Abstract
D.Phil. (Civil Engineering Science)
Silica was leached from fly ash (FA) produced by Camden power station (South Africa). The residue, desilicated fly ash (DFA) was then stabilised using lime and alkaline activation. The stabilised residue was then used as a soil stabiliser.
Camden FA was found to have a basic pH (10.7) whilst DFA was more basic (12.3) due to a higher K2O content.The major difference between DFA and FA was the presence of a zeolite (Phillipsite K) in DFA. Radiological analysis showed that the activity concentrations of 226Ra, 232Th and 40K for both DFA and FA were below the limits of the South African Government Gazette of 2006.
Silica was leached from FA using KOH. The leaching parameters investigated were leaching time, KOH concentration, agitation speed, FA particle size, leaching temperature and liquid solid ratio (L/S). It was found that the optimum leaching conditions were leaching time of 6 h, 3 M KOH, 500 rpm agitation speed, 25 L/S ratio, leaching temperature of 100˚C. The yield was found to be 283.7 g of silica per kg of FA. Silica leaching followed a modified Jander equation kinetics with an N value of 1.48 and an activation energy of 5.9 kJ/mol. DFA was stabilised using lime at elevated temperatures. DFA and lime were mixed in different ratios at optimum moisture content and maximum dry density and then cured at 40ºC, 80ºC and 100ºC for 4 days. 80ºC was found to be the optimum curing temperature. The DFA: lime ratio of 70:30 was found to have an average UCS of 8.8 MPa. Lime consumption was found to follow modified Jander kinetics with activation energy of 16 kJ/mol. Toxicity leaching tests showed a 79% reduction in the leachability of trace and heavy metals. The composite met the minimum requirement of ASTM C34-13 and South African burnt clay standard (SANS227:2007).
The influence of open porosity, sum of hydration products and extent of DFA hydration on the strength of DFA lime composites was also studied. For both FA and DFA it was observed that the three parameters combined (open porosity, extent of fly ash/desilicated fly ash hydration and sum of hydration products) had the greatest influence on the UCS than individually with correlation coefficients above 0.98.
The use of DFA-lime composite as a stabiliser for expansive soil was also studied. A 70:30 DFA: lime composite was used as the stabiliser. The 30% stabilised soil was found to have a UCS of 4.1 MPa. The stabilised soil met the minimum requirements for the American Concrete Institute’s requirements for backfill material. A 40% addition of FA: lime composite was the optimum for the stabilisation of the same expansive soil. DFA composite stabilisation was better than the FA composite stabilisation due to a superior UCS (4.1 vs 2.62 MPa), better 24 h soak UCS (3.62 vs 0.98 MPa) and a better saturation coefficient 0.35 vs 0.68...