DOI: 10.14457/tu.the.2015.1345
title: Utilization of wet fly ash in concrete

description: Fly ash is one of the by-products generated from coal combustion. Fly ash is generally used as a cement replacing material in concrete. About 3 million tons of fly ash is produced annually in Thailand. In the past, almost all produced fly ash was used in the concrete industry. However, some amount of fly ash is in excess of the demand by the concrete industry in Thailand. So, some of the excessive fly ashes are wet-stored at about 0.9 million tons during recent years. The un-used fly ash is water-sprayed to have moisture content about 15% to 20% before being dumped near the power plant. It is defined here as wet fly ash. The wet fly ash should be more properly managed by using it in the concrete production for the sake of both the environment and the cost. This study is aimed to investigate the effect of moisture content and wet exposure period of wet fly ash on basic properties and durability of mixtures with wet fly ash. Chemical and physical properties of wet fly ash are studied. Basic properties and durability of paste, mortar, and concrete mixed with the wet fly ash are also investigated and compared with those mixed with dry fly ash. The tested dry fly ash and wet fly ash samples are collected from the same lot of production. The wet fly ashes, both obtained from real site and prepared in the laboratory, are used in the investigation. The moisture contents of the wet fly ash prepared in the laboratory are 25%, 45% and 65%. The wet exposure periods of the fly ash are 1 week, 1 month, and 3 months. The test results revealed that chemical composition of the wet fly ash is similar to that of the dry fly ash though some particles of the wet fly ash are agglomerate. Setting times of wet fly ash are slightly longer and normal consistency is slightly higher than that of the dry fly ash. In contrast, strength activity index and flow of mortars mixed with wet fly ash are slightly improved. Slump and compressive strength of concrete mixed with wet fly ash are higher than those with dry fly ash. Expansions due to alkali-aggregate reaction of mortars mixed with wet fly ash and dry fly ash are similar. Other durability properties such as autogenous shrinkage, total shrinkage, and carbonation of concrete mixed with wet fly ash are enhanced when compared with dry fly ash. The changes in basic properties of paste, mortar, and concrete are not affected much by the chemical effect, but due to the physical effect of wet fly ash. The effect of different moisture contents of wet fly ash on properties of paste, mortar, and concrete can be explained by an influence of water absorption of fly ash and double mixing method. Moisture content of wet fly ash is considered as the first portion of water added into the mixer during the mixing process. The content of the first portion of water is proven to have an effect on workability, strength, and durability of cement-fly ash mixtures. In order to verify the effect of double mixing method on mixtures, the mechanical property of ITZ of mortar and concrete is studied by using a micro hardness test. On the other hand, the exposure period of wet fly ash has insignificant effect on basic properties of cement-fly ash mixtures since the chemical composition of wet fly ash at different exposure periods is unchanged.