Alternative alkali activator from pulp mill waste – One-part blast furnace slag mortar activated with recovery boiler fly ash

Alternative alkali activators are a growing research area in the field of alkali-activated materials (AAMs) because conventional chemical-based activators, such as sodium hydroxide and sodium silicate, have a large environmental footprint and high costs. Industrial residues are widely studied as substitutional cementitious materials in concrete and precursors in AAMs but not much as activators. In the present study, sodium rich waste from pulp mill was used as an alkali source in AAMs. Recovery boiler fly ash (RBA) is dust-like waste fraction removed from flue gases of recovery boiler by electrostatic precipitators. It consists mainly of alkalis, sulfate, and carbon while sodium sulfate is the main phase. This work utilized RBA as a sole one-part alkali activator for a precursor mixture containing 95 wt% of blast furnace slag and 5 wt% of cement, and the results were compared with two references, one without activator and the other activated with commercial sodium sulfate. The setting properties and strength gain were comparable between the RBA-activated and commercial activator-activated samples. Calorimetry data showed also similar reactivity between them, and X-ray diffractometry, thermogravimetric analysis, and scanning electron microscopy measurements revealed that the same phases were formed. The main issue was the microcracking of the paste samples when using RBA as an activator. Results confirm the earlier findings from the literature, the activator dosage did not greatly impact the properties: in this study, the initial proportion of 1% or 3% Na2O was the most suitable.