الفهرس 
INTRODUCTION & LITERATURE SURVEYOnly 14 pages are availabe for public view
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Abstract
The use of air entrainment has been an accepted practice in cement technology for more than 60 years. Air is intentionally entrained to reduce the potential for damage from freezing and thawing. Another important purpose is to increase thermal insulation of walls and roofs in hot and cold countries beside increase workability of the concrete while in a plastic state. Until the early 1980s, most air-entraining admixtures were based solely on the salts of wood resin (i.e., neutralized Vinsol resin). In recent years, a new generation of air-entraining admixtures was introduced and has been used to entrain air in cement paste in many bridges and pavements. The specifications were developed in the late 1970s to assure users that new admixtures would not have significant adverse effects on cement paste properties and to verify their effectiveness in reducing freeze-thaw damage.
In this study, the current procedures are available for evaluating new surfactant’s ability to entrain air bubbles and stabilize them in the fresh cement paste.
The main concept is to test the surfactants under simulated field conditions and to compare the results with those of non-air-entrained cement paste.
The aim of this study is to investigate the effect of four surfactants each one from a different category (anionic: Sodium dodecyl benzene sulphonate (SDBS),cationic :Alkyl dimethyl hydroxy ethyl ammonium chloride (HYPR), amphoteric : Cocamidopropyl betaine (CAPB) and nonionic: Cocamide DEA (LM)), on the properties of the cement paste and to select the variables that should be included in the testing procedures. It is recognized that all cement paste materials, production procedures and field conditions influence to a degree of the cement paste air void system.
Based on a survey of field performance of air-entraining admixtures and review of the available literature, several potential key factors are chosen for evaluation in this project.
surfactant type, surfactant concentration and mixing time, are the key factors selected because of their reported effects on the air-entrained, density ,strength and the hydration reaction. Based on preliminary laboratory screening involving compressive strength measurements, X-ray diffraction and scanning electron microscope (SEM), four commercially available surfactants, each having different chemical composition and expected to vary in performance, are selected for the evaluation.
Different cement pastes are prepared for each of the four surfactants for five interval times (3,7,28,90 and 360 days). The influence of surfactant concentration and mixing time on the air content, density, compressive strength and hydration reaction of these hardened cement paste specimens are investigated .
The resulting data indicate that the type of surfactant has a great effect on the entrained air. The comparison between the air entrained into the cement pastes by the four surfactants shows that CAPB entrains the highest percentage of air according to the following order : CAPB ˃ HYPR ˃ LM ˃ SDBS. In addition, the mixing duration is found to have a positive correlation with the entrained air content since it increases as the mixing duration increases.
The density and compressive strength are found to decrease with increasing the entraining air. The highest compressive strength was obtained for the mixes having LM.
The effect of the surfactants on the hydration reactions of hardened cement pastes are tested by XRD and SEM. It is indicated that the surfactants vary in their effect and most of the impact occurs in the early ages of the hydration reaction.
Conclusions
1. The type and the chemical structure of the surfactant affect its ability to introduce air in the cement pastes .
The sequence of the used surfactants as air entrainers is found to be as follows :
amphoteric surfactant ˃
(CABP) cationic surfactant ˃
( HYPR) nonionic
Surfactant ˃
(LM) anionic surfactant
(SDBS)
2. The concentration of surfactants has a statistically significant effect on the entrained air content. Generally the entrained air increases by increasing the concentration until a certain maximum then decreases .
3. The entrained air increases with increasing mixing duration. Hence the entrained air in the cement pastes can be controlled by the mixing time beside the surfactant concentration.
As an example, the case of using HYPR , increases the mixing time of the concentration 0.02% from 3 to 8 minutes leads to increase the air content by 14.9% .
4. The efficiency of the surfactant as an air entraining agent in the cement media depends on:
a. The solubility of the surfactant in presence of divalent ions.
b. The mechanism of interaction between the surfactant and the cement grains surface. In other words, the ionic attraction is stronger than hydrogen bonding which determines the ability of the surfactant to stabilize the air bubbles .
c. The electrostatic repulsion which keeps bubbles and cement grains separated leading to media expansion.
5. The density is inversely proportional with the air content of the moulds. Generally, the density decreases about 0.3g/cm3 when the air content increases about 14%.
6. The compressive strength of the prepared cement– surfactant pastes is affected by three factors which depend on the surfactant type, namely, the entrained air content, the hydration time and the bubble size distribution as follows :
a. The compressive strength decreases on increasing the entrained air content.
b. The compressive strength increases on adding a surfactant that accelerates the hydration time.
c. The compressive strength increases by a well distributed small bubbles.
Finally, it can be concluded that LM is preferred if a low dosage of entrained air up to 9% is required. This is because LM enhances the compressive strength, accelerates the hydration reaction and produces a well distributed small bubbles.
CAPB, however, is more favorable than HYPR because it entrains a wide range of air dosage besides it is ability to maintain a compressive strength. As example, the CAPB specimens which have 22% air give the same compressive strength of the HYPR specimen with 14% air only.