Illite-smectite (I-S) mixed-layer minerals from North Sea oil fields and a Danish outcrop were investigated to determine the detailed structure and the diagenetic clay transformation. Clay layers in the chalk and residues obtained by dissolution of the chalk matrix at pH 5 were investigated. The phase compositions and layer sequences were determined by X-ray diffraction (XRD) including simulation with a multicomponent program. The structural formulae were determined from chemical analysis, infrared (IR) and 27Al NMR spectroscopies and XRD, and the particle shape by atomic force microscopy (AFM). A high-smectitic (HS) I-S phase and a lowsmectitic (LS) illite-smectite-chlorite (I-S-Ch) phase, both dioctahedral, together constitute 80 – 90% of each sample. However, two samples contain significant amounts of tosudite and of Ch-Serpentine (Sr), respectively. Most of the clay layers have probably formed by dissolution of the chalk, but one Campanian and one Santonian clay layer in well Baron 2 may have a sedimentary origin. The HS and LS minerals are probably of detrital origin. Early diagenesis has taken place through a fixation of Mg in brucite interlayers in the LS phase, this solid-state process forming di-trioctahedral chlorite layers. During later diagenesis involving dissolution of the HS phase, neoformation of a tosudite or of a random mixed-layer trioctahedral chlorite-berthierine took place. In the tosudite, brucite-like sheets are regularly interstratified with smectite interlayers between dioctahedral 2:1 layers, resulting in ditrioctahedral chlorite layers.

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The effects of the addition of clay minerals on the hydration reactions and kinetics of alite and belite are investigated for a white Portland cement (WPC)–kaolinite (20 wt%) blend and a mixture of 90 wt% WPC and 10 wt% bentonite using 29Si MAS NMR spectroscopy. 29Si MAS NMR spectra, used to follow the hydration from 12 h to one year, demonstrates that the two clay minerals have an accelerating effect on the hydration of both alite and belite. The relative intensities for the resonances from the calcium–silicate–hydrate (C–S–H) phase in these spectra show that addition of kaolinite results in a C–S–H phase that on average has longer chains of SiO4 tetrahedra as compared to the results for pure WPC. On the other hand, addition of bentonite results in a C–S–H with slightly shorter average SiO4 chain lengths. Furthermore, the 29Si MAS NMR spectra demonstrate that the basic structure of the kaolinite admixture is not affected by the hydrating cement and that kaolinite is not consumed during hydration. This observation is supported by 27Al MAS NMR which also shows that the rate of formation of ettringite is very similar for the pure WPC and the WPC–kaolinite mixture. Finally, the 27Al quadrupole coupling parameters and isotropic chemical shift are reported for octahedrally coordinated aluminum in kaolinite.