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Journaiofthe CeramicSocietvofJapan leY l5] 380-385 (2001) Paper

PhaseTransformation and Hydration of Silicate Containing StabilizersDicaicium

Choon-Keun PARKKorea Ihastitute of indttstrial fl!cJmolagv'. 35-3, Hongchon-Ri, lbjang-toun,Chonan 330-8215 Korea

?xxtfiil(ldi v)Trci;-6 {si-r hJLsi r)ASi iJ b'- ts (7)iegpe.. (} zkMNJM iF #tc ptajthgttrtiff1,eewa, 330-825 Jkptftpa JCk"i t,/utfi

fi-dicalcium silicate was stabilized at room temperature and sy]thesized from reagent grade chemicals usi]gB!03, K20 or S03 as stabilizers. B20: acts as the most effective stabilizer for dicalcium silicate. Phase trans-formation from P- to 7-dicalcium silicate depends on the start temperature of quenching. Hydration heat offi-C2S shows that S03 delayed hydration at an early hydration time, but higher heat liberation rate at a laterhydration time than the other stabilizers. 29Si MAS NMR analysis shows that the h)rdrate of dicalcium sili-cate stabilized by B20g is polymerized more than those of diealcium silicates stabilized by K20 a]d SO:.Diealeium silicate prese]ts different shapes of calcium silicate hydrate aecordi"g to stabilizers: dicalciumsilicate stabilized by B203 shows crumbled foil shape and K20 needle shape of C-S-H. FReceived June 29, 1999; Accepted Septenibcr 20, 2000]

Key-words ; Dicalcium silicate, Slabitixeny Phase traitstbrmation,Polymeriaation, llydrates

1, lntroduction 'Dicalcium

silicate is one of the main crystalline phases o'fportland cement clinker, and has many advantages: lowenerg}r consumption during clinkerization, low heat of Iiber-ation and ]ow absorption of superplasticjzer when hydrated,high flowability of concrete , good durability of structure etc.Although dicalcium silicate has many strong points, it has alimit of applications because of its slow hydration rate at anearly stage and its phase transiormation of 6-C2S to v-C2Sphase during the clinkerization. The transformation of fi-C2S to v-C2S cause volume change of clinker, resulting informing dusts and circulating them in the kiln which jn-fluence a sintering process and quality problems, The reac-tivity of 7-C2S is very low and affects low rnechanical

properties of cement. Therefore, the transformation has tobe prohibited in case of rnuch amount of dicalcium silicate inthe clinker for better sintering process and quality ofresultant product, cernent. For stabilizing or- or 6-C2S which is unstable forrn at roomtemperature, many studjesi/''5/' have been conducted to findforeign ions which stabi]ize the 6-C2S phase at roem tcmper-ature and increase the hydration rate, and reported that thcforeign ions, such as I3, K, S, Mn, Cr etc., were goodstabilizers for the fi-C2S phase. Schwiete6) suggested a theo-ry of judgment to determine ions stabilizing fi-C2S, which isbased on ionic radius, Pritts andi Daugherty7) had alsoreported the judgment of CfRratio (C: ionic electrovalence,R: Pauling's ionic radius) to determine stabilization of thedicalcium silicate crystal at room temperature. They alsoreported that the rate of S-C2S hyclration was greatly in-fiuenced by the type and the amount ofa foreign ion dopedto the calcium silicate structure; the stabilizers incorporatedinto the dicaleium silicated struc:ture may rctard or acceler-ated by hydration reaction, In this paper, the characteristics of fi-C2S stabilized byforeign ions, such as B, S and K were invest{gated by XRD,DTA and SEM, and the phase stability and hydration ol

fi-C2S were also studied. In addition, the hydration behavioref fi-C2S was alse discussed with 2gSiMASNMR analysisand SEM observation.

2. Experimental The specimens of 6-dicalcium silicate were synthesizedusing reagent grade chemicals as shown in Table 1. Themixtures of CaO and Si02 were based on a CaOISi02 rnolec-ule ratio of 2.0. The stabilizers, B, K20 and SO/] were sup-pliedbychemicalsH3B03,K2C03CaS042HzO,respectively. For synthesizing the specimens the mixtures were mixedin a ball mill and calcined at 10000C for 1 h, After calcinationthe mixtures were pressed into a pellet, 20 cm in diameterand 20 cm in height, and then sintered at 1400DC for 3h.The sintered samples were coeled down with a cooling rateef 300eClh in a furnace to 1400, 1200, 1000, and 5000C, andthen finally quenched by air blow in order to investigate ef-fects of cooling temperature on the stabilization of dicalciurnsilicatc structure at room temperature. (refer to a coolingcondition in Table 2) XRD was performed on the sintered specimens to inves-tigate phase identification. Free CaO values were measuredfor determining sintering degree of dicalciurn silicate. Tocharacterize the fi-C2S, all the specimens were ground to theBlaine specific surtace are 3000 cm2fg. The phase stabilityef synthes{zed 6-CzS was studied using DTA and SEM ob-servation. Micro-conduction calorimeter measurement, 2DSi

MAS NMR analysis and SEM observation were also carriedout for studying hydrates cured at 200C with selid to liquidratio of O.5. Calcium and silicate ion concentrations of solu-tions filtered from paste slurries were also measured, Thepaste slurries were prepared and stirred on the magneticstirrer with WfC=5.

