Palm oil fuel ash-soft soil geopolymer for subgrade applications: strength and microstructural evaluation

This paper presents the strength and microstructural properties of a geopolymer stabilised soft soil (SS) using palm oil fuel ash (POFA) as a precursor. The liquid alkaline activator, L used was a mixture of sodium silicate solution (Na2SiO3) and sodium hydroxide solution (NaOH). Compressive strengths were measured to assess the viability of POFA-SS geopolymer as a stabilised subgrade. Specialised tests were undertaken on POFA-SS geopolymers to investigate the geopolymerisation mechanisms under various Na2SiO3:NaOH ratio, L content and POFA:SS ratio. These specialised tests included; X-ray diffraction, scanning electron microscope with energy dispersive X-ray spectroscopy (SEM/EDX) and Fourier transform infrared (FTIR) spectroscopy tests. The lower POFA:SS ratios required more NaOH and L to be absorbed by both external and internal negative layer surfaces of SS to develop the stabilised soil structure. The optimum Na2SiO3:NaOH ratios providing the highest strength were at 40:60, 50:50, and 60:40 for POFA:SS ratios of 30:70, 40:60, and 50:50, respectively. The L providing the highest strength was determined at the optimum liquid alkaline activator content, OLC (L = 22.8%), 1.2OLC (L = 31.4%) and 1.4OLC (L = 44.55%) for POFA:SS ratios at 30:70, 40:60, and 50:50, respectively. The main chemical products in POFA-SS geopolymers were calcium aluminate silicate hydrate (C–A–S–H) and calcium sodium aluminate silicate hydrate (C–(N)–A–S–H).