringite was due to a deficiency of water whengradually suppressed Due to the drying process even thevery slow but obvious conversion of crystalline ettringite intoamorphous metaettringite took place in terms of later hydra-tion periods An increasing loss of free bonded molecularwater located in the structural cavities of ettringite proved

this transformation of ettringite into metaettringite Even sothe hydroxyl water molecules still remained in structuralunits of metaettringite

Under exposure to an environment of saturated watervapour the content of the resulting ettringite increased butcalciummonosulfoaluminate hydrate was also formed besideettringite in terms of relatively early periods of hydrationThemaximum amount of ettringite was achieved at a hydrationage of 50 days This content did not change later or itsincrease was negligible Amounts ofmonosulfate were kept atrelatively low levels up to an exposure of 20 days After thatit began to grow fairly steeply Free bonded molecular waterto hydroxyl water ratios also decreased simultaneously withincreasing monosulfate content

It is apparent from the information above that ettringitein the system of hydrated yersquoelimite clinker was relativelyunstable in regard to both humidity conditions and thestorage time periods

Competing Interests

The authors declare that they have no competing interests

Acknowledgments

This work was financially supported by Project no 14-32942SldquoEffect of Fluidized Bed Ash on the Thermodynamic Stabilityof Hydraulic Bindersrdquo This paper was also developed withthe financial support of Project no LO1408 ldquoAdMaS UP-Advanced Materials Structures and Technologiesrdquo supportedby the Czech Ministry of Education Youth and Sports underldquoNational Sustainability Programme Irdquo

References

[1] H F W Taylor Cement Chemistry Thomas Telford LondonUK 1997

[2] H FW Taylor C Famy and K L Scrivener ldquoDelayed ettringiteformationrdquo Cement and Concrete Research vol 31 no 5 pp683ndash693 2001

[3] Q Zhou and F P Glasser ldquoThermal stability and decompositionmechanisms of ettringite at lt 120∘Crdquo Cement and ConcreteResearch vol 31 pp 1333ndash1339 2001

[4] V Satava and O Veprek ldquoThermal decomposition of ettrin-gite under hydrothermal conditionsrdquo Journal of the AmericanCeramic Society vol 58 no 7-8 pp 357ndash359 1975

[5] K Ogawa and D M Roy ldquoC4A3S hydration ettringite for-

mation and its expansion mechanism I expansion ettringitestabilityrdquoCement and Concrete Research vol 11 no 5-6 pp 741ndash750 1981

[6] C Hall P Barnes A D Billimore A C Jupe and X TurrillasldquoThermal decomposition of ettringite Ca

6[Al(OH)

6]

2(SO4)3sdot

26H2Ordquo Journal of the Chemical Society vol 92 no 12 pp 2125ndash

2129 1996[7] S A Abo-el-enein S Hanafi and E E Hekal ldquoThermal

and physiochemical studies on ettringite II Dehydration andthermal stabilityrdquo Cemento vol 85 no 2 pp 121ndash132 1988(Italian)

[8] J Pourchez F Valdivieso P Grosseau R Guyonnet and BGuilhot ldquoKinetic modelling of the thermal decomposition of


ettringite into metaettringiterdquo Cement and Concrete Researchvol 36 no 11 pp 2054ndash2060 2006

[9] Q Zhou E E Lachowski and F P Glasser ldquoMetaettringitea decomposition product of ettringiterdquo Cement and ConcreteResearch vol 34 no 4 pp 703ndash710 2004

[10] M Santhanam M D Cohen and J Olek ldquoSulfate attackresearchmdashwhither nowrdquo Cement and Concrete Research vol31 no 6 pp 845ndash851 2001

[11] Y Shimada and J F Young ldquoThermal stability of ettringite inalkaline solutions at 80∘Crdquo Cement and Concrete Research vol34 no 12 pp 2261ndash2268 2004

[12] C J Warren and E J Reardon ldquoThe solubility of ettringite at25∘Crdquo Cement and Concrete Research vol 24 no 8 pp 1515ndash1524 1994

[13] C W Hargis A P Kirchheim P J M Monteiro and EM Gartner ldquoEarly age hydration of calcium sulfoaluminate(synthetic yersquoelimite C

