Sivkov S., Rugitskaya A.,Hardy H., Glushkov A., Firsayev D. «Properties of cement-polymer compositions hardened in extreme conditions»

The strength and stress-strain properties of cementpolymer
compositions for exterior insulation and finishing
systems hardened under extreme conditions,
such as lowered humidity or freezing have been investigated.
It has been established that fast loss of moisture by
a material at the initial stages of hydration did not worsen
final properties of material, if the material is repeatedly
humidified. The freezing of fresh mortar and its subsequent
thawing considerably decreases the strength of
material. The addition of coalescent and antifreeze agents
preventing formation of ice crystals in hardening material
is offered to improve properties of cement-polymer
compositions exposed to risk of shortterm freezing.

Cement-polymer compositions are widely used as
a basic plaster in systems of buildings’ external heat-insulation.
Such compositions are applied onto a surface of
heat-insulating plates in a form of a 5-8 mm thick layer.

When plaster works are carried out during warm
season, especially on facades exposed to sunlight, the temperature
of fresh basic plaster layer may reach 50-70°С.
Taking into consideration a large open surface of plaster
layer, such temperature contributes to intensive water
evaporation and material drying. At that, hardening of
Portland cement containing in a composition stops. Under
repeated humidifying, a process of cement hydration
resumes, however final properties of hardened material
may differ from those of a material that was hardened
under normal conditions.

When plaster works are carried out at temperatures
below 0°С, water containing in fresh cement-polymer
composition may freeze. It leads to hindering a process
of cement hydration and formation of polymer film in a
hardening material, and therefore, to changes in properties
of hardened plaster coating.

Thus, hardening of cement-polymer compositions
under extreme conditions may cause changes in a structure
and properties of the material, and this provides sufficient influence onto life time of external heat insulation
systems of buildings and constructions.

There has been investigated influence of extreme
hardening conditions, such as drying or freezing, onto
strength and stress-strain properties of cement-polymer
compositions in the present work.

Subject of research was a cement-polymer composition
with the following percentage of components, by
mass %:

ƒƒ* Portland cement PC 500-DO – 30
ƒƒ* quartz sand, Мк = 1,98 – 64,8
ƒƒ* redispersion polymer powder – 5
ƒƒ* cellulose esterWALOCEL MW 40000 PFV – 0,2

Test beams with dimensions of 280x45x5 mm, according
to the standard EN 12002, were taken for investigation
of the material properties.

The samples were hardened under three procedure
modes:

Procedure mode1 – controlling – 7 days at temperature
T=23°С and relative humidity of ϕ=50%;

Procedure mode 2 – drying – immediately after
forming the samples – 1 day at temperature T=40°С, then
humidifying by water spraying during 5 minutes, exposing
during 1 hour at Т=23°С and relative humidity of
ϕ=99,8%, after that – 6 days at temperature T=23°С and
relative humidity of ϕ=50 %

Procedure mode 3 – freezing – immediately after
forming the samples – 1 day at temperature T = -10°С
and relative humidity of ϕ=80%, then 6 days at T = 23 оС
and relative humidity of ϕ=50%.

Investigation of properties of cement-polymer compositions
was carried out in accordance with requirements
of the standard EN 12002 with the use of testing stand Z
010 Allround series by company Zwick Roell AG.

Polymers Vinnapas, main properties of which are
shown in the Table 1, were taken as redispersion polymer
powders.

Pic. 1 shows specific deformation curves of a cement-
polymer composition containing polymer 7210 N hardened under various procedure modes. Pic. 2 and 3
show a change in maximum breaking load and maximum
breaking strain of material, respectively

Table 1. Properties of polymers

Pic. 1 Deformation curves for cement-polymer composition containing polymer 7210 N, hardened under different conditions

As can be seen from obtained results, properties of
cement-polymer compositions change considerably when
hardened under extreme conditions.

Hardening under procedure mode 2 with drying the
samples after their forming helps to formation of strong
polymer films in material at early stages of hardening. At
further humidifying of the material and it hardening under
normal conditions, hydration and hardening of Portland
cement continue. As a result it leads to formation of a
strong structure of composite material with good stressstrain
properties.

A load that breaks hardened material, increases with
a growth of rigidity of the forming polymer film. The
maximum value of load is reached for a composition with polymer 7210 N. Increase of strength of a dried composition
with the use of polymer 8031 Н with hydrophobic
properties is relatively small and comparable to the values
typical for compositions with highly elastic polymer
LL 4040 N. The value of ultimate strain of the material
slightly decreases with a growth of rigidity of forming
polymer film. However, deformability of a dried composition
is no worse than that of a material hardened under
normal conditions. Only for a composition containing
polymer with hydrophobic properties 8031 Н, deformability
is reduced by 20-25% compared to that for a composition
hardened under normal conditions.

