A novel method to perform small-scale laboratory experiments that reproduce concrete–bentonite and concrete–groundwater interactions has been developed. Such interfaces will prevail in engineered barrier systems used for isolation of nuclear waste. With the goal of optimizing the experimental method, this work has analysed the geochemical interaction of distilled water, low-pH cement mortar and FEBEX-bentonite for 75 days. Limited but evident reactivity between the materials was observed, mainly decalcification in cement mortar, carbonation at the interface with bentonite and Mg enrichment in bentonite. These results are consistent with the state-of-the-art literature and were used to validate this small-scale pilot laboratory experiment to establish the basis for further studies comparing the behaviour of different buffer and cement materials.

Artificial and singular geochemical environments are created around the engineered barrier systems (EBS) designed to isolate high level nuclear wastes in deep geological repositories. A concrete-bentonite interface takes place within the EBS and it builds a significant chemical gradient (pH), approximately from pH 8 (bentonite) to pH 12 (low alkali concrete), in a few millimetre thickness. This disequilibrium triggers dissolution and precipitation reactions and form a thin altered region. In this area, poorly ordered authigenic clay minerals, mainly hydrated magnesium silicates, are formed adjacent to hydrated calcium silicates and calcite precipitates adhered to the interface with concrete. This paper presents the development of this authigenic mineral layer comparing 6–18 months to 13 years interfaces. Scanning Electron Microscopy with Energy Dispersive X-ray spectroscopy (SEM-EDX) morphological and chemical characterization with the aid of ternary plots, X-ray diffraction (XRD) and infrared (IR) data show the young to old interface evolution from single brucite layers to stevensite-saponite silicates composition. Geochemical calculations indicate that this layer acts as a pH~11 buffer useful to minimize bentonite alteration and to favour the retention of amphoteric metal ions.

The present study evaluates the formation of magnesium silicates phases as a result of the alkaline alteration of FEBEX bentonite in long-term experiments. The results are relevant in the context of radioactive waste disposal since the bentonite barrier will partly change its original mineralogy and condition its long-term geochemical behavior. Bentonite samples from an in situ experiment of interaction with a CEM-II-type concrete performed for 13 years in a rock gallery and a laboratory experiment of interaction with a CEM-Itype concrete performed for 10 years provide some experimental evidences on the mineralogical alteration.Results required multiple analytical techniques to resolve the nature of the Mg silicates. X-ray diffraction, thermogravimetric analyses, scanning electron microscopy, infrared spectroscopy and 27 Al and 29 Si nuclear magnetic resonance have been used. The mineralogical alteration is complex since several Mg silicates may coexist in the same region not only with themselves but also with carbonates and calcium (aluminium) silicate hydrates. Brucite intercalation in the interlayer of smectite, previously reported as a chlorite-like phase in a former study, is observed. In addition, a serpentine-type mineral phase was better observed in the in situ samples, at least by XRD, and a 2:1 trioctahedral phyllosilicates was better observed in the laboratory samples. Formation of Mg silicates in the bentonite barrier may buffer the Ca alkaline front originated in concrete and may decrease the porosity at the concrete-bentonite interface.

The regeneration of the vallate papilla in the rat was studied by both light and electron microscopy. The papillae were excised and regeneration was studied at time intervals of 3 to 45 days. It was found that the vallate papilla is capable of regeneration after both partial and total papillectomies. The regenerated papillae were asymmetrical in shape. Several invaginations, independent of one another, were the equivalent of the original sulcus. Regeneration of the gustatory system occurred at circumscribed portions of the associated Von Ebner glands. The amounts of newly formed taste buds were proportional to the amount of nerve fibers in the subepithelial plexus. The regenerated taste buds showed normal histochemical and fine-structural characteristics. The results support the contention of some degree of specificity concerning the epithelium where taste bud regeneration occurs.

Reutilization of industrial waste products as cement additives yields a number of secondary materials, yet their identification is not always trivial. Confirmation of the formation of a LDH-type (phyllosilicate/carbonate) material is the main purpose of this study. Mineralogically, organic and inorganic compounds form some industrial wastes, among which the phyllosilicates are prominent. Crystalline and amorphous hydrated phases appeared during the pozzolanic reaction in a dehydroxylated phyllosilicate/lime system, whose controlled activation yielded dehydroxylated products with high pozzolanic properties. The LDH-type (phyllosilicate/carbonate) material was one of the reaction products; this material could be considered as the superposition type 1:1 of tetrahedral layers of silicon and aluminum and octahedral layers of aluminum (dehydroxylated kaolinite), generating a positive charge in the interlaminar region compensated by carbonate anions with a basal spacing (001) measured by XRD at 7.57Å. Therefore, several techniques were used to determine whether they could be included in the structural group named Layered Double Hydroxides (LDHs), as they are, too, laminar layers of oxides and hydroxides with a positive laminar charge in the interlayer region compensated by the presence of anions with similar basal spacing. In this research, the effects of activation temperature and calcite proportion were analyzed on the hydrated phases formed after 28 days of pozzolanic reaction in a metakaolin (MK)/lime system. The MK was obtained from a mix of 1:1; 2:1 and 3:1 kaolinite:calcite mixture, thermally activated at 750°C for 2 hours, comparing it with kaolinite activated at 600°C for 2 hours. As the most LDH-rich sample, the 1:1 LDH-type (phyllosilicate/carbonate) phase was characterized using XRD, SEM-EDX, HRTEM-EDX, FTIR, NMR-MAS, and Raman spectroscopy. K E Y W O R D S calcium silicate hydrate, LDH type (phyllosilicate carbonate), metakaolin, Portland cement, SEM, transmission electron microscopy, X-ray diffraction | 4991 SÁNCHEZ Et al. How to cite this article: Sánchez I, Ramírez M, Casas Angulo M, et al. Characterization of LDH (phyllosilicate/carbonate) type compounds formed in the pozzolanic reaction of phyllosilicate-rich industrial waste. J Am Ceram Soc. 2019;102:4990-4998. https://