Synthesis and Characterization of Fly Ash Geopolymer Sand

Geopolymers are an alternative binder to portland cement in the manufacture of mortars and concrete, as its three-dimensional aluminosilicate network imparts excellent mechanical properties. Use of geopolymers in place of ordinary portland cement is favored owing to the possible energy and carbon dioxide savings. River sand is another construction industry material that needs development of a sustainable alternate in India. Geopolymerization of fly ash amorphous silica mixtures is employed to produce fine aggregates as a possible replacement to river sand. Geopolymerization of fly ash amorphous silica mixtures in 10 M NaOH solution at 100°C for 7 days produced fine aggregates [termed fly ash geopolymer sand (FAPS)] that had comparable grain size distribution, specific gravity, and improved frictional resistance with river sand. The FAPS particles exhibited more alkaline pH (12.5) and higher total dissolved solids (TDS) concentration ($\mathrm{TDS}=747\mathrm{mg}/\mathrm{L}$) in comparison to the river sand specimen ($\mathrm{pH}=7.9$ and $\mathrm{TDS}=32.5\mathrm{mg}/\mathrm{L}$). However, when used as fine aggregate in mortar, FAPS–mortar specimens develop similar pH, lower TDS, similar compressive strength, and modulus in relation to river sand–mortar specimens. The experimental results suggest that FAPS particles have the potential to replace river sand in the manufacture of mortar and concrete.