Low pressure steam hydration of tricalcium silicate

CEMENT and CONCRETE RESEARCH. Vol. 4, pp. 279-288, 1974. Pergamon Press, Inc Printed in the United States.

LOW PRESSURE STEAM HYDRATION OF TRICALCIUM SILICATE

Vito Alunno-Rossetti, Gerardo Chiocchio and Mario Collepardi Ist i tuto di Chimica Applicata ed Industriale

Cattedra di Tecnologia dei Materiali e Chimica Applicata-Facolt~ di Ingegneria

University of Rome

(Refereed) (Received Aug. l , 1973; in final form Jan. 3, 1973)

ABSTRACT Pastes of C3S (w/c ratio = 0.5) were steam cured at 25, 40, 60

and 90°C for l hour to 30 days. The results obtained have shown

that, as the curing temperature rises, the induction period is

shortened and the in i t ia l rate of hydration of C3S is increased;

at longer curings, on the other hand, such hydration rate is con-

siderably lowered. In order to explain the influence of temper-

ature on the hydration reaction a new hypothesis has been proposed,

which takes into account the C/S molar ratio as well as the sur-

face properties of the hydrated sil icate.

Des p~tes de C3S (rapport eau/ciment : 0.5) ont 6t6 hydrat6es

25, 40, 60 et 90°C pour temps differents, ~ part ir d'une heure

jusqu'A 30 jours. Les r6sultats obtenus ont montr6 que l'aug-

mentation de la temperature d'hydratation a l ' e f fe t d'abreger la

p~riode d'induction et d'augmenter la vitesse in i t ia le d'hydratation

du C3S; apr~s un long temps d'hydratation, au contraire, la vitesse

d'hydratation est consid~rablement reduite. Dans le but d'expliquer

l' influence de la temperature sur la r~action d'hydratation on a

propos~ une nouvelle hypoth~se donnant une importance particuli~re

au rapport molaire C/S et ~ les propriet~s superficielles du s i l i -

cate hydrate. 279

280 Vol. 4, No. 2 TRICALCIUM SILICATE, STEAM CURING, LOW PRESSURE

Introduction

The low pressure steam hydration of Portland cement results in an increased

rate of the hardening process which therefore makes possible the large scale

production of cast components in a relatively short time. However, at long

enough steam curing times, the strength of the cured concrete is considerably

lower than that of the material cured at room temperature.

A similar trend has been noticed also for the hydration rate (I-4) and for

the development of other properties of the cement paste, such as heat of hydrat-

ion, non-evaporable water, contraction, elast ic i ty, t5j" " However, there is no

general agreement of the experimental results: Gebauer and Odler, t4)" " for example,

do not find any crossing of the curves degree of hydration versus time for di f-

ferent temperatures, at least up to 28 days of hydration.

Some authors have studied the hydration of C3S and C2S at different tempera-

tures, in order to explain the effect of temperature on the curve degree of hy-

dration versus time and on the development of the Droperties of the cement paste.

Kondo and Ueda, (6) for example, claim that, when C3S and C2S are hydrated above

70-80°C, the products obtained are richer in lime and possess a lower specific

surface area; these modifications are responsible, according to those authors for

the reduction of the strength of the paste. The data reported by Taplin t7)" " indi-

cate that crossing of the curves of degree of hydration vs. time does exist for

the temperatures of 5 and 42°C, although this fact is not expl ic i t ly pointed out.

Kantro, Brunauer and Weise (8) have studied the hydration of C3S at 5 and

50°C; examining the results obtained, Copeland and Kantro (9) came to the conclus-

ion that at early ages the rate of hydration increases as the temperature rises,

but after l day of curing the percentage of hydration is independent of tempera-

ture over the entire range examined. The same data have also been interpreted

in a different way: as the temperature increases, the degree of hydration of C3S

increases at early curings and decreases at later ones. (lO) Skalny and Odler(ll)

have found that at long curings the degree of hydration of C3S decreases as the

temperature is raised from 25 to lO0°C. Van Bemst (12) noticed a small inverse

temperature dependence, whereas Schwiete and Knoblauch (13) indicated a small DOS-

i t ive temperature dependence.

