A reply to discussion by J. Baron and M. Buil of the Paper “Mechanical features of chemical shrinkage of cement paste”

CEMENT and CONCRETERESEARCH. Vol. 9, pp. 549-550, 1979. Printed in the U.S.A. 0008-8846/79/040549-02502.00/0 Copyright (c) 1979 Pergamon Press, Ltd.

A Reply to Discussion by J. Baron and M. Buil of the Paper "MECHANICAL FEATURESOF CHEMICAL SHRINKAGEOF CEMENTPASTE*" Nava Setter and Della M. Roy Materials Research Laboratory The Pennsylvania State University University Park, Pennsylvania 16802 We thank J. Baron and M. Buil for their reference to our work and for the interesting comparison they have made with their experimental results. Combining the data given in the figures i~ the Discussion with our data on cement paste having water/cement ratio (w/c) of 0.3 shows that the picture that we observed is intermediate between the curve obtained by M. Del Campo and that of J. Baron, et al. As Baron and Buil pointed out, the expansion observed between 5-I0 hours age does not occur in every cement; this makes our results in further agreement.

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FIG. 1 Volume Change of Cement Paste (w/c = 0.3) During First 20 Hours. (l) Del Campo (curve taken from figure in above discussion); (2) Setter and Roy*; (3) Baron, et al.

*Cem. Concr. Res. 8, 623-634 (1978).

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Vol. 9, No. 4 N. Setter, D.M. Roy

The question of the effect of bleeding-water is more a question of the conditions under which the dimensional change is measured. We were concerned with measuring the volume change in cement paste when there is no external supply of water. In this case the bleeding water plays the same role as external water, f i l l i n g the empty capillaries while hydration proceeds. With bleeding water removed, the picture immediately afterwards is one of saturated cement paste, covered with a thin water film. Indeed, as hydration proceeds, pressure w i l l be created inside the c a p i l l a r i e s , resulting in further shrinkage When the system is l e f t without removing the water of bleeding, the process of emptying the capillaries is postponed until the bleeding water is consumed. The change in the shrinkage progress w i l l be a function of the physical and chemical characteristics of the cement as expressed in the rate of development of a stress-bearing skeleton. On the other hand, i f there is a continuous supply of water, expansion is observed (1,2). The influence of bleeding water on the volume change of fresh cement paste is more pronounced in pastes of higher w/c ratio, since this means a higher amount of bleeding water. In case of w/c = 0.30 (close to that used by Baron and Buil) bleeding is not significant, and the measured shrinkage in our experiments is similar to the shrinkage observed when bleeding water is not removed. References

I.

R. L. Berger, Cem. Concr. Res. 3_, 837-845 (1973).

2.

T. C. Powers, PCA Journal 3_, 47-56 (1961).