Plasticizers (Based on Lignosulfonate)
These additives, which are based on
lignosulfonate, create an electrostatic repulsion mechanism between the cement
particles, causing them to disperse in the water and thus increase the slump.
Lignosulfonates are among the oldest
chemical additives used in the concrete industry. These compounds have been
used as water reducers or plasticizers since the 1930s. Lignosulfonates which
were added to concrete, had beneficial properties such as delay in setting and
reduction of mixing water. In addition, the higher efficiency of fresh concrete
when applied and the increase in the volume of incoming air was another feature
of this material that made it being used until today.
As a by-product of the paper
industry, lignosulfonate, although low in price, has improved the properties of
concrete. Lignin is part of the structure of wood and is the most abundant
organic molecule on Earth after cellulose. Wood contains 20% to 30% lignin. In
paper production, during a chemical process called sulfonation, lignin is
sulfated and the bonds between lignin and polysaccharides are broken at the
same time as the internal bonds of lignin, and the molecular mass is greatly
reduced. This process causes the molecules to dissolve in water. The liquid
phase that separates from the pulp contains lignosulfonate, salts, and sugar
derivatives. A large part of this material is used as fuel for energy
production and a small part of it is used as a concrete additive after
separating the remaining salts. Due to the fact that the presence of sugars in
high amounts delays the setting of cement, the separation of sugars from
lignosulfonate will improve its performance.
Lignosulfonates are highly branched
polymers composed of phenyl propane units attached to ether or carbon-carbon
groups in aromatic rings. Their molecular mass is between