Search

Thermite welding of Cu–Nb microcomposite wires was investigated. Suitable compositions of thermite material and slag were determined from the equation of the exothermic combustion synthesis reaction. The phase compositions of the thermite mixture and slag determined by X-ray diffraction analysis correspond to those assessed from the equation. According to non-destructive radiographic testing, the joint structure does not have welding defects. Microstructural examination of the joint cross-section with scanning electron microscopy showed that the Cu–Nb wire retained its shape and microstructure and only a thin surface layer of wire was melted during welding. The difference in electrical resistances of the conductor and welded joint was below 20 %. The thermite joint can withstand a maximum load equal to 62.5 % of the load-bearing capacity of microcomposite conductor.

AbstractDemand for plasticisers used in concrete industry is growing in Lithuania and other countries. The paper examines a plasticising admixture based on carboxylate and the influence of temperature on the properties of concrete mixtures. Two types of Portland cement – limestone CEM II/A – LL 42.5R and slag CEM II/A – S 42.5N – have been investigated. To determine the effect of plasticising admixtures on the rheological properties of a concrete mixture, the slump, spread and density of the mixture, along with the depth of cone penetration, were examined. Then, with reference to the obtained data, yield stress was calculated. The research results illustrate the effects specific to certain combinations of plasticising admixtures with different types of cement. A plasticised concrete mixture with limestone Portland cement retains its slump better and remains almost unchanged for 90 min. The slump of the plasticised concrete mixture with slug Portland cement, however, is decreasing gradually for the full period of two hours, from the moment the mixture is ready. At higher temperatures, plastification effect manifests itself in a less intensive manner. Concrete mixtures with slug Portland cement have a higher slump, spread and the depth of cone penetration. A 5 °C increase in temperature (from +15 °C to +20 °C) makes the slump decrease by 32–33% both in concrete mixtures with limestone Portland cement and in concrete mixtures with slug Portland cement.

After analysis of calorimetric tests results of the cement mixtures with PSw prepared at different temperatures and SEM, XRD, physical-mechanical properties results of cement stone hardened for 7 and 28 days, it is determined that PSw can be utilized/used for the preparation of cement mixtures by adding up to 5%. Depending on the environmental working conditions, the preparation of PSw can be selected. To slow down cement hydration processes, it is useful to use only dried PSw, which slows down the hydration of the cement due to the high content of cellulose contained in PSw. To accelerate cement hydration, it is expedient to use PSw which is burned at 700degC. Dried PSw performs an extended induction hydration period and significantly delays the second heat release time. After the addition of 5% dried PSw, the phase III effect time compared to the control sample is 1.8 h, and after 10% addition, it is extended to 4.4 h. After the addition of 5% burnt PSw, the phase III effect time compared to the control sample is hastened to 1.9 h, after inserting 10% - to 2.4 h. The use of PSw saves the environment, reduces the amount of cement in the mixture and improves the properties of cement materials. Using 5% PSw burned at 700degC instead of cement increases the compressive strength of the specimens, and the density as well as ultrasound pulse velocity values are slightly changed compared to the control sample. It is determined that burnt PSw significantly changes mineral composition and structure. It is found that the microstructure of samples without PSw and samples with dried 5% PSw is similar, crystals formed are visible. With a higher (10%) amount of dried PSw, the microstructure of the cement stone differs significantly from the control samples. Larger voids with plenty of etringite are also visible, as well as higher levels of calcite. The microstructure of specimens with burnt PSw is significantly denser. XRD studies show that with a higher amount of PSw burned at 75degC, the main peak intensities of crystallohydrates ettringite and portlandite are lower, while the peak intensities of calcite are higher compared to samples without PSw. By increasing the amount of dried PSw in mixtures and reducing the amount of cement, the peak intensities corresponding to CSH and CASH are lower compared to those of the control samples. Using burnt PSw also reduces the peak intensities of ettringite, portlandite, CSH and belite, but significantly increases peak intensities of calcite and CASH.