Venčeslav Kaučič

The effects of limestone addition, clinker type and fineness on properties of portland cement

Full factorial experimental design was applied to evaluate the effect of 5% limestone addition on various properties of cement in relation to clinker type and fineness. Two clinkers, differing only in lime saturation factor (LSF), were used, and cements with two levels of fineness were prepared. The experiment was performed in real-scale conditions on a ball mill with a capacity of 15 t/h. Effects of different factors on compressive strength, heat of hydration, setting times and water demand as well as the interactions between factors were evaluated.

Understanding controlled drug release from mesoporous silicates: theory and experiment.

Based on the results of carefully designed experiments upgraded with appropriate theoretical modeling, we present clear evidence that the release curves from mesoporous materials are significantly affected by drug-matrix interactions. In experimental curves, these interactions are manifested as a non-convergence at long times and an inverse dependence of release kinetics on pore size. Neither of these phenomena is expected in non-interacting systems. Although both phenomena have, rather sporadically, been observed in previous research, they have not been explained in terms of a general and consistent theoretical model. The concept is demonstrated on a model drug indomethacin embedded into SBA-15 and MCM-41 porous silicates. The experimental release curves agree exceptionally well with theoretical predictions in the case of significant drug-wall attractions. The latter are described using a 2D Fokker-Planck equation. One could say that the interactions affect the relative cross-section of pores where the local flux has a non-vanishing axial component and in turn control the effective transfer of drug into bulk solution. Finally, we identify the critical parameters determining the pore size dependence of release kinetics and construct a dynamic phase diagram of the various resulting transport regimes.

Removal of aqueous manganese using the natural zeolitic tuff from the Vranjska Banja deposit in Serbia.

The natural zeolite tuff from the Vranjska Banja deposit (Serbia) has been studied as sorbent for Mn(II) ions from aqueous solutions. The zeolite sample containing mainly clinoptilolite (more than 70%) removes Mn(II) ions by ion-exchange process, which was confirmed by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDXS). XPS showed that there is no surface accumulation of Mn but an almost uniform distribution inside the sorbent; EDXS confirmed that Mn(II) replaced the clinoptilolite Na counter ions. The sorption isotherms were studied at 298 K by batch experiments showing that the Mn(II) removal is best described by the Langmuir-Freundlich or Sips model. The kinetics followed the pseudo-second-order model, the activation energy being 128 kJ mol(-1). The intra-particle diffusion is not the rate-controlling step in the sorption of Mn(II) on clinoptilolite. Thermodynamic data suggest spontaneity of the endothermic ion-exchange process in the 298-338 K range.

On the possibility of incorporating Mn(II) and Cr(III) in SAPO-34 in the presence of isopropylamine as a template

Hydrothermal synthesis of SAPO materials using isopropylamine (i-PrNH2) as the template leads to the formation of MnAPSO-34 if Mn(II) is present. In the presence of Cr(III), the synthesis yields SAPO-34 containing chromium(III), which is not incorporated in the silicoaluminophosphate lattice. Calcination of MnAPSO-34 results in oxidation of Mn(II); the calcinated product can be subsequently reduced back to MnAPSO-34. The MnAPSO-34 material catalyzes dehydration of methanol, and its catalytic activity is higher than that of SAPO-34.

A novel open framework zincophosphate : synthesis and characterization

Abstract The crystallization of a reactive zincophosphate gel (which may contain Co(II) component) in the presence of sodium cations at ambient temperature and pressure yielded products (abbreviated ZnPO-HEX or CoZnPO-HEX) with a novel anionic open framework topology. The Co(II) ions isomorphously replace zinc(II) in this zincophosphate lattice up to 30%. The hopeite phase appears as a precursor in the crystallization of these new products. The single crystal structure determination of CoZnPO-HEX reveals that the structure belongs to the hexagonal P61 space group with unit cell parameters a = 10.447(3) and c = 15.049(5)A. The structure consists of a three-dimensional tetrahedral framework containing one-dimensional six-ring channels. An infinite helix built up of sodium cations and water molecules has been found inside the channel. The helix plays an important structural role because a complete dehydration of the materials is the cause for the transformation of the hexagonal structure to an unknown crystalline phase.

MnO(x) nanoparticles supported on a new mesostructured silicate with textural porosity.

