Effect of phosphate addition on room-temperature-cured fly ash-metakaolin blend geopolymers

Abstract This paper details the influence of NaOH concentration, solid-to-liquid (S/L) and sodium silicate-to-sodium hydroxide (Na₂SiO3/NaOH) ratios, and the role of aluminum phosphate on the compressive strength development of the fly ash-metakaolin (FA-MK) blend geopolymer. The blended geopolymer was prepared with FA: MK at a ratio of 1:1, activated with a sodium-based alkali activator and then cured at room temperature. The optimum FA-MK blend geopolymer was formulated using a 10 M NaOH solution, a S/L ratio of 1.2, and a Na₂SiO3/NaOH ratio of 2.6. Monoaluminium phosphate (MAP) and aluminum dihydrogen triphosphate (ATP) were added as the source of aluminium phosphate. The FA-MK blend geopolymers have compressive strengths of 63.7 MPa and 55.5 MPa after being cured for 28 days with the addition of 1 wt% of MAP and ATP, respectively. The strength enhancement was assumed to be dictated by the formation of a matrix with a higher cross-linking geopolymer framework. The microstructure of blend geopolymer with MAP and ATP was smoother and compact compared to those without phosphate addition. In comparison, MAP addition in blend geopolymer yielded better physical and mechanical properties.