Effect of rice husk biochar and coal fly ash on some physical properties of expansive clayey soil (Vertisol)

• Rice husk biochar (RHB) and coal fly ash (CFA) change the soil physical properties.
• RHB promotes the formation of macroaggregates and resists aggregate slaking.
• RHB and CFA change the pore size distribution and increase larger pores of soil.
• RHB and CFA decrease the tensile strength and COLE of clayey soil.

The objective of this work is to evaluate the effect of rice husk biochar (RHB) and coal fly ash (CAF) on the formation and stability of aggregates, pore size distribution, water retention, swell–shrinkage, consistency limit, and tensile strength of an expansive clayey soil (Vertisol). For this purpose, RHB and CAF are added to the clayey soil at four levels of 0, 2, 4, and 6% by weight, and incubated for 180 days in a glasshouse. Results indicate that the RHB significantly increases macroaggregates with a diameter larger than 0.25 mm and reduces microaggregates with a diameter of < 0.25 mm. Whereas CFA does not significantly affect the formation of macroaggregates. The RHB- and CFA-amended soils have significantly higher mean weight diameter (MWD) and geometric mean diameter (GWD) as compared with the control soil. The enhanced aggregate stability is attributed to a decrease in the aggregate breakdown by differential swelling and an increase in the aggregate resistance to mechanical breakdown. The RHB-amended soil has a greater water-holding capacity and higher available water content. Pore size distribution (PSD) of RHB- and CFA-amended soils, determined by the mercury intrusion porosimetry (MIP), indicates that the amendment enhances the formation of mesopores having a pore size range between 6 and 45 μm. In the measured pore range (0.003–360 μm), the amended soils are found to have considerably higher porosity than the control soil. The RHB and CAF affect the PSD of clayey soil by binding microaggregates together to form macroaggregate and combining carbon and fly ash particles with clay mineral phases to form a larger complex. Meanwhile, the RHB and CFA significantly decrease the tensile strength and coefficient of linear extensibility (COLE) of clayey soil. For example, adding a 6% RHB can reduce the tensile strength from 936.8 to 353.6 kPa and COLE from 0.63 to 0.56, respectively. The RHB and CFA also decrease the plasticity index of clayey soil. The above results indicate that the RHB and CFA are able to improve the physical quality and swelling–shrinkage status of expansive clayey soils, being a potential soil amendment for improving poor physical characteristics of the clayey soil.