CHARACTERISTICS OF THE SLAG PASTE ACTIVATED BY ALKALINE SUBSTANCES FROM ELECTROLYSIS OF SWRO BRINE

Abstract

Brine generated by reverse osmosis (RO) desalination plant is usually discharged to the sea or inland water bodies, and constitutes a threat to ecosystems. Thus, research for the utilization of brine is needed for introducing environmentally friendly and economically viable management options for RO brines. Brine as a concentrated saline water has a potential as an activator to ground granulated blast furnace slag (GGBFS), one of the primary materials of alkali-activated materials (AAM) because it can be converted to an alkaline solution through electrolysis. However, electrolyzed brine has Cl- or ClO- ions, and a chemical precipitated sludge is produced during the pretreatment of the electrolysis process. This thesis presents the effects of Cl- or ClO- ions, and chemical precipitation sludge on the compressive strength, microstructure, and durability of alkali-activated GGBFS (AAS) paste. Basically, NaCl and NaClO did not act as activators of GGBFS at all. However, when NaCl and NaClO were added to NaOH solution, high compressive strength was obtained. Especially, the solution with NaClO and NaOH combined showed the highest compressive strength. This is the result of the formation of hydrotalcite-like compounds. In addition, AAS paste prepared by alkaline solution obtained by electrolysis after 3 times concentration of synthetic seawater showed a higher compressive strength than that prepared by NaOH solution. Ca(OH)2, Mg(OH)2, CaCO3 and Na2CO3 assumed to be chemical precipitation sludge were added into the NaOH-activated GGBFS paste. When Ca(OH)2 was mixed with NaOH-activated GGBFS, it caused a very high compressive strength loss. On the other hand, mixing of Na2CO3 led to higher compressive strength development. Mg(OH)2 and CaCO3 showed no significant change in compressive strength in the NaOH-activated GGBFS system due to their chemical stability. The precipitation sludge obtained by the addition of NaOH and Na2CO3 to synthetic brine was mixed with GGBFS and electrolyzed synthetic brine and that showed the highest compressive strength. AAS paste specimens prepared by NaOH solution mixed with NaCl or NaClO was exposed to the 5% H2SO4, 5% Na2SO4 and 5% MgSO4 solutions. First of all, the AAS paste showed higher durability than the OPC paste. In addition, the addition of NaCl or NaClO into the NaOH-activated GGBS paste led to a higher durability when exposed to MgSO4 solution. This is because Cl and ClO present in the AAS paste inhibit the transformation of C‒S‒H into M‒S‒H from external MgSO4 attack.