Plasticizers: Materials, Action and Uses | Admixtures | Concrete Technology

In this article we will discuss about:- 1. Introduction to Plasticizers 2. Materials Used as Plasticizers 3. Action.

Introduction to Plasticizers (Water Reducing Agents):

The essence of good quality concrete is the requirement of right workability. Under different situations concrete of different degree of workability is needed. A high degree of workability is required in situations like deep beams, thin sections with high percentage of reinforcement, beam and column junctions, pumping of concrete, tremie concreting, hot weather concreting etc. The conventional methods of improving work­ability are by improving the gradation or increasing the quantity of fine aggregate or by increasing the cement quantity.

In the field there are limitations and difficulties to obtain high workability under the given set of conditions. In the field in most of the cases generally extra water is added to the concrete unmindful of its ill effects on the properties of the concrete. The use of extra water is very harmful and should never be used. The use of extra water will not improve the inherent good quality such as cohesiveness and homo­geneity of the mix, which reduces the bleeding and segregation of the concrete.

Nowadays many water reducing admixtures are available in the market. These admixtures are known as plasticizers. The combination of organic substances or combinations of organic and inorganic substances which cause reduction in water content for a given workability or give a higher workability at the same water content are known or termed as plasticizer admixtures.

These are used for the following proposes:

1. To achieve a higher strength by decreasing the water/cement ratio at the same workability as that of an admixture free mix.

2. To achieve the same workability by decreasing the cement and to reduce the heat of hydration in the mass concrete.

3. To increase the workability to provide ease in placing concrete in inaccessible locations.

The use of plasticizer is found to improve the desirable qualities of the plastic or green concrete. For making reinforced concrete or mass concrete of higher workability nowadays it has become a standard practice to use plasticizer or super plasticizer. Nowadays the use of super plasticizer has become a universal practice to reduce the water/cement ratio for the given workability. The reduction in water/cement ratio increases the strength and improves durability of the concrete. Sometimes plasticizers are used to reduce the cement content and heat of hydration in mass concrete.

Materials Used as Plasticizers:

Generally following materials are used as plasticizers:

1. Ligno sulphates and their derivatives and modifications, salts of sulphonates hydrocarbons.

2. Polyglycol esters, acid of hydroxylated carboxylic acids and their derivatives and modifications.

3. Carbohydrates.

Out of these materials, sodium, calcium and ammonium ligno sulphates are most popular. The amount of plasticizers used varies from 0.1% to 0.4% by weight of cement. At constant workability, the reduction in mixing water by the use of 0.1 to 0.4% of plasticizer is expected from 5% to 15%, which naturally increases the strength. At the same water/cement ratio, the increase in workability may range from 30 mm to 150 mm slump depending upon the initial slump of concrete, type of cement, content of cement and dose of the plasticizer.

A good plasticizer produces fluidity in concrete or mortar in a different way than that of an air entrain­ing agent. However some plasticizers also entrain some air along with improving the workability. A good plasticizer should not entrain air in concrete more than 1 to 2% as air entrainment reduces the strength of concrete.

Generally one of the most common chemical used as plasticizer is ligno sulphonic acid in the form of its calcium or sodium salt. This chemical is a natural product derived from wood processing industries. It has been observed that a normal recommended dose of plasticizer does not affect the setting time signifi­cantly, but higher dose than prescribed may cause excessive retardation.

Action of Plasticizers:

The main action of plasticizers is to produce fluidity in the mix and improve the workability of concrete, mortar or grout.

The mechanism of the process may be explained as follows:

(i) Dispersion:

Portland cement being very fine, its particles have a tendency to move and collect in pockets of the wet concrete forming bunch of particles. In other words cement has a tendency to flocculate in wet concrete. These bunch of cement particles or flocculations entrap certain amount of water used in the mix. Thus the total water used in the mix is not available for producing fluidity in the mix.

The principal active components of water reducing admixtures are surface active agents which con­centrate at the interface between two immiscible phases, which alter the physicochemical forces at the interface.

When plasticizers are used, their surface active agents are attracted and hold at the surface of cement particles i.e. the surface active agents are adsorbed on the cement particles. Due to this adsorption the cement particles get negatively charged and produce repulsive force on the particles, which repulse the particles away from each other and neutralize the force of attraction of the particles resulting in stabilizing their dispersion.

This repulsive force is called Zeta potential, which depends on base, solid contents and the quantity of plasticizer used. The overall effect of adsorbtion of plasticizer is that cement particles get deflocculated and dispersed. On the deflocculation of cement particles the water trapped inside the floes, gets released and is available to produce fluidity in the mix.

Due to the deflocculation of cement particles, there will be inter particle friction between particle to particle and floe to floe. But in the dispersed condition there is water in between the cement particles, which results in inter particle friction reduction.

The dispersing ability of plasticizers (water reducing admixtures) results in a greater surface area of cement exposed to hydration. For this reason there is an increase in early strength of concrete than that of admixture free concrete of the same water/cement ratio. Long term strength may also improve due to a more uniform distribution of the dispersed cement throughout the concrete.

In general terms, these plasticizers are effective with all types of cements, though their influence on strength is more with cements of low C₃A or low alkali content. No detrimental effects on other long term properties of concrete are observed. However if admixture is used correctly, the improved durability of concrete is found.

(ii) Retarding Effect:

The plasticizer gets adsorbed on the surface of cement particles and forms a thin film. This film prevents the surface hydration reaction between the water and cement particles as long as sufficient molecules of Plasticizer are available at the particle or solution interface. The quantity of avail­able plasticizer will decrease progressively as the polymers become entrapped in hydration products.

Many researchers have stated that one or more than one mechanism of the following may take place simultaneously:

1. Reduction in the surface tension of water.

2. Development of electrostatic repulsion between cement particles.

3. Development of lubricating film between cement particles.

4. Dispersion of cement grains and release of entrapped water with in cement floes.

5. Prevention of surface hydration reaction of the cement particles, leaving more water for the fluidity of the mix.

6. Change in the form and structure (morphology) of the products of hydration.

7. Prevention of particle to particle contact.

It may be that all plasticizers are set retarders to some extent depending upon the base of plasti­cizers, dose and concentration etc. Hence it is advised that users should follow the instructions of the manu­factures fully.