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

Abstract 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.