Frost resistance and micro mechanism of metakaolin and polypropylene fiber modified coastal cement soil

In order to investigate the effects of metakaolin and polypropylene (PP) fiber on the macroscopic mechanical properties and microscopic mechanisms of cement soil subjected to freeze-thaw cycles, a series of unconfined compressive strength (UCS) and splitting tensile strength (STS) tests were conducted on samples with varying dosages of metakaolin and PP fiber. Additionally, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were performed. The test results show that the metakaolin and PP fiber effectively reduce the strength loss rate of samples due to freeze-thaw cycles. At 28 days of curing, the maximum UCS of metakaolin-modified cementitious soil (MCS) reached 4.24 MPa, while the STS peaked at 0.54 MPa, representing increases of 22% and 19%, respectively, compared to the control group without metakaolin. In addition, the incorporation of PP fiber considerably enhanced both UCS and STS in metakaolin-modified cementitious soil (FMCS). At curing ages of 7 and 28 days, the UCS and STS of FMCS achieved their highest values with 0.5% fiber content, indicating that the optimal PP fiber dosage was 0.5%. Metakaolin addition enhances the microstructure of cement-stabilized soil by reducing porosity and increasing soil compactness. PP fiber contributes by forming a three-dimensional network structure within the sample, which constrains particle movement and improves the material's continuity. This study provides valuable theoretical insights for the application of metakaolin and PP fiber in cement soil, offering a significant reference for exploring the frost resistance of cement-based materials, particularly in the treatment of soft soil foundations in coastal regions.