Masters Degrees (Civil Engineering)

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Browsing Masters Degrees (Civil Engineering) by Author "Alexandre, Vital Jorge"

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    The performance of locally produced supplementary cementitious materials when incorporated in concrete
    (Stellenbosch : Stellenbosch University, 2016-03) Alexandre, Vital Jorge; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The production of cement has a strenuous impact on the environment. Nonetheless, it is required by the construction industry as a key parameter for socio-economic development. Supplementary cementitious materials (SCMs) are a group of materials that can be used to partially replace cement as a binder in concrete mixtures, two of which includes slag and fly ash (FA). These materials are obtained as waste products from iron smelting and coal combustion processes, respectively. These materials have the capability to generate hydration products similar to that of cement when used. The current study’s main objective was to investigate the performance concrete containing slag and fly ash, produced in South Africa, as a means to reduce the dependency of cement as the only binder used in concrete. The goal of the study was to establish to what degree each of the materials can replace the cement in a typical concrete mix and the impacts thereof. Experiments were conducted on concrete samples made from a series of mixes with a constant binder content and water-to-binder ratio. Cement was replaced on a mass basis, with limits defined by the typical construction replacements: slag at 25, 50 and 75% and FA at 15, 25 and 35 %. The performance is based on: reactivity of the SCM, fresh state, mechanical properties and durability. Setting time was found to be sensitive to the SCM quantity and reactivity, however it was accelerated by more reactive materials and low replacement level. In addition, a wider SCM-particle span increased the bleeding capacity and reduced the bleeding rate. FA was found to increase the plastic settlement of concrete more when compared to slag based concrete, with the maximum plastic settlement occurring at 25 % FA content. In addition, the unrestrained plastic shrinkage of all mixes was significantly greater than that of the reference, yet decreased with increased SCM content. The compressive strength of SCM based concrete was lower than the reference at early ages and improved with curing age. At 91 days the control and a few SCM-based concretes had similar compressive strength of approximately 62 MPa. Moreover, the indirect tensile strength per unit compressive strength of the SCMs based concretes were higher than that of the control, signifying the improvement of the interfacial transition zone. The addition of SCMs also significantly improved the concrete microstructure. Additionally, the durability performance of the SCMs based concrete was better than or equivalent to that of the reference. The chloride resistance of slag-based concrete was four times lower than fly ash concrete or the control mix. The knowledge from the current study shows that SCMs can be used, to a great extent, to replace cement in the construction industry. The early age properties may require attention, yet, in the end, the final product of SCM-based concrete is found to be superior to that of the control. Hence, the use of slag and fly ash, as a binder replacement, does provide a solution to reduce the environmental impact of cement production.