The change in reactivity of silicate anions during the hydration of calcium silicates and cement

CEMENT and CONCRETERESEARCH. V o l . 3, pp. 767-776, 1973. PergamonPress, Inc Printed in the United States.

THE CHANGE IN REACTIVITY OF SILICATE ANIONS DURING

THE HYDRATIONOF CALCIUM SILICATES AND CEMENT Ferenc D. Tam~s and M6ria F~bry Central Research and Design Institute for Silicate Industry Budapest, Hungary

(Refereed)

ABSTRACT The change in reactivity of the silicate anion complex of tricalcium silicate, B-dicalcium silicate and portland cement during hydration was studied by measuring the rate constant (~) of the silicomolybdic acid formation. The three anhydrous starting materials have identical k values indicating their identical monosilicate anion structure. During hydration the value of k decreases continuously; the rate of decrease of k is highest in-case of cement and lowest in case of B-dicalci~m silicate. The decrease of k can be attributed to the formation of bridging oxygen ions in The silicate complex, k tends to reach a final, limiting value; beyond this ~ can s t i l l be-decreased by special techniques as e.g. by regrinding and rehydrating the hardened samples.

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HO HpN~NCEBaT~CR 06pasoBaHZ~ MOCTOBMX KNc~opo~HNX NOHOB B CN~NKaTHOM KCMH~eKce. Be~N~NHa "K" cTpeMNTCH K HeKOTOp0My HOCTOHHHOMy SHaNeHN~; ~ a ~ B H e ~ e e CHN~eHze "K" MOEeT ~OCTNPaTBCH HaHpzMep ]OBTOpHMM HOMO[OM saTBep~eBmzX o6pasaoB c noc~e~y~me~ B¢opz~HOZ ZX rz~pacaaze~.

767

768

Vol. 3, No. 6 REACTIVITY, ANIONS, CALCIUM SILICATES, CEMENTHYDRATION Introduction When anhydrous tricalcium silicate, B-dicalcium silicate or portland cement

react with water, a combined hydrolysis-hydration reaction takes place, the stoichiometry of which can be described as 2Ca3SiO5 + 6H20 = Ca3Si207.3H20 + 3Ca(OH)2

[l]

in case of tricalcium silicate, and 2Ca2SiO4 + 4H20 = Ca3.3Si207.3"3.3H20 + O.TCa(OH)2 in case of B-dicalcium silicate. (1)

[2]

The product is very poorly crystalline, its

structure however resembles that of the natural mineral tobermorite; therefore i t is called "tobermorite gel" by Brunauer(1) and following him, by almost all researchers. compositions.

Tobermorite gel "is a series of hydrates of continuously varying The lowest CaO/SiO2 ratio observed by us for a stable tobermorite

gel was 1.39, the highest was 1.75. ''(1)

Although at f i r s t glance the reaction

product of Eq. l would indicate a d i s i l i c a t e , containing one bri#ging oxygen ion, the chemical formula of Ca2[SiO2(OH)2]2.CaO.H20 is assigned to tobermorite gel prepared of tricalcium silicate. This formula indicates a monosilicate structure (containing no bridging oxygen ions), having principally the same anion arrangement as the parent material.

By this formula however the observed great

variability of the CaO/SiO2 ratio in tobermorite gel cannot be explained. Although a vast literature has dealt with the hydration of calcium s i l i cates and portland cement, the possible changes in the anionic structure have very often been disregarded.

This was primarily due to the fact that the classic

methods of anionic structure determination (by releasing them by a stronger acid) are unsuitable for silicates unless special measures are taken, because the s i l i cic acids released are unstable and tend to instantaneous polycondensation; there. fore they cannot be isolated and identified by the usual methods as to reflect the anionic structure of the parent material. Recently some methods for anionic structure determination in acid soluble silicates have been described.

The method by Lentz(2) prevents the released

s i l i c i c acids from polycondensation by end-blocking their reactive groups by t r i methylsilylation. These end-blocked s i l i c i c acids can be isolated by gas chromatography, and by mass spectrography too. (3) The original Lentz method has been controlled and modified several times. (3'4'5) The Lentz method thus utilizes the principle of "Retention of Configuration." The rapid change of the released s i l i c i c acids can be hindered by using cold and dilute acids for dissolution. The released s i l i c i c acids can be isolated by paper chromatography{6'7) or can be identified by their reactivity with

Vol. 3, No. 6

769 REACTIVITY, ANIONS, CALCIUM SILICATES, CEMENTHYDRATION

complexing agents, usually molybdates.

