Optimization mechanism of electrical conductivity and compressive sensing characteristics in magnesium phosphate repair mortar

• Revealed the influence of PDP replacement rate on the electrical conductivity of CMPRM. • Elucidated the conduction mechanism of CMPRM on base of pores and cation composition. • Clarified The influence law of CF on the piezoresistive behavior and its sensitivity of CMPRM. Magnesium phosphate repair mortar is widely used for emergency repair of cracked foundation components, and its intelligent enhancement provides a carrier for structural health monitoring. This study aimed to develop a composite magnesium phosphate repair mortar (CMPRM) with improved electrical conductivity and piezoresistive sensing capability. CMPRM was prepared by combining ammonium dihydrogen phosphate with potassium dihydrogen phosphate (PDP), and the optimization mechanism of PDP on the electrical properties was investigated. Carbon fibers (CF) were subsequently incorporated to further improve the conductivity and piezoresistive properties, while the latter is assessed via the resistivity response under cyclic axial compressive loading. The results indicated that the cation composition and micropore structure primarily governed the conductivity of CMPRM. The addition of PDP appreciably optimized conductivity of CMPRM, and CF moderately improved its mechanical properties, conductivity, and piezoresistive sensitivity at an appropriate volume fraction. The fractional change in resistivity (FCR) of CF-modified CMPRM exhibited two percolation thresholds at 0.4 vol% and 1.0 vol%, with FCR peak values of 4.32 % and 4.05 %, respectively. The stress electrical sensitivity (SES) increased as the CF volume fraction approached either threshold. Both FCR and SES decreased with the increase in stress rate, whereas the FCR increased with the stress amplitude, the SES demonstrated an opposite trend. These results demonstrate that precise control of PDP replacement rate, CF volume fraction, stress amplitude and loading rate is essential to optimize the multifunctional performance of CF-modified CMPRM. The findings provide guidance for the design of high-performance, self-sensing repair materials for infrastructure health monitoring.