High-temperature performance of the optimized fly ash-metakaolin based geopolymer mixture containing Phosphate

The implementation of sustainable development in construction industries has led to the research on replacement materials that are thermally stable with lower environmental impact. One of the efforts is to promote alternative cementitious binder by utilizing alumina- silicate source (metakaolin) and wastes produce from industrial sector (fly ash). The goal of this research was to examine the optimize mixing formulation in order to produce fly ash-metakaolin blended geopolymers. Ambient temperature aging condition was adopted in this study to widen its application in real construction industry. In this study, the effect of mixing proportions which are sodium hydroxide (NaOH) concentration (6M, 8M, 10M, 12M and 14M), solid-to-liquid (S/L) ratio (0.6, 0.8, 1.0, 1.2 and 1.4) and sodium silicate-to-sodium hydroxide (Na₂SiO₃/NaOH) ratio (2.4, 2.6, 2.8, 3.0 and 3.2) on the physical, compressive strength, microstructural, phase and functional group properties of blended geopolymers were investigated. Monoaluminium phosphate (MAP) and aluminum dihydrogen triphosphate (ATP) were added as the source of aluminum phosphate. The effect of MAP (0 wt.%, 1.0 wt.% and 3.0 wt.%) and ATP (0 wt.%, 1.0 wt.% and 3.0 wt.%) loading on the physical, compressive strength properties and characterization of blended geopolymer at ambient temperature was also elucidated. Moreover, the thermal stability geopolymers under elevated temperature exposures was investigated. Standard fire test such as non-combustibility and fire propagation test were considered to provide an accurate assessment of the geopolymers of its passive fire protection. The optimize blended geopolymer was formulated using a 10 M NaOH solution, a S/L ratio of 1.2, and a Na₂SiO₃/NaOH ratio of 2.6. The highest compressive strength of 54.7 MPa was dictated by the formation of a matrix with a higher cross-linking geopolymer framework. The addition of 1 wt.% MAP and ATP achieved the compressive strengths of 63.7 MPa and 55.5 MPa after 28 days of aging, respectively. The microstructure of blended geopolymers with MAP and ATP was compact compared to those control geopolymers under ambient temperature. The elevated temperature exposures findings identified that with an additive level of 1.0 wt.% MAP and ATP experienced an increase in compressive strength by 1.5 % and 16.5 %, respectively once heated from room temperature up to 200 °C. The free aluminum species drive a rapid gel transition, due to the supplementation of the free Al in the aqueous phase providing the immediate gel formation. At the temperature of 1000°C, the blended geopolymers with1.0 wt.% ATP and MAP have higher compressive strength by 22.9 % and 10.8 % than that of control geopolymers, respectively. The blended geopolymers showed a high thermal stability, with no sign of spalling and disintegration at temperature of 1000°C. The onset of crystallization starts at temperature of 800°C. It was shown that the blended geopolymer had a homogeneous microstructure and formed crystalline phase of nepheline and berlinite, which served as a matrix filler at high temperature. The fire propagation index of 0 with no flame was observed to propagate on the blended geopolymers when the flame test was ignited. Thus, the material was declared to have good fire resistance ability and can be used in the building construction in accordance to Clause 204 in Uniform Building By-Law.