Sustainable enhancement of subgrade bearing capacity of river sludge using rice husk ash–cement stabilisation

Abstract River sludge, characterised by high compressibility and low bearing capacity, poses significant challenges for its utilisation as pavement subgrade material. This study evaluates the sustainable stabilisation of river sludge using 6% cement combined with varying proportions of rice husk ash (RHA: 5?15%) through soaked and unsoaked California Bearing Ratio (CBR) testing, swelling assessment, and microstructural analysis (SEM?EDS). Cement stabilisation (RS + 6C) dramatically improved performance, increasing unsoaked CBR to 97.32% and soaked CBR to 96.35%, while reducing swell index to 0.096%. The addition of RHA further modified the strength behaviour and durability. The RS + 10R + 6C mixture exhibited the lowest swell index (0.024%) and high CBR values (84.11% unsoaked; 93.97% soaked), with a strength retention ratio of 111.73%, indicating continued pozzolanic reaction during soaking. Similarly, RS + 5R + 6C showed soaked CBR (84.91%) exceeding unsoaked values (72.09%), achieving 117.78% strength retention. Even at 15% RHA, the mixture maintained stable performance (78.10% unsoaked; 78.90% soaked). In contrast, untreated sludge showed severe collapsibility (80%), whereas stabilised mixes exhibited negligible or negative collapsibility, confirming structural densification. SEM observations revealed transformation from a dispersed clay matrix to a dense interlocked structure, while EDS analysis confirmed enhanced Ca?Si interactions and secondary C?S?H gel formation due to the pozzolanic reaction between cement hydration products and amorphous silica in RHA. The findings demonstrate that RHA-cement stabilisation not only suppresses swelling and collapse but also ensures exceptional retention of soaked strength, providing a sustainable and durable solution for converting problematic river sludge into high-performance subgrade material while promoting agricultural waste utilisation. Highlights • Cement-RHA stabilisation significantly improves the strength and bearing capacity 31 of river sludge. • Optimum mix (RS+10RHA+6C) shows higher CBR, reduced swelling, and enhanced durability. • OMC increases and MDD decreases with increasing RHA content due to higher water demand. • Microstructural analysis confirms formation of C-S-H and C-A-S-H gels, leading to densification. • Use of RHA promotes sustainable stabilisation by reducing cement demand and CO 2 emissions.