Abstract: Pumping agents can improve the fluidity of paste slurries. However,the effecting mechanism of pumping agents on the coagulability of paste slurries is not made clear. To solve this problem,a pumping agent was added for compression strength tests and environmental scanning electron microscopy(ESEM) observations. Compression strength test results indicate that the pumping agent can improve the coagulability of paste slurries,and the more the pumping agent amount,the better the coagulability of paste slurries.ESEM observation results show that large size floccules and pores decrease after adding the pumping agent,and the paste's microstructure is changed. The fractal dimensions of the paste's microstructure images were calculated based on the image processing technology and fractal theory,and the structure was quantitatively analyzed. The mechanism of improving the coagulability of paste slurries was discussed. It is found that the coagulability of paste slurries is affected by changing the size and number of flocculants,the porosity and the hydration reaction.