Soil treatment with natural materials is an effective method to improve the mechanical properties of the original soil for recycling engineering construction. This research aims to evaluate the synergistic effects of lignin fiber and cement on sandy clayey silt stabilization. A factorial experimental design was employed, testing five lignin fiber contents (0%, 2%, 4%, 6%, and 8%) and three cement contents (0%, 2%, and 4%) across four curing periods (1, 7, 14, and 30 days). Unconfined compressive strength (UCS) tests were conducted in triplicate for each combination (total *n* = 180 samples), and failure surfaces were analyzed using Scanning Electron Microscopy with Energy Dispersive X-ray spectroscopy (SEM-EDX). Results indicate a critical lignin fiber threshold of 4%, beyond which UCS decreased by 15-20% due to increased void ratios. Statistical analysis (ANOVA, *p* < 0.05) confirmed significant interactions between lignin fiber, cement content, and curing time. For instance, 4% lignin fiber and 4% cement yielded a 139% UCS increase after 30-day curing compared to untreated soil. SEM-EDX revealed that lignin fiber networks enhance ductility by bridging soil particles, while cement hydration reduced particle detachment. These findings provide a quantitative framework for optimizing lignin fiber-cement stabilization in sustainable geotechnical applications.

To address the low utilization rate of construction waste soil and the environmental impact of traditional cement solidification, this study investigates the effect of desulfurized gypsum and silica fume in synergy with cement for construction waste soil. The effects of solidifying material dosage, liquid-to-solid ratio, and mixing ratio on mechanical properties were analyzed. Optimal performance was achieved with the dosage of solidifying material was 20%, the liquid-to-solid ratio was 0.2, and the mixing ratio of desulfurized gypsum, silica fume, and cement was 2:1:1, meeting the requirements of the technical specification for application of road solidified soil (T/CECS 737-2020). This formulation is referred to as FS-C type solidified soil. A self-fabricated carbonation device was employed to assess carbonation methods, time, and curing age on the mechanical properties of solidified soil. Carbonation for 6 h post-molding significantly enhanced strength, while carbonation in a loose state led to strength reduction. SEM analysis revealed a denser microstructure in carbonated samples due to calcium carbonate and silica gel formation. Compared to traditional cement solidification, FS-C type solidified soil reduces cement consumption by 15%, decreases CO2 emissions by 299.25 g/m(3), and sequesters 85 g/m(3) of CO2. These findings highlight the potential of FS-C type solidified soil as an environmentally friendly alternative for construction applications.

This study explored an integrated recycling strategy of utilising a typical excavation waste soil, completely decomposed granite (CDG), in geopolymer production as a sustainable alternative of cement for marine clay improvement. Experimental campaign was conducted to evaluate the effects of composite alkali content and sodium silicate modulus on the mechanical performance of CDG-based geopolymer paste. Test results identified an optimal sodium silicate modulus, while it was also found that the incorporation of magnesium oxide (MgO) in alkali activator could improve the general workability. Moreover, regarding the unconfined compressive strength of marine clay improved, it was demonstrated that the combination of calcined CDG and ground granulated blast furnace slag (GGBS) considerably outperformed cement at the same level of dosage. Finally, multi-criteria assessment based on life cycle analysis (LCA) demonstrated the advantage of CDG-based geopolymer compared to conventional binders, in terms of mechanical properties, environmental benefits, and economic cost.

This work presents an analysis of the relationship between strength parameters determined in the laboratory and the results of a cone penetration test with pore water pressure measurement (CPTU) of waste soils in the White Seas area in Cracow. Anthropogenic soil is an alkaline waste formed during the production of soda ash and deposited in the area of the former Solvay Sodium Plant factory in Cracow, Poland. Due to the large area of the land and numerous investment plans and completed buildings, there was a need to identify reliable functional relationships enabling the determination of the strength parameters of these soils based on the results of the CPTU. Statistical analysis showed that the best correlation with the test results was provided by two logarithmic functions in which the dependent variables were the effective friction angle and effective cohesion. The dependent variable for both cases was the corrected cone resistance qt. The functional relationship combined data from labour-intensive, long-lasting and costly laboratory measurements with quick and less expensive measurements, i.e., in situ CPTUs. The obtained relationships enable the determination of the strength properties of the subsoil of these anthropogenic soils.

This study investigates the utilization of titanium gypsum (TG) and construction waste soil (CWS) for the development of sustainable, cement-free Controlled Low Strength Material (CLSM). TG, combined with ground granulated blast furnace slag, fly ash, and quicklime, serves as the binder, while CWS replaces natural sand. Testing thirteen mixtures revealed that a CWS replacement rate of over 40% controls bleeding below 5%, with a water-to-solid ratio between 0.40 and 0.46, ensuring flowability. Higher TG content reduces flowability but is crucial for strength due to its role in forming a crystalline network. Compressive strength decreases with higher TG and water-to-solid ratio, while 3-5% quicklime provides a 56 day strength below 2.1 MPa. Higher CWS reduces expansion, and TG content between 60% and 70% minimizes volume changes. XRD and SEM analyses underscore the importance of controlling TG and quicklime content to optimize CLSM's mechanical properties, highlighting the potential of TG and CWS in creating low carbon CLSM.

Foam lightweight soil (LS) is a cement composite with excellent lightness, but the excessive use of cement causes some negative impacts on the surrounding environment. This study aims to develop a sustainable cement composite by utilizing fly ash and waste soil in LS, providing a practical reference for green construction of road engineering. The physico-mechanical properties of cement composites with different mixing ratios were comparatively evaluated using geotechnical tests, and the micro-mechanisms were investigated using microscopic tests. The testing results showed that the utilization of fly ash and waste soil was unfavorable to improve the mechanical strength and the damage resistance of LS, but significantly decreased the use of cement. The comprehensive performance of cement composite reached the optimum when the replacement rates of fly ash and waste soil were 10% and 20%. Fly ash reacted with the hydration products of cement producing more cementitious gels to make the internal structure of cement composite denser, while waste soil not involved in its chemical reaction. The life cycle assessment indicated that the potential environmental impact of LS was improved after utilizing fly ash and waste soil, and the proposed sustainable cement composite had good feasibility in engineering.

Urban construction has generated substantial amounts of waste soils, impeding urban ecological development. With the aim of promoting waste recycling, waste soils possess a high potential for sustainable utilization in subgrade construction. However, these waste materials exhibit inadequate engineering properties and necessitate stabilization for an investigation into their long-term performance as subgrade filling materials. Initially, a thorough assessment and comparison were conducted to examine the key mechanical properties of lime- and cement-stabilized soils with mixed ratios (total stabilizer contents ranging from 2% to 8%). The results indicated that these soils met the requirements of subgrade materials except for the 2% lime-treated soil. Subsequently, to reveal the improvement in water resistance of stabilized waste soil (e.g., under conditions of rainfall or elevated groundwater table), the effects of soil densities and stabilizer contents on the disintegration characteristics were investigated using a range of disintegration tests. An evolutionary model for the disintegration ratio of stabilized soils was then developed to predict the process of disintegration breakage. This model facilitates the quantification of the lower disintegration rates and elevated disintegration time attributed to higher levels of compactness and stabilizer contents during a three-stage disintegration process. This enhances the understanding and evaluation of sustainable applications in stabilized waste soils used as subgrade filling materials.