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Synthetic zeolites have shown great potential for a number of applications in various fields such as adsorption, separation and ion exchange. They have unique structural properties such as uniform pores, high surface area, high ion exchange capacity and high thermal stability [1]. Synthetic NaX zeolite is composed of eight sodalite cages joined through six-membered oxygen rings in a tetrahedral arrangement. It has also supercage accessible through twelve-membered oxygen rings as seen in Fig. 1 [2]. The negative charges of the AlO 4 units which built the framework are balanced by exchangeable cations. These cations depending heavily on the size, charge density and distribution of cations in the porous structure play a very important role in adsorption, dehydration and ion exchange properties. The water and other adsorbate molecules are loosely bound to these cations. The cations can be introduced into the supercages to affect acid-base properties of zeolite. CO 2 is a small, non-polar, weak acidic molecule and can be adsorbed on the surfaces having basic character. The basic properties can be increased with the aluminum content of the zeolite structure with decreasing cation electronegativity. Besides to the properties mentioned above, the quantitative structural properties of NaX zeolite based on thermal analysis (thermogravimetric analysis (TG), differential scanning calorimetry (DSC)) data are employed to gain quantitative information about the coordinated water in channels and cages of the zeolite [3]. Based on dehydration of zeolites, water is classified as loosely bound water, zeolitic water and crystal water [4]. The cations located in the cavities and the channel walls are coordinated with water molecules which give rise to dehydration behavior of zeolites. Dehydration behavior of NaX zeolite depends on the amount and type of exchangeable cation, the Al/Si ratio, the presence or absence of H 2 O, time and temperature [5]. Dehydration enthalpy change and thermal stability can be determined by using thermoanalytical techniques [6].The objective of this work is to determine the effect of cation (Li + , K + , Ca 2+ and Ce 3+ ) on the CO 2 adsorption and the dehydration behavior of the NaX zeolite. Analysis and Calorimetry, Vol. 94 (2008) It was observed that both dehydration and CO 2 adsorption properties are related to cation introduced into zeolite structure.

Abstract.The goal of the present work was to investigate the effects of several cement preparation parameters on setting and hardening reaction mechanisms and hydroxyapatite (HA) cement properties. A central composite experimental design (CCD) was conducted by choosing particle size, solid to liquid ratio, pH, seed concentration and buffer concentration as design parameters along with compressive strength and setting time being the responses. Tetracalcium phosphate (TTCP) powders were prepared by heat treatment of calcium and phosphate source mixtures in the 1200-1400 o C temperature range followed by quenching to room temperature in a dessicator. The second phase used in the formulations (brushite) was prepared by aqueous chemical methods. A series of HA pastes/cements were prepared by changing the above mentioned design parameters. Cements were characterized by a standardized setting time test, mechanical testing machine, SEM and XRD. HA cements with the desired properties can be formulated by using CCD in which the responses were expressed by a second order polynomial equation of the parameters. Compressive strengths for the majority of HA cements were determined to be in the 100-160 MPa range which is significantly higher than those reported in the literature. IntroductionCalcium phosphate cements (CPCs) have been increasingly used for bone repair and regeneration of defects since their discovery in 1983[1]. Moldable CPC paste self setting in vivo offer significant advantages in several clinical applications. CPCs may also have great potential in drug delivery. CPCs are obtained by mixing one or several calcium phosphate phases with an aqueous solution forming a paste which sets and then hardens in minutes. The nature of these setting and hardening reaction mechanisms are still under investigation. The dissolution rates of the components and the nucleation/growth rate of the cement phase have a determining role in establishing the cement properties. The commonly known most important parameters used for controlling the rate of these reactions are solid to liquid ratio, characteristics of the powder phases, pH and composition of the liquid phase, HA seed amount, particle sizes/ratios of the solid phases, use of inhibitors, etc. [2,3].The ability to control these reaction rates is essential in providing sufficient time to the surgeon, necessary viscosity and hardened strength for the success of the application. The main goal of a significantly large number of CPC formulations developed since their discovery is the control of the development of cement paste/structure/properties as a function of time. Various current and proposed CPC applications all have specific setting and hardened cement properties.The goal of the present work was to investigate the dependence of the cement setting time and hardened cement compressive strength on five important cement formulating parameters by using a five-factor central composite experimental design approach. The use of these generated equations relating the two important pr...