ALTERNATIVE SOIL IMPROVEMENT USING HYDRAULIC CEMENT COMBINATION WITH STEEL SLAG AND BIOCHAR: STRENGTH AND MICROSTRUCTURAL CHARACTERISTICS

Abstract

Soil improvement is a critical challenge in geotechnical engineering, especially for weak soils like dredged sediments. This study proposes an alternative stabilization method using hydraulic cement (HC), partially replaced with steel slag (SS), and enhanced with bamboo biochar in both soaked and dry forms. Various tests, including unconfined compressive strength (UCS), free-free resonance (FFR), X-ray diffraction (XRD), and scanning electron microscopy (SEM), were conducted to evaluate mechanical and microstructural behavior. Results indicated that the mixture with 25% steel slag substitution achieved a 7-day UCS of 900.15 kPa, exceeding the standard subbase requirement of 689 kPa. The inclusion of both kinds of biochar significantly enhanced early-stage strength development, with certain combinations reaching UCS values of 769.37 kPa and 658.47 kPa after only 3 days of curing, whereas the mixture without biochar exhibited a lower UCS of just 562.59 kPa. Stiffness development, reflected by E₅₀ and small-strain elastic moduli (E₀ and G₀) derived from FFR, exhibited trends consistent with UCS, indicating coherent mechanical behavior across different strain levels. XRD and SEM analyses confirmed the formation of calcium silicate hydrate (CSH) and ettringite, validating the chemical stabilization mechanism. This approach promotes sustainability through reduced cement usage and the beneficial reuse of industrial by-products and natural agricultural waste. The findings demonstrate potential for integrating these materials into low-carbon geo-environmental soil stabilization practices, in alignment with circular economy principles and the Sustainable Development Goals (SDGs).