On the role of alkali content on one-part alkali activated slag pastes produced with tri- blend solid activators

The paper investigates the effect of tri-blend solid alkaline activators, based on sodium metasilicate pentahydrate, potassium hydroxide and sodium carbonate, on the physico-chemical features, the fresh and hardened performances of activated slag pastes prepared with the one-part methodology. Specifically, the alkali to slag ratio (Ac) and the total amount of the selected activators were varied at a fixed water/binder ratio, molar silica modulus (Ms) and relative mass activators ratio. Microstructures and reaction products were analyzed by means of X-ray diffraction, thermogravimetry, scanning electron microscopy and FT-IR. Fresh and elasto-mechanical properties of pastes were also evaluated. Amorphous calcium carbonate (ACC) as well as metastable vaterite were found in low reacted systems, produced by the preferential early-age reaction between dissolved carbonate ions present in the pore solution and the calcium ions released by the partial dissolution of the slag and stabilized by a sufficient high concentration of magnesium in the pore solution, not consumed to the form hydrotalcite. Sodium carbonate failed in extending the pot life of the investigated systems. The optimal alkali content was found to be close to 0.075 as it determines the development of compact and dense microstructure (2025 kg/m3), the maximum elasto-mechanical properties (elastic modulus of 26.1 GPa and compressive strength of 102.8 MPa at 28 days) and it allows to produce flowable mixtures (200 mm).