In conclusion i t can be said that the various authors do not agree either

on the experimental results or on their interpretation; this disagreement is

certainly due also to the different techniques used to evaluate the degree of

hydration of C3S.

Furthermore, in the l i terature there are no precise data concerning the

stoichiometry of the hydration reaction and the type of products formed at

Vol. 4, No. 2 281 TRICALCIUM SILICATE, STEAM CURING, LOW PRESSURE

temperatures between 25 and IO0°C. In fact, while the majority of the research-

ers seem to think that, even at higher temperatures, the hydration product is

the C-S-H gel only, Buckle and Taylor ~14]" " report the formation of tricalcium

si l icate hydrate also.

The purpose of the present paper is to examine the influence of the temper-

ature, in the range of 25 to 90°C, on the steam hydration of C3S as well as on

the chemical and physical features (composition, surface properties, morphology)

of the hydrated product.

Experimental Procedure

The C3S prepared by repeated f ir ings at 1600°C of CaCO 3 and SiO 2 (free

CaO = 0.05%) was ground until i t had a specific surface area, according to the

Blaine method, of about 2000 cm2/g. Such relatively low surface area was chosen

in order to enhance the differences in the degree of hydration brought about by

the temperature variations. Tricalcium sil icate was mixed with water (water/

solid ratio = 0.5) and the paste was kept in polytetrafluoroethylene containers

for isothermal hydration at 25, 40, 60 and 90°C + O.l°C. At the end of the

preset time, the sample was cooled at room temperature, the hydration was

blocked with ethyl alcohol and the percentagez ~°f anhydrous C3S of the paste was

determined by quantitative x-ray analysis, ~15j while the amount of free calcium

hydroxide was obtained by a solvent extraction method. (16) The C/S ratio of the

hydrated si l icate was determined. The nature of the hydrated products was deter-

mined by means of x-ray diffraction, thermogravimetric and differential thermal

analyses, performed according to a previously described procedure. ~17)" "

Some samples were examined with the scanning electron microscope (SEM) and

for adsorption of nitrogen at -196°C; from the adsorption isotherm the specific

surface area, the pore size distribution and the cumulative specific volume of

the pores were obtained. (18)

Examination of Results

Products obtained and rate of hydration.

In the time and temperature ranges chosen for the present work the phases

present in the hydrated product are, besides C3S, calcium hydroxide and the

hydrated calcium si l icate, C-S-H. The crystalline hydrated silicates are absent,

in particular C3SHI. 5, which according to Buckle and Taylor is present in the

C3S pastes hydrated at temperatures higher than 70°C.

Fig. l shows the percentage of C3S hydrated in 0 to 8 hours at 25 to 90°C.

I t is evident that the induction period becomes shorter and the rate of the re-

action longer with increasing temperatures. At 25, 40, 65 and 90°C the induction


3 0

z o

X 20 o >- "r

w

z ~ 10

w

ss~ s

s

/ i /

l / . " /

I G i 0 25

VS4 --/ , .... - _ ; " 2 4 6 8

TIME (houri)

FIG. l

Percent hydration of C3S, 0 to 8 hours.

5~

0,0

)</o f 100 101 102 103

TIME (h0ur~)

FIG. 2

periods are about l hour, 30 min.,

lO min. and 5 min. respectively. At

the end of this time lag the average

reaction rate in the following two

hours varies from 1.8%/hour at 25°C

to 13%/hour at 90°C. The activation

energy at the end of the induction

period is 4.7 kcal/mole and decreases

steadily as the degree of hydration

increases: a value of 3.5 kcal/mole

is obtained when the amount of C3S

hydrated is I0% and 2.9 kcal/mole

when the percent hydration reaches

20%.

At the end of the acceleration

period the reaction rate decreases

as the temperature increases (Fig.

2). After l day of hydration, for

example, the average reaction rate

in the following 12 hours is about

O.58.10"l%/hour at 90°C, but becomes

3.6.10-1%/hour at approximately 25°C.

Accordingly, i t can be stated that

higher temperatures increase the

in i t ia l rate of hydration, vJhile

longer curing decrease the reaction

rate.