A two-step synthesis of a novel mesostructured silicate, KIL-2, and its manganese-containing analogue, Mn/KIL-2, has been developed. KIL-2 possesses interparticle mesopores with pore dimensions between 5 and 60 nm and a surface area of 448 m(2). The mesopores are formed by the aggregation of silica nanoparticles, which creates a network with interparticle voids. The particle size and the pore diameters depend on the temperature of the ageing step (first step) and on the solvothermal treatment in ethanol (second step), respectively. Mn/KIL-2 contains octahedrally coordinated Mn(3+) (80%) and tetrahedrally coordinated Mn(2+) (20%) ions. Mn(3+) ions are present in the extra-framework MnO(x) nanoparticles with typical dimensions of 2 nm, which are homogeneously distributed throughout the material. Mn(2+) ions occur as isolated manganese framework sites. The material is also able to retain its structure characteristics after the hydrothermal treatment in boiling water. Because of its non-toxic nature and cost-effective synthesis, Mn/KIL-2 thus exhibits properties that are needed for an environment-friendly catalyst.

Synthesis and structure of [Zn8(HPO4)8(H2PO4)8]•[(C2H8N)8]•4H2O

Abstract A chain-like zincophosphate [Zn 8 (HPO 4 ) 8 (H 2 PO 4 ) 8 ]•[(C 2 H 8 N) 8 ]•4H 2 O was obtained at room temperature from a ZnO/P 2 O 5 /dimethylamine/H 2 O mixture. The crystal structure was determined by single crystal X-ray diffraction. The symmetry is monoclinic a=1.26450(7)nm, b=1.08477(5)nm, c=1.46311(4)nm, β=98.793(5)°, space group Cc. The structure consists of chains of zinc-corner-sharing Zn 2 P 2 O 4 four rings. The negative charge of the chains is compensated by the protonated dimethylamine. The characterization by 31 P solid state nmr spectroscopy is also reported.

The influence of mineral admixtures on sulfate resistance of limestone cement pastes aged in cold MgSO4 solution

Abstract The influence of slag (S), fly ash (FA) and silica fume (SF) on the sulfate resistance of limestone cements was evaluated. Hardened pastes were exposed to MgSO4 solution at 5 °C. Visible changes of the samples during the exposure were followed. Absorption of sulfate was measured and changes in mineralogical composition were evaluated by thermogravimetric analysis and X-ray diffraction (XRD). It was found that among admixtures used, only the addition of silica fume to limestone cement significantly improved its sulfate resistance. Cement with lower contents of C3A and C3S also showed favorable performance compared to cement having higher contents of these minerals.

A zinc-rich CHA-type aluminophosphate

Abstract Zinc(II)-substituted chabazite-like aluminophosphate ZnAPO-34 of excellent phase purity and a high amount of incorporated zinc was synthesized hydrothermally in the presence of tetraethylammonium hydroxide (TEAOH). The characterization of the product indicates that 20% of framework aluminum is replaced by zinc and shows that template is located at the center of the unit cell, in the large interstitial cavity of the chabazite framework. 27 Al and 31 P MAS n.m.r. results are consistent with the X-ray structural determination. Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopic analysis proves the isomorphous substitution of zinc for aluminum in the framework.

Measuring distances between half-integer quadrupolar nuclei and detecting relative orientations of quadrupolar and dipolar tensors by double-quantum homonuclear dipolar recoupling nuclear magnetic resonance experiments

We studied the possibility of using double-quantum homonuclear dipolar recoupling magic angle spinning nuclear magnetic resonance experiments for structural analysis of systems of half-integer quadrupolar nuclei. We investigated symmetry-based recoupling schemes R2(2) (1) and R2(2) (1)R2(2) (-1) and showed that the obtained double-quantum filtered signals depend substantially on magnitudes and relative orientations of dipolar and quadrupolar tensors. Experimental results measured on aluminophosphate molecular sieve AlPO(4)-14, containing dipolar-coupled spin-52 aluminum nuclei, were compared to results of time-consuming numerical simulations. The comparison for short mixing times allowed us to roughly measure internuclear Al-Al distances, if constraints about relative tensor orientations were available. Inspection of relative orientations of dipolar and quadrupolar tensors, using known distances between nuclei, required experimental and simulated data for long mixing times and yielded less accurate results. Two experimental protocols were employed for measuring double-quantum filtered curves, the symmetric protocol, in which excitation and reconversion periods are incremented simultaneously, and the asymmetric protocol, in which only the length of the excitation period is incremented and the length of the reconversion period is kept constant. The former experimental protocol was more convenient for the detection of internuclear distances, and the latter one was more appropriate for the inspection of relative orientations of interaction tensors.