Synthesis of geopolymer from large amounts of treated palm oil fuel ash: Application of the Taguchi method in investigating the main parameters affecting compressive strength

•Geopolymer was synthesized from high content of treated palm oil fuel ash.•TPOFA-based geopolymer mortar gained compressive strength of 47MPa at 7days.•The optimized TPOFA-geopolymer mortar mixture was analysed using XRD and FTIR.•The main binding phases consist of N–A–S–H and C–S–H gels. The aim of this study was to synthesize geopolymers using a large amount of treated palm oil fuel ash (TPOFA). The efficiency of the TPOFA (as the source material) in producing geopolymer products was enhanced via six factors which were optimized using the Taguchi method L25. The six factors were divided into two different components: (i) additive materials i.e. Ca(OH)2, silica fume (SF), Al2(OH)3, and (ii) alkaline activators; i.e. NaOH concentration (moles), Na-silicate: NaOH ratio, and alkali-activator:solid-material ratio. Each of these factors was examined on five levels in order to obtain the optimum mixture. A total of 25 mixtures were prepared in accordance to the L25 array proposed by the method. The performance of the specimens was evaluated by compressive strength tests. The results show that the optimum mixture consisted of 65wt.% TPOFA and 35wt.% additive materials which achieved a compressive strength of 47.27±5.0MPa after 7days of curing. The properties of the optimized mixture were further analyzed via X-ray diffractography (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses. The results show that the main binding phases consist of aluminosilicate type gel “N–A–S–H” (Na2O–Al2O3–SiO2–H2O) and calcium silicate hydrate (C–S–H) gels, formed simultaneously, within the TPOFA-based geopolymer mortar.