Potassium-rich biomass ashes as activators in metakaolin-based inorganic polymers

The use of biomass ashes as an alkaline activator in the synthesis of metakaolin-based inorganic polymers was investigated in the present work. Maize stalk and maize cob ashes reached the highest pH after mixing with water, 13-14, and contained the largest amount of potassium, 30-32 wt.% K2O. Of these two, the maize cob ashes showed a higher reactivity and reaction extent when mixed with water and metakaolin calcined at an optimized temperature of 700 °C. A maximum reaction enthalpy of − 372 J/g was reached with a mixture with an ash to metakaolin mass ratio of 0.9. In attenuated total reflection Fourier-transformed infrared spectroscopy, the wavenumber shift of the Si-O-T (T: Si, Al) stretching band upon activation proved to be linearly related to the compressive strength of pressed samples, cured with all surfaces exposed at 80 °C or 60 °C for 48 h. A maximum strength of 27 MPa and a wavenumber shift of the Si-O-T stretching band of 52 cm -1 were observed for samples with a maize cob ash to metakaolin mass ratio of 0.9 cured with an open surface at 80 °C. This shift and increase in strength is explained by a larger reaction extent, which was also observed using calorimetric techniques, and confirmed by electron probe micro-analysis. The curing conditions were altered to optimize the microstructure. The temperature was lowered to 60 °C, the samples were wrapped in plastic to avoid water evaporation, or the samples were subjected to a pre-cure of 24 h at 20 °C before curing at 80 °C. The latter resulted in the maximum compressive strength of 40 MPa.