Durability Index and Microstructure Evolution of Magnesium Sulfate-Exposed One-Part and Two-Part Alkali-Activated Slag Concretes

Abstract

Durability enhancement of low-carbon alkali-activated cements, as an emerging alternative to portland cement, is imperative for optimal performance in various conditions. This study examines the influence of one-part and two-part activators ( Na 2 SiO 3 · 5 H 2 O , Na 2 SiO 3 -anhydrous, and NaOH + Na 2 SiO 3 ) on the long-term performance of alkali-activated slag concretes exposed to a 5% magnesium sulfate ( MgSO 4 ) solution for 450 days at 23 ° C ± 2 ° C . Experimental results show that the mixture activated with Na 2 SiO 3 -anhydrous exhibits compressive strength 17% and 7% higher than the NaOH + Na 2 SiO 3 and Na 2 SiO 3 · 5 H 2 O mixtures, respectively, after prolonged sulfate exposure. Its electrical resistivity decreases by 32.7%, compared to reductions of 46.8% and 44.6% for the two-part and pentahydrate mixtures. Mass loss after 450 days was 3.5% for Na 2 SiO 3 -anhydrous, 4.8% for Na 2 SiO 3 · 5 H 2 O , and 5.9% for NaOH + Na 2 SiO 3 . A dimensionless durability index, previously developed for evaluating performance in acidic environments, is applied here to compare the mixtures under magnesium sulfate exposure. Based on the outcomes of the durability index calculation, the one-part mixture, activated with Na 2 SiO 3 -anhydrous, exhibits approximately 34% greater durability compared with the two-part mixture. These results confirm that the type of activator plays a crucial role in controlling microstructural stability and durability in sulfate-rich environments.