Enhanced microstructural analysis of black cotton soil stabilized through rice husk ash integration

Black cotton soil (BCS) poses significant complexities in geotechnical applications due to its swelling and shrinkage behavior. It causes significant economic losses globally due to reconstruction and rehabilitation efforts. Reliable soil reinforcement techniques are, therefore, essential to mitigate the deleterious effects of expansive BCS and to ensure the long-term stability of the structures built upon them. The present study explores the application of rice husk ash (RHA) to BCS using nitrogen (N2) gas adsorption techniques such as Brunauer–Emmett–Teller (BET), Langmuir, and adsorption isotherm analyses. These techniques are based on the principle that N2 gas is adsorbed onto the reactive surface sites. The surface of BCS is considered reactive due to its high clay content and the presence of montmorillonite. With the addition of RHA, pozzolanic reactions progress, leading to the development of cementitious phases such as calcium silicate hydrate (C-S-H), which gradually fill these reactive surface sites, leading to a decrease in the material’s gas adsorption capacity. This reduction in N2 gas adsorption provides a measurable indication of pozzolanic activity, allowing for a more detailed microstructural assessment of stabilized soil systems. A sharp reduction in N2 gas adsorption was observed in BET, Langmuir, and adsorption isotherm analyses at 6 % RHA content, conducted on 28-day cured Unconfined compressive strength (UCS)-tested samples. BET results showed a reduction in adsorption from 0.0635 mg/g for untreated BCS to 0.0385 mg/g at 6 % RHA concentration. This 6 % RHA content also corresponds with peak mechanical performance observed in UCS, California bearing ratio (CBR), indirect tensile strength (ITS), and cone penetration test (CPT), highlighting a strong correlation between microstructural improvement and engineering behavior. The UCS of untreated BCS (183 kPa) increased to a maximum of 819 kPa after 7 days and 1370 kPa after 28 days of curing, confirming 6 % RHA as the optimum dosage.