Influence of sodium silicate powder silica modulus for mechanical and chemical properties of dry-mix alkali-activated slag mortar

Sodium silicate powders with different SiO₂/Na₂O (silica modulus) were characterized by solubility rate, pH, and chemical structure (by ²⁹Si MAS-NMR) and compared in the preparation of one-part (or dry-mix) alkali-activated blast furnace slag mortar. The low SiO₂/Na₂O indicated the beneficial presence of less-polymerized silica (Q¹ and Q² Si environments) and thus faster dissolution. Consequently, using sodium silicates with SiO₂/Na₂O of 0.9, 2.1, and 3.4 resulted 28 d compressive strengths of 103, 80, and 2 MPa, respectively, with increasing setting time and decreasing heat release in isothermal calorimetry. Adjustment of activator SiO₂/Na₂O from 2.1 or 3.4 to 0.9 by adding NaOH powder resulted increased or decreased mechanical properties of mortar, respectively, depending on the initial silica modulus. These properties were not, however, similar to those obtained with sodium silicate having SiO₂/Na₂O of 0.9 originally. Depending on the case, the added NaOH can be consumed for dissolving sodium silicate activator, slag, or the forming (C,N)-(A)-S-H gel.