Air-entraining agent

Air-entrainment is the process whereby many small air bubbles are incorporated into concrete. Air-entrainment is essential for increasing the durability of concrete exposed to freeze thaw conditions. Air-entrainment improves workability of concrete and also decreases bleeding and segregation. Salts of wood resins, synthetic detergents, salts of sulfonated lignin, salts of petroleum acids, salts of proteinaceous materials, fatty and resinous 

The air-entraining action is known to involve adsorption at the air-water or solid-water interfaces. The soluble surfactant ions are adsorbed on cement particles making them hydrophobic, so that as bubbles are generated during mixing, they adhere to the cement. This process stabilizes bubbles, preventing their coalescence. Lowering of surface tension may also stabilize bubbles. 

Many factors influence the amount of entrained air. They include dosage, slump, aggregates, temperature, inclusion of other admixtures, chemical composition of cement, mixing, vibration, etc. 

Generally, a strength loss occurs in air-entrained concrete. As it also allows the reduction of w/c ratio, the loss in strength may partly or wholly be offset. 

Air entraining is the intentional creation of tiny air bubbles in concrete (39). The bubbles are introduced by adding to the mix an air entraining agent and a surfactant. The air bubbles are created during the mixing of the fresh concrete. Most of them survive to be part of the hardened concrete. The purpose of an air entrainment is to lengthen the durability of the hardened concrete. 

Entrained air dramatically improves the durability of concrete exposed to moisture during freeze-thaw cycles and greatly improves the resistance of the concrete to surface scaling caused by chemical deicers. Also, it is intended to increase workability of the concrete while it is in the plastic state. 

The air entraining agents are the surfactant adsoibed on the surface of air bubbles, that is on the air-Uquid phase interfacial surface in concrete (Fig. 6.107). 

The sodium abietate, the salt of abietic acid produced from natural pine tree resin, was used as the air entraining agent. The anionic admixtures, mainly alkylaiylsul-phonates, sodium oleate, as well as the non-ionic surfactants such as nonylphe-nolethoxylate are more and more frequently used. This surfactant is efficient air entrainer but does not perform well in freeze-thaw durability tests [358]. 

Among the anionic surfactants the sodium dodecylbenzenesulphonate, sodium sodium oleate sulphate and sodium oleate are the most widely used (Fig. 6.108). The sodium dodecyl-benzene-sulphonate and sodium abietate are very effective and a very low content, about 0.005 % by mass of cement is sufficient to give the air content on the level about 5 %. 

Air entraining agents are composed of the non-polar hydrocarbon chains or the other hydrophobic units, linked with the following hydrophilic groups —COO , —COO, — SOj On the gaseous and hquid phase interface these polar groups are directed to water— they lower its surface tension, and facilitate the formation of air 

The too high addition of air entraining agent is disadvantageorrs because primarily the Itydration is slowed down (the surface of cement grains becomes hydrophobic), and secondly too high air content is introdrrced to the paste. 

Admixtures are known that decrease air entrainment butyl phosphate is an example. 

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A second form of limestone is finely pulverized limestone for masonry cement. This form is achieved by intergrinding roughly equal parts of limestone and cement clinker in a bad mill to which an air-entraining agent is added. 

Naphthalenesulfonic acids are important chemical precursors for dye intermediates, wetting agents and dispersants, naphthols, and air-entrainment agents for concrete. The production of many intermediates used for making a2o, a2oic, and triphenylmethane dyes (qv) involves naphthalene sulfonation and one or more unit operations, eg, caustic fusion, nitration, reduction, or amination. 

Air-Entrainment Agents. Materials that are used to improve the abiUty of concrete to resist damage from freezing are generally known as air-entrainment agents. These surfactant admixtures (see Surfactants) produce a foam which persists in the mixed concrete, and serves to entrain many small spherical air voids that measure from 10 to 250 p.m in diameter. The air voids alleviate internal stresses in the concrete that may occur when the pore solution freezes. In practice, up to 10% air by volume may be entrained in concrete placed in severe environments. 

