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EFFECT OF CALCIUM CHLORIDE AND CITRIC ACID ON THE HYDRATION OF C

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EFFECT OF CALCIUM CHLORIDE AND CITRIC ACID ON THE HYDRATION OF C

3

S AND

~C2S

PASTES

By

Department of Building Materials, Technical University, Budapest (Received May 30, 1977)

1. Introduction

Cement hydration can be influenced by admixtures. According to VAVRIN [1], accelerating agents are cither acids, or salts of strong bases, or alkalic salts of ·weak acids such as Na, K and Ca chlorides, sulphates and nitrates, carbonates, aluminates, K and Na silicates. The most effective accelerator is CaCI2 • superimposing its accelerating effect to that of steam curing at 608C.

As early as by the late 1800's it has been stated that from the aspect of high early strength, 1

%

pure calcium chloride solution was the most efficient [3].

The opinions on the accelerator effect are still not uniform. Catalysation effect of CaClz on C3S and lJCzS hydration 'vithout modifying the final product

~omposition has been demonstrated by KURCZYK and SCHWIETE [4] for the case of natural hardening and hy BAL . .\ZS and TAl\L.\s [2,5] for steam curing.

Others stated the CaCl z to be adsorbed on the C3S surface reducing the alkalin- ity of the liquid phasc, accelerating in turn the C3S hydrolysis. Also the C/S ratio in the CSH has been stated [1] to change upon accelerator admixture.

Setting retarders include lignin sulphonates, sugar, methyl cellulose, borax, tartaric acid, sodium phosphate, silico-fluorids, gluconic acid, citric acid and certain accelerators in high concentration. The citric acid retarder under Hungarian patent produces the 6-h retardation generally required in eoncrete technology in a dosage as low as 0.1

%.

TAl\L.\s [6] generalized earlier statements on setting control admixtures. Zinc oxide tests by LIEBER [7]

showed the early strength to be significantly reduced but the final strength to be increased by about 20%. WALZ and MATHIEU made similar observations [8]. Texture examinations by RICHARTZ [9] showed admixture of 1

%

zinc

oxide to produce 10 to 30 mm silicate hydrate whiskers in the C3S paste.

RICHARTZ and LOCHER [10] came to the final conclusion that setting accelera- tion brings about shorter, setting retardation longer whiskers, these latter being preferable from the aspect of final strength.

These precedences induced us to test the problem little investigated to our knowledge, how strength of CaS and {3C2S pastes of different grinding fine- nesses depends on the CaCl2 and citric acid dosage.

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30 BAL.izs- BOROS

2. Experimental

Clinker minerals C3S and iJC2S grown by Dr. M. Kov_.i'.cs at the Veszprem University of Chemical Engineering have been ground to different finenesses and made to cubes vl'ith 20 mm sides for cube strength tests and to 10 by 10 by 50 mm prisms for prism strength tests with 30% of mixing water, varying CaCl2 and citric acid admixture dosages.

Paste specimens were cast in steel moulds, stripped at 24 h to be stored at 100% r.h. Hydration 'water content has been determined by derivatography and also paste compactness development with time has been examined.

3. Test results and conclusions

28-day euhe strength as a function of CaCl2 dosage and grinding fineness of silicate minerals is seen in Fig. I. Cube strengths of C3S pastes 'were invariably higher than those of flC2S pastes, hut while strength vs. grinding fineness dif-

1200

1000

.:::

E

800

Cl.

-"

<- .c: 0,

c

e

600 Vi

QJ ..0 ::I U

400

200

780

4770

I

/~3400 \~

i

i

"---.----+!~50 ~'

Q 2

CaCl" %

Fig. 1. Effect of CaCl2 on cube strength of C3S and {3C2S pastes

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HYDRATIOS 31

ferences were nearly constant, for higher CaCl z dosages, strength of C3S pastes grew abruptly from 0 to 1

%,

and moderately from 1 to 2

%,

while for pCzS pastes it grew about linearily , .. ith CaCl z dosage.

Comparison of kinetic curves of hoth silicate minerals shows nearly constant cuhe strcngth differences at 28 to 90 days of agc, independent of spe- cific surface and CaCl z dosage, "while for chloride-admixed /3C zS pastes, cube strength grew t,,"o or cvcn three times fastcr than for C3S pastes and increased with CaCl z content. This phenomenon is attributed to the differences partly hetween the initial structural compactnesses of both silicate minerals and partly hetween the hydration product morphologies. Hydration and hardening velocities of pCzS are known to he much lower than those of C3S - especially for low grinding finenesses - to achieye the 90-day strength of C3S only at ahout 360 days of age. Namely, compact texture of /3C zS prevents hydration from starting carlier than at 28 days. From this time its cube strength grows faster than docs that of C3S (with greater corresponding differences), still

0.90

0.85

III ~ 080

~ E"

80.75

1n

..

cf

o 0.1

Citric acid., %

~~

o

+---/----+477D

OJ 02

Citric acid, Ofo

Fig. 2. Effect of citric acid on cube strength of C3S and (JC2S pastes a) at 28 days; b) at 360 days

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32 BAL.AzS-BOROS

O, b,

E sol E

50

t

..f2

Cl. ..f2

-'" Cl.

