CONCRETE CHEMICALS AND APPLICATIONS

Admixture and construction chemicals are chemicals added to the ingredients of concrete to obtain the desired strength and durability.

Admixtures

Of late concrete is used for varied purposes and is made suitable for different occasions and environments. Ordinary concrete is not as versatile and does not suit all purposes. Thus, admixtures and chemicals are added to the ingredients of concrete. On the other hand, additives are materials that are added at the time of grinding cement clinker at cement factories.

The effect of admixture depends on the brand of cement, grading of aggregate, mix proportion and richness of mix. Thus, it is with caution that admixtures should be selected in correctly predicting the behaviour of concrete. There are several admixtures available, some important ones are discussed below.

Plasticizers and Superplasticizers

High degree of workability is needed in different situations. The addition of excess water will only help the fluidity and not the workability of the concrete. But the addition of plasticizers will improve the desirable qualities requires for plastic concrete.

Plasticizers are based on the following constituents:

1. Anionic surfactants such as lignosulphonates and their modifications.
2. Non-ionic surfactants, such as polyglycol acid of hydroxylated carboxylic acids and their modifications.
3. Others such as carbohydrates.

Among the plasticizers, calcium, sodium and ammonium lignosulphonates are mostly used. The quantities used are 0.1–0.4 % by weight of cement.

Superplasticizers constitute a relatively new and improved form of plasticizers. These are chemically different from conventional plasticizers. The special quality of superplasticizers is their powerful action as dispensing agents, and they are high-range water reducers. They are chemically different from plasticizers. Superplasticizers permit reduction of water up to 30% without reduction in workability. Superplasticizers are used for the production of flowing, self-levelling, self-compacting, high-strength and high-performance concrete.

Retarders

A retarder is an admixture which slows the process of hydration because of which the concrete remains plastic and workable. If concreting is prepared in hot weather, retarders overcome the accelerating effect of high temperature. Retarders are used in consolidating large number of pours without the formation of cold joints and in grouting oil wells.

Calcium sulphate is the commonly used retarder. Other admixtures used as retarders are lignosulphonic acids and their salts, hydroxylated carboxylic acid and their salts. These admixtures increase the compressive strength by 10 to 20%.

Retarding plasticizers are available in the market. These important admixtures are often used in the ready-mixed concrete industry for purposes of retaining the slump loss, during high temperatures long transportation distances and to avoid cold joints.

Accelerators

Accelerators are another very useful type of admixture which is added to get early strength. The various purposes are:

1. Early removal of formwork
2. Reduction of period of curing
3. Putting the structure early to use
4. Accelerating the setting time during cold weather
5. Energy repair work

The commonly used accelerator in the past was calcium chloride. However, it is not in use now, instead soluble carbonates, silicates, fluosilicates and some organic compounds are used. Fluosilicates and organic compounds like triethenolamine are comparatively expensive. Some of the accelerators available of late can set cement into a hard state in a matter of five minutes. The availability of such accelerators has made underwater concreting very easy. Further, waterfront structures which need repairs in short time may be done using accelerators. These materials could be used in a cold environment up to 10°C. Accelerating plasticizers are also available.

Air-entraining Admixture

Air-entrained concrete is made by using air-entraining cement or by adding an air- entraining agent. Air-entraining agents produce a large quantity of air bubbles which act as flexible ball bearings and modify the properties of concrete regarding workability, segregation, bleeding and the finishing quality of concrete. Further, the hardened concrete gains resistance to frost action and permeability.

Natural wood resins, animal and vegetable fats, various wetting agents (such as alkali salts), water-soluble soaps of resin acids, etc., are various air-entrained agents used.

Different air-entraining agents behave differently depending on the elasticity of the film of the bubble formed and the extent to which the surface tension is reduced.

Pozzolanic Admixtures

Pozzolanic or mineral admixtures have been in use since the advent of concrete. The application of pozzolanic admixtures modify certain properties of fresh and hardened concrete. The proper addition of pozzolanic admixtures to cement enhances many qualities of concrete, such as lowering the heat of hydration, increasing the water tightness, reducing the alkali–aggregation reaction, resisting sulphate attack, improving workability, etc.

Siliceous materials and aluminous materials do not possess any cementitious materials. But on reacting with cement and moisture, they chemically react with calcium hydroxide liberated on hydration and form compounds possessing cementitious properties. This reaction is called as pozzolanic reaction.

Naturally available pozzolanic materials are clay and shale, diatomaceous earth, volcanic tuffs and pumicites. Artificial pozzolanic materials are fly ash, blast furnace slag, silica fume, rice husk ash, metakaoline and surkhi. Other mineral admixtures are finely ground marble, quartz and granite powder.

Damp-proofing Admixtures

Two important properties that concrete should possess with reference to water are:

1. To resist seepage when subjected to the pressure of water and
2. To protect the absorption of surface water by capillary action. In general, properly designed and constructed concrete should be impermeable. But it has been accepted that the addition of some damp-proofing admixture may prove to be of some advantage in reducing the permeability.

Damp-proofing admixtures are available in powder or liquid form. They have the property of filling pores or being water repellents. The prime materials in pore-filling admixtures are silicate of soda, aluminium and zinc sulphates, and aluminium and calcium chloride. These are also more active and render the concrete more impervious and accelerates the setting time. Hence, mineral oils free from fatty or vegetable oils are used.

The production of low-permeability concrete depends on the uniform spreading of the admixture.