Air-entraining agents
3.3 The effects of air-entraining agents on the water–cement system
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
Fig. 3.4 Nonylphenol ethoxylates.
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Page 109 void system, and to some extent, their effect on the properties of concrete. However, unlike the water-reducing agents, the comparison of behavior between that in cement pastes and that in total concrete systems is not so useful because of the influence of the aggregate component in determining the air content and the resultant rheological characteristics. The data can be conveniently described under the following headings:
1. Rheology.
2. Air content and characteristics.
3. Distribution between solid and aqueous phase.
4. Effect on cement hydration reactions.
5. Interpretation as a mechanism of action.
3.3.1 Rheology
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.
An interesting program of work was carried out to isolate the individual effects [13] by preparing the cement paste/admixture mixes in two ways, as given below.
(a) Without air entrainment
The apparatus shown in Fig. 3.5 was used, and was filled completely with cement paste. Admixture additions were made by injecting with a hypodermic needle through the rubber cap. Pastes prepared in this way had air contents of less than 0.6% by volume, were free from any premature stiffening tendencies and were
homogeneous.
(b) With varying degrees of air entrainment
The same apparatus was used, but quantities of paste were removed to give an air space in the vessel. On rapid agitation the volume increased, dependent on the air content required. Paste viscosities were measured, using a Stormer viscometer, which is a type of concentric cylinder viscometer. Although it is possible to obtain results in absolute terms, for comparative purposes the times for 100 revolutions of the rotor under a fixed applied torque were recorded.
In the absence of entrained air, the results shown in Fig. 3.6 were obtained, where the similarity of behavior of the three anionic materials, sodium dodecyl sulfate, sodium resinate and a petroleum sulfonate, in increasing the paste viscosity is seen. The non-ionic material, phenol
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Fig. 3.5 Mixing apparatus for experiments on air-entrained and non-air-entrained pastes (Bruere).
ethoxylate, has almost no effect on the paste viscosity when no air is entrained.
When sodium resinate was incorporated into pastes made from two cements having significantly differing surface areas, the results given in Table 3.3 were obtained, indicating that the sodium resinate produces a much greater increase in viscosity in paste made from the fine cement than in pastes made from the coarse cement.
When the viscosities of air-entrained pastes were measured by the same means, the results shown in Fig. 3.7 were obtained for sodium abietate at 0.05% by weight of cement. It can be seen that the magnitude of the effect due to the presence of the admixture itself is small in relation to the effect of the air it causes to be entrained.
3.3.2 Air content and characteristics (a) Dosage and admixture type
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.
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Fig. 3.6 Viscosities of cement pastes containing varying concentrations of surface active agents in the absence of entrained air (Bruere). 1 = sodium dodecyl sulfate; 2 = sodium abietate; 3 = petroleum sulfonate; 4 = phenol ethoxylate.
Table 3.3 The effect of air entrainment on paste viscosity for two different cements
Cement
no. Surface area (cm2 g
−1) (Blaine)
Sodium abietate (%
by wt of cement) Time for 100 rev. of
rotor(s) Increase in time for 100 rev. (%)
1 3600 0 26
1 3600 0.05 36 38
2 4350 0 74
2 4350 0.05 130 76
entraining
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Fig. 3.7 The viscosity of a cement paste containing sodium abietate in the presence of entrained air (Bruere).
Fig. 3.8 Variations of air-entraining capacities of surface active agents in cement pastes with varying concentrations of agents (Bruere). 1 = sodium dodecyl sulfate; 2 = sodium tetradecyl sulfate; 3 = sodium abietate.
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Table 3.4 The effect of various air-entraining agents at different concentrations on the specific surface area and computed spacing factor of air bubbles in cement paste
Surface active agent Concentration (% by wt of cement)
Air content (%
by volume)
Specific
surface area of bubble (mm2 mm−3)
Computed spacing factor (mm)
None
Sodium dodecyl sulfate
— 0.3 31 0.813
0.005 6.5 68 0.112
0.05 14.0 73 0.074
0.010 17.5 78 0.066
0.025 21.3 78 0.058
0.050 27.2 78 0.048
Sodium dodecyl- benzene sulfonate
0.005 4.0 56 0.188
0.010 5.6 61 0.135
0.025 11.2 64 0.094
0.050 13.1 61 0.091
Neutralized wood
resins 0.05 4.6 53 0.170
0.010 10.1 56 0.112
0.025 17.4 53 0.094
0.050 22.6 54 0.084
agent than either the sodium dodecylbenzene sulfonate and neutralized wood resins, also results in a high specific surface area of bubbles in closer proximity.
The effect of altering the fatty-acid chain length of sodium salts of linear fatty acids on the air entrained in mortars is illustrated in Fig. 3.9 [5]. The superiority of the 9–11 carbon chain length fatty-acids is clearly shown and, in practice, fatty-acid fractions with a high C10 content are chosen; the C9 and C11 fatty acids do not occur naturally in appreciable quantities.
