Denis Damidot

Investigation of the hydration and bioactivity of radiopacified tricalcium silicate cement, Biodentine and MTA Angelus

OBJECTIVE Novel root-end filling materials are composed of tricalcium silicate (TCS) and radiopacifier as opposed to the traditional mineral trioxide aggregate (MTA) which is made up of clinker derived from Portland cement and bismuth oxide. The aim of this research was to characterize and investigate the hydration of a tricalcium silicate-based proprietary brand cement (Biodentine™) and a laboratory manufactured cement made with a mixture of tricalcium silicate and zirconium oxide (TCS-20-Z) and compare their properties to MTA Angelus™. METHODS The materials investigated included a cement containing 80% of TCS and 20% zirconium oxide (TCS-20-Z), Biodentine™ and MTA Angelus™. The specific surface area and the particle size distribution of the un-hydrated cements and zirconium oxide were investigated using a gas adsorption method and scanning electron microscopy. Un-hydrated cements and set materials were tested for mineralogy and microstructure, assessment of bioactivity and hydration. Scanning electron microscopy, X-ray energy dispersive analysis, X-ray fluorescence spectroscopy, X-ray diffraction, Rietveld refined X-ray diffraction and calorimetry were employed. The radiopacity of the materials was investigated using ISO 6876 methods. RESULTS The un-hydrated cements were composed of tricalcium silicate and a radiopacifier phase; zirconium oxide for both Biodentine™ and TCS-20-Z whereas bismuth oxide for MTA Angelus™. In addition Biodentine™ contained calcium carbonate particles and MTA Angelus™ exhibited the presence of dicalcium silicate, tricalcium aluminate, calcium, aluminum and silicon oxides. TCS and MTA Angelus™ exhibited similar specific surface area while Biodentine™ had a greater specific surface area. The cements hydrated and produced some hydrates located either as reaction rim around the tricalcium silicate grain or in between the grains at the expense of volume containing the water initially present in the mixture. The rate of reaction of tricalcium calcium silicate was higher for Biodentine™ than for TCS-20-Z owing to its optimized particle size distribution, the presence of CaCO₃ and the use of CaCl₂. Tricalcium calcium silicate in MTA hydrated even more slowly than TCS-20-Z as evident from the size of reaction rim representative of calcium silicate hydrate (C-S-H) around tricalcium silicate grains and the calorimetry measurements. On the other hand, calcium oxide contained in MTA Angelus™ hydrated very fast inducing an intense exothermic reaction. Calcium hydroxide was produced as a by-product of reaction in all hydrated cements but in greater quantities in MTA due to the hydration of calcium oxide. This lead to less dense microstructure than the one observed for both Biodentine™ and TCS-20-Z. All the materials were bioactive and allowed the deposition of hydroxyapatite on the cement surface in the presence of simulated body fluid and the radiopacity was greater than 3mm aluminum thickness. SIGNIFICANCE All the cement pastes tested were composed mainly of tricalcium silicate and a radiopacifier. The laboratory manufactured cement contained no other additives. Biodentine™ included calcium carbonate which together with the additives in the mixing liquid resulted in a material with enhanced chemical properties relative to TCS-20-Z prototype cement. On the other hand MTA Angelus™ displayed the presence of calcium, aluminum and silicon oxides in the un-hydrated powder. These phases are normally associated with the raw materials indicating that the clinker of MTA Angelus™ is incompletely sintered leading to a potential important variability in its mineralogy depending on the sintering conditions. As a consequence, the amount of tricalcium silicate is less than in the two other cements leading to a slower reaction rate and more porous microstructure.

Comparison of a bioremediation process of PAHs in a PAH-contaminated soil at field and laboratory scales

A laboratory experiment was carried on the same initial soil and at the same time than a windrow treatment in order to compare results at field and laboratory scales for a soil mainly contaminated with PAHs. After 6 months, laboratory experiments gave similar but less scattered results than those obtained in the field indicating that the field biotreatment was well optimised. The total amount of PAHs degraded after 6 months was ca. 90% and degradation rates followed a negative exponential trend. Relative degradation rates of 3- and 4-ring PAHs were about 32 and 7.2 times greater than those of 5- and 6-ring PAHs, respectively. With respect to the bacterial community, bacteria belonging to Gamma-proteobacteria persisted whereas Beta-proteobacteria appeared after three months of biotreatment when PAH concentration was low enough to render the soil non-ecotoxic.

