Colour and chemical stability of bismuth oxide in dental materials with solutions used in routine clinical practice

Bismuth(III) oxide is included as a radio-opacifier in dental materials, including hydraulic silicate cements, the material of choice for several endodontic procedures. It has been implicated in tooth discoloration after contact with endodontic irrigants, in particular NaOCl solution, To date, there has been no work on the chemistry: all reports have been of clinical findings only. The purpose now was to report the reactions leading to colour change from Bi2O3 in contact with solutions used in routine endodontic practice. Ten-gram portions of Bi2O3 were immersed in either water, NaOH, NaCl, NaOCl or HCl solution, either in the dark or exposed to visible light, and samples retrieved at 1, 4, 12 and 24 weeks. After washing, these were exposed to either added CO2 or not, for 1 week while drying, and under the same dark or light conditions. Changes in appearance were monitored by photography and colour measurement, and chemically by X-ray diffraction and Fourier-transform infrared spectroscopy. 24-week material was studied using electron paramagnetic resonance and Raman spectroscopy; NaOCl-treated material was also examined by scanning electron microscopy. With water, NaCl and NaOH, bismuth subcarbonate was formed. With or without added carbon dioxide, discoloration occurred from pale yellow to light brown when exposed to light, and to a lesser extent in the dark, intensifying with time. In contrast, exposure to NaOCl rapidly formed a dark brown-black sodium bismuthate. With HCl, white BiOCl was formed. Bi2O3 is not at all inert in this context as is commonly believed, denying its principle of use. Previously unreported solution-mediated reaction occurs readily even in water and NaCl solution, forming new compounds that discolour. In contact with NaOCl sodium bismuthate is formed; severe darkening occurs rapidly. The reactivity is such that Bi2O3 is not indicated for dental materials and should be withdrawn from use

On the global hydration kinetics of tricalcium silicate cement

We reconsider a number of measurements for the overall hydration kinetics of tricalcium silicate pastes having an initial water to cement weight ratio close to 0.5. We find that the time dependent ratio of hydrated and unhydrated silica mole numbers can be well characterized by two power-laws in time, $x/(1-x)\sim (t/t_x)^\psi$. For early times $t < t_x$ we find an `accelerated' hydration ($\psi = 5/2$) and for later times $t > t_x$ a `deaccelerated' behavior ($\psi = 1/2$). The crossover time is estimated as $t_x \approx 16 hours$. We interpret these results in terms of a global second order rate equation indicating that (a) hydrates catalyse the hydration process for $t<t_x$, (b) they inhibit further hydration for $t > t_x$ and (c) the value of the associated second order rate constant is of magnitude 6x10^{-7} - 7x10^{-6} liter mol^{-1} s^{-1}. We argue, by considering the hydration process actually being furnished as a diffusion limited precipitation that the exponents $\psi = 5/2$ and $\psi = 1/2$ directly indicate a preferentially `plate' like hydrate microstructure. This is essentially in agreement with experimental observations of cellular hydrate microstructures for this class of materials.Comment: RevTeX macros, 6 pages, 4 postscript figure

Impact of unrestrained Delayed Ettringite Formation-induced expansion on concrete mechanical properties

International audienc

Possible mechanisms of expansion of concrete exposed to elevated temperatures during curing (also known as DEF) and implications for avoidance of field problems

The phenomenon of expansion in concretes which have been exposed to elevated temperatures during curing and subsequently to moisture is discussed; in particular, the relationship of this expansion to the formation of ettringite after initial curing (delayed ettringite formation). Evidence is also presented for the possible implication of the C-S-H gel in the expansion process. The avoidance of this type of expansion in field concrete is also discussed. The data show that expansion occurs only when the concrete temperature during hydration exceeds 65°C and that the proportion of cements that expand under laboratory testing conditions increases as the temperature of exposure increases. Tests on limited numbers of cements have indicated that some factors in the cement that appear to influence the tendency of the concrete to expand after elevated temperature curing (such as, alkalis content, fineness, SO3), but there is no general relationship between these factors and the degree of expansion valid across the range of commercial cements. This means that the selection of a cement within certain limits will not guarantee the avoidance of expansion if the concrete is exposed to a high enough temperature during curing

Mise au point d’une suspension bactérienne épaissie pour l’exobiocicatrisation de matériaux cimentaires fissurés

La précipitation de carbonate de calcium par les bactéries offre la possibilité de réparer les matériaux cimentaires fissurés par une méthode durable et non polluante. Une suspension bactérienne épaissie a été mise au point pour favoriser la biocicatrisation de microfissures entre 150 et 500 μm en utilisant la souche bactérienne Bacillus pseudofirmus. Le fluide obtenu, par synergie entre deux épaississants colloïdaux, est fortement rhéofluidifiant et thixotrope, résistant à la décantation et forme lors d’un repos prolongé un gel physique réversible. Ces propriétés permettent de l’injecter sans drainage post-injection dans des fissures d’ouverture variant de 150 à 500 μm. L’ajout d’épaississants n’a pas d’impact sur la croissance ni sur l’activité de B. pseudofirmus et entraîne la formation d’amas autour des bactéries. Cette structure similaire à un biofilm favorise potentiellement leur survie face aux contraintes du milieu extérieur