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48
ZEB
annual report 2014
Innovation of building materials is a broad
field with many challenges to be addressed,
including both construction and thermal
insulation materials. Naturally, it would be
most ideal to reduce the disadvantages
of these two systems and one possible
pathway is to combine their crucial functions,
i.e. strength and thermal insulation, while
maintaining a minimal thickness of the
material. This can be achieved by the
formation of composite materials through
the incorporation of nanotechnology in
producing a slimmer wall, but yet maintaining
its structural strength and thermal insulating
property. Ideally, for such a composite material
to be feasible particularly as a standalone
structural concrete, it should possess a
strength of minimum 20 MPa while having a
thermal conductivity of 0.1 W/(mK) or below.
Commonly, the mechanical properties of
concrete are enhanced by a reduction in
water/cement ratio (w/c) or improvement in
packing densities. An optimized model of this
system is the ultrahigh performance concrete
(UHPC) where a mixture of coarse, fine and
micro fine aggregates, very low amounts
of water, silica fume and high amounts of
cement are utilized. Silica fume is an essential
ingredient of UHPC which increases packing
due to their nano size nature, but also further
enhances the bond between the cement
paste and aggregate particles through their
hydration reaction with cements. On the other
hand, insulation materials are commonly
employed to improve the thermal resistance of
composite materials.
Based on these reasons, this investigation
utilizes the UHPC model for the modification
of aerogel-incorporated mortars (AIM) to
improve the structural properties such as
compressive strength while maintaining the
thermally insulating properties achieved by
the aerogel incorporation. It was observed
that increased aerogel content reduced the
thermal conductivity of the mortar (Fig.1).
However, a corresponding drastic drop in the
mechanical strength of these AIM samples
was also observed due to the decrease in
packing effectiveness and effective binder.
The most significant drop in strength was
detected when 20 vol% aerogel was present
which may be attributed to the introduction of
micro ‘gaps’ in a well-defined packing matrix
(Fig.2), while deviation from the linear case
increased further with increasing aerogel
content as more gaps could form between the
aerogel granulates and the cement matrix.
The presence of ‘gaps’ can be attributed to
the low adhesion between the hydrophobic
aerogel granulates and the water rich mortar.
SLIM – STRONG – INSULATING CONCRETE?
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SLANK – STERK – ISOLERENDE BETONG?
Serina Ng (SINTEF) and Bjørn Petter Jelle (NTNU and SINTEF)
Achieving a thin and strong thermal
insulator may just prove to be no
longer a dream...