Paper
Mechanical properties of calcium-leached cement pastes: Triaxial stress states and the influence of the pore pressures

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Abstract

Application of concrete in nuclear waste containments requires knowledge of its mechanical behavior when subjected to calcium leaching. In order to address real-life situations, multiaxial stress states of leached material must be considered. This paper reports results from a series of triaxial tests of calcium-leached cement paste obtained from accelerated leaching tests that operate on an acceleration rate of 300, compared with natural calcium leaching. Along with the global strength loss due to chemical decohesion, an important loss of frictional performance is reported. Environmental scanning electron microscope (ESEM) pictures of both leached and unleached material are presented, and they indicate that this loss of frictional performance can be associated with a highly eroded microstructure perforated by the leaching process. In addition, the frictional behavior of leached cement pastes is found to be strongly dependent on the drainage conditions of the material and thus, on the interstitial pore pressure. Through a poromechanical analysis, it is shown that this high pore pressure sensitivity of leached cement paste can be attributed to the low skeleton-to-fluid bulk modulus ratio, Ks/Kf, of the degraded material, which, together with the increase in porosity, leads to the high compressibility of calcium-leached materials. This low Ks/Kf ratio is the consequence of an intrinsic chemical damage of the solid skeleton, which occurs during calcium leaching.

Introduction

Concrete is commonly employed in radioactive waste disposal as an effective and economical construction material for containment barriers, liners, and encasement of containers. Because of the critical nature of nuclear waste, the load-bearing capacity of concrete containment structures must be ensured over several hundred years. A widely accepted reference scenario for the durability design of waste containers is calcium leaching by pure water [6]. This design scenario refers to the risk of water intrusion in the storage system. It is assumed that the concrete is subjected to leaching by permanently renewed deionized water acting as a solvent. The lower calcium ion concentration in the interstitial pore solution leads to the dissolution of the calcium bound in the skeleton as Portlandite crystals, Ca(OH)2, and calcium-silicate-hydrates (C-S-H) characterized by sharp dissolution fronts [1], [8], [15]. This calcium leaching leads to a degradation of the mechanical properties of concrete. Moreover, as leaching by deionized water is a very slow process, monitoring the durability of nuclear waste storage structures involves large time scales that complicate experimental assessment.

In terms of chemo-mechanical effects, the loss of elastic stiffness (chemical damage) and the strength loss in uniaxial compression due to calcium leaching have been subject to first studies [7], [11], [18]. In contrast, little is known about the behavior of leached cementitious materials under bi- or triaxial stress states.

Section snippets

Experimental program

The objective of the experiments is to determine the strength domain of leached cementitious materials under triaxial stress states. This requires (1) an accelerated test method able to reproduce in vitro the intrinsic material response that characterizes the long-term behavior of cementitious materials, and (2) a homogeneous decalcification state to assess the “real” material response in the mechanical tests.

Results

Fig. 4, Fig. 5 show the results of this test campaign in the 3J2×σm stress invariant half-plane. In the triaxial test, the second deviator invariant J2 and the mean stress σm are defined by:J2=12sijsij=13z−σr|;σm=13σii=13z+2σr)where sij=σijσmδij is the stress deviator of stress tensor σij, and δij denotes the Kronecker Delta; σz is the vertical stress, and σr the radial stress, controlled in the triaxial test setup. The slope δc=dJ2/dσm corresponds to the friction coefficient on the

Environmental scanning electron microscope (ESEM) analysis

Fig. 8, Fig. 9 show the microstructure of degraded and the undegraded microstructure of the cement paste obtained with an ESEM at a magnification varying between 300 and 10,000. In contrast to other microscope techniques, e.g. SEM, an ESEM can be operated with wet samples, avoiding cracking due to drying. The material surfaces shown in Fig. 8, Fig. 9 are freshly fractured and nitrogen-cleaned.

The pictures indicate that the significant chemically induced loss of frictional performance can be

Conclusion

(1) The paper deals with triaxial mechanical behavior of calcium-leached cement paste, to which little attention was paid in the past. The triaxial behavior of leached cement paste is characterized by an important overall strength loss, a loss in frictional performance, and a high sensitivity to drained and undrained conditions. The linearized friction coefficient (compressive meridian) decreases from δc=0.82 to δc=0.23 in the drained case. In the undrained case, due to the internal pressure

Acknowledgements

This research was performed as part of Grant No. DE-FG03-99SF21891/A000 of the US Department of Energy (DOE). The authors gratefully acknowledge the support for this work by the Nuclear Energy Research Initiative Program of DOE. This research also benefited from collaboration with the C.E.A., Saclay, France, through Dr. J. Sercombe. We thank Professor B. Völker of the MIT Civil and Environmental Engineering Department for fruitful discussions and helpful comments about the chemical aspects of

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