Experimental study on the properties of concrete containing black fly ashes


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Title: Experimental study on the properties of concrete containing black fly ashes

Author(s): How-Ji Chen, Te-Hung Liu, Yu-Ching Chang, Lih-Wen Quo, San-Der Chyou and Shun-Hung Wang

Publication: TCI Concrete Technology

Volume: 5

Issue: 4

Appears on pages(s): 71-82

Keywords: black fly ash, black fly ash concrete compressive strength, electrical resistance, permeability

Date: 10/20/2011

A large amount of coal ash continues to be produced from the coal-fired power plants of Taiwan Power Company (TPC) every year (e.g. in 2008, the annual production quantity was 1,500 and 380 thousand tons for fly ash and bottom ash, respectively). Fly ash is a valuable byproduct material. It can be used as a mineral admixture for concrete due to its pozzolanic activity. However, when using some fly ashes containing higher content of ferric oxide and magnesium oxide, the surface of mixed concrete may tend to be dark-colored. This phenomenon negatively affects the appearance of concrete surfaces. In addition, the depression of construction industry in recent years leads to the usage of fly ash decreasing year by year. Large amounts of fly ash are deposited in the ash pond and that substantially reduces the pond life. At present, the related problems (such as the required land for ash pond for deposition, the environmental impact assessment, the public protest, and so on) will increase the cost and time for constructing the ash pond. More attention should be paid to the proper disposal of fly ash. For the above reasons, the purpose of this study is to evaluate the effects of black fly ash (BFA) on the concrete properties and optimize the utilization of BFA. The results of this study can provide reference to the concrete industry and help to the follow-up promotion for recycling of BFA. The fly ashes used in this study included 5 fly ashes from TPC (sample no. FA, FB, FC, FD and FE) and one from Formosa Plastics (sample no. FF). The physical and chemical properties of the 6 fly ashes were analyzed. Among them, 2 black fly ashes from TPC (sample no. FA and FB) were selected for used in the concrete mixture design and the resulting concrete properties were tested. The designed concrete strengths were 210 kgf/cm2 and 350 kgf/cm2. The percentage of cement replaced by fly ash was 0%, 15% and 25% (10 groups in total). The experimental study included the fresh concrete tests (slump and time of initial and final setting) and the hardened concrete tests (compressive strength, permeability and electrical resistance). Besides, the mortar properties were tested according to the corresponding concrete mixture proportion in that the coarse aggregate fraction was removed. The experimental items included compressive strength, volume change and potential expansion of mortars exposed to sulfate. The experimental results showed that the 28-day compressive strength of BFA concrete with the replacement of 15% or 25% was comparable to that of normal concrete. The permeability of BFA concrete was significantly lower than that of normal concrete. The higher electrical resistance was also observed for BFA concrete. Therefore, when adding the BFA in the concrete, the engineering properties of concrete are little affected by the difference in the color between BFA and general fly ash. In other words, the BFA concrete can achieve the expected performance in comparison with general fly ash concrete.
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