Dergunov S.A.,Rubtsova V.N.,Nesterenko A.S. «Research of plasticized systems»

Annotation

There are considered chemical bases of different
plasticizers and principles of their effect on mineral
compositions. Some factors are revealed, which define
the choice of a modifier. The article presents the comparative
analysis of the research results of the plasticized
additive influence on the mixture basic rheological
characteristics and physical-mechanical and
structure features of stone on its base. Also the article
presents the economic evaluation of the additives use
in dry building mixtures.

For dry construction mixture modification various
products of construction chemistry are applied. Complex
additives – pre-mixes, which are mixtures consisting
of several components different in their effect
range are used often. Such a complex allows to radically
change mixture and mortar properties, influencing
almost all the indexes. The essential part of each premix,
as well as the one most frequently used as an independent
modifier, is plasticized additive. Therefore
it is not surprising that it is plasticizing agent which occupies
the biggest share of manufacture and market of
additives for dry construction mixtures. In general, to
the experts’ opinion, the powder plasticizing additives’
market is at the stage of development yet [1].

At present in the Russian Federation the following
plasticizers are present, the market shares for which
are present in Pic. 1.

Pic. 1 Use of powdery plasticizers of various chemical bases in Russia

The main factors of additives’ being in demand in
the nearest future are as follows:

ƒƒ* construction growing up due to unsatisfied demand
for housing, targeted Federal development
programs with financing from state budget,
real estate mortgage introduction to provide
for housing demand paying capacity;

ƒƒ* intensification of additives’ application due to cement
deficiency, development of modern technologies
for monolith construction, tendency to
increase construction and iron-concrete products
manufacture effectiveness in order to maintain
defined development rate of the branch;

ƒƒ* increase in additives’ export potential on the account
of their growing use both in the CIS countries
and abroad [2].

Depending on chemical structure there are the
various kinds of super-plasticizers made on the following
bases: melamin- and naphthalene-formaldehyde,
lignosulphonate, carboxylate and ethylene glycol compounds
and complexes on their base (Pic. 2).

The first three kinds are known from the very
beginning of super-plasticizers commercial use, and
therefore they are frequently called traditional ones.
The action mechanism of such surface-active substances
is presented in a simple way in Pic. 3.

As far as super-plasticizers belong to surface-active
substances, their basic property is that molecules
of such substances are absorbed on the surface of cement
particles and new compounds being formed, so
that the thinnest mono- or bi-molecular layer appears,
and with that inter-phase cohesive energy decreases
and particles desaggregation is facilitated.

Simultaneously immobilized water is released
which plays a part of plasticizing lubricant. Apart from that the absorbed layer smoothes particles’ microroughness,
thus reducing friction coefficient between
particles. Finally, appearing of like electric charges as
a result of super-plasticizer’s absorption on solid phase
particles’ surface excludes the possibility of their adhesion
on the account of electro-static forces, and thus
decreases suspension viscosity. In the hydration process,
with new formed crystals growth, the repulsive action
of the like electric charge is gradually ceasing, and mortar
looses its mobility.

Pic. 2 General chemical bases of plasticizing additives a – sulpho-melamine-formaldehyde, b – sulpho-naphthalene-formaldehyde, c – lignosulphonat technical, d – polycarboxylate

Pic. 3 Action mechanism of traditional plasticizers

Contrary to traditional super-plasticizers, action
of carboxylates, due to applied polymers structure’s
peculiarities, is based in general on steric effect, on account
of which friction of mortar suspension components
is reduced (Pic. 4).

In many foreign countries such super-plasticizers
are called over super- or hyper-plasticizers. Additives
of this kind have been developed in 1990s and are quite
widely used in European countries and in Japan.

Application of various super-plasticizers in dry
construction mixtures compositions makes for mortars’
flow ability increase in comparison with control
non-modified composition. Different duration of
plasticizing effect, at other equal conditions, depends
first of all on molecules’ structure of chemical base
and action principles [3]. Therefore the right choice
of the plasticizing additive is very important. Other
factors defining modifier application’s efficiency,
not less important ones, are the following: composition
purpose, binder mineral composition, plasticizing
additive composition, modifier application peculiarities,
active substance absorption rate, additive
effectiveness, quality characteristics stability, technological
possibilities of mixture manufacture (first
of all, obtaining of homogeny compositions), availability
and interchangeability, cost-effectiveness of
application.

Pic. 4 Action mechanism of plasticizers on the base of carboxylate compounds

Reasonable choice of the additive is necessary to
make on the base of comparative analysis of results of
experimental researches complex. Therefore the main
target of the latter became studies of plasticized systems’
rheological and physical-mechanical properties.

