"Effects of Fine Aggregate Replacement on the Rheology, Compressive Strength and Carbonation Properties of Fly Ash and Mortar"

Abstract

Recently, the amount of fly ash generated from electric power plants and other industries in Japan was about 6.4 million tons per year, and it is expected to increase to 12 million tons per year in 2010. Hence, there is a great need to investigate solutions on how to utilize fly ash more effectively. One of the promising solutions is to use high amounts of fly ash in concrete as replacement of the fine aggregate, instead of replacing portland cement. When replacing a part of the cement by a high amount of fly ash, changes in strength development and resistance to carbonation may cause problems in the applications of the concrete to actual building construction in respect to the structural and durability requirement. Though the replacement of a part of fine aggregate by fly ash reduces low strength development, higher rates of carbonation of the concrete may remain as a problem, as calcium hydroxide will be consumed by pozzolanic reactions with the fly ash. This study aims to clarify the effects of high level of replacement of fine aggregate by fly ash on the properties of concrete in the fresh state as well as in the hardened state, and investigate the limit of the replacement in view of the practical application of this type of concrete. Rheological properties, compressive strength and rate of carbonation of mortars of water to portland cement ratio of 0.3, 0.4 and 0.5 in which the fine aggregate was replaced with fly ash at 25 and 50% levels were evaluated experimentally. It was found that the rheological constants increased with higher replacement level of fly ash and that, when water to portland cement ratio was maintained the strength development and carbonation properties were improved.