Hardening of cement-polymer compositions under
procedure mode 3 with freezing-up the samples, immediately
after their forming with consequent hardening
under normal conditions, causes a sharp decrease in the
material strength and increase in deformability. Breaking
load goes down by 45-70% depending on a type of the
polymer used. Minimal drop in strength of the composition
is observed when the most rigid polymer 7210 N, or
a polymer with hydrophobic properties 8031 Н is used
(approximately by 40-46%). At that considerable (by
1,5-4 times) increase of ultimate strain values of hardened
cement-polymer compositions is observed. Plus to
this, characteristics of material breaking changes. There
is practically no a section corresponding to elastic strain
on a load – deformation chart, while there appears a horizontal
section typical for the state of viscous flow for a
wide range of strain values.

Such characteristics of breaking shows that a contribution
of cement into strength properties of a final
composite material is minimal. It is probable that crystallohydrates
that are formed in a process of cement hydration
are separated with a relatively thick non-tight
layer of polymer and, therefore, they can not create a
tight structure typical for a cement brick hardened under
normal conditions. Investigation of cement hydration
rate in a cement-polymer composition that was frozen at
early hardening stages, show that the rate of cement hydration
in such composition is lower compared to check
samples. Still, this rate is sufficient to ensure an acceptable
strength of the material.

Such behavior of the material can be partly explained
by formation of less tight, thick, less rigid and
strong polymer films in a structure of the composite material.
Actually, the temperature of initial storage of the
samples is lower that the minimal temperature of film
forming for polymers 5044 N, 7210 N and 8031 H. At such
temperature, a dense and thick layer of polymer globules
is formed on particles of cement as a result of adsorption
and coalescence, while polymer film is not forming. When
a polymer is kept in alkaline medium of hardening cement
for a long time, partial hydrolysis of polyvinyl acetate
and formation of polymer cross-links inside globules
become possible. Cross-links between globules separated
by layers of ice are also possible. When the temperature
increases again, a process of film forming resumes, however,
partially linked polymer forms nontight, porous
over-thick film that has low affinity and adhesion to crystallohydrates of cement brick. As a result, strength of
composite cement-polymer material becomes low.

Nevertheless, this hypothesis can not provide a full
explanation of changing properties of the composite material,
when a highly elastic LL 4040 N with minimal temperature
of film forming is used as a redispersion polymer
powder.

An obvious way to improve properties of cementpolymer
compositions that undergo freezing after being
applied onto a heat insulation plate, is to introduce additives
reducing the minimal temperature of film forming,
and also anti-freezing additives that would prevent ice
formation in the structure of hardening material.

Sodium acetate CH3COONa and potash К2СО3 were
used as anti-freezing additives in the present investigation.
Additives were introduced into the cement-polymer
composition in quantity corresponding to 1% of cement
weight.

It is found that the use of anti-freezing additives
enables to improve strength properties of hardened material.
Maximal breaking load increases by 35-68% compared
with a sample containing no additives. At the same
time, this load remains lower that for samples that were
hardened under normal conditions. Composite materials
containing a rigid polymer 7210 N or a polymer with hydrophobic
effect 8031 H showed the best strength properties.
Ultimate strain values for breaking the material also
go down, which points onto formation of more compact
and strong polymer film in the structure of the material.

Based on results of the present investigation, the following
practical recommendations can be made:

ƒƒ* Fast removal of moisture from fresh-applied cement-
polymer mortar does not deteriorate strength
and stress-strain properties of the hardened material,
provided that its sufficient humidifying and
hardening under normal conditions is ensured afterwards.
It is necessary to use rigid polymers to ensure
best strength and stress-strain properties of the material.

ƒƒ* Freezing of fresh-applied cement-polymer mortar
leads to drastic deterioration of its strength properties
during its hardening after defrosting. To increase
the strength of hardened material it is recommended
to introduce additives of salts decreasing the water
freeze point and accelerating hydration of cement,
for example, sodium or calcium acetate. The use of
coalescent additives does not provide a considerable
effect because of formation of more elastic film with
low reinforcing effect.

The use of complex additives including cellulose,
polymer or glass-fiber, which would probably improve
properties of cement-polymer compositions hardening
under conditions of short-term freezing or under conditions
of high moisture content, will be investigated in the
following research works.


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