The admixtures are the materials other than water, aggregates, and cement added to the concrete mix (Figure 1) immediately before or during mixing to modify one or more of the specific properties of concrete in a fresh or hardened state. The use of admixture should offer an improvement economically attainable by adjusting the properties of cement and aggregates, and should not affect adversely any properties of the concrete. Admixtures are no substitute for good concreting practices. An admixture should be employed only after appropriate evaluation of its effects on the particular concrete under the conditions in which the concrete is intended to be used. It is often necessary to conduct tests on the representative materials under simulated job conditions in order to obtain information on the properties of the concrete containing admixtures.
The admixtures ranging from the addition of chemicals to waste materials have been used to modify certain properties of concrete. The properties commonly modified are the rate of hydration or setting time, workability, dispersion, and air-entrainment. The admixture is generally added in a relatively small quantity. A degree of control must be exercised to ensure the proper quantity of admixture as an excess quantity may be detrimental. In using any admixture, careful attention should be given to the instructions provided by the manufacturer of the product.
Purpose of admixture
Some of the important purposes for which the admixtures could be used are:
– Acceleration of the rate of strength development at early ages
– Retardation of the initial setting of the concrete
– Increase in strength
– Improvement in the workability
– Reduction in heat evolution
– Increase in durability or the resistance to special condition of exposure
– Control of alkali-aggregate expansion
– Reduction in the capillary flow of water and increase in impermeability to liquids
– Reduction in segregation in grout mixtures
– Production of coloured concrete or mortar
Classification of admixtures
Table 1 shows classification of admixtures as per ASTM C 494.
According to the characteristics effects produced, the admixtures may be classified as follows:
The accelerating admixtures are added to concrete either (a) to increase the rate of hydration of cement, hence to improve the rate of development of strength or (b) to shorten the setting time. An increase in the rate of early strength development may help in (i) earlier removal of forms, (ii) reduction of required period of curing, (iii) earlier placement of structure in service. Accelerating admixtures are also used when the concrete is to be placed at low temperature.
The most widely used accelerator is calcium chloride (CaCl2). IS: 7861 (Part II) – 1981 recommends a maximum of 1.5 percent CaCl2 for plain and reinforced concrete works in cold weather conditions. Large doses of CaCl2 result in the flash set of concrete and also in increased shrinkage.
Retarding and Water-reducing admixtures
The retarding admixtures delay the setting of concrete. They are used primarily to offset the accelerating and damaging effect of high temperature and to keep concrete workable during the entire placing period which should be sufficiently long so that the succeeding lifts can be placed without developments of cold joints or discontinuities in the structural unit. Some of the retarding admixtures also reduce the water requirement of the mixtures.
The water-reducing admixtures are mainly used to improve the quality of concrete, to obtain specified strength at lower cement content, or to increase the slump of a given concrete mix without increasing the water content.
Some retarders are especially useful in cement grout slurries, particularly where the grouting is prolonged, or in cases where the grout must be pumped for a considerable distance, or where hot water is encountered underground.
These admixtures (Figure 2) help to incorporate a controlled amount of air in the form of minute bubbles in the concrete during mixing, without significantly altering the setting or the rate of hardening characteristics of concrete. The capillaries are interrupted by relatively large air voids in the air-entrained concrete. Entrainment of a small amount of air results in the concrete of insufficient durability whereas with the large amounts of entrained air an excessive reduction in strength may occur. This air-entrainment alters properties of both the freshly mixed and hardened concrete. Air-entrained concrete is considerably more plastic and workable than non-air-entrained concrete. The durability of hardened concrete is improved by increased uniformity, decreased absorption and permeability, and by an elimination of planes of weaknesses at the top of lifts.
A pozzolana is a siliceous material which as such does not possess cementitious property in itself, but reacts in the presence of water with the lime at the normal temperatures to form compounds of low solubility having cementitious properties.
The pozzolanas can be used in combination with or for partial replacement of Portland cement. Some of the advantages obtained through their use are: (i) improved workability with lesser amount of water, (ii) reduction in heat of hydration, (iii) increased resistance to the attack from salts and sulphates, and (iv) prevention of calcium hydroxide leaching.
The fly ash or pulverized fuel ash (PFA) is the most commonly used pozzolana. It is the residue from the combustion of pulverized coal collected by the mechanical or electrostatic separators from the fuel gasses of thermal power plants. Its composition varies with the type of fuel burnt, the load on the boiler, and type of separator etc. The fly ash obtained from electrostatic precipitators may have a specific surface of about 3500 to 5000 cm2/g, i.e. it is finer than the Portland cement. The fly ash obtained from cyclone separators is comparatively coarser and may contain a large amount of unburnt fuel. The carbon content in fly ash should be as low as possible, whereas the silica content should be as high as possible.
The fly ash may be used in concrete either as an admixture or in part replacement of cement. As an admixture, the fly ash reduces the segregation and bleeding; whereas, when used as a replacement of cement, the silica content combines with the free lime liberated during the hydration of cement. The optimum amount of pozzolana as a replacement may normally range between 10 to 30 per cent. A fine grinding of silica and high temperature curing increases the reactivity of pozzolana.
Colouring admixtures or pigments
Pigments are the admixtures added to produce coloured cements. The pigments used must be permanent and should not react with free lime in concrete.
– ASTM C494 “Standard specification for chemical admixture for concrete.” ASTM International.
– Gambhir, M.L. (2009). “Concrete technology.” Tata McGraw Hill Education.
– Gambhir, M.L., Jamwal, N. (2014). “Building and construction materials – Testing and quality control.” McGraw Hill Education (India) Private Limited, New Delhi.