The relationship between composition, structure, and solubility in sodium aluminosilicate hydrates
This work seeks to identify the fundamental precursor-to-product composition relationships by which inorganic polymer binders (IPBs) derive their chemical structure and to link that chemical structure to their engineering properties. IPBs, also known as geopolymers, are a new class of construction materials that will potentially serve as a low-energy, low-CO₂ alternative to ordinary portland cement (OPC). These binders are synthesized by the activation of an aluminosilicate solid, such as coal fly ash, by a highly alkaline aqueous solution generally consisting of an alkali hydroxide with added sodium silicate to control the system’s silica content. The IPB products are similar in composition to zeolites, but are amorphous to semi-crystalline. An extensive body of research has demonstrated comparable mechanical properties (compressive strength, stiffness) as well as superior dimensional stability and durability (resistance to corrosion, alkali-silica reaction, acid attack) of IPB concrete compared to OPC concrete. Much of the existing research, however, has focused on the characterization and evaluation of a variety of aluminosilicate sources with significant levels of variation from one source to the next and inhomogeneity within a given source; the basic mechanisms that govern product formation, microstructure development and ultimately engineering properties are still poorly understood. The research presented here aimed to reduce the large number of variables present when using natural precursors to better understand the effects of manipulating each variable on the composition and structure of reaction products. This was accomplished by synthesizing and examining sodium aluminosilicate hydrate, the primary binding phase in IPBs, from reagent grade materials across a range of compositions, allowing complete stoichiometric control of the constituents by eliminating variability in the composition of solid precursors. The impact of temperature and bulk composition on the structure and composition of the solids was investigated. Additionally, temperature dependent solubility products (K [subscript sp] ) of the IPB products were determined by monitoring the ionic concentration of the solutions over time. Solubility products are a necessary prerequisite for the future development of thermodynamic models that can help predict IPB mechanical properties and durability.