I t is evident in Figs. l and 2

that, dif ferently from what has been

observed by other researchers, t8'9)''

the crossing of the curves degree of

hydration vs. time at the various Percent hydration of C3S, 0 to I000 hours, temperatures is very apparent, and

the degree of hydration is relatively small at any temperature. Probably both

the above mentioned phenomena have to be related to the relatively coarse size

of the C3S used in this work. In fact, the finer the material, the higher the

hydration rate and the smaller the temperature influence; i f , for example, all

the particles of C3S were of colloidal size, the hydration would be so fast at


2J 0

,~ 2.2 r~ I'v

~ 2c

~ 1.~

~> 90 °C

- o \ o ~ o ~ 65 " \ ~ ~ 40 ,'

\ ~ ~ o 25 .'

",, ~ . , ~ ~

{3 ~, D 0

I I I o 20 3'o ¢o

PERCENTAGE OF" HYDRATION

FIG. 3

Lime si l ica molar ratio vs. percentage of hydration

60

every temperature, that differences in the degree of hydration would be scarce-

ly perceptible.

Composition of the hydrated calcium si l icate.

In Fig. 3 are reported the C/S molar ratios vs. time of hydration. I t can

be observed that the higher the temperature, the faster the reduction of the C/S

rat io. After 4 hours of reaction, the C/S value is 2.4 at 25°C, 2.2 at 40°C,

1.9 at 60°C and 1.9 at 90°C. At the long curings the C/S ratio is about 1.9,

independently of the reaction temperature. On the other hand, Kantro et al. (8)

and Copeland and Kantro, (9) reported that the C/S ratio (of about 1.5) in the

hydrated si l icate at the end of the hydration reaction does not vary appreciably

with the temperature in the range of 5 to 50°C.

Pore structure and morphology.

In Table l are reported, as a function of the temperature, the specific

surface area and the specific volume of the pores with radius smaller than 250A

for l gram of hydrated C3S after l day. The data show that the values of the

surface area and of the pore volume decrease as the temperature of hydration in-

creases. These results agree with those quoted by other authors, tl9)" " who have

found that, in temperature range 23-75°C, the aging of the "tobermorite gel,"

estimated through the reduction of the specific surface area, takes place more

rapidly as the temperature is increased.

Table l

Specific surfaces and pore volumes of hydrated C3S at l day

T°C 25 40 65 90

S(m2/g) 45 32 25 23

VIcm3/~) 0.091 0.094 0.075 0.066

284 VoI. 4, No. 2 TRICALClUM SILICATE, STEAM CURING, LOW PRESSURE

2 0 ( ] -

s0 II 25 °C

,~, , , . . . . . . . . 90 "

~ ,

L J - - L ~L . I 0 50 100 150 200 250

PORE RAOIUS (A)

FIG. 4

Pore s i z e d i s t r i b u t i o n of hydra ted p roduc t s

The pore size d is t r ibu t ions

(Fig. 4) c lear ly show that the

reduction of the porosity and

surface area is associated with

a remarkable drop of the number

of the pores w i th rad ius between

20 and 40A not compensated by a

s l igh t increase in the number of

pores with radius larger than

IOO

The above mentioned data

have been obtained from adsorpt-

ion isotherms determined with

N 2 as adsorbate. I t is known

that d i f fe ren t opinions exist

concerning the gas to be used in order to evaluate the "true" porous structure

of cement and C3S pastes; some authors claim that water rather than nitrogen

should be chosen as adsorbate. However, in the present case, the values of por-

os i ty and speci f ic surface area calculated from water adsorption isotherms, and

those obtained with nitrogen as adsorbate, although numerically d i f fe ren t , show

simi lar variat ions as the temperature in changed. Skalny and Odler, i l l)" " in fact ,

have found that both the surface area and the porosity of the steam hydrated C3S,

determined by water adsorption, decrease as the temperature is raised.

These data do not confirm, therefore, the conclusions drawn by Verbeck and

Helmuth (I0) concerning the "physical" properties of the products hydrated at d i f -

ferent temperatures. According to these authors the surface and porous propert-

ies of the hydrated products, evalued by means of calor imetr ic and water adsorp-

t ion measurements, do not vary at the d i f fe ren t hydration temperatures.

