Interaction between carbonate rock and cement paste

CEMENT and CONCRETE RESEARCH. Vol. 16, pp. 127-134, 1986. Printed in the USA. 0008-8846/86 $3.00+00. Copyright (c) 1986 Pergamon Press, Ltd.

INTERACTION BETWEEN CARBONATE ROCK AND CEMENT PASTE

P.J.M. Monteiro and P.K. Mehta Department of Civil Engineering University of California Berkeley CA 94720

(Communicated by J.P. Skalny) (Received Sept. 13, 1985)

ABSTRACT A reaction in the transition zone between the portland cement paste and the carbonate rocks is generally attributed to the formation of calcium carboaluminate hydrates. This investigation shows that a reaction product is formed even when an alite cement containing no aluminates is used. From the study of the transition zone between cement paste and carbonate rock using X-ray analysis and scanning electron microscopy, it is concluded that a hydrated calcium carbonatecalcium hydroxide compound is formed at the interface. The substitution of the large and highly oriented crystals of calcium hydroxide by the compound having smaller crystals appears to be responsible for the strenghening of the transition zone.

Introduction The carbonate-cement paste interaction has fascinated researchers ever since Farran (1) in 1956 pointed out that a reaction occurs between the carbonate and the portland cement paste. Summarizing the results of an extensive study, Farran reported: 1.. The bond between cement paste and calcite or dolomite was better than that with other aggregate types 2. A "corrosion" of calcite surfaces, when in contact with cement paste, was observed 3. No epitactic bond between the calcium hydroxide (CH) precipitated from the paste and the aggregates other than calcite, was observed Buck and Dolch (2) reported that limestone aggregates reacted with cement paste, and formed thin, dark bands just within the aggregate surface. These limestones had low absorption, low acid-insoluble residue and contained little or no dolomite; therefore a dedolomitization was ruled out. In their tests, the reaction occurred both with high or low-alkali cement, however the reaction was more apparent in mixtures made with low-alkali cement at 30 to 40 days also no acid-insoluble material was formed in the rock by the reaction. They suggested that alkalies are probably required for the reaction to occur, because there was no reaction in a paste of tricalcium silicate. The intensity of the 3.04 A calcite peak was always less at the

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surface of the reacted rock after the reaction had occurred. The conclusion of their work was that a reaction between limestone and the alkaline solution of the portland cement takes place at the interface, with calcite reacting and forming a small amount of highly oriented calcium hydroxide. Chatterji and Jeffery (3) reported similar etching of grains of limestone in their aggregate-cement paste interface research. Many investigators have raised the possibility of another reaction between the portland cement paste and the calcite aggregate. This reaction occurs between the aluminates in cement and the carbonate rock resulting in the formation of a carboaluminate hydrate. Lyubimova and Pinus (4), and Schwiete et al. (5) originally postulated this reaction. Later on Cussino and Pintor (6), and Cussino et al. (7) showed a reaction between calcite and C3A, or CaAF in the presence of water to produce C3A.CaC03.11H20. More recently Grandet and Ollivier (8) reported the formation of C3A.CaC03.11H20 by the reaction between C3A.1/2CO2.12H20 and carbonate ions produced by dissolution of calcium carbonate. They also showed the "corrosion" of the calcite surface when in contact with cement paste. Hadley (9) did not attribute any special strong bond between calcite and the paste. He suggested that the tendency of calcite to fracture through the mineral rather than at the interface was due to the well developed planes of calcite, and he reported the same behavior for other minerals such as dolomite, siderite, muscovite, biotite, phologopite, fluorite, and galena. In summary, most researchers agree that the carbonate aggregates are not inert because they react with the hydration products in the transition zone. This chemical interaction is alleged to produce a better bond that improves the mechanical properties of concrete. From this point on, the theories depart. The object of the work reported here was to further characterize the interaction product in the transition zone between cement paste and carbonate rock. Materials and Experimental Procedure An ASTM Type I/II portland cement and marble specimens as aggregate were used in this investigation. Table 1 Chemical Analysis of Portland Cement Chemical Analysis Compound Percentage Present Silicon dioxide, Si02 21.0 Aluminum oxide, AI203 4.6 Ferric oxide, Fe203 3.0 Calcium oxide, CaO 64.1 Magnesium oxide, MgO 2.4 Sulfur trioxide, S03 2.7 Sodium oxide, Na20 0.40 Potassium oxide, K20 0.20 Ignition Loss 1.5 Calculated Compound Composition Compound Percentage Present Tricalcium Silicate, C3S 58.46 16.11 Dicalcium Silicate, C2S 7.11 Tricalcium Aluminate, C3A 9.13 Tetracalcium Aluminoferrite, C4AF 4.59 Calcium Sulfate, CaSO4 In order to find out whether an interaction would occur between the cement and the carbonate rock when no aluminates are present in the system, a high-purity alite was also investigated in place of the portland cement. Tables 1 and 2 show the chemical analysis of

