Laboratory Study on Improvement of Expansive Soil by Chemically Induced Calcium Carbonate Precipitation

This paper proposes the use of calcium carbonate (CaCO₃) precipitation induced by the addition of calcium chloride (CaCl₂) and sodium carbonate (Na₂CO₃) solutions as a procedure to stabilize and improve expansive soil. A set of laboratory tests, including the free swell test, unloaded swelling ratio test, unconfined compression test, direct shear test, scanning electron microscopy (SEM) test, cyclic wetting-drying test and laboratory-scale precipitation model test, were performed under various curing periods to evaluate the performance of the CaCO₃ stabilization. It is concluded from the free swell tests and unloaded swelling ratio tests that the addition of CaCl₂ and Na₂CO₃ can profoundly decrease soil expansion potential. The reduction in expansion parameters is primarily attributed to the strong short-term reactions between clay and stabilizers. In addition, the formed cementation precipitation can decrease the water adsorption capacity of the clay surface and then consequently reduce the expansion potential. The results of unconfined compression tests and direct shear strength tests indicated that the addition of CaCl₂ and Na₂CO₃ has a major effect on geotechnical behavior of expansive soils. Based on the SEM analyses, new cementing crystalline phases formatted by sequentially mixing CaCl₂ and Na₂CO₃ solutions into expansive soil were found to appear in the pore space, which results in a much denser microstructure. A laboratory-scale model test was conducted, and results demonstrate the effectiveness of the CaCO₃ precipitation technique in stabilizing the expansive soil procedure. The test results indicated that the concentration of CaCl₂ higher than 22.0% and Na₂CO₃ higher than 21.2% are needed to satisfactorily stabilize expansive soil. It is proposed to implement the precipitation technique in the field by the sequential permeation of CaCl₂ and Na₂CO₃ solutions into soils in situ.