Compaction and strength characteristics of Lime activated flyash with GGBS as an admixture

Abstract

A very fine by-product generated through coal combustion process at thermal power plants is known as flyash and a part of ash falls down at the bottom of the boiler is known as bottom ash. Out of the total production of waste material flyash generated is approximately 80% whereas bottom ash generated is 20% (by weight of total generated waste).
In India, the total production of flyash was 184.14MT in the year of 2014-2015. Out of which total utilization of flyash was 102.59MT or 60.94% and in the year of 2015-16, the production of flyash was 176.74MT. Out of which total utilization of flyash was 107.77MT or 60.97%. Here it can be seen that the production and utilization both are increasing but there is still 40% of flyash that producing as a waste.
The flyash that remained unused will deposited as landfills and brings environmental problems. From these landfills, some of the heavy metals like mercury, cadmium and boron and the very fine particles of flyash leach to groundwater and cause the ground water contamination. And also unused flyash is the major cause of air pollution.
In the present study, a try has been made for effective utilization of flyash as a geoengineering material. Material that has been used in the study was class-F flyash and brought up from Adhunik Metaliks Limited, Sundergarh. The geotechnical properties like specific gravity, OMC, MDD, and UCS strength has been evaluated, of this virgin flyash.
To enhance the properties of the flyash, it was mixed with lime and slag at different proportion. Lime was mixed with flyash at 0%, 2%, 4%, 8% and 12% whereas slag was mixed at 0%, 5%, 10%, 15%, and 20%. A number of combinations of flyash, lime, and slag have been made for testing. The light compaction test has been done to determine the OMC and MDD of different mixes of flyash-GGBS-lime. In total 25 numbers of compaction test has been conducted to find out the OMC and MDD of the above mixes. Further UCS test has been done with different combinations of flyash-GGBS-lime compacted to their respective MDD at OMC. These samples were cured under an average temperature of 28ºC with samples sealed in wax for curing periods of 0, 7, 14 and 28 days and the UCS values were determined.
In the hydrometer analysis, it was found that the flyash particles are uniformly graded and the size of the particles lies between fine sand to silt size. The MDD determined was low at higher OMC. After treatment of flyash with lime and slag, the OMC reduced and MDD increased. The UCS determined for virgin flyash was very less and when treated with lime, it increased immediately marginally. UCS for the lime treated flyash samples were increased with increase in curing period. UCS for the slag-treated flyash samples was very low when tested immediately and with increasing curing periods the UCS values increased up to some extent. The strength of flyash treated with lime and slag was found to be highest when cured for 28 days of curing period.
At a given curing period flyash samples mixed with slag and lime shows a higher UCS value then flyash treated with the same percentage of lime without slag. This indicates a defiant advantage in adding slag to flyash. Slag which is rich in pozzolanic material like silica and alumina and it also contains a substantial amount of lime. However GGBS possess latent hydraulic properties which have to be activated by an alkali environment, here lime has used to provide an alkali environment to initiate the pozzolanic reaction of slag.