3, Results and discussion 3.1 Phase stability of dica[cium silicate Table 2 shows the effects of sintering temperature andcooling condition on the stability of 6-CzS. The forejgn ionsact at not only stabilizers but also mineralizers. Burnabilityof fi-C2S is dependent on the foreign ion. KzO graduallydecreases ACaO from 1.9% to O.7% as the temperature in-creases from 1200 to 14000C. Bz03 shows that the fLCaOabruptly decreases frem 12.7 to O.2% as the tempcrature in-creases from 1200 to 13500C. S03 represents venr lowfCaO(O.5-O.2%) at temperatures between 10000C and 14000C.

380

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Table1 . Compositions of Mixtures IJsed in the Study(mass%)

Specimens CaC03 Si02 H3B03 K2CO, CaS04-2H20

P-C2S(B) 77.5 22.0 O.7 ' -

P-C2S(IC) 74.7 22,3 h 3.0 -

P-C,S(S) 75.5 21.5 - - 3.0

:

za

/.

-s

s tzadets,1

-

th

B -C,S (K,O) B -C,S (SO,) B -C,S(B,O,)

Fig. 1 . SEM micrographs oi the fi-C2S stabilized by various foreign ions.

EXO

:Endo

400

Fig. 2.

6De ueo 1,OO{]

Temperaturec=}

Phasa transformation of C2S stubilized by foreign

1.200 1,4oe

ions (coolimg rate: 20Cfrnin).

As a result the SO:s is the best mincralizer. The foreign ionsalso stabilize the 6-CzS phase as the sintering temperatureincreases. At temperatures higher than 14000C, the d-valuesoi (122) of 6-C2S crystal are almost constant value of O.278nm regardless of stabilizers used.

When the sintered specimens were cooled down, changesin lattice parameters of fi-C2S occurred. The B-C2S stabilizedby K20 shows that d-value of (122) decreases as thequenching temperature decreases, while the d-value of 6-C2S stabilized by SO/s increases. The d-value oE I7-C2S stabi-lized by B203 shows the almost the same value regardless ofsintering temperature, at temperature higher than 1200"C. Conversion reaction from fi- to 7-C2S stabilized by K20and S03 is strongly dependent on the start temperature of

quenching: the phase transformation occurred much at lowstart temperature of quenching, ]ower than 12000C. Dustingphenomena which were caused by volume changes betweentwo dicalcium si]icate phases due to the conversion of S- toy-C2S were observed when the quenching temperature waslow, lower than 12000C, for the specimens stabilized by S03and K20. No dusting phenomena were observed for thespecimen stabilized by B203 regardless of quenching tern-perature. The B203 is a good stabilizer ior the fi-C2S at reomtemperature. The microtexture of fi-C2S stabilized by various foreignions arc shown in Fig. 1 , The microtexture of 6-C2S crystalwas influenced by foreign ions. On the surfaces of dicalciumsilicates stabi]ized with S03 and K20, there are stripes

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382 Phase Transformation and Hydration of Dicalcium Silicate Containing Stabilizers

Table2, Effects of Sintering Temperature on the Formation ef fi-C2S and Effects of Cooling Condition on the Phase Stability of fi-C2S (Sintering)Specimens

SinteringTemp.(OC) dof(122>*"Oo'iom)

f-CaO(O/,)

P-C7S.(K)S 1,2001,2501.3001,3SOI,4002.79402.793S2.79202,79422.78611.91.71.61.5O.7

P-c2s-(s) 1,2001,2501,3001,3501,4002.78232.78172.78252.78212.7805O,5O.2O.202O,2

P-C2S-(B) 1,2oo1,2501,3oe1,3501,4002.77792.77712.77982.78132.7811l2.76.52.3O.2O.2

(Cooling)Specimens

StartTemperatureof dof(122)Quenching(OC) (10'i"m)

