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Inorganic phosphorous based materials, produced at low temperature in an acid environment : mechanism, kinetics, molecular structure and thermal properties

maandag, 15 mei, 2006 - 17:30
Campus: Brussels Humanities, Sciences & Engineering campus
D
2.01
Gunther Mosselmans
doctoraatsverdediging

Phosphate materials, contrary to silicate materials, held long time a minor position in
the history of inorganic materials. These phosphate materials nowadays have many
applications in industry and everyday lifetime not only as fertilizers, but also as cements
for dental applications, as bioactive ceramic implant materials, as architectural and
construction products, … Different ways of producing these materials are possible:
sintering at about 1350°C or precipitation out of an aqueous solution.

Since this study is situated in a large research program in which an inorganic phosphate
cement (IPC) is studied that can be reinforced with E-glass fibres, the cold-setting
method is looked at in this work on a model system for this (also commercially
available) material: the heterogeneous reaction between an aqueous phosphoric acid
solution (which is highly concentrated in comparison to other similar studies) and a
wollastonite solid (CaSiO3, since this is the calcium source in the commercially
available material) is studied in this work. The reaction products depend on the molar
P/Ca ratio (r). For r below 1, only amorphous silica and brushite are formed. If r is
above 1 also monetite and calcium dihydrogenphosphate mono hydrate are formed.
Only r < 1 is studied in more detail in this work, looking at practical applications of this
material.

The influences of many different parameters on the reaction kinetics are examined: the
reaction temperature (T), the amount of water (the molar H2O/P ratio = y), the amount
of wollastonite (r), the amount of phosphoric acid (combined variation of y and r) and
the amount of calcium initially present in the aqueous phosphoric acid solution (molar
Ca/P ratio in the aqueous phosphoric solution = c). It is concluded that although these
influences have different impacts on the kinetics, the reaction mechanism is not
influenced.

During reaction, calcium is extracted out of the wollastonite sample (incongruent
dissolution). This extraction is combined with the removal of hydrogen atoms out of
the aqueous phase in order to compensate the negative charges introduced. As a result,
the pH of the reaction mixture increases. Experiments showed that the dimensions of
the initial wollastonite grains did not change much during reaction. The extraction of
calcium occurs following the shrinking core model.

In the aqueous phase, the extracted calcium is accumulated as Ca2+ and under the form
of ion pairs. In the beginning of the reaction, the increase of the calcium concentration
is quasi-linear. Suddenly, an increase of the calcium extraction is observed. This phenomenon is explained in literature by crazing and the formation of pores, but in this
study it seems also to be pH dependent. From that moment on, also a small silicon
concentration is measured in the aqueous phase. Shortly after this increased calcium
extraction rate, an amorphous calcium phosphate material (monetite-like) precipitates.
For longer reaction times, crystallisation into brushite occurs.

By means of MTDSC, the heat flow and the heat capacity signal were obtained as a
function of the reaction time. In the heat flow, after an induction period two exothermal
phenomena are observed. The complex shape of the exotherm can be caused by a
combination of different successive reaction steps and/or by the specific heterogeneous
reaction conditions. At the start of the reaction, the Cp increases due to the formation of
intermediates and/or end- products with a higher Cp value than the initial components.
At the start of setting, the Cp signal levels off. At higher conversion (about 50 %), the
Cp-signal decreases stepwise to a value lower than the initial Cp. This drop in Cp is
due to the transformation of a viscous reaction suspension into a solid material and
coincides with the minimum between the two maxima in the heat flow signal.

A simulation for the reactions in the liquid phase has been proposed. Notwithstanding
the assumptions, a good agreement between experiment and simulation is found for the
best fitting model (elemental analysis) or the heat conversion model (DSC). On the
contrary, the extraction of calcium out of the wollastonite particles could so far not be
modelled based on a diffusion or a reaction controlled mechanism.

The thermal stability of the reaction products (for r below 1) is checked by different
techniques. Different transitions could be observed: the dehydration of brushite into
monetite (about 130°C and 205°C), the dehydration of monetite into calcium
pyrophosphate (Ca2P2O7) (about 450°C), a possible (re)crystallisation (about 520°C),
and the softening of the material (about 650°C). Since the brushite phase is
thermodynamically not the most stable phase, transformation into octa calcium
phosphate is observed due to ageing during storage. This phenomenon happens faster if
more residual calcium (not incorporated in the brushite phase) is present in the reacted
material.

Different additional species are added to the reaction mixture in order to influence the
reaction kinetics. When borax (Na2B4O7.10H2O) is added to the reaction mixture only a
delay of the reactions takes place. Contrary, when zinc oxide (ZnO) or aluminium
hydroxide (Al(OH)3) are used, besides the influence on the reaction kinetics also the
reaction products are influenced: formation of respectively parascholzite
(CaZn2(PO4)2.2H2O) and aluminosilicates.