Evolution of bacterial community during bioremediation of PAHs in a coal tar contaminated soil

The monitoring of a windrow treatment applied to soil contaminated by mostly 2-, 3- and 4-ring PAHs produced by coal tar distillation was performed by following the evolution of both PAH concentration and the bacterial community. Total and PAH-degrading bacterial community structures were followed by 16S rRNA PCR-DGGE in parallel with quantification by bacterial counts and 16 PAH measurements. Six months of biological treatment led to a strong decrease in 2-, 3- and 4-ring PAH concentrations (98, 97 and 82% respectively). This result was associated with the activity of bacterial PAH-degraders belonging mainly to the Gamma-proteobacteria, in particular, the Enterobacteria and Pseudomonas genera, which were detected over the course of the treatment. This group was considered to be a good bioindicator to determine the potential PAH biodegradation of contaminated soil. Conversely, other species, like the Beta-proteobacteria, were detected after 3months, when 2-, 3- and 4-ring PAHs were almost completely degraded. Thus, presence of the Beta-proteobacteria group could be considered a good candidate indicator to estimate the endpoint of biotreatment of this type of PAH-contaminated soil.

In Situ Assessment of the Setting of Tricalcium Silicate–based Sealers Using a Dentin Pressure Model

INTRODUCTION EndoSequence BC Sealer (Brasseler, Savannah, GA) is a premixed tricalcium silicate-based root canal sealer that requires moisture from the root dentin to hydrate. The aim of this study was to investigate the setting of EndoSequence BC Sealer and other sealers in contact with human dentin in a simulated clinical environment. METHODS EndoSequence BC Sealer, MTA Fillapex (Angelus, Londrina, Brazil), Septodont Sealer (Septodont, Saint Maur-des-Fosses, France), and Apexit Plus (Ivoclar, Schaan, Lichtenstein) were assessed. Caries-free lower premolars extracted for orthodontic purposes in patients aged 13-16 years were standardized to a 10-mm root length and were filled with test sealers and set up in a dentin pressure model for 14 days. In addition, set sealers immersed in physiologic solution for 14 days were also assessed. The set materials in solution and materials retrieved from the dentin pressure setup were characterized by scanning electron microscopy and X-ray diffraction analysis. The setting time and radiopacity were assessed using ISO 6876:2002 specifications. Furthermore, mineral ion leaching was evaluated by inductively coupled plasma mass spectrometry. RESULTS All the sealers tested exhibited formation of a calcium phosphate phase when in contact with physiologic solution. Septodont Sealer and Apexit Plus did not exhibit the formation of a calcium phosphate phase in the dentin pressure setup. The fluid in the system was enough to allow the setting of EndoSequence BC Sealer, which did not set in a dry environment. All materials leached calcium with the Septodont Sealer, exhibiting double the calcium ion leaching compared with EndoSequence BC Sealer. CONCLUSIONS Using the dentinal fluid pressure system resulted in an adequate flow of dentinal fluid that allowed EndoSequence BC Sealer to set inside the root canal. Although the sealers tested were tricalcium silicate based, the hydration reaction and bioactivity in the presence of dentinal fluid were different to hydration in vitro. Thus, clinically, material bioactivity cannot be assumed.

Comparison of solid and liquid-phase bioassays using ecoscores to assess contaminated soils

Bioassays on aqueous and solid phases of contaminated soils were compared, belonging to a wide array of trophic and response levels and using ecoscores for evaluating ecotoxicological and genotoxicological endpoints. The method was applied to four coke factory soils contaminated mainly with PAHs, but also to a lesser extent by heavy metals and cyanides. Aquatic bioassays do not differ from terrestrial bioassays when scaling soils according to toxicity but they are complementary from the viewpoint of ecological relevance. Both aquatic and terrestrial endpoints are strongly correlated with concentrations of 3-ring PAHs. This evaluation procedure allows us to propose a cost-effective battery which embraces a wide array of test organisms and response levels: it includes two rapid bioassays (Microtox(®) and springtail avoidance), a micronucleus test and three bioassays of a longer duration (algal growth, lettuce germination and springtail reproduction). This battery can be recommended for a cost-effective assessment of polluted/remediated soils.

Interactions between municipal solid waste incinerator bottom ash and bacteria (Pseudomonas aeruginosa)

Municipal solid waste incinerator bottom ash (MSWI BA) can be used in road construction where it can become exposed to microbial attack, as it can be used as a source of oligoelements by bacteria. The extent of microbial colonization of the bottom ash and the intensity of microbial processes can impact the rate of leaching of potentially toxic elements. As a consequence, our objective was to highlight the mutual interactions between MSWI bottom ash and Pseudomonas aeruginosa, a common bacteria found in the environment. Experiments were carried out for 133 days at 25 degrees C using a modified soxhlets device and a culture medium, in a closed, unstirred system with weekly renewal of the aqueous phase. The solid products of the experiments were studied using a laser confocal microscopy, which showed that biofilms formed on mineral surfaces, possibly protecting them from leaching. Our results show that the total mass loss after 133 days is systematically higher in abiotic medium than in the biotic one in proportions going from 31 to 53% depending on element. Ca and Sr show that rates in biotic medium was approximately 19% slower than in abiotic medium during the first few weeks. However, in the longer term, both rates decreased to reach similar end values after 15 weeks. By taking into account the quantities of each tracer trapped in the layers we calculate an absolute alteration rate of MSWI BA in the biotic medium (531 microg m(-2) d(-1)) and in the abiotic one (756 microg m(-2) d(-1)).

