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 °C and exposed at elevated temperatures (300–1200 °C) were investigated. It was found that when cured at 60 °C, specimens with replacements exhibited low compressive strength as compared to those without replacement. Conversely, when heated as from 1100 °C, both compressive strength and volume stability of specimens with replacement were improved. Thus, specimens without replacement were morphologically damaged at 1150 °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 °C respectively. Additionally, geopolymers with 30% by mass of aluminium oxy-hydroxide that were heated at 1100 °C showed compressive strength of 29.1 MPa whereas those with 10% by mass of amorphous aluminium hydroxide gave 60.2 MPa at 1150 °C. This was attributed to the improvement of compactness of specimens along with the formation of stable crystalline phases. Yet, heating geopolymers at 1200 °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. [Display omitted] •Aluminium hydroxide and oxy-hydroxide were used to replace kaolin (30% by mass).•The mixtures were used to produce kaolin based-geopolymers via alkaline activation.•Compressive strength of geopolymers with up to 30% of replacement was low.•Heated products with 10% of aluminum hydroxide gave compressive strength of 60 MPa.•Certain proportions of replacement allowed thermal stability of heated products.