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Strongly weathered phyllite is a metamorphic soft rock widely distributed in the central and western regions of China. Due to its inadequate engineering properties, the direct use of this material as a roadbed filler is not feasible and requires improvement. In this paper, calcium acetate Ca(CH 3 COO) 2 , magnesium acetate Mg(CH 3 COO) 2 and calcium chloride CaCl 2 are prepared to improve the filler by microbially induced carbonate precipitation (MICP). The effects of different calcium and magnesium sources on porosity, water absorption, uniaxial compressive strength, crack propagation time and failure patterns are investigated. The results show that CaCl 2 and Ca(CH 3 COO) 2 exhibit the most significant enhancement effect in reducing porosity and water absorption, increasing the uniaxial compressive strength by over 30%, delaying the time of failure occurrence and reducing the number of tensile cracks significantly. X-ray diffraction (XRD), scanning electron microscopy (SEM) and pore size distribution analyses reveal disparities among the rock samples. CaCl 2 and Ca(CH 3 COO) 2 have greater efficacy than Mg(CH 3 COO) 2 in terms of improvement, as they generate relatively many carbonate precipitates to bind the phyllite filler and reduce the size and quantity of pores. To avoid the corrosion of Cl- ions when adding reinforcement bars in actual engineering, Ca(CH 3 COO) 2 is an ideal calcium source for improving the strongly weathered phyllite filler. • The physical and mechanical properties of strongly weathered phyllite filler can be improved after modification. • The physical properties and strength of modified sample have a relationship with carbonate content and crystal morphology. • Calcium acetate is suggested for use as the calcium source to improve the strongly weathered phyllite filler. [ABSTRACT FROM AUTHOR]