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.

The integrated transdisciplinary concepts and technologies for greener and more sustainable innovations for recycling waste materials through geopolymer technique into construction materials were developed in this research. The objective of this research was to study the strength and microstructural development of slag (S)-based geopolymer-stabilized recycled asphalt pavement (RAP) as a pavement base material. An attempt was made to study the influence of a liquid alkaline activator ($\mathrm{L}=\mathrm{NaOH}/{\mathrm{Na}}_2\mathrm{Si}{\mathrm{O}}_3$ ratio) on the strength development of RAP-S geopolymers, which was evaluated by an unconfined compressive strength (UCS) test and examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The XRD and SEM analyses results indicated that the main geopolymerization reaction product of RAP-S geopolymers was calcium alumino-silicate hydrate (C-A-S-H). Without ${\mathrm{Na}}_2\mathrm{Si}{\mathrm{O}}_3$ ($\mathrm{L}=\mathrm{NaOH}/{\mathrm{Na}}_2\mathrm{Si}{\mathrm{O}}_3=100:0$ ), the S-geopolymer exhibited a slow geopolymerization reaction. The silica presented in ${\mathrm{Na}}_2\mathrm{Si}{\mathrm{O}}_3$ was highly soluble; hence, a suitable content of $\mathrm{NaOH}/{\mathrm{Na}}_2\mathrm{Si}{\mathrm{O}}_3=60:40$ can produce RAP+20%S geopolymers as a base material whose 7-day UCS value met the minimum strength requirements specified by the Department of Highways, Thailand. However, when excess ${\mathrm{Na}}_2\mathrm{Si}{\mathrm{O}}_3$ content ($\mathrm{NaOH}/{\mathrm{Na}}_2\mathrm{Si}{\mathrm{O}}_3=50:50$ ) was used, the presence of silica retarded the geopolymerization formation rate, leading to UCS reduction. The outcomes of this research were found to be useful for making progress in innovative technologies and applications of geopolymers, which strongly reflected the improvements made in the practical applications of the S-geopolymer as a cleaner production by using the recycled waste material in road work.