Driven by the environmental concern, novel sustainable geopolymer based syntactic foams (GSFs) were developed for the first time. The different GSFs were prepared by using two types of hollow microspheres, namely K15 glass microspheres and Phenoset BJO-0930 phenolic microspheres with microspheres content up to 50 vol%. The underlying mechanisms of the synergistic effects achieved between the micro spheres and the matrix on the compressive and thermal properties were explored originating from chemical interaction of matrix and hollow microspheres. The introduction of hollow glass microspheres created a unique interfacial bonding between the microspheres and the matrix and simultaneously altered the molar ratio of Si:Al of the matrix from 1.85:1 to 2.23:1, which positively contributed to the compressive strength of the GSFs. More significantly, the interfacial bonding was able to survive under high temperature of 500 degrees C, an attribute lacking in polymer composite or foam systems. Such behavior was beneficial towards compressive strength enhancement by discontinuing the crack propagation caused by water evaporation under high temperature exposure. A 90.1% compressive strength retention was achieved for the GSFs containing 50 vol% of hollow glass microspheres after exposure to 500 degrees C for 2 h. Scanning electron microscope (SEM) studies were performed to determine the failure mode of the GSFs containing different types of hollow microspheres. Direct flame tests validated that the GSFs were superior to polymer based syntactic foams (PSFs) in terms of fireproofing and eco-friendliness. (C) 2016 Elsevier B.V. All rights reserved.