Performance enhancement of geopolymer concrete using aluminosilicate nanotubes: strength, durability, and microstructural insights

This study investigates the effect of aluminosilicate nanotubes (ANTs) on the fresh, mechanical, durability, and microstructural properties of thermally cured geopolymer concrete (GPC) synthesised using Class F fly ash (FA) and metakaolin (MK). ANTs were incorporated at dosages of 0–3.0% by binder weight, and the specimens were cured at 80 °C to replicate geothermal conditions. The experimental program included tests for slump flow, compressive and tensile strength (from 3 h to 28 days), water absorption, drying shrinkage, and pore size distribution. To elucidate the geopolymerization process and the role of ANTs in gel evolution and matrix densification, microstructural analyses were performed using X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and derivative thermogravimetry (DTG). Furthermore, a one-way Analysis of Variance (ANOVA) and Tukey’s HSD analysis were conducted to evaluate the statistical significance of the findings. The inclusion of ANTs reduced workability, with slump decreasing from 122 mm at 1.0% ANT to 89 mm at 3.0%, due to enhanced internal friction and water adsorption. Compressive strength at 1 day peaked at 55.95 MPa with 2.0% ANTs, 32.77% higher than the control. Likewise, splitting tensile strength improved by up to 191% at early ages with 2.0% ANTs but declined at 3.0% due to agglomeration. Drying shrinkage was minimised in the GPC-ANT_2 mix (3967 µm/m), while maximum porosity refinement was observed in GPC-ANT_3. Mercury intrusion porosimetry confirmed significant pore structure densification, and capillary absorption dropped by over 35% with optimal ANT content. TGA/DTG and XRD results revealed enhanced geopolymer gel formation and thermal stability in ANT-modified GPC, while FTIR and SEM confirmed improved bonding and microstructural compactness at 2.0% ANTs. These findings highlight that an optimum ANT dosage of 2.0% enhances early-age strength, durability, and structural integrity of heat-cured geopolymer concrete.