Improvement of Mechanical Properties and Permeability of Plastic Concrete of a Diaphragm Wall under a Sulfate-Destructive Environment Using Nanosilica to Preserve Groundwater

The diaphragm wall is a common structure in geotechnical projects and is placed in the soil to control different flows. Considering that the soil environment is a sulfate environment, the materials of this wall (plastic concrete) are exposed to sulfate attack. The proximity of the sulfate environment and plastic concrete leads to a loss of efficiency of the diaphragm wall. As a result, waste products, such as leachate, pass through the diaphragm wall and contaminate other parts of the soil, or subsurface water resources are scattered in different layers, and it becomes impossible to exploit them. In this research, nanosilica has been added to plastic concrete mortar to improve the mechanical properties and permeability of plastic concrete against the sulfate environment. Therefore, the mechanical properties and permeability of plastic concrete without and with nanosilica (1.5, 3, 4.5, and 6 cement weight replacement) at different ages (7, 14, 28, 56, and 90 days) have been investigated and finally compared with similar results of plastic concrete in the typical environment. In this research, the results of uniaxial and triaxial compressive strength tests and the elastic modulus were studied, to determine the mechanical properties, and the permeability coefficient was obtained by performing a triaxial permeability test. Scanning electron microscopy and X-ray diffraction analysis were also used for microscopic inspection of plastic concrete specimens under sulfate attack. The results show that mechanical properties, such as compressive strength and elastic modulus, increase at an early age and deteriorate over the long term. The permeability coefficient was also significantly reduced, while the changes of elastic modulus were controlled. Finally, microstructural experiments showed a significant effect of nanosilica on specimen cavity filling, justifying the results obtained in mechanical and permeability experiments.