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Optimization of properties of inorganic phosphate cement (IPC) for construction and high-temperature applications

vrijdag, 2 juni, 2006 - 17:30
Campus: Brussels Humanities, Sciences & Engineering campus
K
auditorium P. Janssens
Mazen Alshaaer
doctoraatsverdediging

Inorganic Phosphate Cement (IPC) is a new cementitious material, developed at the Vrije
Universiteit Brussel (VUB), which sets at room temperature. Due to its non-alkaline
environment during and after setting and hardening, IPC can be combined with glass
fibre reinforcement. Such a textile reinforced cementitious is an interesting material in
those applications where high load-bearing capacity, good temperature resistance and
lightweight construction are demanded.

Since IPC is a relatively new cement-based material, this work focuses on engineering
and optimizing the material for a field which has not been thoroughly studied yet.
Although the behaviour of textile (glass-fibre) reinforced IPC under mechanical load has
been subject of study for many years, the chemical, physical and physicochemical
mechanisms and subsequent macro-mechanical behaviour under thermal loading hasn’t
been studied systematically yet. Moreover, the possible rearrangements within the
microstructure and mineral composition of IPC as a function of time under ambient
conditions (ageing) are still not documented in detail. This thesis focuses on the IPC’s
macro properties, its microstructure and its chemical phases. In correlating these levels,
we aim to establish a sturdy base for further research.

The study is conducted in three main stages. In the first stage the thermal and long-term
stability of so-called reference IPC (post cured at 60 °C) are investigated. This stage
characterizes the macro properties of the reference IPC as a function of temperature and
time and defines which evolutions are considered to be undesirable. It was found that
glass fibre reinforced IPC tends to show cracking when kept on the shelf, indicating that
even at ambient conditions restrained shrinkage of the IPC matrix can lead to the
introduction of internal tensile stresses in the IPC, high enough to introduce cracking.

In the second stage, chemical and micro structural changes are identified along with
changes in macro properties, again as functions of time and temperature. It is found that
during ageing of IPC at ambient conditions the meta-stable calcium phosphate phases
show considerable dehydration during several months, even after the substance has set.
These ongoing chemical transformations over a long time contribute significantly to the
chemical instability of the material. Additionally, it is observed that calcium phosphate
transformations are one of the main factors behind thermally induced chemical shrinkage
when heating up from 105 °C to ~ 515 °C. Transformations of the meta-stable calcium
phosphate phases (e.g. brushite) to more stable calcium phosphate phases (e.g. monetite)
are accompanied with considerable shrinkage. The advantage of a heat treatment – which
generally speaking transfers ‘brushite-based’ IPC into more stable ‘monetite-based’ IPC
– is that monetite-based IPC shows excellent chemical stability under long-term ambient
conditions. The drawback of the heat treatment is found in the high shrinkage, which
accompanies the treatment. This unwanted effect is due to the fact that the abovementioned
transformation leads to an increase of the skeletal density.

Building on the work done in the first two stages, a new technique is developed in the
third stage to optimize the properties of the material. A hydrothermal post curing (HTPC)
technique is developed in the last stage of this thesis to overcome the above-mentioned
challenges: long-term evolutions, cracking and shrinkage. HTPC is based on thermally
induced transformation of the unstable calcium phosphates phases into more stable
phases (as discussed in the second stage) with prevention or minimization of the bulk
shrinkage during the treatment. To achieve this effect, sufficient pore moisture is kept in
IPC during the transformation stage. This way, the increase of skeletal density thus not
necessarily lead to global bulk shrinkage, since the contraction of the skeleton is replaced
internally by an increase of the pore size. This effect can be obtained by post-curing in
the autoclave, when a sufficient high temperature is used to provoke the necessary
chemical transformation, combined with a pressure high enough to force the pore
moisture and the released bound water to stay inside the pores. After evaluation of this
technique, it was noticed that the monetite percentage in the Hydrothermally post cure
IPC (HTTP-IPC) products increases from 26 % to 39%, compared to the reference IPC. It
is also verified that additional pore volume replaced the bulk shrinkage.

Compared to reference IPC, monetite-based IPC exhibits a good chemical stability over
time, as a function of temperature and under various pH levels. When for example
heating up to 390 °C, HTPC-IPC preserves its initial chemical phases, i.e. monetite. In
addition, HTPC-IPC retains its mechanical properties under prolonged exposure to high
temperature. As a result of the HTPC very low bulk shrinkage is recorded. For example,
after heating at 105 °C, specimens with hydrothermal post curing show a shrinkage that is
10 times lower than that of the reference specimens. HTPC-IPC does not exhibit
cracking, even by heating up to the glass transition temperature (~ 700 °C), while
reference IPC exhibits cracking in temperature range: 40 °C - 105 °C. One drawback of
HTPC is found in the fact that it causes a high decrement (about 60 %) in compression
strength, compared with reference IPC, mainly due to the additional porosity introduced
by HTPC. One can however overcome this drawback by using functional fillers in
combination with HTPC. For illustration purposes, the introduction of bauxite is
combined with HTPC on IPC specimens. This treatment leaded to an end product, which
could retain most of its compression strength after HTPC and still retains excellent
stability under temperature loading and as a function of time.

The effects of HTPC are also tested within the scope of producing composite materials:
E-glass fiber reinforced IPC laminates were produced and subjected to the HTPC
treatment. It is found that HTPC greatly improves the thermal and dimensional stability
of E-glass reinforced IPC matrix composites. These hydrothermal post cured composites
exhibit more stable stiffness with heating compared to similar reference composites.
Furthermore, HTPC-E - glass reinforced IPC matrix composites exhibit high cracking
resistance when heated up to 700 °C, on the other hand the reference E - glass reinforced
IPC matrix composites exhibit cracking at ambient conditions.

Generally, one can state that this work contributes towards the stabilization of IPC under
temperature load and decreases or even stops unwanted ageing phenomena. The
developed production technique is strongly based on the obtained knowledge of
chemical, physical and transformations within the IPC under various treatment conditions
and thus links basic material science with practical demands for industrial applications.