Characterization and preliminary processing of biomass Ash for possible utilization in alkali-activated materials

This paper presents the characterization of two biomass ashes and investigates their feasibility to be utilized as binder precursor in alkali-activated materials. The investigated Biomass ash (BA1 and BA2), which has CO32- and SO42- containing phases, have a high amount of Ca but a relatively low SiO2 and Al2O3 content with respect to class F fly ash. Both of them shows adequate reactivity for alkaline activation, although the reactive amount of Si and Al is limited. A promising route for the valorisation of BA is using it as a replacement of slag or fly ash for the production of alkaline cements. In this study, two biomass ashes have been used as replacement for fly ash inside a reference alkali-activated system containing 50 wt.% fly ash/50 wt.% blast furnace slag as binder precursor.
Mortar samples prepared with original BA revealed severe expansions due to the existence of metal Al phase within the ashes. Minor Al content in both BA were determined by the gas release potential tests, suggesting elimination of metallic phases in both BA is necessary. For this purpose, both ashes were pre-treated using low concentration NaOH solution in an earth moist state before sample preparation, which has been proved to be highly effective to eliminate expansions producing around 77 % increase in compressive strength compared with sample prepared with original BA ashes. This study largely promotes the understanding on the physical and chemical properties of biomass ash and provides a potential solution to this kind of waste material. In addition, it contributes to a theoretical basis for mixt design in BA as binder precursor in geopolymer concrete.