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129 CARBONATE ROCK, INTERFACE, HYDROXIDE, COMPOUND

the portland cement and alite respectively. The surface of the marble was polished down to 0.25 microns in order to establish a well defined reference plane for the interface. Fig. 1 shows a micrograph of the grain structure of the marble used. Table 2 Chemical Analysis of Alite

Chemical Analysis Compound Silicon dioxide, SiO 2

Percent 23.33

Aluminum oxide, A1203

0.44

Ferric oxide, Fe203

0.87

Calcium oxide, CaO

67.92

Magnesium oxide, MgO

2.14

Sulfur trioxide, S03

0.07

Sodium oxide, Na20

0.02

Potassium oxide, K20

0.03

Free lime, CaO

1.89

Cement paste was cast on top of the polished surface of the aggregate forming a composite specimen. The water/cement ratio for the portland cement paste was 0.35 and for the alite paste was 0.4. The composite specimen was stored in a fog-room until the test. The paste and the aggregate were then fractured at the interface. Special care was taken to avoid carbonation during the preparation and storage of the specimens. Results and Discussions

A typical morphology of the portland cement paste side of the transition zone is shown in Fig. 2 where the CH film with a preferential orientation can be seen. The early development of the transition zone follows the results presented by Hadley (9). However, at later ages manifestations of the development of a chemical bond between the aggregate and the paste were felt. For instance, the tendency of the calcite crystals to adhere to the CH film can be observed in Fig. 3. This could have been attributed either to a better bond or to the weak calcite cleavage planes. However, Figs. 4 and 5 show clearly that a strong bond existed between the CH film and the calcite. From similar investigations with other rock types, such a strong bond was not observed.

Fig. 1 Detail of the Marble Crystal Structure

Fig. 2 General View of the Transition Zone

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Fig. 3 Calcite Grain Adhering to the CH Film

Fig. 4 Evidence of a Better Bond when Carbonate Rock is Used. The Dark Zone is the CH Film

Fig. 5 Removal of the Contact Film During Fracture. The Dark Zone is the CH Film

On the aggregate side, the etching of the calcite can be observed as shown in Fig. 6. The imprint of this etching can be seen on top of the CH film in Figs. 7 and 8. These micrographs show without any doubt that there is a chemical reaction between the carbonate rock and the matrix. The interaction appears to be strong between the aggregate and the CH film. No interaction could be observed by scanning electron microscopy between calcite and the aluminates in the cement. However, to avoid making hasty conclusions, particularly due to the rather localized nature of the SEM analysis, a detailed X R D analysis of the transition zone from age 1 day to 1 year was performed. A typical X R D analysis of the interface is presented in Fig. 9. The peak at I 1.2 ° 20 ( Cu -Kc~ ) should be noted. Grandet and Ollivier (8) observed a peak at I 1.6 ° 20 ( C u - K a ) and proposed that this was due to calcium carboaluminate hydrate. In this work, however, for all ages this peak occurred at 11.2 ° 20 (

Cu -Ko~ ). In order to determine whether the interaction product was indeed the calcium carboaluminate hydrate or whether any other compound formed as a result of the reaction between CH and calcite, it was decided to study the transition zone between the alite cement (which contains almost no alumina) and marble. The first point to check was whetter a reaction similar to the one between marble and portland cement developed when alite was used instead of portland cement. The micrograph shown in Fig. 10 clearly demonstrates that when alite was used, the calcite was etched in the same way as with portland cement. On the CH film the imprint of a reaction with a calcite

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131 CARBONATE ROCK, INTERFACE, HYDROXIDE, COMPOUND

Fig. 6

Fig. 7 Imprint of the Calcite Etching on the CH Film

Etching of the Calcite

Colcium Hydroxide

B

Fig. 8 Detail of Fig. 7

Fig. l0 Etching of Calcite Casted Against Alite Paste

9

~0

.

15

13

~4

~.~

~6

i~

18

K~

20

Fig. 9 XRD Analysis of the Interface Showing Peak at 11.2 ° 20 (Cu-K a)

Fig. 11 General View of the Transition Zone when Alite is Used

Zl

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16, No.