Dustingphenornena**th

P-C2S-(K) 1400,l,2001,OOO500 2.78482.78472.78402.7gll xooo

3-C2S-(S) l,4001,2001,OOO500 2.78112.78152.78172.7827xooo

p-c,s-e) 1,4001,2001,OOO500 2.78132.78172,78112,7811xxxx

*K:K20, S;S03, BIB20s

"" d value ef B diealgfufn silicate

*** O i occurred, X : not occurred.

which seem to be larnella-like waves related to of }fi conver-sion, but the specimens stabilized by B203 was not observed.The grain sizes of fi-C2S stabilized by Bz03 and S03 aresmaller than that of K20. The crystal size is closely relatedwith the burnability as shown in Table Z. The low "CaOshows larger crystal size of the dicalcium silicate. The results of DTA of dicalcium silicates are shown inFig. 2. All specimens were heated to 15000C with heatingrate 10[C!min and then cool down with 20C!min. All speci-mens show exothermic peaks of phase transiormation offrom cr to high temperature forrn (cbi') , from low tempera-ture iorrn (eq.") , to fi-C2S around at 1400 and 7000C, respec-tively. In the specimen stabilized by B203 S- to y-C2S phasetransiormation around at 500"C was not occurred. But in the

specimens stabilized by S03 and K20, and the controlledspecimen the exotherrnic peak of fi- to y-C2S were detectedaround at 5000C. Especially the controlled at the specimenstabilized by SO:i show high exotherrnic peak meaning muchphase transformation of fi- to y-phase. The exothermicpeaks of the specimens were shown different shapes and oc-curred at slightly different temperatures because of differ-ent influences oi foreign ions to crystallization of dicalciumsilicate. The foreign ions incorporated into the dicalciumsilicate structure is locally distorted its lattice and causescrystal imperfections.i),2),6)7) The degree of crystal imper-fection by a foreign ion is strongly dependent on ionicradius, polarization ability related with substitution modeand binding force between metal ions and (SiOD.O2> B3'/

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AoruocobL"ties9Nte2

.c-.rv9di.-, otobuMinutes 10 20 30

- HeursTime

Fig.3. Heat of liberation rates for the 6-C2S's as a function of hydration time cured at different temperatures.

fiEaeE:atcsxb:281500

1000

soo

o110Time

(hr)(A)100 leoo

?v='l

:.;E,

:/

ISOO

1ooO

soo

a110Time

{hr)

(B)

1OO 1000

AegEws':ee'g:e:eL-:.":e"20DO

1500

1000

soo

o110Time

{hr)1OO1OOO

(C)

Fig. 4.B203, Soluble calcium and silicate ion concentrations in the solution filtered from the paste slurries of fi-C2S's.(B) fi-C2S stubilized by KzO, (C) fi-C2S stabilized by S03.

(A) S-C2S stabilizedi by

and S6' substitute for Si4', and K' for Ca2" in the formulaoi (Ca2 .M.) (Sii-,N,)04. The imperfection of crystal byforeign ion is closely related with thermodynamic equation,E= UL71S, where E is free energy for the transformation ofdicalciurm silicate, [J is internal energy and S is entropy.When a foreign ion is incorporated into a dicalcium silicate

structure, the entropy increases, and the free energydecreases. At temperature lower than the phase transiorma-tion temperature of rs- to y- of C2S, the increase of freeenergy is more significant than that of entropy, and the dis-tribution of foreign ion in a crystal causes the lattice energ}rto increase, resulting in unstable dicalcium silicate.i) From