In this case the s i l i c i c acids are trans-

formed into a silico-molybdate heteropoly-acid which has a characteristic yellow color and consequently its formation velocity can be easily followed by photometry, k8]'' but the use of other complexes, e.g. silicomolybdate-quinoline complexes have been reported too. (9) The methods have been used mainly to study the silicate anions of minerals of known structure (e.g. olivine, forsterite, hemimorphite, sodalite, natrolite, etc.) in order to examine the selectivities of the methods( l O ' l l ) but sodium silicate solutions, calcium silicates, their hydration products and related substances have been studied too. (9'13'14'15) Experimental Materials Synthetic tricalcium silicate and B-dicalcium silicate as well as a commercial cement were used as starting materials.

The synthetic substances were pre-

pared at the Chair of Silicate Chemistry, The University of Chemical Engineering, Veszpr~m, Hungary. Their purity was controlled by chemical analysis and no considerable contamination was found (with the exception of 0.87% of B203 in the B-dicalcium silicate). The CaO/SiO2 ratios were correct to the second decimal. The mineralogical identity was controlled by x-ray diffraction. The cement, L600 Brand was manufactured by the LEbatlan Cement Works. Its chemical composition, calculated mineralogical composition and strength data tested by ISO-RILEMCembureau Recommended Practice are contained in Table I. Sample preparation The reactivities of silicate anions of hydrated substances were determined on paste samples. These were prepared as follows: The powdered anhydrous s i l i cates and cement were ignited prior to test in order to decompose hydrates which might have been formed upon air moisture. Ignited samples were then stored in t i g h t l y stoppered bottles. Pastes were prepared with a 0.35 i n i t i a l water-to-cement ratio in a CO2 free glove box. The resulting slurries were de-aerated in a vacuum desiccator; after 15-20 minutes of de-aeration they were sucked into polythene tubes of lO mm ID and lO0 mm length. These paste-filled tubes were then tightly stoppered to prevent carbonation and the loss of water and f i n a l l y cured at room temperature. Method Reactivities of the silicate anions are expressed as rate constant k values. For the determination of k a modified Thilo-Wieker-Stade methodt~was

770

Vol. 3, No. 6 REACTIVITY, ANIONS, CALCIUMSILICATES, CEMENTHYDRATION Table I Properties of L600 Brand "L~batlan" Cement Loss on ignition

1.38%

SiO}

21.17

Al203 Fe203 TiO2

5.60 3.00 0.47

Tricalcium silicate

40.17%

Dicalcium silicate

30.40

Tricalcium aluminate Tetracalcium aluminate ferrite

9.77 9.12

CaO MgO

62.17 2.70

Gypsum Free lime

4.06 0.79

K20

0.36

Insoluble

0.32

Na20

0.26

SO3

2.39

Strength Data Age 6 bending 6 compressive

l 24.5 llO

3

7

40.I

50.6

226

337

28

90

days

73.1

77.7*

kgf/cm 2 kgf/cm 2

446

509*

* No strength data for the age of go days were available. to T600 cement of almost identical composition.

Data refer

used the description of which is given below. Dissolve the substance in d i l u t e , ice-cold hydrochloric acid (0.05N, 2° 0.5°C). The s i l i c a concentration must not exceed 2 mg Si/100 ml solution. Dissolution is complete in 2-3 minutes. Quickly (max. 90 sec) heat the solution to 25°C, add 2 ml 10% ammonium molybdate solution of the same temperature and after rapid s t i r r i n g transfer the solution into the cuvette of the photometer. Record transmission as a function of time at 400 nm wavelength (zero time: the addition of the molybdate solution).

During the measuring cycle the temperature

of the photometering chamber must be kept constant at 25°C. In our studies a Hungarian-made System Jurgny-Kov~cs "Extinctionmeter" was used; the transmission vs. time plot was recorded by an X-t plotter. The method is based on the following reaction: SiO~- + 12 H2MoO4 = (SiMOl2040)4- + 12H20

[3]

Only the SiO~- ions reactA with molybdic acid; all other silicate anions must be depolymerised to SiO;- before reacting with the molybdic acid. As this depolymerisation time depends on the structure of the s i l i c i c acid, by measuring the formation velocity of the yellow silicomolybdate heteropoly-acid the anionic structure of the parent material can be deduced.

Vol. 3, No. 6

771 REACTIVITY, ANIONS, CALCIUMSILICATES, CEMENTHYDRATION

The formation of the heteropolyacid being a first-order reaction the rate constant, ~ can be expressed as dc t~=k

.c

where ~ is concentration and ~ is time. k is expressed in I/min. As extinction (E) in dilute solutions is proportional to ~, the above equation can be written as

dE B~--= k . E Values of ~ were calculated from the E vs. ~ plot by using a least squares

method (see Appendix). Three to ten parallels were made, the standard deviation of k = + o.og. D

The standardization of the method was done with the aid of monomineralic standard samples kindly supplied by the Institute of Inorganic Chemistry of the German Academy of Sciences, Berlin-Adlershof, GDR. This standardization was necessary as our experimental circumstances differed from those used in the German laboratory. Table 2 shows the results. Table 2 Rate Constants of Standard Substances Substance