The photomicrographs shown in Fig. 5 reveal the morphological and micro-

structural features of the pastes hydrated for 24 hours at the temperatures of

25 and 90°C; i t can be noticed that the sizes and the arrangement of the hydrated

part ic les are substant ia l ly identical in the pastes hydrated at 25 and 90°C. The

C3S granules appear coated with f ibers about 1 ~m long and 0.I ~m thick. The

f ibres that s t ick out rad ia l l y from a C3S granule come in contact with each other

or can even jo in with those protruding from an adjacent par t ic le .

Similar observations have been made by Lawrence and Young ~20)" " on C3S pastes

hydrated at room temperature. They have also been able to demonstrate, with the

help of an exceptional photomicrograph showing a fractured anhydrous s i l i ca te


a FIG. 5 b

Micrographs (X 3,000) of C3S paste hydrated for 1 day at 25°C (a) and 90°C (b).

grain, that between the inner anhydrous C3S and the outer acicular hydration

product, an apparently denser "internal" product can be detected. Since the

morphology of the external acicular products examined in the present work does

not seem to vary appreciably with the temperature of hydration, i t is l ikely

that the differences in the porous structure, revealed by means of the adsorpt-

ion measurements, could be related to the microstructure of the "internal" pro-

ducts.

Discussion

During the f i r s t hours of hydration an increase in temperature causes a

corresponding increase in the rate of hydration of C3S and at the same time a

reduction of the induction period (Fig. l) and a rise in the reaction rate dur-

ing the acceleration period (Fig. 2). Moreover, the higher temperature favours,

in the in i t ia l stage, the transformation of a lime-rich product into another

having a lower C/S ratio.

Similar effects on the induction period, the reaction rate and the C/S

ratio are produced by the addition of electrolytes. ~21'22j'' The accelerating

effect of temperature on the rate of hydration of C3S can probably be ascribed

to the more rapid formation of the "nuclei" of the hydrated sil icate that is

poorer in lime and more porous in the in i t ia l stage of the reaction.

The only model which, as far as we know, has been proposed to interpret the

effect of temperature in the 25-I00°C range on the development of the physical

properties of Portland cement and C3S is due to Verbeck and Helmuth. (lO) Accord-

ing to them, the composition and the properties, in particular the surface


properties, of the hydration products are independent of temperature and the

degree of hydration is only sl ight ly influenced. Remarkable differences, on the

contrary, exist in the distribution of the hydration products throughout the

paste, differences which could be accounted for considering that a temperature

variation might affect to a different extent the process of hydration and that

of diffusion of the hydrated products from the surface of the C3S granules to the

interstices between them. According to these authors, at room temperature the

rate of hydration is low, so that the products of the reaction, once formed, can

diffuse uniformly towards the interstices and distribute themselves homogeneously.

At higher temperatures, however, the rate of hydration becomes much higher than

the rate of diffusion of the hydrated products, with the result that a type of

structure more compact near the anhydrous C3S but more open in the interstices is

formed. The above mentioned authors think that at the long curings the lower

strength of the pastes hydrated at high temperature could be ascribed f i r s t l y to

the presence of porous zones with a gel/space ratio lower than the average value

and, secondly, but to a lesser extent, to the lower degree of hydration of C3S

caused by the denser layer of the hydration product.

Our results do not seem to confirm the model suggested by Verbeck and Hel-

muth. ( l l ) In the f i r s t place the scanning electron micrographs of Fig. 5 do not

show, at least up to 24 hours of hydration, striking differences in the d i s t r i -

bution of the hydration products among the cement granules hydrated at different

temperatures. Secondly, remarkable differences in the surface area values have

been observed for samples hydrated at different temperatures; as has already been

pointed out, at 24 hours, the higher the temperature of hydration, the smaller

the specific volume of the pores with radius not larger than lO0 A. The inf lu-

ence of the temperature on the hydration reaction has probably to be attributed

to this phenomenon, since at higher temperatures the C-S-H gel undergoes a more

rapid aging and the diffusion-controlled hydration process becomes slower.

However, one cannot completely rule out the possibi l i ty that during the

aging of the gel, the formation of denser zones in the hydrated particles leaves,

as a consequence, the adjacent areas with a more open structure. This possibil-

i ty seems to be suggested by the sl ight increase of the volume of the pores larger

than lOOA (Fig. 4). The mechanism proposed in the present paper therefore does

not exclude the presence of localized areas of hydrated product more porous and

hence of lower strength, suggested by Verbeck and Helmuth. Additional informat-

ion concerning the size distributions of the macropores in the C3S pastes hydrated

at various temperatures, which can be arrived at by means of the mercury penetrat-

ion under pressure, might throw further l ight on this subject.