As original components the following ones were
used: cement-sand mortar containing 25% of binder
(Portland cement of PTs 500-DO mark), 75% of filler
(quartz sand) and powdery plasticizers of various
chemical bases.

As far as the additive introduction is directly connected
with change in water-solid ratio, to achieve
the set purpose the mathematic planning method has
been applied with implementation of two-factor plan
of the experiment. As the varied factors the following
ones were used: Х1 – additive content (up to 1% of
binder’s weight) and Х2 – water-solid ratio (0,16–0,20).
Variations intervals were selected on the base of referring
to technical literature and recommendations [4].
There have been investigated rheological properties
of systems (flow ability, density, durability) as well as
physical-mechanical properties of stone based on the
systems (strength, density, porosity and porous space
homogeneity).

On the base of the mathematical processing of
experimental data the isolines of mixture and mortar
properties have been plotted according to changes in
modifier’s content and in water-solid ratio.

The analysis of mixture rheological properties
investigation’s results allowed to draw the following
generalizing conclusions:

ƒ* ƒ the development rate of plasticizing effect depends
on a product chemical base, i.e. fast (immediate)
plasticization is registered for carboxylate,
melamine formaldehyde and ethylene glycol bases, the majority of which belong to hyper-plasticizers
group. The slow effect (5 to 10 minutes
after tempering) appears on naphthalene- formaldehyde
additives. The slowest in absorption
rate (15 minutes and more) are lignosulphonates
and modifiers on the base of naphthalene-formaldehyde
and lignosulphonate compounds;

* ƒƒwhen plasticizers are added, there are observed
different degrees of flow ability and durability
increase, depending on dosage. The tendency of
variations in mixture durability depending on
an additive type, its content and water-solid ratio
is presented in Pic. 5 a and b. On the base of
plotted isolines the isosurface of mixture’s durability
is plotted, for common influence of the
additive and water-solid ratio (Pic. 5B).

In spite of the fact that rheological properties of
the system allow to estimate its technological effectiveness,
when selecting dosage of one or another modifier
it is necessary to take into account its influence on variations
of physical-mechanical properties and structure
characteristics.

As the results have shown, with cement-sand
base’s hardening at various water-solid ratios and plasticizing
additive contents, there is no unified tendency
of changes in strength isolines neither for compression,
nor for bending.

The system’s strength change takes place under
influence of both water-solid ratio and plasticizing additive amount. If water-solid ratio’s increase leads to
strength reducing, the influence of modifiers is not so
simple. The majority of them increase this characteristic
of mortar, though not so significantly, in the whole
range of dosages. In some cases (for hyper-plasticizers
group) the so called «over-dose» effect is observed, i.e.
when an optimal quantity of additive is exceeded, the
drop of compression and bending strength takes place
in the system.

Pic. 5 Plasticized mixtures durability

When additives dosage and water-solid ratio are
optimal, the acceleration of strength gain during systems
hardening is registered.

On the final stage of investigations the porous
space homogeneity of stone on the base of plasticized
systems has been studied.

The tendency of variations in porosity indexes for
the factors of average size and open capillary pores
homogenity is similar for all kinds of studied plasticizers.
With growing additives content the pores average
size is decreasing, however this leads to impairment
of the porous space’s homogeneity. On this ground it
is possible to assert that plasticizers’ content must be
minimal but sufficient to maintain necessary rheological
and physical-mechanical properties of mixture and
mortar.

According to the results of investigation of powdery
plasticizers’ influence on cement-sand system
properties the optimal dosage of additives has been
determined, the comparative shares of which are presented
in Pic. 6.

Adding plasticizers to cement-sand mortar allows
to improve mixture’s processing characteristics and
mortar’s physical-mechanical characteristics; however it leads to rising cost of the final product. The
degree of rise in production cost of plasticized systems
depends on the kind of additive applied and on
its content. Imported additives, in spite of their lower
required dosage, are more expensive. Taking into
consideration the economical component of plasticizers’
application, when additives are introduced in optimal
amount (Pic. 6), the mineral base appreciation is
2.71% to 16.33%.

Pic. 6 Comparative shares for optimal dosage of plasticizing additives of various chemical bases

According to comments of imported chemical additives’
customers, the last 2-3 years are characterized
with certain irregularity in the products delivery,
which in its turn leads to speculative demand on the
construction chemistry market. Therefore many plants
manufacturing dry construction mixtures significantly
increase their stock of additives in winter season,
which leads to rise in overhead costs [5]. On the base of
obtained results the diagram for reasonable choice and
interchange for plasticizing additives of various kinds
and manufacturers has been worked out. This diagram
allows not only to select an additive and its amount but
also, if necessary, to exchange of one modifier for another
one which are similar in effectiveness, without
decreasing technical-economic parameters of final
product.

Bibliography

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