Superplasticizer are water
soluble, electrically charged organic polymers having molecular weights in the
range 1000 – 100000 Daltons. The action exerted by these molecules in an
aqueous slurry (paste) of cement particles may viewed as resulting from a
superimposition of “physical” and “chemical” effects.
The “physical” effects are
those which would be observed in any dense slurry of ‘inert’ (e.g limestone,
titanium dioxide); in such slurries, superplasticizer molecules can exert a
strong fluidizing effect at constant water content or a major water reduction
at constant fluidity. The mode of action of superplasticizer molecules in these
systems (and generally in any slurry of mineral particles), first relies on the
adsorption of part of the molecules on the surface of the particles. The
superplasticizer molecules convey an electrical charge to the particle surface
and this favors a better dispersion (de-flocculation) of the particle via
electrostatic repulsive forces. The adsorbed superplasticizer polymers also act
as a physical barrier to keep particles apart (i.e. steric repulsion), further
assisting the dispersion of the particles and fludification of the slurry. The
molecular properties of the superplasticizer which optimize the “physical”
contribution to the mode of action superplasticizer have now been largely
elucidated.
The “chemical” role of
superplasticizer in cement pastes may be qualitatively pictured as the way in
which these polymer molecules can influence the course of cement hydration
reactions and the nature of the hydration products. For example,
superplasticizer molecules can interfere with the nucleation and growth of the
hydration products, thus retarding the formation of connective get between the
hydrating articles; this will help maintain fluidity (slump) for longer
periods. Also depending on their chemical composition, superplasticizer
molecules may become embedded, or intercalated, in the hydration products
leading to new organo-mineral hydrates. In the case of these “chemical”
mechanisms, the relationship between the molecular properties of the
superplasticizer and the effects observed are only partially understood.
Flowability of the cement
matrix over a long period of time depends on the fineness of the cement and
chemical composition as well as water demand for a given flowability and
hydration process. Dispersive action of polymeric admixtures depends on its
affinity towards the particle surface, the dispersing medium and itself, as
well as on dispersion mechanism via electrostatic and/or steric hindrance.
These factors are greatly affected by changes in the cement pore solution
and/or surface conditions. Therefore, extended time flowability of the cement
paste is subjected to a state of competition between the cement pore solution
and the surface conditions.
The mineral phases constituting
Portland cement include alite, belite, aluminate phase, (C3 A) and
ferrite phase, (C4AF) and gypsum. The formation of hydration
products takes place where ionic species dissolve from the cement solids into
the pore solution. Crystalline or amorphous solid hydration products begin to
form in the pore solution or at the solid solution interphase. These hydrates
grow in size via chemical reaction and physical process.
Generally, the dispersion
mechanism of admixture on cement particles is classified as adsorbed and
non-adsorbed polymers. Adsorbed polymers enhance dispersion via electrostatic
and steric effects or both, whereas non-adsorbed polymers can induce depletion
and tribology effects.
Action mode of Non-adsorbed
portion of admixtures:
·
Depletion Effect
Depletion
effect states that, polymers disperse or flocculate even if they are in a free
state without adsorption. At excessive dosage, if the polymer concentration
increased to above a critical value, the polymer coils loose their individual
identity, where the chains interpenetrate and form continuous network.
Therefore, surplus polymer remains in solution and fills the spaces between
particles preventing cement particles flocculation.
·
Tribology Effect
Tribology
is the science of friction, abrasion and lubrication. Low molecular weight
compounds (Hydroxy-carboxylic acid, Glyconates, Ctrates, etc.); reduce the
frictional forces between particles, enhancing flowability. In addition, the
surface tension of the mixing water is also reduced.
It is
interesting to study the effect of different chemical admixtures like Sodium
lignosulphonate, Sulphonated naphthalene formaldehyde, Polyacrylate on the
performance of different cement pastes such as normal Portland cement, sulphate
resisting cement, high slag cement and fly ash cement. Also the production of
High volume Fly Ash, that is with Fly Ash contents from 35 to 60%, the
plasticizer used would need to different and compatible with the mix of two
ingredients.
