DOI: 10.14457/tu.the.2017.1027
title: Effect of LOI of fly ash on properties of concrete

description: The quantity of low LOI fly ash available is decreasing worldwide as an indirect result of controlling toxic gases such as nitrogen oxides (NOx) to meet the emission standards of the 1990 Clean Air Act amendments. More recent coal power plants around the world are equipped with low NOx burners in their boilers, which are operated at lower firing temperature. This approach has an adverse effect on the quality of fly ash produced because it increases the %LOI of the produced fly ash. The term LOI basically stands for Loss on Ignition. Generally, the amount of unburned carbon of fly ash can be easily determined by the LOI test (ASTM D7348). High LOI fly ash is generally known to cause some malfunctions in concrete, which are probably known to include discoloration, poor air entrainment ability, high water requirement and low compressive strength. Therefore it was only utilized in low-value method or disposed at landfills. So, to utilize these high LOI fly ashes in the concrete work, which is the high-value application for fly ash, the unburned carbon needs to be reduced either by optimizing combustion process or by efficient carbon separation techniques. However, both the disposal and the carbon reduction processes of fly ash are complicated and required large budget and time in the process. Therefore, better understanding of high LOI fly ash concrete behavior is crucial to be capable of using it directly in concrete work without needs of additional process.This research aims to investigate and clarify the effect of LOI of fly ash on many basic properties and durability of concrete. To be able to compare the performances of fly ash concrete containing various %LOI and to vary the %LOI of fly ash without any changes in chemical and physical properties, artificial high LOI fly ashes were made and used throughout this entire study. Low LOI fly ash, having %LOI of 0.77% from Mae-Moh power plant, Thailand, and powder activated carbon were used to make other 4 artificial high LOI fly ashes, having %LOI of 6%, 12%, 18% and 25%. Two replacement percentages of fly ash were used at 20% and 40%. Two different curing conditions, which are air curing and water curing, were used to investigate the curing sensitivity of high LOI fly ash concrete. Basic properties of low and high LOI fly ashes such as moisture content, specific gravity, Blaine fineness, water retainability and water requirement were preliminarily investigated. After that, experiments on the slump, compressive strength, shrinkage, carbonation and chloride resistance were carried out. Moreover, slump model and investigation on the microstructure of low and high LOI fly ash concrete were done for clarifying the test results.Moisture content of fly ash increases with the increase of its %LOI. However, the moisture content of fly ash having %LOI of 25% is still lower than the limit in ASTM standard specification, which limits the maximum moisture content of fly ash used in concrete at 3%. Particle size distributions of the prepared high LOI fly ashes are coarser than the low LOI fly ashes, whereas the Blaine fineness of high LOI fly ashes are higher. This is because high LOI fly ashes used in this study have more porous structure and contain irregular particles, because of the added PAC particles. Water retainability of fly ash increases when %LOI of fly ash increases due to the porous and rough-texture particles. Therefore, high LOI fly ashes increase the water requirement of  the mixtures.Using low LOI fly ash significantly improves the slump of concrete compared to the cement-only mixture. However, slump of fly ash concrete was significantly affected by the %LOI of fly ash. The initial slump of concrete gradually decreases with the increase of %LOI of fly ash. Nevertheless, using fly ash having %LOI of 0.77% to 6% with replacement percentage of 20% in the mixture seems to improve the workability of concrete comparing to the cement-only mixture. Increase percent replacement of fly ash  from 20% to 40% significantly enhances slump of fly ash concrete having %LOI of 0 to 12%. On the contrary, the slump of concrete with 40% fly ash replacement gradually decreases and becomes worse than that of 20% replacement when %LOI of fly ash is over 12%. This phenomenon is because when the high amount of high LOI fly ash is used in the mixture, its water retainability plays the more important role than its lubrication effect.The reduction in compressive strength of high LOI fly ash concrete was obtained in the case of the controlled slump by adjustment of water. However, the increase in compressive strength of high LOI fly ash concrete was obtained in the cases of the controlled slump by the use of superplasticizer and controlled w/b. In the latter 2 cases, the compressive strength of concrete gradually increases when %LOI of fly ash increases from 0.77 to 12%. Although the compressive strength tends to gradually decrease when %LOI of fly ash is beyond 12%, the overall compressive strength of high LOI fly ash concrete is comparable to fly ash concrete with the lowest %LOI (LOI=0.77%). Increase the replacement percentage of fly ash from 20% to 40% resulted in lower compressive strength for mixtures with fly ash with all %LOI. Using high LOI fly ashes in the mixtures can reduce the curing sensitivity of fly ash concrete, especially the one that containing %LOI of 12% due to its internal curing effect. The increase in compressive strength of high LOI fly ash concrete was found to be due to its internal curing effect. SEM pictures of polished high LOI fly ash concrete showed that cement paste infiltrated into the rough surface and pores of the carbon particles. Cement and fly ash react with the additional water, absorbed by the carbon particles, resulting in better bonding between cement and carbon particles. Moreover, the result from micro hardness test of high LOI fly ash concrete also revealed that the hardness values near to carbon particles were higher than those near to fly ash particles, proofing the existence of hard shell around the carbon particles.Carbonation and chloride resistances of high LOI fly ash concrete are worse than the low LOI fly ash concrete. However, the effect of LOI of fly ash was less significant when using in low w/b concrete. Autogenous shrinkage of high LOI fly ash was significantly decreased. This result is one of the evidences indicating the internal curing ability of high LOI fly ash. Total shrinakge of high LOI fly ash concrete gradually increases with the increase of %LOI of fly ash. However, the use of fly ash with %LOI of 0.77 to 25% can reduce the total shrinkage when compared to the OPC mixture.