Stabilising expansive subgrades with agro-industrial waste: An experimental investigation

Abstract

Reusing aluminosilicate wastes in subgrade stabilisation offers three benefits: improving subgrade performance, minimising waste, and reducing dependence on carbon-intensive stabilisers. Previous studies have focused on cement or high NaOH concentrations to achieve strength gains, often at environmental and economic costs. This study explores an alternative approach using low activator doses to control swelling and enhance the viability of aluminosilicate wastes for subgrade stabilisation. An extensive program of one-dimensional free-swell tests under monotonic and cyclic wet–dry conditions was conducted to assess the effects of low activator dosages on soil behaviour, underlying mechanisms, performance, and durability. Mercury intrusion porosimetry, scanning electron microscopy, unconfined compressive strength, x-ray fluorescence, x-ray diffraction, and pH tests were performed to provide deeper insight into the stabilisation process. Results show that swelling stabilisation relies on a distinct but complementary mechanism to strength development, requiring much lower activator concentration (1 M NaOH) and shorter curing (1 day). Swelling in five soils, initially ranging from 24 % to 53 %, was consistently reduced to below 3 %. A fourfold increase in 7-day strength was achieved, driven by activator content and initial dry density. Microstructural analyses show that stabilisation produces denser, less porous material. Stable pH and reduced leachability are recorded over 30-day wetting cycles. pH levels in adjacent soils resemble those in concrete, neutralising over time due to carbonation and dilution. The study hence shows that using aluminosilicate waste for stabilising soils is more likely to be environmentally and economically viable if geopolymerisation is used for swelling reduction and mechanical compaction for strength development, to avoid reliance on expensive high-molarity systems.