Anionic Structure

Rate constant, k (I/min) [8]

This study

y-Ca2SiO4

monosilicate

1.7

3.54

Ca2Na2Si207 KHSiO3

disilicate cyclo-tetrasilicate

0.90 0.67

0.86 0.63

[N(CH3)418Sis020

dicyclo-tetrasilicate

-

0.27

Ca4Na4Si6018

cyclo-hexasilicate

-

0.43

Results and Discussion As seen in Fig. l the three starting materials (tricalcium silicate, Bdicalcium silicate and cement) have almost identical k values (3.68, 3.49 and 3.44 respectively). By the results of standardization i t can be assumed that they are all monosilicates (k_monosilicate = 3.54). Reactivities of the silicate anions is decreasing gradually during hydration (Fig. I). The rate of this decrease is high in case of cement, medium in case of tricalcium silicate and very low in case of B-dicalcium silicate. A linear plot is obtained, i f not ~, but the half-life period of the reaction, ] - ~ is plotted against the logarithm of time (Fig. 2). This shows clearly that the hydration rate of calcium silicate in cement is affected by the

772

Vol. 3, No. 6

REACTIVITY, ANIONS, CALCIUMSILICATES, CEMENTHYDRATION K

(l/mi n)l

2-

~ ' - " " ' ~ " = - - - - r egr ound cement

'

'

stort 2

3

~

'

'

10

14

'

'

' s ' ' '

21 28

42

I

6 ?0 90 time(doys)

FIG. l The change of rate constant k as a function of curing time. T (sec) 50-

~

reground cement

10-

~=.ort 2

$

?

10 1~

The change o f the h a l f - l i f e

21 28

2 6?090 tim (days)

FIG, 2 period as a f u n c t i o n o f c u r i n g t i m e .

Vol. 3, No. 6

773 REACTIVITY, ANIONS, CALCIUM SILICATES, CEMENTHYDRATION

other cement phases too. As the decrease of reactivity of the silicate anions can be attributed to the increase of "bridging" oxygen ions being present in the anion, the essence of hydration can be considered as the disproportionation of oxygen ions: 20"

> 0° + 02+

[4]

where O" represents an oxygen ion attached to Si by one of its valencies and to Ca (or other cation) by the other; 02- represents an oxygen where both valences are neutralized by a cation; and 0° a bridging oxygen ion, i.e. coordinated to two Si ions. Among the starting cement minerals neither dicalcium silicate, nor t r i calcium silicate contain 0° ions (in other words: both are monomers): B-Ca2SiO4 contains four O- ions(16) while Ca3SiO5 contains four O- ions and an additional 02" ion too. (17) In the special case of calcium silicate hydration the equations by Brunauer[l,2] could be interpreted in such a way that the monosilicate anion group in the starting material is dimerised, one-eighth of the O" ions being present in the starting structure are degraded to 0°; and this is counteracted by the formation of one 02- ion which is bonded to the expelled Ca2+ ion, forming CaO [or, more correctly Ca(OH)2]. The process may continue further on, longer SiOSiOSiOSi chains, and by their linkage, rings, sheets, frameworks, etc. may develop.

As various anions with different degrees of polymerisation are pre-

sent simultaneously, the reactivity of the silicate anions of our samples will have an average value. The lowest value of ~ was 1.04; this is slightly higher than the respective value for disilicate.

This however, does not mean that the gO days old sample

of hydrated cement paste contains dimerised molecules in a predominating quantity. First, the reactivity is not a direct function of molecular length or weight; and further, even the samples subjected to longest hydration contain a considerable percentage of anhydrous calcium silicates. This is well reflected by their x-ray diffraction patterns showing only a slight decrease in intensity of the tricalcium silicate and B-dicalcium silicate reflections. An alternate proof has been made too: for this the cement sample hydrated and hardened for ten weeks (during this time its k value decreased from the i n i t i a l 3.44 to 1.22) was ground to cement fineness again, mixed with water and cured in the usual way. This sample hardened again with a simultaneous further decrease of (marked by the symbol [ ] in Fig. l) and reached finally a value of k = 0.87, i.e. identical with that of pure disilicate. And even in this reground-rehy-

774

Vol. 3, No. 6

REACTIVITY, ANIONS, CALCIUM SILICATES, CEMENTHYDRATION drated cement there is a certain, rather high amount of the anhydrous monomer, mainly ~dicalcium silicate as proved by x-ray diffraction, indicating that silicate groups of longer chain length or otherwise containing 0° ions are present too.

However, as the degree of polymerisation is low as contrasted to

organic plastics; i t should therefore rather be called "oligomerisation"!18)"" This oligomerisation is reflected by the continuous decrease of specific surface of tobermorite gel during agingtlgj'" and could be followed by measuring the change of the CaO/SiO2 ratio of tobermorite gel during hardening, obviously after the selective dissolution of the free lime formed. This will show a continuous decrease with time.

