Study of mechanical properties of concrete with low concentration of magnetic nanoparticles

Concrete samples containing small amounts (0.6 wt%) of agglomerated magnetic nanoparticles of cobalt (Co/C) or iron carbide (Fe3C/C) coated with carbon have been synthesized. The time dependence of the compression, bending and contraction strength of samples with and without magnetic nanoparticles have been investigated. Twelve different samples have been prepared and tested during 28 days to detect variation in compression, bending and contraction properties. The temporal change of the bending strength for concrete with and without nanoparticles has showed measurable differences. During the initial stage (first 3 days) of hardening all samples with nanoparticles were less resistant to bending than samples without nanoparticles. After 1 week samples with iron carbide agglomerates and cobalt nanoparticles showed an increase in resistance to bending in comparison with undoped concrete, 3% and 6%, respectively. After 28 days in both cases the bending strength slowly decreased for samples containing magnetic nanoparticles. The compression strength after 28 days decreased by about 9% for samples containing cobalt and 12% with iron carbide. The contraction increased by about 38% for sample with cobalt ions, however no change was observed for samples containing iron carbide. The FMR investigation has shown that the resonance line has shifted more for samples with cobalt and it has been suggested that this process could be connected with contraction caused by decreasing temperature after freezing [P.C. Aitcin, Cement Concrete Res. 30 (2000) 1349]. This study can be very important for magnetic shielding effects and the selection of optimal conditions for building materials with magnetic nanoparticles. It can also be useful for new technological solutions aimed at increasing the functionality of these materials. © 2008 Elsevier B.V. All rights reserved

Magnetic properties of the micro-silica/cement matrix with carbon-coated cobalt nanoparticles and free radical DPPH

Samples of micro-silica/cement containing iron oxide, Fe2O3, and doped with carbon-coated cobalt nanoparticles and free radical DPPH were prepared and studied by the magnetic resonance method. The concrete's main components (silica and cement) produced very complicated FMR/EPR (ferromagnetic and electron paramagnetic resonance) spectra. The temperature dependence of the FMR/EPR spectra was recorded in the 90-300 K temperature range. The cement/micro-silica matrix produced a very broad FMR line originating from iron oxide particles and two EPR lines originating from iron(III) ions in the crystal field of low-symmetry (centered at geff ∼ 4.3) and from manganese(II) ions (geff ∼ 2) of hyperfine structure. Additionally, a very narrow line and a very broad EPR/FMR line were registered and, respectively, attributed to DPPH and cobalt nanoparticles. The isolated paramagnetic iron(III) and manganese(II) centers displayed increasing intensity of the EPR spectra with decreasing temperature, while no influence of the magnetic nanoparticles was observed. The intensity of the FMR spectrum of iron oxide decreased strongly and the resonance field was effectively shifted toward low magnetic fields with decreasing temperature. The observed FMR behavior is similar to what was registered for iron oxide magnetic nanoparticles. The introduction of magnetic nanoparticles influenced the EPR spectrum of the free radical DPPH significantly: its intensity decreased above 260 K and increased slightly below this temperature, while the resonance field changed with decreasing temperature. This behavior may be associated with the porous state of cement and/or the reaction of the multi-component magnetic system. The FMR/EPR method could be very useful for the characterization of matrices containing small amounts of magnetic nanoparticles. © 2008 Elsevier B.V. All rights reserved