Accelerated curing of cementitious systems by carbon dioxide

CEMENT and CONCRETERESEARCH. Vol. 2, pp. 647-652, 1972. Printed in the United States.

Pergamon Press, Inc

ACCELERATED CURING OF CEMENTITIOUS SYSTEMS BY CARBONDIOXIDE

Part I I .

Hydraulic Calcium Silicates and Aluminates

R. L. Berger Depts. of Civil and Ceramic Engineering University of I l l i n o i s , Urbana, I l l i n o i s 61801

W.A. Klemm General P. C. Company Dallas, Texas 75240

(Communicated by D. M. Roy) ABSTRACT The treatment of Ca3SiO5, B-Ca2SiO4, Ca3AI206 and CaI2AII4033 low water/solids r a t i o compacted pastes with carbon dioxide immediately after preparation resulted in a rapid strength development of the calcium s i l i c a t e s and a negligible strength development of the calcium aluminates. The strength gain a f t e r 5 minutes of carbon dioxide treatment was over 3000 psi for both the Ca3SiO5 and B-Ca2SiO4Pastes. There was no apparent difference between the rate of reaction of the Ca3SiO5 and B-Ca2SiO4 pastes as based on strength gain characteristics. R~sum~ On montre i ci que le traitement de p~tes compact~es de Ca3Si05, B-Ca2SiO4, Ca3AI206 et de CaI2AII4033 ~ faibles rapports eau/solide au moyen de dioxyde de carbone imm~diatement apr~s preparation produit un d~veloppement de la r~sistance rapide pour les s i l i c a t e s de calcium et i n s i g n i f i a n t dans les aluminates de calcium. On a constat~ un accroissement de r~sistance de plus de 3000 psi (environ 200 kg/cm 2) apr~s 5 minutes de traitement au CO2, aussi bein pour le Ca3SiO5 que pour le B-Ca2SiO4. Aucune difference n'est apparue, si l'on consid~re les r~sistances, entre la vitesse de r~action des p~tes de Ca3SiO5 et de B-Ca2SiO4 •

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648

Vol. 2, No. 6 ACCELERATION, CARBONATION, CURING, CEMENTS Introduction Recent work on carbon dioxide (CO2) treatment of portland

cement mortars immediately a f t e r molding to form high strength units in the matter of minutes, ~,2 of pure cement minerals.

led to the present study of CO2 treatment

Previous workers have studied the carbonation

of the hydration products of portland cements and the influence of post carbonation of hydrated cement on strength and shrinkage. 3-7 The reaction products of C3S and B-C2S with CO2 immediately a f t e r mixing with water appear to be CaCO3 and a calcium s i l i c a t e hydrate (C-S-H). 8 These two products were intimately mixed with the CaCO3 present in less than 2 um particles and the C-S-H could not be unequivocally i d e n t i f i e d with the scanning electron microscope (SEM) even with the aid of a nondispersive x-ray fluorescence attachment to the SEM. The strength of the individual

cement compounds studied by

Bogue and Lerch 9 showed that the calcium s i l i c a t e s both had s i g n i f i c a n t strength upon hydration whereas the hydrated aluminate and f e r r i t e phases were r e l a t i v e l y weak.

As the strength of pure cement minerals

carbonated immediately a f t e r mixing with water had not been previously evaluated, a program was set up to test the strength of carbonated C3S, B-C2S, C3A and CI2A7. Experimental Approximately 50 gram samples of pure C3S, B-C2S, C3A, and C12A7 and water were placed in vials which contained two I/2 in diameter l u c i t e balls.

The water/cement r a t i o (w/c) varied from 0.05 to 0.175.

Samples were thoroughly blended by mechanical mixing on a paint shaker for 5 minutes.

After blending, samples were normally compacted at

850 psi into 1.225 inch diameter by 1

1.5 inch high cylinders.

Immedi-

ately a f t e r compacting, the cylinders were exposed for 5 minutes to CO2 at 56 psig.

Additional tests were made in which both the time

and pressure of carbonation were varied. Results and Discussion The compressive strengths resulting from CO2 treatment of the various compounds studied are shown in Table I .

Standard cement nomenclature C = CaO, S = SiO 2, A = AI203, H = H20

Vol. 2, No. 6

649 ACCELERATION, CARBONATION, CURING, CEMENTS TABLE l Compressive Strength of CO2 Treated Pastes of

C3S, B-C2S, C3A, CI2A7, Type I I , and White Cement Sample

W/C

Compact'ion Pressure (psi)

Carbonation Pressure Time (psig (min)

Compressive Strength (psi)

C3S

0.08

850

56

5

3250

B-C2S

0.08

850

56

5

3520

C3A

0.08

850

56

5

140

CI2A7

0.08

850

56

5

190

Type II

0.I0

850

56

5

2665

White

O.lO

850

56

5

3430

C3S, B-C2S, Type II and white cement samples became noticeably hot upon carbonation and water was condensed on the sides of the reaction vessel. The C3A and CI2A7 samples exhibited no appreciable temperature rise. As the formation of CaCO3 is strongly exothermic, i t was assumed that appreciable reaction with the formation of CaCO3 took place only in the C3S, B-C2S and cement samples. The compressive strengths of the various samples indicate that this was the case. Both B-C2S and C3S samples developed over 3000 psi while the C3A and CI2A7 had strengths equivalent to the strength of unreacted compacted cylinders. The Type II and white cements which were tested for comparison purposes had compressive strengths similar to the calcium silicates.

