O. Petrov

Characterization of calcium phosphate phases obtained during the preparation of sintered biphase Ca-P ceramics

Abstract A complex of chemical, powder X-ray diffraction (XRD), scanning and transmission electron microscopic (SEM and TEM) methods is specified for a controlled preparation of calcium-phosphate (Ca-P) biphase ceramics for the needs of the clinical practice to be used as bone implants. Reproducibility in the synthesis of hydroxyapatite+alpha-tertiary calcium phosphate (HAP+α-TCP) and hydroxyapatite+beta-tertiary calcium phosphate (HAP+β-TCP) bioceramics is achieved under controlled conditions: the calcium-to-phosphorus (Ca/P) ratio, the phases contents, and the texture of the sintered material.

Hydration process of cement containing fly ash and silica fume: The first 24 hours

Results from studies on the early hydration (till the 24th hour) of cements mixed with fly ash, silica fume or a combination of both, called Pozzolit, are presented. The active role of the Pozzolit mineral additive was revealled by DTA, XRD and IRS analyses. The effect of the additive is expressed in increased total amount of hydration products and decreased portlandite content.

Structural state of microcrystalline opals: A Raman spectroscopic study

Abstract The structure of natural hydrous silica is complex and its study requires the complementary application of several methods. To elucidate the structural state of opaline silica of different geneses, microcrystalline opals from siliceous rocks, geodes, and bentonite clays from East Rhodopes, Bulgaria, were analyzed by Raman spectroscopy, X-ray powder diffraction, electron microscopy, and thermogravimetric and differential thermal analysis. Comparison of X-ray diffraction and spectroscopic data for a series of microcrystalline opals showed that the fraction of tridymite-like structural units can be estimated using the relative intensity of the Raman scattering near 350 cm-1. Opals displaying an intense, poorly resolved Raman band centered near 330.360 cm-1 contain a larger proportion of nanosized spatial regions with tridymite-type atomic arrangements as opposed to cristobalite-type arrangements. The results demonstrate the ability of Raman spectroscopy to characterize the finescale structure of opal and to better distinguish opals showing similar XRD patterns. The application of Raman micro-spectroscopy showed that on intermediate-range scale the atomic structure of opal lepispheres is closer to the framework topology of tridymite than to that of cristobalite

Mineralogy, geochemistry and pyrite content of Bulgarian subbituminous coals, Pernik Basin

Abstract The mineralogy and geochemistry of Pernik subbituminous coals (coal bed A) and some genetic peculiarities related to the mineral formation were studied. The mineral matter of the coal consists chiefly of pyrite, kaolinite, siderite, quartz and calcite. Other minerals (dolomite, ankerite, plagioclase and some sulphates) are present in minor amounts, some occurring as accessory single crystals. Pyrite is the main mineral in these coals and exhibits a large array of textures and morphology. Isolated and clustered euhedral, bacterial and inorganic framboidal, cluster-like, homogeneous and microconcretional massive, infilling and replacing anhedral, and cleat-filling and fracture-filling infiltrational pyrite types were observed. Four stages of mineralization were distinguished: pyrite-kaolinite, pyrite, pyrite-siderite and sulphate stages. The amount of pyrite present in two sections of coal bed A was determined by quantitative powder X-ray diffraction analysis. The concentrations of 37 trace elements were determined. As, Cu, Co, Ni, Zn, Pb, V Ti, Mo, Rb, Cr and Mn are typomorphic for this coal. On the basis of their relation to organic or inorganic matter, four groups of trace elements were subdivided; and on the basis of cluster analysis four associations were differentiated.

Properties and hydration products of lightweight and expansive cements Part II: Hydration products

Abstract The type and quantity of hydration products in cement stone (plain cement, lightweight cement with cenospheres, and cement mixed with expansive additive) hydrated at 20 and 75°C were studied. The changes in the cement stone structure under the influence of lightweight and expansive additives and raised hardening temperature were studied with complex thermal analysis, infrared spectroscopy, powder X-ray diffraction analysis, and scanning electron microscopy. In the case of raised hardening temperature, the cenospheres and the expansive additive improve the crystallization of hydration products. A chemical reaction between the cenospheres and portlandite formed from the cement hydration was observed, accompanied by a decrease of the portlandite quantity. The expansive additive stimulates the formation of hydration products, which were thermally stable and decompose at temperatures above 600°C.

