Suitability of Fly Ash-Foundry Sand Mixes in Geotechnical Applications

Abstract

Fly ash (FA), a by-product of coal combustion in thermal power plants, generates approximately 271 million tons annually in India (CEA report, 2022). Its high pozzolanic reactivity makes it a promising precursor for geopolymer cement when activated with alkaline solutions such as sodium hydroxide (NaOH). Foundry sand (FS), a silica-rich granular waste from metal casting industries, further enhances the cementitious potential of FA due to its significant silicate content. This study investigates the index and engineering properties of FA-FS mixes. It assesses their geoengineering performance under treatments of different concentrations of NaOH (0M to 12M) at curing ages of 0 and 7 days. The FA-FS mixture's maximum dry density (MDD) increased from 1.3 g/cc to 1.51 g/cc, and the optimum moisture content (OMC) reduced from 24.5% to 19.8% with the increase in FS content from 0% to 30%. UCS increased significantly while hydraulic conductivity decreased at higher alkali concentrations and for longer curing durations. The maximum UCS of 1.86 MPa was obtained at 20% FS with 12M NaOH cured at 20°C for 7 days. SEM analyses indicate significant phase changes, evidencing the formation of geopolymeric N-A-S-H gels and better particle packing in the alkali-activated mixtures. These observations show that FA-FS mixtures can serve as green replacement for conventional cement-treated base materials. They also comply with IRC:37-2018 and IRC: SP:89 base and sub-base standards. Improved strength of FA-FS mixes make them durable and environmental friendly.