Unburned carbon removal effect on compressive strength development in a honeycomb briquette ash-based geopolymer

The potential of honeycomb briquette ash as a new source material for the manufacture of geopolymers was examined. The successful geopolymerization of honeycomb briquette ash was attained by means of NaOH treatment. The concentrations of the major and minor elements of honeycomb briquette ash are in the normal range of the chemical composition of fly ash and appropriate for the synthesis of geopolymers. Honeycomb briquette ash contains 9.88 wt.% unburned carbon, however, all but 0.04% of the unburned carbon was removed in clean ash by means of a froth flotation process. The development of the compressive strength of the geopolymer bodies is affected by the unburned carbon content, the molar concentration of the NaOH solution, the curing temperature, and the curing time. With a few exceptions, a high compressive strength was achieved for the unburned carbon-free ash-based geopolymer bodies regardless of the curing temperature and the curing time. A geopolymer body was prepared by the dissolution of unburned carbon-free ash in 12 M of NaOH solution and curing at 80 °C for 7 days. Under these conditions, the compressive strength of the geopolymer body increased to 65.63 MPa. This value is almost double the highest value of a geopolymer body, prepared by the dissolution of as-received ash in 12 M of NaOH solution and curing at 80 °C for 28 days. In conclusion, unburned carbon-free honeycomb briquette ash could be an alternative to Portland cement as a prospective cementitious material.