Biomedical Engineering Reference
In-Depth Information
5.4
Various Properties
5.4.1
Setting and Hardening
Generally, self-setting calcium orthophosphate formulations must
set slowly enough to provide sufficient time to a surgeon to perform
implantation but fast enough to prevent delaying the operation.
Ideally, good mechanical properties should be reached within
minutes after initial setting. Two main experimental approaches
are used to study the cement setting process: a batch approach and
a continuous approach. In the batch approach, the setting reaction
is stopped at various times and the resulting samples are analyzed
to determine, e.g.,
the composition and compressive strength of the
samples [207, 208]. There are currently two standardized methods to
apply this approach, namely, Gillmore needles method (ASTM C266-
89) [239] and Vicat needle method (ASTM C191-92) [240]. The idea
of both methods is to examine visually the surface of cement samples
to decide whether the cement has already set, i.e. if no mark can be
seen on the surface after indentation. Besides, the setting process
might be monitored in real time by non-destructive methods (the
continuous approach), e.g.,
pulse-echo ultrasound technique [241,
242], isothermal differential scanning calorimetry [174, 175, 243-
248] and alternating current (AC) impedance spectroscopy [249].
For example, calorimetry measurements suggested that in Eq. (5.2)
the endothermic MCPM dissolution and the highly exothermic β-TCP
dissolution occurred simultaneously, followed by the exothermic
crystallization of DCPD [247]. Moreover, acid-base reactions (5.1-
5.3) can be and have been analyzed by measuring the pH evolution
of a diluted cement paste [207]. Finally yet importantly, methods of
Fourier-transform infrared spectroscopy [24-26, 248, 250], solid
state NMR [221], X-ray diffraction [24, 26, 50, 156, 251], and energy
dispersive X-ray diffraction [24-26, 252, 253] might be applied as
well. The latter techniques proved to be powerful even though they
have limitations such as the time required for each measurement
(250 s for an X-ray diffraction scan is a problem for fast setting
reactions); besides the analysis is located at the surface of the sample
where evaporation and thermal effects can modify the reaction
rate of the surface compared to that of the bulk. Furthermore, the
continuous approach is an indirect one, which markedly complicates
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