Physical and microstructural aspects of iron sulfide degradation in concrete

The microstructural aspects of iron sulfide degradation in dam concrete were investigated by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) in both dam concrete samples and laboratory concrete. The results show that iron sulfide inclusions with a diameter of a few micrometers in the aggregates are reactive and appear to generate expansion first in the aggregates and consequently in the cement paste. The expansion from the iron sulfides is a consequence of the increase in volume of the reaction products formed. The types of iron sulfide present in the aggregate, mainly pyrrhotite (FeS) and pyrite (FeS2), show similar reaction behavior in the aggregates. The released sulfate can lead to a secondary ettringite formation in the concrete matrix, but the degradation associated with this appears to be minor. The reaction of the iron sulfides was found to be very slow even when laboratory samples were exposed to elevated temperatures. (C) 2010 Elsevier Ltd. All rights reserved.

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Emmanuel Gallucci, Karen Scrivener
2011
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https://infoscience.epfl.ch/record/166467?ln=fr

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Béton de ciment

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La température est une grandeur physique mesurée à l’aide d’un thermomètre et étudiée en thermométrie. Dans la vie courante, elle est reliée aux sensations de froid et de chaud, provenant du transfe

What causes differences of C-S-H gel grey levels in backscattered electron images?

Karen Scrivener

Backscattered electron (BSE) images of heat-cured concretes show alite grains surrounded by inner C-S-H gel of two distinct grey levels (referred to as two-tone inner C-S-H gel). The lighter rim forms at elevated temperature whereas the darker rim develops during subsequent exposure to moisture at 20 °C. This microstructural feature can potentially be used as an indicator to assess the curing history of a concrete. However, microstructural examinations of room-temperature concretes containing silica fume or which have been exposed to severe conditions (external sulfate, carbonation) also show distinct rims of two-tone inner C-S-H gel. The chemical compositions of the rims were determined by EDX microanalysis in the scanning electron microscope (SEM). Our results show that for heat-cured samples, the different grey levels of the two-tone inner C-S-H are caused by relative differences in microporosity and water content and not by ones in chemical composition. However, in silica-fume blended concrete, sulfate attacked or carbonated specimens the different grey levels of the two-tone inner C-S-H gel were associated with significant differences in chemical composition. This difference allows two-tone inner C-S-H gel arising from heat curing to be distinguished from that arising from these other causes. © 2002 Elsevier Science Ltd. All rights reserved.

2002

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Some cementitious materials cured at elevated temperature may expand on subsequent exposure to moisture. Concern has been expressed as to the relevance of small laboratory specimens to field concretes, in particular due to the potential leaching of ions into the storage solution. The work presented here aims at investigating this effect. Mortars cured at elevated temperature in identical fashion were subsequently stored at 90-100% relative humidity (RH) and in water, a LiOH solution, and KOH solutions of various concentrations. The largest and most rapid expansion occurred with mortars immersed in water. The samples stored at 90-100% RH expanded more slowly. Expansion was even slower when subsequent storage was in LiOH or KOH solutions. The same samples were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). These techniques indicated that ettringite had formed over time in all samples, with little apparent relation to the degree of expansion. The composition of the C-S-H gel changed over time, differently for the different storage conditions. © 2001 Elsevier Science Ltd.

2001

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Nanocrystals as Precursors in Solid-State Reactions for Size- and Shape-Controlled Polyelemental Nanomaterials

Raffaella Buonsanti, Laia Castilla Amorós, Chethana Janardhana Gadiyar, Pranit Srinivas Iyengar, Anna Loiudice, Valeria Mantella, Emadeddin Oveisi, Dragos-Constantin Stoian

Solid-state reactions between micrometer-size powders are among the oldest, simplest, and still widely used methods for the fabrication of inorganic solids. These reactions are intrinsically slow because, although the precursorsare "well mixed" at the macroscale, they are highly inhomogeneous at the atomic level. Furthermore, their products are bulk powders that are not suitable for device integration. Herein, we substitute micrometersize particles with nanocrystals. Scaling down the size of the precursors reduces the reaction time and temperature. More importantly, the final products are nanocrystals with controlled size and shape that can be used as active materials in various applications, including electro- and photocatalysis. The assembly of the nanocrystal precursors as ordered close-packed superlattices enables microscopy studies that deepen the understanding of the solid-state reaction mechanism. We learn that having only one of the two nanocrystal precursors dissolving and diffusing toward the other is crucial to obtain a final nanocrystalline product with homogeneous size and shape. The latter are regulated by the nanocrystal precursor that is the most stable at the reaction temperature. Considering the variety of controlled nanocrystals available, our findings open a new avenue for the synthesis of functional and tunable polyelemental nanomaterials.

AMER CHEMICAL SOC2020

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