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

Application of concrete in nuclear waste containments requires knowledge of its mechanical behavior when subjected to calcium leaching. In order to ad dress real-life situations, multiaxial stress states of leached material mu st be considered. This paper reports results from a series of triaxial test s 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, a n important loss of frictional performance is reported. Environmental scann ing electron microscope (ESEM) pictures of both leached and unleached mater ial are presented, and they indicate that this loss of frictional performan ce can be associated with a highly eroded microstructure perforated by the leaching process. In addition, the frictional behavior of leached cement pa stes is found to be strongly dependent on the drainage conditions of the ma terial and thus, on the interstitial pore pressure. Through a poromechanica l 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 r atio, K-s/K-f, of the degraded material, which, together with the increase in porosity, leads to the high compressibility of calcium-leached materials . This low K-s/K-f ratio is the consequence of an intrinsic chemical damage of the solid skeleton, which occurs during calcium leaching. (C) 2001 Else vier Science Ltd. All rights reserved.