Steinherz (20) used the inverse of this method,

measuring the percentage of the free i.e. acetoacetic ester soluble lime during hardening where a continuous increase was l i k e l y .

However, all conclusions

based on selective dissolution methods ought to be evaluated with utmost care as dissolution is affected by particle size to a high extent; usually a part of bound lime is also extracted from the calcium silicate hydrate.

This is the

probable cause why an undulating free lime vs. hardening time plot was obtained by Steinherz, ~20j'" although the general trend of his curves is convincingly increasing. Conclusions I.

The reactivity of silicate

anions formed during the hydration of t r i -

calcium silicate, B-dicalcium silicate and portland cement is continuously decreasing with time. 2.

The rate of this continuous decrease is lowest in case of 6-dicalcium

silicate, medium in case of tricalcium silicate and highest in case of portland cement. 3.

The decrease in reactivity can be attributed to the formation of bridg-

ing oxygen ions in the silicate anion complex, or, in other words, the polymerisation of the SiO~- monomers to units of higher molecular weight (chains, rings, sheets, etc.). However, as this polymerisation is low as compared with that of organic plastics, the term "oligomerisation" is recommended 4. The determination of reactivity gives only an average measure of oligomerisation; in fact, an equilibrium of silicate anions of different compositions and structures is hypothesized. Acknowledgement Sincere thanks are due to Dr. W. Wieker, of the Institute o f Inorganic Chemi s t r y of the German Academy of Sciences, Berlin, GDR, for supplying us with monomineralic samples for standardization. His help is gratefully acknowledged.

Vol. 3, No. 6

775 REACTIVITY, ANIONS, CALCIUM SILICATES, CEMENTHYDRATION

Appendix Calculation of k by the method of least squares The transmission (T) vs. time (t) plot is recorded by the plotter. tinction values (E) are calculated by E:Ig

Ex-

lO0 T

As Lambert's law is valid for solutions of such low concentrations, i t can be written

AEi At i

= k

. Ei

+ a

(Ei is the extinction at t i time; a is a constant); i.e. a linear equation is obtained.

Let us introduce the following abbreviation: AEi/At i = Yi and select

n pairs of corresponding Yi " Ei values. Calculate ~by the following two equations to ensure a minimum sum of squares of differences between measured and calculated data:

n i=l

n Yi - n'a - k ~ i=l

Ei = 0

and n Yi " i=l

n Ei- a ~ i=l

n Ei- k F i=I

2 Ei

=0

I.

References S. Brunauer, American Scientist 50, 210 (1962).

2. 3.

C. W. Lentz, Inorg. Chem. 3, 574 (1964). F. F. H. Wu, J. G~tz, W. D. Jamieson and C. R. Masson, J. Chromatography

4.

48, 515 (lg70). J. G6tz and C. R. Masson, Journ. Chem. Soc., Sect. A, 2683 (1970).

5. 6. 7. 8. 9.

J. W. W. E. H.

GBtz and C. R. Masson, Journ. Chem. Soc., Sect. A, 686 (1971). Wieker and D. Hoebbel, Z. anorg, allg. Chem. 366, 139 (1969). Wieker, Epit~anyag 24, 188 (1972). Thilo, W. Wieker and H. Stade, Z. anorg, allg. Chem. 340, 261 (1965). Funk, Proc. 5th Symp. Chemistry of Cement, Tokyo, 2, 128 (1969).

lO. II. 12. 13. 14.

E. W. H. C. K.

Thilo, Silikattechnik 18, 171 (1967). Wieker, Silikattechnik 19, 240 (1968). Stade and W. Wieker, Z. anorg, allg. Chem. 384, 53 (1971). W. Lentz, Highway Res. Board, Spec. Report No. 90, p. 269 (1966). Komatsu, I. Tohyama, A. Kawahara and T. Nakamura, Kogyo Kagaku Zasshi

74, 160 (1971).

776

Vol. 3, No. 6

REACTIVITY, ANIONS, CALCIUMSILICATES, CEMENTHYDRATION

15.

W. Wieker and H. Stade, Proc. Int. Sjnnp. on Autoclaved Calcium Silicate Products, London, p. 125 (1967).

16.

J. Cruishank, Acta Cryst. ]]7, 685 (1965).

]7. 18.

J. W. Jeffery, Acta Cryst. 5, 26 (1952). F. TamAs, Proc. Symp. on Science and Research in Silicate Chemistry, Brno,

19.

p. 70 (1972). M. Collepardi, L. Massidda and G. Usai, i l cemento 68, 3 (1971).

20.

A. R. Steinherz, Revue Mat~riaux, No. 652-653, 36 (1971).