Effect of green liquor dregs as an alkali source for alkali-activated blast furnace slag mortar

Sodium-rich green liquor dregs (GLD) are one of the main landfilled residues generated in pulp mills, and to date, no economically profitable ways to utilise GLD has been identified. In this study, GLD have been studied to be used as an alternative alkali source together with alkali-activator, sodium metasilicate (Na2SiO3), for blast furnace slag (BFS) mortar. Microsilica was used as an additional silica source to ensure steady Na2O:SiO2-ratio (0.16). In purpose to remove the remaining organic carbon and increase the suitability of GLD to be used in alkali-activated materials, it has been first thermally treated in 525 °C. The amount of GLD in the recipes varied from 0 to 45 wt%. Calorimetry measurements showed that increasing the amount of GLD reduced the hydration heat and postponed the reaction. With thermogravimetric analysis, X-ray diffractometry and scanning electron microscopy it was found that hydrated samples with more GLD contained more carbonate phases, which indicated that the reactivity of GLD was lower in comparison with that of other raw materials. Compressive and flexural strength measurements showed that increasing the amount of GLD affected the early strength gain (2 and 7 days) of the samples, but at the age of 29 days, the difference was not as high. The compressive strength of the reference sample at 29 days was 59 MPa, while the compressive strength of four sample recipes containing GLD (up to 36 wt%) was higher than 49 MPa. According to the results, green liquor dregs have potential to be used as a secondary alkali source in alkali-activated materials, but more detailed study should be conducted for the recipe in the future.