Experimental evaluation of strength and resilient modulus of expansive subgrade stabilized with cement and pumice

Expansive subgrade is one of the most problematic soils encountered in road construction due to its high swelling/shrinkage properties and low bearing capacity. Cement is conventionally employed in treating such soils; however, the augmented production of cement, consequent to mounting urbanization and industrialization, has engendered deleterious environmental consequences. In response to these restrictions, this study is the first to systematically assess the effect of partial cement replacement with pumice on the mechanical and microstructural behavior of cement-stabilized expansive subgrades. Four different cement contents (3, 6, 9, and 12%) and various percentages of cement replacement with pumice (0, 10, 30, 50, 70, and 90%) were selected. A series of experimental tests, including modified proctor compaction, unconfined compressive strength (UCS), California bearing ratio (CBR) and resilient modulus (M_r) were performed under different curing conditions, ranging from 7 to 90 days. Microstructural investigations were conducted by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses. The results showed that adding cement increased the maximum dry density (MDD) and optimum moisture content (OMC) of the soil-cement mixtures, while an increase in the pumice content resulted in opposite tendencies. Replacing cement with 30% pumice achieved the maximum UCS, CBR and M_r for each curing period. The UCS, CBR and M_r values of 12% cement-stabilized soil increased by about 9%, 7% and 9% with a 30% pumice replacement, respectively, after 90 days of curing for UCS and CBR, and 28 days of curing for M_r. Adding pumice to the cement-stabilized mixtures reduced the brittleness of the specimens. The active composition parameter was established, showing the highest strength improvement rate can be achieved by cement replacement with 30% pumice at different cement contents. In addition, equivalent carbon emissions (CO₂-e) of the specimens were assessed and compared with those of cement, demonstrating that soil with pumice has lower CO₂-e than cement at higher subgrade strengths, making it more environmentally friendly. Approximately 30% lower carbon emission was achieved when cement was replaced with 30% pumice. Microstructural investigations revealed a more compact, dense and less porous structure of the cement-stabilized specimens with pumice addition. Pumice is therefore a viable replacement material that could be used to improve the engineering properties of subgrade materials for road construction.