4D imaging of microstructural evolution in alkali-activated slag cement pastes: linking pore-network topology to performance

This study employs 4D X-ray micro-computed tomography (μCT) imaging to directly capture the microstructural evolution of sodium hydroxide-, silicate-, and carbonate-activated slag cement pastes during curing (14–180 days). By combining time-lapse μCT with electron microscopy, pore-network topology parameters — including connected porosity, tortuosity and percolating clusters — were quantified, to elucidate links to their mechanical performance. The results demonstrate that, although activator chemistry governs reaction pathways and overall densification, performance is influenced primarily by the geometry and connectivity of the pore-network rather than the degree of reaction or total porosity. Silicate activation generated the most refined networks, with connected porosity reduced to ∼1.5 % and permeability to ∼10⁻¹⁶ m² by 180 days, giving the highest compressive strength. Hydroxide activation produced crystalline, heterogeneous gels that left open transport pathways, whereas carbonate activation followed a delayed densification route, progressively converging at later ages. These mechanistic insights show that strength in alkali-activated slag cements depends on how reaction products disrupt percolating pores and refine network topology, highlighting the unique capability of 4D μCT imaging to guide the predictive design of clinker-free binders.