Investigation of the effect of using metakaolin in soil-cement samples improved by th deep mixing method

The use of soil–cement columns constructed by the deep mixing (DM) method has attracted considerable attention as an effective ground improvement technique for enhancing the strength and mechanical performance of both granular and fine-grained soils. In this method, cement is commonly used as the primary stabilizing agent; however, the relatively high environmental impacts associated with cement production have encouraged the development of alternative and supplementary cementitious materials. The incorporation of sustainable pozzolanic additives alongside cement, such as metakaolin, can potentially improve the engineering performance of stabilized soils while simultaneously reducing the required cement content and its associated environmental impacts. In addition, the use of supplementary cementitious materials may modify the hydration products and microstructure of treated soils, thereby affecting their long-term mechanical behavior under aggressive environmental conditions.The present study investigates the feasibility and effectiveness of incorporating metakaolin as a supplementary material in cement-stabilized sandy soil obtained from the Anzali region, with particular emphasis on its mechanical behavior under seawater curing conditions. The experimental program was designed to evaluate the effects of cement content, metakaolin content, water-to-binder ratio, and curing time on the strength development of treated specimens. Unconfined compressive strength (UCS) tests were initially performed to evaluate the influence of these parameters and to determine an appropriate mixture proportion. The specimens were cured under seawater environmental conditions for different curing periods to assess both the early-age and longer-term strength development of the stabilized soil. Based on the UCS results, an optimum mixture was subsequently selected for further mechanical and microstructural investigations.The optimum mixture consisted of 20% cement, 3% metakaolin, and a water-to-binder ratio of 1. The results demonstrated that the incorporation of metakaolin generally improved the mechanical performance of the cement-treated sand. In particular, the presence of metakaolin contributed to continued strength development at later curing ages and reduced the tendency for long-term deterioration in strength and elastic modulus under seawater exposure. This behavior can be associated with the pozzolanic activity of metakaolin and its contribution to the formation of additional cementitious hydration products, particularly calcium silicate hydrate (C–S–H) gel. However, increasing the metakaolin content beyond the optimum level resulted in a reduction in both compressive strength and elastic modulus, indicating that an excessive amount of metakaolin does not necessarily provide further mechanical benefits and may adversely affect the performance of the stabilized mixture.Following the selection of the optimum mixture, Brazilian tensile strength and unconsolidated-undrained (UU) triaxial tests were conducted to further characterize the mechanical behavior of the treated soil. The Brazilian tensile strength results indicated a progressive increase in tensile strength with increasing curing time, demonstrating the continued development of cementitious bonding within the stabilized soil matrix. The UU triaxial test results also showed that both curing time and confining pressure influenced the strength response of the treated specimens. An increase in curing time resulted in higher strength, reflecting the progressive development of cementation and bonding, while increasing the confining pressure enhanced the overall resistance of the specimens under axial loading. To provide further insight into the mechanisms responsible for the observed mechanical behavior, the microstructure and mineralogical characteristics of the optimum mixture were investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The microstructural observations and mineralogical analyses supported the formation of cementitious hydration products and the development of a denser and more interconnected soil–cement matrix. Overall, the findings demonstrate that the incorporation of an optimum amount of metakaolin can enhance the mechanical performance and long-term stability of cement-stabilized Anzali sand under seawater curing conditions, while providing a promising approach for the partial incorporation of supplementary pozzolanic materials in deep mixing ground improvement applications