Fly ash based geopolymer stabilisation of silty clay/blast furnace slag for subgrade applications

The mechanical and microstructural properties of problematic silty clay (SC) stabilised with fly ash (FA) based geopolymer and blast furnace slag (BFS) replacement are presented in this research. The influence factors evaluated included FA:BFS replacement ratio, NaOH concentration, and curing temperature. A series of geotechnical laboratory tests and microstructural analyses were also undertaken. The strength of the stabilised material was found to be governed by interparticle forces, mainly from the contribution of the chemical bonding strength. The results indicated that the unconfined compression strength (UCS) values of FA based geopolymers stabilised with SC/BFS blends increased with increasing NaOH concentration, at the various FA:BFS replacement ratios when cured at various temperatures (25, 50 and 80°C). The high concentration of NaOH could dissolve FA particles to leach silica and alumina which reacted with NaOH to produce a geopolymerization (N-A-S-H gel) process, which resulted in high UCS results. The decrease in FA:BFS ratio however reduced the specific area of particles to be welded by FA geopolymerization products and reduces the geopolymer gel due to the reduction in quantity of FA precursor. As such, the interparticle forces increased as the FA:BFS reduced up to the optimum value and then decreased as the FA:BFS reduced. The optimal FA:BFS ratio was found to be 20:10. An elevated curing temperature accelerated the geopolymerization reaction, leading to the higher UCS at higher temperature. The use of waste by-products BFS and FA to stabilise problematic soil in civil engineering applications will contribute to a significant reduction in construction costs and the sustainable development of the project life cycles.