4A3119878) in the presence of gypsum and

varying amounts of calcium hydroxiderdquo Cement and ConcreteResearch vol 48 pp 105ndash115 2013

[14] C W Hargis A Telesca and P J M Monteiro ldquoCalcium sul-foaluminate (Yersquoelimite) hydration in the presence of gypsumcalcite and vateriterdquo Cement and Concrete Research vol 65 pp15ndash20 2014

[15] M Fridrichova K Dvorak K Kulısek A Masarova andK Havlıckova ldquoSynthetic preparation of ettringiterdquo AdvancedMaterials Research vol 1000 pp 55ndash58 2014

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ettringite decomposes into calcium sulfate hemihydratewater and an amorphous compound that is probably AFmphase

When the ettringite is exposed to very low values ofwater vapour pressures (6mm) metaettringite is the primaryproduct of decomposition but it spontaneously and graduallytransforms into AFm phase

Satava and Veprek [4] found a spontaneous decom-position of ettringite at a temperature of 125∘C Certainsimilar results were also found by Ogawa and Roy [5] underhydrothermal conditions at temperatures over 100∘C Hallet al [6] claimed that ettringite begins to decompose at awater vapour pressure of 163 bar (approximately 163 kPa) ata temperature of 114 plusmn 1∘C The study of Abo-el-enein etal [7] determines weight losses in ettringite at temperaturelower than 100∘C Taylor [1] determines the temperatureof decomposition to be 110∘C Artificial ettringite becomesnearly amorphous due to either higher temperature orvacuum In an environment with ambient relative humidityettringite already begins to lose water molecules at 50∘CThe endotherm of ettringite from DTA analysis is located attemperatures ranging between 110 and 150∘CWhen ettringiteoriginates from cement paste the typical peak can be foundat temperatures between 125 and 130∘C

Pourchez et al [8] claim that ettringite may be ther-mally unstable at temperatures lower than 120∘C dependingon the pressure of water vapour Decomposing ettringiteformsmetaettringite containing 10ndash13 molecules of water itsamount depending upon time

Zhou et al [9] also mention temperatures of decompo-sition in a range between 114 and 116∘C at water vapourpressures greater than 760 Torr (approximately 1333 Pa)In the presence of water ettringite decomposes forminghemihydrate CaSO

4sdot05H2O water and a compound that

may be monosulfate depending on its CaOAl2O3SO3ratio

The stability of ettringite depends among other factorson pH value According to Santhanam et al [10] ettringite isunstable in a low-lime environment when pH value decreasesmore than 115ndash120 Ettringite decomposes into gypsumbelow this pH value Shimada and Young [11] extend thestable range of ettringite pH value to 90 to 134

Warren andReardon [12] studied the stability of ettringitewith respect to its solubility They presume that ettringitedissociates according to the following reaction

Ca6Al2O6(SO4)

3sdot 32H2O

997888rarr 6Ca2+ + 2Al (OH)4

minus+ 3SO

4

2minus+ 4OHminus

+ 26H2O

(3)

Zhou and Glasser [3] as well as Hargis et al [13 14]claim that yersquoelimite hydrates to give off monosulfoaluminateand aluminium hydroxide as can be seen in the followingreaction


4+ 18H

2O

997888rarr 3CaO sdot Al2O3sdot CaSO

4sdot 12H2O + 4Al (OH)

3

(4)

When yersquoelimite is accompanied by calcium and sulfateions and then reacts with water ettringite is formed see thefollowing reaction


4+ 8CaSO

4+ 6Ca (OH)

2

+ 90H2O

997888rarr 3 (3CaO sdot Al2O3sdot 3CaSO

4sdot 32H2O)

(5)

When gypsum is depleted formations of monosulfatebecome dominant Fridrichova et al [15] probably performedthe latest experiment that synthesised ettringite by a pro-cedure involving the direct hydration of mineral yersquoelimite(3CaOsdot3Al

2O3sdotCaSO

4) for 28 days at laboratory tempera-

tures However the long-term stability of this ettringite hasnot yet been fully described Based on the above-mentionedfacts because of its low temperature of decomposition andhigh content of relatively free bonded water we can presumethat ettringite will gradually decompose due to the evapora-tion of free bonded water under dry conditions and be morestable in conditions of higher humidity