Influence of hardened cement paste content on the water absorption of fine recycled concrete aggregates

A linear relationship was found between the mean size of four granular classes (0/0.63, 0.63/1.25, 1.25/2.5, 2.5/5 mm) of different laboratory-made fine recycled concrete aggregates (FRCA) and their hardened cement paste content (CPC). A method based on salicylic acid dissolution was specifically developed for the measurement of CPC. Results showed that bound water and density of FRCA were strongly correlated with their CPC. Identically, the water absorption coefficient also followed a linear trend as a function of the CPC but only for the three coarser granular classes. Indeed, the water absorption coefficient of the finer fraction of FRCA (0/0.63 mm) cannot be correctly measured using European standard method EN 1097-6 or method no. 78 of IFSTTAR; but it can be obtained by extrapolation from the previous linear trend. As a consequence, the accurate total water absorption of FRCA (fraction 0/5 mm) can be estimated.

Stabilization of ZnCl2-containing wastes using calcium sulfoaluminate cement: Leaching behaviour of the solidified waste form, mechanisms of zinc retention

To assess the potential of calcium sulfoaluminate cement to solidify and stabilize wastes containing high amounts of soluble zinc chloride (a strong inhibitor of Portland cement hydration), a simulated cemented waste form was submitted to leaching by pure water at a fixed pH of 7 for three months, according to a test designed to understand the degradation processes of cement pastes. Leaching was controlled by diffusion. The zinc concentration in the leachates always remained below the detection limit (2 μmol/L), showing the excellent confining properties of the cement matrix. At the end of the experiment, the solid sample exhibited three zones which were accurately characterized: (i) a highly porous and friable surface layer, (ii) a less porous intermediate zone in which several precipitation and dissolution fronts occurred, and (iii) the sound core. Ettringite was a good tracer for degradation. The good retention of zinc by the cement matrix was mainly attributed to the precipitation of a hydrated and well crystallized phase with platelet morphology (which may belong to the layered double hydroxide family) at early age (≤ 1 day), and to chemisorption onto aluminum hydroxide at later age.

Stabilization of ZnCl2-containing wastes using calcium sulfoaluminate cement: cement hydration, strength development and volume stability.

The potential of calcium sulfoaluminate (CSA) cement was investigated to solidify and stabilize wastes containing large amounts of soluble zinc chloride (a strong inhibitor of Portland cement hydration). Hydration of pastes and mortars prepared with a 0.5 mol/L ZnCl(2) mixing solution was characterized over one year as a function of the gypsum content of the binder and the thermal history of the material. Blending the CSA clinker with 20% gypsum enabled its rapid hydration, with only very small delay compared with a reference prepared with pure water. It also improved the compressive strength of the hardened material and significantly reduced its expansion under wet curing. Moreover, the hydrates assemblage was less affected by a thermal treatment at early age simulating the temperature rise and fall occurring in a large-volume drum of cemented waste. Fully hydrated materials contained ettringite, amorphous aluminum hydroxide, strätlingite, together with AFm phases (Kuzels salt associated with monosulfoaluminate or Friedels salt depending on the gypsum content of the binder), and possibly C-(A)-S-H. Zinc was readily insolubilized and could not be detected in the pore solution extracted from cement pastes.

Beneficial use of a cell coupling rheometry, conductimetry, and calorimetry to investigate the early age hydration of calcium sulfoaluminate cement

A specific cell was designed to monitor simultaneously the evolution of the viscoelastic properties, electrical conductivity, and temperature of a cement paste with ongoing hydration. Hydration of calcium sulfoaluminate cement by demineralised water or by a borated solution was then investigated as an example. Borate anions acted as set retarders but to a smaller extent than with ordinary Portland cement. The delay in cement hydration resulted from the precipitation of an amorphous or poorly crystallized calcium borate, which also caused a rapid stiffening (and thus a loss of workability) of the paste after mixing. The gypsum content of the CSA cement was shown to play a key role in the control of the cement reactivity.