Process-engineered immobilization of ureolytic biocatalysts in diatomite for in-situ biomineralization under high-alkalinity solid-phase conditions.

Problematic clays are widely stabilized with lime to improve strength and durability, yet slow early-age strength development, pronounced brittle failure, and limited densification often constrain performance in high-alkalinity environments. This study explores a synergistic route that integrates diatomite-immobilized ureolytic microbially induced calcium carbonate precipitation with lime stabilization. Mixtures were prepared with 6% hydrated lime and 3 to 7% diatomite or a diatomite-based microbial curing agent; where applicable a 1.0 M urea-calcium chloride solution supplied substrates for mineralization. Mechanical properties were assessed by unconfined compressive strength, unconsolidated-undrained triaxial testing, and ultrasonic pulse velocity, and microstructure and phase assemblage were characterized using scanning electron microscopy, X-ray diffraction, and thermogravimetric analysis. Results show clear dosage-dependent gains. At 28 d, unconfined compressive strength reached 1987.18, 2278.17, and 2563.00 kPa for 3%DE-B, 5%DE-B, and 7%DE-B, exceeding the corresponding diatomite-only groups by 75.65%, 83.17%, and 88.50%. Under 300 kPa confining pressure, cohesion increased to 382.52 to 498.72 kPa and the internal friction angle to 38.88 to 47.88°. Ultrasonic pulse velocity rose with curing age, with 7%DE-B increasing by 45.01% from 7 d to 28 d. Triaxial responses followed linear elasticity, strain hardening, peak strength, and softening, while failure shifted from through-crack brittleness to non-through diffuse cracking with pronounced bulging. Microstructural evidence indicates clustered calcium silicate hydrate (C-S-H) progressively encapsulating diatomite and abundant ellipsoidal CaCO₃ forming interconnected three-dimensional networks. These observations support a synergy between pozzolanic reaction and microbial mineralization that constructs a multi-scale cementation network, densifies the matrix, and strengthens interparticle contacts, yielding reproducible improvements in strength, ductility, and structural integrity. These results provide reference value for performance enhancement and process optimization of lime-stabilized clays and cementation-enhanced clay systems.