Reactivity of waterglass in cementitious systems

Abstract

This research investigates the chemical reactivity of waterglass, a sodium silicate (Na₂O·nSiO₂·yH₂O). This research establishes a framework for thermodynamic modeling of waterglass systems that contain calcium hydroxide and potassium hydroxide. Conventional pozzolanic reactivity tests used for supplementary cementitious materials (e.g., fly ash) that rely on heat release and calcium hydroxide consumption cannot adequately capture the waterglass reactivity, primarily due to the high reaction rates. Capturing the heat released using isothermal calorimetry requires procedural changes in the testing protocol. Specifically, the test is modified by lowering the temperature of the test to slow the reaction rate and using internal mixing to capture the initial reaction. The heat release and calcium hydroxide consumption are used to quantify the reactivity. The theoretical relationship between heat, reactivity, and calcium hydroxide consumption is related to the molar ratio of SiO₂ to Na₂O, also known as the waterglass modulus (n). Thermodynamic modeling and X-ray powder diffraction results demonstrate that the mixtures react to produce Tobermorite-like calcium-silicate-hydrate (C–S–H, C/S = 1.42), which increases in amount with waterglass modulus. Finally, the developed approach demonstrates how the quantified reactivity is used in thermodynamic calculations to predict the reaction products and paste properties.