Ettringite

About: Ettringite is a research topic. Over the lifetime, 2702 publications have been published within this topic receiving 67056 citations. The topic is also known as: woodfordite.

Microscopic observations of samples affected by delayed ettringite formation (DEF)

Abstract: This article, which deals with the study of the microscopic modifications of DEF-affected materials, has two main objectives The first one is to study the influence of sample preparation on the microcracks of specimens observed in Scanning Electron Microscopy (SEM) From the results of this study, it can be concluded that direct observations of the samples are inappropriate because they produce cracks that are not ascribable to the pathology It is therefore preferable to use an indirect technique of observation such as the replica technique This technique was developed in the 1980s and used to study the evolution of microcracking due to mechanical damage to the concrete The results presented in this article show that it is possible to use this technique to study the microcracking associated with delayed ettringite formation The second objective is to study ettringite formation during the swelling of DEF-affected specimens The main results show that secondary ettringite is initially formed in the paste–aggregate interface even at low levels of expansion For these levels of expansion, there is no trace of secondary ettringite or microcracks observable in the paste However, for higher expansions, cracks filled with ettringite appear in the paste

Remediation of chromite ore processing residue using ferrous sulfate and calcium polysulfide

Abstract: Batch tests were conducted to assess the potential use of ferrous sulfate and calcium polysulfide for the remediation of chromite ore processing residue (COPR). The remediation process entails addition of ferrous sulfate or calcium polysulfide to chemically reduce hexavalent chromium [Cr(VI)] to trivalent chromium [Cr(III)] in slurry form and pH adjustment to precipitate Cr(III) as chromium hydroxide. The present study investigates the effects of COPR particle size, treatment pH, and chemical dosage on the performance of the treatment. Smaller particle size resulted in increases in alkaline digestion and Toxicity Characteristic Leaching Procedure (TCLP) Cr(VI) concentrations for the untreated samples. The chemical reduction of Cr(VI) with ferrous iron and sulfides was non-stoichiometric. Four times the stoichiometric amount of ferrous iron of two times the stoichiometric amount of polysulfide were needed to meet both the New Jersey Department of Environmental Protection (NJDEP) regulatory limit of 240 mg/kg for Cr(VI) and EPA TCLP regulatory limit of 5 mg/L for chromium [C,r]. pH adjustment was necessary to prevent the formation of ettringite, a swell causing mineral, upon the introduction of sulfate to the COPR material via ferrous sulfate or calcium polysulfide. The slow hydration of some COPR minerals caused the pH of the treated COPR to creep upward during the curing period. However, when sufficient acid was added, the pH value was controlled at less than 9.27 for a curing period of 1.5 years, which prevented the formation of ettringite.

Water as the Key to Expansion of Ettringite in Cementitious Materials

Abstract: Damage in sulfate-bearing soils and aggregate systems stabilized with additives containing lime, including lime and portland cement, has drawn considerable attention over the past two decades. Researchers and practitioners have made considerable contributions to the understanding of the problem, including the mechanisms involved in the formation of the two minerals, ettringite and thaumasite, that are most often associated with this damage. This paper provides a case history analysis of the expansion history compared with the ettringite growth history of three controlled low-strength mixtures containing fly ash with relatively high sulfate contents. Samples were subjected to three curing conditions: a dry cure, in which only mixing water was available for curing; a moist cure, in which an external source of water was available for curing; and a sulfate cure, in which an external source of sulfate-bearing water was made available for the duration of cure. Ettringite was quantified by using both differential scanning calorimetry and X-ray diffraction; the resulting volume changes in the samples were measured. Results suggested that sorption of water by the ettringite molecule was at least part of the reason for expansion. The importance of sorption was based on the fact that although expansion increased in moist cure compared to dry cure systems, the quantities of ettringite formed under each regime were about the same and remained about the same throughout the experiments. As expected, samples exposed to sulfate cure responded with the greatest expansion, which was concomitant with continued ettringite crystal growth due to a supply of the limiting reagent, sulfate.