Solid Reactant-Based Geopolymers from Rice Hull Ash and Sodium Aluminate

High carbon rice hull ash and solid sodium aluminate were used as silica, alkali and alumina sources to synthesise one-part "just add water'' geopolymer binders. Three binders with different Si/Al ratios and different water contents were studied. Due to the high carbon content of the samples, using a high amount of water is required to satisfy the workability of the binders. Similar to traditional geopolymer systems, high water content increases the crystallinity, decreases the reaction rate and negatively affects the microstructure of samples. In high carbon rice hull ash system, silica concentration is not a suitable indication of the silica availability for reaction, and the amount of unburned carbon in ash particles affects silica release rate. Increasing the silica content of raw materials leads to higher amount of Si/Al ratio in the final geopolymer binder and improves the mechanical and microstructural properties of samples. All samples studied here successfully made geopolymer binders. The highest strength achieved was 22 MPa after 3 weeks.