"'"

.c: 40

:g,

40

0, c: c:

e

~

U; 1ii

.!!! ~

·Vi c: 30 c: 30

.8 , 2 Ol I

0>

~ c: ~

~ 0.

(f) 20 (f) 20

10

\ L - - - . - - - - + ' , 2050

o

1

CaCl" %

0.1 0.2 Citric acid I %

o

Fig. 3. 28-day splitting-tensile strength vs. clinker mineral grinding fineness and a) CaCI2 ;

b) citric acid A

I

0.95

1

i

0.90

Vl I

f85 1

8 0.80 2 Vl 0.. o

0.75

0.70-

1'#)--.;---74770

3400 c;

(5'\

c;

~---.---_Y2050

x' o

1

CaClz, % 2

Fig. 4. 28-day compactnesses of CaCl2-admixed CaS and {JC2S pastes

(5)

100~

] 800 I

sz

.c;'"

c;, c

~ 600 tl

QJ .0 ::J U

200 i

o

DJ

Citric acid %

HYDRATIOX

nC2S - 4770

1000

] 800 -" 0-

.c;'

g>

600 tl ~

33 bJ

0.1

Cjt~ic acid,%

Fig. 5. Compactnesses of citric acid-admixed CJS and j3C2S pastes at a) 28 days: b) 360 days

enhanced by the strength-reducing effect of crystalline portlandite released by C3S hydration. Portlandite (or better Ca(OH)3) released according to the reac- tion equation of IJC2S hydration is not crystalline. It cannot be demonstrated by thermal or X-ray methods, but - according to the recent theory by

LOCH ER and RICHARTZ [11] supported by our test results - it fills out the space between tobermorite whiskers of pseudohexagonal position and adds thereby to strength.

28-day (a) and 360-day (b) cube strength values of citric acid-admixed silicate pastes are seen in Fig. 2. Apparently, 28-day strength increased "with that of grinding fineness and strength decrease upon citric acid admixture was much higher for C3S pastes than for pC2S pastes with the same characteristics.

At 360 days, these differences are seen in Fig. 2b to be about equalized. The cube strength of C3S pastes of high initial specific surface even hardly decreased for higher citric acid dosages as against the important losses of pC2S pastes.

Comparison of the corresponding values in kinetic curves shows the strength loss due to higher citric acid dosage to increase "with decreasing grind- ing fineness of C3S and inversely "with increasing grinding fineness of pC2S.

3

(6)

34 BAL4ZS - BORDS

a) b)

300

2

300

~

.8 Cl 250 28 days

E

~ 250

~ "0

g C Cl

0

"0

~ QJ 200

:5 200 -2

;£! 0 tJl

13 .0

u Cl

E E

g 150 g .150

~ e:!

:..~ ;f!.

§ 100 c

100

0

"2

"0 :;:; "0

e

>. >.

I :r:

/ /

50

/3l.0~i~

1/ /

h

<{'

2050

0 2 0

CcCI" % CcCI" °io

Fig. 6. 28-day hydration percentages of CaCl2-admixed C3S and pC2S pastes computed from a) constitutional water content: b) absorption and layer water content

28-day splitting-tensile strength ,-alues of both silicate minerals have been plotted in Fig. 3 as a function of increasing grinding finenesses, of (a) CaClz and (b) citric acid dosages. With increasing CaCl z the splitting-tensile strength of pCzS pastes invariahly grew hut that of C3S pastes only for high grinding finenesses. Fig. 3b points out that strength loss due to citric acid is much higher in C3S than in j3C zS pastes, especially for high grinding finenesses.

Paste compactnesses at 28 days are seen in Fig. 4 for CaCl2 admixture and in Fig. 5 for citric acid admixture, diagrams similar to those of cuhe strength.

Hydration of silicate minerals 'with admixtures is hest compared by hydration percentages. 28-day hydration percentages computed from constitu- tional water content (a) and adsorption and layer water content (b) are seen in Fig. 6 as a function of CaC12 dosage and silicate mineral grinding fineness.

Hydration rates of both silicate minerals are seen to develop similarly (Fig.