Conclusions

The results obtained in this work show that an increase in the temperature of

hydration up to 90°C causes a reduction in the induction period and an increase

in the in i t ia l rate of hydration of the C3S pastes; however, at the longer cur-

ings this rate is considerably reduced.

Together with such variations in the hydration rate one can make the fol-

lowing observations:

- The C/S ratio of the hydrated si l icate drops to the final value of 1.9

more and more rapidly as the temperature is raised.

At the higher temperatures the hydration product after 24 hours is char-

acterized by lower values of the specific surface area and pore volume.

I t can therefore be assumed that, on the analogy of what happens in the

presence of accelerating electrolytes, the rise of temperature speeds up the

hydration of C3S because i t favours the transformation of a f i r s t calcium s i l i -

cate hydrate rich in lime into a second one poorer in lime and more porous.

At the long curings the high temperature causes a reduction of the hydrat-

ion rate since i t is responsible for the decrease of the porosity of the hydrated

product.

Acknowledgement

Financial support of Consiglio Nazionale delle Ricerche is acknowledged.

References

I. J. of PCA Research and Development Laboratories ~, No. 3, G. J. Verbeck.

57 (1965).

J. H. Taplin. 2. Australian J. Appl. Sci. L3, 164 (1962).

3. L. E. Copeland, D. L. Kantro and G. J, Verbeck. Proceedings of the Fourth

International Symposium on the Chemistry of Cement, Vol. I , p. 429, Washing-

D.C. (I 960).

4. J. Gebauer and I. Odler. Zement Kalk Gips 55, 277 (1966).

5. G. M. Idorn. Proceedings of the Fifth International Symposium on the Chem-

istry of Cement, Vol. I l l , p. 411, Tokyo (1969).

6. R. Kondo and S. Ueda. Proceedings of the Fifth International Symposium on

the Chemistry of Cement, Vol. I I , p. 203, Tokyo (1969).

7. J. H. Taplin. Proceedings of the Fifth International Symposium on the Chem-

istry of Cement, Vol. I I I , pg. 343, Tokyo (1969).

8. D. L. Kantro, S. Brunauer and C. H. Weiseo J. Phys. Chem. 66, 1804 (1962).

9. L. E. Copeland and D. L. Kantro. Proceedings of the Fifth International

Symposium on the Chemistry of Cement, Vol. I I , p. 387, Tokyo (1969).


lO. G. J. Verbeck and R. H. Helmuth. Proceedings of the Fifth International

Symposium on the Chemistry of Cement, Vol. I l l , p. l , Tokyo (1969).

I I . J. Skalny and I. Odler. Cement and Concrete Research 2, 387 (1972).

12. A. van Bemst. Bull. Soc. Chim. Belges 64, 333 (1955).

13. H. F. Schwiete and H. Knoblauch. Forschungsberichte des Landes Nordrhein-

Westfalen, No. 748 (1959).

14. E. R. Buckle and H. F. W. Taylor. J. Appl. Chem. 9, 163 (1959).

15. L. E. Copeland and R. H. Bragg. Anal. Chem. 30, 196 (1958).

16. E. E. Pressler, S. Brunauer, D. L. Kantro and C. H. Weise. Anal. Chem.

33, 877 (1961).

17. V. Alunno-Rossetti, G. Chiocchio and M. Collepardi. Cement and Concrete

Research 3, 665 (1973).

18. M. Collepardi, G. Rossi and G. Usai. Ind. I ta l . Cem. 38, 657 (1968).

19. M. Collepardi, L. Massidda and G. Usai. II Cemento 68, 3 (1971).

20. F. V. Lawrence, Jr. , and J. F. Young. Cement and Concrete Research 3,

149 (1973).

21. M. Collepardi and L. Massidda. J. Am. Cer. Soc. 54, 419 (1971).

22. M. Collepardi, G. Rossi and M. C. Spiga. Ann. Chim. (Rome) 2, 137 (1971).

Low pressure steam hydration of tricalcium silicate

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