Biomedical Engineering Reference
In-Depth Information
5.2CaHPO
·2H
O + 3.6CaCO
Æ
4
2
3
O
(5.8)
The initial chemical reaction (5.7) was very fast and provoked
DCPD formation and setting of the cement pastes within seconds.
The second step was slower: DCPD reacted completely within
several hours with remaining α-Ca
Ca
(HPO
)
(PO
)
(CO
)
(OH)
+ 2.9CO
+ 12H
8.8
4
0.7
4
4.5
3
0.7
1.3
2
2
forming
carbonatapatite according to equation (5.8). The latter step caused
the cement hardening. A similar two-step hardening mechanism was
established for a cement consisting of MCPM and CaO: in the first
step, during the mixing time, MCPM reacted with CaO immediately
to give DCPD, which, in the second step, reacted more slowly with
the remaining CaO to give CDHA [58].
In addition, the setting mechanism of an apatite cement was
investigated in details for a three-component mixture of TTCP, β-TCP,
and MCPM dry powders in convenient proportions and with the
overall atomic Ca/P ratio equal to 1.67. Two liquid phases in a raw
were used to damp the cement powder, initially it was water + ethanol
(ethanol was added to slow down the hardening) and afterwards
orthophosphoric acid and sodium glycerophosphate were added to
water to prepare a reactive liquid [135]. At the very beginning, DCPD
was found to form according to two chemical reactions:
(PO
)
and CaCO
3
4
2
3
Ca(H
PO
)
·H
O + β-Ca
(PO
)
+ 7H
O
Æ
4CaHPO
·2H
O (5.9)
2
4
2
2
3
4
2
2
4
2
Æ
O (5.10)
The formation reactions of DCPD were fast and corresponded
to the setting stage. Afterwards, TTCP reacted with the previously
formed DCPD and with β-TCP to give CDHA according to the
reactions:
Ca
(PO
)
O + 2H
PO
+ 7H
O
4CaHPO
·2H
4
4
2
3
4
2
4
2
Æ
2Ca
(PO
)
O + 2CaHPO
·2H
O
4
4
2
4
2
Ca
(HPO
)
(PO
)
(OH)
+
x
Ca(OH)
+ (4−
x
)H
O (5.11)
10−
x
4
x
4
6−
x
2-
x
2
2
2Ca
(PO
)
O + 4β-Ca
(PO
)
+ (2+2
x
)H
O
Æ
4
4
2
3
4
2
2
(5.12)
The formation reactions of the CDHA phase were quite slow
and corresponded to the hardening stage. Although OCP was not
detected in that study, its formation as an intermediate phase was
2Ca
(HPO
)
(PO
)
(OH)
+ 2
x
Ca(OH)
10−
x
4
x
4
6−
x
2−
x
2
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