A change in the supply situation of Vinsol resin with a commensurate price increase as well as the more widespread use of supplementary materials led almost all admixture companies to reformulate their air-entraining agents using synthetic or alternative natural products such as tall oil or rosins. In a period of about 2 years, a product that had been in dominant use for 40 years was relegated to a lesser role in the industry. 

Fig. 1.3 The effect of air entrainment on compressive strength of concrete containing a water-reducing air-entraining agent and a normal air-entraining agent.

Water-reducing admixture Dosage Air-entraining agent type Water-cement ratio Freeze-thaw data Durability factor (%) Weight loss (%)... 

Table 1.27 Freeze-thaw resistance of corresponding mixes containing water-reducing admixtures and air-entraining agents (after Mielenz)...

Series Mix Air- entraining agent Water-reducing admixture Cement content (kg m - ) Water- cement ratio 28-day compressive strength (N mm-2) Air content (%) Slump (mm) Durability factor ASTM C290 1967... 

The literature describes many different chemical surfactants as suitable for the formulation of air-entraining agents for concrete. However, in practice, the major proportion of commercial products are based on a relatively small number of raw materials and these are set out below ... 

Table 3.1 Fatty acids used as air-entraining agents...

Although the vast majority of commercially available air-entraining agents are simple solutions of materials within one of the above categories, it is possible to produce mixtures and this is occasionally carried out. 

Numerous studies have been made on the effect of additions of air-entraining agents to cement pastes which enable an insight to be gained into the mechanism by which these materials produce the stable microscopic air... 

The effect that air-entraining agents have on the rheology of fresh cement pastes can be considered from the point of view of changes due to the admixture itself, and those due to the presence of entrained air. 

The effect of varying the quantity of several air-entraining agents is shown in Fig. 3.8 [10]. The similarity of behavior of sodium dodecyl sulfate and sodium resinate is again illustrated. 

The specific surface area of bubbles entrained by each air-entraining agent is largely independent of its concentration and there are indications that sodium dodecyl sulfate, apart from being more effective as an air-... 

Table 3.4 The effect of various air-entraining agents at different concentrations on the specific surface area and computed spacing factor of air bubbies in cement paste...

A study of the foaming capacities and stabilities [10] of a variety of air-entraining agents in a solution of cement extracts showed that commonly used anionic air-entraining agents, such as sodium dodecyl sulfate and sodium resinate (1) were visually precipitated from solution, (2) retained their ability to form stable foams after precipitation with only minor amounts of admixture left in solution, and (3) lost the major part of their ability to form stable foams after filtration. It was further shown from studies in cement pastes firstly that the admixture should be adsorbed on the solid particles of the paste with the non-polar ends of the molecule pointed towards the water phase, imparting a hydrophobic character to the cement... 

There is little published data on the effect of air-entraining agents on the chemistry and morphology of cement hydration. However, the limited studies [15] indicate that the normal hydration pattern under isothermal conditions for ordinary Portland cement shown in Fig. 3.14 is modified as follows ... 

For sodium-oleate-based air-entraining agents, the C3S peak is not affected, but the C3A peak is... 

For the other anionic air-entraining agents, such as neutralized wood resins and sulfates or sulfonates, high dosages lead to a retardation of the C3S peak, whilst the C3A peak is accelerated, and sometimes... 

Air-entraining agents are predominantly anionic surfactants which, on addition to cement pastes, are adsorbed on to the cement particles with their polar groups orientated towards the particles. This sheath is of limited solubility and only a minor, but finite, proportion remains in solution as the calcium salt. 

Fig. 3.15 The interactions between cement, air, water and molecules of air-entraining agent (Kreijger).

There is no evidence to suggest that the presence of air-entraining agents of the type normally available commercially alter, in any way, the eventual hydration products of the cement. 

Table 3.12 The amount of air-entraining agent required to obtain 4% air is increased for higher surface area cements...

If the addition of the air-entraining agent is maintained at a constant level, a more workable mix will entrain more air than a less workable one. However, for very workable concrete of slump greater than 180 mm, the air will be more rapidly lost before placing. 

Fig. 3.20 The effect of sand content on the air entrainment of concrete at two addition levels of air-entraining agent, the fineness modulus (FM) of the sand also being varied (Craven).

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