Hydration of dicalcium silicate and diffusion through neo-formed calcium-silicate-hydrates at weathered surfaces control the long-term leaching behaviour of basic oxygen furnace (BOF) steelmaking slag

Alkalinity generation and toxic trace metal (such as vanadium) leaching from basic oxygen furnace (BOF) steel slag particles must be properly understood and managed by pre-conditioning if beneficial reuse of slag is to be maximised. Water leaching under aerated conditions was investigated using fresh BOF slag at three different particle sizes (0.5–1.0, 2–5 and 10 × 10 × 20 mm blocks) and a 6-month pre-weathered block. There were several distinct leaching stages observed over time associated with different phases controlling the solution chemistry: (1) free-lime (CaO) dissolution (days 0–2); (2) dicalcium silicate (Ca₂SiO₄) dissolution (days 2–14) and (3) Ca–Si–H and CaCO₃ formation and subsequent dissolution (days 14–73). Experiments with the smallest size fraction resulted in the highest Ca, Si and V concentrations, highlighting the role of surface area in controlling initial leaching. After ~2 weeks, the solution Ca/Si ratio (0.7–0.9) evolved to equal those found within a Ca–Si–H phase that replaced dicalcium silicate and free-lime phases in a 30- to 150-μm altered surface region. V release was a two-stage process; initially, V was released by dicalcium silicate dissolution, but V also isomorphically substituted for Si into the neo-formed Ca–Si–H in the alteration zone. Therefore, on longer timescales, the release of V to solution was primarily controlled by considerably slower Ca–Si–H dissolution rates, which decreased the rate of V release by an order of magnitude. Overall, the results indicate that the BOF slag leaching mechanism evolves from a situation initially dominated by rapid hydration and dissolution of primary dicalcium silicate/free-lime phases, to a slow diffusion limited process controlled by the solubility of secondary Ca–Si–H and CaCO₃ phases that replace and cover more reactive primary slag phases at particle surfaces

Hydration of tricalcium aluminate in the presence of various amounts of calcium sulphite hemihydrate : Conductivity tests.

Hydration of calcium aluminate C3A (3CaO·Al2O3) in the presence of calcium sulphite hemihydrate (CaSO3·0.5H2O), with the molar ratio of substrates close to 1, produces the C3A·CaSO3·11H2O calcium monosulphite aluminate phase. Small amounts of calcium sulphite added to calcium aluminate (the ratio of CaSO3·0.5H2O/C3A equalling 0 : 1) change the rate of C3A hydration and influence the whole reaction. Reaction processes for various ratios of the C3A–CaSO3·0.5H2O mixture were examined in pure distilled water with a considerable amount of liquid W/ S=38–50 (constant W/C3A). Processes in the liquid phase were monitored with conductivity equipment, and the XRD analysis was used to identify the phases precipitated during the examined reactions

Effect of the hydration temperature on mechanical resistance of Portland cement mortar and paste.

The hydration of white cement and tricalcium silicate has been investigated at three temperatures (5, 25 and 45°C). Samples were tested after eight periods of hydration during which samples were immersed in saturated lime solution, using: compressive strength determination, and thermogravimetry. Increasing temperature initially accelerated the hydration but a lower hydration degree was observed in the long term comparatively to lower temperatures. The same kind of effect was observed for the evolution of the compressive strength. Whatever the hydration temperature, it was found that compressive strength is a linear function of the hydration degree of which the characteristic is the same for all the temperatures. However the given hydration degree strength is greater for the lower temperature. An explanation of this phenomenon was proposed. It was postulated that mechanical properties are mostly influenced by the product which is created at early hydration period

Mechanisms and parameters controlling the tricalcium aluminate reactivity in the presence of gypsum.

International audienceTo understand the mechanisms and the parameters controlling the reactivity of tricalcium aluminate in the presence of gypsum at an early age, a study of the hydration of the “C3A–sulphate” system by isothermal microcalorimetry, conductimetry and a monitoring of the ionic concentrations of diluted system suspensions have been carried out with various gypsum quantities. The role of C3A source and its fineness were also studied. This work shows the fast initial formation of AFm phase followed by ettringite formation during the period when the sulphate is consumed. It has been highlighted that the time necessary to consume all the gypsum varies with the type of C3A and it has been attributed to the intrinsic reactivity of each one and mainly to the change of fineness from one C3A to another. Results are discussed alongside hypothesis from the literature to explain the slowing down of C3A hydration process in the presence of calcium sulphate

Influence of orthophosphate ions on the dissolution of tricalcium silicate.

International audienceTricalcium silicate dissolution in the presence of orthophosphate ions was monitored by measuring the concentrations of calcium and silicate ions in dilute suspensions using a special dissolution cell coupled to an optical emission spectrometer. Results show that increasing adsorption of orthophosphate ions slows down the dissolution of Ca3SiO5 and that a calcium-phosphate precipitate may form at certain orthophosphate concentrations. These observations are correlated with results of calorimetric experiments carried out during the hydration of silica-rich cement pastes in the presence of the same salts

Dimensional stability under wet curing of mortars containing high amounts of nitrates and phosphates.

International audienceInvestigations were carried out in order to solidify in cement some aqueous streams resulting from nuclear decommissioning processes and characterized by a high salinity (300 g/L), as well as important concentrations of nitrate (150–210 g/L) and phosphate ions (0–50 g/L). Special attention was paid to the influence of these compounds on the dimensional variations under wet curing of simulated solidified waste forms. The length changes of mortars containing nitrate salts only (KNO3, NaNO3) were shown to be governed by a concentration effect which involved osmosis: the higher their concentration in the mixing solution, the higher the swelling. The expansion of mortars containing high amounts of phosphates (≥ 30 g/L in the mixing solution) was preceded by a shrinkage which increased with the phosphate concentration, and which could be suppressed by seeding the cement used with hydroxyapatite crystals. This transitory shrinkage was attributed to the conversion into hydroxyapatite of a precursor readily precipitated in the cement paste after mixing