Study of a new potassium phosphate-based waste as an alkaline activator in alkali-activated binders: The açai seed ash

•Alkali-activated binders (AAB) were produced with açai seed ash (ASA) as alkaline activator.•Physicochemical characterization of ASA showed soluble K2O and P2O5 content.•ASA was combined with calcium hydroxide (CH) to remove harmful soluble phosphate.•Applications of ASA, CH and ASA/CH in AAB based on blast-furnace slag were carried out.•Results showed that the combination of ASA/CH led to a proper alkaline activator. Açai seed ash (ASA) was assessed as a new alkaline activator in alkali-activated binders (AAB) based on blast-furnace slag (BFS). ASA was produced by calcination of açai seed in the laboratory, and it is mainly composed of K2O (33.75 wt%) and P2O5 (22.59 wt%), with a pH of 12.3 and solubility in water of over 35 wt%. Due to the harmful soluble phosphate, ASA was combined with Ca(OH)2 (CH) to form insoluble calcium phosphate. Four AAB mixtures were assessed after 1 day of curing at 60 °C: BFS/water, BFS/CH/water, BFS/ASA/water, and BFS/ASA/CH/water. Microstructural studies of pastes showed that the BFS/ASA/CH/water mixture presented the formation of insoluble hydroxylapatite by X-ray diffraction, the highest reaction product content (26.52 %) by Fourier transform infrared spectroscopy, and densest microstructure by scanning electron microscopy. The compressive strength of mortars showed that BFS/ASA/CH/water mixture presented the highest value of 8.7 MPa.