Gel composition and molecular structure of alkali-activated metakaolin-limestone cements

The influence of the compositional parameters Na2O/Al2O3 = 0.60–1.08, SiO2/Al2O3 = 2.42–3.17 and of 20–60% limestone on the compressive strength, gel composition and structure of alkali-activated metakaolin-limestone cements was studied. The reference formulation of SiO2/Al2O3 = 3.17, Na2O/Al2O3 = 1.08 and 20%limestone, formed sodium aluminosilicate hydrate (N-A-S-H) intermixed with a Ca-containing N-A-S-H (N-(C)-A-S-H), both with 3D-network structure. A reduced SiO2/Al2O3 = 2.42 resulted in the formation of Zeolites and calcium-aluminosilicate-hydrate (C-A-S-H) as secondary phases that reduced the strength. A lower Na2O/Al2O3 = 0.60 reduced the reactivity of metakaolin and the Al uptake in the gels, forming silica gel and reduced the strength of mortars. Increasing the limestone load to 60% increased the Ca2+ uptake in N-(C)-A-S-H via ion exchange Na+↔Ca2+ as charge balancers, without modifying the 3D-network structure nor the strength of pastes. Limestone can be considered as a precursor; its addition reduced the alkali consumption, changed the chemical composition and the assemblage of the reaction products, while preserving their 3D-network structure. •Compositional parameters were studied on Limestone-Metakaolin alkaline cements.•The main reaction products were amorphous 3D structured N-A-S-H and N-(C)-A-S-H.•Using 60% limestone increased Ca2+ uptake in N-(C)-A-S-H via ion exchange Na+↔Ca2+.•The 3D-network of N-(C)-A-S-H was preserved with the higher Ca2+ contents.•Reduced SiO2/Al2O3 formed zeolites and C-A-S-H and reduced the strength.