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Flow behavior of fresh very high strength portland cement pastes prepared at various water/cement ratios was studied by using a rate-controlled coaxial cylinder viscometer (Rotovisko-Haake 20, system M5-osc., measuring device MV2P with serrated surfaces). The tests were performed under continuous flow conditions. Experimental shear stress and shear rate data were fitted very satisfactorily with various models, one of which has already been proposed by the authors. Excellent results were achieved also by applying the Quemada equation. In addition, the influence of two different commercial superplasticizing agents (Concretan RX and Ergomix 1000, the former based on polycyclic copolymers with modified structures, carrying hydroxylated side chains, and the latter on a modified polyacrylic resin) was studied with the aim of determining their optimum dosage and verifying the effectiveness by comparing rheological results with those obtained in a previous work on an ordinary portland cement. The use of superplasticizers modified rheological behavior of the pastes; however, no value of optimum dosage was found for any additive, but only a superplasticizer concentration range within which pastes presented very low values of viscosity. Moreover, both superplasticizers showed a greater effectiveness when added to HSPC pastes rather then the OPC ones formulated with the same water/cement ratio. [ABSTRACT FROM AUTHOR]

A method for the determination of cadmium and lead in vegetables by stripping chronopotentiometric analysis, after digestion of the sample with concentrated sulphuric acid and dry-ashing, is described. Metal ions were concentrated as their amalgams on a glassy carbon-working electrode previously coated with a thin mercury film and then stripped by a suitable oxidant. Potential and time data were digitally derived and E was plotted versus dt/dE-1, thus increasing both the sensitivity of the method and the resolution of the analysis. Quantitative analysis was carried out by the method of standard addition; good linearity was obtained in the range of examined concentrations, as was shown by the determination coefficients, which were 0.998 (n = 4) for cadmium and 0.993 (n = 4) for lead. Recoveries of 85-100% for cadmium and of 84-97% for lead were obtained from a sample spiked at different levels. Accuracy was demonstrated by analysis of a matching reference sample of cabbage. The detection limits were 1.8 ng g-1 of wet mass for cadmium and 5.1 ng g-1 of wet mass for lead. The relative standard deviations (mean of nine determinations), evaluated on a real sample, were 6.7 and 6.2%, respectively. Results obtained on 10 different commercial samples of pepper (Capsicum annuum), and egg plant (Solanum melongena) were not significantly different from those obtained by graphite furnace atomic absorption spectrophotometry technique. The average content was in the range 3.1-18.6 ng g-1 for cadmium and 38.2-64.3 ng g-1 for lead. [ABSTRACT FROM AUTHOR]