Physical and mechanical properties of unheated and heated kaolin based-geopolymers with partial replacement of aluminium hydroxide

Amorphous aluminium hydroxide and aluminium oxy-hydroxide were used as replacements in the synthesis of kaolin based-geopolymers. Physical and mechanical properties of geopolymers cured at 60 degrees C and exposed at elevated temperatures (300-1200 degrees C) were investigated. It was found that when cured at 60 degrees C, specimens with replacements exhibited low compressive strength as compared to those without replacement. Conversely, when heated as from 1100 degrees C, both compressive strength and volume stability of specimens with replacement were improved. Thus, specimens without replacement were morphologically damaged at 1150 degrees C whereas those with 10% by mass of amorphous aluminium hydroxide or with 30% by mass of aluminium oxy-hydroxide showed good volume stability at 1150 and 1200 degrees C respectively. Additionally, geopolymers with 30% by mass of aluminium oxy-hydroxide that were heated at 1100 degrees C showed compressive strength of 29.1 MPa whereas those with 10% by mass of amorphous aluminium hydroxide gave 60.2 MPa at 1150 degrees C. This was attributed to the improvement of compactness of specimens along with the formation of stable crystalline phases. Yet, heating geopolymers at 1200 degrees C led to partial dissolution of nepheline and carnegieite which generates closed pores hence decrease of compressive strength. Partial replacement of amorphous aluminium hydroxide or aluminium oxy-hydroxide in the synthesis of kaolin-based geopolymers allows both improvement of compressive strength and thermal stability of heated kaolin-based geopolymers.