(c) Water–cement ratio
An increase in the water–cement ratio of cement pastes leads to greater air entrainment and a decrease in the specific surface area of bubbles. However, the spacing factor is relatively unchanged, as shown in Table 3.5 [14].
(c) Cement type
water–cement ratio of 0.45 [10].
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Fig. 3.9 The effect of increasing chain length of the sodium salts of linear fatty acids on air-entraining capacity (Rixom).
The large differences, however, can largely be attributed to differences in viscosities of the different pastes at the same water–cement ratio. When the pastes were prepared to the same consistencies, similar levels of air
entrainment were obtained.
Table 3.5 The effect of water–cement ratio of cement pastes on the air content, specific surface area and computed spacing factor
Surface active
agent Water–cement
ratio (by wt) Air content (% by
vol.) Specific surface
area of bubble Computed spacing factor (mm)
sulfate 0.50 25.8 56 0.069
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Table 3.6 The relationship between particle size of cement and the level of air entrainment
Particle size of cement (BSS mesh) Air content of paste (% by vol.)
20–52 44.1
52–100 32.0
140–200 24.8
Passing 200 21.0
Table 3.7 Variations in cement characteristics have a considerable effect on the level of air entrainment at constant water–cement ratio
Cement batch
number Surface area of cement (cm2 g
−1) (air permeability)
Total alkali content of cement
SO4 content of cement
Air content of paste (% by vol.)
1 3300 1.09 1.99 14.7
2 3730 0.57 2.56 6.1
3 3480 0.12 1.85 11.0
4 3500 0.25 1.82 5.7
(d) Temperature
Over the range 18–35°C, the effect of temperature is not great and is illustrated in Fig. 3.10 for a paste containing 0.0125% sodium dodecyl sulfate at a water–cement ratio of 0.45 [10]. It can be concluded that the degree of air entrainment obtained in cement pastes of a given consistency is largely a function of the type and dosage of air- entraining agent incorporated, whilst other variables have only a minor effect.
3.3.3 Distribution between solid and aqueous phases
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
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Fig. 3.10 The effect of temperature on the air-entraining capacity of sodium dodecyl sulfate in a cement paste (Bruere).
particle to which the air bubbles can adhere, and secondly that the residual concentration of admixture in the mixing water, although not necessarily high, must be sufficient to generate bubbles during mixing.
In cement paste, the residual concentration of calcium resinate is approximately 0.5 g1−1 in the aqueous phase, which compares well with the aqueous solubility of calcium caprate, as shown in Fig. 3.11. It will be noted that the C9 fatty acid has a solubility of about 1 g1−1, whilst the C11 is about 0.05 g1−1. The solubility of calcium dodecyl sulfate is approximately 0.1 g1−1, so it is indicated that the optimum value lies in the region of 0.1– 0.5 g1−1 solubility of the calcium salt. It is interesting to note that the abilities of the soluble sodium and insoluble calcium salts to entrain air in cement pastes are very similar [6] (Figs 3.12 and 3.13).
3.3.4 Effects on the hydration chemistry of 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:
1. For sodium-oleate-based air-entraining agents, the C3S peak is not affected, but the C3A peak is
accelerated and splits into two up to a 10 times normal dosage level. It is believed that the ettringite and
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Fig. 3.11 The solubility of linear fatty-acid calcium salts in water.
Fig. 3.12 Comparison of the air-entraining capacities of sodium and calcium abietates. ○ = sodium abietate dissolved in mixing water and added to cement; ● = made with filtrates from mixtures of sodium abietate and cement paste extract (Bruere).
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Fig. 3.13 Comparison of the air-entraining capacities of sodium and calcium resinates (wood resins). ○ = sodium resinate dissolved in mixing water and added to cement; ● = made with filtrates from mixtures of sodium resinate and cement paste extract.
Fig. 3.14 Schematic diagram of the development of the heat of hydration of Portland cement under isothermal conditions (Bruere).
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monosulfate reactions are retarded due to an impermeable layer of a calcium-oleate–aluminate-hydrate salt.
2. 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 splits into two.
3. Non-ionic materials, such as ethoxylates, do not appear to alter the heat output pattern of ordinary Portland cement.
Any changes in morphology of hydration products have not been published.
3.3.5 Interpretation as a mechanism of action
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.
The weak surfactant solution forms bubbles on agitation in the aqueous phase, which are stabilized as
microscopic spheres from coalescing into large bubbles by the orientation of insoluble surfactant across the air/
liquid interface and by adhering to the hydrophobic surface created on the cement particle by the adsorbed surfactant. This is shown diagrammatically in Fig. 3.15 [16].
The bubbles are less than 0.25 mm diameter and probably do not exist in the fresh paste with diameters less than 10 µm because the high pressure present in such small bubbles would cause the air to be dissolved.
Fig. 3.15 The interactions between cement, air, water and molecules of air-entraining agent (Kreijger).
Page 120 Air-entrained pastes possess a higher viscosity than pastes with little or no air content, mainly because of the bridging effect of cement particles by air bubbles increasing the structure of the system.
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.