P.J.M. Monteiro and P.K. Mehta

Fig. 12 Detail of Fig. 11 Showing the Precipitation of Crystals on the Transition Zone

Fig. 13A Detail of Fig. 13 Showing the Imprint of Calcite Etching

Fig. 13 General View of the Transition Zone

Fig. 14 Detail of Fig. 13 Showing the Reprecipitation of Crystals

crystal can be seen in Fig. 11; a magnification of the circled area in Fig. 11 is presented in Fig. 12, which shows the presence of small crystals precipitated on top of the CH film. O f particular interest are the series of micrographs presented in Figs. 13 and 14 showing the morphology of the transition zone on the alite paste side. The lower magnification micrograph (Fig. 13) shows in the left hand side the imprint of the calcite decomposition and on the right hand side the precipitation of smaller crystals which were formed after a reaction with the film. E D A X analysis was performed on the site of the calcite decomposition and crystal precipitation of the micrograph in Fig. 15, and no trace of A1 was found. Likewise, E D A X analysis was performed on the zones shown in Fig. 16. The foregoing results seem to rule out that the formation of carboaluminate hydrate in the transition zone is the primary reason for a better bond between marble and cement paste. As a final check, an extensive X R D study was performed on the transition zone in the alite cement system from age 1 day to 220 days. A typical X R D diagram is shown in Fig. 17. It is important to note the presence of the diffraction peak at 11.2 ° 20 which also appears when portland cement is used. From what was discussed earlier it is natural to expect that the reaction between calcite and the CH film, probably involved the formation of a basic calcium carbonate hydrate of composition CaCO3-Ca(OH)2-H20. A review of the published literature shows the existence of basic magnesium carbonate hydrates of composition, Mga(OH)2CO3.3H20 and Mg2(OH)2CO2.3H20. It is difficult to synthesize a similar basic

2

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133 CARBONATE ROCK, INTERFACE, HYDROXIDE, COMPOUND

Fig. 15 General View of the Transition Zone Showing the Reaction in the CH Film. EDAX Shows no Trace of AI

AREA A Element 6

Weight %

Atomic %

Precision 2¢

K-ratio

Mg Ks AI Ks Si Ks Ca Ka

9.14 3.23 26.55 61.07

12.68 4.04 31.88 51.39

2,07 1.35 1.91 2.20

0.0468 0.0175 0.1720 0.5516

0.63 0.09 0.56 0.65

0.0361 0.0114 0.1693 0.5945

AREA B Mg Ka AI Kc~ Si Ka Ca Ka

Fig. 16 EDAX Analysis

7.27 2.08 25.41 65.24

CH

,~

,

i

,4

16

le

2~

22

z,

26

~s

~

s2

Fig. 17 XRD Analysis of the Interface When Alite Paste is Cast Against A Marble

10.29 2.65 31.10 55.96

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hydrate of calcium because of experimental difficulties, however, Schimmel (10,11) was able to prove the existence of a basic calcium carbonate with 1.5 molecules of crystallized water, Ca3(OH)2(CO3)2.1.5H20. His work showed that the X-ray diffraction pattern was extremely sensitive to the amount of water of crystallization. It should be mentioned however that when the water of crystallization is removed, the second highest peak occurs at 11.4 ° 20 ( C u - K a ) which is very close to the one obtained in this research. The differences in XRD diffraction can be attributed to a different amount of water of crystallization. Therefore it is proposed here that the hydration product from the dissolution of calcite in the cementcalcite-water system is probably a basic calcium carbonate with a variable composition,

CaCO3-Ca(OH)2-H~O. Conclusions The results of this study confirm that carbonate rock reacts with the cement paste at the interface. The etching of calcite and the subsequent precipitation of much smaller crystals than calcium hydroxide may be responsible for the reported increase of mechanical strength of concrete containing limestone. Since this interaction occurs both for portland cement and alite cement, the formation of the carboaluminate hydrate in the transition zone does not appear to be the cause of this phenomenon. Instead, it is shown here that the reaction between calcite and calcium hydroxide resulted in the formation of a basic calcium carbonate hydrate.

Acknowledgments Paulo Monteiro wishes to acknowledge the support provided by the Carlson-Polivka Fellowship for postdoctoral research at U.C. Berkeley.

References 1.

J. Farran, Revue Mater. Constr. Trav. Publ. 490,491,492 (1956).

2.

A.D. Buck and W.L. Dolch, J. Am. Concr. Inst., 6 3 , 7 5 5 , (1966).

3.

S. Chatterji and J.W. Jeffery, Indian Contr. J., 4 5 , 3 4 6 , (1971).

4.

T.Y. Lyubimova and E.R. Pinus, Colloid J. USSR., 2 4 , 4 9 1 , (1962).

5.

H.E. Schwiete, U. Ludwig and J. Albeck, Wiss. Z. Tech. Univ. Dresden, 17 , 1587, (1968). L. Cussino and G. Pintor, I1Cemento, 4 , 2 5 5 , (1972). L. Cussino, M. Murat and A. Negro, II Cemento, 7 3 , 7 7 , (1976).

6. 7. 8. 9. 10. 11.

J. Grandet and J.P. Ollivier, Cement and Concrete Research, 10,759, (1980). D.H. Hadley "The Nature of the Paste-Aggregate Interface". Ph.D. Thesis. University, (1972). G. Schimmel, Nature, 5 7 , 3 8 , (1970). G. Schimmel, Phys. Blutter, 5 , 2 1 3 , (1970).

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