Mechanical performance and microstructure of phosphate-based geopolymer modified by nano-SiO2 and nano-Al2O3

Fly ash phosphate-based geopolymer (FPGEO) has emerged as a promising alternative to conventional metakaolin phosphate-based geopolymer. However, relatively low strength limits its wide application. This study systematically investigated the influence of different dosage of nano-SiO 2 and nano-Al 2 O 3 on the workability, mechanical performance and microstructure of FPGEO. The result showed that the incorporation of nano-SiO 2 and nano-Al 2 O 3 contributed to a decrease of setting time and fluidity. Besides, the pH of the nano-doped samples increased with the prolonged curing age, while the electricity conductivity exhibited a decreasing trend, regardless of the dosage. Additionally, the compressive strength of the specimens with 1 wt% nano-SiO 2 and 5 wt% nano-Al 2 O 3 were 15.2 MPa and 22.6 MPa at 28 days, showing an increase of 14.3 % and 69.9 % compared to the control specimen, respectively. The enhancement can be attributed to the synergistic effects of nanomaterial filling and nucleation effects., which effectively fill the voids and promote the reaction, leading to more AlPO 4 gels formation. While incorporating nano-SiO 2 and nano-Al 2 O 3 had slight influence on the mineralogical composition of the resultant reaction products, it significantly enhanced the amount of reaction gels, particularly Al-containing gel phases. Meanwhile, the nanomaterial modified geopolymer exhibited enhanced structural homogeneity and density compared to the control sample. Both samples containing 1 % nano-SiO 2 and 5 % nano-Al 2 O 3 effectively reduced the cumulative pore volume and converted the less harmful pores (20–50 nm) to harmless pores (< 20 nm). In addition, the CO 2 emissions of samples with incorporation of 1 % NS and 5 % NA increased 2.9 % and 31.6 % respectively while the CO 2 emission index (CI) showed a reduction of 9.9 % and 22.5 % respectively compared to that of the control sample. In contrast, the cost and the cost index (C p ) of the NS-modified and NA-modified samples were higher than that of the control sample. • The optimal dosages of NS and NA for improving the compressive strength of FPGEO was 1 % and 5 %, respectively. • Compared to the NS-modified sample, NA-modified sample produced more Al–P–O gels and resulted a more compact microstructure. • In addition to the nucleation effect, NA could react with acid activator to produce additional AlPO 4 gels.