A reply to a discussion by D. Constantiner of the paper “Mechanism of dedolomitization and expansion of dolomitic rocks”

Cement and Concrete Research, Vol. 24, No. 8, pp. 1584-1586, 1994 Copyright ©1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0008-8846/94 $6.00+00

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A Reply to a Discussion by D. Constantiner of the Paper "MECHANISM OF DEDOLOMITIZATION AND EXPANSION OF DOLOMITIC ROCKS"* Deng Min and Tang Mingshu Department of Materials Science and Engineering Nanjing Institute of Chemical Technology Nanjing 210009, P.R. China

The authors thank Mr. Daniel Constantiner for his constructive comments on our paper. We agree with the point raised by Mr. Constantiner that the ionization constant of water is a function of temperature and Fig. 1 in the paper[l] should be revised. We are apologized for another mistake. The pH value listed along the horizontal axis in Fig. I of the paper[l] should be reduced by 1. The discussed part of the pz,per is a theoretical consideration on the possibility and limit of the dedolomitization. Thermodynamically, the reaction is in equilibrium when tee Gibbs free energy of the reaction equals to zero. According to the equation (2) in the paper[l], the relationship between activities of CO32- and OH- ions at equilibrium state can be expressed as follows A G° T

[CO32-] =10 - 2.303RT ' [OH-]2 o represents the standard Gibbs free energy of the reaction at temperature T; Here A G T

(1)

[CO32- ]

and [OH-] are the activities of CO32- and OH- ions, respectively. The CO32- ions are generated by the dedolomitization and are quantitatively related to degree of the reaction. By the equation (1) of the paper[l] the concentration of CO32- ions (C) may be given by

C-

(z M(L/S)

(2)

Where ot is degree of the dedolomitization; L/S is a ratio of involved alkali solution volume (in litre) to dolomite (powdered) weight (in gram); M is molecular weight of dolomite, being 184.41. Considering that activity is activity coefficient by concentration, we can obtain the following equation by substituting equation (2) into equation (1) and rearranging: &G°

184.41(L/S) 10

1' ~.3O3~T " [OH-J2

* C C R 24(6), 1397-1408 (1994) 1584

(3)

Vol. 24, No. 8

DISCUSSIONS

1585

Here y is the average activity coefficient of alkali carbonate; ot is the degree of the reaction at equilibrium, representing the maximum of the reaction degree. When the solution is so dilute that it may be considered as a real solution, equation (3) may be rewritten as L

& GT

ct = 184.4t g • 10 "2.303RT [OH']2

(4)

"

In view of audio-visual illustration, it is batter to evaluate the relationships between maximum degree of the dedolomitization and activity of OH" ions. Fig. 1 is the calculated results by data listed in Table 1 of the paper[ 1] and equation (4). In the light of equation (2) and Fig. 1, the maximum concentration of CO32- ions may reach 0.0271 mol/l in the case of 2~C and L/S=0.2 and the maximum activity of OH" ions needed to consume dolomite may be 0.0375 mol/l in the case of 300°C and L/S=0.5. This indicates that concentrations of the solutions are in a magnitude of 10-2 mol/1 or less. Therefore, the solutions are dilute ones, but they should not be all dealt as real solutions. As a result, the actual maximum degree of tile dedolomitization corresponding to a certain activity of OH" ion is slightly larger than the calculated value in Fig. 1, especially at higher activity of OH" ion according to equation (3), because ~, is less than 1 so as to a dilute solution is concerned[2]. 100 8O o

i

60 40

I

20

0.4,_. 104

I tll

- ~ - 25C & L/S=10 --o-- 25C & L/S=1.0 +

25C & L/S=0.5

q~- 25C & L/S=0.2

// /

- A - 80C & L/S=0.5 --o- 150C & L/S=•.5 -~,- 200C & L/S=0.5 -~::- 300C & L/S=0.5

;:;

i

i

i

10-2 10-3 Activity of hydroxyl ions (mol/1)

i

lit

10-1

Fig. 1 The influences of activity of GH" ion on the possibility and limit of the dedolomitization at different ratios of alkali solution to dolomite and temperatures when the solutions are dealt as real ones These revised results show that the tendency in the influences of activity of OH" ion, temperature and ratio of alkali solution to dolomite (powdered) on the possibility and limit of the dedolomitization is quite similar to that reported in the paper[l]. Under the discussed conditions, the dedolomitization tends to complete in an alkali solution of a higher activity of OH- ion than 5x10 -2 mol/1. At a larger ratio of alkali solution to dolomite such as 10, the reaction may proceed to some extent even the activity of OH" ion in the solution is low as

1586

DISCUSSIONS

Vol.24, No. 8

5x10 -4 mol/l. However, it should be noted that the reaction also depends on kinetic processes. From Fig. 2 in the paper[l], the dedolomitization carried out very slowly when the activity of OH" ion was lower than about 10-2 mol/1.

References

[1] Deng Min and Tang Mingshu, Mechanism of dedolomitization and expansion of dolomtic rocks. Cement and Concrete Research, 23~ 1379-1408(1993) [2] Harned, H.S. and Owen, B.B., The Physical Chemistry of Electrolytic Solutions, 2nd Edition, Reinhold Pub., Corp., 423,453,560-561(1950)