Improved mechanical and thermal properties of sustainable ultra-high performance geopolymer concrete with cellulose nanofibres

Although the application of the cellulose nanofibre (CNF) on the mechanical performance of conventional ultra-high performance concrete (UHPC) and geopolymer materials have been extensively studied, the effects of CNF on the mechanical and thermal behaviour of ultra-high performance geopolymer concrete (UHPGC) have not been well understood. This paper presents a laboratory study on the mechanical properties, thermal resistance, and microstructures of CNF-incorporated UHPGC prepared under the ambient (23 degrees C) and elevated (80 degrees C) temperatures and volumetric contents of CNF from 0 to 0.4 %. The precursors included fly ash (19 %), ground granulated blast-furnace slag (76 %), and silica fume (5 %). The volumetric contents of the steel fibres (length of 13 mm and diameter of 0.2 mm) ranged from 0 % to 3 %. The flowability, compressive and flexural strengths, and residual compressive strength after the thermal exposure to 200, 400, and 600 degrees C were determined. The flowability of CNF-UHPGC dropped with CNF proportions. For the specimens cured under the ambient temperature, the highest compressive strength (143.6 MPa) and flexural strength (22.3 MPa) were determined with 0.2 % and 0.4 % CNF, respectively. For the specimens cured under the elevated temperature, the maximum compressive strength (156.4 MPa) and flexural strength (21.5 MPa) were determined with 0.1 % and 0.2 % CNF, respectively. Furthermore, the residual compressive strength of UHPGC with 0.1 % CNF cured under the ambient temperature was 6.2 %-6.8 % and 2.4 %-4.4 % higher than that without CNF after being exposed to 200 and 400 degrees C, respectively. Systematic microscopic investigations, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA), revealed that the addition of the CNF enhanced geopolymerisation reactions and promoted calcium (aluminate) silicate hydrate (C-(A)-S-H) gel formation, enhancing the strength. Based on the experimental results, adding 0.1 % v/v CNF not only enhanced the mechanical performance but also improved the thermal resistance of UHPGC.