Alina Badanoiu

CEMENTITIOUS COMPOSITES REINFORCED WITH CONTINUOUS CARBON FIBRES FOR STRENGTHENING OF CONCRETE STRUCTURES

Abstract “Continuous fibre wrapping technique” is becoming a common repair and strengthening technique for concrete structures. In this method continuous fibre sheets are bonded to the surface of a concrete structure using organic resins. The main disadvantages of this method are: (i) the use of resins (organic compounds) which are hazardous for the manual worker, and (ii) the low permeability of this type of organic materials. These problems could be solved if a cement-based matrix is used instead of epoxy matrix. In the current development stage one of the limiting parameter in a cementitious carbon fibre composite is the bond between the fibres and the cementitious matrix. The main objective of the study presented in this paper was to improve the bond between the carbon fibres and the cementitious matrix. Two possible methods were studied: (i) the design of a complex binder based on Portland cement with polymer and silica fume additions and (ii) surface treatment of the carbon fibres prior to their casting in the cementitious composite.

Hydration process in 3CaO.SiO2-silica fume mixtures

Abstract The hydration process and hydrate compounds formed in the C3S-silica fume mixtures, were investigated using thermal analysis and BET specific surface determinations. Polymerization of silicates anions in the calcium silicate hydrates was studied using the molybdate method. The addition of silica fume to C3S, increases the kinetics of hydration-hydrolysis, for short periods of time (one day). An increase of Type I calcium silicate hydrates proportion was observed. The results of the molybdate complex method confirm that the presence of silica fume in a hydrating system influences the kinetics of the silicate anion polymerization process because of its pozzolanicity.

THE STUDY OF 'DSP' BINDING SYSTEMS BY THERMOGRAVIMETRY AND DIFFERENTIAL THERMAL ANALYSIS

The so-called DSP (Densified Systems containing homogenously arranged Particles) systems represent a high-performance class of inorganic binders. The hydration and hardening processes of some DSP systems, based on calcium silicates (C3S and C2S) or Portland cement/clinker with silica fume additions, were assessed, in this paper, using the thermogravimetry (TG) and differential thermal analysis (DTA). These data permit a qualitative and quantitative study of the formed hydrates as well as the estimation of hydration process kinetics.

Highly reactive dicalcium silicate synthesised by hydrothermal processing

Abstract The paper brings information concerning the synthesis and reactivity of a highly reactive dicalcium silicate using calcium silicate hydrates as precursors. The calcium silicate hydrates were prepared by hydrothermal treatments of mixtures consisting in CaO, amorphous SiO2 (ROMSIL) and water, under various hydrothermal conditions. C2S was obtained by thermal treatment – at low temperatures (700–800°C) – of these silicates hydrates. Due to this way of processing (hydrothermal synthesis and low temperature treatment), the dicalcium silicate obtained is very reactive and develops good compressive strengths in pressed mortar specimens even after one day of hardening.

Synthesis and characterization of ZnO nanostructures obtained in mixtures of ionic liquids with organic solvents

AbstractZnO nanoparticles were synthesized in mixtures of ionic liquids based on imidazolium cation with organic solvents (dimethyl sulfoxide and ethylene glycol) by a simple, one-step solution route at low temperature. The effect of these mixtures on the morphology, size and properties of as obtained ZnO nanopowders was investigated. The obtained nanopowders have been characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), UV-Vis absorption spectroscopy (UV-Vis) and photoluminescence (PL). The effect of the ionic liquid mixture on the photocatalytic degradation of methylene blue has been analysed. The XRD studies confirmed the hexagonal wurtzite structure of the obtained ZnO powder. The UV-Vis absorption spectra present the typical shape for ZnO, with a broad band situated in the UV region, with the maximum around 360 nm. The calculated band-gap energy is in interval 3.25–3.28 eV. The synthesized ZnO nanopowders have high photocatalytic activity against methylene blue, the best results being obtained when 1-ethyl-3-methylimidazolium tetrafluoroborate was used as the solvent.

Influence of Thermal Treatment Conditions on the Properties of Dental Silicate Cements.

In this study the sol-gel process was used to synthesize a precursor mixture for the preparation of silicate cement, also called mineral trioxide aggregate (MTA) cement. This mixture was thermally treated under two different conditions (1400 °C/2 h and 1450 °C/3 h) followed by rapid cooling in air. The resulted material (clinker) was ground for one hour in a laboratory planetary mill (v = 150 rot/min), in order to obtain the MTA cements. The setting time and mechanical properties, in vitro induction of apatite formation by soaking in simulated body fluid (SBF) and cytocompatibility of the MTA cements were assessed in this study. The hardening processes, nature of the reaction products and the microstructural characteristics were also investigated. The anhydrous and hydrated cements were characterized by different techniques e.g., X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (FT-IR) and thermal analysis (DTA-DTG-TG). The setting time of the MTA cement obtained by thermal treatment at 1400 °C/2 h (MTA1) was 55 min and 15 min for the MTA cement obtained at 1450 °C/3 h (MTA2). The compressive strength values were 18.5 MPa (MTA1) and 22.9 MPa (MTA2). Both MTA cements showed good bioactivity (assessed by an in vitro test), good cytocompatibility and stimulatory effect on the proliferation of cells.

Biocements with potential endodontic use

Abstract Mineral trioxide aggregate (MTA) is a new type of biocement for endodontic use. MTA, also known as silicate binder, contains, as its main components 3CaO·SiO2, 2CaO·SiO2, 3CaO·Al2O3, 4CaO·Al2O3·Fe2O3. Depending on the amount of coloring ions (mainly Fe) MTA can be gray (GMTA) or white (WMTA). In this study the sol-gel synthesis (Pechini method) of 2CaO·SiO2 and 3CaO·Al2O3 is presented, the main components of WMTA, as well as of WMTA and partially stabilized cements (PSCs). PSCs are also MTA type silicate cements in which Zn and F were added in order to create crystal defects and a high energy state in order to decrease the cement setting time. X-ray diffraction analysis (XRD) was used for the assessment of phased composition of the intermediate products and final oxide powders. Electron microscopy techniques (SEM and TEM) were used to provide information about the oxide powder morphology and to estimate the average particle size and crystallinity degree of the synthesized materials. The bioactivity of 2CaO·SiO2, 3CaO·Al2O3, and WMTA was assessed by soaking in simulated body fluid and the nature of formed compounds was determined by XRD or infrared spectroscopy. In vitro cytotoxicity assay was also performed for WMTA and PSCs obtained by this method.