Aggregates extraction in Serbia: multiple structural breaks analysis

This short letter investigates the dynamics of aggregates extraction in Serbia between 1973 and 2015. The change point analysis identifies 5 structural breaks in the intercept and linear time trend of aggregates extraction: 1980, 1989, 1995, 2003 and 2009. The estimated break dates correspond to the major business cycle turning points of the Serbian economy in the last 40 years. These findings suggest how aggregates extraction industry in Serbia was largely affected by the overall economic activity between 1973 and 2015

The extraction of aggregates in Serbia, 1973-2015, a statistical exploration

We contribute to the fields of mineral economics, environmental and economic history in two respects: first, we construct a novel time series data set for the levels of aggregates extraction in Serbia between 1973 and 2015; the second, we estimate a change point ordinary least squares regression to model the dynamics of aggregates extraction in the aforementioned period. Our cliometric estimates are in great accordance with the major business cycle facts of the Serbian economy in the last 40 years. The most important policy implication of our results pertains to the potential slowdown of aggregates extraction from the onset of the Great Recession

Analytical modeling of activation procedure applied in α-alumina thermo-mechanical synthesis

The impact of the mechanical processing parameters on the alumina grain-size distribution affiliated characteristics and on the γ to α phase transformation rate was investigated. The moderation in the alumina samples behavior has been correlated to the granulometric and mineralogical changes induced by activation via an ultra-centrifugal mill. The assessment of the activation process variables influence on the final quality of the product parameters was conveyed in order to optimize the mechanical treatment of the alumina, which otherwise could be regarded as either energetically or economically unsustainable procedure. The Response Surface Method, Standard Score Analysis and Principal Component Analysis were applied as means of the mechanical activation optimization. The r 2 values obtained by developed models were in range from 0.816 to 0.988. The established mathematical models were able to precisely predict the quality parameters in a broad range of processing parameters. The Standard Score Analysis emphasized that the optimal output sample was obtained using a sieve mesh of 120μm set of processing parameters (SS=0.96). Diverse comparison analyses disclosed that the optimal set of activation process parameters could reduce the negative effect of γ-alumina samples immanent properties on the final score, and furthermore to enhance the rate of γ to α transition which would improve energetic and economic sustainability of the alumina phase transformation procedure. © 2015 Elsevier Ltd and Techna Group S.r.l

The Mechanisms of Cavitation Erosion of Raw and Sintered Basalt

The paper analyzes the morphology of cavitation damage of raw and sintered basalt samples. The experiment was conducted using the ultrasonic vibratory cavitation test method according to the ASTM G-32 standard. During the determination of the resistance to the effect of cavitation, a change in the mass of samples was observed in the function of the cavitation time of operation. The morphology of damage caused by the effect of cavitation was followed by scanning with an electron microscope, and the level of degradation of the surface of the samples was quantified using the image analysis. The results showed a significantly higher degree of resistance of sintered basalt, with a cavitation rate of 0.019 mg/min relative to raw basalt, with a cavitation rate of 0.738 mg/min. After 120 minutes of exposure to the cavitation effect, a smaller number of small pits on the surface of sintered basalt were observed, while a higher level of damage to the surface with the appearance of numerous pits was found in raw basalt, which can be connected in some places to larger and deeper pits in some places. The obtained results indicate the possibility of using sintered basalt for the production of parts that will be exposed to the effects of high cavitation loads

Cavitation resistance of composite polyester resin/basalt powder

The paper presents the results of research on cavitation resistance of the composite based on unsaturated polyester resin and basalt powder, as reinforcement. Basalt powder was obtained by grinding and micronising basaltic rocks from the Vrelo-Kopaonik deposit. Different amounts of basalt powder as reinforcement were applied (g): 0.15; 0.30; 045; 0,50. The mechanical properties (tensile strength, bending strength, hardness) and cavitation resistance properties were determined for the resulting composite. An ultrasonic vibration method (with stationary specimen) was applied according to ASTM G32 standard. Studies have shown that the mechanical properties and cavitation resistance of the composites increase with the addition of basalt powder as reinforcement

Mechanical activation as sintering pre-treatment of talc for steatite ceramics

The effect of vibratory mill induced mechanical activation on the change of the particle size, crystallinity and the phase transformations of the minerals present in the activated material, was studied with the purpose of decreasing of the sintering temperature of talc (Mg3Si4O10(OH)2) as raw material which is the basic component of the steatite ceramics. The aims of the conducted investigation were, also, increasing of the reactivity of the comminuted raw material and establishing of the optimal activation period. The properties of the activated talc induced by mechanical force were expressed in form of the grain inertia change which was measured by means of automatic grain counter. Mechanically activated grains are the most convenient mineral form for physical concentration since the energy change of the mill-material system is recorded on them. The effect of dry grinding on the structure, particle size and shape of talc was studied by means of XRD, DTA and SEM/EDS methods. Activation of talc produced an increase of the starting surface area value progressively from 4.5 m2/g up to a maximum of 108.5 m2/g achieved at 30 min. A subsequent decrease of rate of surface area change and the rate of size reduction were observed following the prolonged grinding. Talc activated in vibratory mill for optimal 30 min showed properties which positively influence the decrease of sintering temperature and the increase of the sintering rate of steatite ceramics. [Projekat Ministarstva nauke Republike Srbije, br. 172057, br. 45008 i br. 33007

