Testing of models of the dissolution of cements—leaching of synthetic CSH gels

Abstract

The exhaustive leaching of a range of synthetic calcium silicate hydrate (CSH) gels in demineralised water has been studied. This has tested the applicability of experimental data and models based on the static dissolution of synthetic CSH gels in pure water (dissolution of a set of CSH gels with a range of compositions, each at effectively constant composition) to the description of the slow leaching of cementitious materials by flowing waters. This provides additional confidence in the modelling of the long-term evolution of conditions in a repository backfilled with cementitious materials and thereby helps underpin the assessment of the performance of such a repository. The results demonstrate that the initial stages of the dissolution of a range of CSH gels in demineralised water are apparently incongruent, followed by an approach to congruent behaviour. This behaviour is broadly similar to that predicted from the static dissolution of CSH gels. The congruently dissolving composition is found at a Ca/Si ratio of approximately 0.8 to 0.9. This is comparable to the ratio of about 0.8 suggested by static dissolution. However, sequential leaching of CSH gels apparently results in a lower solubility for the congruently dissolving composition than is predicted from static dissolution. The experimental results are compared with data obtained from static dissolution measurements for synthetic CSH gels and with the predictions of a computer model based on the Berner model of cement dissolution. The need to revise the model of dissolution to take account of the behaviour of CSH gels during sequential leaching is examined.