The aim of this paper is to describe the long-term stabilityof ettringite formed by yersquoelimite clinker hydration underconditions of varying humidity

2 Materials and Methods

In order to observe the long-term thermodynamic stabilityof ettringite it was prepared by the hydration of yersquoelimiteclinker 3CaOsdot3Al

2O3sdotCaSO

4 Samples of hydrated yersquoelimite

clinker were next exposed to two varied conditionsThe raw meal for yersquoelimite clinker preparation con-

sisted of 3 compounds limestone CaCO3with a purity of

99 corundum Al2O3with a purity of 100 and gypsum

CaSO4sdot2H2O with a purity of 986 Their ratios were

calculated according to the stoichiometric ratio of yersquoelimiteThe raw meal then contained 3844 limestone 3921corundum and 2235 gypsumThe three components werehomogenized andmilled under wet conditions in a PlanetaryMonoMill PULVERISETTE 6The volume of the capsule was500mL and the size of milling bodies was 20mm Isopropylalcohol was chosen as the inert liquid for milling andhomogenization Milling and homogenization were carriedout gradually in 200 g doses at 500 rpm for 15min Theindividual doses were next blended using an electric blenderThe resulting mixture was then dried in a laboratory drier ata temperature of 50∘C for 24 h 3 kg of rawmeal was preparedin total

The raw meal was then placed into 30mL platinumcrucibles and compacted by stomping A 4mm diametercavity was made in the centre of the material to enable theeasy release of gas The firing process was performed in aclassic 1700 kiln at a temperature of 1200∘C soaking for3 h and a heating rate of 10∘Cmin The firing process wassubsequently terminated by quenching using pressurized air

The resulting clinker was then ground in a planetarymill under dry conditions at 300 rpm allowing no particleslarger than 0063mm at completion Prior to that the samplefor the powder diffraction was refined using a McCrone



1205721=











4(d-spacing = 3758


4



YY

Y

Y

Y

YYYY

YY

Y

YY

Y

CoP

PA

A

A A Co

12 16 20 24 28 32 36 40 44 4882120579(

∘)

Y yersquoelimite



AY

Y


100 120 140 160 180 200 220 240 260 280 300802120579(

∘)

Y yersquoelimite

1 day


154 days










1) 3CaOsdotAl

2O3sdotCaSO

4sdot




AY

Y

E

G GG

E E E

E

E E

100 120 140 160 180 200 220 240 260 280 300802120579(

∘)

M1M2

M1

M1M2HM1M2H

E + M1

E + M1G

E + Y

Y+ EE + Y + P

M1 + M2G

1 day


154 days


H C4AHX



2O3sdotCaSO



3(d-spacing =









40 90 140 190 240 290 340 390 440 490 540

Weight loss89186581271368751653754251955418271818180217781913

EE




t (∘C)

minus250

minus200

minus150

minus100

minus050

DTG

(m

in)

50 days

15 days159 days

1day5 days


Weight loss

8911011985

11031134

516658793984

1067

18272078227226292774

minus320

minus270

minus220

minus170

minus120

minus070

minus020

DTG

(m

in)

90 140 190 240 290 340 390 440 490 54040

t (∘C)




1 day5 days15 days

50 days159 days

E

(A)

E +

+

M


















8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159



0

5

10

15

20

25

30

Am

ount

of e

ttrin

gite

()


8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159



002040608

112141618

Mol

ecul

arh

ydro

xyl w

ater

(mdash)






05

1015202530354045

1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159A

mou

nt o

f ettr

ingi

te m

onos

ulfa

te (

)




1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159



08

1

12

14

16

18

2

22

Mol

ecul

arh

ydro

xyl w

ater

(mdash)




4 Conclusion






Competing Interests


Acknowledgments


References








6[Al(OH)

6]

2(SO4)3sdot























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





1205721=











4(d-spacing = 3758


4



YY

Y

Y

Y

YYYY

YY

Y

YY

Y

CoP

PA

A

A A Co

12 16 20 24 28 32 36 40 44 4882120579(

∘)

Y yersquoelimite



AY

Y


100 120 140 160 180 200 220 240 260 280 300802120579(

∘)

