الفهرس 
Characterization and analysis of calcium aluminate cementOnly 14 pages are availabe for public view
 |  | from | 160 |  |  |
| | | from | 160 | | |
Abstract
Seven batches of calcium aluminate cement powder containing different ratios of aluminum oxide and calcium oxide were prepared by thermal decomposition method. Hydrated aluminium nitrate Al(NO3)3.9H2O and hydrated calcium nitrate Ca(NO3)2.4H2O were used as starting materials for the preparation of calcium aluminate batches. Aluminium nitrate and calcium nitrate masses were mixed with about 50 ml of water. The paste was heated on a sand bath up to 300°C. After evaporation of all water and with continuous heating, brown fumes of nitrogen oxides were detected. The samples were left until all fumes disappeared and the mixture solidified again. A tough white solid was formed at the end of the heating process. The produced solids were fired at 500°C and at 950°C for 1 hour each. After the firing process, all solids were grinded and mixed with adequate amount of water to study the hydration process as well as pressed into tablets to study the sinterability and other properties. XRD patterns showed that, mono calcium aluminate phase (CaO. Al2O3) is the major constituent of the prepared calcium aluminate binary system but its quantity differs according to their aluminium oxide to calcium oxide molar ratios. Also it has been indicated that do-deca-calcium hepta aluminate (C12A7) is formed in all batches. For batches containing lower CaO/ Al2O3 ratio, i.e., 4, 5, 6 and 7, C5A3 (5CaO. 3Al2O3 ) phase is formed. Also SEM photomicrographs showed that, small crystals in the nano range are formed with non regular shapes. Also there is a large particle size distribution which indicates the formation of heterogeneous materials.
XRD & SEM of hydrated calcium aluminate cement batches showed that, CAH10 and C2AH8 were the main hydration product of CA, CA2 and C12A7 and they play the bonding role in such materials. The higher C/A ratio of C12A7 favours the formation of C2AH8 with very little amounts of CAH10. Increasing the time of hydration, all phases are converted into C3AH6. Alumina gel (AH3) was generally observed as structureless grains. The mechanical properties in terms of cold crushing of the hydrated samples indicated that, the compressive strength was different for calcium aluminate batches due to the variation of CaO/Al2O3 molar ratio which consequently affects the type and amount of the formed phases. The hydration of batches containing high CaO/ Al2O3 ratio i.e. ,batches No. 1, 2 &3 showed higher strength than batches containing low CaO/ Al2O3 ratios due to the presence of CA and C12A7 as a major component, since they react rapidly with water.
The produced powders were pressed at 50 MPa into pellets (2.5cm diameter and 0.5cm height) and then sintered in an electrical furnace between 1350°C and 1550°C. Bulk density and apparent porosity were measured by Archimeds technique. The mechanical properties in terms of cold crushing strength were also determined. XRD & SEM were used to identify the phase composition and microstructure of the sintered samples. It has been found that, CA is the major constituent in all batches in a varying percent according to aluminium oxide to calcium oxide ratios. Also small amounts of CA2 were detected in batches 4, 5, 6 and 7, while as small amounts of C12A7 were formed in batches 1, 2 and 3. The amount of those phases affects the properties of the sintered calcium aluminate. Also, the first three batches exhibited higher bulk density as compared with the other batches. This is due to the variation of CaO/Al2O3 molar ratios and consequently formation of low melting point phase (C12A7). This phase tends to lower the apparent porosity. After the sintering process, the obtained nano powder was converted into micro-sized grains due to the gain growth developed by heat treatment.
The bioactivities of calcium aluminate cement batches as well as the improvement in the bioactivity due to the addition of lithium fluoride and maleic acid to calcium aluminate powders in different amounts were studied. This was estimated by examining the hydroxyapatite formation on their surfaces in simulated body fluid (SBF). Calcium aluminate powders were dry mixed with lithium fluoride and maleic acid. The pellets were cleaned with ethyl alcohol, dried and immersed in an enough amount of the SBF solution for 30 days at 37°C. After that, the specimens were removed from the SBF solution, gently rinsed with distilled water and dried at 60°C for 24 hours. The prepared cement specimens were analyzed by the Scanning Electron Microscope to prove the formation of hydroxyapatite layer on their surface. Also the remaining SBF was analyzed by X-ray fluorescence spectrometer to follow up the ion concentration changes after immersion. The samples containing both additives were pressed into tablets at 50 MPa in order to study the compressive strength by means of cold crushing at room temperature. It is concluded that, lithium fluoride has an important role in the formation rate of hydroxylapatite phase on the surface of calcium aluminate batches. Also it was found that the presence of LiF accelerated the hydroxyapatite formation whereas maleic acid had little effect on the phase formation. In addition, lithium has high ionization potential (5.39 eV) which makes it highly reactive and electropositive in chemical reactions. Thus, the addition of LiF to calcium aluminate cement leads to degradation of CA structure because it dissolves more quickly in the SBF solution. Lithium ion reacts with Al ion dissolved from the calcium aluminate into SBF solution to form LiAl2(OH)7.2H2O which includes OH groups within a short time. Although a large amount of calcium in calcium aluminate enhances the ability of hydroxyapatite formation, it takes a long time to release Ca into SBF solution. The presence of LiF enhances the release of Ca and increase the dissolution rate of calcium. In contrast, maleic acid retarded the formation of hydroxyapatite because an acid-base reaction takes place between a proton donated liquid (maleic acid) and a base (CA). A salt hydrogel is formed which binds the unreacted powder particles together into a paste mass. This salt prohibits calcium and aluminium ions to be released into the SBF solution. SEM of soaked samples and XRF analysis of the remained SBF showed that, series 2 which contains 0.075g of lithium fluoride and 1.3125g of maleic acid per 15g of calcium aluminate and series 4 which contains 0.15g of lithium fluoride and 1.3125g of maleic acid per 15g of calcium aluminate gave the best results and the surface of the cement samples was covered by the rounded shape hydroxyapatite particles. However, they still need more time for complete coverage of CAC surface by hydroxyapatite. It has been found that, batch 3 gave better results than the other batches because it contains the highest amount of CA. Also the results of compressive strength showed that, the presence of maleic acid improved the compressive strength of CAC. The mechanism of hydroxyapatite formation on calcium aluminate cement with lithium fluoride and maliec acid in the SBF could be explained as follows: first, the Li+ ion is released from the substrate into the SBF, and then the Ca2+ and Al3+ ions were released through an ion exchange with H3O+ in the fluid to form LiAl2(OH)7.2H2O and 3CaO.Al2O3.6H2O. Then these compounds were incorporated with Ca2+ and P5+ ions in the SBF to form hydroxyapatite nuclei.