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the results B/i' substitutes for Sid' easier than Si6', result-ing in stabilizing high temperature fi form of dicalcium silj-cate at room temperature. ,K- substitutes for Ca2 , differ-ent mode from those of B and S and shows different shape ofexothermic pcak, lo"rer exothermic peaks at lower tempera-tures, compared Lo B and S. 3.2 Hydration of fi-C2S stabilized by foreign ions fi-C2S is not reactive with water cornpared with other ce-ment rninerals, such as tricalcium silicatc, calciumaluminate etc. Fierens and TirlocaS'] reported that the typesof foreign ions greatly affected the degree of hydration of6-C2S. Figure 3 shows the heat ef liberation of fi-C2S speci-mens stabj]ized by various stabilizers. The specimens werecured at 20 and 400C for 36 h, and mcasured liberation heatrate as a function of hydration time, The specimens contain-ing S03 liberates high liberation heat rate immediately aftercontact oi the spceimen with water, compared to the otherspecimcns which are very similar each other, But the en-vironmental temperature docs not infiuence the very earlyliberation heat rate, After contact water the specimens liber-ated heat continuously, and show diflerent trends accordingto the stabilizers. The specimen containing SO/t liberatesheat little at an early hydration time and much until/ 36 h, butthe environmental temperature infiuences hydration rate;high temperature accelerates hydration resulting in ealymaximum peak from at 2-3h at 200C to 9 at 40MC. Thespccimen containing S03 also shows the hydration is retard-ed, especially at 200C, and hydrated centinuously and veryactively at a later hydration time. The specimens containingKuO and B203 represent similar hydration except that B203higher heat liberatjon rate than K20. The environmentaltemperature affects to accelerate hydration, but the effectsof temperature on the liberation heat rate are not so sig-nificant, compared to the S03. The results measured the concentrations of Ca2' and Si4-in the solution tiltered from the paste slurries are shown inFig. 4. Three specimens represent that the ca]cium ion con-centration is much higher than the silicate ion concentrationin the solution. The trends of soluble cation concentrationsare in agreement with the results of Trettin et al.9) that cal-cium ion concentration is higher than silicate ions, resultingin increasing CIS ratio in thc liquid phase with hydrationtime. In the specimens of BzO/s and K20 the distinguishedmaximum concentrations of silicatc are occurred within 20min. The specimen containing KzO shows higher concentra-tion of calcium and higher maximum concentration of sili-cate than the other specimens. This specimons also showssoluble K ion which increases concentration with time. TheK ion is considered to accelerated hydration of the fi-C2S atan early hydration time and forming high calcium and maxi-mum silicatc ion concentration. For the of specimcn stabi-lized by SO/s, the calcium ien concentration increases linear-ly and represents high silicate ion concentration at all overhydration time. This result means the hydration is occurredcontinously more actively than two other specimens.

BeU et al.iO) studied hydrates using ZUSi MAS NMR andreported that the total range of 2YSi shifts in silicates wasrelated to the degree of pelymerization o[ the Si04 tetra-hedra Q", where O" represcnts the Si04 tetrahedron whilethe superscript refers to the number of other tetrahedra towhich is linked by sharing of O and OII. It can be dividedinto separate ranges 'for monosilicated (QO; -66--74ppm), disilicate and chain end groups (Q'; -75--82ppm), chain middle groups ((}2; -85 to -89 ppm), chainbranching site (Q3; -95--100ppm) and three-dimen-sional frame work (Q`; -10S--115 pprn). 20Si MAS NMRspectra of hydrates of 6-C2S cured at room temperature for7 d are shown in Fig. 5. The hydrate of fi-CzS stabilized byB203 show Qi and OZ spectra at 7 d, The other hydrates of

t

QO L

r

S03

tspmvWww

QlK20

bu,.NwhNN

rr--7"-rr ---"-T----T

-60

-so -100

PPM

Fig, 5, 29Si MAS NMR spectra of hydrates of fi-C2S stabilized byforeign ions (The specimens were bydratad fer 7 d at room temper-ature),

fi-C2S's stabilized by S03 and' K20 do not show Q2. Theresults suggest that the hydration of 6-C2S stabilized byBz03 is more polyrnerized tban thosc of fi-C2S stabilized byK20 and S03, The rnicrestructures of 6-C2S hydrate are dependcnt onthe types of stabilizers, especially the shapes of C-S-H gel,(Fig. 6) The morphology of hydrate o'i fi-C2S stabilized by

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B203 and S03 in crumbled foils, while the morphology of hy-drate of fi-C2S stabilized by KzO is needle shape. The speci-men stabilized by S03 shows very short calcium si]icate hy-drate, compared with K20 and B203. SEM micrographsshow that the crystallinity of C-S-H for the fi-C2S's stabi-lized with Bz03 and K20 are higher than that of S03.

4. Conclusions 6-CzS was prepared at 14000C ior 3h in order to inves-tigate phase stability and the hydration reaction. The resultsobtained from the study are as follows (1) fi-C2S stabilized by B203 shows a very stable phaseregardless of the start temperature of quenching, (2) The phase stability of fi-CzS stabilized by KL,O andS03 is dependent on a quenching condition: fi-C2S is stabi-lized by K20 and S03 when the quenehing temperature isgreater than 12000C. (3) Heat of hydration of the 6-C2S also depends on thestabilizers: S03 delayed hydration at an early hydrationtime, but showed high liberation heat rate at a later hydra-tion time, compared to two other stabilizers. The B203 simi-lar to KzO except a couple of hours delayed hydration andhigh liberation heat. (4) 6-C2S stabilized by various foreign ions showsdifferent crystallinity and morphology of calcium silicate hy-drate: the specimens stabilized by B203 and S03 presentcrumbled foil and K20 presents neeclle shape. The crystal-linity of C-S-I{ prepared by S03 is lower than by B203 andK20,

References

1) Feng, X. and Long, S., Cem. Concr'. Res'., 16, 587-601