Effect of the Alkali-Sulphate Activators on the Hydration Process of Blast-Furnace Slag Mortars and Pastes

The alkali-activation of blast-furnace slags (BFSs) is a topic largely studied today. However, some types of activators, more environmentally friendly, have been less studied such as alkali-sulphate activators. In this study, the effect of four alkali-sulphate activators (Na₂SO₄, K₂SO₄, MgSO₄, CaSO₄.2H₂O) is investigated to better understand the effect of cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) and of a high content of sulphate ions (SO₄^2-) on the hydration process of BFS and the nature of the hydrates. To reach this objective, a large experimental campaign is carried out to characterize the pore solution, the hydration products and the kinetics of the chemical reactions. As the temperature seriously affects the hydration advancement, the activation energy coefficient is also determined experimentally to compare the results as function of the equivalent time. Finally, a new method is proposed to determine the evolution of the hydration degree of BFSs, a key parameter for predicting the evolution of the hydrates through a thermodynamic modeling. The results indicate that the use of sodium sulphate results in faster hydration kinetics and shorter setting times due to a higher pH of their pore solution, leading to a larger rate of C-A-S-H type gel precipitation from the initial setting time to the long term and a higher hydration advancement. These hydration products are characterized by a higher content of Na⁺ and a denser rim around the surface of anhydrous particles. The effect of K₂SO₄, MgSO₄ and CaSO₄.2H₂O on the BFS activation efficiency is limited compared to Na₂SO₄ due to their lower rate of C-S-H type gel evolution at early age. It is directly related to the pH of the pore solution and the effect of cations on the nature of hydrates. However, the compressive fis research study, a large strength beyond 28 days is more significant for mortars activated with Na₂SO₄ and MgSO₄, satisfying the strength requirement of the repaired mortars (R2 and R3) due to the larger contents of C-(N)-A-S-H/M-S-H-type gels, ettringite and hydrotalcite.