6a). Neither curve slopes differ significantly. Hence hydration mechanisms seem to be identical. Also the hydration percentage of C3S pastes is higher than

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E o .l::

250

~ 100

<f?

c o

~ "R 5 ::r:

28 days

o

0.1 0.2 050 Citric acid, %

HYDRATION

300

J!! '-

~ 250

}

-g o 200

"C

-e

QJ

"C 5l o E o .l::

c o :.;::;

"R

e

50 ::r:

35

4770

/3400 ,.\<:0

/ c,$'

~----~---n.~~ ~,

o 0.1 0.2

Citric acid, %

Fig. 7. Hydration -percentages of citric-acid-admixed C3S and (JCzS pastes computed from a) constitutional water content; b) adsorption and interstitial water content

that of other-wise similar (JC2S pastes achieving the former percentage as late as at 360 days. On the other hand, hydration percentages computed from adsorption and layer water (Fig. 6b) exhibit extreme differences between cor- responding values ("'--' 120%rel)' while (3C2S paste hydration percentages com- puted from adsorption and layer water lag far behind the corresponding values of otherwise similar C3S pastes. This phenomenon is attributed to the initial texture compactness differences between the two silicate minerals. C3S is of looser texture, and can adsorb more water on surface and between layers than the much compacter (JC2S.

The same is true of silicate pastes admixed with citric acid (Fig. 7) excepted that increasing the citric acid percentage reduces the hydration per- centage.

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35 BALAzs-BOROS

Summary

Hardening and hydration kinetics of silicate minerals C3S and {3CzS in dependence of grinding fineness and setting control admixtures (CaCl z and citric acid) were examined and seen the cube strength of both to increase upon CaClz admixture, the increase being higher for C3S than for {3CzS, while for increasing grinding finenesses the latter exhibited twice and even three times the strength increase of the former. More of citric acid admixture reduced the cube strength; the higher the grinding fineness of C3S and the lower of {3C zS, the greater the loss.

The tendency of splitting-tensile strength was similar to that of cube strength.

The differences between hardening kinetics of both silicate minerals are attrihuted partly to different initial compactnesses, and partly to different hydration product morphologies.

Irrespective of the setting control admixture, the similarity of hydration percentages of both silicate mineral pastes suggests the identity of hydration mechanisms.

Increased hydration percentage of C3S paste computed from adsorption and interstitial water content is due to the looser texture, enabling it to adsorb more surface and layer water, irrespective of the grinding fineness, than the much compacter {3C zS paste.

References

1. VAvRrN. F.: Effect of Chemical Additions on Hydration Processes and Hardening of Ce- ment. A principal paper. The VIth International Congress on the Chemistry of Cement.

Moscow, Sept. 1974. II-6.

2. B.u.us, Gy.: Concrete Curing by Combined Steam Curing and Calcium Chloride Admix- ture. * Candidate's Thesis. 1963.

3. G .. L>l.L A.: Influence of Cement Composition on Setting and Hardening Times. * :\1}IEEK 32. 1898, pp. 356-358.

4. KURCZYK. H. G.-SCHWIETE, H. E.: Elektronmikroskopische, thermochemische Unter- suchungen iiber die Hydratation der Kalziumsilikate 3 CaO . SiO z und 2 CaO . SiO z und Einflu13 von Kalziumchlorid und Gips auf die Hydratationsvorgiinge. Tonindustrie Zeitung nnd Keramische Rundschau 1960. Heft 24. pp. 585-598

5. BALAzs, Gy.-TAlIL>l.S, F.: Investigations into ?Iechanism of Calcium Chloride Effect in Case of Natural and Steam Curing Conditions. (RILEM Conf. Moscow, 1964)

6. TAlIIc>l.S, F.: Research in the Field of Cement Hardening Chemistry. * SZIKKTI Tudoma- nyos Kozlemenvek. Vo!. 17. 1958

7. LrEl3ER, W.: Einihi13 von Zinkoxyd auf das Erstarren und Erhiirten von Portlandzementen.

Zement-Kalk-Gips, H. 3., 26. 1967. pp. 91-95

8. WALZ, K.-MATHIEU. H.: EinfluB der Zusatzmenge von Betonverfliissigern anf die Festig-

keitsent",-jckl~ng. Beton, H. 9, 11. 1961. pp. 619-624

9. RrcHARTz, W.: Uber die Gefiige- und Festigkeitsentwicklung des Zemelltsteins. Betontech- nische Berichte. Beton, 5-69. pp. 203-205.

10. RrcHARTz. W.-LOCHER, F. W.: Ein Beitrag zur ~Iorphologie und Wasserbindung von Kalziumsilikathydraten und zum Gefiige des Zementsteins. Zement-Kalk.Gips, H. 9.

18. 1965. pp. 449 -459.

11. LOCHER, F.-RrcHARTz, W: Study of the Hydration Mechanism of Cement. The VI.

International Congress on the Chemistry of Cement. Moscow, 1974

Ass. Prof. Dr. Gyorgy BALJZS, head of department } H-lt:;_".l B d

u / U apest

Dr. Margit BOROS (Mrs)

* in Hungarian

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