Contents no. 2/2005                            Romanian Journal of Materials

STEFAN CARCIUN, EUGEN CRACIUN, BOGDAN-ALEXANDRU SAVA The silicone nitride ceramics obtaining by microwave field sintering method


Bentonite geocomposites, tightening materials for hydrotechnic building works and environment protection   


Low alkali cement  


Measuring Cr6+ in cements and admixtures


Low - heat special cements designed for massive concrete


Use of blast furnace slag as additive in cement 


Use of rigid sphere theory for assesing viscosity of vitreous Na2O.SiO2  


Testing colorimetric method of alumina - zircon suspensions t

The silicone nitride ceramics obtaining by microwave field sintering

The silicone nitride ceramics obtaining is an actual domain, due to its special properties and to the potential utilization in top industries of this kind of material. A new and performant technology for the sintering of those ceramics is the one that utilize the microwave field’s absorbtion of the so-called susceptor materials.

The paper presents the researches made for the establishement of the proper pressing and sintering additives, such as yttrium, aluminium and magnesium oxides. The polyvynil alcohol is considered the most suitable for pressing this kind of ceramics.

For microwave field sintering one utilize a microwave applicator of monomode resonant cavity type. The temperature is measured with the aid of an infrared radiation pyrometter Raytech Thermalert type.

For the sintered samples is measured the a to b transformation using the XRD diffractometter Dron type, the microhardness with the aid of Reichardt apparatus and the texture using the MOTIC digital optical microscop.

This method of sintering ceramics presents important advantages, such as high homogeneity of sintered ceramic body, lower sintering temperatures and improved materials properties.



Bentonite geocomposites, tightening materials for hydrotechnic building works and environment protection

This paper presents the fundamentals for the design and laboratory achievement of two categories of geosynthetic clay liners with role of tightening (by this hydraulic barrier) for ecologic landfill covers.

      For the design of bentonite geocomposite liner usable as new product for tightening we have considered mainly the following advantages :

Ø      high hydraulic performance and low-permeability;

Ø       represent a hydraulic barrier for water and levigate

Ø      it is a superior solution due to physical and chemical characteristics;

Ø      fast and easy to install;

Ø      highly competitive installed cost.

The first category by Bentonite Geocompozite Liner projection it was in 3 proceedings in a sandwich structure dispose which combine two durable geotextile outer layers with a uniform core of activate bentonite clay to form hydraulic barrier.

The second category by Bentonite Geocompozite Liner it was in 2 proceedings in a sandwich structure dispose which combine a one geomebrane, a one durable geotextile outer layers with a uniform core of activate bentonite clay .

This projects have complete viability and accordingly there are obtained new geocomposite tightening materials, respectively bentonite geocomposites of high demand on the domestic market (presently all geosiyntetic materials are imported).



Low alcali cement  

One prior requirement for concrete is durability. Ensuring of such a property demands, among other things, precluding of factors entailing deterioration in the concrete mass. The presence of alkalis above a certain limit in the cement employed in preparing concrete with reactive aggregates is one of the main factors to incite unwitting defects as a result of the alkali–aggregate reactions (expansion, cracking, deformation, etc.) in concrete.

To develop low alkali cement, the siliceous aluminous component in the raw mix was replaced by materials low in alkalis. So, raw mixes were prepared with the siliceous aluminous component (clay or marl) replaced by non-traditional natural raw materials (siliceous materials, bauxite) as well as industrial by-products (granulated blast furnace slag, fly ash).

The paper presents a comparison between the results obtained in the laboratory investigations on the performance (grindability, clinker burnability, volatility) of raw mixes made with traditional and non-traditional raw materials as well as the properties of clinkers and cements obtained therewith.



Measuring Cr6+ in cements and admixtures

Quantitatively, the Cr6+ proportion in cement is meagre, yet the significance to its measuring comes from the effects it inflicts upon the health of the cement-placing workers. In March 2003, the European Union adopted an amendment to the European Directive 76/769/EEC restricting the Cr6+ presence in cement to 2 mg/kg. This amendment has raised the necessity of developing a method for measuring Cr6+ in accordance with the requirements in the European norms in the field. The method is applied in measuring Cr6+ in raw materials, cements and admixtures, and uplifts the competitiveness of Romanian cements on the European Union market. The principle of the method is based on measuring spectrocolourimetrically Cr6+ dissolved in water after eliminating any possible interference from reducing elements. The efficiency of the method has been fine tuned by measuring Cr6+ in controlled amounts in clinker samples. The paper also reveals the results obtained at measuring Cr6+ in Romanian raw materials, cements and admixtures.


Low-heat special cements designed for massive concrete

Because of the stress complexity massive concrete, such as the concrete for dams, has to undergo, it needs to feature good strength, good corrosion and freeze-thaw resistance, low shrinkage, low permeability and low exothermicity.

In order that the concrete may show lower exothermicity, the cement used in its fabrication needs to release as low a heat amount as possible from the exothermic processes of hydration and hydrolysis taking place at hardening.

The paper presents the results of laboratory investigations on developing low-heat special cements. Major properties, such as water demand, setting time, soundness, standard strength, heat of hydration, etc., of the obtained low-heat cements are revealed. Furthermore, the effect of the nature and ratio of the main additional constituents on the heat of hydration of the low-heat special cements is disclosed.



Use of blast furnace slag as additive in cement 

The paper presents in brief the qualities of blast furnace slag cements as compared to Portland cement, the procedures of manufacturing blast furnace slag cements by intergrinding and separate grinding of the components, and the results of a number of laboratory investigations that have shown the possibility of obtaining cements with high blast furnace slag ratios and featuring comparable strength to Portland cement. In addition, performances obtained with various industrial grinding facilities are revealed along with elements to compare technically and economically such facilities.


Use of rigid sphere tehory for assesing viscosity of vitreous Na2O.SiO2  

The non-crystalline solids may be classified, for structural reasons, as vitreous, amorphous systems respectively.

Rigid glasses, as vitreous systems, are usually obtained by fast cooling of some melts. For this reason, they are considered to preserve a number of characteristics of the melts, with properties influenced by this “memory”.

This paper uses the rigid sphere theory, which was successfully used for assessing some properties of glass-forming melts, for calculation of the viscosity of glass melt with Na2O.SiO2 chemical composition.

The results obtained by calculation are compared with a series of experimental data. A proper concordance of calculated data with the experimental ones is obtained, which proves a correct judgement of the starting hypotheses.



Testing colorimetric method of alumina - zircon suspensions

The paper presents a preparing, optimizing and characterizing method from physical-chemical point of view of aqueous suspensions based on alumina-zirconium sand. Suspensions characterization has made by rheological and size distribution methods. Viscosity and average diameter of grains influence the quality of ceramic aqueous suspensions utilized for shaping through pouring in plaster moulds. These suspensions are semi-products used for the obtaining of some ceramic products with good refractory characteristics. A check colorimetric method of these suspensions quality is proposed. The colourimetry, like as the rheology, permits estimation of suspensions and oxide ceramic pastes quality. Proposed colorimetric method has not been met, into literature of speciality, for the aqueous ceramic suspensions estimation.                                                                                                                                                    


Romanian Journal of Materials