Study of the pozzolanic activity and hydration products of cement pastes with addition of natural zeolites

Abstract This paper presents results from comparative thermogravimetric, calorimetric and pozzolanic activity analyses of five natural zeolite samples from Bulgaria, Slovakia, Philippines, USA and North Korea. The zeolites actively participate in the hydration processes of cement. Their activity in the early stage of hydration is based mainly on the large surface area of the particles while, in the later stages of activation, chemical reactions occur between the products of the hydration of cement and the soluble SiO2 that is present in the bulk of the zeolites. It has been shown that in all cement pastes which contain zeolite additives, the quantity of portlandite is lower than that in pure cement paste or is even totally absent. The amounts of hydration products are greater when 30% zeolite is used than when 10% zeolite is added (excluding the sample with chabazite). The lowest pozzolanic activity is shown by chabazite, which possessed the lowest SiO2/Al2O3 ratio.

Effect of calcium phosphate ceramics on gram-negative bacteria resistant to antibiotics.

This paper discusses the results of an experimental study on the effect of biphase calcium phosphate ceramics (BCPC) on laboratory-isolated polyresistant Gram-negative bacteria. Monitoring of this effect in a dynamical regimen was carried out upon Enterobacter cloacae 313, Klebsiella pneumoniae 227, Serratia marcescens 206, Klebsiella oxytoca 202, as well as on the standard strain Klebsiella pneumoniae 52145 (Institute Pasteur, Paris). The results show a significant antimicrobial effect of the ceramics. Antimicrobial properties are manifested during direct contact with BCPC and these depend on the quantity and grain size of the particles, as well as on the microbiological characteristics of the test microorganisms, and particularly on their cell size.

Effects of high energy dry milling on biphase calcium phosphates

Abstract Abstract The effect of high energy dry milling on the structural and crystalline state of sintered biphase calcium phosphates was studied. After various periods of grinding, the initial biphase calcium phosphate material alters its crystalline structure and phase composition. The phase transformations achieved during milling were recorded by powder X-ray diffraction, scanning electron microscopy, infrared spectroscopy and chemical analysis. X-ray diffraction analysis of samples milled for 20 h showed that the initial composition of the biphase ceramics changed and part of β-tertiary calcium phosphate was nanocrystalline and partially in amorphous state. Hydroxylapatite fully transformed into nanocrystalline phase. Cytotoxicity tests of samples milled for 20 h clearly present cellular viability with pronounced biological activity.

Mg- and Zn-modified calcium phosphates prepared by biomimetic precipitation and subsequent treatment at high temperature

Powders of magnesium-modified as well as zinc-modified calcium phosphates (Me-β-TCP and HA) with a (Ca2++Mg2++Zn2++Na++K+)/P ratio of 1.3–1.4 and various Me2+/(Me2++Ca2+) ratios (from 0.005 to 0.16) were prepared in biomimetic electrolyte systems at pH 8, mother liquid maturation and further syntering at 600–1000°C. Some differences in zinc and magnesium modifications have been prognosed on the basis of thermodynamic modeling of the studied systems and explained by the Mg2+ and Zn2+ ion chemical behaviour. The temperature as well as the degree of Zn2+ and Mg2+ ions substitutions were found to stabilize the β-TCP structure and this effect was more prononced for zinc. Thus, zinc-modified β-TCP powders consisting of idiomorphic crystals were obtained through sintering of Zn2+ ion substituted calcium phosphates precursors at 800–1000°C. The Mg2+ ion substitution leads to obtaining magnesium-modified β-TCP with spherical grains.

Physical and mechanical characteristics of cement mortars and concretes with addition of clinoptilolite from Beli Plast deposit (Bulgaria), silica fume and fly ash

Abstract Cement mortars and concretes incorporating clinoptilolite, silica fume and fly ash were investigated for changes in their physical and mechanical properties. It was found that additions of 10% clinoptilolite and 10% Pozzolite (1:1 mixture of silica fume and fly ash) were optimal for improvement of the quality of the hardened products, giving 8% and 13% increases in flexural and compressive strength respectively. The specific pore volume of the mortars incorporating zeolite decreased between the 28th and 180th day to levels below the values for the control composition due to the fact that clinoptilolite exhibits its pozzolanic activity later in the hydration. In these later stages, pores with radii below 500 nm increased at the expense of larger pores. The change in the pore-size distribution between the first and sixth months of hydration occurs mostly in the mortars with added zeolite.