Evolution of hydration in cement blends with incorporation of activated low-kaolinite clays: Insights into the preferred aluminum uptake by C-(A)-S-H

Despite their global abundance, heterogenous clays are often excluded from SCM applications, due to their limited pozzolanicity. This study investigates hydration evolution, particularly aluminum uptake pathways, in statistically designed cement blends incorporating thermo-mechanochemically activated low-kaolinite clays. Despite kaolinite contents below 40%, a 30% binary blend achieved 110% and 125% of OPC strength at 7 and 56 days, respectively, while reducing total porosity by 42% at 56 days. 29 Si NMR indicated an increase in silicate chain length in C-(A)-S-H, correlating with pore structure refinement and strength gain in 56 days of hydration. 27 Al NMR revealed a preferential incorporation of aluminum into C-(A)-S-H rather than AFm phases. This behavior is attributed to the lower alumina availability in the system compared to LC3 blends, suggesting that in such environments, C-(A)-S-H becomes the dominant host phase for aluminum. This incorporation pathway reduces the Al availability for carbonate-AFm formation, limiting the synergy typically observed in LC3 systems with added limestone. • Thermo-mechanochemical activation of 2:1 clays yields high strength binders. • Strength and pore refinement are driven by high reactivity and matrix densification. • At 56 days 29 Si NMR shows longer silicate chains in C-(A)-S-H for activated clay blends. • 27 Al NMR shows Al entering C-(A)-S-H rather than forming AFms in low-aluminate systems. • Al uptake by C-(A)-S-H reduces CO 3 -AFm formation, limiting the limestone reactivity and synergy unlike in LC3 systems.