The slightly

higher strength of the white cement relative to Type II is probably due to its higher total calcium silicate content. The high strength developed in the B-C2S was unexpected since i t shows such a slow strength gain under normal hydraulic conditions. 9 In the case of the C3S, a great amount of heat was liberated making the specimen too hot to handle with bare hands. I t is believed that steam formation caused microcracking and resulted in a lower strength. Additional tests were run on CO2 treated B-C2S to determine the influence on compressive strength of w/c and time and pressure of carbonation. The results of these tests are shown in Table 2.

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Vol. 2, No. 6 ACCELERATION, CARBONATION, CURING, CEMENTS TABLE 2 Compressive Strength of CO2 treated B-C2S with Varying W/C, Time, and Pressure of Carbonation

Sample

W/C

Compaction Pressure (psi)

1

0.I0

850

56

I0

6200

2

0.13

850

56

I0

6320

3

0.15

850

56

I0

6040

4

0.175

850

56

I0

4405

5

0.05

850

56

5

3905

6

0.08

850

56

5

3235

7

0.I0

850

56

5

5625

8

0.13

850

56

5

5660

9

0.13

850

2

5

685

2

1

56

1

2

1

56

1

I0

0.13

850

Carbonation Pressure Time (psig) (min)

Compressive Strength (psi)

1770

In general, the compressive strength of samples carbonated both for 5 and I0 minutes at 56 psig increased in strength with increasing w/c up to 0.13, a f t e r which a decrease in strength was noted.

The

decrease in strength at high w/c is due to the blocking of pores by water which impedes the diffusion of CO2 into the sample.

Lower strengths

at w/c less than 0.13 may be a function of poor sample preparation of the dry mixes, or not enough water available to act as

transport

media for calcium ions from the B-C2S particles to react with the CO2 and form CaCO3. Increasing the time of carbonation from 5 to I0 minutes increases compressive strengths a l i t t l e

over I0 percent, indicating that the

strength gain curve has already started to plateau a f t e r 5 minutes of carbonation.

In the course of the above tests, i t was observed

that an apparent second exothermic reaction occurred at the moment pressure was released following carbonation.

Thus, although the reaction

Vol. 2, No. 6

651 ACCELERATION, CARBONATION, CURING, CEMENTS

vessel became warm during the 5 or I0 minutes of carbonation, further pronounced heating occurred immediately after venting the CO2.

A possible

explanation of the second exotherm would be i f the carbonation reaction was for some reason inhibited by applied carbon dioxide pressure. This p o s s i b i l i t y was tested by carbonating 0.13 w/c samples at high, low, and a combination of alternating low and high CO2 pressure treatments. Samples 8 to I0 in Table 2 give the results of these tests. Since Specimen 8, which was exposed to the highest pressure of carbon dioxide for the longest period of time exhibited the highest strength, i t would appear that there is no pressure inhibition of the reaction.

The higher CO2 pressure in fact accelerated the reaction

since increasing the pressure from 2 to 56 psiq resulted in over an 8-fold increase in strength. A reasonable explanation for the secondary heating effect would be that sufficient heat is evolved by the carbonation reaction to vaporize a portion of the water present within the samples at atmospheric pressure.

However, in the pressurized system, carbonation occurs

at almost 4 atmospheres gauge pressure and vaporization is prevented until the carbon dioxide pressure is released.

At this time, the water

flashes off the specimen and subsequently condenses on the cool wall of the bomb, releasing its heat of vaporization of over 500 calories per gram of steam. Conclusions Carbonation of C3S, B-C2S, C3A and CI2A7 immediately after mixing with water and compacting into cylinders resulted in the high strength development of C3S and B-C2S samples, but essentially no strength development in C3A and CI2A7.

The strength development of B-C2S and

C3S treated with CO2 for 5 minutes at 56 psig were about equal, but the carbonation of C3S was a more strongly exothermic reaction.

Extend-.

ing the CO2 treatment time from 5 to I0 minutes increased the compressive strength of B-C2S compacted pastes by about I0 percent indicating that the strength development had essentially plateaued by 5 minutes under the conditions used in the test.

The high heat evaluation observed

from the carbonation of the C3S specimens could be lowered, and microcracking avoided, by the preparation of mortar specimens in which the sand would act as a diluent.

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Vol. 2, No. 6 ACCELERATION, CARBONATION,CURING, CEMENTS

The rapid strength development of B-C2S with CO2 treatment may open up the possibility of producing a new low-lime cement which could be used in carbonation cementing. However, questions of durability and s t a b i l i t y of this system remain to be answered as does the influence of the high and rapid temperature rise on larger samples. Acknowledgments This program was carried out at the American Cement Corporation Technical Center where both authors were employed. We thank Dr. G. J. C. Frohnsdorff for his helpful discussions and the American Cement Corporation for releasing the work for publication. References I.

W. A. Klemm and R. L. Berger, Cement and Concrete Res., 2, 567-576 (1972).

2.

K. G. Bierlich, "Manufacture of Cement Products," U.S. Patent 3,468,933 (1969).

3.

G. Verbeck, "Carbonation of Hydrated Portland Cement," Am. Soc. Testing Mater. Sepc. Tech. Rept. No. 205, pp. 17-36 (1958).

4.

I. Leker and F. A. Blakey, J. Am. Conc. Inst., 28, pp. 295-308 (1956).

5.

B. Kroone and F. A. Blakey, J. Am. Conc. I n s t . , 31, pp. 479-510 (1959).

6.

S. L. Meyers, Rock Products, 52, pp. 96-98 (1949).

7.

G. E. Bessey, J. Soc. Chem. Ind., 52, pp. 387-319 0933).

8.

R. L. Berger, J. F. Young and K. Leung, submitted to Science (1972).

9.

R. H. Bogue and W. Lerch, Industrial Engg. Chem., 26, 837 (1934).