Superplasticizer
Encyclopedia
Superplasticizers, also known as high range water reducers, are chemicals used as admixtures where well-dispersed particle suspension are required. These polymers are used as dispersant
s to avoid particle aggregation, and to improve the flow characteristics (rheology
) of suspensions such as in concrete
applications. Their addition to concrete
or mortar
allows the reduction of the water to cement ratio, not affecting the workability of the mixture, and enables the production of self-consolidating concrete
and high performance concrete. This effect drastically improves the performance of the hardening fresh paste. Indeed the strength of concrete increase whenever the amount of water used for the mix decreases. However, their working mechanisms lack of a full understanding, revealing in certain cases cement-superplasticizer incompatibilities.
copolymer (side chain) grafted with methacrylic acid
copolymer (main chain). The carboxylic acid
COONa dissociates in water, providing a negative charge along the PCE backbone. The polyethylene oxide (PEO) group affords a not uniform distribution of electron cloud, which gives a chemical polarity
to the side chains. The number and the length of side chains are flexible parameters easy to change. When the side chains have a huge amount of PEO units, they lower with their high molar mass the charge density of the polymer, which enables poor performances on cement suspensions. To have both parameters on the same time, long side chain and high charge density, one can keep the number of mian-chain-units much higher than the number of side-chain-units.
on the positively charged colloidal particles. As a consequence of PCE adsorption, the zeta potential
of the suspended particles changes, due to the adhesion of the COO- groups on the colloid surface. This displacement of the polymer on the particle surface ensures to the side chains the possibility to exert repulsion forces, which disperse the particles of the suspension and avoid friction. These forces can be directly detected by the use of the atomic force microscopy (AFM), working with model substances in liquid environment.
Dispersant
A dispersant or a dispersing agent or a plasticizer or a superplasticizer is either a non-surface active polymer or a surface-active substance added to a suspension, usually a colloid, to improve the separation of particles and to prevent settling or clumping...
s to avoid particle aggregation, and to improve the flow characteristics (rheology
Rheology
Rheology is the study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force....
) of suspensions such as in concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
applications. Their addition to concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
or mortar
Mortar (masonry)
Mortar is a workable paste used to bind construction blocks together and fill the gaps between them. The blocks may be stone, brick, cinder blocks, etc. Mortar becomes hard when it sets, resulting in a rigid aggregate structure. Modern mortars are typically made from a mixture of sand, a binder...
allows the reduction of the water to cement ratio, not affecting the workability of the mixture, and enables the production of self-consolidating concrete
Self-consolidating concrete
Self-consolidating concrete is characterized by a low yield, high deformability, and moderate viscosity necessary to ensure uniform suspension of solid particles during transportation, placement , and thereafter until the concrete sets.Such concrete can be used for casting heavily reinforced...
and high performance concrete. This effect drastically improves the performance of the hardening fresh paste. Indeed the strength of concrete increase whenever the amount of water used for the mix decreases. However, their working mechanisms lack of a full understanding, revealing in certain cases cement-superplasticizer incompatibilities.
Polycarboxylate ether superplasticizer
The new generation of this kind of admixtures is represented by polycarboxylate ether based superplasticizers (PCEs). With a relatively low dosage (0.5–1.5% by cement weight) they allow a water reduction up to 40%, due to their chemical structure which enables good particle dispersion.Chemical structure
PCEs are composed by a methyl-polyethylene glycolPolyethylene glycol
Polyethylene glycol is a polyether compound with many applications from industrial manufacturing to medicine. It has also been known as polyethylene oxide or polyoxyethylene , depending on its molecular weight, and under the tradename Carbowax.-Available forms:PEG, PEO, or POE refers to an...
copolymer (side chain) grafted with methacrylic acid
Methacrylic acid
Methacrylic acid, abbreviated MAA, is an organic compound. This colourless, viscous liquid is a carboxylic acid with an acrid unpleasant odor. It is soluble in warm water and miscible with most organic solvents. Methacrylic acid is produced industrially on a large scale as a precursor to its...
copolymer (main chain). The carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
COONa dissociates in water, providing a negative charge along the PCE backbone. The polyethylene oxide (PEO) group affords a not uniform distribution of electron cloud, which gives a chemical polarity
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
to the side chains. The number and the length of side chains are flexible parameters easy to change. When the side chains have a huge amount of PEO units, they lower with their high molar mass the charge density of the polymer, which enables poor performances on cement suspensions. To have both parameters on the same time, long side chain and high charge density, one can keep the number of mian-chain-units much higher than the number of side-chain-units.
Working mechanisms
PCE's backbone, which is negatively charged, permits the adsorptionAdsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
on the positively charged colloidal particles. As a consequence of PCE adsorption, the zeta potential
Zeta potential
Zeta potential is a scientific term for electrokinetic potential in colloidal systems. In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta, hence ζ-potential...
of the suspended particles changes, due to the adhesion of the COO- groups on the colloid surface. This displacement of the polymer on the particle surface ensures to the side chains the possibility to exert repulsion forces, which disperse the particles of the suspension and avoid friction. These forces can be directly detected by the use of the atomic force microscopy (AFM), working with model substances in liquid environment.