Permeability characteristics and empirical prediction of fine sandy soils stabilized by cement and metakaolin

The permeability of cementitious soils is substantially controlled by the permeability of the soil itself, the dosage of binder, the water-to-binder ratio, and other factors. This study employed metakaolin as the additive to enhance underground engineering construction's economic benefit since it could improve the impermeability of cement-stabilized soils and replace cement partly. A series of indoor permeability tests on cement- and metakaolin-stabilized fine sandy soils (CMSFSSs) with different cement-to-metakaolin ratios, water-to-binder (the mixture of cement and metakaolin) ratios, total binder contents, and curing times were conducted. The influences of these factors on the impermeability of CMSFSSs were investigated. Their impermeability improvement mechanism at the microscale was explored by Scanning Electron Microscopy and Mercury Injection Porosimeter tests. The empirical permeability coefficient prediction formulas about these influence factors, compressive strength, and porosity were discussed. The results showed that the best impermeability of CMSFSSs was achieved when the cement-to-metakaolin ratio was 5:1, saving 1/6 cement consumption. This ratio did not vary with the total binder content. The permeability coefficient of CMSFSSs increased nonlinearly with the water-to-binder ratio but decreased rapidly at first and then slowly with the increase of total binder content and curing time. The optional water-to-binder ratio should be less than 0.6 if both their liquidity and impermeability requirements were met together. The total binder content for fine sandy soil stabilization should be less than 15% since it was not more reliable to improve the impermeability of fine sandy soils by using excessive binder in terms of economic benefits. The hydrated gels in CMSFSSs formed rapidly at the early curing time. The calcium hydroxide formed by cement hydration disappeared over the curing time. The internal pore volume and sizes in CMSFSSs decreased over the curing time, resulting in worse and worse connectivity. All of them proved the contribution of metakaolin to cement-stabilized soil's impermeability improvement. Six empirical formulas for the permeability coefficient of binder-stabilized soils were summarized regarding the water-to-binder ratio, total binder content, curing time, unconfined compressive strength, and porosity. The results of this study provide theoretical and technical references for improving the impermeability of binder-stabilized soils.