The effect of heat treatment on the mechanical and structural properties of one-part geopolymer-zeolite composites

This contribution presents the results of structural and compressive strength investigations on cured and high-temperature treated silica-based one-part geopolymer-zeolite composites. The specimens were synthesized from two different silica sources, sodium aluminate and water. The phase content as well as the compressive strength of the cured composites varied depending on the starting mix-design and the silica feedstock. Besides geopolymeric gel, A-type zeolites and hydrosodalites were the major reaction products. One of the silica feedstocks yielded significantly higher compressive strength (19 MPa), while the other one appears to cause less variation in phase content. Strength testing indicated an improvement on heating up to 200–400 °C (28 MPa) followed by a moderate decrease up to 700 °C. Above 700 °C the systems underwent new phase formation and shrinkage (volume decrease) deformations. After exposure at 1000 °C the different mixes consisted of a mix of several stuffed silica phases, almost pure hexagonal nepheline or amorphous phase. Depending on the mix-design, the onset temperature of the high temperature phase transformations varied.