Durability and microstructural evolution of metakaolin-based geopolymers under various coupled temperature and relative humidity conditions

This study investigates the long-term durability of a metakaolin-based geopolymer composite formulated with potassium silicate, fine sand, limestone filler, and short carbon fibers, after 9 months of exposure to three hygrothermal conditions: 20 ◦C – 100 % RH, 90 ◦C – 100 % RH, and 90 ◦C – 0 % RH. A comprehensive evaluation was carried out, including mechanical tests, shrinkage, mass loss, water-accessible porosity, gas permeability, chloride migration, capillary absorption, TGA, and MIP. The results show that dry thermal exposure (90 ◦C – 0 % RH) induces pronounced drying shrinkage, nearly an order of magnitude higher than at 20 ◦C – 100 % RH and shifts the pore size distribution toward coarser capillary pores, with the modal pore diameter increasing from approximately 5 nm to 35 nm. Despite this, specimens exposed to dry heat exhibit the
highest mechanical performance, reaching a compressive strength of 69 MPa, compared to 48 MPa at 20 ◦C – 100 % RH, reflecting matrix densification. In contrast, high temperature saturated conditions (90 ◦C – 100 % RH) results in slightly reduced strength, attributed to moistureinduced softening effects. The dry heated specimens also exhibit higher gas permeability, increasing from the order of 10-18 to 10-17 m2, which is associated with enhanced connectivity of fine capillary pores and preferential transport pathways rather than changes in total porosity. The findings highlight the strong influence of long term exposure to extreme hygrothermal environment on microstructure-property relationships and support the use of metakaolin-based geopolymers in thermally demanding applications such as nuclear waste containment.