Degradation Mechanisms of Mechanical Properties of Cement-Stabilized Bentonite Under Highly Alkaline NaOH Solutions from 1 to 8 mol/L

Cement stabilization is widely used to improve the mechanical performance of bentonite-rich soils; however, the behavior of cement-stabilized bentonite under highly alkaline conditions remains unclear. This study aims to elucidate the degradation mechanisms of the mechanical properties of cement-stabilized bentonite exposed to NaOH solutions with concentrations ranging from 1 to 8 mol/L. Unconfined compressive strength (UCS) tests were combined with XRD, 29Si NMR, and MIP to characterize mineral phases, silicate structure, and pore structure of the stabilized soils. Increasing alkalinity led to pronounced strength deterioration, with the 90 d UCS decreasing from 2.56 MPa to 0.25 MPa, corresponding to a reduction of approximately 90%. Microstructural analyses indicate that elevated alkali content inhibits cement clinker hydration, promotes the formation of zeolitic crystalline phases, induces depolymerization of the silicate network with the mean chain length decreasing from 6.4 to 3.5, partially transforms montmorillonite from a 2:1 to a 1:1 layer structure, and results in significant pore coarsening, as reflected by an increase in the most probable pore size from 62.53 to 1054.52 nm. These coupled effects weaken the integrity of the gel network and account for the continuous reduction in mechanical strength with increasing alkali concentration, providing a mechanistic basis for understanding the alkali-induced weakening behavior of cement-stabilized bentonite and offering guidance for its engineering application in alkaline environments.