Air entrained cement

The characteristics of the cement used to prepare air-entrained cement pastes have a marked influence on the amount of air entrained and are illustrated in Tables 3.6 and 3.7 for pastes prepared under standard conditions at a... 

Fig. 3.37 Length changes (shrinkage) of plain and air-entrained cement pastes (Feldman).

Eight types of cement are covered in ASTM C 150 and AASHTO M 85. These types and brief descriptions of their uses are listed in Table 31.1. Although lA, IIA, and IIIA (air-entraining cements) are available as options, concrete manufacturers prefer to use an air-entraining admixture during concrete manufacture, where they can get better control in obtaining the desired air content. 

Fig. 4. Variation of thermal conductivity with temperature in a mortar sample (1 p.b.w. type lA air-entraining cement, 2.75 p.b.w. graded Ottawa sand, 0.47 p.b.w. water) [ ].

Indian Red. A red ferric oxide pigment made by calcining ferrous sulphate. Indiana Method. A technique for the determination of the quantity of entrained air in concrete on the basis of the difference in the unit weights of a concrete sample with and without air. The name derives from the fact that air-entrained cement was first used to any extent in Indiana, USA. (Proc. A.S.T.M., 47, 865,1947.)... 

Figure 39. Length changes of air-entrained cement paste saturated with brine of various concentrations.

Type lA—Air-entraining cement for the same uses as Type I, where air-... 

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. 

Type V (High Sulfate Resistance). Type V Pordand cement is used in concrete exposed to severe sulfate attack of 1,500 to 10,000 ppm. Low concentrations of tricalcium aluminate [12042-78-3] give Type V its sulfate resistance. The sulfate resistance is improved with air entrainment and low water to cement ratios in the wet concrete. U.S. production of Type V Pordand cement in 1989 was 0.9% of the total Pordand cement production. 

Most masonry cements are finely iaterground mixtures where Portland cement is a principal constituent. These cements also iaclude finely grouad limestones, hydrated lime, aatural cement, po22olans, clays, or air-entraining ageats. Secoadary materials are used to impart the required water reteatioa and plasticity to mortars. 

When concrete hardens, the cement paste shrinks. The gravei, of course, is rigid, so that smaii shrinkage cracks are created. It is found that air entrainment (mixing small bubbles of air into the concrete before pouring) helps prevent the cracks spreading. 

Air-entraining water-reducing admixtures require special consideration the presence of entrained air leads to a reduction in compressive strength, whilst the water reduction results in a compensatory increase in strength. The effect can be quantified, however, by considering the amount of entrained air in terms of an equivalent volume of water to calculate the (air and water)-cement ratio. This new factor can be used to estimate the expected strength from Fig. 1.37. 

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

It can be concluded from the assessment of the data in this section that inclusion into a concrete mix of a water-reducing admixture of the lignosulfonate, hydroxycarboxylic acid and air-entraining type should not lead to any deterioration in the durability of that concrete to freeze-thaw cycling. Indeed there are strong indications that, when used either as a means of reducing the water-cement ratio or, alternatively, of reducing the cement content, more durable concrete may result. 

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... 

When superplasticizers are used to reduce the water-cement ratio, normally any increase in air content will be minimal, especially in high-cement-content mixes. When it is a requirement to intentionally air-entrain such mixes, the dosage required to obtain a given air content is often considerably increased, presumably because of the reduced aqueous medium in the concrete. 

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. 

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. 

Table 3.3 The effect of air entrainment on paste viscosity for two different cements...

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.

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...

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]. 

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 3.6 The relationship between particle size of cement and the level of air entrainment...

Table 3.7 Variations in cement characteristics have a considerable effect on the level of air entrainment at constant water-cement ratio ...

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