Y yersquoelimite

1 day


154 days










1) 3CaOsdotAl

2O3sdotCaSO

4sdot




AY

Y

E

G GG

E E E

E

E E

100 120 140 160 180 200 220 240 260 280 300802120579(

∘)

M1M2

M1

M1M2HM1M2H

E + M1

E + M1G

E + Y

Y+ EE + Y + P

M1 + M2G

1 day


154 days


H C4AHX



2O3sdotCaSO



3(d-spacing =









40 90 140 190 240 290 340 390 440 490 540

Weight loss89186581271368751653754251955418271818180217781913

EE




t (∘C)

minus250

minus200

minus150

minus100

minus050

DTG

(m

in)

50 days

15 days159 days

1day5 days


Weight loss

8911011985

11031134

516658793984

1067

18272078227226292774

minus320

minus270

minus220

minus170

minus120

minus070

minus020

DTG

(m

in)

90 140 190 240 290 340 390 440 490 54040

t (∘C)




1 day5 days15 days

50 days159 days

E

(A)

E +

+

M


















8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159



0

5

10

15

20

25

30

Am

ount

of e

ttrin

gite

()


8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159



002040608

112141618

Mol

ecul

arh

ydro

xyl w

ater

(mdash)






05

1015202530354045

1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159A

mou

nt o

f ettr

ingi

te m

onos

ulfa

te (

)




1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159



08

1

12

14

16

18

2

22

Mol

ecul

arh

ydro

xyl w

ater

(mdash)




4 Conclusion






Competing Interests


Acknowledgments


References








6[Al(OH)

6]

2(SO4)3sdot























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





AY

Y

E

G GG

E E E

E

E E

100 120 140 160 180 200 220 240 260 280 300802120579(

∘)

M1M2

M1

M1M2HM1M2H

E + M1

E + M1G

E + Y

Y+ EE + Y + P

M1 + M2G

1 day


154 days


H C4AHX



2O3sdotCaSO



3(d-spacing =









40 90 140 190 240 290 340 390 440 490 540

Weight loss89186581271368751653754251955418271818180217781913

EE




t (∘C)

minus250

minus200

minus150

minus100

minus050

DTG

(m

in)

50 days

15 days159 days

1day5 days


Weight loss

8911011985

11031134

516658793984

1067

18272078227226292774

minus320

minus270

minus220

minus170

minus120

minus070

minus020

DTG

(m

in)

90 140 190 240 290 340 390 440 490 54040

t (∘C)




1 day5 days15 days

50 days159 days

E

(A)

E +

+

M


















8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159



0

5

10

15

20

25

30

Am

ount

of e

ttrin

gite

()


8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159



002040608

112141618

Mol

ecul

arh

ydro

xyl w

ater

(mdash)






05

1015202530354045

1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159A

mou

nt o

f ettr

ingi

te m

onos

ulfa

te (

)




1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159



08

1

12

14

16

18

2

22

Mol

ecul

arh

ydro

xyl w

ater

(mdash)




4 Conclusion






Competing Interests


Acknowledgments


References








6[Al(OH)

6]

2(SO4)3sdot























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials














8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159



0

5

10

15

20

25

30

Am

ount

of e

ttrin

gite

()


8 12 15 22 36 38 50 71 82 85 89 92 117

131

138

145

152

159



002040608

112141618

Mol

ecul

arh

ydro

xyl w

ater

(mdash)






05

1015202530354045

1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159A

mou

nt o

f ettr

ingi

te m

onos

ulfa

te (

)




1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159



08

1

12

14

16

18

2

22

Mol

ecul

arh

ydro

xyl w

ater

(mdash)




4 Conclusion






Competing Interests


Acknowledgments


References








6[Al(OH)

6]

2(SO4)3sdot























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




05

1015202530354045

1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159A

mou

nt o

f ettr

ingi

te m

onos

ulfa

te (

)




1 2 5 6 7 8 12 15 22 36 50 71 92 117

138

145

152

159



08

1

12

14

16

18

2

22

Mol

ecul

arh

ydro

xyl w

ater

(mdash)




4 Conclusion






Competing Interests


Acknowledgments


References








6[Al(OH)

6]

2(SO4)3sdot























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Research Article Thermodynamic Stability of Ettringite...

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