Synergistic effect of elevated curing temperatures and halloysite nanotubes on the mechanical properties and reaction mechanism of geopolymer concrete

In view of the problem that geopolymer concrete (GPC) is prone to insufficient reaction or pore defects under elevated curing temperature conditions, which leads to the degradation of mechanical properties, this study aims to explore the synergistic mechanism of elevated curing temperature and halloysite nanotubes (HNTs) for the performance regulation of GPC adaptable to high geothermal environments. GPC was prepared using fly ash (FA) and ground granulated blast furnace slag (GGBS) as precursor materials, with 2.0wt% HNTs as the reinforcing and modifying material, and temperature gradients of 20, 40, 60, and 80℃ were set. Through micro-testing techniques such as FTIR, XRD, TGA, and SEM, the synergistic effects of curing temperature and HNTs on the mechanical properties of GPC and the underlying mechanisms were systematically investigated. The results showed that at 20~40℃, the precursors reacted insufficiently, with a large amount of unreacted particles, sol phases, and moisture remaining in the system, leading to a loose matrix structure and relatively low strength. When the temperature rose to 60℃, the wavenumber of the Si-O-T absorption peak was the lowest, and moisture diffusion promoted the geopolymerization reaction, significantly increasing the gel content and improving the matrix compactness. Among all groups, the 60-H group (with HNTs) exhibited the most prominent optimization effect: its 28-day compressive strength reached 66.6 MPa and splitting tensile strength reached 3.40 MPa, which were 11.0% and 12.9% higher than those of the 60-C group (without HNTs) cured at the same temperature. HNTs accelerated the polymerization reaction through the "seed nucleation" effect, promoted the dissolution of quartz phase, and reduced the number and size of pores, thereby enhancing the mechanical properties of GPC. At 80℃, however, the wavenumber of the Si-O-T absorption peak increased and the degree of polymerization decreased. Excessively high temperature caused a sharp increase in gel content that wrapped around raw material particles and induced rapid moisture evaporation, hindering the continuation of the reaction and forming a large number of connected and loose pores, which ultimately led to the degradation of GPC mechanical properties.