Interface microstructure and compressive behavior of fly ash/phosphate geopolymer hollow sphere structures

• Novel FPGHSS were prepared by the pre-bonding and curing technology.
• Compressive properties of FPGHSS showed a typical character of cellular materials.
• Fracture morphology was observed during compression to reveal failure mechanisms.
• Microstructures of fly ash, phosphate geopolymer and interface area were observed.
• Interface microstructure and its role on the compressive behavior were discussed.

In the present work, novel fly ash/phosphate geopolymer hollow sphere structures (FPGHSS) were prepared by the pre-bonding and curing technology. Moreover, the interface microstructure and its role on the compressive behavior were studied. The compressive properties of FPGHSS demonstrated a typical character of cellular materials, with three well defined stages in stress–strain curve. The compressive strength was approximately 5.8 MPa, and the failure of FPGHSS was mainly due to the evolution of the multi-collapsed layers of fly ashes and large macro-cracks during the compression. The microstructure of fly ash was composed of aluminosilicate glass phase, crystalline quartz and mullite. The phosphate geopolymer comprised of aluminum-phosphate phase and α-Al2O3, and the nano-pore structure was observed. Moreover, the chemical reaction interface of FPGHSS was generated. It should be noted that the horizontal cracks were mainly produced in fly ashes, and the evolution of horizontal cracks leaded to the clasped layers. Moreover, the large macro-cracks propagated preferentially in the phosphate geopolymer and along the interface region of FPGHSS, due to the dense glass phase of fly ash and the chemical reaction interface. Further, the interface structure between α-Al2O3 and amorphous phase could increase the propagation path of cracks in phosphate geopolymer.