Abstract: To explore the modification effect of CO2 foam on the properties of cementbased grout, the cementbased grout modified by CO2 foam is used to replace the original grout with the same watercement ratio. Then, a range analysis is conducted for the ratios of waterbinder, fly ashcement, bindersand, and bentonitewater of the slurry to comprehensively determine the main factors affecting the performance of the modified slurry and optimal mixing proportions. In addition, the performance parameters of the modified slurry are compared with those of the synchronous grouting slurry currently being used in the shield tunneling of the Nanjing metro line 7. Further, microscopic mechanism tests are performed using the scanning electron microscope and Xray diffraction. The test results show that: (1) For the slurryfoam ratio of 1∶4.5, watercement ratio of 1∶2.4, foaming pressure of 0.02 MPa, and rotational speed of 70 r/s, the compressive strength of the slurry is high, and the density, water separation rate, specific gravity, and other properties can meet the actual site requirements. (2) The carbonation of CO2 reduces the time for foamed grout to achieve a higher strength. In addition, the results of the range analysis show that the optimal waterbinder, fly ashcement, bindersand, and bentonitewater ratios are 0.8, 2.0, 0.5, and 0.16, respectively. The setting time of the slurry increases with the increasing waterbinder and bentonitewater ratios. In addition, the bentonitewater ratio has a significant effect on the consistency, initial fluidity, and water separation rate of the slurry. Compared with the synchronous grouting slurry currently being used in the shield tunneling of the Nanjing metro line 7, the modified slurry has lower fluidity and density, higher consistency, initial fluidity, density, stone rate properties, and strength, as well as a shorter setting time under the action of carbonization by the CO2 foam.

Key words: cementbased CO2 foam, carbonation effect, performance of modified slurry, influencing factors, orthogonal test, range analysis, microscopic mechanism analysis