This work aims to study the effect of partial substitution of ordinary Portland cement (OPC) by rice husk ash (RHA) and metakaolin (MK) on the physico-chemical and mechanical properties of the hardened OPC-RHA-MK blended cement pastes. OPC was partially replaced by different ratios of MK (10, 15 and 20%) and a constant ratio of RHA (5%) and the resulted cement blends were hydrated in the paste form by using the water/cement ratios required for the standard water of consistency; the pastes thus obtained, were hydrated for 1, 3, 7, 28, and 90 days. At the end of hydration period, the cement pastes were tested for compressive strength, total porosity and hydration kinetic via determination of free lime contents.The phase composition of the formed hydration products was investigated using X-ray diffraction (XRD) and differential thermal analysis (DTA) techniques. It was found that, the substitution of ordinary Portland cement (OPC) by rice husk ash (5% RHA) enhances the physico-chemical and mechanical properties of the hardened blended cement pastes as compared with the neat OPC. The results of compressive strength indicated slightly higher values for the pastes made of OPC-RHA-MK blends containing 5% RHA blended with 10, 15 % MK. However, the blended cement paste derived from OPC-RHA-MK blend containing 5% RHA blended with 20% MK, for economic reasons, was taken as the most suitable mix containing both RHA and MK. The partial substitution of OPC by RHA and MK leads to higher porosity values with a consequent decrease in the compressive strength values especially during the early ages of hydration. It was found that, the increase of MK content in OPC-RHA-MK blended cement pastes resulted in an increase in water consistency and setting times. Lower values of free lime contents were obtained for OPC-RHA-MK blended cement pastes, with the formation of further additional amounts of CSH, as a result of the pozzolanic reaction.
The complexation of solid urea with (Co 2þ ), (Cu 2þ ) or (Ni 2þ ) ions has been studied by using dielectric spectroscopy technique over a wide frequency range at different temperatures. Samples' structure were investigated by XRD, FTIR and FT-Raman spectroscopy. FTIR and Raman analysis indicated that the urea coordinates with the metal atoms through the same oxygen-metal bond (O-M). Furthermore, XRD analysis showed that the samples have polycrystalline structure with single phase. The permittivity of complexes was found at much higher values than that of the ligand (pure urea), showing structure dependency. Furthermore, two relaxation peaks were observed in the dielectric loss spectra corresponding to the orientation polarization of urea molecule, and some parts of urea molecule may be NH 2 groups. Both relaxation peaks correspond to thermally activated because they were shifted to higher frequency with the temperature increase. The peak position for the low frequency relaxation peak was observed at the same relaxation time ( ¼ 7:6 Â 10 À4 s), corresponding to the coordination mode through the same O-M bond. Novelty of this work is the successful finding of the relationship between the crystalline phase, coordination mode and the dynamic molecular behavior of solid urea and its complexes using the dielectric spectroscopy technique.
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