Title: The Use of Beneficiated Fly Ash as a Component of Cement in Concrete
Author(s): J. B. Newman and P. L Owens
Publication: Symposium Paper
Appears on pages(s): 73-90
Keywords: : additions; cement; CO2 emissions; compressive strength; consistence; durability; fly ash; free water; hydration; k-values; mixture proportioning; pozzolanicity; W/C
For almost 70 years fly ash has been used as a constituent of concrete by replacing (substituting) a minor proportion of normal portland cement (PC). However, in this process the two complementary materials have simply been combined without much consideration being given to the specification of each to provide concrete with the best possible performance. Cements based on lime and pozzolanic materials such as volcanic ash have been used successfully for more than 2000 years and it is timely to re-view this experience in the context of contemporary technology. The paper firstly discusses the need to reconsider the chemical composition and particle size distribution of normal PC clinker and the requirement to incorporate gypsum in the final product. Secondly, the beneficiation of fly ash, normally discarded as a non-hazardous waste material from electricity generating power stations fired with pulverised coal, is considered with particular reference to the removal of carbon (char), cenospheres and sulfates. In-formation is given on the specification and performance of concretes designed to exploit and maximise the technical advantages and sustainability of PC clinker and beneficiated fly ash combinations. The test method for the determination of pozzolanic activity index has been successfully adapted using the concept of cement-factor for the evaluation of the effectiveness of a fly ash as a cementitious material when combined with a normal PC. A similar procedure has been used to evaluate the water demand of combinations of normal PC and fly ash using the concept of water demand-factor. The cement-factor and water demand-factor concepts can be used in concrete mixture proportioning procedures to achieve strengths no less than that required for structural design and a free-W/C of no greater than the maximum specified for durability. The concepts can be used to optimise concrete mixtures for price and environmental impact.