Mineralogical and Microstructural Change Accompanying the Interaction of Boom Clay with Ordinary Portland Cement

The results of in situ tests conducted at the HADES underground laboratory, Mol, Belgium, indicate significant reaction between Boom Clay and ordinary Portland cement over a period of 18 months. Mass transfer of calcium, magnesium, aluminium, iron, silicon and sulphur leads to the development of a distinct zonal structure extending 100-250 mum into both the cement and the clay. The associated mineralogical changes have modified the microstructure of the altered region and there is clear evidence of increased porosity, in the zone of portlandite dissolution. Additionally, experiments conducted at 85 degreesC show the presence of a narrow Mg-Al-Si rich band in the clay close to the contact. Analyses indicate the formation of a di-phasic (Mg-aluminate hydroxide and Mg-silicate hydroxide) gel with low crystallinity and compositions close to hydrotalcite and sepiolite, respectively. Reactive transport modelling has been used to simulate phase transformations within the active region and relate these to porosity changes based on simple molar volume considerations. Given the close coupling of mineral chemistry and microstructure in cement-clay systems, such an approach is regarded as essential if reliable estimates are to be made of long-term reactivity.