A Dissertation Submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in partial fulfillment for the Degree of Master of Science in Engineering. Johannesburg, 1979. / Soil stabilization is the generic term for any process which has as its aim the upgrading or improvement of one or more soil properties. In South Africa, stabilization is usually effected using a combination of mechanical densification and the addition of a binder to the soil. Two waste materials, PFA (a power station waste) and phosphogypsum (a waste from the fertilizer industry) have been shown to be useful binders overseas. The purpose of this project was to investigate local samples of these waste materials to establish their potential as soil stabilizers. A limited amount of research into the use of these two materials has been carried out in South Africa. The results of these programmes are discussed. Generally, however, none of these programmes were comprehensive enough to reach categoric conclusions regarding either binder. The soils used in the investigation were taken from various locations in the Johannesburg Municipal area in which it is intended to construct surfaced roads for light traffic in the near future. The soils chosen cover a range of plasticity indices from 0 to 30, but unfortunately, were very similar mineralogically. The phosphogypsum used was supplied by Triomf Fertilizer (Pty) Limited. Two samples of PFA were used. One of them was supplied by Darling and Hodgson, Limited, who are extracting a "selected" ash from Grootvlei power station, and intend to make it commercially available at some stage. The other ash was obtained from Kelvin power station. Tests were done to establish both the short and long term effects of the binders on the soils. The short term effects were investigated by looking at changes in the plasticity and moisture density relationships for the soils immediately after the addition of the binders. The long term effects were investigated by looking at the changes in unconfined compressive strength of moulded soil samples with the addition of the binders. (Samples were moulded at 100% Mod AASHO density and optimum moisture content in a tapered cylindrical mould by static compaction). These effects were looked at from 3 different angles. i) the effect of binder composition (i.e. different ratios of PFA/lime and PFA/cement) ii) the effect of binder content iii) the effect of age on strength gain The results of the testing programme were not very encouraging. For the binder contents used, there were minimum short term effects for both binder types. The addition of phosphogypsum to the soils resulted in only very small strength increases. For the PFA binders, it appears that the lime content of many of the PFA/lime binders fell in a range where only short term reactions could be expected. However, even where sufficient lime was present to satisfy the initial lime demand of the soil, the PFA appeared to act mainly as a diluent to the lime. Similar conclusions apply to the PFA/cement binders. The general nature of PFA/lime stabilization was uncovered in this testing programme, but further tests will have conducted before conclusions regarding the quality of our local materials can be drawn.

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Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 249-257). Also available via the Internet.

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