Corundum and Bauxite Refractory Shotcretes based on Activated Waste Coal Ash: Investigation of Thermally Induced Properties Change

The necessity for application of activated secondary raw materials in refractory industry is caused by a growing demand for refractory castables with advanced properties and continuous technological evolution of high-temperature materials. In this investigation, refractory shotcretes with the same matrix composition were prepared from 15 wt.% of high aluminate cement and 45 wt.% of bauxite aggregate + 30 wt.% of chamotte filler, i.e. 75% of corundum aggregate. The request for obtaining a low-cement castable is fulfilled by application of 10 wt.% of mechanically activated coal ash as the cement substitution in the shotcretes. The ash was activated by means of various high energy mechano-activators. Results were compared in order to choose the most efficient activation procedure. The properties have been studied at temperatures ranging from room temperature to adopted maximal temperature 1400°C. Mechanisms of hydration and sintering were investigated by means of differential thermal analysis at three different heating rates. The measurements showed different activation energies for ordinary shotcretes and shotcretes with activated ash. The evolution of the refractory shotcretes properties was investigated and correlated to microstructural changes induced by temperature and microfiller addition. The combination of advantages in investigated refractory shotcretes makes them suitable for use in severe conditions at high temperature applications especially in refractory industries

Performances of Vermiculite and Perlite Based Thermal Insulation Lightweight Concretes

This experimental study was conducted with an aim to investigate the effect of the elevated temperature on the mineral phase composition, microstructure and mechanical properties of the thermal insulation lightweight concretes. The first group of experimental concretes was based on the expanded vermiculite and expanded perlite used as lightweight aggregates (in 65 wt%) in combination with either ordinary Portland cement or refractory calcium aluminate cement. The mix-design of the second group of concretes comprised standard quartz aggregate, vermiculite or perlite as aggregate replacement (25 wt%) and binder (PC or CAC). A total of 10 concrete mix-designs were fabricated in form of 40x40x160 mm samples which were submitted to heat-treatment at 400 degrees, 600 degrees, 800 degrees and 1000 degrees C upon standard 28-days period of curing and hardening. The changes in crystallinity and mineral phase composition induced by temperature were monitored by X-ray diffraction technique. Microstructural visualizations of the non-fired and fired concrete samples were conducted by scanning electron microscopy accompanied with EDX analysis. The results indicated that despite the decrease in compressive strengths upon firing, investigated lightweight concretes can be categorized both as thermal insulators and structural materials

The flaws of aluminium-magnesium alloy elements: Influence of inclusions

Rezultati ispitivanja uticaja uklanjanja gasa iz tečnog metala na kvalitet livenih elemenata od aluminijum-magnezijum legure, a na bazi recikliranih materijala su dati ovom radu. Dobijeni rezultati praktično definišu proceduru livenja kojom se može umanjiti prisustvo uključaka i gasova u livenim elementima, što je osnovni preduslov za postizanje projektovanog kvaliteta livenog materijala i krajnjih proizvoda. Ispitivani su kinetika i mehanizmi procesa degasifikacije, zatim su definisani parametri procesa degasifikacije, i na posletku primenjeni novi agensi za degasifikaciju legura. Rezultati ovog ispitivanja su značajni sa ekološkog aspekta i sa aspekta održivog razvoja proizvodnje livenih elemenata od aluminijumskih legura.The investigation results of gas removal from smelted metal and the effect on performances of aluminium-magnesium alloy cast elements are presented in this paper. The investigated alloy is based on recycled material. The obtained results define the specific procedure which is used for removal and reduction the presence of inclusions of various types in the cast elements, which is the basic step in achieving the designed quality of cast material and quality final products. Kinetics and mechanisms of degasification process were investigated, parameters of degasification procedures were defined, and the new agents for degasification of alloy cast were implemented. The results of this investigation are significant from the aspect of sustainable development the production of cast elements of aluminium alloys

The Chemometric Study of Limestone Physico-chemical Properties and Thermal Behavior for Application in Construction Composites

The limestone, as an economically sustainable and easily available basic raw material, is frequently utilized in the building industry for resolving of the environmental protection issues. The limestone is incorporated in a cementitious system either by grinding with cement clinker, or by blending with the binder during concrete production. The employing of powdery limestone as partial cement replacement gives the construction composites with properties comparable to that of conventional concrete. The study of limestone thermal behavior and its chemistry is crucial for the prognosis of the designed composites properties. In this work, the instrumental techniques (atomic emission spectroscopy, differential thermal and thermo-gravimetric analysis, Fourier transform infrared spectroscopy) and the Principal component analysis were employed to discriminate and classify 22 limestone types. The PCA statistical method, as a means of spectra and experimental data fingerprinting, grouped the samples in a multi-dimensional factor space producing four graphical prognosis - one for each instrumental method. DTA/TG peak values varied the most in a short range between 830-870 degrees C, while FTIR spectra showed the highest diversity in the 867-887 cm(-1) and 1237-1647 cm(-1) ranges. This research was governed by an idea to reveal whether it is possible to differentiate various limestone types and to predict the possibility of their employment in construction composites on the basis of the results of instrumental and mathematical analyses