الفهرس 
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Abstract
In this work, some approaches for improvement of Portland cement as a matrix for immobilization of radioactive waste were conducted. Incorporation of waste materials or small amounts of prepared inexpensive nanomaterials was considered in this study as a viable strategy for cement reinforcement.
Iron slag as waste material and titanate nanofibers were introduced to cement paste separately or as a mixture to evaluate the mechanical, morphological and chemical properties of the solidified cement blend.
The experimental results showed that 35% water/cement was the proper ratio for higher compressive strength of solidified cement blocks without any additives.
Addition of titanate nanofibers attains two vital achievements:
(i) Chemical achievement by adsorption activity of more than 27% of 137Cs present as radioactive waste.
(ii) Mechanical and physical achievement by improving the mechanical integrity by 5.7% by addition of only 0.04% of titanate nanofibers.
Presence of 6% iron slag solely improved the physicomechanical properties (expressed as compressive strength) of cement by about 11.5%. However, the dual advantages of adsorptivity and integrity was required in order to prevent the release of immobilized radionuclides exposed to immersion.
A low amount of 0.04% titanate nanofibers and optimal ratio of 6% iron slag were mixed with cement to obtain a modified composite of compressive strength of 40 MPa and porosity of about 24%. Simultaneously, spectroscopic investigations with FT-IR spectroscopy, XRF and X-ray diffraction were conducted to explain the mechanical improvement achieved to blended cement by incorporation of titanate nanofibers and/or slag on a molecular basis.
Based on the obtained results, it could be deduced that the produced composite consisting of Portland cement, slag and negligible amount of titanate nanofibers is a promising material for rigid constructions and building applications, and for safe stabilization of radioactive wastes due to the economic and mechanical integrity of this material.
We evaluated the promising composite material by exposing it to challenging environmental conditions including freezing/thawing cycles and flooding in aggressive media.
We investigated the stability of composites of Portland cement and iron slag combined with small amount of titanate nanofibers under flooding in three types of aqueous media and during freezing/thawing cycles up to 180 days. Moreover, the chemical stability of the novel composite including dissolved radioactive 137Cs was evaluated by assessing different variables in studies lasting up to 180 days. Outcomes of this experimental approach can be summarized as follows:
1. Mechanical integrity has been improved by reducing the porosity; this was achieved by improving the Portland cement by incorporation of 6% iron slag and 0.04% titanate nanofibers.
2. Flooding tests have been performed by immersion of the hardened composite for 6 months in various media including three types of water and acidic and alkaline aqueous solutions. Compressive strength and spectroscopic analyses of the examined composites have confirmed the mechanical integrity and its durability under flooding conditions.
3. Freezing/thawing cycles only have a non-significant effect on the mechanical integrity of the solidified cementitious matrices, showing an only low decrease in compressive strength of less than 9%, and a small increase in porosity of about 15%. Thus, the produced compound achieved proper stability when exposed to frost attack during up to 90 cycles of freezing/thawing.
4. The chemical stability of the new composite material has been demonstrated based on the low release of 137Cs when varying various parameters such as temperature, type, amount and mobility of the leaching media.
5. Finally, the produced composite prepared from Portland cement, iron slag and small amounts of titanate nanofibers has subjected towards several mechanical, physical and chemical testing after drastic weathering events. Based on the elaborated results, the new composite formulation can be suggested as matrix both for safe immobilization of radioactive wastes and as a valuable material to be used in the construction and building industry.