Browsing by Author "Bessinger, Johandre Marthinus Hubert"
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- ItemInfluence of calcium sulphate to plasticiser/superplasticiser interaction on concrete performance(Stellenbosch : Stellenbosch University, 2020-12) Bessinger, Johandre Marthinus Hubert; Combrinck, Riaan; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: With the continuously increasing use of concrete as a construction material the development of materials to enhance concrete performance has received a great deal of attention over the past few decades. However, one cause of inadequate concrete performance is found to stem from the cement production process itself. During the grinding of clinker, calcium sulphate dihydrate (gypsum) is added to produce cement. It is noted that the milling of clinker and calcium sulphate, to produce cement, causes peaks in temperature at localised areas within the mil. These increased temperatures result in the dehydration of calcium sulphate dihydrate to hemihydrate and to a lesser extent, anhydrite. Calcium sulphate regulates the hydration of tricalcium aluminates; the reaction of tricalcium aluminates with calcium sulphate dominates the early hydration stage of cement. This facilitates a dormant period during which cementitious materials, such as concrete, maintain its workability which allows sufficient time for transport, placement and finishing of concrete. This dehydration of dihydrate to hemihydrate, and to a lesser extent anhydrite, influences the solubility and dissolution rate of the calcium sulphate in cementitious materials. The type of calcium sulphate therefore influences the hydration kinetics of cement. This dehydration of dihydrate to hemihydrate particularly influences the setting time and susceptibility to false/flash set of cementitious materials. Modern concrete design often includes the use of chemical and mineral admixtures to provide improved fresh and hardened state properties that produce higher quality concrete structures. However, chemical admixtures such as plasticiser/superplasticiser, incorporated into concrete to increase workability or reduce water demand while maintaining workability, also have an affinity towards tricalcium aluminates in cement. This gives rise to cement – plasticiser/superplasticiser compatibility issues caused by the variation in type of calcium sulphate present in cement. This study therefore aims to establish the influence of the relation between calcium sulphate and plasticiser/superplasticiser on concrete performance. The influence of this interaction on concrete performance is evaluated through the use of slump,slump flow, rheology, setting time and compressive strength properties. An attempt is made to explain the influence of the relation between calcium sulphate and plasticiser/superplasticiser on the aforementioned concrete properties by considering the influence of plasticiser/superplastisers on calcium sulphate solubility. Solubility tests showed that the presence of plasticiser/superplasticiser increases the solubility of calcium sulphate hemihydrate and to a lesser extent, dihydrate. A correlation is found between the solubility of calcium sulphate and concrete fresh state properties. The presence of hemihydrate in cement is found to cause a decrease in concrete workability while increasing the rate at which concrete stiffens. The presence of hemihydrate also causes the rate of early compression strength gain to increase. This is verified by evaluating the influence of calcium sulphate – plasticiser/superplasticiser combinations on hydration temperature curves. From the evaluation of temperature hydration curves it is observed that the type of calcium sulphate present in cement influences the length of dormant period during hydration; hemihydrate causing a reduction of the dormant period. The magnitude of this reduction in length of dormant period is observed to be dependent on the presence and type of plasticiser/superplasticiser.