Soft Soil Stabilisation Using a Novel Blended Cementitious Binder Produced From Waste Fly Ashes

Soil stabilisation is one of the most common techniques used to mitigate the undesirable properties of soft soils such as low compressive strength and high compressibility. Cement is the most commonly used binder for soil improvement applications in the UK and worldwide due to its high strength performance. However, its manufacture is energy intensive and expensive, contributing approximately 7% of global carbon dioxide (CO2) emissions. Therefore, the search for alternative raw materials, such as waste and by-products, is becoming critical in order to develop cost effective and more environmentally friendly binders to replace cement and reduce its negative environmental impact. Blended waste material fly ashes have been identified as promising alternatives to traditional binders (cement CEM-I) in different construction industries including ground improvement. The reuse of waste material fly ashes such as waste paper sludge ash (WPSA), palm oil fuel ash (POFA) and rice husk ash (RHA) has many advantages, specifically in terms of eliminating the cost of their transportation and eventual landfill, their continuous supply and the negligible, or zero, cost of production. This research project details the process of the development of a new cementitious binder, produced by blending cement-free WPSA, POFA and RHA under physico-chemical activation using flue gas desulphurisation (FGD) gypsum, for use in soft soil stabilisation. The effects of different binders produced from unary (WPSA), binary (WPSA and POFA) and ternary (WPSA, POFA and RHA) blended mixtures, along with ground and FGD gypsum activated ternary mixtures, on the geotechnical properties of soft soils, were extensively investigated. Comparisons of Atterberg limits, strength (unconfined compressive strength (UCS)), compressibility characteristics and durability (wetting-drying cycles effect) of untreated soil and soil stabilised with the optimum unary, binary, ternary and activated ternary mixtures and a reference cement treated soil, have been carried out. An investigation of the microstructural and mineralogical composition of the newly developed binder, in comparison to those of the reference cement, was also carried out using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) imaging and energy dispersive X-ray (EDX) spectroscopy analysis. The results indicate that the soil stabilised with the ternary mixture activated by FGD gypsum (T+FGD), had the greatest compressive strength, compressibility and durability improvement; the performance of the newly developed cementitious binder was comparable to that of the reference cement. This binder comprises 8% WPSA + 2% POFA + 2% RHA activated with 5% of FGD, by the total mass of binder. The addition of FGD gypsum has been observed to enhance the pozzolanic reaction, leading to improved geotechnical properties; mainly UCS which increased over time of curing and exceeded that for the soil treated with reference cement, after 180 days. The results obtained from XRD analysis, SEM testing and EDX analysis revealed the formation of hydrated cementitious products represented by calcium silicate hydrates (C-S-H), Portlandite (CH) and ettringite. The formation of these hydrates reveals the developments gained in the geotechnical properties of the treated soil. A solid, coherent and compacted soil structure was achieved after using T+FGD, as confirmed by the formation of C-S-H, CH and ettringite. Therefore, a new, Cost effect, eco-friendly and sustainable cementitious binder has been successfully developed and can be used with confidence for soft soil stabilisation, as a 100% replacement of conventional cement