Effects of Fly Ash and Silica Fume on the Rheological Properties of Magnesium Phosphate Cement-Emulsified Asphalt (MPC-EA) Composite Repair Materials

Fly ash (FA) and silica fume (SF) are two commonly used materials for modifying magnesium phosphate cement (MPC). MPC- emulsified asphalt (MPC-EA), a new type of pavement rehabilitation material, poses a challenge in combining MPC hydration and EA emulsion breaking to enhance its performance. In this study, FA and SF were used to modify MPC-EA, and their effects on the rheological properties of MPC-EA at different substitution amounts were analyzed. Rheological curves were obtained using an RST-SST rheometer. Three commonly used rheological models—the Bingham model, the Herschel-Bulkley (H-B) model, and the modified Bingham model—were used to fit the rheological curves. The matching relationship between yield stress and plastic viscosity in these three models was analyzed to reveal the different rheological loss mechanisms of the two dopants on the MPC-EA slurry. Results indicated that the H-B model was the most suitable for fitting the modified MPC-EA. Substituting magnesium oxide (MgO) with FA and SF did not alter the flow pattern of MPC-EA but did change its rheological parameters. In the FA and SF substitution groups, both yield stress and plastic viscosity were proportional to the substitution amount and negatively affected the fluidity of the MPC-EA slurry. By analyzing the rheological parameters, the different modification mechanisms of the two dopants at high dosages were clarified: FA provided more nucleation sites, increasing the number of flocculated structures, while SF absorbed more water, leading to increased EA emulsion breaking behavior and, consequently, an increase in the volume of flocculated structures. This study established the rheological constitutive equations of MPC-EA for the first time, considering FA and SF substitution amounts ranging from 5% to 25%. The research findings can provide theoretical support for the field application design of FA- and SF-modified MPC-EA for pavement remediation.