الفهرس 
Introduction and Object of InvestigationOnly 14 pages are availabe for public view
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Abstract
The objectives of the present investigation are to study the effect of organic admixtures on the physico-chemical and mechanical characteristics of the fresh and hardened oil well cement pastes, hydrated for various time intervals, and their reflection on the mechanical properties of concrete constructions.
The concrete admixture is defined as a material, other than cement, water and aggregate, that is used as an ingredient of concrete and is added to the batch immediately before or during mixing or after the mixing by short time.
The admixtures have the ability not only to give much workability to concrete but also to effect high water reductions. The dispersion action caused by adding the admixtures increases the workability of concrete, which leads to the production of concrete of normal workability but with an extremely high strength due to the reduction of the W/C ratio (initial porosity) of the concrete.
In this investigation, the main physico-chemical and mechanical properties of the neat and admixed oil well cement pastes were studied. Three types of admixtures were selected for this study. A conventional type which is melment (M) and two laboratory prepared admixtures, the first is melamine formaldehyde sulfanilate (MFS) and the second is cyclohexanone formaldehyde sulfonate (CFS). The admixtures dosages used were 0.25, 0.5, 0.75 and 1.00% of cement weight.
The various cement pastes were made by using the optimum water/cement (W/C) ratio corresponding to the standard water of consistency for each cement paste.
Each cement paste was mixed for three minutes continuously, and then moulded in 4 x 4 x 4 cm cubic moulds. The pastes were then cured at 100% relative humidity up to 24 hours, followed by curing under water for various intervals time of 3, 7, 28 and 90 days.
At the end of each hydration period, the specimens were tested for compressive strength, hydration kinetics; X-ray diffraction analysis (XRD) as well as scanning electron microscopy (SEM). The hydration kinetics were studied by determining the chemically combined water. from the results of the chemically combined water, degree of hydration and gel/space ratios were calculated for different time intervals. Mathematical correlations between gel/space ratio and compressive strength were also deduced.
The rheological measurements were then performed using a mini-slump cone. The water/cement ratio used was 0.38. The admixtures dosages were 0.0, 0.25, 0.5, 0.75 and 1.00% at temperatures of 20 1C.
The main conclusions derived from this investigation are summarized as follows:
1. As the dosage of admixtures increases, the required W/C ratio decreases (water reduction increases) to maintain the different cement pastes with a standard consistency. The maximum water reduction was 29.7% for the dosage of 1% melment (M), while it was 22.4% and 21.5% in case of the same dosage of melamine formaldehyde sulfanilate (MFS) and cyclohexanone formaldehyde sulfonate (CFS) respectively.
2. The chemically combined water values (Wn,%) of the cement pastes admixed with the three admixtures are lower than that of the control sample. The admixtures affect the mechanism of the hydration process. At the early stages, up to 7 days, an increased rate of hydration is observed depending on the admixture type and its dosage which is related to how much admixture can reduce the water of paste consistency. At the later stages, after 7 days of curing time, the rate goes slower due to the limited space available for new precipitates as a result of accumulation of the initially formed hydration products.
3. The values of the degree of hydration of hardened cement pastes admixed with the different admixtures are lower than those of the control samples (neat cement paste). In addition, as the dosage of admixture increases the degree of hydration decreases. The effect of the different admixtures on the degree of hydration of the hardened cement pastes is arranged ascendingly in the following order melment< cyclohexanone formaldehyde sulfonate < melamine formaldehyde sulfanilate.
4. The compressive strength values of the admixed cement pastes are higher than those of the control samples, indicating that these admixtures improve the plastic and hardened properties of the cement pastes. For all cement pastes made with the three admixtures, as the dosage increases the compressive strength increases which is dependent on the interrelated parameters namely, W/C ratio and degree of hydration that affect the main characteristics of the fresh and hardened cement pastes.
5. The gel/ space ratio of cement pastes mixed with the three admixtures are higher than those of the control cement pastes. Also it is found that the curves of the cement pastes containing the admixtures are shifted to higher values of gel/space ratio and higher values of compressive strength at each time of hydration. In other words, as the gel/space ratio increases the compressive strength increases.
6. The mini-slump values of the all admixed cement slurries are higher than those of the control samples, indicating that these admixtures improve the plastic and hardening properties of the oil well cement pastes. Also the mini-slump increases with increasing the dosages of the admixtures. This is due to the increase of cement paste fluidity with admixture content. As admixture was added to cement paste, the rapid adsorption of admixture molecules on cement particles which is related to the electrostatic repulsion forces, causes strong dispersion, thus generating high fluidity of cement paste. The values of the fluidity of M containing pastes is higher than those of CFS containing pastes and higher than those of MFS containing pastes at the same dosages.
7. Addition of the admixtures doesn’t affect the phase composition of cement hydration product; these admixtures affect only the physical state and the degree of crystallinity of the formed hydrates.
8. The addition of the Admixtures to the cement pastes results in a reduction of the degree of crystallinity of the hydration products leading to a dense structure of the admixed cement pastes.