Influence of Na2CO3 content on the properties of alkali-activated materials based on biomass fly ash and phosphogypsum

Abstract

Biomass fly ash (BFA) and phosphogypsum (PG) are by-products of wood biomass energy and phosphoric acid production. PG is formed during the decomposition of phosphate raw materials by sulfuric and phosphoric acids. About 1500 thousand tons of PG waste is generated in Lithuania each year, which poses serious environmental problems. This study reveals the possibility of using these two difficult-to-utilize waste products in alkali-activated materials (AAM). Less alkaline Na2CO3 and Na2SiO3 were used as the activator. This paper systematically investigated the effect of Na2CO3 content on the rheological properties, structure formation, product synthesis, and physical-mechanical properties of BFA and PG-based AAM pastes. Increasing the Na2CO3 content in the alkaline activator solution (AAS) from 0 to 30 wt% increases the density (28 d, from 1335 to 1433 kg/m3) and compressive strength (28 d, from 14.17 to 18.36 MPa) of the AAM samples, but shortens the setting time of the AAM pastes. X-ray diffraction (XRD) analysis shows that increasing the Na2CO3 content in the AAS up to 30 wt% significantly reduces gypsum crystallization. When the Na2CO3 content varies between 5-10 wt%, the ratio of gypsum to the new phase kottenheimite is almost one. With an increase in the Na2CO3 content up to 30 wt%, this ratio increases more than two times. The findings of this study contribute to the applicability of waste materials (e.g., BFA and PG) and the development of sustainable AAM.