The flow, setting, and aging characteristics of a newly developed calcium phosphate/calcium aluminate composite orthopaedic cement were studied. The effect of vibration on the flow of the cement paste was studied and found to greatly enhance placement. The setting times of this cement were dependent on temperature and decreased with increasing temperatures. At 37 degrees C, the working and setting times were 6.3 +/- 0.3 and 12.8 +/- 0.4 minutes, respectively. Hydration and conversion of the cement phases continued while specimens were stored under simulated, physiological conditions. A cumulative increase in mass of 8.23 +/- 0.65% was observed over a 14 month test period. During this time, the cement was found to expand slightly, 0.71 +/- 0.39%. X-ray diffraction was used to characterize the crystalline phases present during hydration and conversion. The calcium aluminate in the cement hydrated and formed calcium aluminate chloride hydrates, while no changes were observed in the beta-tricalcium phosphate during the testing period.
A temperature and stress cycling technique was developed to examine the effects of thermaVstrain history on creep of refractory concrete. Creep of a 90+ % AlzOJ refractory concrete and a high-purity calcium aluminate cement was investigated under stresses of 3.4 to 20.7 MPa at 500" to 1200°C. It was found that on initial heat-up of a cured specimen of either the concrete or cement, the AHc=130 to 170 kJ/mol. After subsequent cycling of the temperature, AH,=620 to 720 kJ/mol. The stress exponent for the initial application of stress was 2.5. It was determined from these and other results that multiple "deformation" processes are acting simultaneously on initial heat-up of the form:
i=cAif(Si)u"l expA€Zi/RTThese processes include crystallization, phase changes, sintering, stress-aided sintering, and steady-state creep. Mechanisms of creep and failure are discussed.
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