Production and characterization of geopolymer blocks based on hydroxyapatite rich biomass ashes

Vegetal and animal derived biomass ashes were selected to realize new sustainable chemically consolidated ceramics via acid or alkaline activation. Mixed biomass ashes are mainly based on calcium phosphate and secondly on aluminosilicate compounds; they still represent an unexplored source material to be used in alternative ceramics production for building industry by low-cost manufacturing processes. Mixed biomass ashes were used to produce room-temperature hardened materials by chemical activation with solutions of phosphoric acid or potassium poly-silicate. Reactive mineral powders (metakaolin, magnesia and wollastonite) were also added to fasten the consolidation and the formation of a binding phase. The obtained formulations were finalized to maximize the amount of ashes disposed. Microstructural, physical and mechanical properties of the obtained materials were evaluated, along with their water stability. Acid-activated samples, even in presence of secondary reactive precursors, maintained a crystalline structure, while alkali-activated ones were able to form a partially amorphous and well consolidated matrix, with compressive strength of about 20 MPa.