The role of CaO and MgO incorporation in chloride resistance of sodium carbonate-activated slag

Abstract

Partial substitution of CaO or MgO for slag accelerates the reaction kinetics and strength gains of sodium carbonate (Na2CO3)-activated slags, yet their impact on the durability performance remains unknown. This study compares the effects of CaO and MgO substitution at various contents (2.5% and 5%) on the fresh properties, reaction kinetics, phase assemblage, pore structure, chloride binding capacity and chloride diffusion resistance of Na2CO3-activated slags. The results show that in addition to its higher effectiveness in boosting strength development, the substitution of CaO for slag results in superior enhancement in chloride binding and resistance of Na2CO3-activated slag compared to the system modified by MgO. Hydrotalcite is identified to have a comparable contribution to the chloride binding of CaO- and MgO-modified AAS, but the greater Friedel's salt formation is the main reason for the higher chloride immobilisation capacity of the CaO-modified AAS. The results demonstrate the technical promises of CaO and MgO as substitutes to control reaction kinetics of Na2CO3-activated slag while improving its chloride durability.