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To improve the swelling–shrinkage characteristics of expansive soil, excessive amounts of cement are often used to solidify it. This study aims to solidify expansive soil by using industrial graphene (G) and graphene oxide (GO)-modified cement to avoid the problem of environmental pollution resulting from the excessive use of cement. A series of unconsolidated-undrained triaxial and microstructure tests were conducted to investigate the engineering properties and micromechanisms of G and GO composite cement-solidified soils. Testing results showed that the deviatoric stress–strain curves of G and GO composite cement-solidified soils showed softening types. The compressive strength, toughness, shear strength, and ductility of cement-solidified soil were significantly improved by adding both G and GO. G and GO not only acted as a template to promote cement hydration and generate regular hydration products but also played a role in pore filling. Consequently, it is feasible to apply G and GO composite cement-solidified soils in practical projects. [ABSTRACT FROM AUTHOR]

The novel alkali-activated filling grout (AAG) for shield tunnels requires detailed understanding of its sulfate resistance prior to practical application. This paper presents a comparative study of the sulfate resistance of a typical ordinary portland cement (OPC)-based grout and two groups of representative AAG formulated with industrial by-products including fly ash, ground granulated blast-furnace slag, and steel slag as the solid precursors. In the study, the grout samples after 28 days of standard curing were exposed to the 5% by weight Na2SO4 (pH = 7.92) and 5% by weight MgSO4 (pH = 7.25) solutions, respectively, to simulate different cases of external sulfate erosion. The sulfate resistance and deterioration mechanism of various grout samples were evaluated and analyzed via multitechnical examinations including visual appearance, mass loss, compressive/flexural strength, strength retention coefficient, mineral composition, microstructure morphology, and pore-size distribution at different test ages. The experimental results indicated that whether under Na2SO4 or MgSO4 erosion, AAG always exhibited more slight deterioration than the OPC-based grout in terms of visual appearance, mass loss, and strength reduction. The reasons for these observations are as follows. The main gel products of AAG were mainly C-(A)-S-H and N-A-S-H, which interweaved and coexisted to form a denser matrix structure than that of the OPC-based grout. The resulting poorer permeability of the AAG matrix was not conducive to the ion exchange between the matrix and the environmental solution. In addition, compared with the OPC-based grout, the lower calcium content and more stable bonded aluminum phases in AGG limited the formation of expansive products responsible for microcracking under sulfate erosion. MgSO4 could cause more severe deterioration than Na2SO4 , mainly due to the promotion of decalcification of calcium-containing phases by the presence of Mg2+ and the resulting gel products harmful to the matrix. This study demonstrated the excellent sulfate resistance of AAG, which can provide scientific basis for its future engineering application. [ABSTRACT FROM AUTHOR]

Cement composite soil is a more stabilized method in expansive soil foundation projects, but it is essential to find an ecofriendly modification material to reduce the content of cement and its pollution for the environment. A series of unconsolidated–undrained (UU) triaxial tests were conducted to investigate the modification effect of graphene oxide (GO) on cement-stabilized expansive soil (CS), and the contents of GO were 0.05%, 0.1%, 0.15%, and 0.2%, respectively. The test results show that (1) the deviatoric stress–strain curves of CS gradually change from a weakly softening type to a strongly softening one after adding GO. (2) The compressive strength, shear strength, and toughness of GO-modified cement-stabilized expansive soil are better than CS, and the optimal GO content is 0.05%. (3) Microscopic tests show that GO can promote the hydration reaction of cement and reduce the internal pores of CS. Consequently, the mechanical properties and microstructure of CS can be significantly improved after adding GO, and it is feasible to replace a part of the cement with GO in expansive soil foundation projects. [ABSTRACT FROM AUTHOR]