Microstructural evolution and its impact on the mechanical strength of typical alkali-activated slag subjected to accelerated carbonation

This study aims to comprehensively investigate the evolution of microstructure, mechanical strength, and their correlation in alkali-activated slag (AAS) mortars, designed for application in the immobilization of liquid radioactive waste, under accelerated carbonation conditions (1% CO 2 , 20 °C and 60% RH). To gain insights into the underlying microstructural changes, CO 2 uptake and decalcification of C-A-S-H were analyzed using TGA/DSC and EDS. The pore structure of AASs was systematically assessed across nano- to macro-scales, employing N 2 -adsorption, MIP, and SEM segmentation. Generally, carbonation led to a decrease in total porosity, primarily attributed to the reduction in meso-macropore volume. However, the pore size distribution of AAS exhibited a complex alteration over varying carbonation durations. Carbonation significantly reduced flexural strength, whereas its effect on compressive strength was comparatively milder. Notably, an evident linear correlation emerged between porosity and compressive strength in both reference and carbonated AASs. • Increasing w/b ratio from 0.35 to 0.55 sped up the carbonation and decalcification. • Total porosity decreased by up to 12.5%, while PSD changed complexly. • Compressive strength was retained or increased by up to 18%. • Flexural strength decreased by more than half. • AAS's compressive strength and porosity showed a linear correlation both before and after carbonation.