AbstractUltra-low-density fiberboards (ULDFs) were prepared by a liquid frothing technique using plant fibers as the matrix and Si-Al compounds as the filler to be used as a versatile bio-based composite. Si-Al compounds played an important role in the fire properties of ULDFs. Fire intensity and the amount of volatiles were significantly restrained because of the Si-Al compounds. To determine the combustion mechanism of ULDFs treated by Si-Al compounds, the microstructure of burned specimens was tested by chemical analysis, X-ray diffractometer (XRD), and infrared spectrometer (IR). According to the results from gas chromatography, glucose, xylose, and mannose disappeared in the bottom ashes. After combustion, the XRD profiles of the two ashes became weaker and broader; the sharpest peaks at 18.6° (2q) that represented Si-Al compounds remained; the obvious peaks at 22° (2q) from cellulose were gone. The results from IR suggested the characteristic functional groups OH, CH, and C=O from carbohydrate also disappeared, and absorbance at 1200 to 400 cm-1, which attributed to the vibration of Si-O, Al-O, and Si-O-Si bonds, increased. In conclusion, fibers are almost completely pyrolyzed at 780 °C. The crystalline structure of Si-Al compounds is rearranged and more amorphous silicon oxide and aluminum oxide are generated.